Design, Analysis and On-sun Testing of Efficient Nanofluid Based Volumetric Absorption Solar Thermal Systems

Abstract

Majority of the installed solar energy conversion platforms either convert the incident solar radiant energy into electricity (solar-photovoltaic) or into thermal energy (solar-thermal). At present, owing to steep decline in the cost of photovoltaic cells; photovoltaic technology has more presence opposed to solar thermal technologies. In relation to meeting heating and cooling energy demand (which accounts for nearly 50% of the total energy demand), solar thermal technologies potentially promise much greater dividends. Paradoxically, the current worldwide deployment of solar-thermal platforms is meager; this may be ascribed to their relatively low thermal efficiencies and high capital investments. Therefore, there is an urgent need to significantly improve the existing solar thermal systems. To this end, nanofluid based volumetrically absorbing systems have emerged as one of the potent candidates that promise high energy conversion efficiencies and lower material requirements. However, these promising novel systems have not been able to outperform the incumbent solar thermal platforms under the sun owing to instability of nanofluids in real-world service conditions - nanoparticles tend to agglomerate and hence settle down. In order to subjugate the stability barrier, and to operate the volumetric receiver in real world applications; we report a low cost and scalable method to synthesize solar selective nanofluids from 'used engine oil'. The as-prepared nanofluids exhibit excellent long-term stability and photo-thermal conversion efficiency. Moreover, these were found to retain their stability and functional characteristics even after extended periods of high temperature (300°C) heating, ultra violet light exposure and thermal cyclic loading. Building upon it, a nanofluid based volumetrically absorbing solar receiver having reflecting inner surfaces has been tested under outdoor conditions. Results show that steady-state thermal efficiency peaks at an optimum nanoparticles volume fraction (ηth = 59 ± 5.5% at fv = 1%). Furthermore, the as-prepared nanofluid shows excellent stability i.e. it retains its optical characteristics and particle size distribution even after undergoing pumping and thermal cycles and moving in flow loops (circulation through pipes/valves) during on-sun testing. Moreover, the as-prepared nanofluid has negligible impact on the surface and optical properties of solar receiver constituent materials. Furthermore, the present work investigates efficacy of ZnO based transparent heat mirrors as thermal loss mitigators in 'direct volumetrically absorbing' solar thermal platforms. Comprehensive experimental and theoretical modeling frameworks have been developed to understand and quantify the heat loss mechanisms. Detailed analysis reveals that performance characteristics are strong functions of the 'side' of the glass that has been coated (i.e. whether 'receiver facing' (RF) or 'sky facing' (SF) sides of the cover has been coated). Results show that the employing ZnO based heat mirror as a cover significantly reduces the thermal losses relative to uncoated glass cover (25.12% and 21.43% reduction for RF and SF side coated heat mirrors respectively). Moreover, fundamental performance limits of ideal heat mirrors have also been determined for both RF and SF side coated cover configurations. Relative to the uncoated glass covers, ideal heat mirror covers (viz., RF and SF side coated) promise 50.03% and 38.23% thermal loss reduction respectively (@ 400˚C receiver surface temperature and 1.5μm cut-off wavelength). Overall, the present work represents a significant step in improving the existing volumetric absorpion based solar thermal systems; particularly aiming at intermediate temperature applications (viz. industrial and domestical heating/cooling).