© German Aerospace Center/Ernsting

The operating temperatures used in solar-thermal power plants span an extremely broad range, from over 400 degrees Celsius by day to below-zero temperatures at night. At the Plataforma Solar de Almería’s PROMETEO testing facility in Andalusia, southern Spain, parabolic mirrors are used to direct sunlight to tubes filled with a heat transfer fluid. The solar energy is then converted into electrical current based on the use of heat exchangers and steam turbines. The higher the temperature of the heat transfer medium, the more efficiently a solar thermal power plant can operate.

Temperature raised to 425 degrees Celsius

Within the SITEF project, researchers from the Institute for Solar Research, which belongs to the German Aerospace Center, are now testing the viability of using the new silicone oil in solar thermal power plants. The oil, known as HELISOL®, was developed by Wacker Chemie AG. Its ability to operate at temperatures of up to 400 degrees Celsius has already been proven. At the installation in Andalusia, the test temperature is now being raised to 425 degrees Celsius, with test operations at this higher temperature to last for several months.

The heat transfer mediums that are currently used at parabolic trough power plants consist of a mixture of biphenyl and diphenyl oxide (BP/DPO). This organic oil mixture can be used at temperatures of up to 400 degrees Celsius, but solidifies when it falls below twelve degrees Celsius. Such a low cooling tolerance temperature means complex heating systems must be used which are expensive to operate.

No hazardous substances like benzol produced

Under laboratory conditions, the new silicone oil has even been able to withstand being heated to a temperature of 425 degrees Celsius, without decomposing. A heat tolerance of this magnitude could increase the efficiency of the power station and increase the current yield. Even in the lower temperature range, the silicone oil is more flexible than the BP/DPO mixture. Its solidification point is about minus 55 degrees Celsius, which would mean that no heating systems would be required to prevent the heat transfer fluid from solidifying. And there are also further advantages. The silicone oil being tested ages more slowly and forms much less hydrogen than conventional heat transfer media. No hazardous substances like benzol are produced in the process, helping to ensure that the power plants can be operated sustainably and even more safely. This makes it possible for solar thermal power plants to be used at other sites that hitherto been unsuitable.

German companies global leaders in solar thermal power plant technology

The Federal Ministry for Economic Affairs and Energy is providing funding of around €850,000 for this research project. Due to a lack of direct sunlight, the German climate is not suited to the commercial operation of solar thermal power plants. However, German firms and research establishments are global leaders in this field of technology, meaning there a high level of potential for exports. A large proportion of solar thermal power plants around the world use key components developed in Germany.