5. Receiver

Receivers are the parts of a PTC where the solar radiation is concentrated and collected. PTC receivers are tubes of various diameters located in the focal line of the concentrator and can be distinguished into two parts: the absorber, i.e. the metallic tube in which the heat transfer fluid (HTF) flows, and the cover, an external tube generally made of glass, adopted to reduce convective and radiative thermal losses. The absorber collects the solar radiation reflected and focused by the concentrator, and it transfers the absorbed heat to the HTF by conduction and then by convection. Due to their function, receivers should have high thermal conductivity and present high absorptance to the solar spectrum. The latter condition is generally realized by painting the outer surface of the receiver with special black coatings. This chapter presents an overview of the receivers used in PTC prototypes, providing some characteristics about the adopted materials. The performance of the prototypes are also discussed. Finally, PTC protoypes using nanofluids, novel promising HTFs, are presented.