Scoping analysis for a large Aquifer Thermal Energy Storage (ATES) system in the London Chalk aquifer

Aquifer Thermal Energy Storage (ATES) systems deliver low-carbon heating and cooling to the built environment. ATES offers higher efficiency (higher coefficient of performance, COP) compared to other low carbon heating and cooling technologies, because it captures, stores and re-uses both heat and cool that would otherwise be wasted.  ATES also offers long term sustainable operation when the system is balanced, so there is no net extraction of heat or cool from the aquifer.

We consider here the design of a large ATES system to supply heating and cooling to a site in London.  The target aquifer is the heterogeneous London Chalk, a dual porosity system in which groundwater flow occurs primarily within fractured and/or karstified intervals.  A small number of ATES and other open-loop geothermal systems currently operate in London.  Most comprise a single doublet and supply only a small proportion of the site heating and/or cooling demand.  The largest operating systems comprise four doublets and supply peak heating and cooling demand of order 2-3 MW.

The analysis reported here is novel for four reasons.  First, we implemented a probabilistic method to assess the initial capacity of the system.  The Monte-Carlo approach assigns input distributions for uncertain design parameters prior to detailed analysis, such as borehole flow rates, injection temperature, thermal recovery factor (and hence production temperature), heat pump COP, and heating and cooling periods.  The approach produces a distribution of predicted values for peak and annual heating and cooling supply, that provide insight into the range of potential system capacity.

Second, for detailed analysis we implemented a controller in our groundwater numerical simulator that continually updates borehole flowrates to meet a predefined heating and cooling demand for a given period.  The controller accounts for the heat pump contribution and the difference between the produced groundwater temperature and the target heating or cooling temperature.  The controller manages changes in groundwater temperature observed during production, modifying the flow rate to meet the heating or cooling demand.

Third, we investigated a large system installed on a relatively small site (ca. 500 m x 400 m) with peak heating demand of order 14 MW.  A key challenge for ATES in urban environments is to deliver the energy and power density (energy per unit area, and power per unit area) commensurate with demand.  Finally, we tested the impact of heterogeneity in the Chalk aquifer, using realistic geological models informed by operational data and modelling of nearby ATES systems.  Our results suggest that a large system on the site is feasible and could meet a substantial proportion (and in some cases, all) of the heating and cooling demand.  The most significant limitation on system capacity is the potential for aquifer heterogeneity to create laterally spreading plumes that result in thermal breakthrough.