隨著全球能源需求日益增長以及環保意識的提升，低溫熱發電系統由於可增加能源運用效率和減少化石能源的使用，而成為各國的發展重點。近年來，穿臨界二氧化碳循環於低溫工業廢熱以及地熱的應用逐漸引起各界關注，因其擁有相當優異的能源轉換效率，且與有機朗肯循環相較之下較為環保。然而，文獻指出穿臨界二氧化碳循環系統尚在發展階段並未商業化，在所有主要元件中，轉動元件 (Turbomachinery) 的性能對整體系統表現有最為顯著的影響，且其發展的挑戰性也最高。因此，本研究旨在提供開發者針對穿臨界二氧化碳循環的全面性參數分析，以利縮短其商業化進程。於是，本研究針對轉動元件的性能對穿臨界二氧化碳循環的系統表現進行敏感度分析，主要研究的循環結構為基本型及回熱型 (recuperative) 朗肯 (Rankine) 循環。本研究主要以MatLab®及NIST REFPROP®資料庫做為參數最佳化分析工具，所分析的操作條件涵蓋8 MPa至30 MPa的工作壓力、120oC至180oC的熱源溫度以及65% 至95% 的轉動元件效率。此外，本研究也針對穿臨界二氧化碳循環及有機朗肯循環的輸出功及熱回收效率進行比較，所比較的有機流體為R245fa、R152a、R600a、R1234ze(E)以及R1234yf。研究結果顯示，在穿臨界二氧化碳循環中，膨脹機 (Expander) 比起泵 (Pump) 對系統性能有較顯著的影響。隨著轉動元件效率提升或是熱源溫度增加，系統性能可獲得提升，然而設備成本也會因較高的工作壓力而增加。本研究也利用熱源和工作流體的質量流率比來推估熱交換器的體積和潛在成本，整體來說，熱源溫度增加會使得回熱型穿臨界二氧化碳循環的優勢更為顯著，同時也會降低轉動元件效率對回熱器(Recuperator) 效率的影響。藉由此研究所提供之多種操作條件下的穿臨界二氧化碳循環性能表現，開發者可依據其實際面臨到的操作情況或特定需求而得到一最具經濟效益的設計點。而在與有機朗肯循環的比較研究中，穿臨界二氧化碳循環在低溫環境下不論是在輸出功亦或是熱回收效率上皆有較佳的表現，而在高溫環境中，穿臨界二氧化碳循環有較低的膨脹機出口體積流率，使其與有機朗肯循環相比下具有較小的元件體積和佔地成本，或可彌補其在熱力性能上的不足。

As global energy demands surges and the environmental awareness rises, the development of low-temperature heat-to-power technologies has become a crucial objective for nations worldwide. These technologies not only increase energy conversion efficiency but also reduce fossil fuel consumptions. In recent years, transcritical carbon dioxide (T-CO2) cycles have drawn attentions in exploiting low-temperature heat sources, including industrial waste heat and geothermal energy. This is attributed to their high energy conversion efficiency and environmental friendliness compared to the organic Rankine cycles (ORCs). However, T-CO2 cycles have yet to be commercialized according to literature. Within the T-CO2 cycles, the turbomachinery efficiencies have been the greatest influence on the overall system performance, and also the biggest developing challenge. This study seeks to shorten the T-CO2 cycle commercialization process by offering a comprehensive parametric analysis. To this end, a sensitivity analysis of T-CO2 cycles regarding the turbomachinery efficiencies is carried out. The Rankine cycles with and without recuperation are investigated. Numerical parametric optimization using MatLab® and NIST REFPROP® is applied to the T-CO2 cycles under various working conditions, including 8 MPa to 30 MPa working pressures, 120oC to 180oC heat source temperatures, and 65% to 95% turbomachinery efficiencies. In addition, a comparative study between the T-CO2 cycle and the ORCs using R245fa, R152a, R600a, R1234ze(E), and R1234yf, has been conducted. The net power output and heat recovery efficiency of both systems are compared under various working temperatures. Results show that, in the T-CO2 cycles, the expander plays a more significant role on improving the system performance compared to the pump. As the system performances increase with increasing turbomachinery efficiencies or heat source temperatures, the equipment costs increase due to the higher optimal working pressure. The mass flowrate ratio of heat source to working fluid has been analyzed for estimating the heat exchanger size and costs in the T-CO2 cycles. In general, as the heat source temperature increases, the advantages of the recuperative T-CO2 cycle are more pronounced, meanwhile, the turbomachinery efficiencies become less influential on the recuperator effectiveness. The comprehensive performance analysis of T-CO2 cycles helps developers to arrive at a more cost-effective design with respect to the actual situation encountered or to the specific preferences. Finally, as compared to the ORCs, T-CO2 cycle is more recommended for lower-temperature environment due to its dominant advantages in both net power output and heat recovery efficiency. For higher working temperatures, the T-CO2 cycle has an advantage in lower volumetric flowrate at the expander outlet over the ORCs, potentially compensating for lower thermodynamic performance through smaller component size and footprint cost.

摘 要-I
ABSTRAC-II
誌 謝-IV
CONTENTS-V
LIST OF FIGURES-VIII
LIST OF TABLES-IX
NOMENCLATURE-XI
1.INTRODUCTION-1
1.1 Motivation-1
1.2 Literature review-4
1.2.1 T-CO2 cycles in low-temperature heat-to-power applications-4
1.2.2 Comparisons between T-CO2 cycles and other power cycles-6
1.3 Thesis aims and organization-7
2. TRANSCRITICCAL CO2 CYCLES-11
2.1 Simple cycle-11
2.2 Recuperative cycle-13
3. PROPOSED METHODOLOGY-14
3.1 Energy analysis-14
3.2 Simulation process-16
3.2.1 Sensitivity analysis of T-CO2 cycles-16
3.2.2 Comparative study of T-CO2 cycle and ORCs-20
3.3 Validation-24
3.3.1 Validation of simple T-CO2 cycle model-25
3.3.2 Validation of recuperative T-CO2 cycle model-26
3.3.3 Validation of ORC model-28
4. T-CO2 CYCLES SENSITIVITY ANALYSIS-29
4.1 Standard case of the T-CO2 cycles-29
4.2 Simple T-CO2 cycle-30
4.2.1 Design point setting for the simple T-CO2 cycle-30
4.2.2 Sensitivity analysis of the simple T-CO2 cycle regarding ηp and ηe-32
4.3 Recuperative cycle-37
4.3.1 Design point setting for the recuperative T-CO2 cycle-37
4.3.2 Sensitivity analysis of the recuperative T-CO2 cycle regarding ηp and ηe-38
4.3.3 Sensitivity analysis of recuperator effectiveness regarding ηp and ηe-43
4.4 Surrounding conditions-44
4.4.1 Mass flow rate ratio of heat source to CO2-44
4.4.2 Mass flow rate ratio of heat sink to CO2-45
4.5 Case study based on the sensitivity analysis-46
4.5.1 Scenario 1: if the turbomachinery efficiencies are given-46
4.5.2 Scenario 2: if the target net power output is given-47
4.5.3 Scenario 3: if the expander efficiency has to be enhanced-48
4.6 Summary of the T-CO2 cycles sensitivity analysis-50
5. COMPARISON BETWEEN T-CO2 CYCLE AND ORCS-52
5.1 Design point setting for the ORCs-52
5.2 Comparisons based on the net power output-54
5.3 Comparisons based on the heat recovery efficiency-59
5.4 Economic assessment-65
5.5 Summary of the comparative study of T-CO2 cycle and ORCs-66
6. CONCLUSIONS-68
REFERENCES-70
APPENDIX-77
A.1 Validation results of the T-CO2 cycles-77
A.2 Simulation results of the T-CO2 cycles at the standard case setting-79
A.3 T-CO2 cycle vs ORCs-81
A.3.1 T-CO2 cycle vs R245fa-ORC-81
A.3.2 T-CO2 cycle vs R152-ORC-82
A.3.3 T-CO2 cycle vs R600a-ORC-83
A.3.4 T-CO2 cycle vs R1234ze(E)-ORC-84
A.3.5 T-CO2 cycle vs R1234yf-ORC-85

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