Performance evaluation of combined passive/active thermal charge/discharge latent heat tank with stabilized-solid-liquid storage material for intermediate thermal storage system

Abstract

This study proposes a new model operation for an intermediate thermal storage (ITS) tank for solar water heater system. It combines passive-charge active-discharge (PCAD) in a single storage tank, potentially reducing the number of components of the system and improving the net energy balance. The charge process is done by heating the storage material using an electric heater, which is also suitable for direct photovoltaic heating. The temperature profile of stabilized storage material in this stage shows a close plateau line in the solid-liquid transition, indicating a steady phase transition takes place. It minimizes the temperature deviation between the solid/liquid region, which is advantageous for determining the operation protocol. For the discharge process, the working fluid circulates from the mantle side and continues to the inner side of the tank, resulting in two-consecutive heating. It allows the fluid to harness the stored heat up to 72.8%. It is also achieved by the stabilized storage material, which has a better solidification mechanism. Thus, the proposed PCAD tank is able to meet the criteria of an ITS tank, which requires energy exchange during the charge/discharge stage. The finding demonstrates that a simplification of the ITS tank is achievable. The implication of the finding can be used as a fundamental basis for developing new solar water heating systems, including for direct charge operation using photovoltaic-heating systems where the heat is taken as alternative energy storage instead of electricity.