Performance Evaluation of Cooling-DHW Combined GSHP System Using Condensation Heat = 응축열을 이용한 냉방급탕복합형 지열히트펌프 시스템의 성능분석

다국어 초록 (Multilingual Abstract)

Nowadays, environment pollution and global warming is causing people to spend more time indoors, and the development of building-material technology has improved the insulation performance of building. Therefore, the cooling load and cooling time of buildings are increasing. Ground source heat pump (GSHP) system is one of the most efficient and clean methods for cooling and heating. That is because the energy source of GSHP is geothermal energy which is a clean energy and maintains a stable temperature throughout the year.
As a cooling system, GSHP delivers the heat from a condenser to ground during cooling time. However, after a cooling GSHP system operating for a long time, the ground temperature will increase. The increased ground temperature can affect the coefficient of performance (COP) of GSHP system. To keep the balance of ground temperature and COP of GSHP system during cooling time, the condensation heat from cooling GSHP can be used by other heating system before it if transferred to ground.
GSHP system can used as a domestic hot water (DHW) system, and DHW GSHP system absorbs heat from ground during heating water. However, DHW GSHP system is difficult to make high temperature hot water with geothermal energy, A favorable method to improve the performance of DHW GSHP system is to increase the heat source temperature.
Therefore, a cooling GSHP system combined with DHW heat pump (combined GHSP) is proposed in this study. In the combined GSHP system, the condensation heat of the cooling GSHP system can be used as the heat source of the DHW GSHP system. Therefore, the ground temperature will be balanced and the performance of the cooling and DHW heat pump of the combined GSHP system will improve during the system operating. To evaluate the performance of the combined GSHP system, experiments and simulations were conducted in this study. All the experiments and simulations were conducted in three operation modes: cooling mode, DHW mode and cooling-DHW mode. The ratio of cooling capacity of cooling heat pump to the heating capacity of the DHW heat pump was set to 2:1. The summary of this study is as follows:

(1) Mock-up experiments were conducted to investigate the effect of different ground temperatures on the performance of the GSHP system and to analyze the COP of the combined GSHP system. As the results of the experiments, when the heat source temperature increased, the COP of the cooling heat pump decreased, and when the heat source temperature increased, the COP of the DHW heat pump increased. In the combined GSHP system. The COP of the cooling heat pump of the combination system was 12.93% higher than that of the cooling GSHP system, and the COP of the DHW heat pump of the combination system was 15.47% higher than that of the DHW GSHP.

(2) Based on the experiments, simple zone simulations were conducted. In the cooling mode simulation, the ground temperature gradually increased after a long operating time of the GSHP, and COP of the cooling GSHP decreased. In the DHW mode simulation, the ground temperature and COP of the DHW GSHP decreased after a long operation time of the GSHP. The COP of the cooling heat pump of the cooling-DHW GSHP increased by 25.39% when compared with the COP of cooling GSHP. The COP of the DHW heat pump of the cooling-DHW GSHP increased by 21.82%, when compared with that of the DHW GSHP.

(3) The ambient temperature and ground temperature vary for different regions owing to the geographical location, and the cooling loads of buildings differ in different places. Therefore, the performances of the combination GHSP system under different weather conditions were analyzed by simulations. Seoul, Daejeon, Busan, and Jeju were selected through location and ground temperature surveys. The COP of combined GSHP when cooling heat pump and DHW heat pump operated simultaneously was analyzed. As the results, the COP of the GSHP under Seoul’s weather conditions was the highest compared with other three weather conditions, because cooling load and ground temperature under Seoul weather conditions are lowest. Therefore, the climatic conditions directly affect the cooling load, and the combined GSHP system working under the lower ground temperatures will allow for better efficiency.

국문 초록 (Abstract)

과학기술이 발전함에 따라 건축의 단열성능이 점점 증가되고 있고, 환경오염, 지구 온난화 등 환경문제로 인해 사람들의 재실 시간이 증가되는 실정이다. 지열히트펌프 (Ground source heat pump; GSHP) 시스템은 온도가 일정하게 유지되는 지열을 이용함으로써 상대적으로 높은 성적계수 (Coefficient of performance; COP)를 가지고 있고 건물의 냉, 난방 시스템으로 적용되는 빈도가 늘어나고 있다. 하지만 증가되는 냉방부하로 인해 GSHP를 냉방 시스템으로 사용 할 때에는 운전시간이 증가되어 지중으로 방출되는 열량이 많아 일정하게 유지되는 지중온도가 상승된다. 열원의 온도가 증가되면 냉방용 GSHP 시스템의 성능에 영향을 미친다. 한편, GSHP는 급탕 시스템으로 사용할 수 있는데 연중 평균온도가 약 15 °C (한국)인 지중열원을 이용하여 급탕을 진행하면 높은 온도의 급탕수를 만드는데 어려움이 있다.
이러한 문제점들을 고려하면서 본 연구에서는 냉방과 급탕이 동시 가능한 냉방급탕복합형 GSHP 시스템을 제안하였다. 제안된 복합형 GSHP에서는 냉방용 히트펌프와 급탕용 히트펌프로 이루어진다. 기존 냉방만 진행하는 GSHP 시스템은 응축기에서 방출되는 응축열을 순환수를 통해 지중열교환배관을 거쳐 지중으로 방출하는데, 제안된 복합형 GSHP 시스템은 이 부분 응축열을 흡수한 순환수를 급탕용 히트펌프로 보내 급탕용 히트펌프의 증발기와 열량교환을 진행한다. 급탕용 히트펌프에서 열량을 방출하고 순환수의 온도는 일정하게 감소되고, 이렇게 강온(降溫)된 순환수는 지중열교환기로 유입한다. 그래서 제안된 복합형 GSHP 시스템은 냉방만 진행하는 GSHP 시스템에 비해 냉방모드 운전을 장시간 수행할 때 지중온도가 증가되는 부분을 대폭 완화시키는 결과를 얻을수 있어 냉방 히트펌프의 COP를 증진시킴과 동시에 비교적 높은 온도의 응축열을 이용하는 급탕용 히트펌프의 COP도 개선시킬 수 있다. 지중온도가 지열히트펌프시스템의 성능에 미치는 영향을 파악하고 제안된 냉방급탕복합형 지열히트펌프 시스템의 성능을 평가하기 위하여 다음과 같이 연구를 진행하였다.

(1) Mock-up 실험을 통해 지중온도가 냉방용 GSHP 급탕용 GSHP의 성능에 미치는 영향을 분석하였고, 냉방용 GSHP와 급탕용 GSHP의 COP 산출하고 각각 복합형 GSHP 시스템의 냉방 히트펌프와 급탕 히트펌프의 COP와 비교를 통해 복합형 GSHP 시스템의 성능을 분석하였다. 실험 결과에 따르면, 지중온도가 증가될수록 급탕용 GSHP의 성능이 증가되고, 지중온도가 감소될수록 냉방용 GSHP의 성능이 감소된다. 또한 열원온도가 15 °C인 조건에서 복합형 GSHP 시스템의 냉방용 히트펌프의 COP는 냉방용 GSHP보다 12.93% 증가되었고, 복합형 GSHP 시스템의 급탕용 히트펌프의 COP는 급탕용 GSHP보다 15.47% 증가되었다.

(2) Mock-up 실험을 바탕으로 지중온도가 GSHP 시스템에 미치는 영향을 시뮬레이션으로 분석하고, 장시간 운전으로 인해 냉방용 GSHP, 급탕용 GSHP 및 복합형 GSHP를 각각 사용 시 지중온도의 변화 패턴을 파악하고, 각 GSHP의 COP를 비교분석하였다. 시뮬레이션 결과에 따르면, GSHP 시스템 장시간의 운전을 거쳐 냉방을 진행 할 때 지중온도는 상승되곤, 급탕을 진행 할 때 지중온도는 감소된다. 복합형 GSHP 시스템이 장시간 운전 할 때 지중온도가 역시 증가되는 양상을 보이지만 냉방용 GSHP 시스템에 비해 지중온도 상승되는 폭이 감소되었다. 그리고 복합형 GSHP 시스템의 냉방용 히트펌프의 COP는 냉방용 GSHP보다 25.39% 증가되었고, 복합형 GSHP 시스템의 급탕용 히트펌프의 COP는 급탕용 GSHP보다 21.82% 증가되었다.

(3) 건물이 위치해 있는 지역에 따라 기상데이터 및 지중온도가 달라진다. 그래서 본 연구에서는 지중온도의 조사를 통해 서울, 대전, 부산, 제주 4개 지역을 선정하여 동일 건물모델로 냉방용 GSHP, 급탕용 GSHP 및 복합형 GSHP의 COP를 분석하면서 기온, 일사 및 지중온도가 시스템의 성능과의 관계를 파악하였다. 4개 지역에서 평균 기온이 낮은 지역의 지중온도는 상대적으로 낮게 나타났다. 지역별 복합형 GSHP 시스템을 사용 시 각 히트펌프의 COP는 지중온도에 영향을 받는다. 지역별 복합형 GSHP 시스템의 성능 분석한 결과, 지중온도 및 평균 기온이 낮은 지역에서 냉방용 히트펌프의 COP가 높게 나타났고, 급탕용 히트펌프의 COP 증가율이 제일 높게 나타났다.

목차 (Table of Contents)

• I. Introduction 1
• 1.1 Background 1
• 1.2 Objectives and scopes of the study 6
• II. Geothermal theory and research trend on GSHP system 11
• 2.1 GSHP theory 12
• 2.1.1 Geothermal energy 12
• 2.1.2 Heat pump and GSHP system 15
• 2.1.3 Energy efficiency of GSHP system 21
• 2.2 Literature survey of GSHP system 25
• 2.2.1 Performance characteristics of GSHP system 25
• 2.2.2 Performance enhancement of GSHP system 28
• 2.3 Summary 36
• III. Combined GSHP system description 38
• 3.1 Definition of combined GSHP system 40
• 3.1.1 GSHP system under cooling mode 40
• 3.1.2 GSHP system under heating mode 42
• 3.1.3 Combined GSHP system 43
• 3.2 Evaluation method of combined GSHP system 48
• 3.2.1 Methodology of validation 48
• 3.2.2 Analyzed parameters 50
• 3.3 Summary 54
• IV. Experimental analysis of the performance of the combined GSHP system 56
• 4.1 Experimental setup 58
• 4.1.1 Description of the experimental systems 58
• 4.1.2 Experimental conditions and measurement 61
• 4.2 Experimental evaluation on GSHP performance under cooling mode 65
• 4.2.1 Cooling mode experiments overview 65
• 4.2.2 Results and discussion of cooling mode experiments 68
• 4.3 Experimental evaluation on GSHP performance under DHW mode 73
• 4.3.1 DHW mode experiments overview 73
• 4.3.2 Results and discussion of DHW experiments 75
• 4.4 Experimental evaluation on GSHP performance under cooling-DHW mode 80
• 4.4.1 Cooling-DHW mode experiment overview 80
• 4.4.2 Results and discussion of cooling-DHW mode experiment 81
• 4.5 Summary 88
• V. Simulation analysis of the performance of the combined GSHP system 90
• 5.1 Simulation overview 92
• 5.1.1 Determination of cooling load and DHW load 92
• 5.1.2 Determination of GHX length 94
• 5.1.3 Setting of the simulation conditions 98
• 5.2 Simulation evaluation of GSHP performance under cooling mode 102
• 5.2.1 Change of ground temperature 103
• 5.2.2 Performance of cooling GSHP 104
• 5.3 Simulation evaluation of GSHP performance under DHW mode 108
• 5.3.1 Change of ground temperature 109
• 5.3.2 Performance of DHW GSHP 110
• 5.4 Simulation evaluation of GSHP performance under cooling-DHW mode 113
• 5.4.1 Change of ground temperature 114
• 5.4.2 Cooling and heating capacity of combined GSHP 115
• 5.4.3 Results of Electricity consumption of combined GSHP 117
• 5.4.4 COP of combined GSHP 119
• 5.5 Summary 122
• VI. Performance analysis under different weather conditions 124
• 6.1 Simulation overview 126
• 6.1.1 Determination of the ground temperature 126
• 6.1.2 Determination of the weather data 131
• 6.1.3 Simulation Setting 134
• 6.2 Results and discussion 137
• 6.2.1 Inlet and outlet temperature of GHX under different weather conditions 137
• 6.2.2 Cooling and heating capacity under different weather conditions 139
• 6.2.3 Electricity consumption under different weather conditions 142
• 6.2.4 COP under different weather conditions 146
• 6.3 Summary 154
• VII. Conclusions and future work 156
• 7.1 Conclusions 156
• 7.2 Future work 159
• Acknowledgment 161
• References 163