Direct absorption receivers for high temperatures

Kurzfassung

Three concepts of direct absorption receivers for concentrating solar power (CSP) systems are compared. They are characterized by advantages like simple design, high working temperatures and good storage possibilities leading to a potential reduction of the levelized electricity costs. The design of the first concept, the liquid film receiver, is based on a face-down cylindrical barrel, whose inner surface is cooled by a directly irradiated molten salt film. Detailed investigations regarding film stability and system management strategies reveal increased receiver efficiency by implementing a slow rotation and inclined receiver walls. The second concept resembles the first one, but instead of molten salt small ceramic particles are used as heat transfer medium. The solar radiation is directly absorbed by a falling particle curtain whereas appropriate recirculation strategies of the particles can lead to high receiver efficiencies for all load conditions. While the above described systems are suitable for the 50 to 400 MWth power range the third concept – also a particle receiver – can be applied in decentralized small-scale CSP plants ranging from 100 kWth to 1 MWth as well as in larger systems with up to 200 MWth. Due to centrifugal acceleration the particles are forced against the cylindrical receiver wall where they form a thin layer which is directly heated up by incoming radiation. The particle retention time and with it the mass flow can be adjusted to all load conditions by regulating the rotation speed. Heliostat field layout calculations for different design power levels were carried out comparing the annual performance and levelized cost of heat for a face-down receiver and a receiver with an optimized inclination angle. Only small differences between both concepts could be noticed. However, simplified assumptions regarding thermal receiver losses were made neglecting e.g. convection losses. Thus, in order to give reliable information about the thermal efficiency of the introduced concepts, computational models considering the main heat loss mechanisms are developed. The models are accompanied by experimental validation and support the numerical findings.