Cavity insulation thermally insulates receivers that are located in a cavity at the top of solar towers. The solar radiation penetrates through a small opening in the cavity to the receiver. As a result, the radiation is distributed more uniformly in the cavity and the losses due to thermal radiation and convection are reduced.

The insulation around solar tower receivers must be able to withstand high loads. Temperatures above 1,000 °C can occur. In addition, wind, rain and sometimes strong temperature fluctuations can also occur. Previous types of insulation have often revealed damage after only short periods of use, such as gaps between individual insulation segments or lower stability. If the damage affects safety-relevant areas such as the receiver suspension or pressure lines, this can lead to the complete system failing.

Four segments with new material combinations

The new insulation consists of several segments that are assembled together during its installation in the solar tower. Johannes Hertel, head of the ISOL research project at the German Aerospace Center (DLR): “The segmental structure is particularly necessary for larger amounts of insulation, as this is beneficial both in manufacturing terms as well as during transport and installation. We tested the handling and assembly of this segmented solution in trials. It was important, for example, to choose a size that would fit in the solar tower's lift.”

The new insulation is built up in several layers. The outer, irradiated plane consists of a fabric or ceramic sheet based on ceramic fibres. The inner insulation layers are composed of different combinations of high-temperature wool, for example based on polycrystalline fibres. The insulating mats are arranged transversely to the cavity wall. Since the front layers are exposed to higher temperatures, they are made of a higher quality material than the rear ones. These consist of a microporous material that has a much higher insulation effect but is not resistant to high temperatures.

100 hours at over 1,000 °C in the solar tower

The scientists tested the cavity under solar loading in the experimental tower power plant at the Plataforma Solar de Almería in Spain: 47 hours at 1,000 to 1,100 °C and 52 hours at 1,100 to 1,200 °C. Here, for example, thermocouples were distributed at different material depths on the surface and a radiometric determination made of the incident flux density. Further investigations were conducted with the infra-red camera. Researchers were therefore able to record for the first time the actual load on a solar-irradiated cavity at a high resolution. “There was no damage to the test cavity during the trial period. And there was no material failure even in heavily stressed areas. The planned joints and overlaps have successfully compensated for shrinking material surfaces and different thermal expansions,” says Hertel in summing up the test results.

The new insulation is now available from Eugen Arnold GmbH.

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