液態氮與水批次於鍋爐內混合以獲得液態氮的可用能

Abstract

本研究開發了一種利用液態氮發電的技術並架設一套液態氮可用能(exergy)的回收系統。此系統能將液態氮噴入裝有不同水量的鍋爐中。液態氮與水接觸時會吸熱並加壓水，開啟鍋爐即可釋放高壓水柱作功。實驗得到的功是透過壓力與體積量測而得。本文提出兩種模型以探討上述程序: 等容-等壓-等溫程序(constant v-P-T process)與準力學平衡程序(quasi-mechanical equilibrium process)。前者的計算簡單，不過需要猜中最大壓力；後者需要迭代並預測系統最大壓力約為14 MPa。兩程序模型預測每公斤氮的熱、功傳輸量相差小於4%。本研究中進行了45組實驗並改變液態氮噴射時間(0.25、0.50、0.75、1秒)與鍋爐內的初始水量(0、1062、1931、2124 mL)。液態噴射時間對可用功、噴入鍋爐的液態氮量、可用能效率都沒有顯著影響。一開始鍋爐內的水量影響較明顯。若將鍋爐內的自由空間納入考慮，準力學平衡程序與實驗得到的可用能效率會落在相同數量級。此系統最大效率為17.6%，因為鍋爐內的自由空間會讓氮沿著低壓的路徑膨脹，造成可用功大幅減少。然而鍋爐內仍需保留自由空間，液態氮才噴得進去。

This study develops a technology to utilize liquid nitrogen as a fuel for power generation. A liquid nitrogen (LN2) exergy recovery system has been constructed. In this system, LN2 is injected into a boiler filled with different amounts of water. LN2 absorbs heat when it is in contact with water, thus producing pressurized water. Then the boiler is opened to release pressure water jets for power generation. In experiments, the energy produced by the compressed water/LN2 is obtained from pressure and volume measurement. Two models are proposed to simulate the processes: a constant v-P-T process, and a quasi-mechanical equilibrium process. The former is simple for analysis but requires an adequate guess for the maximum pressure. The latter requires iteration and predicts a maximum pressure of 14 MPa for the system in this study. The energy transferred between LN2 and water (heat, work, and useful work) are predicted by the two models. The error between the two models are less than 4%. In this study, 45 experiments were conducted with 4 different duration of LN2 injection (0.25, 0.50, 0.75, 1 second) and 4 different initial volume of water in the boiler (0, 1062, 1931, 2124 mL). The injection duration had no significant effect on the useful work output, the mass of LN2 injected into the boiler, or the exergy efficiency. In contrast, the effect of the initial volume of water in the boiler is more obvious. When the free space in the boiler is explicitly considered, the exergy efficiency predicted by quasi-mechanical equilibrium model and that from the experiments are of the same order of magnitude. The maximum exergy efficiency achieved is 17.6% since the free space in the boiler makes N2(g)/LN2 expand along low pressure paths, thereby reducing the work output. However, the free space in the boiler is still needed to admit the injected LN2.