Underground Thermal Energy Storage at Scale: A Review of Techniques and a Case Study for Calgary, Alberta

Renewable energies, such as solar and wind, traditionally suffer from temporal incongruity. Society's energy demand peaks occur at different times of day than the electricity generation potential of a photovoltaic panel or, often, a wind turbine. Heat demand in particular, is subject to a significant mismatch between the availability of heat (in the summer) and the need for heat (the winter). Thus, a future energy system design should incorporate underground thermal energy storage (UTES) to avoid this temporal mismatch and emphasize thermal applications. Such a basis of design would introduce new methods of energy arbitrage, encourage adoption of geothermal systems, and decrease the carbon intensity of society. UTES techniques are becoming increasingly sophisticated. These methods of storage can range from simple seasonal storage for residential structures in a grouted borehole array (BTES), to aquifer thermal energy storage (ATES), deep reservoir storage (RTES) in basins, among others. The method that each of these techniques shares is the use of earth as a storage medium. UTES can also be characterized for electricity production, but this work largely explores applications in heating and cooling. Heating and cooling processes - residential, commercial, and industrial - make up large fractions of energy demand in North America. This is also true of other locales. With the increasing concerns of climate change, exacerbated by anthropogenic greenhouse gas emissions, developers and municipal planners are strategizing to decarbonize building heating and cooling at district scales. This work focuses on a review of UTES techniques, specific applications in cold climates, and elaborates on the experimental designs of UTES in Calgary. The research team at the University of Calgary is working with major oil and gas companies, real estate developers, and other energy service companies to implement next generation geothermal energy systems. With a new storage method such as UTES, the techno-economic barrier for low-carbon district energy decreases, paving a low- and no-subsidy pathway for geothermal projects in Alberta.