THERMAL PERFORMANCE STUDY OF BOREHOLE HEAT EXCHANGER

Abstract

The increasing pollution, global warming, and simultaneous increase in the demand of comfort among population has made us think about the available resources. The focus has shifted towards the renewable resources. We can reduce the consumption of fossil fuels for the purpose of space cooling and space heating by using ground. Ground can be utilized as a source or sink in applications like space heating and cooling using borehole heat exchangers and in energy independent buildings. To transfer/take heat into/from the ground effectively, ground property like thermal conductivity is to be obtained. The main purpose of this report is to conduct experimental simulation studies to understand the thermal performance of single and double U-tube borehole heat exchangers (BHEs) during space cooling applications. Initially the thermal conductivity of ground using line source model has been determined through a thermal response test (TRT) by injecting heat into the ground continuously for 50 hours. An experimental facility for TRT consisting of a water heater, flow meter, temperature sensors and piping system, was fabricated. The value of thermal conductivity obtained is 2.70 W/m-K. The TRT test results obtained compare reasonably well with the data obtained from NETRA, Greater Noida set-up and Architecture & Planning Department GSHP set-up. In the final step of this work, experimental simulation is conducted for part load condition for space cooling application by injecting 3.8 kW of heat through the heat transfer fluid in the borehole heat exchangers for 10 hours duration. Soil recovery experiment was performed for 14 hours duration after the end of space cooling simulation experiment. The effect of mass flow rate on thermal performance of the single U-tube and double U-tube BHEs were carried out for three different mass flow rates of the heat transfer fluid. Using a design of experiments method, 11 experimental simulation trials were conducted for space cooling application. From the experimental data obtained, it is noticed that the effectiveness of series type double U-tube BHE is found to be higher than the parallel type. Double U-tube BHEs show 8% higher effectiveness compared to single U-tube BHE for the same heat load condition.