Diffractive Gratings to Improve TiAlN Based Spectrally Selective Solar Absorbers

Based on an optimized TiAlN multilayer structure achieving high absorption and low thermal emission at temperatures above 500 degrees C for CSP receivers, the authors demonstrate that - sub-wavelength period gratings associated with the multilayer can further increase the absorption and thus the efficiency of the CSP system. The C-method (Chandezon Method) is used to theoretically optimize the grating profiles. An experimental demonstration of the combination of lithographic grating fabrication and absorptive layer deposition using Inconel 625 as substrates is also presented. Experimental results show that the microtexturing increases the absorption from 91% to 95% while the emissivity at 550 degrees C is kept at an acceptable level, increasing from 32% to 38%, representing an enhancement of almost 3% of the photothermal efficiency in comparison to non-structured coatings. Textured absorbers were annealed in air at 500 degrees C and demonstrated high thermal stability of their optical properties during more than 336h. Textured absorber would also be greatly efficient for CSP plants working at lower temperature (250-300 degrees C), such as linear Fresnel CSP for process heat production, since optical properties and thermal stability will be excellent at these temperatures.