Design and Operating Strategy of A Novel Hybrid Ground Source Heat Pump System|RISS 상세보기

다국어 초록 (Multilingual Abstract)

A ground-source heat pump (GSHP) system coupled with a borehole heat exchanger (BHE) is a widely used renewable energy system that supplies heating and cooling energy to buildings. As the underground homeothermic zone located 20–500 m below the ground surface exhibits a stable temperature to 10–15 °C throughout the year, the GSHP system can stably supply the heating and cooling energy to buildings with high efficiency. The formation of a homeothermic zone stems from the large heat capacity of the ground, which encounters challenges in recovery after the temperature varies. Therefore, the GSHP system can be beneficially used in an environment in which the cooling and heating balance of the building is appropriately maintained.
Most buildings exhibit a thermal load imbalance between cooling and heating. The ground thermal imbalance results in long-term ground heat accumulation and causes long-term variations in ground temperature. In such scenarios, the temperature of the underground heat source varies continuously upon using the GSHP system, and its efficiency decreases inversely with the variations.
Recent studies suggest various methodologies for resolving the ground thermal imbalance issues in GSHP systems. In particular, studies on hybrid GSHP (HyGSHP) systems report that the HyGSHP system can mitigate the ground thermal imbalance presented in GSHP systems. In addition, they demonstrate that the HyGSHPs can improve the COPs of the heat pump, and simultaneously, reduce the total required length of BHEs compared to GSHP systems. Nonetheless, most studies on HyGSHP employed the existing GSHP system analysis method to analyze their HyGSHP systems.
In the HyGSHP system, complex behaviors occur through the heat carrier fluid shared with the hybrid heat source and BHEs. Thus, the existing GSHP analysis method is not preferred in the HyGSHP system. Owing to these limitations, the existing research on HyGSHP systems has been restricted to parallel hybrid configurations that can assign the energy to be transferred to the BHEs, such that the hybrid heat source can supplement a portion of the heating or cooling energy demand in buildings. In particular, analyzing a HyGSHP system with a high degree of freedom requires a methodology that can dynamically calculate the complex thermal behavior of the heat source side.
This study presents the analysis results of a HyGSHP system high degree of freedom, which is beyond the ability of the existing HyGSHP system analysis method reported in previous studies. In this dissertation, a novel HyGSHP system is proposed using an hourly simulation-based analysis method. This study selected a small-scale unused groundwater resource corresponding to 10% of the building peak load and capable of supplying both cooling and heating as a supplemental hybrid heat source.
The fundamental objective of this dissertation is to mitigate the ground thermal imbalance and preclude long-term ground temperature variations using the proposed HyGSHP system. Based on the hourly simulation and optimization technique, the restraining ability of the proposed HyGSHP systems to the long-term ground temperature was evaluated. In addition, the impact of the long-term ground temperature restraining on the BHE length and heat pump COPs are discussed including its economic cost analysis. Finally, the proposed HyGSHP system analysis results were validated via inter-model comparison with the FLS-based analysis method provided in the GLHE software.

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목차 (Table of Contents)

Chapter 1. Introduction 1
1.1 Research background 1
1.2 Research process and objectives 7
Chapter 2. Literature Review 13

Chapter 1. Introduction 1
1.1 Research background 1
1.2 Research process and objectives 7
Chapter 2. Literature Review 13
2.1 Review of long-term ground temperature variations in GSHP systems 13
2.2 Review of studies for mitigating long-term ground temperature variations in GSHP systems 18
2.3 Review of HyGSHP system analysis method 24
2.4 Chapter summary 29
Chapter 3. Proposition of Groundwater-Coupled HyGSHP Systems 31
3.1 Target HyGSHP systems 33
3.2 Permanent-dewatering modules 37
3.2.1 Groundwater resources in Korea 39
3.2.2 Permanent dewatering module 47
3.2.3 PD module capacity (kW) according to groundwater inflows 52
3.3 Sample building load 56
3.4 Heat pump 58
3.5 Borehole heat exchanger 63
3.6 Source-side flow rate control strategies 71
3.6.1 GSHP: Reference case 73
3.6.2 HyGSHP: Parallel case 74
3.6.3 HyGSHP: Serial cases 76
3.7 Chapter summary 79
Chapter 4. Evaluation Method of Proposed HyGSHP Systems 81
4.1 Evaluation of long-term ground temperature variations by proposed HyGSHP systems 83
4.2 Mitigation effect of long-term ground temperature variation on BHE length and heat pump COP 87
4.3 Economic evaluation of proposed HyGSHP systems 96
4.4 Validation of proposed HyGSHP systems 99
4.5 Chapter summary 103
Chapter 5. Evaluation of Proposed HyGSHP Systems 107
5.1 Results of long-term ground temperature evaluation 109
5.2 Comparison of BHE length and heat pump COPs 114
5.2.1 BHE total length reduction 114
5.2.2 Heat pump COP evaluation 126
5.3 Life cycle cost evaluation results 132
5.4 Validation results of proposed HyGSHP systems 136
5.5 Chapter summary 143
Chapter 6. Conclusions 147
Bibliography 152
Appendix A PD module sizing method 159
Appendix B Experimental data of heat pump cooling COPs 162
Appendix C Equivalent bore field modification method for DST model 165
Appendix D Example of BHE sizing against long-term groundwater inflow uncertainty 168
Appendix E Detailed data of heat pump COP evaluation results 170
E.1 Heat pump COPs without BHE total length reductions 170
E.2 Heat pump COPs with the BHE total length reductions 176

참고문헌 (Reference)

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