Thermal performance of borehole heat exchangers in different aquifers: a case study from Shouguang

摘要

Groundwater flow plays an important role in affecting the thermal performance of borehole heat exchangers (BHEs). In the present work, a few field tests are conducted in Shouguang, China, to compare the thermal performance of three BHEs in different aquifers. The results show that there is a good linear correlation between the heat transfer rate of BHEs at steady state and the average fluid temperature. A larger slope usually means a higher heat transfer rate of a BHE, under the same borehole conditions. The thermal performance of BHEs is extremely enhanced, especially in those regions with multiple aquifers, and even the thermal response test results can be enlarged to a great extent by groundwater flow. The enhanced effect of groundwater flow depends mainly on the amount, thickness and depth of aquifers. It is found that when the depth of aquifers covers the middle or lower part of BHEs, at least exceeding the depth of the constant temperature ground layer, the enhancement on the thermal performance of BHEs becomes more intense due to the increase of the heat transfer temperature difference and the decrease of the total thermal resistance from the inner fluid to the surrounding ground. Groundwater flow in aquifers is helpful to reduce the required length of BHEs and the construction cost of ground source heat pump systems.