Thermal analysis of meso-scale high-temperature Borehole Thermal Energy Storage (BTES) systems with varying operational cycles

Murat Aydin , Bo Wang, Jens Lingenauer, Sebastian Bauer

High temperature and short-term subsurface heat storage using BTES is a promising option and an emerging technology for increasing the fraction of renewable energy in the heat sector and
supplying stored heat at high and directly usable temperatures. Investigation of thermal interactions of multiple BHEs employed for high-temperature cyclic storage operations is required to understand the system behavior and the relevant thermal processes involved. This work therefore presents highly controlled meso-scale experiments for high temperature borehole thermal energy storage.
The experiment is set up at Kiel University, using a sand pit with two-meter depth and a volume of 37 m³ filled with partially saturated fine sand. Five BHEs are constructed, with four positioned at the edges of a square of 0.7 m side length and the fifth one in the center. The temperatures are measured at 224 locations at varying distances and depths from the center BHE. For the tests,
inflow temperatures of the BHEs were set to represent a high temperature storage system for both stationary and cyclic heat loads by using 70°C and 10-15°C inflow temperature for heating and
cooling cycles, respectively. The range of the cycles was changed from 12 to 120 hours.
All BHEs were jointly operated using the same inflow temperatures, to determine the effect of their thermal interactions on the recovery factor of cyclic storage operations. Thermal interaction due to the simultaneous operation of the BHEs reduced the heat transfer rate by about 30% after 12 hours of continuous heating in the center BHE, while for the outer BHEs the heat transfer rate was reduced by approximately 24%. After about three days of continuous heating, heat transfer rates have stabilized at about 60% in the outer and 40% in the center BHE. Based on these values, a thermal recovery factor of 55% is obtained. For the cyclic heat storage experiments, similar utilization ratios were found, although average heat transfer rates for the individual BHEs increase with decreasing cycle time. Furthermore, although heat transfer rates are lower in the joint
operation of the BHEs, temperatures in the sand are higher. Temperatures in the sand at 0.2 m from the center BHE increase from 30°C for individual BHE operation to 57 °C in the joint
operation, thus providing higher storage temperatures.