Thermal Performances of Three Types of Ground Heat Exchangers in Short-Time Period of Operation

The thermal performances of three types of ground heat exchangers (GHEs) installed in a steel pile foundation including double-tube, U-tube and multi-tube have been investigated in short-time period of operation. The performance of the GHEs descends gradually due to the heat buildup in the surrounding ground with operating time in continuous operation. Alternative operation modes (cooling, heating, and hot water heating) over a short-time period of operation for ground source heat pump (GSHP) system can be alternative solution to increase its performance. Experimental study was carried out in discontinuous operation in 2 hours operation with constant interval of off-time. As a result, the heat exchange rate in the short-time period of operation in discontinuous operation increases of 53.1 % for double-tube, 33.3 % for U-tube, and 29.1 % for multi-tube compare with in continuous operation. Geothermal energy system is receiving interest as environmentally friendly energy source with wide range of applicability such as space heating and cooling, hot water heating and applications in the agricultural field. The wellknown application is for space heating and cooling in residential and commercial building with using ground source heat pump (GSHP) systems. Ground heat exchanger (GHE) is used in this system to exchange heat with the ground. The vertical type of GHE is widely used in the GSHP system due to the higher thermal performance than the horizontal type. The thermal performance of the GHE is an important subject in GSHP system design. A number of research have investigated the thermal performance of GSHP system with many design and simulation techniques using experimental, detailed analytical or numerical methods. The performance of the GHEs descends gradually due to the heat buildup in the surrounding ground with operating time in continuous operation. Cooling/heating alternative operation modes in short-time scales improve the performance of the GSHP system. The heat transfer behavior in alternative operation modes (heating and cooling modes) investigated in short-time scale and the temperature distribution of borehole field was analyzed using finite element method (Cui et al., 2008). Short-time analysis of the GSHP system was discussed in many researches. Yavuzturk and Spitler (1999) use numerical heat diffusion problem in this analysis, Sutton et al. (2002) and Lamarche and Beauchamp (2007) proposed analytical solutions. In applying the GSHP system in alternative operation modes, it is crucial to investigate the thermal performance in short-time period of operation. The present research investigated the thermal performance of three types GHEs installed in a steel pile foundation in short-time period of operation. The temperature distribution of borehole field is shown from the experimental data. The thermal performances of the GHEs in short-time period of operation in discontinuous operation are compared with that of in continuous operation.