Effect of the borehole thermal resistance on the performance of a ground-coupled heat pump system

The effects of the borehole thermal resistance on the annual performance of a ground coupled heat pump system are analyzed by means of a dynamic simulation code developed by the authors, based on the g-functions determined by Zanchini and Lazzari (Energy 59, 2013, 570–580; Energy 70, 2014, 444–455). The code simulates the borehole heat exchangers, the heat pump and the storage tanks, and applies to multifunction heat pumps with or without a variable-frequency inverter. The hourly heat loads for heating, cooling-dehumidifying and domestic hot water production of a residential building with 6 apartments located in Bologna, North-Center Italy, are considered. The ground heat exchanger examined is composed of two BHEs with diameter 152 mm, each 150 m long, at a distance of 6.08 m from each other, placed in a soil with thermal conductivity 1.8 W/(mK). Four different BHE kinds are considered: single U-tube with shrank spacing 94 mm and grout thermal conductivity either 1 W/(mK) or 1.6 W/(mK), double U-tube with shrank spacing 85 mm and grout thermal conductivity either 1 W/(mK) or 1.6 W/(mK). The working fluid is a mixture of water and 25% ethylene glycol, with flow rate 16 liters per minute per BHE. The results show that, due to higher shrank spacing and higher Reynolds number, the annual performance of single U-tube BHEs is almost equal to that of double U-tube ones, while the grout thermal conductivity has a slightly more relevant effect. Replacing single U-tube BHEs with grout thermal conductivity 1 W/(mK) by double U-tube BHEs with grout thermal conductivity 1.6 W/(mK) yields about 1% reduction of the annual use of electric energy by the heat pump.