Fabrication of high efficacy selective solar absobers.
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High efficiency tandem selective solar absorber materials of carbon in nickel oxide (C-NiO) composite were fabricated on an aluminium substrate using a simple and cost effective sol-gel process. The process involved preparation of carbon and nickel oxide precursor sols which were homogeneously mixed to form a final C-NiO precursor sol. The carbon precursor sol was prepared by dissolving sucrose (SUC) in 8 ml of distilled water. The NiO precursor sol was prepared by dissolving 7.5 g nickel acetate in 50 ml ethanol, then adding 6.3 g diethanol amine (DEA) to stabilise the solution followed by addition of a structure directing template of polyethylene glycol (PEG). The final C-NiO precursor sol was spin coated on pre-cleaned aluminium substrate to form thin films which were then heat treated in nitrogen ambient inside a tube furnace. The final heat treatment temperature of the sols was determined by thermal studies using thermo gravimetric analytic (TGA) and differential scanning calorimetric (DSC) techniques. TGA and DSC studies of the final precursor sol showed that the weight loss of the precursors stabilised at around 450 °C. The impact of the sol-gel process parameters namely heat treatment temperature, PEG content, SUC content as well as spin coating speed on the optical properties i.e. solar absorptance (αsol) and thermal emittance (εtherm) was investigated. It was found that the optical properties as well as photo-thermal conversion efficiency, η = αsol - εtherm, improved with an increase in heat treatment temperature in the range studied (300-550 °C). This is in good agreement with the results obtained from thermo-gravimetric analysis which showed the weight loss of the precursor to stabilise around a temperature of 450 °C. Results obtained from the Raman studies showed a progressive increase in the graphitic domain in C-NiO samples with an increase in temperature. Heat treatment temperatures above 450 °C gave the best optical properties. Scanning electron microscopy (SEM) results showed that samples that did not have PEG in the precursor sol were compact and an addition of PEG in the precursor sol caused an increase in the size and density of pores in the films produced which affected the optical properties. As a result, the optical properties increased with an increase in PEG content from 0 g to 2 g then decreased with further increase in PEG content. It was found that addition of SUC of up to 8 g in the sol did not change the optical properties of the fabricated materials because SUC contributed little carbon to the final composite material. Further increase in SUC content resulted in materials with poor photo-thermal conversion efficiency. An increase in spin coating speed did not change the absorptance of the materials but it improved their thermal emittance. The best spin coating speed was found to be 7000 RPM. A solar absorptance of 0.81 and thermal emittance of 0.06 have been achieved for an optimum sample in this study yielding a photo-thermal conversion efficiency of 0.75. The optimum sample fabricated in this study showed superior optical properties compared to the widely used commercial solar absorber paint. This suggests that the C-NiO composite material has the potential for possible use as a selective solar absorber in a solar collector.