Harvesting environmental thermal energy using solid/liquid phase change materials

Abstract

This article investigates the feasibility of using solid/liquid phase change materials to harvest the renewable thermal energy in various natural environments, which is often associated with a low temperature differential. The basic idea is to move the phase change material cyclically through the temperature differential and convert a fraction of the energy absorbed by the phase change material in its melting process into mechanical or electrical energy. In this work, we first develop a thermodynamic model for an idealized setting, thereby deriving a theoretical upper limit of the thermal efficiency. Next, we couple the thermodynamic model with a structural mechanics model based on Kirchhoff–Love plate theory, in order to predict the performance of specific devices. To validate the thermomechanical model and demonstrate the feasibility of the underlying approach, we develop a prototype that uses pentadecane (C₁₅H₃₂) as the phase change material. The measured specific energy agrees favorably with the model prediction. Finally, we employ the validated model to conduct a parameter study. The result implies that stiffer structures and phase change materials with high solid/liquid density ratio are preferred. The study also suggests that compared to bismuth telluride (Bi₂Te₃)-based thermoelectric generators, the phase change material–based approach may yield significantly higher efficiency when the temperature differential is less than 100°C.