Energy and exergy parameters of low-temperature solar thermal energy storage systems: empirical estimation and optimisation

This study presents the energy and exergy analyses of three low-temperature solar thermal energy storage (STES) systems. These STESs were of the same design but, contained different heat storage materials (benzoic acid, stearic acid and palm olein) evaluated during heat charging and discharging periods. The energy and exergy inputs and outputs were analysed and factored to estimate the efficiencies during heat charging, while the energy and exergy drop during heat discharge were also estimated. The energy and exergy outputs were optimised using a set constraint employed on Design-Expert 11 software. The energy and exergy outputs ranged between 25.89-95.96 W and 13.15-52.37 W, respectively, depending on the type of heat storage materials used. Under optimised conditions, the energy and exergy outputs increased to 117.1-202.7 W and 81.2-107.5 W, respectively. The STESs were found to have a positive and significant effect on the energy and exergy efficiencies and the energy and exergy drop. The quantity of water heated only affected the energy and exergy drop. This paper reveals that the energy and exergy parameters of a solar thermal system can be optimised by the integration of a STES.