Thermal efficiency and endurance enhancement of tubular solar receivers using functionally graded materials

Combinations of pure metals –Alloy 316H, Haynes 230, Inconel 625, Inconel 740H and Incoloy 800H– are analysed as functionally graded materials (FGM) to test their potential suitability for the solar central receiver tubes manufacturing, aiming to reduce their elastic stresses under operation to extend their lifetime, and to increase the solar power tower thermal efficiency. The proposed FGMs rely on a circumferential variation of their properties, with one material at the tube front side and the other at the rear one. The analytical investigation of the complete receiver shows lower elastic stresses at the tubes middle length when two materials with very disparate properties are combined since the temperature profile presents a peak at the front and gradually decreases towards the rear side. The mechanical properties are homogenized if the front material has a greater Young's modulus at a certain temperature and lower thermal expansion coefficient than the rear-side one. Yet, a compromise must be reached since the thermal gradients at the tube ends are almost negligible, and so greater elastic stresses are observed there when two very different materials are combined. Various FGMs tested manage to reduce the overall elastic stresses, with the Inconel 625-Incoloy 800H FGM showing the most promising results and a potential lifetime extension. Moreover, such stress reduction shows room for receiver thermal efficiency and heliostat field optical efficiency improvement: around 0.13% each in the case analysed since the proposed FGM can withstand the concentrated heat flux from more equatorial aiming strategies than the pure metals.