Life cycle assessment of two deep geothermal heating plants and the potential of auxiliary energy to reduce environmental burden

• Life cycle assessment of two heat plants in the Southern Molasse Basin. • Results vary due to geological conditions and auxiliary energy use. • Major impacts: electricity demand and fossil-based peak load and redundancy. • Using biomethane for peak load and redundancy coverage could improve impact. Geothermal energy is promising as a low-carbon alternative for heat generation. This paper presents a life cycle assessment (LCA) of two currently operational deep geothermal plants in the Southern German Molasse Basin according to ISO 14040 and 14044. The plants significantly differ in geothermal water temperature and auxiliary energy usage. Additionally, one plant is coupled to a cogeneration unit. The environmental impact of the plants varies greatly, with 81 and 195 g CO 2 eq./kWh for global warming impact. In comparison to the typical German mix utilized for district heating in the year 2023, geothermal heat exhibits a substantial reduction potential. The primary impact of both plants is mainly attributed to the operational phase, specifically the use of auxiliary energy to meet the electricity demand of the downhole pumps. Another major contribution is the peak load and redundancy coverage. Conducting scenario analyses regarding auxiliary energy usage, it was found that switching to a more renewable electricity mix could lead to a reduction of up to 39 % in Global Warming Potential (GWP). Utilizing biomethane for peak load and redundancy coverage results in a reduction of up to 77 %. Hereby, the improvement potential significantly depends on both the type and quantity of auxiliary energy used. The findings underscore the significance of auxiliary energy in mitigating the environmental impact of deep geothermal heating facilities, which can contribute to achieving the EU's net-zero targets.