EXPERIMENTAL STUDY ON THERMAL PERFORMANCE OF BOREHOLE HEAT EXCHANGER FOR SEASONAL VARIATIONS

Abstract

Geothermal energy, which is renewable source of energy, can be utilized by means of geothermal heat pumps or ground source heat pumps (GSHP) which are considered as a common solution for space heating/cooling in many European countries. It can cover a wide range of energy demand, from small residences to large commercial buildings. The most important part of a GSHP, both thermal and economic point of view, is the ground heat exchanger,. which consists of a closed loop of pipes buried in the ground. Borehole Heat Exchangers (BHEs) are the most frequently adopted solution for ground coupled heat pump applications. They require for installation, a small ground space and are more effective as compared to horizontal ground heat exchangers. A careful design analysis is needed to either assure long time performance or reduce the payback period, both parameters related to overall BHE length. However, for both types i.e., horizontal as well as vertical ground heat exchangers, installation cost is very high. In the present report, fabrication and installation details of a vertical ground heat exchanger is given which has been given a slight modification by using a copper loop for joining two pipes at the bottom. The length of vertical ground heat exchanger has been taken equal to 3 m and thermal response test has been carried out for winter as well as for summer season for evaluation of ground thermal properties and to seek any change, if observed, for in-situ ground thermal properties for Roorkee (Uttrakhand). Four tests trials were carried out and very interesting results were obtained from the tests. A significant variation of soil thermal conductivity was observed for winter and summer. Assessment of ground thermal properties has been carried out by slope determination method and curve fitting method. The results obtained from these two methods are found to be coming very close to each other. For instance, first thermal response test conducted in April month results soil thermal conductivity equal to 3.68 and 3.7 W/m-K for slope determination method and curve fitting method respectively. The borehole thermal resistance was found to be slightly smaller for summer season as compared to its value for winter. Apart from the thermal properties of ground formation, it was observed that the copper loop played a significant role in heat transfer from heat carrying water to the ground. With this observation it can be said that incorporation of such loops can greatly enhance the total heat transfer between borehole heat exchanger and ground formation surrounding the borehole and this will definitely lead to cost cutting by reducing the length of borehole.