SIEMENS

The basic principle underlying solar-thermal electricity generation (concentrated solar power — CSP) is simple: Energy from the sun heats water, either directly or indirectly. The water vaporizes, and the resulting steam drives a turbine whose motion is converted into electricity in a generator. The large turbines used in today’s coal-fired power plants operate at over 600 degrees Celsius and at pressures of up to 285 bar, thereby enabling an efficiency as high as 46 percent. CSP plants have much lower steam parameters and outputs, which is why smaller turbines, like the Siemens SST-700, are used at such facilities. In addition, many CSP power plants (especially those not equipped with heat storage) need to be started up very quickly at sunrise, which in turn requires highly flexible turbines. There’s also another important difference between CSP units and coal power plants: Power generated at the former is completely CO2 free. All CSP plants concentrate solar energy using mirrors distributed across a small area in order to generate high temperatures. The most widely used technology today employs half-open parabolic mirrors, with a receiver tube mounted along the focal line (top). A liquid flows through this tube as a heat transfer agent; a special synthetic oil is the most commonly used substance today. The oil is heated to approximately 370 degrees Celsius, after which it transfers its heat via a heat exchanger to water, which drives a turbine in the form of steam. Alternatively, special salts can be used instead of thermal oils. These salts can be heated up to 550 degrees, thereby increasing the efficiency of the plant. Some companies are now also testing direct steam generation systems in which water is used as the heat transfer agent in the receivers and is sent on to the turbine as hot steam in a closed loop. As a result, a heat exchanger is no longer required. Many solar-thermal plants are also equipped with heat storage so that they can produce electricity at night as well. Here, steam is either stored directly in heat-insulated pressure containers or the heat from the steam is transferred to an additional storage medium — usually in the form of the special salts that are also used in the receiver tubes. Utilizing salt as both the transfer agent and storage medium eliminates the need for a heat exchanger, which lowers both investment and operating costs. CSP power plants can also be built as central receiver systems that use flat mirrors to reflect sunlight onto a small area on the top of a centrally located tower (bottom) that is often taller than 100 meters. This approach enables the highest possible temperatures to be achieved (up to 850 degrees Celsius). However, the farther away the mirrors are from the tower, the lower the efficiency, which is why such plants must be kept small. A cost-saving alternative is offered by Fresnel technology. Here, long strips of flat mirrors (which are cheaper to produce than parabolic troughs) reflect light onto a receiver tube suspended above them (middle). However, the low initial investment cost for Fresnel power plants comes at the price of lower efficiency. Experts believe that the market for solar-thermal power plants will post double-digit annual growth between now and 2015. A number of competing technologies will probably continue to exist side by side as they undergo further development. Andreas Kleinschmidt