Quantification of losses from borehole thermal energy storage through distributed temperature sensing and numerical modelling

Borehole Thermal Energy Storage (BTES) is a successful technology for storing and supplying heat and cooling for buildings and industry. The losses from the storage can be drastically reduced by changes in design and operation. According to Fourier’s law of conduction, heat losses are proportional to the thermal gradient. The thermal gradient in five boreholes within a BTES in southern Norway, constructed in granite, was measured using Distributed Temperature Sensing (DTS). The measurements show that the downwards losses are linearly proportional to the temperature level in the BTES as expected, but also that the linear relationship shifts towards lower losses for the same temperature level as the ground is heated. After a half year of operation, the losses are approximately 50% lower than those in the first heating season, which had measured temperatures of about 35°C.||Modeling of the BTES shows that the conductive losses can be reduced if as much heat as possible is injected in the center versus if even heat injection is used. The outwards losses cannot be directly estimated from DTS results unless the BTES has been unused for at least 12 hours, due to the close relationship between borehole temperature and recent heat injection.