Exploring the feasibility of uni-directional ATES in high ambient groundwater flow aquifers

In the context where the member states of the European Union are asked to reach the goal of net zero emission of greenhouse gases before 2050, many cities all over Europe are adopting sustainable energy systems. One of the primary sources of CO₂ emission are building heating and cooling systems. As aquifers are commonly present in many urban areas, ground coupled heat pumps are a renewable solution that many countries are adopting to replace fossil based heating and cooling techniques.

Aquifer thermal energy storage (ATES) is attained by storing thermal energy in groundwater aquifers. Hence, a key aquifer characteristic for ATES is the natural groundwater flow velocity. For velocities less than 25 m/y, bi-directional ATES is usually possible. For groundwater flow velocities greater than 25 m/y, ATES is still possible, but requires multiple doublets with a specific well placement to be able to compensate for the groundwater flow via upstream injection and downstream extraction. Alternatively, “pump and dump” systems, which generate thermal plumes that affect the downstream groundwater temperature and are not desirable for other users, are adopted.

A possible way to tackle the issue of aquifers with high ambient groundwater flow velocity is to combine the two solutions described above by applying a uni-directional pumping scheme allowing to compensate for the groundwater flow by constantly injecting in the upstream well and extracting from the downstream well. Spacing of the wells should be adjusted to the storage cycle length and natural groundwater flow velocity to ensure re-capture the energy injected within the aquifer in the previous season from the upstream well and transported to the down-gradient extraction well by the groundwater flow. This concept would also mitigate the downstream effect of a thermal plume. In this research the results of a sensitivity and feasibility analysis of this concept are presented. The results show that optimal inter-well distance not only depends on storage cycle length and groundwater flow velocity, but also on storage volume. Downstream thermal pollution can be avoided and recovery of the heat/cold stored can reach values between 50-60% depending on the conditions.