Horizontal axis wind turbines
Contenido
Son aquellos en los que el eje de rotación del equipo se encuentra paralelo al suelo. Esta es la tecnología que se ha impuesto, por su eficiencia y confiabilidad y la capacidad de adaptarse a diferentes potencias.
Las partes principales de un aerogenerador de eje horizontal son:.
Todos los aerogeneradores de eje horizontal tienen su eje de rotación principal en la parte superior de la torre, que tiene que orientarse hacia el viento de alguna manera. Los aerogeneradores pequeños se orientan mediante una veleta, mientras que los más grandes utilizan un sensor de dirección y se orientan por servomotores o motorreductores.
Existen dos tipologías principales de generadores eléctricos: con y sin caja multiplicadora. Los primeros funcionan a velocidades del orden de 1000-2000 rpm. Dado que la velocidad de rotación de las aspas es baja (entre 8 y 30 rpm), requieren el uso de una caja multiplicadora para conseguir una velocidad de rotación adecuada. Los aerogeneradores que no precisan multiplicadora se conocen como direct-drive y sus generadores se llaman habitualmente multipolo, ya que para conseguir una frecuencia elevada con una baja velocidad de giro tienen más de una decena de polos.
En la mayoría de los casos la velocidad de giro del generador está relacionada con la frecuencia de la red eléctrica a la que se vierte la energía generada (50 o 60 Hz).
En general, las palas están emplazadas de tal manera que el viento, en su dirección de flujo, las encuentre antes que a la torre (rotor a barlovento). Esto disminuye las cargas adicionales que genera la turbulencia de la torre en el caso en que el rotor se ubique detrás de la misma (rotor a sotavento). Las palas se montan a una distancia razonable de la torre y tienen alta rigidez, de tal manera que al rotar y vibrar naturalmente no choquen con la torre en caso de vientos fuertes. El rotor suele estar inclinado entre 4 y 6 grados para evitar el impacto de las palas con la torre.
A pesar de la desventaja en el incremento de la turbulencia, se han construido aerogeneradores con el rotor localizado en la parte posterior de la torre, debido a que se orientan en contra del viento de manera natural, sin necesidad de usar un mecanismo de control. Sin embargo, la experiencia ha demostrado la necesidad de un sistema de orientación para orientar la máquina hacia el viento. Este tipo de montaje se justifica debido a la gran influencia que tiene la turbulencia en el desgaste de las aspas por fatiga. La mayoría de los aerogeneradores actuales son de este último modelo.
Wind power
The kinetic energy of air () depends on the square of the speed of air () and its density ():.
The power (), in watts per unit area, can be expressed as:.
Therefore, the wind power to which a turbine will be exposed is determined by multiplying the previous expression by the swept area of the turbine, which is the circle covered by the blades.[5] For example, the swept area of a wind turbine with a rotor of 82 meters in diameter will be 5,281 m².
However, not all the power of the air can be used by the wind turbine. The limit of power that can be extracted is given by the limit established by the physicist Albert Betz. This limit, which bears his name, is derived from the conservation of the mass and moment of inertia of the air flow. The Betz limit indicates that a wind turbine cannot harness more than 59.3% of the kinetic energy of the wind. The number (0.593) is known as the Betz coefficient. For example, if a wind turbine 82 meters in diameter were exposed to a wind of 15 m/s with an air density of 1.28 kg/m³, it could extract, assuming a perfect wind (without turbulence) and perfect performance, up to 6.76 MW of electrical energy.
Modern wind turbines obtain between 75 and 80% of the Betz limit.[6] One of the factors that most influences not reaching 100% of the Betz limit is the roughness of the ground. This roughness is influenced by the presence of vegetation or buildings on the ground, which reduce wind speed and increase air turbulence. Therefore, a greater height of the rotor and the installation in the sea ("offshore wind")) contribute to better use of the energy in the air.
Power control
In general, modern horizontal axis wind turbines are designed to work with wind speeds that vary between 3 and 25 m/s on average. The first is the so-called connection speed and the second is the cutting speed. Basically, the wind turbine begins producing electrical energy when the wind speed exceeds the connection speed and, as the wind speed increases, the power generated is greater, following the so-called power curve.
The blades have a control system so that their angle of attack varies depending on the wind speed. This allows the rotation speed to be controlled to achieve a fixed rotation speed under different wind conditions.
Likewise, a rotation speed control system is necessary so that, in the event of excessively strong winds, which could endanger the installation, it rotates the rotor in such a way that the blades present the minimum opposition to the wind, which would cause the machine to stop.
For high-power wind turbines, some types of passive systems use aerodynamic characteristics of the blades that cause the rotor to stop even in very strong wind conditions. This is because it itself enters a regime called "aerodynamic stall".
Impact on the environment
This type of generators have quickly become popular as they are considered a clean source of energy, since they do not require, for energy production, combustion that produces polluting waste or gases involved in the greenhouse effect. However, its use is not free of environmental impact. Their location—frequently in remote places of high ecological value, such as mountain peaks, which because they are not inhabited, preserve their landscape and fauna richness—can cause harmful effects, such as the visual impact on the horizon line, the large surface area they occupy due to the necessary separation between them—between three[7] and ten[8] rotor diameters—or the intense noise generated by the blades, in addition to the effects caused by the infrastructure that must be built for transportation. of electrical energy to the points of consumption. Despite research to minimize them, bird deaths continue to occur due to them,[9] in addition to bat populations being affected.[10] In some wind power plants, about 14 birds and 40 bats die each year for every MW installed.[11] More recently, the possibility has been proposed that their widespread use could even contribute to global warming by blocking air currents.[12].
On the other hand, taking into account the greenhouse gases that are produced by the tasks derived from the construction, transportation and maintenance of the wind turbine, terrestrial wind energy, with 12 g of CO per kWh, is the second least polluting energy,[13] after hydroelectric energy (with 4 g of CO per kWh); This is followed by nuclear energy (with 16 g of CO per kWh), and solar thermal energy (with 22 g of CO per kWh). To this we must add the problem of shovels, which cease to be useful after about 20 years of use, and which usually end up in landfills (called "shovel cemeteries") due to the complexity of their recycling.[14].