수직형 지중열교환기의 열확산 특성에 관한 연구

Abstract

ABSTRACT

A Study on the Characteristics of Thermal Diffusion with the Vertical-type Geothermal Heat Exchanger

Sun, Jong-Cheol Advisor. Prof. Kim, Byung-Chul Ph.D. Department of Mechanical Engineering, Graduate School of Chosun University

Recently the whole world are looking for a wide variety of ways to solve the problem of global warming caused by greenhouse gases. As part of that, ground source heat pump system(GSHP) has received a lot of attention as renewable energy facilities keeping pace with government's low-carbon green growth paradigm. Research has shown that domestically supply and installation capacity of GSHP continues to grow more than 1.5 times each year, passing over a total installed capacity of 104,922 kW at the end of 2008 and 307,501 kW in 2011. This growth is expected to continue for the future because the combination of social awareness on renewable energy and economic value of GSHP creates the synergies. GSHP is the heating and cooling system that largely consists of a heat exchanger and heat pump. It's the highly efficient system because it is not almost affected by the ambient temperature and takes advantage of the underground's constant temperature. It's environmentally friendly because it emits less the greenhouse gas such as carbon dioxide relatively compared to the existing air conditioning and heating equipment. Up to now, as to GSHP, the research and demonstration on the development mainly was carried out to improve the various underground heat exchanger applicability and thermal conductivity. To the optimal design of the vertical ground loop heat exchanger that mostly installed in the country, the accurate calculation of the effective thermal conductivity should be applied to its design. If the underground heat exchanger's interval is too close, heat pump performance that bumped to the limits of the heat capacity by long time operation, does not fulfill the role of heat sink and heat source. If the gap is too large, it will be disadvantageous economically because the installation of underground heat exchanger takes up a lot in a limited land area. Therefore, the distance between the ground heat exchanger installations is very important for the performance of the system. To improve the performance and ensure the reliability of the underground heat exchanger as a key element of the geothermal source heat pump system, the heat transfer characteristics of the underground heat exchanger should be reflected in the design. The distance between the boreholes is so important that the effect on the performance of ground heat exchanger is taken according to the nature of the thermal conductivity and thermal diffusivity in the process of absorbing heat from the ground or dissipating heat to the underground. Even now the analysis of heat transfer and thermal diffusivity in the direction of the radius of the bore hole is insufficient. In this study, to analyze the thermal conductivity calculated by performing In-situ ground thermal conductivity test, depending on the depth of the underground through the experimental device operated at the actual building, In first, by installing each 15 thermocouples according to a vertical depth of the Ground in the ambient (1.5 m), the surface, from 1 m below the surface to 10 m at the intervals of 1 m, and in 12 m, 15 m, 20 m places, the temperature variations of the each points are respectively measured at one-minute intervals. Secondly, by installing each 6 thermocouples from a point 10 meters underground to 2.5 m horizontally at intervals of 0.5 m in six places, the changes in temperature due to the thermal diffusivity by the distance and by the season are collected and analyzed to study the temperature distribution by distance between the operating underground geothermal heat pumps. Finally, by installing K-type thermocouples on the Heat Pump entrance & exit, the building entry & exit, the underground trench entry & exit (underground 1.5 m),and on the surface of each 10 m, 60 m, 150 m underground geothermal heat pump, this paper presents the basic data for the optimization of the underground heat exchanger distance using a DA-100 Data acquisition system. This paper shows the following results by measuring and analyzing the temperature distribution of the vertical ground heat exchanger. 1) On the in-situ thermal response test, the initial temperature of the soil in the ground for 50 minutes without heat with no difference between the Inlet and outlet of the circulation was 16.6℃ regularly. 2) On the in-situ thermal response test, to the average temperature change curve, the equation of the slope (Logarithmic time) was , and the slope value was 1.81. 3) Temperature changes of the ground heat exchanger inlet-outlet were measured and the Line-source model was applied to the calculation. The ground thermal conductivity value was 2.65 W/m∙K at the target area of the experiments. 4) The influence of the ambient over the underground temperature is significantly reduced between 1 m and 2 m of underground depth, is least affected from 10 m or more. 5) The annual average temperature 20 m below the surface was 16.4℃. Time leg was shown for the deeper we went below the surface the less affected by outside temperature, the later the effect of temperature changes appeared. 6) The depth of the trench pipe at least over 2 m below the surface is little affected by the ambient and the insulation is needed more under that depth. 7) The annual temperature change value at the point 0.5 m distant from the underground heat exchanger of the underground 10 m is less than 1.6℃ and at the point 2.5 m is 0.1℃. 8) If the interval between the bore holes of the underground heat exchangers is more than 5 m, the proximity has no influence.