Solar Thermal Electricity

Solar power towers use one central receiver mounted on top of a tower which is surrounded by a field of heliostats - concentrating mirrors which follow the sun. Reflected light is focused onto the receiver and absorbed by the heat transfer medium, which could be sodium, water, molten salt or air. Temperatures of 500-1000 C can be achieved and proposed systems also incorporate energy storage using molten salts. System sizes up to 200MW are possible.

Since water at air temperature will be about 25C, there is a large temperature differential between the molten salt bucket and the water. Any time two fluids of different temperature are coupled, energy can be extracted. The larger the temperature difference, the higher the efficiency. This is how Ocean Thermal Electric Conversion works, which we will be discussing.

So, basically, the hot fluid is the source of the heat input to the steam turbine generator.

In addition, a solar concentrator system can be used in a hybrid capacity with an existing fossil fuel plant. In this capacity, the solar collector is used as an additional energy source for heating the cold air to allow it to mix with the natural gas in the natural gas turbine. This means that less natural gas is needed for the combuster and there is an overall savings of fuel. This is a particularly viable option when the price of natural gas is very high.








The overall efficiency of this system depends upon temperature of the molten salt fluid and the concentration ratio. With suitably high temperatures, relatively high efficiency can result. The dashed line called Carnot represents the maximum theoretical efficiency based on thermodynamics.

There are several advantages to solar thermal electricity generation:

(1) electricity and hot water can be provided at the same time,

(2) plants can be scaled to match the application (up to a point, see below),

(3) utilities can match power production to daytime electricity demand peaks,

(4) there are few pollutants associated with solar thermal power production, and

(5) permits can be issued and plants built in as little as 18 months, far faster than conventional fossil fuel or nuclear plants.

Note, however, that solar power towers are not infinitely scalable. For capacities larger than 200 MW, the heliostats have to be located too far away from the tower so that optical/ refraction losses start to become important. In other words, it becomes too difficult to effectively focus the light from the sun. However, one could easily make an array of 200 MW units of solar power towers all clustered together in some desert location.