The German Aerospace Centre (Deutsches Zentrum für Luft- und Raumfahrt, DLR) has developed a solution by which small ceramic pellets are directly heated by the solar field of a CPS power plant. They serve as a storage medium and allow for higher temperatures in the storage and in the power plant process. This and other system advantages reduce the costs of power from CSP systems. The solution is also suited to the provision of process heat.
Developers of concentrating solar thermal power plants (concentrated solar power – CSP) have long discovered ceramic as the material of choice to reach higher temperatures in the system. A receiver - the component that absorbs heat from the solar field and transmits it to the storage - made of ceramic has long been tested. This collects the concentrated sunlight and heats up the aspirated air up to 750 to 800 degrees Celsius.
These high temperatures are necessary to reduce the costs of power generation from CSP power plants, since generators that are operated with the heat from the solar field do not work efficiently at the temperatures of 400 degrees Celsius used so far. These reach their optimal operating point at 550 to 600 degrees Celsius. Additionally, the hotter the storage medium, the longer the heat can be stored at a sufficient temperature level.
Pellets are heated to 1000 degrees
Some efforts have already been made to reach these high temperatures. From thermal oils to emulsifying salts to solutions where sulphur is used as a storage medium, temperatures were getting higher and higher. Researchers of the German Aerospace Centre (DLR) have now developed a solution with which they can store solar energy from the solar field to up to 1,000 degrees Celsius. The storage medium consists of small ceramic pellets. These have a diameter of about one millimetre and are transported into a rotating drum. With centrifugal force, they are pushed to the outer layer of the drum, on which the concentrated solar radiation strikes. Once the ceramic particles have reached the target temperature, they fall from the receiver sloping downwards into a thermally insulated container.
The heat stored through the hot ceramic pellets can then be used directly for the power plant process. The advantage is that the entire system works at standard pressure, and researchers can therefore save on expensive pressure containers. In this way, heat can safely be stored for longer at a sufficiently high temperature, since, even after hours, the temperature is still high enough to operate a conventional steam turbine at an optimal operating point, despite the heat losses created during storage.
“With ceramic particles, power plant operators can work with higher process temperatures between 600 and 800 degrees Celsius and can reach a higher efficiency rate in the power plant process,” Reiner Buck, department manager for point-focused systems at the DLR Institute of Solar Research, describes the advantage of the new solution. So the motto for the development of the system was: The higher the initial temperature, the higher the usable temperature at the end of the storage phase.
The system is additionally more flexible than previous solutions. “Through the rotational speed of the drum, the retention period of the particles in the receiver and their temperature at the exit can be determined - depending on which process the thermal energy should be used for,” Reiner Buck explains. In this way, the system can directly supply the heat for production processes at the temperature directly required there between 600 and 900 degrees Celsius. The advantage: the hot ceramic pellets can be directly transported to the operation area without great effort.
Successfully tested in the laboratory
So far, DLR researchers have tested their development in a laboratory on a small scale. A high-power radiator was used as a replacement for the sun. With this, they still reached a power of ten kilowatts. The receiver has now been installed in the solar tower in the trial power plant in Jülich. The centrifugal receiver there is illuminated by the giant mirrors and thus reaches a power of 500 kilowatts.
Researchers expect the production costs of power from CSP power plants to sink even further thanks to their new solution. Along with increased efficiency due to the higher temperatures, they note that low-cost components and storage media are used here. In addition to this, they save on the heat exchanger at the beginning of the entire process, through which the receiver transmits the heat to the storage medium, because the ceramic pellets, i.e. the storage medium, are heated directly in the receiver.