High energy and power density latent heat storage based on boric acid/expanded graphite composites for medium-temperature applications

Latent heat energy storages for the medium-temperature applications within 150–250 °C are typically restricted by low enthalpy change and low thermal conductivity of frequently used phase change materials (PCMs) like molten salts and sugar alcohols. This study employs boric acid (BA) as a PCM and expanded graphite (EG) as thermal conductivity enhancer to prepare BA/EG composite PCMs for high energy and power density latent heat storage. Pure BA is characterized by a phase change temperature of 170 °C, a high intrinsic enthalpy of 525 J·g −1 and a superior phase change behavior. EG acts as an inert filler to form an efficient laminar conductive network to accelerate thermal transport within the composite PCMs. With 20 wt% EG and a 1400 kg·m −3 packed density, the BA/EG composite block has the radial thermal conductivity of up to 12.8 W·m −1 ·K −1 , 16 times higher than pure BA. The corresponding energy storage density is highly competitive among PCMs within the same transition temperature region. Experimental tests on the dynamic thermal performance are performed based on a concentric tube thermal storage unit, and the results show that the average charging power of the BA/EG system is increased to 210% compared with the pure BA system. Overall, the BA/EG composite PCMs developed in this study combine the multiple advantages of high enthalpy, high thermal conductivity and excellent cyclic stability, providing a highly promising option for developing medium-temperature latent heat thermal energy storage.