STUDY OF THERMAL INTERACTION OF BOREHOLE HEAT EXCHANGERS WITH GROUND DURING GROUND SOURCE HEAT PUMP APPLICATIONS

Abstract

Energy consumption for indoor climate control systems has increased to one-third of total worldwide energy consumption due to fast civilization and increased level of standard of living of people. The commonly used indoor climate control systems such as air-conditioning units and centralized air-conditioning systems reject/extract heat to/from ambient whose temperature varies continuously in a diurnal cycle. Ground source heat pump (GSHP) system is a promising technology that makes use of the ground at a depth of about 100 m as sink/source. The temperature of ground at this depth almost remains constant throughout the year at a given location. Hence, when heat interaction takes place at constant temperature during heat rejection/extraction, the performance of the system increases, thereby electricity power consumption decreases. Borehole heat exchangers are vertical single or double U-tube pipes installed inside the borehole to reject/extract heat to/from the ground using water as the heat transfer fluid. Effective heat interaction between BHEs and the ground determines the overall performance of a GSHP system. A detailed literature review carried out as part of the present research work indicates that only very few studies are available on the investigation of effectiveness of BHEs during real time operation of GSHP systems for space cooling and heating applications. For double U-tube BHEs, the effect of series and parallel for configurations on effective heat interaction with the ground needs to be fully understood. At the end of operation of a GSHP system, the ground has to regain its initial temperature after absorbing/losing heat from/to the BHEs and this phenomenon is called soil recovery. It is important to understand the nature of soil recovery after cooling/heating process, so that the GSHP system operates without compromising on its performance during the next cycle of operation. Based on a detailed literature review the following objectives are finalized for the present dissertation work. The transient variation of effectiveness of BHEs has to be analyzed in detail during the space cooling and heating mode operation of the GSHP system. With the help of experimental data obtained, it is essential to estimate any other performance parameters for the BHEs so that the heat interaction with the ground is fully understood. The working of a BHE is influenced by many design, operating parameters and thermo-physical properties of the ground formation in the vicinity of BHEs. Hence, optimization of such controlling parameters on the effectiveness of BHE is considered as one of the objectives of the present dissertation work. It is also proposed to explore the performance of a retro-fitted commercial split air-conditioner with a slinky type of ground heat exchanger for space cooling and heating applications.