Modelling aquifer thermal energy storage systems

Abstract

Aquifer thermal energy storage systems can decarbonise space heating and cooling, provide large-scale long-term storage and buffer the seasonal mismatch in supply and demand. In the winter, the system provides a heat source by extracting warm water from a subsurface reservoir. The extracted fluid cools as it passes through a heat exchanger at the surface before being injected into a second, colder reservoir. In the summer, the cold fluid is extracted to provide a source of cooling, absorbing heat rejected from buildings. The extracted cold fluid heats up and is injected into the hot aquifer, recharging it for the following winter. A numerical model of the complex heat transfer in the geological formation is developed to analyse the thermal evolution of a double well aquifer thermal energy storage system and identify optimal operating principles. First, the key controls of a system delivering an equal amount of heating and cooling are explored. The starting conditions impact the distribution of thermal energy between the hot and the cold aquifer in the first few cycles. After many cycles, the mean extraction temperature of the system tends towards a steady state. The impact on the heat pump, which raises the temperature of the heat source to provide space heating, is analysed. Second, the thermal evolution of a system delivering an uneven heating and cooling load is explored. Two operating scenarios are considered to meet the imbalance in demand. Their distinct impact on the long-term thermal behaviour of the system is analysed, as well as the effect on the electricity demand of the heat pump. One mode of operation extracts and injects different fluid volumes, eventually causing the temperature of the reservoir with a net loss of fluid to adjust to the far-field temperature of the subsurface. While the temperature of the other reservoir, with a net accumulation of fluid, approaches the injection temperature. The other mode of operation imposes a different temperature change on the extracted aquifer fluid as it passes through the heat exchanger during the summer and winter cycles. A greater temperature change in the winter compared to the summer leads to a gradual cooling of both reservoirs. While a smaller temperature change in the winter compared to the summer leads to a gradual heating of the system. The magnitude of the imbalance can enhance the thermal drift of both aquifers. Again, the starting conditions control the early-time evolution of the system before the imbalance dominates the long-term behaviour. Finally, the relationship between different operating modes and the energy consumption of the full system is examined. The operating modes established in this thesis, which exhibit fluctuating injection temperatures, are compared with a hypothetical system operated with a constant injection temperature. The latter often relies on carbon-intensive technology to control the recharge of the reservoirs, decreasing the efficiency and increasing the carbon intensity of the full system. The benefits of an ATES system recharging only via the waste output of space heating and cooling are highlighted. These findings have important implications for the efficient operation of an ATES system, especially in scenarios where the heating and cooling demands differ.