Optimal doublet spacing for Aquifer Thermal Energy Storage (ATES) and open-loop unidirectional shallow geothermal (OLG) systems

Aquifer Thermal Energy Storage (ATES) and Open-Loop unidirectional shallow Geothermal (OLG) systems are delivering low-carbon heating and cooling to the built environment. Both technologies require similar infrastructure: two or more boreholes, hydraulic pumps, heat exchanger(s) and heat pump(s). Both systems also offer sustainable operation when the heat and cool energy injected into, and extracted from, the aquifer is balanced. However, ATES systems benefit from energy storage and re-use, with thermal energy recovery reaching 70-90% for systems with no interference between warm and cool plumes in the aquifer. Here we compare ATES and OLG energy production and energy production per area (energy density) for a suite of common aquifer properties and design decisions. Energy production is an important metric because systems must be engineered to meet an identified demand, but energy density is equally important when systems must operate within a limited surface footprint.  All systems investigated here are energy balanced.

Results indicate that ATES systems always produce more energy (on average 80%) than equivalent OLG systems, even when thermal recovery is low. Maximum energy delivery depends on large boreholes spacing and short screen lengths. Maximum energy density increases with increasing screen length and reduced well spacing. The optimal combination for energy production and energy density combines long screens with boreholes spaced just far enough to prevent thermal breakthrough. The thermal plumes produced in ATES deployments have an areal extent that is, on average, 30% larger than that of equivalent OLG systems, but their energy production is much higher, enabling 44% higher energy production for the same area. Given the higher energy production and energy density offered by ATES, and the higher system coefficient of performance resulting from the use of pre-warmed and cooled groundwater, we argue that ATES systems should always be considered ahead of equivalent OLG systems. Furthermore, this means from a planning perspective that more deployments can be packed in the same area.