A nanofluid phase change material (PCM) is prepared by adding a small amount of CuO nanopowders to paraffin, for use in solar thermal conversion, thermal storage, and thermoelectric applications. Results show that adding CuO nanopowders to paraffin can greatly improve the solar thermal conversion capacity by enhancing the light absorption ability of PCMs. The steady temperature increased with increasing mass fraction of CuO NPs at low mass concentrations (0-0.1%). The largest increase was about 2.3 times that of the pure paraffin with comparable latent heat. In the solar thermoelectricity experiments, the open-circuit voltage improved (1.35 V) with the CuO/Paraffin composite (f(m) = 0.1%) under the same conditions, and was almost 1.8 times that of the pure paraffin. More importantly, a unique feature of the CuO/Paraffin composite PCM is the extension of thermal release, which enables the continuation of the voltage output when the solar simulator is switched off. The introduction of a very low mass concentration of CuO nanopowders can endow the composite PCMs with strong solar absorption ability, and contribute to realizing efficient solar thermal and solar thermoelectric energy conversion and storage. This provides a new prospect for solar radiation usage efficiency and direct solar energy conversion and utilization. (C) 2018 Elsevier Ltd. All rights reserved.