Material selection and testing for thermal energy storage in solar cooling

The goal of this study is to implement and to test a thermal energy storage (TES) system using different phase change materials (PCM) for solar cooling applications. A high temperature pilot plant able to test different types of TES systems and materials was designed and built at the University of Lleida (Spain). This pilot plant is composed mainly by three parts: heating system, cooling system, and different storage tanks. The pilot plant uses synthetic thermal oil as heat transfer fluid (HTF) and has a working temperature range from 100 to 400 °C. Two different PCM were selected after a deep study of the requirements of a real solar cooling plant and the available materials in the market through literature review and DSC analysis. Finally d-mannitol with phase change temperature of 167 °C and hydroquinone which has a melting temperature of 172.2 °C were used at pilot plant scale. For both PCM, no hysteresis was detected, and at pilot plant only d-mannitol showed subcooling even though both showed it during the DSC analysis. An effective heat transfer coefficient between the storage material and the heat transfer fluid (HTF) was calculated. For the same boundary conditions, the energy stored by d-mannitol was higher than that for hydroquinone. Moreover, d-mannitol has polymorphism that needs to be taken into account when the material is used as PCM, but experiments in this paper showed that polymorphism did not interfere its performance as PCM.