Experimental study on latent thermal energy storage system with gradient porosity copper foam for mid-temperature solar energy application

Latent thermal energy storage is a promising option for the flexible and efficient use of solar energy. However, the low conductivity of phase-change materials limits its practical applications. This study proposes a type of gradient porosity metal foam as a heat transfer-enhancement system to overcome the above-mentioned drawback. Specifically, the thermal performances of a gradient copper foam and commonly used homogeneous copper foam are experimentally investigated and compared in a mid-temperature solar energy storage system. In this study, two lab-scale shell-and-tube units are built with A153 as the phase-change material in the annulus. The two types of copper foam are embedded in the units, gradient porosity copper foam in one unit and homogeneous copper foam in the other. Silicon oil is used as the heat transfer fluid flowing in the inner tube. The charging and discharging processes of the two thermal energy storage units are analyzed in detail. Compared to the embedded homogeneous metal foam in the phase-change material, which has been studied by numerous researchers previously, the experimental data indicate that the gradient porosity copper foam can significantly enhance the heat transfer capacity. Moreover, it improved temperature uniformity in the thermal energy storage unit and reduced the overall melting time by 37.6%. This study is the first to confirm that the application of gradient porosity metal foam can enhance the performance of an energy storage system. This conclusion is important for developing thermal energy storage systems, and indeed, can promote the utilization of solar energy at medium temperatures.