Experimental investigation on thermal performance of phase change materials–metal foam composites in ocean thermal engine

Because battery-powered underwater vehicles have limited long-term mission capacity, the development of ocean thermal engines that can harvest ocean thermal energy (OTE) is a potential solution to this “energy limitation” issue. Ocean thermal engines using solid-liquid phase change materials (PCM) for thermal–mechanical energy conversion. This study examined the effects of combining PCMs with metal foams (MF) on OTE harvesting. Two phase change materials and three MFs were considered. Experiments were conducted to measure the temperature change inside the PCM, phase change duration, and volume change to evaluate the thermal and energy-harvesting performances under different temperature and pressure conditions. Results show that (1) Mixed PCM composed of 95% n-hexadecane and 5% n-octane melting point decreased by 0.9-1.6°C and volume change rate reduced by 16-40% compared to pure n-hexadecane. (2) The mixed PCM has by 14-18% higher output power than pure n-hexadecane. (3) Within the test range, to maximize the power density, the optimal porosity of copper foam and back pressure during melting were 94% and 20 MPa, respectively. This study provides useful insights into the design and optimization of ocean thermal engines.