تحلیل عملکرد حرارتی پمپ حرارتی زمین‌گرمایی ادغام‌شده با پی ساختمان در تابستان

Geothermal energy has been provided to improve the energy performance of buildings with great support from the government in Korea. However, despite the many advantages of using a ground-coupled heat pump (GCHP) with geothermal energy, the high construction cost of the ground-coupled heat exchanger (GCHE) is the primary obstacle to prevent the supply and spread of GCHPs. In this study, in order to overcome the problems of the conventional GCHP, a GCHP integrated with a PHC (prestressed high-strength concrete) pile, which is used in the foundation of buildings, was introduced and its thermal performance was analyzed through experiments conducted in summer. To increase the thermal performance, a coil-type pipe was used. The PHC-pile-integrated GCHP was installed at a depth of 15 m. However, because it was installed in the beneath of the building, it was not largely affected by the outdoor temperature. The measured effective thermal conductivity was 3.69 W/m °C, which is similar to that of a conventional vertical GCHP. Also, the COP was determined to be 3.9–4.3, which is slightly lower than the conventional vertical GCHP. However, considering the fact that the expensive drilling cost could be mitigated by 83.7%, the thermal performances were satisfactory.

Recently in Korea, the energy consumption of buildings has rapidly increased. Technologies and policies for energy conservation to enhance the energy performance of buildings are being enacted with the great support of government. A representative policy promoted by the government is a building code, which has been enforced since 2004, which stipulates that 5% or more of new public building construction cost must be invested in renewable energy facilities. Due to this building code, the renewable energy market in Korea has increased. Along with this, many Korean engineers have tried to find and develop effective applications for renewable energy. The ground-coupled heat pump (GCHP), which uses geothermal energy, has especially attracted much attention. Geothermal energy is an environmentally friendly energy source. Because most energy used in Korea is imported from overseas, it is considered to be a strong alternative that can reduce energy consumption in the building sector [1] and [2]. Geothermal research is being conducted in other countries, as well as in Korea. Yang et al. introduced the theoretical background for a vertical GCHP and methods to evaluate its thermal performance. They stated that energy can be saved even in cold or hot areas using such a GCHP [3]. Esen et al. installed a horizontal GCHE in Turkey in an experiment and reported that it was more efficient than general heating systems in terms of coefficient of performance (COP) [4] and [5]. Petit and Meyer compared the thermal performances of a GCHP with an air source air conditioner, finding that a horizontal or vertical GCHP was more favorable in terms of economic feasibility [6] and [7]. However, GCHPs are still expensive when compared to other systems, and the majority of the energy and cost is consumed in the installing process of the ground-coupled heat exchanger (GCHE). Healy and Ugursal indicated that a GCHP was economically efficient compared to conventional systems. However the cost to install the GCHE is still high about 38% of the total cost [8]. Also, Genchi et al. reported that 87% of the amount of CO2 produced by installing the GCHE was generated during the drilling process [9]. There are many other researchers who also agree that the initial cost for a GCHP is high [10], [11], [12] and [13]. To mitigate high construction cost of a GCHP, this study introduces a PHC-pile-integrated GCHP that can reduce the costs generated during the drilling process and increases the efficiency of a GCHE. To analyze the thermal performance of the proposed GCHP, the effective thermal conductivity was analyzed through a thermal response test (TRT). Also, the COP of the GCHP was analyzed through an experiment within a house in summer. The analyzed results were compared with the findings of existing research for an evaluation.

This study introduced a GCHP system, which included a coil-type pipe in the PHC pile and analyzed its thermal characteristics through experiments. To do this, an underground heat conductivity experiment and a cooling experiment on an actual residential building were conducted. The findings are as follows: (1) The effective heat conductivity measured by the experiment using a TRT was 3.69 W/m °C. This value was shown to be at a similar level to the U-type GCHP previously measured by other researchers. (2) At the initial stage of the cooling experiment period, the input and output fluid temperatures of the GCHE slightly increased, but they became stable after 6 days. (3) The GCHE of this system is not buried deep compared to existing vertical GCHEs. However because it was installed beneath of the building, the fluid temperatures at the GCHE exit were not largely influenced by the outdoor temperature. (4) In a situation in which the indoor temperature of the residential building was held at 26 °C, the cooling energy reached a maximum of 3.9 kW and the electricity energy consumed by the heat pump was 0.06–1.0 kW. The COP was 3.9–4.3. This is slightly lower when compared to existing GCHP measurement data. However considering that the installation cost was reduced to 83.7%, the COP results can be considered to be satisfactory. (5) It is expected that in circumstances such as in Korea, where the building density is high, there is very limited ground to install a GCHE, and the construction costs for drilling are expensive, the results of this study can be helpful as a reference to increase the utilization and improve the design of the GCHP.