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

A borehole heat exchanger is a ground heat exchanger devised for the extrac tion or injection of thermal energy from/into the ground. The thermal perfo rmance of a borehole heat exchanger can be assessed with a response test. T he response test method allows the in situ determination of the thermal con ductivity of the ground in the vicinity of a borehole heat exchanger, as we ll as the effective thermal resistance of this latter. The response test method is briefly described before it is applied to sever al 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 sp acers 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 tempe rature 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. (C) 2001 Elsevier Science B.V. All rights reserved.