The potential of intermediate-to-deep geothermal boreholes for seasonal storage of district heat
Eero Hirvijoki, Janne Hirvonen
Abstract
This paper explores intermediate-to-deep coaxial geothermal boreholes for seasonal storage of district heat. Extracting heat from within the bedrock creates a “temperature pothole” which can be filled and utilized for thermal storage. Unlike in shallow boreholes, the pothole effect minimizes losses and offers a novel heat-storage concept. Exploiting this idea, two scenarios are studied: one to maximize the capacity factor of the district heating network and another to utilize intermittent wind power as a heat source on annual basis. With the borehole serving as a load-shifting storage facility in both cases, simulating the minimum annual delivery temperature as a function of the borehole depth and the system’s average power rating enables determining the necessary sizing for the borehole to function as a storage for space heating on annual basis. The first-principle simulation results show that, in co-operation with the borehole, intermittent wind power as well as a district heat network operating at full capacity throughout the year both could cover the hourly heat demand with effectively 100% storage efficiency, provided that the borehole depth and system power rating are chosen in proportion.