Oxidized copper for photothermal conversion of solar energy

Abstract

A high absorptance (>0.90) is an essential factor in the efficient use of a .solar collector. However, equally important is the need to keep the emittance as low as possible (<0.20). Both the.absoptance and emittance depend on the optical properties of the coating and the substrate. The solar optical properties of copper oxide coatings on copper substrate have been investigated. The coatings were formed by three different methods, namely, dry thermal oxidation, sodium chlorite oxidation and the Ebanol C treatment. Temperature and time have been the most fundamental parameters. Specular reflectance* profiles are presented in the wavelength range 0.35 - 15 p.m and the total reflectance profiles in the interval 0.35 - 2.0 jam for some representative samples, of those that have shown a remarkably low reflectance in the wavelength region 0.3 - 2.0 fim.Scanning electron microscope analysis of the sample surfaces has provided evidence of how changes in the topo-graphical structure of the coatings could alter their reflect¬ance characteristics and hence absoptance and emittance. The surface structure is in turn related to the treatment temperature and time. X-ray diffraction analysis has also revealed some interesting results. Whereas the thermal and sodium chlorite treatments tended to give CU2O coating at least up to the temperatures and treatment durations inve¬stigated, the Ebanol C method tended to form both oxide types with the CuO phase being the most dominant. Evaluation of the reflectance data shows that it is possible by proper choice of treatment parameters to form coatings with Q - 0.90, e < 0.20, Q = 0.96, e < 0.20 S JL ™ S J- ™" and Q =0.96, eT= 0.10 for the thermal, Ebanol C and o J-chlorite treatments respectively.

All heated bodies emit thermal infrared radiation whose wavelengths and intensities depend on the absolute temperature of the body and the emittance of the surface, Reflectance data is often used to evaluate selective coatings because it is easy to measure. However the values of emittance derived from reflectance data would not nece¬ssarily represent the actual values .at the operating temperature of the collector. It is therefore necessary w to know the emittance of coatings is related to the increase in the temperature. This is because the collector must not be allowed to operate at temperatures that might result in the degradation of its optically- and thermally-functional properties. A vacuum calorimeter for measurement of the emittance of the coatings has been designed, constructed tested and calibrated. The emittance, as a function o£ temperature, of some of the representative samples have been measured and the result are presented.