현장열응답시험을 이용한 지중열교환기 열성능 평가에 관한 연구 = A study on the thermal performance evaluation of a borehole ground heat exchanger using in-situ thermal response test|RISS 상세보기

다국어 초록 (Multilingual Abstract)

The geothermal energy is one of the most reliable and stable renewable energy resources in the world. During the last twenty years, commercial applications of the geothermal energy have been noticeably increased by the development of various types of ground heat exchangers (GHX) coupled with the ground source heat pump systems (GSHP).
The vertical closed U-loop GHX is the most widely chosen and applied to the ground heat exchanger. In the vertical U-loop GHX, a circulating fluid extracts heat from or injects it into the ground as the fluid travels down one leg of U-tube and returns up the other leg. The space between the U-tube and the borehole is usually backfilled with grout.
Standing column wells (SCW) have been in use in limited numbers since advent of SCW geothermal systems are recently receiving much more attention because of its high heat capacity and improved overall performance in the regions with suitable hydrological and geological conditions. Standing column well is configured with a vertical borehole filled with ground water up to the level of water table. A pump submerged in the well circulates the water through the borehole and GSHP cycle.
Thermal response tests (TRT) are frequently used to determine thermal properties such as thermal conductivity and borehole thermal resistance, which are needed to size ground heat exchangers for commercial ground coupled heat pump systems. A defined heat is added or extracted from the ground in the test and resulting water temperature variations are measured at the inlet and outlet of ground heat exchangers for a period of time.
In this study, the in-situ thermal response test associated with a line source method is applied to predict the thermal performances of various types of the vertical closed U-loop ground heat exchangers and staning column wells currently applied in Korea. a defined heat load is put into the borehole and the resulting temperature changes of the circulating water are measured for a period of time. A line source method is applied to estimate the effective thermal conductivity and the borehole thermal resistance. Results are presented in terms of design configurations and operating parameters including the tube diameter, number of tubes and borehole depth, heat injection rate, water flow rate, bleeding rate and filler for SCW GHX.
TRT coupled with a line source method is also applied to evaluate thermal characteristics of the straight horizontal GHX. Load tests of straight horizontal GHX are also performed to examine the daily variations of ground and fluid temperatures associated with daily intermittent operation of GSHP.
The thermal performances of vertical GHX are considerably enhanced with the increase of tube surface area(tube diameter, number of tubes and borehole depth). Heat injection rate and water flowrate has little influence on the ground thermal conductivity. Of the borehole thermal resistance components evaluated for the vertical GHX the grout thermal resistance is the most governing one in the borehole heat transfer(more than 65% of the total borehole resistance).
The SCW which is configured such that return pipe exit is located at the bottom well and the pump suction at the upper well, the thermal conductivity increases by about 11.7%, compared to the case of opposite configuration. Of operating parameters tested for SCW, bleed is the most significant one for the improvement of the well performance for a given heat load. Filler inserted in the well provides a considerable enhancement of the thermal conductivity, but the borehole thermal resistance also increases. Effects of heat injection rate and flowrate on the effective thermal conductivity are relatively small, while borehole thermal resistances considerably decrease with the increase of heat injection rate or flow rate.
The variations of ground thermal conductivity of horizontal GHX during one year are relatively small with the range of 1.41 ～ 1.64 W/m․K, and the maximum and minimum values appear in December and May, respectively. Load tests with heat injection rate of 6.0 kW for 10 hours per day to horizontal GHX during 12 days were performed in December, September and June, and resulted in a ground initial temperature rise of 1.21 ℃, 3.14 ℃, and 4.31 ℃ during these days, respectively.

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