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Phase change material emulsion (PCME) is a latent functionally thermal fluid prepared by dispersing a phase change material into a carrier fluid with the aid of surfactants. The PCME has a high specific heat capacity and enhanced heat transfer performance in the phase change region, and it shows great potential as a novel heat transfer fluid in air-conditioning systems and solar thermal systems. There have been few literature reviews on PCMEs. In this paper, a comprehensive summary of the research on PCMEs, including the fabrication process, thermophysical properties, rheological behavior, heat transfer performance, and applications in solar thermal systems, is presented. Additionally, directions for further studies and applications of PCMEs are proposed. more...

Carbon nanomaterials decorated phase change material emulsions (PCMEs) are promising working fluids for direct absorption solar collectors (DASCs). Aiming at realizing an optimization, eleven PCMEs with different mass fractions of graphite nanoplatelets (GNPs) (0.04%, 0.055%, 0.07%, 0.085% and 0.10%) and paraffin (15%, 20% and 25%) were prepared, and their thermophysical characteristics, optical absorption properties, and photo-thermal conversion performance were investigated systematically. It is shown that the thermal conductivity and optical absorption of the GNPs decorated PCMEs increase with the mass fraction of the GNPs, but show a reverse trend as the mass fraction of the paraffin is increased. The viscosity increases with the increase of the paraffin mass fraction at the same temperature, and gradually decreases along with the temperature increases. The photo-thermal conversion performance of the 0.04% GNPs decorated PCMEs is inferior to that of the ones at higher loadings of the GNPs; while, at high loadings of the GNPs, an interaction between the effect of the GNPs and that of the paraffin should be considered. It is revealed that thermal conductivity is an important factor influencing the temperature distributions within the PCMEs. And to avoid the evaporation of the water included in a PCME to weaken the incident light consequently, its optical absorption property should match with its thermal conductivity. The 0.07% GNPs decorated 20% PCME possesses the best photo-thermal conversion performance, and the relative heat storage capacity is 1.64 times as higher as that of distilled water. The 100 heating-cooling cycles test indicates that the 0.07% GNPs decorated 20% PCME exhibits good thermal reliability, making it show great potentials for use in DASCs. more...

A mixed polymeric emulsifier consisting of polyvinyl alcohol (PVA) and polyethylene glycol-600 (PEG-600) was explored for preparing high-performance phase change material emulsions (PCMEs) containing the paraffin with a melting point of 62–64 °C. After the effects of the mass ratio of PVA to PEG-600, the mass ratio of the mixed emulsifier to the paraffin and the homogenization rate on the size distribution, viscosity and dispersion stability of the PCMEs containing 20 wt% paraffin were systematically investigated, the optimal emulsifying process parameters have been determined to be the mass ratio of PVA to PEG-600 of 50:50, the mass ratio of the mixed emulsifier to paraffin of 1:5 and the homogenization rate of 10,000 rpm. Accordingly, the PCMEs with the paraffin mass fractions varying from 10 wt% to 30 wt% were prepared. It is found that the paraffin has been well dispersed in water in the form of sphere-like droplets with average diameters ranging from 3 µm to 11 µm in the obtained PCMEs. Significantly, the PCMEs with different mass fractions of the paraffin exhibit no supercooling, owing to the function of the mixed polymeric emulsifier as a nucleating agent. The apparent specific heat of PCMEs are 1.51–2.18 times as high as that of water at the phase transition temperature region, due to the existent of the paraffin in them. Their apparent thermal conductivity gradually decreases with an increase in the mass fraction of the paraffin, while the viscosity increases significantly with mass fraction of the paraffin at the same temperatures. Furthermore, the pumping power consumption of the PCMEs shows a drastic reduction as compared to that of water at the same heat storage capacity. It is revealed that the PCMEs show great potential for use as a novel heat transfer fluid (HTF) in thermal energy storage (TES) systems. more...

In this paper, graphite nanoparticles have been dispersed into a paraffin/water emulsion with the purpose of improving the thermal conductivity and photothermal conversion performance of the emulsion. The melting enthalpy and apparent specific heat of 20 wt% paraffin/water emulsion is 39.2 J g−1 and 9.105 J g−1 K−1 and its melting and freezing temperatures are very close to those of paraffin. Moreover, the 300 heating–cooling cycles test indicates that the paraffin/water emulsions containing 0.1 wt% graphite exhibits good thermal reliability. The viscosities of all samples are lower than 0.0750 Pa s, meeting the transportablilty requirements in pump systems for applications. In addition, the thermal conductivity of the emulsion containing 0.1 wt% graphite increases by 20.0% as compared to the base fluid and its receiver efficiency is higher than 86.0% as the temperature ranges from room temperature to 80 °C. As a result, the paraffin/water emulsion containing graphite with high energy storage capacity, enhanced thermal conductivity and excellent photo-thermal performance, good thermal reliability and good fluidity shows great potential for use as advanced heat transfer fluid (HTF) in the low temperature applications of direct absorption solar collector (DASCs). more...