Analytical dimensioning of ATES system size and well spacing

Aquifer thermal energy storage (ATES) is a well-established technology that bridges the seasonal mismatch between summertime heat supply and wintertime heat demand. Due to the storage of available excess heat, such as waste heat, it promotes the decarbonization of the space heating sector. Typically, a minimum transmissivity of the storage formation is required for ATES to achieve the necessary capacity of the pumping wells. However, depending on local hydrogeological conditions or existing subsurface usage, such high-transmissivity aquifers may not always be available, and the use of low-transmissivity aquifers may be required. In such cases, the applicable pumping rate per well may be significantly limited to prevent aquifer depletion and mechanical uplift of the confining layer, increasing both the required number of well doublets and the complexity of the system design.
Here, an analytical approach is presented to determine the minimum required number of well doublets and their spacing, considering both thermal and hydraulic design constraints. Commonly used well separation rules based on the thermal radius serve as the thermal design constraint to avoid thermal interference between wells and limit the minimum well distance. The maximum allowable head change defines the hydraulic design constraint and limits the maximum well spacing. In multi-doublet well fields, the superposition of pressure fields of adjacent wells may notably affect the observed head change, which in turn impacts the possible pumping rate. Consequently, this approach is derived for typical ATES-well field configurations, i.e., the “lane” and “checkerboard” layout.
This method is demonstrated in a case study of a low-temperature ATES on the campus of Kiel University in Germany. Results show that for lower transmissivities, the “checkerboard” layout requires fewer well doublets than the “lane” layout to achieve the specific target pumping rate of 200 m³/h. Depending on the assumed geological and operational conditions, up to eleven well doublets are required for the “lane” layout, whereas nine well doublets are sufficient for the “checkerboard” layout. This is because the increased hydraulic superposition between injecting and extracting wells reduces the observed head change and, in turn, increases the possible maximum pumping rate per well. This method allows to facilitate better ATES system design during an initial feasibility study or potential assessment by integrating both thermal and hydraulic constraints, thereby potentially reducing overall capital and well maintenance costs.