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.
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