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Posted: April 25, 2010
New Solar Receivers Boost Efficiency
(Nanowerk News) Siemens has optimized the receiver tubes for solar thermal facilities. The latest model, the UVAC 2010, is currently the most efficient solution on the market and improves the efficiency of a 50-megawatt (MW) facility by five percent. The tubes run along the longitudinal axis of parabolic troughs, on which the sunlight is focused. Synthetic thermo oil is used as a heat transfer medium that absorbs the solar energy and heats up water. A heat exchanger converts the heated water into steam, which is used to drive a turbine. The efficiency of a solar power plant largely depends on how well the receivers can absorb the sun’s rays and prevent the stored heat from being lost.
The principle of concentrated solar power (CSP) plants is simple: curved sun-tracking mirrors capture sunlight and concentrate it on a solar receiver tube. A transfer medium, mainly oil, flows through this tube, and is heated by the concentrated solar radiation. This medium transfers its heat to water, generating steam. The steam drives a turbine, which then drives a generator. Finally, the generator produces electricity.While the technical principles of CSP were established a long time ago, the current challenge is enhancing plant efficiency. Experts expect that CSP plants could be economically competitive within 15 years, allowing the gradual phase-out of subsidies for electricity generatedin these plants.
According to the International Energy Agency (IEA), solar thermal facilities only generated one gigawatt (GW) of electricity worldwide in 2009. However, additional power plants with a total capacity of 15 GW are either being developed or under construction. The latest information from the IEA indicates that solar thermal energy could cover 12 percent of the world’s electricity needs in 2050. State-of-the-art technology for solar thermal plants is based on long parabolic troughs, in the center of which receiver tubes absorb the solar energy. Such facilities are currently generating tens of megawatts of electricity and have an overall efficiency of slightly more than 25 percent on average.
Special selective coatings play a key role in the new solar receiver from Siemens Renewable Energy. These coatings can absorb over 96 percent of the solar energy and also reduce heat loss to less than nine percent, depending on the temperature within the tube. The heat transfer medium heats up to around 390 degrees Celsius as it flows through a stainless steel tube that is housed in a glass cylinder. In the gap between the tube and the cylinder is a vacuum to provide additional thermal insulation. The new components can increase the output of a 50 MW facility by about 6,500 MWh per year, compared to conventional solutions. More than 220,000 UVAC receivers are currently in use worldwide.
At the end of March, Siemens obtained another order for solar receivers from the solar thermal power plant at Les Borges Blanques in northern Spain. The technology was developed by the successful solar thermal specialist Solel Solar Systems Ltd., which was acquired by Siemens in fall 2009. As a result of the acquisition, Siemens Energy’s portfolio now contains all of the key components for parabolic trough power plants (solar fields and receivers, as well as the power-station unit containing the steam turbine). As a result, Siemens Energy will be able to further optimize the overall efficiency of the facilities.