Photothermal properties and performance of hybrid carbon-paraffin/water emulsions

Solar thermal energy has attracted renewed research interest for its excellent energy efficiency among different types of solar technologies. However, the limiting factor of them all is the intermittent nature of solar radiation. To overcome this drawback, it is possible to store absorbed light in form of heat using emulsions with phase-change materials (PCMs) dispersed in solar nanofluids by combining the advantage of solar thermal storage characteristics of PCMs and the higher light absorption capacity of solar nanofluids. In this study, paraffin/water and hybrid carbon-paraffin/water emulsions with 5 wt% paraffin wax RT44HC as PCM are proposed. Low-cost emulsions with proven colloidal stability were successfully produced, with and without small amounts of oxidized carbon black nanoparticles (0.01 wt%) dispersed in the emulsion. Thermal energy storage capacity improved up to 16 %, while maintaining good thermal conductivity properties. Optical properties were evaluated by means of a spectrophotometer with an integrating sphere, including both spectral absorptance and reflectance of the samples. Two experimental setups were developed with concentrated light, to evaluate the photothermal conversion efficiency under simulated and natural sunlight. The improvement of the light to heat conversion properties provided by the oxidized carbon black nanoparticles was confirmed, with PTE values of 1.5- and 2.7-fold better than the base fluid (water) using a solar simulator and natural sunlight, respectively.