Aquifer Thermal Energy Storage (ATES) is the most efficient heat-pump technology for heating and cooling that you have never heard of. It involves the capture, storage and re-use of waste heat and cool. ATES is a type of shallow geothermal technology. Some of these technologies extract heat or cool from underground rocks and some store heat or cool underground for later use. Systems can be further classified as closed- or open-loop. In the former, a fluid is circulated through a network of buried pipes or boreholes to exchange thermal energy with the surrounding rock or soil. In the latter, thermal energy is stored or produced by injecting or extracting groundwater via boreholes. Open-loop extractive systems are sometimes termed ‘Groundwater Heat Pump’ (GWHP) systems.
ATES is an open-loop technology that stores warmed or cooled groundwater in naturally porous, permeable underground rocks (aquifers) and uses this to provide low carbon heating and cooling. Systems employ pairs of boreholes (termed ‘doublets’) equipped with reversible pumps which inject or produce groundwater depending on the demand for heating or cooling. The boreholes are defined by the temperature of the stored groundwater. ATES systems are ‘non-consumptive’ which means there is no net groundwater extraction: all groundwater produced to supply heating or cooling is re-injected into the aquifer.
The basic requirements for ATES deployment are:
- distinct periods of heating and cooling demand, and
- a suitable storage aquifer at shallow depth (typically up to around 300m below ground surface) beneath the building(s) supplied by the system. The temperate UK climate is well suited to ATES and the UK hosts numerous aquifers that are geographically widespread.
ATES is a proven technology capable of delivering low carbon heating and cooling at scale with significant international uptake. There are more than 3,000 ATES systems installed worldwide, with the majority operating in the Netherlands. In the UK, there are just 11 systems, mostly serving large residential and commercial developments in London. ATES systems are characterised by large storage (of the order of hundreds to thousands of MWh thermal (MWhth)) and power (of the order of MW thermal (MWth) to tens of MWth) capacities. Systems typically operate at storage temperatures of 15–25°C at the warm borehole(s) and 5–10°C at the cool borehole(s).
Storage capacity is large compared to man-made reservoirs (including thermochemical reservoirs) because of the very large volumes naturally available underground. Power capacity is large because pumping groundwater into and out of the storage aquifer allows rapid transport of energy via advection, especially compared to closed-loop systems that rely on conductive heat exchange. Storage and power capacity of ATES systems can be scaled by increasing the number of doublets. Capacity is typically limited by the maximum sustainable groundwater flowrate and the recovery efficiency of the stored heat.
Inter-seasonal recovery efficiencies greater than 80 per cent are observed in many operating systems, because rocks are relatively good thermal insulators. Recovery efficiency is typically lower when there is natural groundwater flow in the aquifer that moves the stored energy away from the boreholes, or when thermal interference occurs as the warm and cool plumes interact in the aquifer. Interference may be observed if warm and cool wells are insufficiently spaced. Systems installed in aquifers with natural groundwater flow can be engineered to increase recovery.
An important design consideration is energy balance – the storage and extraction of equal amounts of heat and cool. Energy balance ensures sustainability: a balanced system extracts no net heat or cool from the aquifer, so causes no long-term change in average aquifer temperature. In the Netherlands, balanced operation is a regulatory requirement. Balance is typically ensured, where necessary, by providing additional sources of low carbon heating or cooling.
ATES systems offer lower electricity consumption and associated CO₂ emissions than equivalent GWHP systems. Heating is more efficient because ATES supplies pre-warmed groundwater, so the heat pump boosts the temperature less and operates with a higher Coefficient of Performance (COP). Cooling is highly efficient because ATES often delivers cooled groundwater which can deliver direct cooling without a heat pump. ATES offers a 7–23 per cent reduction in electricity consumption and CO₂ emissions for heating, and a 19–93 per cent reduction for cooling, compared to an equivalent GWHP system. Cooling is a close-to-free byproduct of heating. ATES also offers significant reductions in grid load and CO₂ emissions compared to Air-Source Heat Pumps (ASHP) of 41 per cent for heating and 94 per cent for cooling.
ATES deployments elsewhere provide evidence that they can be economically as well as environmentally attractive. ATES typically has higher Capital Expenditure (CAPEX) than conventional Heating, Ventilation, and Air-Conditioning (HVAC) systems (typically two to three times more expensive), but significantly lower Operating Expenditure (OPEX) (typically five to seven times less expensive, depending on energy prices). Slow and complex permitting procedures in the UK can be a barrier to deployment in cases where ATES is considered. However, the main barrier to widespread deployment of ATES in the UK is an almost complete lack of awareness of the technology.