Life Cycle Assessment of Borehole Thermal Energy Storage

Sammanfattning: Borehole thermal energy storage (BTES) is a technology used to store energy between seasons. It could therefore be a solution to the seasonal mismatch between the use and production of (renewable) energy, which can be used to increase energy efficiency and reduce greenhouse gas emissions. The installation of a BTES system is, however, an ex- tensive procedure that requires the drilling of several deep boreholes. Different drilling methods, mainly down-the-hole (DTH) hammers, can be used to produce boreholes.  The decision of drilling method is commonly done from a technical or economic per- spective, without concern for the environment. The aim of this report is therefore to evaluate different drilling methods used for the installation of BTES, in regards to their environmental impact. The potential environmental impact of two different DTH ham- mers, air and water powered, have been compared from a life cycle perspective using life cycle assessment (LCA). The LCA is based on a BTES system in Sweden, consisting of 80 boreholes. The study shows that water powered DTH is preferred when com- paring the impact on climate change, ozone depletion, acidification and freshwater eutrophication, whereas the air powered DTH has a lower impact on water use.  The result of the LCA also speaks to the importance of making comparisons from a life cycle perspective, as the largest environmental impact of a BTES stems from its use phase. So even though a smaller borehole size is the aspect that results in the largest difference in environmental impact during the installation process, it can be canceled out by the higher energy consumption during the use phase.  The environmental impact from installation and use of a BTES system was also com- pared with a reference scenario, in which district heating was used. The comparison showed that there is a significant benefit with a BTES installation, as compared to a system with district heating. This comparison was, however, only conducted for a European case.