Nanofluids are promising heat transfer fluids for a wide range of energy management applications. Probe ultrasonication-mediated preparation of copper - propylene glycol nanofluid is accomplished through dispersion of biosurfactant-functionalized copper nanoparticles in propylene glycol. Copper - propylene glycol nanofluids are colloidally stable; retain their thermal conductivity enhancement despite repeated heating-cooling cycles and storage for more than 15 days. These nanofluids exhibit temperature-independent thermal conductivity enhancement, with about similar to 11% enhancement for 1 vol % nanofluid, attributable to Brownian motion and interfacial layering. The viscosity of nanofluids is lower than that of base fluid (propylene glycol) due to interactions between biosurfactant and propylene glycol. Our data clearly demonstrate that the use of 1 vol % copper - propylene glycol nanofluid as coolant can lead to 13.2% improvement in the rate of energy recovery from a constant-temperature hot bath when the heat transfer resistance in the hot fluid side is low. The results of the present study have implications for energy management in solar thermal systems. (C) 2016 Elsevier Ltd. All rights reserved.