Numerical analysis of the fluid flow and heat exchange characteristics in a low-temperature and high-permeability reservoir of a heat storage and heating system

It is a promising energy strategy to utilize a shallow, low-temperature geothermal reservoir for seasonal heat storage and heating, with the potential to meet winter heating needs in decentralized rural buildings. The fluid flow and heat exchange in the reservoir govern the thermal performance of this heat storage and heating mode. However, for a U-shaped closed-loop geothermal system with long-term cyclic operation, the characteristics of fluid flow and heat exchange in a low-temperature and highly permeable water-saturated reservoir lacking background groundwater flow remain poorly understood. In this study, we develop a finite-element model to simulate the fluid flow and heat exchange processes in this reservoir. Then we analyze the characteristics of fluid flow and heat transfer in the reservoir and evaluate the effects of main factors on the thermal efficiency of the heat storage and heating system. The results show that, in a high-permeability reservoir, the heat exchange between the reservoir and the well can induce thermal convection, which significantly enhances both heat storage and extraction. In the cyclic processes of heat storage and extraction, thermal convection patterns show periodic reversals that alternate between domination by hot and cold fluids, and these periodic reversals also determine the heat storage and extraction regions. In a reservoir dominated by thermal convection, the injected fluid temperature and reservoir permeability govern the thermal efficiency; the heat injection modes within the well have a moderate effect; and separate horizontal wells in the gravitational direction for heat storage and extraction may offer a limited improvement.