Contenido

El término «fotovoltaico» se comenzó a usar en Reino Unido en el año 1849.[21]​ Proviene del griego φώς: phos, que significa «luz», y de -voltaico, que proviene del ámbito de la electricidad, en honor al físico italiano Alessandro Volta.[nota 1]​.

El efecto fotovoltaico fue reconocido por primera vez unos diez años antes, en 1839, por el físico francés Alexandre-Edmond Becquerel,[22]​[23]​ pero la primera célula solar no se fabricó hasta 1883. Su creador fue Charles Fritts, quien recubrió una muestra de selenio semiconductor con pan de oro para formar la unión. Este primitivo dispositivo presentaba una eficiencia menor del 1 %, pero demostró de forma práctica que, efectivamente, producir electricidad con luz era posible.[24]​ Los estudios realizados en el siglo por Michael Faraday, James Clerk Maxwell, Nikola Tesla y Heinrich Hertz sobre inducción electromagnética, fuerzas eléctricas y ondas electromagnéticas, y sobre todo, el trabajo realizado por Albert Einstein en 1905, por el cual le fue otorgado el premio Nobel en 1921, proporcionaron la base teórica y práctica del efecto fotoeléctrico,[25]​ que es el fundamento de la conversión de energía solar en electricidad.

Working principle

When a doped semiconductor is exposed to electromagnetic radiation, an incident photon hits an electron and knocks it out, creating a hole in the atom. Normally, the electron quickly finds another hole to fill again, and the energy provided by the photon is therefore dissipated as heat. The principle of a photovoltaic cell is to force the electrons and holes to move towards the opposite side of the material instead of simply recombining in it: thus, a potential difference, and therefore voltage, will be produced between the two parts of the material, as occurs in a battery.

To do this, a permanent electric field is created, through a pn junction, between two doped layers respectively, p and n "Doping (semiconductors)"). In silicon cells, which are mostly used, there are therefore:

• - The top layer of the cell, which is composed of n-type doped silicon#doping_type_n_and_type_p "Doping (semiconductors)").[note 2]​ In this layer, there are a greater number of free electrons than in a layer of pure silicon, hence the name n-doping, negative. The material remains electrically neutral, since both the silicon atoms and those of the dopant material are neutral: but the crystalline lattice has an overall greater presence of electrons than in a pure silicon lattice.

• - The bottom layer of the cell, which is composed of p-doped silicon "Doping (semiconductors)").[note 3]​ This layer therefore has a lower average amount of free electrons than a layer of pure silicon. The electrons are linked to the crystal lattice which, consequently, is electrically neutral, but has gaps, positive (p). Electrical conduction is ensured by these charge carriers, which move throughout the material.

At the time of creation of the p-n junction, free electrons from the n-shell instantly enter the p-shell and recombine with holes in the p-region. There will thus exist throughout the life of the junction, a positive charge in the n region along the junction (because electrons are missing) and a negative charge in the p region along the junction (because the holes have disappeared); The set forms the "Space Charge Zone" (ZCE) and there is an electric field between the two, from n to p. This electric field makes the ZCE a diode, which only allows the flow of current in one direction: the electrons can move from the p to the n region, but not in the opposite direction and on the contrary the holes only pass from n to p.

In operation, when a photon knocks an electron out of the matrix, creating a free electron and a hole, under the effect of this electric field each one goes in the opposite direction: the electrons accumulate in the n region (to become the negative pole), while the holes accumulate in the p-doped region (which becomes the positive pole). This phenomenon is most effective in the CEZ, where there are almost no charge carriers (electrons or holes), since they are nullified, or in the immediate vicinity of the CEZ: when a photon creates an electron-hole pair, they are separated and are unlikely to find their opposite, but if the creation takes place at a site further away from the junction, the electron (turned into a hole) maintains a great opportunity to recombine before reaching the n zone. But the CEZ is necessarily very thin, so it is not useful to give a large thickness to the cell.[note 4]​ Indeed, the thickness of layer n is very small, since this layer is basically only needed to create the CEZ that makes the cell work. On the other hand, the thickness of the p layer is greater: it depends on a compromise between the need to minimize electron-hole recombinations, and on the contrary allow the capture of the greatest number of photons possible, for which a certain minimum thickness is required.

In short, a photovoltaic cell is the equivalent of a power generator to which a diode has been added. To achieve a practical solar cell, it is also necessary to add electrical contacts (that allow the generated energy to be extracted), a layer that protects the cell but allows light to pass through, an anti-reflective layer "Reflection (physics)") to guarantee the correct absorption of photons, and other elements that increase its efficiency.

First modern solar cell

The American engineer Russell Ohl patented the modern solar cell in 1946,[26]​ although other researchers had advanced in its development previously: the Swedish physicist Sven Ason Berglund had patented a method in 1914 that tried to increase the capacity of photosensitive cells, while, in 1931, the German engineer Bruno Lange had developed a photocell using silver selenide instead of copper oxide.[27]​.

The modern era of solar technology did not arrive until 1954, when American researchers Gerald Pearson, Calvin S. Fuller and Daryl Chapin, from Bell Laboratories,[28] accidentally discovered that silicon semiconductors doped "Doping (semiconductors)") with certain impurities were very sensitive to light. These advances contributed to the manufacture of the first commercial solar cell. They used a diffuse p–n silicon junction, with a solar energy conversion of approximately 6%, an achievement compared to selenium cells that barely reached 0.5%.[29]​[30]​.

Later, the American Les Hoffman, president of the Hoffman Electronics company, through its semiconductor division, was one of the pioneers in the large-scale manufacturing and production of solar cells. Between 1954 and 1960, Hoffman managed to improve the efficiency of photovoltaic cells up to 14%, reducing manufacturing costs to achieve a product that could be marketed.[31]​.

First applications: space solar energy

At first, photovoltaic cells were used in a minority way to electrically power toys and in other minor uses, since the cost of producing electricity using these primitive cells was too high: in relative terms, a cell that produced one watt of energy using sunlight could cost $250, compared to the two or three dollars that a watt from a coal-fired thermoelectric plant cost.

Photovoltaic cells were rescued from oblivion thanks to the space race and the suggestion of using them in one of the first satellites placed in orbit around the Earth. The Soviet Union launched its first space satellite in 1957, and the United States would follow a year later. The first spacecraft to use solar panels was the American satellite Vanguard 1, launched in March 1958 (today the oldest satellite still in orbit). Its design used solar cells created by Peter Iles in an effort led by the company Hoffman Electronics.[32]​ The photovoltaic system allowed it to continue transmitting for seven years while the chemical batteries "Battery (electricity)") ran out in just 20 days.[33]​.

In 1959, the United States launched Explorer 6. This satellite had a series of solar modules installed, supported on external wing-like structures, made up of 9,600 solar cells from the Hoffman company.[31] This type of device later became a common feature of many satellites. There was some initial skepticism about how the system would work, but in practice the solar cells proved to be a great success, and were soon incorporated into the design of new satellites.

A few years later, in 1962, the Telstar "Telstar (satellite)") became the first communications satellite equipped with solar cells, capable of providing a power of 14 W.[34] This milestone generated great interest in the production and launch of geostationary satellites for the development of communications, in which the energy would come from a device for capturing sunlight. It was a crucial development that stimulated research by some governments and drove the improvement of photovoltaic panels.[35]​

Gradually, the space industry opted for the use of gallium arsenide (GaAs) solar cells, due to their greater efficiency compared to silicon cells. In 1970, the first highly efficient gallium arsenide heterostructure solar cell was developed in the Soviet Union by Zhorés Alfiorov and his research team.[36]​[37]​.

Starting in 1971, the Soviet space stations of the Salyut program were the first manned orbital complexes to obtain their energy from solar cells, attached to structures on the sides of the orbital module,[38]​ as was the North American Skylab station, a few years later.[39]​.

In the 1970s, after the first oil crisis, the United States Department of Energy and the space agency NASA began studying the concept of solar energy in space, which aimed to supply terrestrial energy through space satellites. In 1979 they proposed a fleet of satellites in geostationary orbit, each of which would measure 5 x 10 km and produce between 5 and 10 GW. The construction involved the creation of a large space infrastructure where hundreds of astronauts would work continuously. This gigantism was typical of a time when the creation of large space cities was planned. Technical difficulties aside, the proposal was scrapped in 1981 because it was too expensive.[40] In the mid-1980s, with oil prices again low, the program was cancelled.[41]

First terrestrial applications

Since its appearance in the aerospace industry, where it has become the most reliable means of supplying electrical energy in space vehicles,[59]​ photovoltaic solar energy has developed a large number of terrestrial applications. The first commercial installation of this type was carried out in 1966, at the Ogami Island lighthouse (Japan), allowing the use of flare gas to be replaced by a renewable and self-sufficient electrical source. It was the first lighthouse in the world powered by photovoltaic solar energy, and was crucial in demonstrating the viability and potential of this energy source.[60]​.

Improvements came slowly over the next two decades, and the only widespread use was in space applications, where its power-to-weight ratio was greater than that of any competing technology. However, this success was also the reason for its slow growth: the aerospace market was willing to pay any price to obtain the best possible cells, so there was no reason to invest in lower cost solutions if this reduced efficiency. Instead, the price of cells was largely determined by the semiconductor industry; Its migration to integrated circuit technology in the 1960s led to the availability of larger bars at relatively lower prices. As their price fell, the price of the resulting photovoltaic cells fell in equal measure. However, the cost reduction associated with this growing popularization of photovoltaics was limited, and in 1970 the cost of solar cells was still estimated at $100 per watt ($/Wp).[61]​.

Price reduction

In the late 1960s, American industrial chemist Elliot Berman was researching a new method for producing the raw material silicon from a tape process. However, he found little interest in his project and was unable to obtain the necessary financing for its development. Later, in a chance meeting, he was introduced to a team from the Exxon oil company that was looking for strategic projects 30 years into the future. The group had concluded that electrical energy would be much more expensive in the year 2000, and considered that this price increase would make new alternative energy sources more attractive, solar energy being the most interesting among these. In 1969, Berman joined Exxon's laboratory in Linden, New Jersey, called Solar Power Corporation (SPC).[61]​.

His effort was first directed at analyzing the potential market to identify the possible uses that existed for this new product, and he quickly discovered that if the cost per watt was reduced from $100/Wp to around $20/Wp, significant demand would arise. Knowing that the “ribbon silicon” concept could take years to develop, the team began looking for ways to reduce the price to $20/Wp using existing materials. The realization that the existing cells were based on the standard semiconductor manufacturing process was a first breakthrough, even if it was not an ideal material. The process began with the formation of a silicon ingot, which was cut transversely into disks called wafers. The wafers were then polished and then, for use as cells, they were coated with an anti-reflective layer. Berman realized that rough-cut wafers already had a perfectly good anti-reflective front surface, and by printing the electrodes directly on this surface, two important steps in the cell manufacturing process were eliminated.[61]​.

His team also explored other ways to improve the assembly of cells in arrays, eliminating the expensive materials and manual wiring previously used in space applications. Their solution consisted of using printed circuits on the back, acrylic plastic on the front, and silicone glue between them, embedding the cells. Berman realized that the silicon on the market was already “good enough” for use in solar cells. The small imperfections that could ruin a silicon ingot (or individual wafer) for use in electronics would have little effect on solar applications. Photovoltaic cells could be manufactured from material discarded by the electronics market, which would result in a great improvement in their price.[61]​.

Putting all these changes into practice, the company began purchasing rejected silicon at very low cost from existing manufacturers. By using the largest wafers available, which reduced the amount of wiring for a given panel area, and packaging them into panels with their new methods, by 1973 SPC was producing panels for $10/Wp and selling them for $20/Wp, decreasing the price of photovoltaic modules by a fifth in just two years.[61]​.

The maritime navigation market

SPC began contacting navigation buoy manufacturing companies offering them the product, but found a curious situation. The main company in the sector was Automatic Power, a manufacturer of disposable batteries. Realizing that solar cells could eat into the business and profits that the battery sector produced, Automatic Power bought a solar prototype from Hoffman Electronics to end up cornering it. Seeing no interest from Automatic Power, SPC then turned to Tideland Signal, another battery supply company formed by former Automatic Power managers.[61]​ Tideland brought a photovoltaic-powered buoy to market and was soon ruining Automatic Power's business.

The timing could not have been better, the rapid increase in the number of offshore oil platforms and other loading facilities produced a huge market among oil companies. As Tideland had been successful, Automatic Power then began sourcing its own supply of photovoltaic solar panels. They found Bill Yerkes of Solar Power International (SPI) in California, who was looking for a market to sell his product. SPI was soon acquired by one of its largest clients, oil giant ARCO, forming ARCO Solar. The ARCO Solar factory in Camarillo (California) was the first dedicated to the construction of solar panels, and was in continuous operation from its purchase by ARCO in 1977 until 2011 when it was closed by the company SolarWorld.[61]​.

This situation was combined with the oil crisis of 1973. The oil companies had enormous funds at their disposal at that time due to their enormous income during the crisis, but they were also very aware that their future success would depend on some other source of energy. In the following years, large oil companies began the creation of a series of solar energy companies, and were for decades the largest producers of solar panels. The companies ARCO, Exxon, Shell, Amoco (later acquired by BP), and Mobil maintained large solar divisions during the 1970s and 1980s. Technology companies also made significant investments, including General Electric, Motorola, IBM, Tyco, and RCA.[62]

Perfecting the technology

In the decades since Berman's advances, improvements have reduced production costs below $1/Wp, with prices below $2/Wp for the entire PV system. The price of the rest of the elements of a photovoltaic installation now represents a higher cost than the panels themselves.[63]​.

As the semiconductor industry developed toward larger and larger ingots, older equipment became available at reduced prices. The cells grew in size when these older equipment became available on the surplus market. The first ARCO Solar panels were equipped with cells 2 to 4 inches (51 to 100 mm) in diameter. Panels in the 1990s and early 2000s generally incorporated 5-inch (125 mm) cells, and since 2008 almost all new panels use 6-inch (150 mm) cells.[64] Also the widespread introduction of flat-panel televisions in the late 1990s and early 2000s led to the wide availability of large sheets of high-quality glass, which are used in the front of the panels.[65]​.

In terms of the cells themselves, there has only been one major change. During the 1990s, polysilicon cells became increasingly popular.[64]​ These cells offer less efficiency than those made of monosilicon, but are grown in large vats that greatly reduce the cost of production.[64]​ By the mid-2000s, polysilicon dominated the low-cost panel market.[64]​.

La producción industrial a gran escala de paneles fotovoltaicos despegó en la década de 1980, y entre sus múltiples usos se pueden destacar:.

Telecommunications and signaling

Photovoltaic solar energy is ideal for telecommunications applications, including, for example, local telephone exchanges, radio and television antennas, microwave repeater stations and other types of electronic communication links. This is because, in most telecommunications applications, storage batteries are used and the electrical installation is normally carried out in direct current (DC). In rugged and mountainous terrain, radio and television signals may be interfered with or reflected due to undulating terrain. In these locations, low power transmitters (LPT) are installed to receive and retransmit the signal among the local population.[66]​.

Photovoltaic cells are also used to power emergency communications systems "Emergency (disaster)"), for example in SOS (Emergency Telephone) poles on roads, railway signaling, beacons for aeronautical protection, weather stations or environmental data and water quality monitoring systems.[61]​.

Isolated devices

The reduction in energy consumption of integrated circuits made it possible in the late 1970s to use solar cells as a source of electricity in calculators, such as the Royal Solar 1, Sharp EL-8026 or Teal Photon.[67]​.

Other fixed devices that use photovoltaic energy have also increased in use in recent decades, in places where the cost of connecting to the electrical grid or using disposable batteries is prohibitively expensive. These applications include, for example, solar lamps, solar street lights, water pumps, parking meters,[68][69]​ emergency telephones, trash compactors,[70]​ temporary or permanent traffic signs, charging stations[71][72]​ or remote surveillance systems.

Rural electrification

In isolated environments, where little electrical power is required and access to the grid is difficult, photovoltaic panels have been used as an economically viable alternative for decades. To understand the importance of this possibility, it is worth taking into account that approximately a quarter of the world's population still does not have access to electrical energy.[73]​.

In developing countries, many villages (rural population) are located in remote areas, several kilometers from the nearest electricity grid. Due to this, photovoltaic energy is increasingly being incorporated to provide electricity to homes or medical facilities in rural areas. For example, in remote parts of India a rural lighting program has provided lighting using solar-powered LED lamps to replace kerosene lamps. The price of solar lamps was approximately the same as the cost of supplying kerosene for a few months.[74]​ Cuba and other Latin American countries are working to provide photovoltaic energy in areas far from conventional electrical power supplies.[75]​ These are areas where the social and economic benefits for the local population offer an excellent reason to install photovoltaic panels, although such initiatives have typically been relegated to one-off humanitarian efforts.[76]​.

Pumping systems

Photovoltaics are also used to power pumping installations for irrigation systems, drinking water in rural areas and water troughs for livestock,[77]​[78]​ or for water desalination systems.[61]​.

Photovoltaic pumping systems (as well as those powered by wind energy) are very useful where access to the general electricity grid is not possible or is prohibitively expensive.[79]​ Their cost is generally cheaper due to their lower operation and maintenance costs, and they have a lower environmental impact than pumping systems powered by internal combustion engines, which also have lower reliability.[80]​[81]​.

The pumps used can be both alternating current (AC) and direct current (DC). Normally direct current motors are used for small and medium-sized applications of up to 3 kW power, while for larger applications alternating current motors are used coupled to an inverter "Inverter (electronics)") that transforms the direct current from the photovoltaic panels for use. This allows systems to be sized from 0.15 kW to more than 55 kW of power, which can be used to supply complex irrigation or water storage systems.[82][83]​.

Hybrid solar-diesel systems

Due to the decreasing costs of photovoltaic solar energy, the use of solar-diesel hybrid systems is also expanding, which combine this energy with diesel generators to produce electricity in a continuous and stable manner.[84]​ This type of installations are normally equipped with auxiliary equipment, such as batteries and special control systems to achieve the stability of the system's electrical supply at all times.[85]​.

Due to its economic viability (transporting diesel to the point of consumption is usually expensive) in many cases old generators are replaced by photovoltaics, while new hybrid installations are designed in such a way that they allow the solar resource to be used whenever it is available, minimizing the use of generators, thus reducing the environmental impact of electricity generation in remote communities and in facilities that are not connected to the electrical grid. An example of this is mining companies,[84]​[86]​ whose operations are normally located in open fields, far from large population centers. In these cases, the combined use of photovoltaics allows us to greatly reduce the dependence on diesel fuel, allowing savings of up to 70% in the cost of energy.[87]​.

This type of system can also be used in combination with other sources of renewable energy generation, such as wind energy.[88]​.

Transport and maritime navigation

Although photovoltaics are not yet widely used to provide traction in transportation, they are increasingly being used to provide auxiliary power in ships and cars. Some vehicles are equipped with air conditioning powered by photovoltaic panels to limit the interior temperature on hot days,[89]​ while other hybrid prototypes use them to recharge their batteries without having to connect to the electrical grid.[90]​[91]​ The practical possibility of designing and manufacturing vehicles powered by solar energy, as well as boats[92]​[93]​ and airplanes,[94]​ has been amply demonstrated, with road transport being considered the most viable for the photovoltaic.[95]​.

Solar Impulse is a project dedicated to the development of an aircraft powered solely by photovoltaic solar energy. The prototype can fly during the day powered by the solar cells that cover its wings, while charging the batteries that allow it to stay in the air at night.[96][97]​.

Solar energy is also commonly used in lighthouses, buoys and maritime navigation beacons, recreational vehicles, charging systems for electrical accumulators on ships, and cathodic protection systems.[61]​ The recharging of electric vehicles is becoming increasingly important.[95]​.

Building Integrated Photovoltaics

Many photovoltaic installations are often located on buildings: they are usually located on an existing roof, or they are integrated into elements of the building's own structure, such as skylights, skylights or facades.[98]​.

Alternatively, a photovoltaic system can also be located physically separate from the building, but connected to the building's electrical installation to supply energy. By 2010, more than 80% of the 9,000 MW of photovoltaics that Germany had in operation at the time had been installed on rooftops.[99]​.

Building integrated photovoltaics (BIPV) are increasingly being incorporated as a primary or secondary source of electrical energy in new domestic and industrial buildings,[100]​ and even in other architectural elements, such as bridges.[101]​ Roof tiles with integrated photovoltaic cells are also quite common in this type of integration.

According to a study published in 2011, the use of thermal imaging has shown that solar panels, provided there is an open gap through which air can circulate between the panels and the roof, provide a passive cooling effect on buildings during the day and also help maintain the heat accumulated during the night.[102]​.

Grid connection photovoltaics

One of the main applications of photovoltaic solar energy, most developed in recent years, consists of power plants connected to the grid for electricity supply,[103]​ as well as photovoltaic self-consumption systems, of generally lower power, but also connected to the electrical grid.

floating photovoltaic

Although solar panels are usually installed on land, it is possible to install them floating on the waters of reservoirs or calm lakes.[104]​ Although it is more expensive, it has many advantages: it reduces losses due to evaporation of the reservoir water, improves its quality (because less algae grows),[105]​ the installation is simpler, the cooling of the panels themselves is facilitated (which increases the energy they produce)[104]​ and it is an alternative way for reservoirs to hydroelectric plants generate electricity, without wasting the water they store or occupying additional land.[105]​.

Una planta solar fotovoltaica cuenta con distintos elementos que permiten su funcionamiento, como son los paneles fotovoltaicos para la captación de la radiación solar, y los inversores "Inversor (electrónica)") para la transformación de la corriente continua en corriente alterna.[106]​ Existen otros, los más importantes se mencionan a continuación:.

Photovoltaic solar panels

Generally, a photovoltaic module or panel consists of an association of cells, encapsulated in two layers of EVA (ethylene-vinyl-acetate), between a front sheet of glass and a back layer of a thermoplastic polymer (tedlar is frequently used) or another sheet of glass when it is desired to obtain modules with some degree of transparency.[107] Very frequently this assembly is framed in an anodized aluminum structure with the aim of increasing the mechanical resistance of the assembly and facilitating anchoring. from the module to the support structures.[107]​.

The most commonly used cells in photovoltaic panels are silicon, and can be divided into three subcategories:

• - Monocrystalline silicon cells are made up of a single silicon crystal, normally manufactured using the Czochralski process.[108]​ This type of cell has a uniform dark blue color.

• - Polycrystalline silicon cells (also called multicrystalline) are made up of a set of silicon crystals, which explains why their performance is somewhat lower than that of monocrystalline cells.[64]​ They are characterized by a more intense blue color.

• - Amorphous silicon cells. They are less efficient than crystalline silicon cells but also less expensive. This type of cell is, for example, the one used in solar applications such as watches or calculators.[109]​.

The current versus voltage curve of a module provides us with useful information about its electrical performance.[110] Manufacturing processes often cause differences in the electrical parameters of different photovoltaic modules, even in cells of the same type. Therefore, only the experimental measurement of the I – V curve allows us to precisely establish the electrical parameters of a photovoltaic device. This measurement provides very relevant information for the design, installation and maintenance of photovoltaic systems. Usually, the electrical parameters of PV modules are measured by indoor testing. However, outdoor testing has important advantages such as no expensive artificial light source is required, there is no limitation on the size of the samples, and the illumination of the samples is more homogeneous.

The performance of a photovoltaic (PV) module depends on the environmental conditions, mainly on the global incident irradiance G in the plane of the module. However, the temperature T of the p–n junction also influences the main electrical parameters: the short-circuit current ISC, the open-circuit voltage VOC and the maximum power Pmax. In general, it is known that VOC shows a significant inverse correlation with T, while for ISC that correlation is direct, but weaker, so this increase does not compensate for the decrease in VOC. As a consequence, Pmax decreases as T increases. This correlation between the output power of a solar cell and the working temperature of its junction depends on the semiconductor material, and is due to the influence of T on the concentration, lifetime and mobility of the intrinsic carriers, that is, electrons and holes, within the photovoltaic cell.

Temperature sensitivity is usually described by temperature coefficients, each of which expresses the derivative of the parameter to which it refers with respect to the junction temperature. The values ​​of these parameters can be found in any PV module data sheet; They are the following:

• β: Coefficient of variation of VOC with respect to T, given by ∂VOC/∂T.

• α: Coefficient of variation of ISC with respect to T, given by ∂ISC/∂T.

• δ: Coefficient of variation of Pmax with respect to T, given by ∂Pmax/∂T.

Techniques for estimating these coefficients from experimental data can be found in the literature[111].

Investors

The direct electric current provided by the photovoltaic modules can be transformed into alternating current using an electronic device called an inverter "Inverter (electronics)")[106]​ and injected into the electrical grid (for energy sale) or into the interior grid (for self-consumption).

The simplified process would be as follows:

• - Energy is generated at low voltages (380-800 V) and direct current.

• - It is transformed with an inverter "Inverter (electronics)") into alternating current.

• - In power plants less than 100 kW, the energy is injected directly into the distribution network at low voltage (400 V in three-phase or 230 V in single-phase).

• - And for powers greater than 100 kW, a transformer is used to raise the energy to medium voltage (up to 36 kV) and it is injected into the transport networks for subsequent supply.

In the initial stages of the development of photovoltaic inverters, the requirements of the operators of the electrical networks to which they were connected only requested the contribution of active energy and the disconnection of the inverter from the network if it exceeded certain voltage and frequency limits. With the progressive development of this equipment and the increasing importance of smart electrical networks, inverters are now capable of providing reactive energy and even providing stability to the electrical grid.[112]​[113]​.

solar trackers

The use of trackers with one or two axes allows for a considerable increase in solar production, around 30% for the former and an additional 6% for the latter, in places with high direct radiation.[114]​[115]​.

Solar trackers are quite common in photovoltaic applications.[116] There are several types:

• - In two axes: the surface always remains perpendicular to the Sun.

• - On a polar axis: the surface rotates on an axis facing south and inclined at an angle equal to the latitude. The gyration is adjusted so that the normal to the surface coincides at all times with the terrestrial meridian that contains the Sun.

• - On an azimuthal axis: the surface rotates about a vertical axis, the angle of the surface is constant and equal to the latitude. The gyration is adjusted so that the normal to the surface coincides at all times with the local meridian that contains the Sun.

• - On a horizontal axis: the surface rotates on a horizontal axis and oriented in a north-south direction. The gyration is adjusted so that the normal to the surface coincides at all times with the terrestrial meridian that contains the Sun.

Cabling

It is the element that transports electrical energy from its generation, for its subsequent distribution and transportation. Its sizing is determined by the most restrictive criterion between the maximum allowable voltage drop and the maximum allowable intensity. Increasing the conductor sections obtained as a result of the theoretical calculations provides added advantages such as:

• - More discharged lines, which prolongs the useful life of the cables.

• - Possibility of increasing plant power without changing the driver.

• - Better response to possible short circuits.

• - Improvement of installation performance.

Photovoltaic concentration plants

Another type of technology in photovoltaic plants are those that use a concentration technology called CPV (Concentrated Photovoltaics)[117]​ to maximize the solar energy received by the installation, just like in a solar thermal power plant. Photovoltaic concentration facilities are located in locations with high direct solar irradiation, such as the countries on both shores of the Mediterranean, Australia, the United States, China, South Africa, Mexico, etc. Until 2006, these technologies were part of the scope of research, but in recent years larger facilities have been launched, such as the ISFOC (Institute of Concentrated Photovoltaic Solar Systems) in Puertollano (Castilla-La Mancha) with 3 MW supplying electricity to the electrical grid.[118]​[119][120]​.

The basic idea of ​​photovoltaic concentration is the replacement of semiconductor material with (cheaper) reflective or refracting material. The degree of concentration can reach a factor of 1000,[117]​ in such a way that, given the small surface area of ​​the solar cell used, the most efficient technology can be used (triple junction, for example). On the other hand, the optical system introduces a loss factor that recovers less radiation than flat photovoltaics. This, together with the high precision of the monitoring systems, constitutes the main barrier to be solved by concentration technology.

The development of large plants (above 1 MW) has recently been announced.[121] Concentrated photovoltaic plants use a double-axis tracker to enable maximum use of the solar resource throughout the day.

Entre los años 2001 y 2016 se ha producido un crecimiento exponencial de la producción fotovoltaica, duplicándose aproximadamente cada dos años.[122]​ La potencia total fotovoltaica instalada en el mundo (conectada a red) ascendía a 16 gigavatios (GW) en 2008, 40 GW en 2010, 100 GW en 2012, 180 GW en 2014, 300 GW en 2016 y 500 GW en 2018.[123]​[124]​[125]​[126]​[127]​[9]​[10]​.

Históricamente, Estados Unidos lideró la instalación de energía fotovoltaica desde sus inicios hasta 1996, cuando su capacidad instalada alcanzaba los 77 MW, más que cualquier otro país hasta la fecha. En los años posteriores, fueron superados por Japón, que mantuvo el liderato hasta que a su vez Alemania la sobrepasó en 2005, manteniendo el liderato desde entonces. A comienzos de 2016, Alemania se aproximaba a los 40 GW instalados.[130]​ Sin embargo, por esas fechas China, uno de los países donde la fotovoltaica está experimentando un crecimiento más vertiginoso superó a Alemania, convirtiéndose desde entonces en el mayor productor de energía fotovoltaica del mundo.[130]​ Se espera que multiplique su potencia instalada actual hasta los 200 GW en 2020.[128]​[131]​[132]​.

World production

The total installed capacity already represents a significant fraction of the electricity mix in the European Union, covering on average 3.5% of electricity demand and reaching 7% in periods of highest production.[127]​ In some countries, such as Germany,[133][134]​ Italy,[135][136][137][note 5]​ United Kingdom[138]​ or Spain,[139]​ it reaches higher maximums. 10%, just like in Japan[140]​ or in some sunny states in the United States, such as California.[141]​ The annual production of electrical energy generated by this energy source worldwide was equivalent in 2015 to about 184 TWh, enough to supply the energy needs of millions of homes and covering approximately 1% of global electricity demand.[127]​.

China

Photovoltaics has become one of the largest industries in the People's Republic of China. The Asian country is the world leader in photovoltaic capacity, with an installed power of more than 170 GW at the beginning of 2019.[142]​ It also has about 400 photovoltaic companies, among which Trina Solar, Jinko Solar and JA Solar, global giants in the manufacturing of solar panels, stand out. In 2014, it produced approximately half of the photovoltaic products manufactured in the world (China and Taiwan together account for more than 60% share). The production of photovoltaic panels and cells in China has increased significantly over the last decade: in 2001 it maintained a share of less than 1% of the world market, while at the same time, Japan and the United States accounted for more than 70% of world production. However, the trend has reversed and China currently far surpasses the rest of the producers.

Chinese solar panel production capacity practically quadrupled between 2009 and 2011, even exceeding global demand. As a result, the European Union accused the Chinese industry of dumping, that is, selling its panels at prices below cost, imposing tariffs on the import of this material.[143]​[144]​.

The installation of photovoltaic energy has developed spectacularly in the Asian country in recent years, even exceeding initial forecasts. Due to such rapid growth, Chinese authorities have been forced to reevaluate their photovoltaic power target on several occasions.

The total installed power in China grew to 77 GW at the end of 2016, after connecting 36 GW in the last year, according to the country's official statistics.[145]​ In 2017, China had exceeded the objective set by the government for 2020, a photovoltaic power of 100 GW.[146]​ Therefore, at the end of 2018 it was announced that China could raise its solar target to 2020 to more than 200 GW.[147]​.

This growth reflects the abrupt decline in the costs of photovoltaic energy, which is currently beginning to be a cheaper option than other energy sources, both at retail and commercial prices. Chinese government sources have stated that photovoltaics will have more competitive prices than coal and gas (also providing greater energy independence) by the end of this decade.[148]​.

USA

Since 2010, the United States has been one of the countries with the most activity in the photovoltaic market, it has large companies in the sector, such as First Solar or SolarCity, as well as numerous grid connection plants. At the beginning of 2017, the United States exceeded 40 GW of installed photovoltaic power,[149]​ enough to provide electricity to more than 8 million homes, after doubling its solar capacity in less than two years.[150]​.

Although the United States does not maintain a uniform national energy policy across the country regarding photovoltaics, many states have individually set renewable energy goals, including solar energy in different proportions in this planning. In this sense, California Governor Jerry Brown has signed legislation requiring that 33% of the state's electricity be generated by renewable energy by the end of 2020.[151] These measures have been supported by the federal government with the adoption of the Investment Tax Credit (ITC), a tax exemption established in 2006 to promote the development of photovoltaic projects, and which has recently been extended until 2023.[152]​.

A private report[153]​ states that photovoltaic solar energy has expanded rapidly over the last 8 years, growing at an average of 40% each year. Thanks to this trend, the cost of the kWh produced by photovoltaic energy has been greatly reduced, while the cost of electricity generated by fossil fuels has continued to increase. As a result, the report concludes that PV will reach grid parity with conventional energy sources in many regions of the United States by 2015. But to achieve a 10% energy market share, the report continues, PV companies will need to further streamline installations, so that solar becomes a plug-and-play technology. That is, it is easy to acquire the components of each system and their interconnection is simple, as is their connection to the network.[153]​.

Currently, most facilities are connected to the grid and use net balancing systems that allow nighttime electricity consumption from energy generated during the day. New Jersey leads the States with the least restrictive net balancing law,[154]​ while California leads the total number of homes with solar energy. Many of them were installed during the million solar roof initiative.[155]​.

The current trend and growth rate indicate that in the coming years a large number of photovoltaic plants will be built in the south and southwest of the country, where available land is abundant, in the sunny deserts of California, Nevada and Arizona. Companies are increasingly acquiring large areas in these areas, with the intention of building larger large-scale plants.[156]​.

Japan

Photovoltaic energy in Japan has expanded rapidly since the 1990s. The country is one of the leaders in the manufacturing of photovoltaic modules and is among the first in terms of installed power, with more than 23 GW at the end of 2014, most of it connected to the grid.[157]​[158][159]​ Irradiance in Japan is optimal, standing between 4.3 and 4.8 kWh·m²·day, making it an ideal country for the development of this type of energy.

The sale of photovoltaic modules for commercial projects has grown rapidly following the introduction by the Japanese Government in July 2012 of a tariff to encourage photovoltaics following the Fukushima nuclear accident and the paralysis of most of the country's nuclear power plants.

The majority of modules come from local manufacturers, among which Kyocera, Sharp Corporation, Mitsubishi or Sanyo stand out, while a small part are imported, according to data from the Japan Photovoltaic Energy Association (JPA). 2012.[161]​ Although this trend is reversing, still more than 75% of the cells and modules sold in Japan at the beginning of 2012 were destined for residential projects, while about 9% were used in commercial photovoltaic installations.[162]​.

In 2014, the total photovoltaic power installed in the country was around 23 GW, which contributed approximately 2.5% to the country's electricity demand.[127] During the summer of 2015, it was reported that photovoltaic production in Japan had at certain times covered 10% of the total national demand.[140] Two years later, in 2016, it was around 42 GW,[149]​ and the forecast suggests that the Japanese photovoltaic market will grow even more in the coming years.[163]​.

Germany

At the beginning of 2016, Germany has an installed capacity of around 40 GW.[130]​ In 2011 alone, Germany installed around 7.5 GW,[164]​ and photovoltaics produced 18 TW·h of electricity, 3% of the total consumed in the country.[134][165]​.

The photovoltaic market in Germany has grown considerably since the beginning of the century thanks to the creation of a regulated tariff for the production of renewable energy, which was introduced by the "German Renewable Energy Act", a law published in 2000. Since then, the cost of photovoltaic installations has fallen by more than 50% in five years, since 2006.[166]​ Germany has set the goal of producing 35% of its electricity through renewable energies in 2020 and reach 100% in 2050.[167]​.

In 2012, the introduced tariffs cost Germany around €14 billion per year for both wind and solar installations. This cost is distributed among all taxpayers through an additional cost of €3.6 cents per kWh[168]​ (approximately 15% of the total cost of electricity for the domestic consumer).[169]​.

The considerable power installed in Germany has set several records in recent years. During two consecutive days in May 2012, for example, the solar photovoltaic plants installed in the country produced 22,000 MWh in the midday hour, which is equivalent to the generating power of twenty nuclear power plants working at full capacity.[170]​[note 6]​ Germany shattered this record on July 21, 2013, with an instantaneous power of 24 GW at noon.[171]​[172]​ Due to the highly distributed nature of German PV, approximately 1.3-1.4 million small PV systems contributed to this new record. Approximately 90% of the solar panels installed in Germany are located on roofs.[173]​.

In June 2014, German photovoltaics once again broke records for several days, producing up to 50.6% of all electricity demand during a single day, and surpassing the previous record for instantaneous power to 24.24 GW.[174][175][176]​.

At the beginning of summer 2011, the German Government announced that the current regulated tariff scheme would end when installed power reached 52 GW. When this happens, Germany will apply a new injection tariff scheme, the details of which are not yet known.[177]​.

However, aware that energy storage through batteries is essential for the massive deployment of renewables such as wind energy or photovoltaics, given their intermittency, on May 1, 2013, Germany launched a new aid program to encourage photovoltaic systems with storage batteries.[178]​ In this way, photovoltaic installations under 30 kW that install batteries and accumulate electricity are financed with 660 euros. for each kW of battery storage. The program is endowed with 25 million euros annually distributed in 2013 and 2014, and in this way energy is available when the resource is not available - there is no wind or it is at night -,[178]​ in addition to facilitating the stability of the electrical system.[179]​.

India

India is densely populated and also has high solar irradiation, making the country one of the best candidates for the development of photovoltaics. In 2009, India announced a program to accelerate the use of solar installations in government buildings, as well as hospitals and hotels.[180]​.

The drop in the price of photovoltaic panels has coincided with an increase in the price of electricity in India. Government support and the abundance of the solar resource have helped drive the adoption of this technology.[181]​.

The 345 MW Charanka solar park (one of the largest in the world) was commissioned in April 2012 and expanded in 2015, along with a total of 605 MW in the Gujarat region.[182] The construction of other large solar parks has been announced in the state of Rajasthan.[183] The 40 MW Dhirubhai Ambani solar park was also inaugurated in 2012.[184]​.

In January 2015, the Indian government significantly increased its solar development plans, setting an investment target of $100 billion and 100 GW of solar capacity by 2022.[185][186]​.

At the beginning of 2017, the total installed capacity in India was above 10 GW.[187]​ India hopes to quickly reach 20 GW installed,[188]​ meeting its goal of creating 1 million jobs[189]​ and reaching 100 GW in 2022.[190][191]​.

Italy

Italy is among the first countries producing electricity from photovoltaic energy, thanks to the incentive program called Conto Energia.[192]​ The growth has been exponential in recent years: installed power tripled in 2010 and quadrupled in 2011, producing in 2012 5.6% of the total energy consumed in the country.[135]​.

This program had a total budget of €6.7 billion. Once this limit was reached, the Government has stopped encouraging new installations, having reached network parity. A report published in 2013 by Deutsche Bank concluded that grid parity had indeed been achieved in Italy and other countries around the world.[193]​ The sector has come to provide work for around 100,000 people, especially in the sector of the design and installation of said solar plants.[194]​.

Since mid-2012, new legislation has been in force that requires the registration of all plants greater than 12 kW; those with lower power (rooftop photovoltaics in residences) are exempt from registration.[195]​ At the end of 2016, the total installed power was above 19 GW,[149]​ assuming such significant energy production that several gas plants operated at half their potential during the day.

United Kingdom

Solar energy in the United Kingdom, although relatively unknown until recently,[196]​ has taken off very rapidly in recent years, due to the drastic fall in the price of photovoltaic panels and the introduction of regulated tariffs from April 2010.[197]​ In 2014, there were already some 650,000 solar installations registered in the British Isles, with a total capacity close to 5 GW.[198]​ The largest solar plant in the country is located on Southwick Estate, near Fareham, and has a capacity of 48 MW. It was inaugurated in March 2015.[199]​.

In 2012, David Cameron's British government committed to powering four million homes with solar energy in less than eight years,[200]​ which is equivalent to installing around 22 GW of photovoltaic capacity by 2020.[197]​ By early 2016, the United Kingdom had installed more than 10 GW of solar photovoltaic energy.[201]​.

Between the months of April and September 2016, solar energy produced more electricity in the United Kingdom (6,964 GWh) than that produced by coal (6,342 GWh), both representing around 5% of demand.[138]​.

France

The French market is the fourth most important within the European Union, after the markets of Germany, Italy and the United Kingdom. At the end of 2014 it had more than 5 GW installed, and currently maintains sustained growth, it is estimated that in 2015 it will connect 1 GW additional to the current capacity to the electrical grid.[202] Recently, the French country increased the quota of its auctions for photovoltaic energy from 400 to 800 MW, as a consequence of government recognition of the increasingly competitiveness of solar energy.[202]​.

In France there is one of the largest photovoltaic plants in Europe, a 300 MW project called Cestas.[203]​[204]​[205]​ Its entry into operation took place at the end of 2015, providing the photovoltaic sector with an example for the rest of the European industry to follow.[203]​.

Spain

Spain is one of the countries in Europe with the highest annual irradiation.[43] This makes solar energy more profitable in this country than in others. Regions such as northern Spain, which are generally considered unsuitable for photovoltaic energy, receive more annual irradiation than the average in Germany, a country that has maintained leadership in the promotion of photovoltaic solar energy for years.[43]​.

Since the beginning of the 2000s, in accordance with the support measures for renewable energies that were being carried out in the rest of Europe, the regulation that establishes the technical and administrative conditions had been approved, and which marked the beginning of a slow takeoff of photovoltaics in Spain. In 2004, the Spanish government eliminated economic barriers to connecting renewable energy to the electrical grid. Royal Decree 436/2004 equalized the conditions for its large-scale production, and guaranteed its sale through generation premiums.[206]​.

Thanks to this regulation, and the subsequent RD 661/2007,[207] Spain was in 2008 one of the countries with the most installed photovoltaic power in the world, with 2708 MW installed in a single year. However, subsequent modifications in the legislation of the sector[208]​ slowed down the construction of new photovoltaic plants, so that in 2009 only 19 MW were installed, in 2010, 420 MW, and in 2011 354 MW were installed, corresponding to 2% of the total in the European Union.[133]​.

In terms of energy production, in 2010 photovoltaic energy covered approximately 2% of electricity generation in Spain, while in 2011 and 2012 it represented 2.9%, and in 2013 3.1% of electricity generation according to data from the operator, Red Eléctrica.[209]​[210][211]​

In 2018, the share of photovoltaic solar energy in Spain reached 3.2% of all energy produced nationally.[212]​.

At the beginning of 2012, the Spanish Government approved a Royal Decree Law by which the installation of new photovoltaic plants and other renewable energies was paralyzed.[213]​ At the end of 2015, the photovoltaic power installed in Spain amounted to 4,667 MW.[214]​ In 2017, Spain fell for the first time from the list of the ten countries with the highest installed photovoltaic capacity, being surpassed by Australia and Korea. of the South.[215]​ However, in July 2017, the Government organized an auction that awarded more than 3,500 MW of new photovoltaic energy plants,[216]​ that will allow Spain to achieve the renewable energy generation objectives established by the European Union for 2020. As a novelty, neither the construction of the awarded plants nor their operation will entail any cost for the system, except in the event that the market price falls below a floor. established at the auction. The great drop in costs of photovoltaic energy has allowed large companies to bid at market price.[217]​.

In 2019, photovoltaics has increased the installed power in Spain by more than 3,000 MW with a total installed power of 7,800 MW.[218] Spain has the largest connected photovoltaic plant in Europe, located in the town of Mula (Murcia), with 494 MW.[219]​.

Latin America

In Latin America, photovoltaics has begun to take off in recent years. The construction of a good number of solar plants has been proposed in various countries, throughout the entire region.[221]​.

Photovoltaic solar energy is expanding in Brazil, while in 2020 the country had 7.8 GW of installed solar energy, the fourteenth country in the world in terms of that energy, as of October 2022 the installed capacity reached a total of 21 GW, with an average capacity factor of 23%. In 2023, Brazil will be among the 10 countries in the world with the most installed solar energy.[222]​ Some of the most irradiated Brazilian states are Minas Gerais, Bahia and Goiás, which have an irradiation between 4.5 to 6.0 kWh/m²/day.[223]​[224]​.

Mexico is the second Latin American country with the highest installed capacity (7.0 GW in 2021),[225]​ and still has enormous potential when it comes to solar energy.[226]​[227]​ 70% of its territory has an irradiation greater than 4.5 kWh/m²/day, which makes it a very sunny country, and implies that using current photovoltaic technology, a 25 km² solar plant in Any place in the state of Chihuahua or the Sonoran Desert (which would occupy 0.01% of Mexico's surface) could provide all the electricity demanded by the country.[228]​.

The Aura Solar project, located in La Paz (Baja California Sur) "La Paz (Baja California Sur)"), inaugurated at the beginning of 2014, which intended to generate 82 GWh per year, enough to supply the consumption of 164,000 inhabitants (65% of the population of La Paz "La Paz (Baja California Sur)")), but was devastated by Hurricane Odile in September of the same year and the plant stopped operating for several months.

In 2016, the reconstruction of the plant was carried out, which was completed at the end of the same year and from 2017 to date it has been in operation again.[229]​.

Another 47 MW photovoltaic plant is in the planning phase in Puerto Libertad (Sonora) "Puerto Libertad (Sonora)").[230]​ The plant, originally designed to house 39 MW, was expanded to allow the generation of 107 GWh/year.[231]​.

Mexico already has more than 3000 MW installed. It is expected to experience greater growth in the coming years, in order to reach the goal of covering 35% of its energy demand from renewable energy in 2024, according to a law approved by the Mexican government in 2012.[232][233]​.

Until a few years ago, Chile led solar production in Latin America (today it is in third place - 4.4 GW in 2021[225]​). The first photovoltaic solar plant in Chile was El Águila, of 2.2 MWp located in Arica, completed in 2012. This country inaugurated a 100 MW photovoltaic plant in June 2014, which became the largest to date in Latin America.[234] The high price of electricity and the high levels of radiation that exist in the north of Chile have promoted the opening of an important free market for subsidies.[235]​ At the end of 2018, the Andean country had 2,427 MW of photovoltaics in operation. Chile has a potential of more than 1800 GW of possible solar energy in the Atacama Desert, according to a study carried out by the German GIZ in Chile (German Society for International Cooperation, 2014). The Atacama Desert is the most irradiated place in the world with global irradiation levels (GHI) above 2700 kWh/m²/year.

Temporal evolution

The following table shows the details of the installed global power, broken down by each country, from 2002 to 2022:

Long term forecast

According to the Renewable Capacity Statistics 2023 report from the International Renewable Energy Agency (IRENA), the photovoltaic power installed in the world reached 773.2 GW in 2020, which represents a growth of 22% compared to the year previous.[1] This growth was double that of renewables as a whole, and represented 39% of the installed power of all electricity generation technologies in 2020.

China was the main photovoltaic market in 2020, with 48 GW installed, followed by the United States, with 19 GW, and Vietnam, with 11 GW1. Spain was among the countries with the highest percentage increase in photovoltaic power, with a 31.99% increase, reaching 18.2 GW.[2] Photovoltaics in countries such as Brazil, Chile and Saudi Arabia have not yet been developed as expected, and are expected to be developed in the coming years. By 2021, the photovoltaic power installed in the world was expected to exceed 900 GW, and that by 2025 it would reach 1,500 GW, according to SolarPower forecasts Europe.[3] The consulting firm Frost & Sullivan estimates that photovoltaic power will reach 1,000 GW in 2024, and that China, India and the United States will follow being the market leaders.

Photovoltaic energy has great potential to contribute to the energy transition and the mitigation of climate change, since it is a clean, renewable, decentralized and competitive source of energy. According to Irena, photovoltaic energy could cover 13% of global electricity demand in 2030, and 23% in 2050. In addition, photovoltaic energy could generate more than 23 million jobs in 2050, and avoid the emission of more than 4 billion tons of CO2 per year.

The PV Market Alliance (PVMA) organization, a consortium made up of several research entities, estimates that global capacity will be between 444-630 GW in 2020. In the most pessimistic scenario, it predicts that the annual installation rate will be between 40 and 50 gigawatts at the end of the decade, while in the most optimistic scenario it estimates that between 60 and 90 GW will be installed annually over the next five years. The intermediate scenario estimates that they will be between 50 and 70 GW, to reach 536 GW in 2020.[247]​[248]​ The PVMA figures agree with those previously published by . In June 2015, Greentech Media (GTM) published its report for 2020, which estimates that annual installations will increase from 40 to 135 GW, reaching a total global capacity of almost 700 GW in 2020. GTM's estimate is the most optimistic of all those published to date, estimating that 518 GW will be installed between 2015 and 2020, which is more than double other estimates.[249]​.

En Europa y en el resto del mundo se han construido un gran número de centrales fotovoltaicas a gran escala.[103]​ Actualmente las plantas fotovoltaicas más grandes del mundo son, de acuerdo a su capacidad de producción:[103]​.

Las mayores plantas solares del mundo se encuentran situadas en China e India. Kurnool Solar, en el estado indio de Andhra Pradesh alberga 1 GW de capacidad, equivalente en potencia a una central nuclear. La planta Yanchi Solar, en la provincia de Qinghai (China) cuenta asimismo con dicha capacidad. Entre los primeros puestos se encuentra también Longyangxia Hydro-Solar PV Station, situada junto a la presa de Longyangxia en China. Consiste en un macrocomplejo hidroeléctrico de 1280 MW, al que posteriormente se le añadió una central fotovoltaica de 320 MW, completada en 2013. A finales de 2015 se inauguró una segunda fase de 530 MW, lo que elevó la potencia total de la planta solar hasta los 850 MW.[251]​[252]​[253]​.

Otros proyectos de gran escala se encuentran situados en Estados Unidos. Solar Star, tiene una potencia de 579 MW y se encuentra en California.[254]​[255]​ Las plantas Topaz Solar Farm y Desert Sunlight Solar Farm en Riverside County, también en California, tiene asimismo una potencia de 550 MW.[256]​[257]​ El proyecto Blythe Solar Power consiste en una planta fotovoltaica de 500 MW, situada igualmente en Riverside County, cuya construcción está prevista próximamente.[258]​ En Europa, la planta de mayor envergadura está ubicada en Murcia (España). Cuenta con una capacidad de 494 MW y entró en operación en 2019.[218]​.

Hay otras muchas plantas de gran escala en construcción. El McCoy Solar Energy Project,[259]​[260]​ en Estados Unidos, tendrá una potencia de 750 MW una vez completado.[261]​ En los últimos años, se ha propuesto la construcción de varias plantas de potencias superiores a los 1000 MW en diferentes lugares del mundo. La planta Quaid-e-Azam Solar Park, situada en Pakistán y cuya primera fase ya se encuentra operativa con 100 MW,[262]​[263]​[264]​ tiene previsto ampliar su capacidad hasta los 1500 MW.[265]​ Los Emiratos Árabes Unidos planean también la construcción de una planta de 1000 MW.[266]​[267]​[268]​ El Ordos Solar Project,[269]​ situado en China, alcanzará los 2000 MW.[270]​ El proyecto Westlands Solar Park tiene una capacidad prevista de 2700 MW,[271]​ a ser completado en varias fases. El proyecto de Ladakh, en India, planea albergar 5 GW de capacidad fotovoltaica.[272]​.

En lo que respecta a instalaciones fotovoltaicas sobre tejado, la mayor instalación se encuentra en las instalaciones de Renault Samsung Motors en Busan (Corea del Sur), y cuenta con 20 MW distribuidos sobre las diferentes cubiertas, aparcamientos e infraestructuras del complejo. Inaugurada en 2013, proporciona energía a la fábrica y miles de hogares cercanos.[273]​.

Photovoltaic energy storage using batteries

Energy storage is presented as a major challenge to enable a continuous supply of energy, given that solar energy cannot be generated at night. Rechargeable batteries have traditionally been used to store excess electricity in isolated systems. With the emergence of grid-connected systems, excess electricity can be transported via the electrical grid to points of consumption. When renewable energy production represents a small fraction of demand, other energy sources can adjust their production appropriately to support the variability of renewable sources, but with the growth of renewable sources, more adequate control is necessary to balance the grid.

With the decline in prices, photovoltaic plants begin to have batteries to control the output power or store excess energy so that it can be used during the hours when renewable plants cannot generate directly. This type of batteries allows the electrical grid to be stabilized by smoothing demand peaks for minutes or hours. It is expected that in the future these batteries will play an important role in the electrical grid, as they can be charged during periods when generation exceeds demand and pour said energy into the grid when demand is greater than generation.

For example, in Puerto Rico a system with a capacity of 20 megawatts for 15 minutes (5 megawatt hours) is used to stabilize the frequency of the grid on the island. Another 27 megawatt system for 15 minutes (6.75 megawatt hours) with nickel-cadmium batteries was installed in Fairbanks, Alaska in 2003 to stabilize the voltage of transmission lines.[274]​.

Most of these battery banks are located next to the photovoltaic plants themselves. The largest systems in the United States include the 31.5 MW battery at the Grand Ridge Power plant in Illinois, and the 31.5 MW battery in Beech Ridge, Virginia.[275] Notable projects include the 400 MWh system (100 MW for four hours) of the Southern California Edison project and a 52 MWh project in Kauai, Hawaii, which completely displaces the production of a 13MW for use after sunset.[276]​ Other projects are located in Fairbanks (40 MW for 7 minutes using nickel-cadmium batteries)[277]​ and in Notrees (Texas) (36 MW for 40 minutes using lead-acid batteries).[278]​.

In 2015, a total of 221 MW with battery storage was installed in the United States, and it is estimated that the total power of this type of systems will grow to 1.7 GW in 2020. The majority installed by wholesale companies in the US market.[279]​.

Photovoltaic self-consumption consists of the small-scale individual production of electricity for one's own consumption, through photovoltaic panels. This can be complemented with the net balance. This production scheme, which allows electricity consumption to be offset by what is generated by a photovoltaic installation at times of lower consumption, has already been successfully implemented in many countries. It was proposed in Spain by the Photovoltaic Industry Association (ASIF) to promote renewable electricity without the need for additional financial support,[280]​ and was in the project phase by the IDAE.[281]​ It was later included in the Renewable Energy Plan 2011-2020,[282]​ but has not yet been regulated.

However, in recent years, due to the growing rise of small renewable energy installations, self-consumption with net balance has begun to be regulated in various countries around the world, being a reality in countries such as Germany, Italy, Denmark, Japan, Australia, the United States, Canada and Mexico, among others.

Among the advantages of self-consumption with respect to network consumption are the following:.

• - With the lower cost of self-consumption systems and the increase in electricity rates, it is increasingly profitable for one to produce one's own electricity.[17]​.

• - Dependency on electricity companies is reduced.

• - Photovoltaic self-consumption systems use solar energy, a free, inexhaustible, clean and environmentally friendly source.

• - A distributed electricity generation system is generated that reduces the need to invest in new networks and reduces energy losses due to the transport of electricity through the network.[283]​.

• - The country's energy dependence on the outside world is reduced.

• - Problems are avoided in supplying all demand at peak times, known for power outages and power surges.

• - The impact of electrical installations on their surroundings is minimized.

• - Companies reduce their energy costs, improve their image and reinforce their commitment to the environment.[284]​.

In the case of photovoltaic self-consumption, the return on investment time is calculated based on how much electricity is no longer consumed from the network, due to the use of photovoltaic panels.

For example, in Germany, with electricity prices of €0.25/kWh and an insolation of 900 kWh/kWp, a 1 kWp installation saves about €225 per year, which with installation costs of €1,700/kWp means that the system will pay for itself in less than 7 years.[285] This figure is even lower in countries like Spain, with irradiation higher than that existing in the north. of the European continent.[43]​.

El efecto de la temperatura sobre los módulos fotovoltaicos se suele cuantificar mediante unos coeficientes que relacionan las variaciones de la tensión en circuito abierto, de la corriente de cortocircuito y de la potencia máxima a los cambios de temperatura. En este artículo, directrices experimentales integrales para estimar los coeficientes de temperatura[286]​.

Las eficiencias de las células solares varían entre el 6 % de aquellas basadas en silicio amorfo hasta el 46 % de las células multiunión.[288]​[289]​ Las eficiencias de conversión de las células solares que se utilizan en los módulos fotovoltaicos comerciales (de silicio monocristalino o policristalino) se encuentran en torno al 16-22 %.[290]​[291]​.

El coste de las células solares de silicio cristalino ha descendido desde 76,67 $/Wp en 1977 hasta aproximadamente 0,36 $/Wp en 2014.[287]​[292]​ Esta tendencia sigue la llamada ley de Swanson, una predicción similar a la conocida Ley de Moore, que establece que los precios de los módulos solares descienden un 20 % cada vez que se duplica la capacidad de la industria fotovoltaica.[293]​.

En 2014, el precio de los módulos solares se había reducido en un 80 % desde el verano de 2008,[294]​[295]​ colocando a la energía solar por primera vez en una posición ventajosa respecto al precio de la electricidad pagado por el consumidor en un buen número de regiones soleadas.[296]​ En este sentido, el coste medio de generación eléctrica de la energía solar fotovoltaica es ya competitivo con el de las fuentes convencionales de energía en una creciente lista de países,[297]​ particularmente cuando se considera la hora de generación de dicha energía, ya que la electricidad es usualmente más cara durante el día.[298]​ Se ha producido una dura competencia en la cadena de producción, y asimismo se esperan mayores caídas del coste de la energía fotovoltaica en los próximos años, lo que supone una creciente amenaza al dominio de las fuentes de generación basadas en las energías fósiles.[299]​ Conforme pasa el tiempo, las tecnologías de generación renovable son generalmente más baratas,[300]​[301]​ mientras que las energías fósiles se vuelven más caras:.

En 2011, el coste de la fotovoltaica había caído bastante por debajo del de la energía nuclear,[303]​ y se espera que siga cayendo:[304]​.

La tendencia es que los precios disminuyan aún más con el tiempo una vez que los componentes fotovoltaicos han entrado en una clara y directa fase industrial.[306]​[307]​ A finales de 2012, el precio medio de los módulos fotovoltaicos había caído a 0,50 $/Wp, y las previsiones apuntan que su precio seguirá reduciéndose hasta los 0,36 $/Wp en 2017.[308]​.

En 2015, el Instituto alemán Fraunhofer especializado en energía solar (ISE) realizó un estudio que concluía que la mayoría de los escenarios previstos para el desarrollo de la energía solar infravaloran la importancia de la fotovoltaica.[309]​ El estudio realizado por el instituto Fraunhofer estimaba que el coste levelizado (LCOE) de la energía solar fotovoltaica para plantas de conexión a red se situará a largo plazo entre 0,02 y 0,04 €/kWh, niveles inferiores a los de las fuentes de energía convencionales.[310]​.

Thin film photovoltaic energy

Another low-cost alternative to crystalline silicon cells is thin-film photovoltaics, which is based on third-generation solar cells.[312] They consist of a solar cell that is manufactured by depositing one or more thin layers (thin film) of photovoltaic material on a substrate.

Thin film solar cells are usually classified according to the photovoltaic material used:.

• - Amorphous silicon (a-Si) and other thin film silicon (TF-Si)[313]​.

• - Cadmium telluride (CdTe)[314]​.

• - Copper indium gallium and selenide (CIS or CIGS)[315]​.

• - Dye-sensitized solar cells (DSC)[316]​ and other organic solar cells.[317]​.

The International Low Cost Solar Energy Conference in Seville, held in February 2009, was the first showcase of the same in Spain.[318]​ This technology caused great expectations in its beginnings. However, the sharp drop in the price of polycrystalline silicon cells and modules since late 2011 has meant that some thin film manufacturers have been forced to exit the market, while others have seen their profits greatly reduced.[319]​.

La cantidad de energía solar que alcanza a la superficie terrestre es enorme, cerca de 122 petavatios (PW), y equivale a casi 10 000 veces más que los 13 TW consumidos por la humanidad en 2005.[320]​ Esta abundancia sugiere que no pasará mucho tiempo antes de que la energía solar se convierta en la principal fuente de energía de la humanidad.[321]​ Adicionalmente, la generación eléctrica mediante fotovoltaica presenta la mayor densidad energética (una media global de 170 W/m²) de todas las energías renovables.[320]​.

A diferencia de las tecnologías de generación de energía basadas en combustibles fósiles, la energía solar fotovoltaica no produce ningún tipo de emisiones nocivas durante su funcionamiento,[1]​ aunque la producción de los paneles fotovoltaicos presenta también un cierto impacto ambiental. Los residuos finales generados durante la fase de producción de los componentes, así como las emisiones de las factorías, pueden gestionarse mediante controles de contaminación ya existentes. Durante los últimos años también se han desarrollado tecnologías de reciclaje para gestionar los diferentes elementos fotovoltaicos al finalizar su vida útil,[322]​ y se están llevando a cabo programas para incrementar el reciclaje entre los productores fotovoltaicos.[323]​.

La tasa de retorno energético de esta tecnología, por su parte, es cada vez mayor. Con la tecnología actual, los paneles fotovoltaicos recuperan la energía necesaria para su fabricación en un período comprendido entre 6 meses y 1 año y medio; teniendo en cuenta que su vida útil media es superior a 30 años, producen electricidad limpia durante más del 95 % de su ciclo de vida.[324]​.

Greenhouse gas emissions

Greenhouse gas emissions throughout the life cycle for photovoltaics are close to 46 g/kWh, and can even be reduced to 15 g/kWh in the near future.[325]​

In comparison, a combined cycle gas plant emits between 400-599 g/kWh,[326]​ a diesel plant 893 g/kWh,[326]​ a coal plant 915-994 g/kWh[327]​ or with carbon capture technology about 200 g/kWh (excluding emissions during coal extraction and transportation), and a power plant high temperature geothermal, between 91-122 g/kWh.[326]​ The intensity of emissions for the life cycle of hydraulic, wind and nuclear energy is lower than that of photovoltaic energy, according to data published by the IPCC in 2011.[326]​.

Like all energy sources whose emissions depend primarily on the construction and transportation phases, the transition to a low-carbon economy could further reduce carbon dioxide emissions during the manufacturing of solar devices.

A 1 kW photovoltaic system saves the combustion of approximately 77 kg (170 pounds) of coal, avoids the emission into the atmosphere of about 136 kg (300 pounds) of carbon dioxide, and saves the use of about 400 liters (105 gallons) of water monthly.[328]​.

Degradation of photovoltaic modules

The power output of a photovoltaic (PV) device decreases over time. This decrease is due to its exposure to solar radiation as well as other external conditions. The degradation rate, which is defined as the annual percentage of output power loss, is a key factor in determining the long-term production of a photovoltaic plant. To estimate this degradation, the percentage of decrease associated with each of the electrical parameters is quantified separately. It should be noted that the degradation of an individual PV module can significantly influence the performance of an entire string. Furthermore, not all modules in the same installation decrease their performance at exactly the same rate. Given a set of modules exposed to long-term outdoor conditions, the individual degradation of the main electrical parameters and the increase in their dispersion must be considered. As each module tends to degrade differently, the behavior of the modules will become increasingly different over time, negatively affecting the overall performance of the plant.

There are several studies that deal with the analysis of power degradation of modules based on different photovoltaic technologies available in the literature. According to a recent study,[329]​ the degradation of crystalline silicon modules is very regular, ranging between 0.8% and 1.0% per year.

On the other hand, if we analyze the performance of thin-film photovoltaic modules, an initial period of strong degradation is observed (which can last several months and even up to 2 years), followed by a subsequent stage in which the degradation stabilizes, being then comparable to what crystalline silicon can have.[330]​ In these thin-film technologies, strong seasonal variations are also observed because the influence of the solar spectrum is much greater. For example, for amorphous silicon, micromorphic silicon or cadmium tellulide modules, we are talking about annual degradation rates for the first years of between 3% and 4%.[331]​ However, other technologies, such as CIGS, have much lower degradation rates, even in those first years.

Recycling of photovoltaic modules

A photovoltaic installation can operate for 30 years or more[332]​ with little maintenance or intervention after its start-up, so after the initial investment cost necessary to build a photovoltaic installation, its operating costs are very low compared to the rest of the existing energy sources. At the end of their useful life, most of the photovoltaic panels can be treated. Thanks to technological innovations that have been developed in recent years, up to 95% of certain semiconductor materials and glass can be recovered, as well as large quantities of ferrous and non-ferrous metals used in modules.[333]​ Some private companies[334]​ and non-profit organizations, such as PV CYCLE in the European Union, are working on collection and recycling operations for end-of-life panels.[335]​.

Two of the most common recycling solutions are:.

• - Silicon panels: The aluminum frames and connection boxes are manually dismantled at the beginning of the process. The panel is crushed and the different fractions are separated: glass, plastics and metals.[336]​ It is possible to recover more than 80% of the incoming weight[337]​ and, for example, the extracted mixed glass is easily accepted by the foam glass and insulation industries. This process can be carried out by flat glass recyclers, since the morphology and composition of a photovoltaic panel is similar to the flat glass used in the construction and automotive industries.

• - Panels made of other materials: Today there are specific technologies for recycling photovoltaic panels that do not contain silicon; some techniques use chemical baths to separate the different semiconductor materials.[338]​ For cadmium telluride panels, the recycling process begins by crushing the module and, subsequently, separating the different parts. This recycling process is designed to recover up to 90% of the glass and 95% of the semiconductor materials.[339]​ In recent years, some private companies have launched commercial-scale recycling facilities.[340]​.

Since 2010, an annual conference has been held in Europe that brings together producers, recyclers and researchers to discuss the future of photovoltaic module recycling. In 2012 it took place in Madrid.[341][342]​.

• - Portal:Energy. Content related to Energy.

• - Portal:Ecology. Content related to Ecology.

• - Wikimedia Commons hosts a multimedia category on Photovoltaic Solar Energy.

• - UNEF website (Spanish Photovoltaic Union) Main association of the photovoltaic sector in Spain.

• - Photovoltaics in Spain website (Universitat Politècnica de València).

• - PV Magazine Latin America News from the photovoltaic sector in Spanish.

• - PVGIS. Accurate calculation of solar radiation and the performance of photovoltaic plants.

• - IRENA Global Atlas Database for Solar and Wind.

• - Photovoltaic Program of the International Energy Agency.

Contenido

El término «fotovoltaico» se comenzó a usar en Reino Unido en el año 1849.[21]​ Proviene del griego φώς: phos, que significa «luz», y de -voltaico, que proviene del ámbito de la electricidad, en honor al físico italiano Alessandro Volta.[nota 1]​.

El efecto fotovoltaico fue reconocido por primera vez unos diez años antes, en 1839, por el físico francés Alexandre-Edmond Becquerel,[22]​[23]​ pero la primera célula solar no se fabricó hasta 1883. Su creador fue Charles Fritts, quien recubrió una muestra de selenio semiconductor con pan de oro para formar la unión. Este primitivo dispositivo presentaba una eficiencia menor del 1 %, pero demostró de forma práctica que, efectivamente, producir electricidad con luz era posible.[24]​ Los estudios realizados en el siglo por Michael Faraday, James Clerk Maxwell, Nikola Tesla y Heinrich Hertz sobre inducción electromagnética, fuerzas eléctricas y ondas electromagnéticas, y sobre todo, el trabajo realizado por Albert Einstein en 1905, por el cual le fue otorgado el premio Nobel en 1921, proporcionaron la base teórica y práctica del efecto fotoeléctrico,[25]​ que es el fundamento de la conversión de energía solar en electricidad.

Working principle

When a doped semiconductor is exposed to electromagnetic radiation, an incident photon hits an electron and knocks it out, creating a hole in the atom. Normally, the electron quickly finds another hole to fill again, and the energy provided by the photon is therefore dissipated as heat. The principle of a photovoltaic cell is to force the electrons and holes to move towards the opposite side of the material instead of simply recombining in it: thus, a potential difference, and therefore voltage, will be produced between the two parts of the material, as occurs in a battery.

To do this, a permanent electric field is created, through a pn junction, between two doped layers respectively, p and n "Doping (semiconductors)"). In silicon cells, which are mostly used, there are therefore:

• - The top layer of the cell, which is composed of n-type doped silicon#doping_type_n_and_type_p "Doping (semiconductors)").[note 2]​ In this layer, there are a greater number of free electrons than in a layer of pure silicon, hence the name n-doping, negative. The material remains electrically neutral, since both the silicon atoms and those of the dopant material are neutral: but the crystalline lattice has an overall greater presence of electrons than in a pure silicon lattice.

• - The bottom layer of the cell, which is composed of p-doped silicon "Doping (semiconductors)").[note 3]​ This layer therefore has a lower average amount of free electrons than a layer of pure silicon. The electrons are linked to the crystal lattice which, consequently, is electrically neutral, but has gaps, positive (p). Electrical conduction is ensured by these charge carriers, which move throughout the material.

At the time of creation of the p-n junction, free electrons from the n-shell instantly enter the p-shell and recombine with holes in the p-region. There will thus exist throughout the life of the junction, a positive charge in the n region along the junction (because electrons are missing) and a negative charge in the p region along the junction (because the holes have disappeared); The set forms the "Space Charge Zone" (ZCE) and there is an electric field between the two, from n to p. This electric field makes the ZCE a diode, which only allows the flow of current in one direction: the electrons can move from the p to the n region, but not in the opposite direction and on the contrary the holes only pass from n to p.

In operation, when a photon knocks an electron out of the matrix, creating a free electron and a hole, under the effect of this electric field each one goes in the opposite direction: the electrons accumulate in the n region (to become the negative pole), while the holes accumulate in the p-doped region (which becomes the positive pole). This phenomenon is most effective in the CEZ, where there are almost no charge carriers (electrons or holes), since they are nullified, or in the immediate vicinity of the CEZ: when a photon creates an electron-hole pair, they are separated and are unlikely to find their opposite, but if the creation takes place at a site further away from the junction, the electron (turned into a hole) maintains a great opportunity to recombine before reaching the n zone. But the CEZ is necessarily very thin, so it is not useful to give a large thickness to the cell.[note 4]​ Indeed, the thickness of layer n is very small, since this layer is basically only needed to create the CEZ that makes the cell work. On the other hand, the thickness of the p layer is greater: it depends on a compromise between the need to minimize electron-hole recombinations, and on the contrary allow the capture of the greatest number of photons possible, for which a certain minimum thickness is required.

In short, a photovoltaic cell is the equivalent of a power generator to which a diode has been added. To achieve a practical solar cell, it is also necessary to add electrical contacts (that allow the generated energy to be extracted), a layer that protects the cell but allows light to pass through, an anti-reflective layer "Reflection (physics)") to guarantee the correct absorption of photons, and other elements that increase its efficiency.

First modern solar cell

The American engineer Russell Ohl patented the modern solar cell in 1946,[26]​ although other researchers had advanced in its development previously: the Swedish physicist Sven Ason Berglund had patented a method in 1914 that tried to increase the capacity of photosensitive cells, while, in 1931, the German engineer Bruno Lange had developed a photocell using silver selenide instead of copper oxide.[27]​.

The modern era of solar technology did not arrive until 1954, when American researchers Gerald Pearson, Calvin S. Fuller and Daryl Chapin, from Bell Laboratories,[28] accidentally discovered that silicon semiconductors doped "Doping (semiconductors)") with certain impurities were very sensitive to light. These advances contributed to the manufacture of the first commercial solar cell. They used a diffuse p–n silicon junction, with a solar energy conversion of approximately 6%, an achievement compared to selenium cells that barely reached 0.5%.[29]​[30]​.

Later, the American Les Hoffman, president of the Hoffman Electronics company, through its semiconductor division, was one of the pioneers in the large-scale manufacturing and production of solar cells. Between 1954 and 1960, Hoffman managed to improve the efficiency of photovoltaic cells up to 14%, reducing manufacturing costs to achieve a product that could be marketed.[31]​.

First applications: space solar energy

At first, photovoltaic cells were used in a minority way to electrically power toys and in other minor uses, since the cost of producing electricity using these primitive cells was too high: in relative terms, a cell that produced one watt of energy using sunlight could cost $250, compared to the two or three dollars that a watt from a coal-fired thermoelectric plant cost.

Photovoltaic cells were rescued from oblivion thanks to the space race and the suggestion of using them in one of the first satellites placed in orbit around the Earth. The Soviet Union launched its first space satellite in 1957, and the United States would follow a year later. The first spacecraft to use solar panels was the American satellite Vanguard 1, launched in March 1958 (today the oldest satellite still in orbit). Its design used solar cells created by Peter Iles in an effort led by the company Hoffman Electronics.[32]​ The photovoltaic system allowed it to continue transmitting for seven years while the chemical batteries "Battery (electricity)") ran out in just 20 days.[33]​.

In 1959, the United States launched Explorer 6. This satellite had a series of solar modules installed, supported on external wing-like structures, made up of 9,600 solar cells from the Hoffman company.[31] This type of device later became a common feature of many satellites. There was some initial skepticism about how the system would work, but in practice the solar cells proved to be a great success, and were soon incorporated into the design of new satellites.

A few years later, in 1962, the Telstar "Telstar (satellite)") became the first communications satellite equipped with solar cells, capable of providing a power of 14 W.[34] This milestone generated great interest in the production and launch of geostationary satellites for the development of communications, in which the energy would come from a device for capturing sunlight. It was a crucial development that stimulated research by some governments and drove the improvement of photovoltaic panels.[35]​

Gradually, the space industry opted for the use of gallium arsenide (GaAs) solar cells, due to their greater efficiency compared to silicon cells. In 1970, the first highly efficient gallium arsenide heterostructure solar cell was developed in the Soviet Union by Zhorés Alfiorov and his research team.[36]​[37]​.

Starting in 1971, the Soviet space stations of the Salyut program were the first manned orbital complexes to obtain their energy from solar cells, attached to structures on the sides of the orbital module,[38]​ as was the North American Skylab station, a few years later.[39]​.

In the 1970s, after the first oil crisis, the United States Department of Energy and the space agency NASA began studying the concept of solar energy in space, which aimed to supply terrestrial energy through space satellites. In 1979 they proposed a fleet of satellites in geostationary orbit, each of which would measure 5 x 10 km and produce between 5 and 10 GW. The construction involved the creation of a large space infrastructure where hundreds of astronauts would work continuously. This gigantism was typical of a time when the creation of large space cities was planned. Technical difficulties aside, the proposal was scrapped in 1981 because it was too expensive.[40] In the mid-1980s, with oil prices again low, the program was cancelled.[41]

First terrestrial applications

Since its appearance in the aerospace industry, where it has become the most reliable means of supplying electrical energy in space vehicles,[59]​ photovoltaic solar energy has developed a large number of terrestrial applications. The first commercial installation of this type was carried out in 1966, at the Ogami Island lighthouse (Japan), allowing the use of flare gas to be replaced by a renewable and self-sufficient electrical source. It was the first lighthouse in the world powered by photovoltaic solar energy, and was crucial in demonstrating the viability and potential of this energy source.[60]​.

Improvements came slowly over the next two decades, and the only widespread use was in space applications, where its power-to-weight ratio was greater than that of any competing technology. However, this success was also the reason for its slow growth: the aerospace market was willing to pay any price to obtain the best possible cells, so there was no reason to invest in lower cost solutions if this reduced efficiency. Instead, the price of cells was largely determined by the semiconductor industry; Its migration to integrated circuit technology in the 1960s led to the availability of larger bars at relatively lower prices. As their price fell, the price of the resulting photovoltaic cells fell in equal measure. However, the cost reduction associated with this growing popularization of photovoltaics was limited, and in 1970 the cost of solar cells was still estimated at $100 per watt ($/Wp).[61]​.

Price reduction

In the late 1960s, American industrial chemist Elliot Berman was researching a new method for producing the raw material silicon from a tape process. However, he found little interest in his project and was unable to obtain the necessary financing for its development. Later, in a chance meeting, he was introduced to a team from the Exxon oil company that was looking for strategic projects 30 years into the future. The group had concluded that electrical energy would be much more expensive in the year 2000, and considered that this price increase would make new alternative energy sources more attractive, solar energy being the most interesting among these. In 1969, Berman joined Exxon's laboratory in Linden, New Jersey, called Solar Power Corporation (SPC).[61]​.

His effort was first directed at analyzing the potential market to identify the possible uses that existed for this new product, and he quickly discovered that if the cost per watt was reduced from $100/Wp to around $20/Wp, significant demand would arise. Knowing that the “ribbon silicon” concept could take years to develop, the team began looking for ways to reduce the price to $20/Wp using existing materials. The realization that the existing cells were based on the standard semiconductor manufacturing process was a first breakthrough, even if it was not an ideal material. The process began with the formation of a silicon ingot, which was cut transversely into disks called wafers. The wafers were then polished and then, for use as cells, they were coated with an anti-reflective layer. Berman realized that rough-cut wafers already had a perfectly good anti-reflective front surface, and by printing the electrodes directly on this surface, two important steps in the cell manufacturing process were eliminated.[61]​.

His team also explored other ways to improve the assembly of cells in arrays, eliminating the expensive materials and manual wiring previously used in space applications. Their solution consisted of using printed circuits on the back, acrylic plastic on the front, and silicone glue between them, embedding the cells. Berman realized that the silicon on the market was already “good enough” for use in solar cells. The small imperfections that could ruin a silicon ingot (or individual wafer) for use in electronics would have little effect on solar applications. Photovoltaic cells could be manufactured from material discarded by the electronics market, which would result in a great improvement in their price.[61]​.

Putting all these changes into practice, the company began purchasing rejected silicon at very low cost from existing manufacturers. By using the largest wafers available, which reduced the amount of wiring for a given panel area, and packaging them into panels with their new methods, by 1973 SPC was producing panels for $10/Wp and selling them for $20/Wp, decreasing the price of photovoltaic modules by a fifth in just two years.[61]​.

The maritime navigation market

SPC began contacting navigation buoy manufacturing companies offering them the product, but found a curious situation. The main company in the sector was Automatic Power, a manufacturer of disposable batteries. Realizing that solar cells could eat into the business and profits that the battery sector produced, Automatic Power bought a solar prototype from Hoffman Electronics to end up cornering it. Seeing no interest from Automatic Power, SPC then turned to Tideland Signal, another battery supply company formed by former Automatic Power managers.[61]​ Tideland brought a photovoltaic-powered buoy to market and was soon ruining Automatic Power's business.

The timing could not have been better, the rapid increase in the number of offshore oil platforms and other loading facilities produced a huge market among oil companies. As Tideland had been successful, Automatic Power then began sourcing its own supply of photovoltaic solar panels. They found Bill Yerkes of Solar Power International (SPI) in California, who was looking for a market to sell his product. SPI was soon acquired by one of its largest clients, oil giant ARCO, forming ARCO Solar. The ARCO Solar factory in Camarillo (California) was the first dedicated to the construction of solar panels, and was in continuous operation from its purchase by ARCO in 1977 until 2011 when it was closed by the company SolarWorld.[61]​.

This situation was combined with the oil crisis of 1973. The oil companies had enormous funds at their disposal at that time due to their enormous income during the crisis, but they were also very aware that their future success would depend on some other source of energy. In the following years, large oil companies began the creation of a series of solar energy companies, and were for decades the largest producers of solar panels. The companies ARCO, Exxon, Shell, Amoco (later acquired by BP), and Mobil maintained large solar divisions during the 1970s and 1980s. Technology companies also made significant investments, including General Electric, Motorola, IBM, Tyco, and RCA.[62]

Perfecting the technology

In the decades since Berman's advances, improvements have reduced production costs below $1/Wp, with prices below $2/Wp for the entire PV system. The price of the rest of the elements of a photovoltaic installation now represents a higher cost than the panels themselves.[63]​.

As the semiconductor industry developed toward larger and larger ingots, older equipment became available at reduced prices. The cells grew in size when these older equipment became available on the surplus market. The first ARCO Solar panels were equipped with cells 2 to 4 inches (51 to 100 mm) in diameter. Panels in the 1990s and early 2000s generally incorporated 5-inch (125 mm) cells, and since 2008 almost all new panels use 6-inch (150 mm) cells.[64] Also the widespread introduction of flat-panel televisions in the late 1990s and early 2000s led to the wide availability of large sheets of high-quality glass, which are used in the front of the panels.[65]​.

In terms of the cells themselves, there has only been one major change. During the 1990s, polysilicon cells became increasingly popular.[64]​ These cells offer less efficiency than those made of monosilicon, but are grown in large vats that greatly reduce the cost of production.[64]​ By the mid-2000s, polysilicon dominated the low-cost panel market.[64]​.

La producción industrial a gran escala de paneles fotovoltaicos despegó en la década de 1980, y entre sus múltiples usos se pueden destacar:.

Telecommunications and signaling

Photovoltaic solar energy is ideal for telecommunications applications, including, for example, local telephone exchanges, radio and television antennas, microwave repeater stations and other types of electronic communication links. This is because, in most telecommunications applications, storage batteries are used and the electrical installation is normally carried out in direct current (DC). In rugged and mountainous terrain, radio and television signals may be interfered with or reflected due to undulating terrain. In these locations, low power transmitters (LPT) are installed to receive and retransmit the signal among the local population.[66]​.

Photovoltaic cells are also used to power emergency communications systems "Emergency (disaster)"), for example in SOS (Emergency Telephone) poles on roads, railway signaling, beacons for aeronautical protection, weather stations or environmental data and water quality monitoring systems.[61]​.

Isolated devices

The reduction in energy consumption of integrated circuits made it possible in the late 1970s to use solar cells as a source of electricity in calculators, such as the Royal Solar 1, Sharp EL-8026 or Teal Photon.[67]​.

Other fixed devices that use photovoltaic energy have also increased in use in recent decades, in places where the cost of connecting to the electrical grid or using disposable batteries is prohibitively expensive. These applications include, for example, solar lamps, solar street lights, water pumps, parking meters,[68][69]​ emergency telephones, trash compactors,[70]​ temporary or permanent traffic signs, charging stations[71][72]​ or remote surveillance systems.

Rural electrification

In isolated environments, where little electrical power is required and access to the grid is difficult, photovoltaic panels have been used as an economically viable alternative for decades. To understand the importance of this possibility, it is worth taking into account that approximately a quarter of the world's population still does not have access to electrical energy.[73]​.

In developing countries, many villages (rural population) are located in remote areas, several kilometers from the nearest electricity grid. Due to this, photovoltaic energy is increasingly being incorporated to provide electricity to homes or medical facilities in rural areas. For example, in remote parts of India a rural lighting program has provided lighting using solar-powered LED lamps to replace kerosene lamps. The price of solar lamps was approximately the same as the cost of supplying kerosene for a few months.[74]​ Cuba and other Latin American countries are working to provide photovoltaic energy in areas far from conventional electrical power supplies.[75]​ These are areas where the social and economic benefits for the local population offer an excellent reason to install photovoltaic panels, although such initiatives have typically been relegated to one-off humanitarian efforts.[76]​.

Pumping systems

Photovoltaics are also used to power pumping installations for irrigation systems, drinking water in rural areas and water troughs for livestock,[77]​[78]​ or for water desalination systems.[61]​.

Photovoltaic pumping systems (as well as those powered by wind energy) are very useful where access to the general electricity grid is not possible or is prohibitively expensive.[79]​ Their cost is generally cheaper due to their lower operation and maintenance costs, and they have a lower environmental impact than pumping systems powered by internal combustion engines, which also have lower reliability.[80]​[81]​.

The pumps used can be both alternating current (AC) and direct current (DC). Normally direct current motors are used for small and medium-sized applications of up to 3 kW power, while for larger applications alternating current motors are used coupled to an inverter "Inverter (electronics)") that transforms the direct current from the photovoltaic panels for use. This allows systems to be sized from 0.15 kW to more than 55 kW of power, which can be used to supply complex irrigation or water storage systems.[82][83]​.

Hybrid solar-diesel systems

Due to the decreasing costs of photovoltaic solar energy, the use of solar-diesel hybrid systems is also expanding, which combine this energy with diesel generators to produce electricity in a continuous and stable manner.[84]​ This type of installations are normally equipped with auxiliary equipment, such as batteries and special control systems to achieve the stability of the system's electrical supply at all times.[85]​.

Due to its economic viability (transporting diesel to the point of consumption is usually expensive) in many cases old generators are replaced by photovoltaics, while new hybrid installations are designed in such a way that they allow the solar resource to be used whenever it is available, minimizing the use of generators, thus reducing the environmental impact of electricity generation in remote communities and in facilities that are not connected to the electrical grid. An example of this is mining companies,[84]​[86]​ whose operations are normally located in open fields, far from large population centers. In these cases, the combined use of photovoltaics allows us to greatly reduce the dependence on diesel fuel, allowing savings of up to 70% in the cost of energy.[87]​.

This type of system can also be used in combination with other sources of renewable energy generation, such as wind energy.[88]​.

Transport and maritime navigation

Although photovoltaics are not yet widely used to provide traction in transportation, they are increasingly being used to provide auxiliary power in ships and cars. Some vehicles are equipped with air conditioning powered by photovoltaic panels to limit the interior temperature on hot days,[89]​ while other hybrid prototypes use them to recharge their batteries without having to connect to the electrical grid.[90]​[91]​ The practical possibility of designing and manufacturing vehicles powered by solar energy, as well as boats[92]​[93]​ and airplanes,[94]​ has been amply demonstrated, with road transport being considered the most viable for the photovoltaic.[95]​.

Solar Impulse is a project dedicated to the development of an aircraft powered solely by photovoltaic solar energy. The prototype can fly during the day powered by the solar cells that cover its wings, while charging the batteries that allow it to stay in the air at night.[96][97]​.

Solar energy is also commonly used in lighthouses, buoys and maritime navigation beacons, recreational vehicles, charging systems for electrical accumulators on ships, and cathodic protection systems.[61]​ The recharging of electric vehicles is becoming increasingly important.[95]​.

Building Integrated Photovoltaics

Many photovoltaic installations are often located on buildings: they are usually located on an existing roof, or they are integrated into elements of the building's own structure, such as skylights, skylights or facades.[98]​.

Alternatively, a photovoltaic system can also be located physically separate from the building, but connected to the building's electrical installation to supply energy. By 2010, more than 80% of the 9,000 MW of photovoltaics that Germany had in operation at the time had been installed on rooftops.[99]​.

Building integrated photovoltaics (BIPV) are increasingly being incorporated as a primary or secondary source of electrical energy in new domestic and industrial buildings,[100]​ and even in other architectural elements, such as bridges.[101]​ Roof tiles with integrated photovoltaic cells are also quite common in this type of integration.

According to a study published in 2011, the use of thermal imaging has shown that solar panels, provided there is an open gap through which air can circulate between the panels and the roof, provide a passive cooling effect on buildings during the day and also help maintain the heat accumulated during the night.[102]​.

Grid connection photovoltaics

One of the main applications of photovoltaic solar energy, most developed in recent years, consists of power plants connected to the grid for electricity supply,[103]​ as well as photovoltaic self-consumption systems, of generally lower power, but also connected to the electrical grid.

floating photovoltaic

Although solar panels are usually installed on land, it is possible to install them floating on the waters of reservoirs or calm lakes.[104]​ Although it is more expensive, it has many advantages: it reduces losses due to evaporation of the reservoir water, improves its quality (because less algae grows),[105]​ the installation is simpler, the cooling of the panels themselves is facilitated (which increases the energy they produce)[104]​ and it is an alternative way for reservoirs to hydroelectric plants generate electricity, without wasting the water they store or occupying additional land.[105]​.

Una planta solar fotovoltaica cuenta con distintos elementos que permiten su funcionamiento, como son los paneles fotovoltaicos para la captación de la radiación solar, y los inversores "Inversor (electrónica)") para la transformación de la corriente continua en corriente alterna.[106]​ Existen otros, los más importantes se mencionan a continuación:.

Photovoltaic solar panels

Generally, a photovoltaic module or panel consists of an association of cells, encapsulated in two layers of EVA (ethylene-vinyl-acetate), between a front sheet of glass and a back layer of a thermoplastic polymer (tedlar is frequently used) or another sheet of glass when it is desired to obtain modules with some degree of transparency.[107] Very frequently this assembly is framed in an anodized aluminum structure with the aim of increasing the mechanical resistance of the assembly and facilitating anchoring. from the module to the support structures.[107]​.

The most commonly used cells in photovoltaic panels are silicon, and can be divided into three subcategories:

• - Monocrystalline silicon cells are made up of a single silicon crystal, normally manufactured using the Czochralski process.[108]​ This type of cell has a uniform dark blue color.

• - Polycrystalline silicon cells (also called multicrystalline) are made up of a set of silicon crystals, which explains why their performance is somewhat lower than that of monocrystalline cells.[64]​ They are characterized by a more intense blue color.

• - Amorphous silicon cells. They are less efficient than crystalline silicon cells but also less expensive. This type of cell is, for example, the one used in solar applications such as watches or calculators.[109]​.

The current versus voltage curve of a module provides us with useful information about its electrical performance.[110] Manufacturing processes often cause differences in the electrical parameters of different photovoltaic modules, even in cells of the same type. Therefore, only the experimental measurement of the I – V curve allows us to precisely establish the electrical parameters of a photovoltaic device. This measurement provides very relevant information for the design, installation and maintenance of photovoltaic systems. Usually, the electrical parameters of PV modules are measured by indoor testing. However, outdoor testing has important advantages such as no expensive artificial light source is required, there is no limitation on the size of the samples, and the illumination of the samples is more homogeneous.

The performance of a photovoltaic (PV) module depends on the environmental conditions, mainly on the global incident irradiance G in the plane of the module. However, the temperature T of the p–n junction also influences the main electrical parameters: the short-circuit current ISC, the open-circuit voltage VOC and the maximum power Pmax. In general, it is known that VOC shows a significant inverse correlation with T, while for ISC that correlation is direct, but weaker, so this increase does not compensate for the decrease in VOC. As a consequence, Pmax decreases as T increases. This correlation between the output power of a solar cell and the working temperature of its junction depends on the semiconductor material, and is due to the influence of T on the concentration, lifetime and mobility of the intrinsic carriers, that is, electrons and holes, within the photovoltaic cell.

Temperature sensitivity is usually described by temperature coefficients, each of which expresses the derivative of the parameter to which it refers with respect to the junction temperature. The values ​​of these parameters can be found in any PV module data sheet; They are the following:

• β: Coefficient of variation of VOC with respect to T, given by ∂VOC/∂T.

• α: Coefficient of variation of ISC with respect to T, given by ∂ISC/∂T.

• δ: Coefficient of variation of Pmax with respect to T, given by ∂Pmax/∂T.

Techniques for estimating these coefficients from experimental data can be found in the literature[111].

Investors

The direct electric current provided by the photovoltaic modules can be transformed into alternating current using an electronic device called an inverter "Inverter (electronics)")[106]​ and injected into the electrical grid (for energy sale) or into the interior grid (for self-consumption).

The simplified process would be as follows:

• - Energy is generated at low voltages (380-800 V) and direct current.

• - It is transformed with an inverter "Inverter (electronics)") into alternating current.

• - In power plants less than 100 kW, the energy is injected directly into the distribution network at low voltage (400 V in three-phase or 230 V in single-phase).

• - And for powers greater than 100 kW, a transformer is used to raise the energy to medium voltage (up to 36 kV) and it is injected into the transport networks for subsequent supply.

In the initial stages of the development of photovoltaic inverters, the requirements of the operators of the electrical networks to which they were connected only requested the contribution of active energy and the disconnection of the inverter from the network if it exceeded certain voltage and frequency limits. With the progressive development of this equipment and the increasing importance of smart electrical networks, inverters are now capable of providing reactive energy and even providing stability to the electrical grid.[112]​[113]​.

solar trackers

The use of trackers with one or two axes allows for a considerable increase in solar production, around 30% for the former and an additional 6% for the latter, in places with high direct radiation.[114]​[115]​.

Solar trackers are quite common in photovoltaic applications.[116] There are several types:

• - In two axes: the surface always remains perpendicular to the Sun.

• - On a polar axis: the surface rotates on an axis facing south and inclined at an angle equal to the latitude. The gyration is adjusted so that the normal to the surface coincides at all times with the terrestrial meridian that contains the Sun.

• - On an azimuthal axis: the surface rotates about a vertical axis, the angle of the surface is constant and equal to the latitude. The gyration is adjusted so that the normal to the surface coincides at all times with the local meridian that contains the Sun.

• - On a horizontal axis: the surface rotates on a horizontal axis and oriented in a north-south direction. The gyration is adjusted so that the normal to the surface coincides at all times with the terrestrial meridian that contains the Sun.

Cabling

It is the element that transports electrical energy from its generation, for its subsequent distribution and transportation. Its sizing is determined by the most restrictive criterion between the maximum allowable voltage drop and the maximum allowable intensity. Increasing the conductor sections obtained as a result of the theoretical calculations provides added advantages such as:

• - More discharged lines, which prolongs the useful life of the cables.

• - Possibility of increasing plant power without changing the driver.

• - Better response to possible short circuits.

• - Improvement of installation performance.

Photovoltaic concentration plants

Another type of technology in photovoltaic plants are those that use a concentration technology called CPV (Concentrated Photovoltaics)[117]​ to maximize the solar energy received by the installation, just like in a solar thermal power plant. Photovoltaic concentration facilities are located in locations with high direct solar irradiation, such as the countries on both shores of the Mediterranean, Australia, the United States, China, South Africa, Mexico, etc. Until 2006, these technologies were part of the scope of research, but in recent years larger facilities have been launched, such as the ISFOC (Institute of Concentrated Photovoltaic Solar Systems) in Puertollano (Castilla-La Mancha) with 3 MW supplying electricity to the electrical grid.[118]​[119][120]​.

The basic idea of ​​photovoltaic concentration is the replacement of semiconductor material with (cheaper) reflective or refracting material. The degree of concentration can reach a factor of 1000,[117]​ in such a way that, given the small surface area of ​​the solar cell used, the most efficient technology can be used (triple junction, for example). On the other hand, the optical system introduces a loss factor that recovers less radiation than flat photovoltaics. This, together with the high precision of the monitoring systems, constitutes the main barrier to be solved by concentration technology.

The development of large plants (above 1 MW) has recently been announced.[121] Concentrated photovoltaic plants use a double-axis tracker to enable maximum use of the solar resource throughout the day.

Entre los años 2001 y 2016 se ha producido un crecimiento exponencial de la producción fotovoltaica, duplicándose aproximadamente cada dos años.[122]​ La potencia total fotovoltaica instalada en el mundo (conectada a red) ascendía a 16 gigavatios (GW) en 2008, 40 GW en 2010, 100 GW en 2012, 180 GW en 2014, 300 GW en 2016 y 500 GW en 2018.[123]​[124]​[125]​[126]​[127]​[9]​[10]​.

Históricamente, Estados Unidos lideró la instalación de energía fotovoltaica desde sus inicios hasta 1996, cuando su capacidad instalada alcanzaba los 77 MW, más que cualquier otro país hasta la fecha. En los años posteriores, fueron superados por Japón, que mantuvo el liderato hasta que a su vez Alemania la sobrepasó en 2005, manteniendo el liderato desde entonces. A comienzos de 2016, Alemania se aproximaba a los 40 GW instalados.[130]​ Sin embargo, por esas fechas China, uno de los países donde la fotovoltaica está experimentando un crecimiento más vertiginoso superó a Alemania, convirtiéndose desde entonces en el mayor productor de energía fotovoltaica del mundo.[130]​ Se espera que multiplique su potencia instalada actual hasta los 200 GW en 2020.[128]​[131]​[132]​.

World production

The total installed capacity already represents a significant fraction of the electricity mix in the European Union, covering on average 3.5% of electricity demand and reaching 7% in periods of highest production.[127]​ In some countries, such as Germany,[133][134]​ Italy,[135][136][137][note 5]​ United Kingdom[138]​ or Spain,[139]​ it reaches higher maximums. 10%, just like in Japan[140]​ or in some sunny states in the United States, such as California.[141]​ The annual production of electrical energy generated by this energy source worldwide was equivalent in 2015 to about 184 TWh, enough to supply the energy needs of millions of homes and covering approximately 1% of global electricity demand.[127]​.

China

Photovoltaics has become one of the largest industries in the People's Republic of China. The Asian country is the world leader in photovoltaic capacity, with an installed power of more than 170 GW at the beginning of 2019.[142]​ It also has about 400 photovoltaic companies, among which Trina Solar, Jinko Solar and JA Solar, global giants in the manufacturing of solar panels, stand out. In 2014, it produced approximately half of the photovoltaic products manufactured in the world (China and Taiwan together account for more than 60% share). The production of photovoltaic panels and cells in China has increased significantly over the last decade: in 2001 it maintained a share of less than 1% of the world market, while at the same time, Japan and the United States accounted for more than 70% of world production. However, the trend has reversed and China currently far surpasses the rest of the producers.

Chinese solar panel production capacity practically quadrupled between 2009 and 2011, even exceeding global demand. As a result, the European Union accused the Chinese industry of dumping, that is, selling its panels at prices below cost, imposing tariffs on the import of this material.[143]​[144]​.

The installation of photovoltaic energy has developed spectacularly in the Asian country in recent years, even exceeding initial forecasts. Due to such rapid growth, Chinese authorities have been forced to reevaluate their photovoltaic power target on several occasions.

The total installed power in China grew to 77 GW at the end of 2016, after connecting 36 GW in the last year, according to the country's official statistics.[145]​ In 2017, China had exceeded the objective set by the government for 2020, a photovoltaic power of 100 GW.[146]​ Therefore, at the end of 2018 it was announced that China could raise its solar target to 2020 to more than 200 GW.[147]​.

This growth reflects the abrupt decline in the costs of photovoltaic energy, which is currently beginning to be a cheaper option than other energy sources, both at retail and commercial prices. Chinese government sources have stated that photovoltaics will have more competitive prices than coal and gas (also providing greater energy independence) by the end of this decade.[148]​.

USA

Since 2010, the United States has been one of the countries with the most activity in the photovoltaic market, it has large companies in the sector, such as First Solar or SolarCity, as well as numerous grid connection plants. At the beginning of 2017, the United States exceeded 40 GW of installed photovoltaic power,[149]​ enough to provide electricity to more than 8 million homes, after doubling its solar capacity in less than two years.[150]​.

Although the United States does not maintain a uniform national energy policy across the country regarding photovoltaics, many states have individually set renewable energy goals, including solar energy in different proportions in this planning. In this sense, California Governor Jerry Brown has signed legislation requiring that 33% of the state's electricity be generated by renewable energy by the end of 2020.[151] These measures have been supported by the federal government with the adoption of the Investment Tax Credit (ITC), a tax exemption established in 2006 to promote the development of photovoltaic projects, and which has recently been extended until 2023.[152]​.

A private report[153]​ states that photovoltaic solar energy has expanded rapidly over the last 8 years, growing at an average of 40% each year. Thanks to this trend, the cost of the kWh produced by photovoltaic energy has been greatly reduced, while the cost of electricity generated by fossil fuels has continued to increase. As a result, the report concludes that PV will reach grid parity with conventional energy sources in many regions of the United States by 2015. But to achieve a 10% energy market share, the report continues, PV companies will need to further streamline installations, so that solar becomes a plug-and-play technology. That is, it is easy to acquire the components of each system and their interconnection is simple, as is their connection to the network.[153]​.

Currently, most facilities are connected to the grid and use net balancing systems that allow nighttime electricity consumption from energy generated during the day. New Jersey leads the States with the least restrictive net balancing law,[154]​ while California leads the total number of homes with solar energy. Many of them were installed during the million solar roof initiative.[155]​.

The current trend and growth rate indicate that in the coming years a large number of photovoltaic plants will be built in the south and southwest of the country, where available land is abundant, in the sunny deserts of California, Nevada and Arizona. Companies are increasingly acquiring large areas in these areas, with the intention of building larger large-scale plants.[156]​.

Japan

Photovoltaic energy in Japan has expanded rapidly since the 1990s. The country is one of the leaders in the manufacturing of photovoltaic modules and is among the first in terms of installed power, with more than 23 GW at the end of 2014, most of it connected to the grid.[157]​[158][159]​ Irradiance in Japan is optimal, standing between 4.3 and 4.8 kWh·m²·day, making it an ideal country for the development of this type of energy.

The sale of photovoltaic modules for commercial projects has grown rapidly following the introduction by the Japanese Government in July 2012 of a tariff to encourage photovoltaics following the Fukushima nuclear accident and the paralysis of most of the country's nuclear power plants.

The majority of modules come from local manufacturers, among which Kyocera, Sharp Corporation, Mitsubishi or Sanyo stand out, while a small part are imported, according to data from the Japan Photovoltaic Energy Association (JPA). 2012.[161]​ Although this trend is reversing, still more than 75% of the cells and modules sold in Japan at the beginning of 2012 were destined for residential projects, while about 9% were used in commercial photovoltaic installations.[162]​.

In 2014, the total photovoltaic power installed in the country was around 23 GW, which contributed approximately 2.5% to the country's electricity demand.[127] During the summer of 2015, it was reported that photovoltaic production in Japan had at certain times covered 10% of the total national demand.[140] Two years later, in 2016, it was around 42 GW,[149]​ and the forecast suggests that the Japanese photovoltaic market will grow even more in the coming years.[163]​.

Germany

At the beginning of 2016, Germany has an installed capacity of around 40 GW.[130]​ In 2011 alone, Germany installed around 7.5 GW,[164]​ and photovoltaics produced 18 TW·h of electricity, 3% of the total consumed in the country.[134][165]​.

The photovoltaic market in Germany has grown considerably since the beginning of the century thanks to the creation of a regulated tariff for the production of renewable energy, which was introduced by the "German Renewable Energy Act", a law published in 2000. Since then, the cost of photovoltaic installations has fallen by more than 50% in five years, since 2006.[166]​ Germany has set the goal of producing 35% of its electricity through renewable energies in 2020 and reach 100% in 2050.[167]​.

In 2012, the introduced tariffs cost Germany around €14 billion per year for both wind and solar installations. This cost is distributed among all taxpayers through an additional cost of €3.6 cents per kWh[168]​ (approximately 15% of the total cost of electricity for the domestic consumer).[169]​.

The considerable power installed in Germany has set several records in recent years. During two consecutive days in May 2012, for example, the solar photovoltaic plants installed in the country produced 22,000 MWh in the midday hour, which is equivalent to the generating power of twenty nuclear power plants working at full capacity.[170]​[note 6]​ Germany shattered this record on July 21, 2013, with an instantaneous power of 24 GW at noon.[171]​[172]​ Due to the highly distributed nature of German PV, approximately 1.3-1.4 million small PV systems contributed to this new record. Approximately 90% of the solar panels installed in Germany are located on roofs.[173]​.

In June 2014, German photovoltaics once again broke records for several days, producing up to 50.6% of all electricity demand during a single day, and surpassing the previous record for instantaneous power to 24.24 GW.[174][175][176]​.

At the beginning of summer 2011, the German Government announced that the current regulated tariff scheme would end when installed power reached 52 GW. When this happens, Germany will apply a new injection tariff scheme, the details of which are not yet known.[177]​.

However, aware that energy storage through batteries is essential for the massive deployment of renewables such as wind energy or photovoltaics, given their intermittency, on May 1, 2013, Germany launched a new aid program to encourage photovoltaic systems with storage batteries.[178]​ In this way, photovoltaic installations under 30 kW that install batteries and accumulate electricity are financed with 660 euros. for each kW of battery storage. The program is endowed with 25 million euros annually distributed in 2013 and 2014, and in this way energy is available when the resource is not available - there is no wind or it is at night -,[178]​ in addition to facilitating the stability of the electrical system.[179]​.

India

India is densely populated and also has high solar irradiation, making the country one of the best candidates for the development of photovoltaics. In 2009, India announced a program to accelerate the use of solar installations in government buildings, as well as hospitals and hotels.[180]​.

The drop in the price of photovoltaic panels has coincided with an increase in the price of electricity in India. Government support and the abundance of the solar resource have helped drive the adoption of this technology.[181]​.

The 345 MW Charanka solar park (one of the largest in the world) was commissioned in April 2012 and expanded in 2015, along with a total of 605 MW in the Gujarat region.[182] The construction of other large solar parks has been announced in the state of Rajasthan.[183] The 40 MW Dhirubhai Ambani solar park was also inaugurated in 2012.[184]​.

In January 2015, the Indian government significantly increased its solar development plans, setting an investment target of $100 billion and 100 GW of solar capacity by 2022.[185][186]​.

At the beginning of 2017, the total installed capacity in India was above 10 GW.[187]​ India hopes to quickly reach 20 GW installed,[188]​ meeting its goal of creating 1 million jobs[189]​ and reaching 100 GW in 2022.[190][191]​.

Italy

Italy is among the first countries producing electricity from photovoltaic energy, thanks to the incentive program called Conto Energia.[192]​ The growth has been exponential in recent years: installed power tripled in 2010 and quadrupled in 2011, producing in 2012 5.6% of the total energy consumed in the country.[135]​.

This program had a total budget of €6.7 billion. Once this limit was reached, the Government has stopped encouraging new installations, having reached network parity. A report published in 2013 by Deutsche Bank concluded that grid parity had indeed been achieved in Italy and other countries around the world.[193]​ The sector has come to provide work for around 100,000 people, especially in the sector of the design and installation of said solar plants.[194]​.

Since mid-2012, new legislation has been in force that requires the registration of all plants greater than 12 kW; those with lower power (rooftop photovoltaics in residences) are exempt from registration.[195]​ At the end of 2016, the total installed power was above 19 GW,[149]​ assuming such significant energy production that several gas plants operated at half their potential during the day.

United Kingdom

Solar energy in the United Kingdom, although relatively unknown until recently,[196]​ has taken off very rapidly in recent years, due to the drastic fall in the price of photovoltaic panels and the introduction of regulated tariffs from April 2010.[197]​ In 2014, there were already some 650,000 solar installations registered in the British Isles, with a total capacity close to 5 GW.[198]​ The largest solar plant in the country is located on Southwick Estate, near Fareham, and has a capacity of 48 MW. It was inaugurated in March 2015.[199]​.

In 2012, David Cameron's British government committed to powering four million homes with solar energy in less than eight years,[200]​ which is equivalent to installing around 22 GW of photovoltaic capacity by 2020.[197]​ By early 2016, the United Kingdom had installed more than 10 GW of solar photovoltaic energy.[201]​.

Between the months of April and September 2016, solar energy produced more electricity in the United Kingdom (6,964 GWh) than that produced by coal (6,342 GWh), both representing around 5% of demand.[138]​.

France

The French market is the fourth most important within the European Union, after the markets of Germany, Italy and the United Kingdom. At the end of 2014 it had more than 5 GW installed, and currently maintains sustained growth, it is estimated that in 2015 it will connect 1 GW additional to the current capacity to the electrical grid.[202] Recently, the French country increased the quota of its auctions for photovoltaic energy from 400 to 800 MW, as a consequence of government recognition of the increasingly competitiveness of solar energy.[202]​.

In France there is one of the largest photovoltaic plants in Europe, a 300 MW project called Cestas.[203]​[204]​[205]​ Its entry into operation took place at the end of 2015, providing the photovoltaic sector with an example for the rest of the European industry to follow.[203]​.

Spain

Spain is one of the countries in Europe with the highest annual irradiation.[43] This makes solar energy more profitable in this country than in others. Regions such as northern Spain, which are generally considered unsuitable for photovoltaic energy, receive more annual irradiation than the average in Germany, a country that has maintained leadership in the promotion of photovoltaic solar energy for years.[43]​.

Since the beginning of the 2000s, in accordance with the support measures for renewable energies that were being carried out in the rest of Europe, the regulation that establishes the technical and administrative conditions had been approved, and which marked the beginning of a slow takeoff of photovoltaics in Spain. In 2004, the Spanish government eliminated economic barriers to connecting renewable energy to the electrical grid. Royal Decree 436/2004 equalized the conditions for its large-scale production, and guaranteed its sale through generation premiums.[206]​.

Thanks to this regulation, and the subsequent RD 661/2007,[207] Spain was in 2008 one of the countries with the most installed photovoltaic power in the world, with 2708 MW installed in a single year. However, subsequent modifications in the legislation of the sector[208]​ slowed down the construction of new photovoltaic plants, so that in 2009 only 19 MW were installed, in 2010, 420 MW, and in 2011 354 MW were installed, corresponding to 2% of the total in the European Union.[133]​.

In terms of energy production, in 2010 photovoltaic energy covered approximately 2% of electricity generation in Spain, while in 2011 and 2012 it represented 2.9%, and in 2013 3.1% of electricity generation according to data from the operator, Red Eléctrica.[209]​[210][211]​

In 2018, the share of photovoltaic solar energy in Spain reached 3.2% of all energy produced nationally.[212]​.

At the beginning of 2012, the Spanish Government approved a Royal Decree Law by which the installation of new photovoltaic plants and other renewable energies was paralyzed.[213]​ At the end of 2015, the photovoltaic power installed in Spain amounted to 4,667 MW.[214]​ In 2017, Spain fell for the first time from the list of the ten countries with the highest installed photovoltaic capacity, being surpassed by Australia and Korea. of the South.[215]​ However, in July 2017, the Government organized an auction that awarded more than 3,500 MW of new photovoltaic energy plants,[216]​ that will allow Spain to achieve the renewable energy generation objectives established by the European Union for 2020. As a novelty, neither the construction of the awarded plants nor their operation will entail any cost for the system, except in the event that the market price falls below a floor. established at the auction. The great drop in costs of photovoltaic energy has allowed large companies to bid at market price.[217]​.

In 2019, photovoltaics has increased the installed power in Spain by more than 3,000 MW with a total installed power of 7,800 MW.[218] Spain has the largest connected photovoltaic plant in Europe, located in the town of Mula (Murcia), with 494 MW.[219]​.

Latin America

In Latin America, photovoltaics has begun to take off in recent years. The construction of a good number of solar plants has been proposed in various countries, throughout the entire region.[221]​.

Photovoltaic solar energy is expanding in Brazil, while in 2020 the country had 7.8 GW of installed solar energy, the fourteenth country in the world in terms of that energy, as of October 2022 the installed capacity reached a total of 21 GW, with an average capacity factor of 23%. In 2023, Brazil will be among the 10 countries in the world with the most installed solar energy.[222]​ Some of the most irradiated Brazilian states are Minas Gerais, Bahia and Goiás, which have an irradiation between 4.5 to 6.0 kWh/m²/day.[223]​[224]​.

Mexico is the second Latin American country with the highest installed capacity (7.0 GW in 2021),[225]​ and still has enormous potential when it comes to solar energy.[226]​[227]​ 70% of its territory has an irradiation greater than 4.5 kWh/m²/day, which makes it a very sunny country, and implies that using current photovoltaic technology, a 25 km² solar plant in Any place in the state of Chihuahua or the Sonoran Desert (which would occupy 0.01% of Mexico's surface) could provide all the electricity demanded by the country.[228]​.

The Aura Solar project, located in La Paz (Baja California Sur) "La Paz (Baja California Sur)"), inaugurated at the beginning of 2014, which intended to generate 82 GWh per year, enough to supply the consumption of 164,000 inhabitants (65% of the population of La Paz "La Paz (Baja California Sur)")), but was devastated by Hurricane Odile in September of the same year and the plant stopped operating for several months.

In 2016, the reconstruction of the plant was carried out, which was completed at the end of the same year and from 2017 to date it has been in operation again.[229]​.

Another 47 MW photovoltaic plant is in the planning phase in Puerto Libertad (Sonora) "Puerto Libertad (Sonora)").[230]​ The plant, originally designed to house 39 MW, was expanded to allow the generation of 107 GWh/year.[231]​.

Mexico already has more than 3000 MW installed. It is expected to experience greater growth in the coming years, in order to reach the goal of covering 35% of its energy demand from renewable energy in 2024, according to a law approved by the Mexican government in 2012.[232][233]​.

Until a few years ago, Chile led solar production in Latin America (today it is in third place - 4.4 GW in 2021[225]​). The first photovoltaic solar plant in Chile was El Águila, of 2.2 MWp located in Arica, completed in 2012. This country inaugurated a 100 MW photovoltaic plant in June 2014, which became the largest to date in Latin America.[234] The high price of electricity and the high levels of radiation that exist in the north of Chile have promoted the opening of an important free market for subsidies.[235]​ At the end of 2018, the Andean country had 2,427 MW of photovoltaics in operation. Chile has a potential of more than 1800 GW of possible solar energy in the Atacama Desert, according to a study carried out by the German GIZ in Chile (German Society for International Cooperation, 2014). The Atacama Desert is the most irradiated place in the world with global irradiation levels (GHI) above 2700 kWh/m²/year.

Temporal evolution

The following table shows the details of the installed global power, broken down by each country, from 2002 to 2022:

Long term forecast

According to the Renewable Capacity Statistics 2023 report from the International Renewable Energy Agency (IRENA), the photovoltaic power installed in the world reached 773.2 GW in 2020, which represents a growth of 22% compared to the year previous.[1] This growth was double that of renewables as a whole, and represented 39% of the installed power of all electricity generation technologies in 2020.

China was the main photovoltaic market in 2020, with 48 GW installed, followed by the United States, with 19 GW, and Vietnam, with 11 GW1. Spain was among the countries with the highest percentage increase in photovoltaic power, with a 31.99% increase, reaching 18.2 GW.[2] Photovoltaics in countries such as Brazil, Chile and Saudi Arabia have not yet been developed as expected, and are expected to be developed in the coming years. By 2021, the photovoltaic power installed in the world was expected to exceed 900 GW, and that by 2025 it would reach 1,500 GW, according to SolarPower forecasts Europe.[3] The consulting firm Frost & Sullivan estimates that photovoltaic power will reach 1,000 GW in 2024, and that China, India and the United States will follow being the market leaders.

Photovoltaic energy has great potential to contribute to the energy transition and the mitigation of climate change, since it is a clean, renewable, decentralized and competitive source of energy. According to Irena, photovoltaic energy could cover 13% of global electricity demand in 2030, and 23% in 2050. In addition, photovoltaic energy could generate more than 23 million jobs in 2050, and avoid the emission of more than 4 billion tons of CO2 per year.

The PV Market Alliance (PVMA) organization, a consortium made up of several research entities, estimates that global capacity will be between 444-630 GW in 2020. In the most pessimistic scenario, it predicts that the annual installation rate will be between 40 and 50 gigawatts at the end of the decade, while in the most optimistic scenario it estimates that between 60 and 90 GW will be installed annually over the next five years. The intermediate scenario estimates that they will be between 50 and 70 GW, to reach 536 GW in 2020.[247]​[248]​ The PVMA figures agree with those previously published by . In June 2015, Greentech Media (GTM) published its report for 2020, which estimates that annual installations will increase from 40 to 135 GW, reaching a total global capacity of almost 700 GW in 2020. GTM's estimate is the most optimistic of all those published to date, estimating that 518 GW will be installed between 2015 and 2020, which is more than double other estimates.[249]​.

En Europa y en el resto del mundo se han construido un gran número de centrales fotovoltaicas a gran escala.[103]​ Actualmente las plantas fotovoltaicas más grandes del mundo son, de acuerdo a su capacidad de producción:[103]​.

Las mayores plantas solares del mundo se encuentran situadas en China e India. Kurnool Solar, en el estado indio de Andhra Pradesh alberga 1 GW de capacidad, equivalente en potencia a una central nuclear. La planta Yanchi Solar, en la provincia de Qinghai (China) cuenta asimismo con dicha capacidad. Entre los primeros puestos se encuentra también Longyangxia Hydro-Solar PV Station, situada junto a la presa de Longyangxia en China. Consiste en un macrocomplejo hidroeléctrico de 1280 MW, al que posteriormente se le añadió una central fotovoltaica de 320 MW, completada en 2013. A finales de 2015 se inauguró una segunda fase de 530 MW, lo que elevó la potencia total de la planta solar hasta los 850 MW.[251]​[252]​[253]​.

Otros proyectos de gran escala se encuentran situados en Estados Unidos. Solar Star, tiene una potencia de 579 MW y se encuentra en California.[254]​[255]​ Las plantas Topaz Solar Farm y Desert Sunlight Solar Farm en Riverside County, también en California, tiene asimismo una potencia de 550 MW.[256]​[257]​ El proyecto Blythe Solar Power consiste en una planta fotovoltaica de 500 MW, situada igualmente en Riverside County, cuya construcción está prevista próximamente.[258]​ En Europa, la planta de mayor envergadura está ubicada en Murcia (España). Cuenta con una capacidad de 494 MW y entró en operación en 2019.[218]​.

Hay otras muchas plantas de gran escala en construcción. El McCoy Solar Energy Project,[259]​[260]​ en Estados Unidos, tendrá una potencia de 750 MW una vez completado.[261]​ En los últimos años, se ha propuesto la construcción de varias plantas de potencias superiores a los 1000 MW en diferentes lugares del mundo. La planta Quaid-e-Azam Solar Park, situada en Pakistán y cuya primera fase ya se encuentra operativa con 100 MW,[262]​[263]​[264]​ tiene previsto ampliar su capacidad hasta los 1500 MW.[265]​ Los Emiratos Árabes Unidos planean también la construcción de una planta de 1000 MW.[266]​[267]​[268]​ El Ordos Solar Project,[269]​ situado en China, alcanzará los 2000 MW.[270]​ El proyecto Westlands Solar Park tiene una capacidad prevista de 2700 MW,[271]​ a ser completado en varias fases. El proyecto de Ladakh, en India, planea albergar 5 GW de capacidad fotovoltaica.[272]​.

En lo que respecta a instalaciones fotovoltaicas sobre tejado, la mayor instalación se encuentra en las instalaciones de Renault Samsung Motors en Busan (Corea del Sur), y cuenta con 20 MW distribuidos sobre las diferentes cubiertas, aparcamientos e infraestructuras del complejo. Inaugurada en 2013, proporciona energía a la fábrica y miles de hogares cercanos.[273]​.

Photovoltaic energy storage using batteries

Energy storage is presented as a major challenge to enable a continuous supply of energy, given that solar energy cannot be generated at night. Rechargeable batteries have traditionally been used to store excess electricity in isolated systems. With the emergence of grid-connected systems, excess electricity can be transported via the electrical grid to points of consumption. When renewable energy production represents a small fraction of demand, other energy sources can adjust their production appropriately to support the variability of renewable sources, but with the growth of renewable sources, more adequate control is necessary to balance the grid.

With the decline in prices, photovoltaic plants begin to have batteries to control the output power or store excess energy so that it can be used during the hours when renewable plants cannot generate directly. This type of batteries allows the electrical grid to be stabilized by smoothing demand peaks for minutes or hours. It is expected that in the future these batteries will play an important role in the electrical grid, as they can be charged during periods when generation exceeds demand and pour said energy into the grid when demand is greater than generation.

For example, in Puerto Rico a system with a capacity of 20 megawatts for 15 minutes (5 megawatt hours) is used to stabilize the frequency of the grid on the island. Another 27 megawatt system for 15 minutes (6.75 megawatt hours) with nickel-cadmium batteries was installed in Fairbanks, Alaska in 2003 to stabilize the voltage of transmission lines.[274]​.

Most of these battery banks are located next to the photovoltaic plants themselves. The largest systems in the United States include the 31.5 MW battery at the Grand Ridge Power plant in Illinois, and the 31.5 MW battery in Beech Ridge, Virginia.[275] Notable projects include the 400 MWh system (100 MW for four hours) of the Southern California Edison project and a 52 MWh project in Kauai, Hawaii, which completely displaces the production of a 13MW for use after sunset.[276]​ Other projects are located in Fairbanks (40 MW for 7 minutes using nickel-cadmium batteries)[277]​ and in Notrees (Texas) (36 MW for 40 minutes using lead-acid batteries).[278]​.

In 2015, a total of 221 MW with battery storage was installed in the United States, and it is estimated that the total power of this type of systems will grow to 1.7 GW in 2020. The majority installed by wholesale companies in the US market.[279]​.

Photovoltaic self-consumption consists of the small-scale individual production of electricity for one's own consumption, through photovoltaic panels. This can be complemented with the net balance. This production scheme, which allows electricity consumption to be offset by what is generated by a photovoltaic installation at times of lower consumption, has already been successfully implemented in many countries. It was proposed in Spain by the Photovoltaic Industry Association (ASIF) to promote renewable electricity without the need for additional financial support,[280]​ and was in the project phase by the IDAE.[281]​ It was later included in the Renewable Energy Plan 2011-2020,[282]​ but has not yet been regulated.

However, in recent years, due to the growing rise of small renewable energy installations, self-consumption with net balance has begun to be regulated in various countries around the world, being a reality in countries such as Germany, Italy, Denmark, Japan, Australia, the United States, Canada and Mexico, among others.

Among the advantages of self-consumption with respect to network consumption are the following:.

• - With the lower cost of self-consumption systems and the increase in electricity rates, it is increasingly profitable for one to produce one's own electricity.[17]​.

• - Dependency on electricity companies is reduced.

• - Photovoltaic self-consumption systems use solar energy, a free, inexhaustible, clean and environmentally friendly source.

• - A distributed electricity generation system is generated that reduces the need to invest in new networks and reduces energy losses due to the transport of electricity through the network.[283]​.

• - The country's energy dependence on the outside world is reduced.

• - Problems are avoided in supplying all demand at peak times, known for power outages and power surges.

• - The impact of electrical installations on their surroundings is minimized.

• - Companies reduce their energy costs, improve their image and reinforce their commitment to the environment.[284]​.

In the case of photovoltaic self-consumption, the return on investment time is calculated based on how much electricity is no longer consumed from the network, due to the use of photovoltaic panels.

For example, in Germany, with electricity prices of €0.25/kWh and an insolation of 900 kWh/kWp, a 1 kWp installation saves about €225 per year, which with installation costs of €1,700/kWp means that the system will pay for itself in less than 7 years.[285] This figure is even lower in countries like Spain, with irradiation higher than that existing in the north. of the European continent.[43]​.

El efecto de la temperatura sobre los módulos fotovoltaicos se suele cuantificar mediante unos coeficientes que relacionan las variaciones de la tensión en circuito abierto, de la corriente de cortocircuito y de la potencia máxima a los cambios de temperatura. En este artículo, directrices experimentales integrales para estimar los coeficientes de temperatura[286]​.

Las eficiencias de las células solares varían entre el 6 % de aquellas basadas en silicio amorfo hasta el 46 % de las células multiunión.[288]​[289]​ Las eficiencias de conversión de las células solares que se utilizan en los módulos fotovoltaicos comerciales (de silicio monocristalino o policristalino) se encuentran en torno al 16-22 %.[290]​[291]​.

El coste de las células solares de silicio cristalino ha descendido desde 76,67 $/Wp en 1977 hasta aproximadamente 0,36 $/Wp en 2014.[287]​[292]​ Esta tendencia sigue la llamada ley de Swanson, una predicción similar a la conocida Ley de Moore, que establece que los precios de los módulos solares descienden un 20 % cada vez que se duplica la capacidad de la industria fotovoltaica.[293]​.

En 2014, el precio de los módulos solares se había reducido en un 80 % desde el verano de 2008,[294]​[295]​ colocando a la energía solar por primera vez en una posición ventajosa respecto al precio de la electricidad pagado por el consumidor en un buen número de regiones soleadas.[296]​ En este sentido, el coste medio de generación eléctrica de la energía solar fotovoltaica es ya competitivo con el de las fuentes convencionales de energía en una creciente lista de países,[297]​ particularmente cuando se considera la hora de generación de dicha energía, ya que la electricidad es usualmente más cara durante el día.[298]​ Se ha producido una dura competencia en la cadena de producción, y asimismo se esperan mayores caídas del coste de la energía fotovoltaica en los próximos años, lo que supone una creciente amenaza al dominio de las fuentes de generación basadas en las energías fósiles.[299]​ Conforme pasa el tiempo, las tecnologías de generación renovable son generalmente más baratas,[300]​[301]​ mientras que las energías fósiles se vuelven más caras:.

En 2011, el coste de la fotovoltaica había caído bastante por debajo del de la energía nuclear,[303]​ y se espera que siga cayendo:[304]​.

La tendencia es que los precios disminuyan aún más con el tiempo una vez que los componentes fotovoltaicos han entrado en una clara y directa fase industrial.[306]​[307]​ A finales de 2012, el precio medio de los módulos fotovoltaicos había caído a 0,50 $/Wp, y las previsiones apuntan que su precio seguirá reduciéndose hasta los 0,36 $/Wp en 2017.[308]​.

En 2015, el Instituto alemán Fraunhofer especializado en energía solar (ISE) realizó un estudio que concluía que la mayoría de los escenarios previstos para el desarrollo de la energía solar infravaloran la importancia de la fotovoltaica.[309]​ El estudio realizado por el instituto Fraunhofer estimaba que el coste levelizado (LCOE) de la energía solar fotovoltaica para plantas de conexión a red se situará a largo plazo entre 0,02 y 0,04 €/kWh, niveles inferiores a los de las fuentes de energía convencionales.[310]​.

Thin film photovoltaic energy

Another low-cost alternative to crystalline silicon cells is thin-film photovoltaics, which is based on third-generation solar cells.[312] They consist of a solar cell that is manufactured by depositing one or more thin layers (thin film) of photovoltaic material on a substrate.

Thin film solar cells are usually classified according to the photovoltaic material used:.

• - Amorphous silicon (a-Si) and other thin film silicon (TF-Si)[313]​.

• - Cadmium telluride (CdTe)[314]​.

• - Copper indium gallium and selenide (CIS or CIGS)[315]​.

• - Dye-sensitized solar cells (DSC)[316]​ and other organic solar cells.[317]​.

The International Low Cost Solar Energy Conference in Seville, held in February 2009, was the first showcase of the same in Spain.[318]​ This technology caused great expectations in its beginnings. However, the sharp drop in the price of polycrystalline silicon cells and modules since late 2011 has meant that some thin film manufacturers have been forced to exit the market, while others have seen their profits greatly reduced.[319]​.

La cantidad de energía solar que alcanza a la superficie terrestre es enorme, cerca de 122 petavatios (PW), y equivale a casi 10 000 veces más que los 13 TW consumidos por la humanidad en 2005.[320]​ Esta abundancia sugiere que no pasará mucho tiempo antes de que la energía solar se convierta en la principal fuente de energía de la humanidad.[321]​ Adicionalmente, la generación eléctrica mediante fotovoltaica presenta la mayor densidad energética (una media global de 170 W/m²) de todas las energías renovables.[320]​.

A diferencia de las tecnologías de generación de energía basadas en combustibles fósiles, la energía solar fotovoltaica no produce ningún tipo de emisiones nocivas durante su funcionamiento,[1]​ aunque la producción de los paneles fotovoltaicos presenta también un cierto impacto ambiental. Los residuos finales generados durante la fase de producción de los componentes, así como las emisiones de las factorías, pueden gestionarse mediante controles de contaminación ya existentes. Durante los últimos años también se han desarrollado tecnologías de reciclaje para gestionar los diferentes elementos fotovoltaicos al finalizar su vida útil,[322]​ y se están llevando a cabo programas para incrementar el reciclaje entre los productores fotovoltaicos.[323]​.

La tasa de retorno energético de esta tecnología, por su parte, es cada vez mayor. Con la tecnología actual, los paneles fotovoltaicos recuperan la energía necesaria para su fabricación en un período comprendido entre 6 meses y 1 año y medio; teniendo en cuenta que su vida útil media es superior a 30 años, producen electricidad limpia durante más del 95 % de su ciclo de vida.[324]​.

Greenhouse gas emissions

Greenhouse gas emissions throughout the life cycle for photovoltaics are close to 46 g/kWh, and can even be reduced to 15 g/kWh in the near future.[325]​

In comparison, a combined cycle gas plant emits between 400-599 g/kWh,[326]​ a diesel plant 893 g/kWh,[326]​ a coal plant 915-994 g/kWh[327]​ or with carbon capture technology about 200 g/kWh (excluding emissions during coal extraction and transportation), and a power plant high temperature geothermal, between 91-122 g/kWh.[326]​ The intensity of emissions for the life cycle of hydraulic, wind and nuclear energy is lower than that of photovoltaic energy, according to data published by the IPCC in 2011.[326]​.

Like all energy sources whose emissions depend primarily on the construction and transportation phases, the transition to a low-carbon economy could further reduce carbon dioxide emissions during the manufacturing of solar devices.

A 1 kW photovoltaic system saves the combustion of approximately 77 kg (170 pounds) of coal, avoids the emission into the atmosphere of about 136 kg (300 pounds) of carbon dioxide, and saves the use of about 400 liters (105 gallons) of water monthly.[328]​.

Degradation of photovoltaic modules

The power output of a photovoltaic (PV) device decreases over time. This decrease is due to its exposure to solar radiation as well as other external conditions. The degradation rate, which is defined as the annual percentage of output power loss, is a key factor in determining the long-term production of a photovoltaic plant. To estimate this degradation, the percentage of decrease associated with each of the electrical parameters is quantified separately. It should be noted that the degradation of an individual PV module can significantly influence the performance of an entire string. Furthermore, not all modules in the same installation decrease their performance at exactly the same rate. Given a set of modules exposed to long-term outdoor conditions, the individual degradation of the main electrical parameters and the increase in their dispersion must be considered. As each module tends to degrade differently, the behavior of the modules will become increasingly different over time, negatively affecting the overall performance of the plant.

There are several studies that deal with the analysis of power degradation of modules based on different photovoltaic technologies available in the literature. According to a recent study,[329]​ the degradation of crystalline silicon modules is very regular, ranging between 0.8% and 1.0% per year.

On the other hand, if we analyze the performance of thin-film photovoltaic modules, an initial period of strong degradation is observed (which can last several months and even up to 2 years), followed by a subsequent stage in which the degradation stabilizes, being then comparable to what crystalline silicon can have.[330]​ In these thin-film technologies, strong seasonal variations are also observed because the influence of the solar spectrum is much greater. For example, for amorphous silicon, micromorphic silicon or cadmium tellulide modules, we are talking about annual degradation rates for the first years of between 3% and 4%.[331]​ However, other technologies, such as CIGS, have much lower degradation rates, even in those first years.

Recycling of photovoltaic modules

A photovoltaic installation can operate for 30 years or more[332]​ with little maintenance or intervention after its start-up, so after the initial investment cost necessary to build a photovoltaic installation, its operating costs are very low compared to the rest of the existing energy sources. At the end of their useful life, most of the photovoltaic panels can be treated. Thanks to technological innovations that have been developed in recent years, up to 95% of certain semiconductor materials and glass can be recovered, as well as large quantities of ferrous and non-ferrous metals used in modules.[333]​ Some private companies[334]​ and non-profit organizations, such as PV CYCLE in the European Union, are working on collection and recycling operations for end-of-life panels.[335]​.

Two of the most common recycling solutions are:.

• - Silicon panels: The aluminum frames and connection boxes are manually dismantled at the beginning of the process. The panel is crushed and the different fractions are separated: glass, plastics and metals.[336]​ It is possible to recover more than 80% of the incoming weight[337]​ and, for example, the extracted mixed glass is easily accepted by the foam glass and insulation industries. This process can be carried out by flat glass recyclers, since the morphology and composition of a photovoltaic panel is similar to the flat glass used in the construction and automotive industries.

• - Panels made of other materials: Today there are specific technologies for recycling photovoltaic panels that do not contain silicon; some techniques use chemical baths to separate the different semiconductor materials.[338]​ For cadmium telluride panels, the recycling process begins by crushing the module and, subsequently, separating the different parts. This recycling process is designed to recover up to 90% of the glass and 95% of the semiconductor materials.[339]​ In recent years, some private companies have launched commercial-scale recycling facilities.[340]​.

Since 2010, an annual conference has been held in Europe that brings together producers, recyclers and researchers to discuss the future of photovoltaic module recycling. In 2012 it took place in Madrid.[341][342]​.

• - Portal:Energy. Content related to Energy.

• - Portal:Ecology. Content related to Ecology.

• - Wikimedia Commons hosts a multimedia category on Photovoltaic Solar Energy.

• - UNEF website (Spanish Photovoltaic Union) Main association of the photovoltaic sector in Spain.

• - Photovoltaics in Spain website (Universitat Politècnica de València).

• - PV Magazine Latin America News from the photovoltaic sector in Spanish.

• - PVGIS. Accurate calculation of solar radiation and the performance of photovoltaic plants.

• - IRENA Global Atlas Database for Solar and Wind.

• - Photovoltaic Program of the International Energy Agency.