Comparison of the thermal performance of double U-pipe borehole heat exchangers measured in situ

Abstract

A borehole heat exchanger is a ground heat exchanger devised for the extraction or injection of thermal energy from/into the ground. The thermal performance of a borehole heat exchanger can be assessed with a response test. The response test method allows the in situ determination of the thermal conductivity of the ground in the vicinity of a borehole heat exchanger, as well as the effective thermal resistance of this latter. The response test method is briefly described before it is applied to several designs of double U-pipe borehole heat exchangers. The tests have shown the viability of the method. They reveal that the thermal resistance can be decreased by 30% when quartz sand is used instead of bentonite and when spacers are used to keep the plastic pipes in contact with the borehole wall. With a common heat extraction rate of 50 W/m of borehole length, the temperature gain in a heat pump evaporator is +2 K. Finally, a mobile device has been developed to offer the possibility of accomplishing a response test.