Measurements and simulations of high temperature borehole thermal energy storage in Drammen, Norway - evaluation of thermal losses and thermal barrier

• Distributed temperature sensing measurements from high temperature borehole thermal energy storage (HT-BTES) are presented. • Thermal losses from HT-BTES are found as function of loss mechanism and time. • The HT-BTES is a more economic alternative than the traditional low temperature BTES. • The effect on loss reduction by a thermal barrier is found to be beneficial, yet currently uneconomic. High temperature borehole thermal energy storages (HT-BTESs) have a huge potential in enabling green cities by storing and supplying a large share of the required heating/cooling demand in buildings and industry. A new concept to minimize losses is the use of a thermal barrier ring of boreholes around the inner boreholes. The barrier is charged with low temperature heat to reduce heat losses from the inner boreholes to the surrounding ground. An HT-BTES with this concept has been built at Fjell Primary School, Drammen, Norway, and temperature profiles in some of the boreholes have been measured for periods of up to 13 months during three years using distributed temperature sensing (DTS). The HT-BTES consists of 100 boreholes, 36 in the barrier ring and 64 inside it, and is charged at ≈50–60 °C starting in April 2020. DTS measurements from inside the HT-BTES have been used to calibrate a Comsol Multiphysics model. The model shows good qualitative agreement with measurements. The calibrated simulations show that after three years more than half of the injected thermal energy is stored in or extracted from the HT-BTES while the remainder is lost to and stored in the surroundings. The HT-BTES reduces yearly costs by 1.8–43.8 kNOK (155–3800 EUR) compared to a low temperature BTES, and additionally provides cooling and reduction of electricity peak demand. The thermal barrier leads to 88 MWh (76–112 MWh) additional thermal energy stored in the HT-BTES, giving a ∼2.5 °C (1.8–3.2 °C) higher average storage temperature. The thermal barrier increases the amount of stored thermal energy after the first three years of operation by 20 %. The cost of the barrier is still too high to make it economically viable in Norway.