STUDY OF HEAT INTERACTION BETWEEN A DOUBLE U-TUBE GROUND HEAT EXCHANGER AND THE GROUND

Abstract

In developing countries like India electricity consumption for indoor climate control such as space heating and cooling is increasing day by day due to the improved living conditions and increased commercial activities. Ground source heat pump (GSHP) systems have become popular in Western and European countries in the past decades. Few hundred meters below the ground the temperature almost remains constant irrespective of the atmospheric temperature and hence this can be used as sink/source. When ground is used as a sink/source compared to the atmosphere as used in the conventional space cooling/heating systems, the overall performance of these systems can be improved, thus resulting in electricity saving. Heat interaction between GSHP and the space to be conditioned takes place through ground heat exchangers. It is highly essential to understand the thermal behavior of these systems. As part of the present dissertation work, the thermal performance of double U-tube ground heat exchanger (GHE) also called borehole heat exchanger (BHE) is carried out. Based on a detailed literature survey, the governing equations for temperature distribution and heat transfer between the GHE and the ground are derived using thermal resistance concept. The required thermal properties of ground formation have been obtained from a thermal response test (TRT) carried at the 5 TR closed loop GSHP facility established at NETRA, NTPC, Greater Noida by the Department of Mechanical & Industrial Engineering, IIT Roorkee. Five double Utube BHEs with 120 m depth were used in the above GSHP system. The data obtained from test trial runs from the above GSHP system have been used as the reference data for computing the temperature distributions along the length of the four legs of BHE and the total heat interaction between the BHE and the ground. The GSHP system may be operating throughout the year to provide cooling and heating effect in the space and depending upon the heating/cooling load on the system, the inlet water temperature to the BHE may vary. Similarly, the borehole temperature also may vary for different seasons. In this dissertation work, inlet water temperature was assumed to vary from 34ºC to 40ºC during space cooling mode and 12ºC to 18ºC during space heating. The borehole temperature was assumed to vary from 26ºC to 30ºC for space cooling and from 22ºC to 26ºC for space heating. Results indicate that maximum temperature change takes place in the first U-tube resulting in about 77% heat transport whereas only 21% of heat transport takes place in the second U-tube