Sun
Thermodynamic solar power plants - multiplying an abundant source of energy
09/14/2010
In contrast to the direct production of electricity via photovoltaic cells, electricity production in a thermodynamic solar power plant is broken down into two distinct phases: first, the sun's rays are concentrated and converted into high temperature heat, then this heat is used in a thermodynamic cycle to drive a turbine (generally a steam turbine) and an electric generator attached to the turbine. The various types of solar power plants can achieve very high temperatures.
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A conventional power plant - with no fuel
Electricity production in a thermodynamic solar plant follows the same principle as other methods of converting heat into electricity (conventional power plants, nuclear power plants, etc.).
The point is to convert heat into mechanical energy (i.e. that can cause movement) which is then used to drive a turbine, which in turn drives an alternator that produces electricity. The difference is that no fuel is used to obtain steam: it is the sun that heats the heat-transfer fluid (i.e. water, gas or oil capable of circulating its heat inside the distribution circuit). The sun's rays are concentrated in a single point, like a magnifying glass used to set fire to paper.
To ensure good profitability, these solar plants are generally set up in areas that have:
• Lots of sun for most of the year - as is the case in southern Europe (Portugal and Spain), Australia, California, etc.
• Good air transparency so that no particles deflect the sun's rays (low humidity and no pollution): e.g. desert regions that are far removed from large urban centers.
• Lots of space for setting up mirrors and parabolic collectors over a wide area: 2 hectares are required to produce 1 megawatt.
• A flat area to save on the cost of pumping the heat-transfer fluid - it is cheaper to find and circulate water in a flat region than in a mountainous region.
• A nearby power grid to which the electricity produced can be transmitted.
One of the disadvantages of solar plants is that they only work during daylight. To ensure that thermodynamic solar power plants operate continuously, the hot heat-transfer fluid can be stored for use at night. Another option is to use fossil fuels (e.g. gas or coal) after the sun goes down. This solution partly reduces the environmental benefits of the solar power plant.
Total-Abengoa Solar as partners in the development of the Shams 1 solar plant. Located 120km south-west of Abu Dhabi, this plant will have 768 parabolic mirrors covering a surface area of 2.5km2 and capacity of 100 MW.
Temperatures from 500 to 800°C
There are 3 types of solar plants, according to how the sun's rays are focused
(or concentrated). Each type provides very high temperatures that increase yield
(400-800°C).
• Parabolic trough power plants
These are parallel rows of half-cylindrical parabolic mirrors which rotate around a horizontal axis to follow the course of the sun throughout the day, like sunflowers. The sun's rays are concentrated on a black horizontal tube (to increase heat absorption) inside which the
heat-transfer fluid circulates (oil, air or liquid sodium). The fluid is then directed to the central plant unit where it heats water using a heat exchanger. The water, converted into steam, drives a turbine, which in turn drives an alternator, which produces electricity. In this, the most common type of power plant, the temperature of the fluid can go up to about 400°C.
Gas can be used to produce steam after sunset. Over the course of several hours, the heat accumulated during the day can also be stored in a tank containing eutectic salts.
These have a set of ground-based swiveling mirrors that concentrate the sun's rays onto a boiler located at the top of a tower. These rays heat the heat-transfer fluid circulating inside the boiler to a very high temperature. The steam generated in this way in turn drives turbines to produce electricity.
• Photovoltaic panels
These plants comprise a set of parabolic collectors with a diameter of 10-20m (similar to radar antennae). Each collector follows the sun's movement like a sunflower and concentrates its rays to a focal point fitted with a miniature power plant. The temperature reaches 800°C and the miniature power plant produces electricity through a Stirling engine, which is driven not by fuel but by heat from an external source. The incoming heat causes the gas inside the engine to expand. It is then transferred to a cooler point where it contracts. These alternating expansion and contraction phases drive pistons, thus causing movement.
In 1970, the Odeillo furnace, near Font-Romeu (in the Pyrenees in southwestern France) was opened. It has 63 flat heliostats (mirrors that track the course of the sun) measuring 45m2 that direct beams of light to a parabolic reflector 40m high and 54m wide. In turn, the reflector sends a heat flow to a focal point 18m in front of it.
The experiments carried out at the Odeillo furnace have enabled the development both of current solar power plants and of domestic applications.
