  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{\deg}C) heating, ultra violet light
exposure and thermal cyclic loading. Building upon it, we have been able to
successfully engineer an efficient volumetric absorption solar thermal platform
that employs the as-prepared nanofluids and achieves higher steady state
temperatures (approximately 5% higher) relative to the conventional surface
absorption based solar thermal system under the sun. The developed volumetric
absorption solar thermal platform could prove to be significant step in the
evolution of efficient solar thermal systems which could potentially be
deployed for host of applications ranging from solar driven heating,
air-conditioning, and desalination units to solar energy electricity generation
systems.
