Ultra-High-Temperature Underground Thermal Energy Storage (UHT-UTES) for Large-scale, Inter-seasonal Electricity Storage

Low-temperature (LT-) and high-temperature underground thermal energy storage (HT-UTES) are used to provide low carbon heating and cooling.  Here, we investigate the potential for ultra-high temperature underground thermal energy storage (UHT-UTES) to balance seasonal fluctuations in electricity supply and demand. During periods of excess supply, groundwater is pumped from naturally porous, permeable underground reservoirs at depths > 500 m, heated to high temperature (>150°C) and pumped back underground where it is stored. When demand exceeds renewable supply, the high temperature water is pumped back to surface and used to generate electricity.

UHT-UTES offers several potential advantages over comparable underground storage technologies such as hydrogen storage: the surface facilities are conventional, comprising boilers and turbines, and expertise can be shared from hydrothermal electricity production.  The technology can utilise deep saline aquifers and end-of-life hydrocarbon reservoirs which are geographically widespread and offer large storage and flow capacity. 

Despite the potential for UHT-UTES, significant uncertainties remain concerning storage efficiency when the stored water temperature is significantly higher than the ambient reservoir temperature.  There are also possible risks to sustainable operation, such as scaling and associated loss of storage and flow capacity, and/or the potential for rock weakening and seismicity. 

This study reports the results of numerical modelling to determine key controls on the storage efficiency of UHT-UTES considering a range of operational storage temperatures, reservoir geological settings and heterogeneities, and real-life patterns of intermittent electricity generation and demand.  Results show that the underground storage efficiency of UHT-UTES for thermal energy is high and losses are due to both thermal conduction and the convection of hot, buoyant groundwater, and varies depending on how heterogeneity impacts on thermal plume migration.

Despite the high underground storage efficiency, the round-trip efficiency for electricity generation from UHT-UTES is constrained by the conversion efficiency of the turbines. Round-trip efficiency can be maximized by choosing a storage temperature that yields maximum turbine efficiency and minimal thermal storage losses in the reservoir. Current work is focused on addressing the potential for chemical reaction to impact sustainable operation.