Components
Contenido
Un sistema fotovoltaico para el suministro de energía residencial, comercial o industrial está formado por el conjunto solar y una serie de componentes que a menudo se resumen en el balance del sistema (BOS). Este término es sinónimo de "Balance de planta" q.v. Los componentes del BOS incluyen equipos de acondicionamiento de energía y estructuras para su montaje, normalmente uno o más convertidores de potencia de CC a CA, también conocidos como inversores "Inversor (electrónica)"), un dispositivo de almacenamiento de energía, un sistema de estanterías que soporta el conjunto solar, el cableado eléctrico y las interconexiones, y el montaje de otros componentes.
Opcionalmente, un sistema de balance puede incluir alguno o todos los siguientes elementos: medidor de ingresos de créditos de energía renovable, rastreador de punto de máxima potencia (MPPT), sistema de baterías y cargador, rastreador solar por GPS, software de gestión de la energía, sensores de irradiación solar, anemómetro o accesorios específicos diseñados para satisfacer los requisitos especializados de un propietario de sistema. Además, un sistema CPV requiere lentes o espejos ópticos y a veces un sistema de refrigeración.
Los términos "panel solar" y "sistema fotovoltaico" a menudo se utilizan incorrectamente de manera intercambiable, a pesar de que el panel solar no abarca todo el sistema. Además, "panel solar" se utiliza a menudo como sinónimo de "módulo solar", aunque un panel consta de una cadena de varios módulos. El término " sistema solar " también es un nombre inapropiado para un sistema fotovoltaico.
Solar panel
The basic components of a photovoltaic system are solar cells. A solar cell is the electrical device that can directly convert photon energy into electricity. There are three technological generations of solar cells: the first generation (1G) of crystalline silicon (c-Si) cells, the second generation (2G) of thin film cells (such as CdTe, CIGS, amorphous silicon and GaAs), and the third generation (3G) of organic cells, sensitized by dyes, Perovskite and multijunction.[26][27].
Conventional c-Si solar cells, typically connected in series, are encapsulated in a solar module to protect them from the elements. The module consists of a tempered glass cover, a soft and flexible encapsulant, a backsheet of a weather- and fire-resistant material, and an aluminum frame around the outer edge. Electrically connected and mounted on a support structure, solar modules form a chain of modules, often called a solar panel.[28] A solar array is made up of one or more of these panels. A photovoltaic array, or solar array, is a linked collection of solar modules. The energy that a single module can produce is rarely enough to meet the needs of a home or business, so modules are joined together to form a set. Most photovoltaic arrays use an inverter to convert the direct current produced by the modules into alternating current that can power lights, motors, and other loads. The modules of a photovoltaic array are usually first connected in series to obtain the desired voltage; The individual strings are then connected in parallel so that the system can produce more current. Solar panels are typically measured under standard test conditions (STC) or PVUSA test conditions (PTC), in watts.[29] Typical panel values range from less than 100 watts to more than 400 watts. The classification of the assembly consists of the sum of the classifications of the panels, in watts, kilowatts or megawatts.
A typical 150 watt photovoltaic module is about one square meter in size. Such a module can be expected to produce 0.75 kilowatt-hours (kWh) every day, on average, after taking into account climate and latitude, for an insolation of 5 hours of sunshine/day. The output and life of the module are degraded by the increase in temperature. Allowing ambient air to flow upwards and, if possible, backwards of the PV modules reduces this problem, as the air flow reduces the operating temperature and consequently increases the efficiency of the module. This almost always occurs for installations with a single photovoltaic module, mistakenly leading to this phenomenon being considered the same for installations with many modules. However, it has recently been shown that, in photovoltaic generators, increasing wind speed increases energy losses.[30] This seemingly counterintuitive result follows well-known laws of fluid mechanics: the interaction of the wind with the photovoltaic generator induces airflow variations that change the heat transfer from the modules to the air. [3].
The effective useful life of modules is typically 25 years or more.[31] The payback period for an investment in a solar PV installation varies widely and is generally less useful than a return on investment calculation.[32] While typically estimated at 10 to 20 years, the financial payback period can be much shorter with incentives.[33].
The effect of temperature on photovoltaic modules is usually quantified using coefficients that relate variations in open circuit voltage, short circuit current and maximum power to changes in temperature. In this work, complete experimental guidelines for estimating temperature coefficients are presented[34].
Due to the low voltage of a single solar cell (typically about 0.5 V), several cells are wired in series in the manufacture of a "laminate". The laminate is assembled into a weather-resistant protective box, thus forming a photovoltaic module or solar panel. The modules can then be chained together to form a photovoltaic array. In 2012, solar panels available to consumers had an efficiency of up to approximately 17%,[35] while panels available on the market in 2020 can reach up to 27%. A group at the Fraunhofer Institute for Solar Energy Systems is reported to have created a cell that can reach an efficiency of 44.7%, making scientists' hopes of reaching the 50% efficiency threshold much more feasible.[36][37][38][39].
The electrical output of photovoltaic cells is extremely sensitive to shading ("Christmas light effect").[40][41][42] When even a small part of a cell, module or assembly is in shade, while the rest is in sunlight, performance decreases dramatically due to internal "short circuit" (electrons reverse course through the shaded part of the p-n junction). If the current drawn from the chain of cells in series is no greater than that which the shaded cell can produce, the current (and therefore power) developed by the chain is limited. If sufficient voltage is available in the other cells in the chain, current will be forced through the cell by breaking the junction in the shaded portion. This breakdown voltage in common cells is between 10 and 30 volts. Instead of increasing the energy produced by the panel, the shaded cell absorbs energy, converting it into heat. Because the reverse voltage of a shaded cell is much greater than the forward voltage of a lit cell, a shaded cell can absorb energy from many other cells in the chain, disproportionately affecting the panel's output. For example, a shaded cell can drop 8 volts, instead of adding 0.5 volts, at a given current level, thus absorbing the energy produced by 16 other cells.[43] Therefore, it is important that a photovoltaic installation is not shaded by trees or other obstacles.
Several methods have been developed to determine tree shading losses in photovoltaic systems, both over large regions using LiDAR,[44] and at the individual system level using sketchup.[45] Most modules have bypass diodes between each cell or string of cells that minimize the effects of shading and only lose power from the shaded part of the array. The main function of the bypass diode is to eliminate hot spots that form in the cells and that can damage the assembly and cause fires.
Sunlight can be absorbed by dust, snow or other impurities on the surface of the module (collectively referred to as dirt). Dirt reduces the light that hits the cells, which in turn reduces the power of the photovoltaic system. Dirt losses accumulate over time and can become large if proper cleaning is not carried out. In 2018, the global annual energy loss due to dirt was estimated to be at least 3% - 4%.[46] However, losses due to dirt vary greatly from region to region,[47] and within regions.[48] Maintaining a clean module surface will increase production performance over the life of the PV system. In a study conducted in an area with a lot of snow (Ontario), cleaning flat-mounted solar panels after 15 months increased their output by almost 100%. However, the sets inclined at 5° were adequately cleaned with rainwater. In many cases, especially in arid regions or in locations close to deserts, roads, industry or agriculture, regular cleaning of solar panels is cost-effective. In 2018, the estimated revenue loss induced by fouling was estimated between €5 billion and €7 billion.[49].
The long-term reliability of PV modules is crucial to ensure the technical and economic viability of PV as a successful energy source. The analysis of the degradation mechanisms of photovoltaic modules is key to guaranteeing a current useful life of more than 25 years.[50].
Solar insolation is composed of direct, diffuse and reflected radiation. The absorption factor of a photovoltaic cell is defined as the fraction of the incident solar irradiation that is absorbed by the cell.[51] At noon on a cloudless day at the equator, the sun's power is approximately 1 kW/m²,[52] on the Earth's surface, in a plane perpendicular to the sun's rays. Therefore, photovoltaic installations can follow the sun throughout the day to improve energy capture. However, tracking devices are an additional cost and require maintenance, so it is more common for photovoltaic arrays to have fixed supports that tilt the array and orient it toward solar noon (approximately south in the northern hemisphere or north in the southern hemisphere). The angle of inclination, from the horizontal, can vary depending on the season,[53] but if it is fixed, it must be adjusted to obtain optimal production of the assembly during the part of maximum electrical demand of a typical year for a stand-alone system.[53] This angle of optimal inclination of the module is not necessarily identical to the angle of inclination for the maximum annual energy production of the assembly. Optimizing the PV system for a specific environment can be complicated, as issues such as solar flux, dirt, and snow losses must be taken into account. Furthermore, subsequent work has shown that spectral effects can play a role in optimal photovoltaic material selection. For example, spectral albedo can play an important role in production depending on the surface area surrounding the PV system and the type of solar cell material. For the climate and latitudes of the United States and Europe, typical sunshine ranges from 4 kWh/m²/day in northern climates to 6.5 kWh/m²/day in the sunniest regions. A photovoltaic installation in the northern latitudes of Europe or the United States can expect to produce 1 kWh/m²/day. A typical 1 kW PV installation in Australia or in the southern latitudes of Europe or the United States can produce between 3.5 and 5 kWh per day, depending on location, orientation, inclination, insolation and other factors. In the Sahara Desert, with less cloud cover and a better solar angle, around 8.3 kWh/m²/day could ideally be obtained, as long as the wind, almost always present, does not blow sand onto the units. The area of the Sahara Desert is more than 9 million km². Of these, 90,600 km² (that is, about 1%) could generate as much electricity as all the power plants in the world combined.[54].
Mounting
Modules are assembled in arrays on some type of mounting system, which can be classified as ground, roof, or pole mounting. In the case of solar parks, a large frame is assembled on the ground and the modules are mounted on it. In the case of buildings, many different frames have been devised for pitched roofs. For flat roofs, racks, containers and building-integrated solutions are used. Pole-mounted solar panel racks can be fixed or mobile. Pole side supports are suitable for situations where a pole has something else mounted on its top, such as a light fixture or antenna. Pole mounting elevates what would otherwise be a ground-mounted assembly above the shadows of brush and livestock, and can satisfy electrical code requirements for inaccessibility of exposed wiring. Pole-mounted panels are open to more cooling air at their bottom, increasing performance. A multiplicity of pole-mounted racks can form a parking carport or other shade structure. A frame that does not follow the sun from left to right may allow for seasonal adjustment up or down.
Cabling
Due to their outdoor use, solar cables are designed to be resistant to ultraviolet radiation and extremely high temperature fluctuations and are generally not affected by weather. Standards specifying the use of electrical wiring in photovoltaic systems include the International Electrotechnical Commission's IEC 60364, in section 712 "Solar Photovoltaic (PV) Supply Systems", the British Standard BS 7671, which incorporates regulations relating to microgeneration and photovoltaic systems, and the US Standard UL4703, in topic 4703 "Photovoltaic Cable".
Tracker
A solar tracking system tilts a solar panel throughout the day. Depending on the type of tracking system, the panel is aimed directly at the sun or at the brightest area of a partly cloudy sky. Trackers greatly improve early morning and late afternoon performance, increasing the total amount of power produced by a system by 20-25% for a single-axis tracker and by 30% or more for a two-axis tracker, depending on latitude.[55][56] Trackers are effective in regions that receive a large portion of sunlight directly. In diffuse light (i.e. under clouds or fog), tracking has little or no value. Since most concentrated photovoltaic systems are very sensitive to the angle of sunlight, tracking systems allow them to produce useful power for more than a short period per day.[57] Tracking systems improve performance for two main reasons. First, when a solar panel is perpendicular to sunlight, it receives more light on its surface than if it were tilted. Second, direct light is used more effectively than angled light.[58] Special anti-reflective coatings can improve the efficiency of the solar panel for direct and angled light, somewhat reducing the tracking benefit.[59].
Trackers and sensors to optimize performance are often considered optional, but can increase viable output by up to 45%.[60] Arrays approaching or exceeding one megawatt often use solar trackers. Taking into account the clouds, and the fact that most of the world is not at the equator, and that the sun sets at night, the correct measure of solar energy is insolation, the average number of kilowatt-hours per square meter per day. For the climate and latitudes of the United States and Europe, typical insolation ranges from 2.26 kWh/m/day in northern climates to 5.61 kWh/m/day in the sunniest regions.[61][62].
For large systems, the energy gained from using monitoring systems can offset the added complexity. For very large systems, the added maintenance of tracking is a substantial detriment.[63] Tracking is not necessary for flat, low-concentration PV systems. For highly concentrated photovoltaic systems, dual-axis tracking is a necessity.[64] Price developments influence the balance between adding more fixed solar panels and having fewer tracked panels.
As the prices, reliability and performance of single-axis trackers have improved, the systems have been installed on an increasing percentage of utility-scale projects. According to data from WoodMackenzie/GTM Research, global shipments of solar trackers reached a record 14.5 gigawatts in 2017. This represents 32 percent year-over-year growth, with similar or greater growth projected as large-scale solar deployment accelerates.[65].
Investor
Systems designed to supply alternating current (AC), such as grid-tied applications, require an inverter to convert direct current (DC) from solar modules to AC. Grid-tied inverters must supply AC electricity in a sinusoidal manner, synchronized with the grid frequency, limit the supply voltage to no higher than the grid voltage, and disconnect from the grid if the grid voltage is off.[66] Islanded inverters only need to produce regulated voltages and frequencies in a sine wave form, as no synchronization or coordination with grid supplies is required.
A solar inverter can be connected to a string of solar panels. In some installations a solar microinverter is connected to each solar panel.[67] For safety reasons, a circuit breaker is provided on both the AC and DC sides to allow for maintenance. The AC output can be connected to the utility grid through an electricity meter.[68] The number of modules in the system determines the total DC watts the solar array can generate; However, the inverter ultimately governs the amount of AC watts that can be distributed for consumption. For example, a PV system comprised of 11 kilowatts DC (kW) of PV modules, paired with a 10 kilowatt AC (kW) inverter, will be limited to the inverter power of 10 kW. In 2019, the conversion efficiency of the latest generation converters reached more than 98%. While string inverters are used in residential and medium-sized commercial PV systems, central inverters cover the commercial and large-scale market. The market share of central and string inverters is approximately 44% and 52%, respectively, and less than 1% for microinverters.[69].
Maximum power point tracking (MPPT) is a technique used by grid-connected inverters to obtain the maximum possible power from the photovoltaic array. To do this, the inverter's MPPT system digitally samples the ever-changing power output of the solar panel and applies the appropriate resistance to find the optimal maximum power point.[70].
Anti-islanding is a protection mechanism to immediately shut down the inverter, preventing it from generating AC power when the connection to the load no longer exists. This occurs, for example, in the event of a blackout. Without this protection, the supply line would become a powered "island" surrounded by a "sea" of unpowered lines, as the solar array continues to supply DC power during the blackout. Isolation is a hazard to power company workers, who may not realize that an AC continues to receive power, and may prevent devices from automatically reconnecting.[71] Anti-islanding is not required for complete off-grid systems.
Battery
Although still expensive, photovoltaic systems increasingly use rechargeable batteries to store surplus that will then be used at night. Batteries used for grid storage also stabilize the electrical grid by leveling peak loads and play an important role in a smart grid as they can charge during periods of low demand and feed their stored energy to the grid when demand is high.
The most widely used battery technologies in current photovoltaic systems are the valve-regulated lead-acid battery - a modified version of the conventional lead-acid battery -, nickel-cadmium batteries and lithium-ion batteries. Compared to the other types, lead-acid batteries have a shorter lifespan and lower energy density. However, due to their high reliability, low self-discharge and low investment and maintenance costs, they are currently the predominant technology in small-scale residential photovoltaic systems, since lithium-ion batteries are still in the development phase and are about 3.5 times more expensive than lead batteries. Furthermore, as the storage devices of photovoltaic systems are stationary, the lower energy and power density and therefore the greater weight of lead-acid batteries are not as critical as, for example, in electric transportation[9] Other rechargeable batteries considered for distributed photovoltaic systems are sodium-sulfur and vanadium-redox, two prominent types of molten salt and flow batteries, respectively[9] In 2015, Tesla Motors launched the Powerwall, a rechargeable lithium-ion battery with the aim of revolutionizing energy consumption.[73].
Photovoltaic systems with an integrated battery solution also need a charge controller, as the variable voltage and current of the solar panel requires constant adjustment to prevent damage from overcharging.[74] Basic charge controllers can simply turn photovoltaic panels on and off, or they can measure pulses of power as needed, a strategy called PWM or pulse width modulation. More advanced charge controllers will incorporate MPPT logic into their battery charging algorithms. Charge controllers can also divert power for some purpose other than battery charging. Instead of simply turning off the free PV power when it is not needed, a user can choose to heat air or water once the battery is full.
Control and measurement
The counter must be capable of accumulating units of energy in both directions, or two counters must be used. Many meters accumulate bidirectionally, some systems use two meters, but a unidirectional meter (with detent) will not accumulate energy from any resulting feed to the grid.[75] In some countries, a frequency and voltage monitor with disconnection of all phases is required for installations over 30 kWp. This is done when more solar energy is generated than the power company can accommodate, and the excess cannot be exported or stored. Historically, grid operators have had to provide transmission lines and generation capacity. Now they have to provide storage as well. Typically this is hydraulic storage, but other means of storage are also used. Initially, storage was used so that baseload generators could operate at full capacity. With variable renewable energy, storage is necessary to allow energy to be generated whenever available, and consumed whenever necessary.
The two variables a grid operator has are storing electricity for when it is needed or transmitting it to where it is needed. If both fail, installations larger than 30 kWp may automatically shut down, although in practice all inverters maintain voltage regulation and stop supplying power if the load is inadequate. Grid operators have the option of reducing excess generation from large systems, although this is more commonly done with wind power than solar power, and results in a substantial loss of revenue.[76] Three-phase inverters have the unique option of supplying reactive power, which can be advantageous in meeting load requirements.[77].
Photovoltaic installations need to be monitored to detect faults and optimize their operation. There are several photovoltaic monitoring strategies depending on the performance of the installation and its nature. Monitoring can be done on site or remotely. You can measure only production, retrieve all data from the inverter, or retrieve all data from communication equipment (probes, meters, etc.). Monitoring tools can be dedicated to monitoring only or offer additional functions. Individual inverters and battery charge controllers may include monitoring using manufacturer-specific protocols and software.[78] An inverter's energy metering may have limited accuracy and may not be suitable for revenue metering purposes. A third-party data acquisition system can monitor multiple inverters, using the inverter manufacturer's protocols, and also acquire information related to weather conditions. Stand-alone smart meters can measure the total energy production of a PV installation. Independent measurements, such as satellite image analysis or a solar radiation meter (a pyranometer), can be used to estimate total sunshine for comparison.[79] Data collected by a monitoring system can be viewed remotely via the World Wide Web, such as OSOTF.
Sizing of the photovoltaic system
Knowing the annual energy consumption in kWh of an institution or a family, for example 2300Kwh, legible on their electricity bill, it is possible to calculate the number of photovoltaic panels necessary to satisfy their energy needs. When connecting to the site https://re.jrc.ec.europa.eu/pvg_tools/es/, after selecting the location in which to install the panels or clicking on the map or writing the name of the location, you must select "Network PV" and "Show results" obtaining the following table for example referring to the city of Palermo:.
Using the wxMaxima program "Maxima (software)"), the number of panels required for an annual consumption of 2300 kWh and for a crystalline silicon technology with an inclination angle of 35°, an orientation angle of 0° and total losses equal to 21.88 % is 6 rounded:.
On average, each family manages to consume 30% of energy directly from photovoltaics. The storage system can bring its self-consumption to a maximum of 70%, therefore the storage capacity of the battery that must be in the specific case is: 4.41 kWh, which is rounded to 4.8 kWh.
If the price of energy is €0.5/Kwh, the cost of energy without taxes will be €1,150 per year:.
So if a 300W panel costs €200, the 4.8Kwh battery costs €3000, the inverter to convert direct current into alternating current €1000, the charge regulator €100, the installation costs €1000, the total cost will be €6300:.
that are amortized in 5.46 years:.
the battery having a life of 10 years and the panels 25-30 years.