Very high temperature BTES: A potential for operationally cost-free and emission-free heating

In cold climates, the heating load is much higher than the cooling load and borehole thermal energy storage (BTES) systems offer an opportunity for high-efficient heating. Heat energy from different sources is stored in the ground to use in wintertime. Higher storage temperatures lower the operating cost of heating and the size of the BTES field. BTES systems, up to 90℃, are studied in the literature. In this study, we consider the charge temperatures beyond 100℃ to analyze the possibility of free-heating for Nordic countries. The system is called here very high-temperature BTES (VHT-BTES). A residence field of 25 houses, 125m² each, is chosen as a mid-scale target and concentrated solar collectors (CSC) are used to charge VHT-BTES up to 140℃. A double-ring layout of ten boreholes is optimized to minimize the heat pump consumption. Free-heating and heat pump modes are activated for high and low borehole temperatures, respectively. The actual meteorological data for Uppsala/Sweden is used. The gradually increasing very high seasonal coefficient of performance values (SCOP), 8-23, are achieved using free-heating and heat pump modes together for the first four years. From the fifth year, all of the heating demand is basically met by the stored energy (free-heating). The results show that VHT-BTES provides practically operationally cost-free and emission-free heating even for a Nordic country. The return of investment is calculated as ten to fourteen years, depending on the cost of the additional land for CSC.