Experimental investigation on heat extraction performance of deep borehole heat exchanger for ground source heat pump systems in severe cold region

Abstract

Deep borehole heat exchanger (DBHE), whose depth can reach hundreds even thousands of meters, has large and stable heat extraction capacity and is thus expected to be a competitive heat source for the heating in winter in severe cold regions. In this study, field experiments on the performance of the DBHEs at multiple depths (400 m–2000 m) were conducted in a ground source heat pump system in Harbin, a city located in the severe cold region of China. Different water flow rates were set in the DBHEs. The results indicated that, the heat extraction capacity climbs linearly with the increase of borehole depth that the heat exchange rate per unit depth increases by 8.34 W/m for every 100 m growth. When circulating flow rate rise from 11.6 m3/h to 17.0 m3/h, the heat exchange rate of 1000 m DBHE was decreased by 7.0% and the optimal thermal performance of 1200 m DBHE occurred in 14.5 m3/h that the maximum and minimum heat exchange rate differed by only 1.3%. However, the heat exchange rate of 2000 m DBHE leapt by 11.2% with the climb of flow rate that had not yet attained the maximum heat extraction capacity. Properly setting a flow rate is vital for the DBHE application in severe cold regions. This study can provide design and operation guidance for medium and deep geothermal heating in severe cold regions.