Emissivity at high temperature of Ni-based superalloys for the design of solar receivers for future tower power plants

In the framework of the H2020 European project POLYPHEM, whose main objective is to improve the flexibility and performance of small-scale solar power plants, studies have been conducted on various nickel-based alloys for the design of the solar receiver. Inconel 600 alloy is used as a reference material in the advanced manufacturing of high temperature solar receivers but other oxidation resistant alloys have been selected such as Inconel 617, Haynes 230, Haynes HR120 and Hastelloy X and the best candidate will be used for making the prototype. Thus, two essential quantities, namely the solar absorptivity alpha - the higher the better - and the total hemispherical emissivity e - the lower the better - were measured in high vacuum and under atmospheric air up to 1400 K in the MEDIASE reactor installed at the focus of the 1000 kW solar furnace. The objective is to reach the best solar absorptivity and the alpha/e ratio the highest as possible. The emissivity measurement was made by a direct method requiring the measurement of the true temperature of the material using a bi-chromatic pyroreflectometer and that of its radiance by spectro-radiometry. Total hemispherical emissivity data are presented at the same time as microstructural (SEM, XRD) and topographic (3D profilometry) characterizations allowing the interpretation of the emissivity evolution with temperature and the growth of the oxide layer. Finally, the alloy Haynes 230 was chosen to build the solar receiver.