Felix Göhring

Jülich Solar Power Tower—Experimental Evaluation of the Storage Subsystem and Performance Calculation

Background. Storage technology based on solid media heated in direct contact – so called regenerators – are well suited to promote the market introduction of solar central receiver plants with air receivers. However, starting from existing technologies, several design issues need to be addressed. Method of Approach. A test campaign was performed at the Solar Power Tower Julich, an experimental central receiver plant, to experimentally verify the functionality and to quantify the performance of the plant’s storage subsystem. To this end, a gas burner used during commissioning of the plant, was reactivated and used to run a series of operation sequences. Computer simulations have been set up and applied to retrace the storage behaviour to confirm the validity of the underlying models and to gain further insight into the relevant phenomena. Results. The test results confirm the full functionality of the storage subsystem, the ability to perform cycling at high discharge heat rates and relatively low heat losses, supporting the view that the technology represents a promising basis for up-scaled implementations. Conclusions. Measurement data and simulation results are in good agreement, confirming the maturity of existing design tools.

Techniques to Measure Solar Flux Density Distribution on Large-Scale Receivers

Flux density measurement applied to central receiver ystems delivers the spatial distribution of the concentrated solar radiation on the receiver aperture, measures receiver input power, and monitors and might control heliostat aimpoints. Commercial solar tower plants have much larger aperture surfaces than the receiver prototypes tested in earlier research and development (R&D) projects. Existing methods to measure the solar flux density in the receiver aperture face new challenges regarding the receiver size. Also, the requirements regarding costs, accuracy, spatial resolution, and measuring speed are different. This paper summarizes existent concepts, presents recent research results for techniques that can be applied to large-scale receivers and assesses them against a catalog of requirements. Direct and indirect moving bar techniques offer high measurement accuracy, but also have the disadvantage of large moving parts on a solar tower. In the case of external receivers, measuring directly on receiver surfaces avoids moving parts and allows continuous measurement but may be not as precise. This promising technique requires proper scientific evaluation due to specific reflectance properties of current receiver materials. Measurement-supported simulation techniques can also be applied to cavity receivers without installing moving parts. They have reasonable uncertainties under ideal conditions and require comparatively low effort.

Jülich Solar Power Tower – System Behavior During Downtime

At the Julich Solar Power Tower, two new shut-off dampers have been installed in the hot air piping system in order to reduce thermal losses during downtime of the plant. The thermodynamic behavior of the thermal energy storage and the steam boiler has been investigated with respect to heat losses after shut-down of the plant. Also, the change of temperature and the pressure drop at both shut-off dampers during downtime has been analyzed. Results show that for the storage, a reduction of thermal losses can be achieved by closing the damper overnight. Regarding the steam boiler, no improvements on heat losses were observed. If only two of the four storage chambers have been charged during operation, heat transfer between the chambers is observed after shut-down. It is concluded that natural convection is not the main source of thermal overnight losses, however, it can be lowered thanks to the dampers. Heat conduction and convectional heat transfer at the surfaces also contribute to the decrease of thermal energy, which can be lowered by thicker insulation layers in the future.