Gradient Metal Foam and Nanoparticle Coupling Enhancement on Phase Change Heat Storage for Improving Thermal Performance of a Heat Pump

Enhancing heat transfer in latent heat thermal energy storage (LHTES) can help further alleviate the negative effects brought about by excessive consumption of fossil energy. This study proposes to couple and enhance LHTES through gradient pore foam and the addition of nanoparticles. Three types of storage tanks with positive gradient porosity, uniform porosity, and negative gradient porosity were designed, and three concentrations of nanoparticle addition ratios were equipped. The research on phase change heat storage was carried out through verified numerical models. The analysis indicates that complete melting time of a tank designed with a positive gradient is decreased by 984 s and reduced by 11.23% compared with a tank without a gradient design. Tanks designed with negative gradient porosity delayed the complete melting time by 2451.8 s, which was extended by 28.00%. Adding an appropriate number of nanoparticles can help enhance heat exchange rate and improve efficiency, while excessive addition of nanoparticles will inhibit heat storage rate, causing a counterproductive effect on heat storage. When the nanoparticle filling concentration was 10%, the storage rate of the tank with positive gradient was the highest, reaching 0.04445 kW, which was 0.00605 higher than the tank without nanoparticle addition, representing a 15.76% increase. Coupling the heat storage tank to the ideal heat pump system for heating can increase its COP from 1.82 to 1.97, which represents an improvement of 8.24%.