Experimental and numerical investigation of thermal interactions between high-temperature borehole heat exchangers in unsaturated and low permeable soil

High-temperature borehole thermal energy storage in the subsurface offers a promising solution to balance the seasonal mismatch between heat demand and supply from renewable sources. For most borehole thermal energy storage systems, a considerable fraction of the storage volume close to the ground surface will be in unsaturated conditions, characterized by comparably low thermal conductivity and heat capacity. This affects the achievable heat transfer rates of the borehole heat exchangers as well as the thermal interaction of neighboring borehole heat exchangers and thus influences the overall borehole thermal energy storage performance. In order to investigate the heat transfer behavior of individual borehole heat exchangers and the thermal interactions between these borehole heat exchangers at high storage temperatures, a series of experiments with up to five borehole heat exchangers in unsaturated low-permeable sandy soil is performed under well controlled conditions with extensive in-situ temperature monitoring. First, a high-temperature thermal response test at inlet temperatures close to 70 °C was performed for each borehole heat exchanger individually. The thermal conductivity of the unsaturated soil derived from the thermal response test was approximately 30 % higher than values obtained in the laboratory or reported in the literature for standard temperature conditions. This demonstrates that high-temperature borehole thermal energy storage can be as effective for partially saturated conditions as for fully saturated conditions. In a second step, the high-temperature thermal response test were repeated using different numbers, spatial combinations and separation distances of the active borehole heat exchangers. Notably, there was a significant reduction of up to 50 % in the thermal performance of an individual borehole heat exchanger due to thermal interactions with neighboring BHEs, particularly for smaller separation distances or higher active borehole heat exchanger numbers. Finally, a geometrically detailed numerical heat transport model was developed, which accurately predicts the individual and combined borehole heat exchanger operations in terms of the achieved heat transfer rates and cumulative energy input with a precision of ± 3 %, as well as the measured soil temperatures with mean absolute errors on the order of 0.6 to 3.4 °C, if the thermal conductivity derived from the high-temperature thermal response tests is used. The numerical model thus is capable of assessing the energy balance, storage operation, and induced thermal impacts of high-temperature borehole thermal energy storage in the unsaturated subsurface. This study for the first time demonstrates that dedicated high-temperature thermal response tests at the scheduled temperature level of a high-temperature borehole thermal energy storage are required for a reliable characterization of unsaturated soil thermal properties and the parametrization of simulation models used to design and operate such a storage site. The combined experimental and numerical work shown here substantially improves the process understanding of heat transport and thermal interactions of borehole heat exchangers in unsaturated porous media and therefore benefits the design and operation of future large-scale high-temperature borehole thermal energy storage systems.