STUDY OF THERMAL PERFORMANCE OF A GROUND SOURCE HEAT PUMP SYSTEM

Abstract

Space heating and cooling appliances consume a significant amount of energy in buildings. Western and European countries have been using gas, oil or coal fired boilers to meet their energy demand for space heating applications. In recent decades, ground source heat pump (GSHP) systems have become popular in these countries for space heating applications because they make use of ground energy, which is green and sustainable. With the help of a heat pump, a GSHP transports heat from the warmer ground to the interior of the building during winter season. Hence with this technology one can obtain heat energy equivalent to at least three times the work input to the system. Thus the use of GSHP technology results in significant reduction in the use of centrally generated electricity and gives rise to an indirect benefit of reduction in CO2 emission from thermal power plants. Ground heat exchangers (GHX) are used to absorb heat from the ground for onward transmission to the room through the heat pump. This technology has an important Thermodynamic advantage in that as the heat is absorbed from the ground which is at a higher temperature than the ambient, its performance index for transporting heat is higher than a conventional heat pump system which transports heat from the low temperature ambient in winter. A detailed literature survey indicates that though GSHP technology is well established in Western and European countries, this technology has to be reformed for applications in Asian countries which demand both heating and cooling modes of operations. In the recent past China, Japan and South Korea have focused on the development of GSHP technology for space heating and cooling applications. China had used the GSHP technology to supply complete heating and cooling demand of Beijing Olympic village in 2008. As far as the Indian climatic and geographic conditions are concerned, only northern region of the country requires space heating during winter for a couple of months, however, the demand for space cooling spans through a longer period. Indian cities located near the Himalayan region experiences severe winter compared to other northern states of India. From the literature it is found that there is a good scope for implementing the GSHP technology for northern region of India both for space heating and cooling applications. This can be achieved only after addressing some technical challenges such as understanding the thermal response of ground for both heat extraction and injection, performance of ground heat exchangers for both heating and cooling modes of operation, thermal performance of GSHP system in both modes of iv operations. With the motivation for the present research work derived from the above opportunities, the present research has been carried out with objectives such as, conducting a detailed study to estimate the amount of saving in electricity power and indirect benefit of reduction in CO2 emission, understanding the thermal properties of ground for heat interaction during winter and summer, performance of GHX and GSHP during space heating and cooling applications, optimization of ground heat exchanger length and COP of GSHP system for both heating and cooling application, exergy analysis of the GSHP system and finally a study on technical and economical viability of GSHP for Indian climatic conditions. The objectives of the research work are accomplished by experimental and theoretical approach. Initially a study to estimate electricity saving and CO2 emission reduction potential was carried out for ten states in northern India. In order to investigate the thermal resistance of borehole heat exchanger and thermal conductivity of soil in Roorkee weather conditions, a thermal response test (TRT) set up has been fabricated and installed in Mechanical & Industrial Engineering Department of IIT Roorkee, Roorkee. A GSHP experimental set up has been fabricated using a 1.5 ton heat pump and a U-tube ground heat exchanger buried at a depth of about 3.5 m from the surface. In this set up, provisions have been made to conduct thermal performance tests on the GHX. In order to understand the thermal behavior of soil during heat extraction and injection, thermocouples are installed near the GHX as well as far-off from the GHX. Theoretical study on GSHP system for ten Indian states shows that by shifting from conventional electric heating and air conditioner to GSHP, India can avoid emission of 1.2 to 14.2 million ton of CO2 into atmosphere and addition of 187 MW to 2141 MW power plant in a year. Experimental studies on single and double U-tube GHX were carried out at BRGM, Orleans, France. The performance of single U-tube heat exchanger in heating and cooling mode is less by 26 % and 30 % respectively compared to double U-tube heat exchanger. Thermal response tests were carried out at IIT Roorkee during summer and winter with each test being conducted continuously for a duration of 50 hours. Results obtained for Roorkee weather conditions indicate that the average thermal conductivity of the ground during summer was 25 % less than the value obtained for winter. The borehole thermal resistance is found to be 30 % more for the test conducted during summer than in winter. v The performance of GSHP was studied during summer and winter by carrying out five trials for each season and each trial run was performed for a duration of eight hours. Measurements of temperature of GHX fluid, mass flow rate, inlet and outlet temperatures of evaporator and condenser were monitored during the test runs. Temperature variations near GHX in the ground were also monitored and plotted. Using the experimental data the COP of heat pump and GSHP were obtained. The average COP of the GSHP system obtained during space cooling was found to be 21 % less than the value obtained for heating mode operation. Using Taguchi method and utility concept the optimum length of GHX was determined to achieve maximum COP and minimum thermal resistance. The optimum COP of GSHP for both heating and cooling modes of operations was obtained using the above optimization technique and it is found that the COP was 8 % higher for cooling and 15 % higher for heating compared to the COP estimated using the Taguchi method. A detailed exergy analysis of different components and whole of GSHP indicates that the exergy efficiency of GSHP is found to be 6 % more for heating mode compared to cooling mode. The technical and economical viability study on the application of GSHP for ten Indian cities located in and around the Himalayan region was carried out using RETScreen energy tool software. Among the ten cities considered, with 0 % subsidy, Darjeeling has the lowest payback period of 5.2 years and Delhi has the higher payback period of 23.2 years.