Solar electricity production follows the same principles as other systems for transforming heat into electricity (thermal power stations, nuclear power plants …). The heat is transformed into mechanical energy, in the form of a gas that is taken to a high temperature at a high pressure and used to drive a turbine. Then the mechanical energy produced is transformed into electricity by using the turbine to drive an alternator. If the system is to produce good yields, it is essential to obtain high temperatures to heat the gas.
Solar captors, even vacuum captors, are incapable
of attaining the necessary temperatures. The principle
of a solar power plant is therefore to concentrate solar rays on a single point, much as a magnifying glass can be used to burn a piece of paper!
There are 3 types of solar power plants, differing according to the method used to focus the solar rays:
Parabolic trough power plants: these consist of parallel alignments of long half- cylindrical mirrors, which turn about a horizontal axis to follow the path of the sun. Solar rays are concentrated onto a horizontal tube carrying fluid that is used to transport the heat to the power plant itself. The fluid temperature can rise to 500°C. This type of power plant is the most common.
Solar power towers: sets of swivelling mirrors, situated on the ground, concentrate the solar radiation on a single point: a boiler located on the top of a tower. A temperature of 600°C is obtained in this installation.
Parabolic reflector plants: the same form as our parabolic TV dishes, but on a larger scale! The dishes have a diameter of 10 to 20 meters and can be oriented to follow the sun’s movement. The solar radiation is concentrated at the focal point of the parabola where a mini electric power plant is situated. Temperatures of 800°C are attainable.
Solar power plants only work during the day. To ensure continuous working, the hot thermal carrying fluid is stored for use at night, or the power plant can burn traditional fuels (gas, coal …), once the sun goes down.
Solar power plants cannot be installed just anywhere. For them to be profitable, there must be:
- Ample sunlight during the majority of the year;
- Good air transparency (i.e.
a non-polluted environment);
- A lot of space available to install the mirrors or parabolic dishes (2ha/MW of power);
- A flat region, if possible (to make savings on the costs of pumping the thermal carrying fluid);
- An electricity network not too far away, to distribute the electricity produced.
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