低全球暖化潛勢流體應用於有機朗肯循環進行資料中心超低溫廢熱回收之可行性評估

Abstract

本研究中旨在評估有機朗肯循環 (Organic Rankine Cycle, ORC) 在資料中心中進行廢熱回收的可行性，並透過DWSIM進行ORC的參數分析及優化。使用HFC-245fa、HFO-1233zd(E) 及HFO-1234ze(E) 作為工作流體，以熱源溫度、泵送壓力及質量流率作為關鍵參數，設定5°C及15°C兩種冷卻水條件，探討熱源溫度、泵送壓力與質量流率變化對膨脹機輸出功率、泵功耗、ORC效率與熱交換器性能的影響。 研究結果顯示，使用三種不同的工作流體時，隨熱源溫度增加，膨脹機輸出功率、泵功耗、熱效率及不可逆性均上升，而第二定律效率則隨熱源溫度增加先上升後下降。不可逆性的值由大到小為冷凝器、蒸發器、膨脹機及泵，但是在使用HFO-1234ze(E) 時蒸發器具有最高的不可逆性，但是其不可逆性為所有工作流體中最低。隨著熱源溫度提高，蒸發器的趨近溫度也增加，但冷凝器的趨近溫度先增加至極大值後就不再變化。對於蒸發器及冷凝器而言，蒸發器的夾點溫差發生在熱交換過程中，而冷凝器的夾點溫差則發生於熱交換器出入口之間。隨著泵送壓力的增加，膨脹機輸出功率、熱效率及第二定律效率均上升，但在高泵送壓力下增幅變小，泵送壓力對膨脹機輸出功率的影響越小。隨著質量流率的提升，膨脹機輸出功率與泵功耗均增加，且在高質量流率時其增幅變大，而熱效率則不受質量流率變化的影響，蒸發器之趨近溫度隨質量流率增加而減少，於高質量流率時其降幅變小。使用HFO-1234ze(E) 時擁有最高的輸出功率、泵功耗、熱效率及第二定律效率，顯示HFO-1234ze(E) 是三種工作流體中的最佳選擇，能作為低全球暖化潛勢流體應用於ORC中。

This study investigates the feasibility of utilizing an Organic Rankine Cycle (ORC) system for waste heat recovery in data centers. DWSIM is applied to simulate the operation of the ORC in real-world data center conditions, and the influence of using three different working fluids, HFC-245fa, HFO-1233zd(E), and HFO-1234ze(E), is evaluated. In addition, key parameters such as the temperature of the heat source, pumping pressure, and mass flow rate of the working fluid, are varied to discuss their impacts on the output power of the expander, the power consumption of the pump, thermal efficiency, second-law efficiency, and the performance of heat exchangers. The results show that increasing the temperature of the heat source leads to higher output power of the expander and higher thermal efficiency, but also higher power consumption of the pump and greater irreversibility. The second-law efficiency reaches its maximum when the heat-source temperature is 50°C. The highest irreversibility is usually found in heat exchangers such as the evaporator or the condenser, whereas the pump has the lowest irreversibility. When HFO-1234ze(E) is used, the overall irreversibility is the lowest among the three fluids. The approach temperature of the evaporator increases as the heat source becomes warmer. For the condenser, the approach temperature also increases with the heat-source temperature up to 10°C then remains constant as the heat-source temperature continues to elevate. The pinch-point temperature occurs inside the evaporator, which limits the maximum saturation pressure of the ORC working fluid during the process of heat absorption. On the other hand, increasing the pumping pressure augments the output power of the expander, thermal efficiency, and second-law efficiency. Increasing the mass flow rate enhances both the output power of the expander and the power consumption of the pump, so that the thermal efficiency remains relatively constant. The approach temperature of the evaporator increases with increasing mass flow rate, but the influence diminishes at high mass flow rates. HFO-1234ze(E) shows the highest output power of the expander, thermal efficiency, and second-law efficiency among the three fluids, despite its higher power consumption of the pump. These results indicate that HFO-1234ze(E) is the most promising low-GWP working fluid for waste heat recovery in data centers using the ORC.