Microstructural optimization and extended durability studies of low-cost rough graphite- aluminium solar absorber surfaces

Mechanically manufactured low-cost selective inhomogeneous rough graphite–aluminium (C/Al2O3/Al) solar absorber surfaces provide, at the current state of development, solar absorptance α=0.90 and thermal emittance e=0.22. Based on a literature review, significantly lower emittance values could theoretically be achieved by utilization of surface plasmons in rough surfaces with an optimal graphite layer thickness, groove depth and sinusoidal surface profile periodicity. It is possible that an arbitrary form of roughening could produce equally good optical properties. In order to achieve lower emittance manufacturing parameters, the composition of silicon carbide grinding pad and the corresponding grinding pattern are required to be enhanced. The commercially available grinding pads used so far have not yielded optimal results. An antireflection coating could improve α to >0.90. Absorber samples were subjected to 383 days of temperature and irradiance cycling. In total, the samples were exposed to ultraviolet (UV) irradiation equivalent to 5–15 years of normal outdoor use. The results show that the samples are not sensitive to natural levels of UV irradiation or cycling-induced degradation of optical properties. A clear increment in absorptance was observed after the first 50 days of cycling. The elevated temperature of 130 °C is the probable cause for the increase. Reference samples indicated similar aging behaviour both after 4 years of natural exposure and after relatively short constant temperature tests at 120 and 180 °C.