Concentrating Solar Power Technologies 1: Introduction to Alternatives

There is a lot of work being done on many fronts to improve the conversion efficiency for solar energy to electricity and to lower the cost of such solar power plants. One area of effort has been development and improvement of concentrated solar power technologies. Four such technologies that will be covered in this article are: heliostat solar power tower, parabolic dish- stirling engine, parabolic trough, and linear fresnel solar energy to electricity systems.

 A Parabolic trough solar power system uses single axis, tracking, parabolic, trough shaped reflectors that concentrate direct solar radiation onto a receiver tube located along the focal line of each reflector. The diagram at the left shows the general nature of the reflectors and the receiver/absorber tubes. The fluid heated in the receiver tube (usually an oil) is used to generate steam, which drives a Rankine cycle steam power plant.

The parabolic trough solar plant is the most commercially developed of the four technologies. Parabolic trough solar electric systems have been operating at a capacity of 350 MW for over 15 years in the Mojave Desert of the U.S.

The heliostat powered system uses a large array of solar tracking heliostat mirrors that reflect incoming solar rays to a central receiver at the  top of a tall tower. The picture at the right shows a 10 MW demonstration solar plant near Barstow, CA that was operated as Solar One from 1982 to 1988, and as Solar Two from 1996 to 1999. This demonstration solar plant used more than 1800 heliostat mirrors. Solar One used water/steam as the heat transfer fluid that was heated in the receiver and used to generate solar electricity. Solar Two used molten salt, heated to over 1000^oF in the receiver at the top of the tower. The hot molten salt was used to produce high pressure steam to produce electricity in a Rankine cycle, and also could be stored during the daytime to allow electricity generation at night. A 20 MW heliostat powered solar tower power plant started operation in Seville, Spain, in mid 2009.

The heliostat powered system achieves a higher operating temperature than either the parabolic trough or linear fresnal systems, and hence has a higher potential efficiency. Its operating temperature is not as high as that of the parabolic dish - stirling engine system.

 The parabolic dish - stirling engine system uses a two axis tracking parabolic dish reflector that focuses the  solar energy on a stirling cycle engine/generator that generates electricity directly without production of steam. Each parabolic dish has its own individual stirling engine and generator as shown in the image of a single parabolic dish - stirling engine unit at the left. The image at the right shows a large array of these units.

The parabolic dish - stirling engine system has the highest operating temperature of the four concentrating solar power technologies that we're discussing. This gives this option the highest potential efficiency for conversion of solar energy to electricity (~30%). The typical electricity production rate for a single unit ranges from 1 to 25 kW.

 A linear fresnel concentrating solar power system uses single axis tracking reflectors as the parabolic trough  system does, but it uses less expensive flat reflectors. Also, it uses a single receiver above the array of linear fresnel reflectors, rather than individual tubes as used with parabolic trough reflectors.

Although the linear fresnel system may produce solar electricity more efficiently and at less expense than the parabolic trough system in the long run, it is not as far along toward commercial availability now.

Reference:

National Renewable Energy Laboratory (NREL), Concentrating Solar Power Projects

Bengtson, H. H., Concentrating Solar Power and Water Use, an online, continuing education course for Professional Engineers

Bengtson, H. H., Solar Energy Fundamentals, an online, continuing education course for Professional Engineers

Images:

Parabolic Trough Reflectors: National Renewable Energy Laboratory (NREL)

Solar Two Solar Tower Demonstration Plant: Alternative Energy Information

Parbolic Dish/Sterling Engine Unit and Array: ecowanderer

Linear Fresnel Reflector and Receiver Tube: U.S. Department of Energy, Energy Efficiency and Renewable Energy

Linear Fresnel Reflector Solar Power Plant: Clean Energy Wonk