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Direct steam generation for more efficient solar power plants
At the Almería Solar Centre in Spain, a test plant for direct steam generation has commenced operation. Instead of thermal oil, water is heated in the receiver tubes in the parabolic troughs. This allows higher temperatures and saves on plant components. This year the scientists are testing the control system in the 3-megawatt plant.
In Almería scientists from the German Aerospace Center (DLR) have begun operating a test plant with a 1,000-metre-long collector array. The steam temperature at the end of the parabolic troughs is intended to reach 500°C. That is a good 100°C more than would be possible with thermal oil, which is the conventional heat transfer medium. The scientists are hoping that the higher operating temperatures will increase the efficiency. In the parabolic trough power plant, water is evaporated directly in the receiver tubes. “The once-through concept dispenses with the need for heat exchangers and many other additional components, for example the processing plant for the oil,” says project leader Fabian Feldhoff from the DLR Institute of Solar Research in describing the advantages. “This should enable a reduction in the investment costs for solar power plants,” hopes Feldhoff. The preliminary studies for the test plant estimate potential savings of around ten per cent.
Direct steam generation: Everything in a tube
The aim of the project in Almería, Spain, is to prove that the plant can function on an industrial scale. With good irradiation in summer, the 1,000-metre-long collector string corresponds to a thermal output of three megawatts. For an actual power plant, several of these strings will be connected in parallel until the desired output is achieved.
Spanish parabolic trough power plants mostly use turbines with a 50-megawatt electrical output. Thanks to the more than 40 per cent efficiency of the steam turbines, this corresponds to a minimum thermal output of 120 megawatts. This means that at least 40 parallel strings must be installed.
The main focus of the current research project is the control system for the power plant: if the irradiation lowers due to clouds moving past, the researchers feed less water into the inlet. The mass flow rate is therefore lowered to such an extent that 500 °C is still attained at the outlet. The project is aiming to develop and demonstrate a rapid control system for changing the mass flow rate. Feldhoff explains: “The challenge with this kind of direct steam generation plant is the increased operating pressure of around 110 bar in the receiver tubes and the control of the overall process.” The water deployed in the collector string is used again and again, which means that the water consumption is low.
Excellent market opportunities for parabolic trough power plants
The researchers are testing the control system for the plant until the middle of 2014. Parabolic trough systems are the most established power plant technology in the solar thermal sector and the scientists believe that they will continue to have excellent market opportunities in the future. “Direct steam generation can help the systems make further necessary cost reductions,” estimates Feldhoff. He considers direct steam generation to be currently the cheapest solution, particularly for power generators with short-term storage that are combined with coal- and gas-fired power plants for securing electricity generation. In the short to medium term, he believes that these power plants are particularly predestined for the major market in North Africa and the Middle East. The greater the direct irradiance per year at any given location, the greater the yields produced by parabolic trough power plants. The direct irradiance in Germany, however, is too small for this type of power plant.
In the “Once-through Concept – Development and Testing” project (DUKE ), the DLR Institute of Solar Research is collaborating with Spain’s CIEMAT (Centro de Investigaciones Energéticas, Medioambientales y Tecnológicas) on the Plataforma Solar de Almería (PSA) research plant. The DUKE project is being funded by the German Federal Ministry for the Environment, Nature Conservation and Nuclear Safety.
A test plant with parabolic collectors for generating industrial process steam is described in the BINE-Projektinfo brochure “The sunny side of saturated steam” (11/2011).
Deutsches Zentrum für Luft- und Raumfahrt (DLR) - Institut für Solarforschung
Plataforma Solar de Almería (PSA)