Controls on the temperature of the produced fluid in a double well ATES system

We investigate the temperature evolution of a double-well low-temperature aquifer thermal energy storage system consisting of a hot and a cold permeable reservoir in the subsurface. The wells are used cyclically to provide a supply of thermal energy in the winter and a thermal sink in the summer. The system is paired with a heat pump at the surface which can raise the temperature of the aquifer fluid, to meet the heating demand in the winter, and can also drop the temperature of the aquifer fluid, to meet the cooling demand in the summer. These systems provide a low-carbon solution for space heating and cooling, which currently makes up over a third of the greenhouse gas emissions in the UK. Our results show how fundamental modelling of the complex heat transfer in the geological formation can help identify optimal operating principles for ATES systems. Our modelling focuses on coupled wells where the extraction temperature of one well, as well as the temperature change imposed by the heat pump, determines the injection temperature of the other well. Our results highlight that the heat transfer between the injected volume and the subsurface leads to a continuous change in the extraction temperature during each cycle. We find that after many cycles, the mean extraction temperatures of the hot and cold wells tend to ΔT2 and −ΔT2, respectively, where ΔT is the temperature difference between the extraction temperature of one well and the injection temperature of the other well. Furthermore, we find that the season in which the system is started has a significant impact on the extraction temperatures of both wells in the first 5–10 cycles. If a system is started in the winter, to initially provide space heating, we observe the extraction temperature of both wells gradually increase from cycle to cycle towards the equilibrium temperatures. But if a system is started in the summer, to initially provide space cooling, the extraction temperatures gradually cool down towards the equilibrium temperatures. We compare the electricity usage in the heating season of a double well ATES system with a simple system which extracts at the ambient temperature of the aquifer. We show that a double well system started in the summer can have an average reduction of 9.9% in its electricity usage for heating, over 20 years. While, a system started in the winter can have an average reduction of 7.1 %, over 20 years. Our modelling therefore provides a framework to optimise operation of such systems.