HYDROGEOLOGIC AND GEOCHEMICAL CHALLENGES FOR URBAN GEOTHERMAL ENERGY

With heating and cooling accounting for approximately one-third of U.S. and one-half of European energy consumption, utilizing geothermal heat and thermal energy storage options offer both cost-effective and green energy alternatives, particularly for cities. Heating and cooling with geothermal direct-use (e.g., groundwater used as heat source or sink) technologies, which are viable options nearly everywhere, greatly reduces the demand for electricity. Despite the proven efficacy of geothermal as a city/community-scale resource, especially in Europe and China, it is currently only a niche resource in the renewable energy sector globally. While heating and cooling are possible from natural ambient groundwater resources, there is also growing need for long-duration storage of large amounts of energy from episodic renewable electricity (e.g., wind and solar), preventing energy gluts and scarcity at timescales of weeks to seasons. Underground thermal energy storage (UTES), wherein surplus or waste heat is stored underground for later use, presents a long-duration high-capacity energy storage solution that creates local jobs and increases energy resilience/security without the need for rare critical minerals. However, using underground geologic reservoirs presents a broad suite of geochemical challenges depending on temperatures, flowrates, and treatment of working fluids. We present the range of geology- and hydrogeology-dependent UTES technologies that are common or under-consideration globally - highlighting identified geochemical challenges to operating geothermal systems and addressing city-specific issues (e.g., urban heat islands, maintaining urban habitat). Geochemical challenges include zonal dissolution or precipitation of minerals within the reservoir (e.g., loss of structural strength and/or permeability) and chemical interactions with engineered system components (e.g., scaling and plugging of heat exchangers, pipes, and wells). Because thermal energy is lost to the environment via conduction and advection, geochemical changes can propagate away from storage reservoirs, potentially affecting habitat or other uses of subsurface resources.