50kW有機朗肯循環系統分析

Abstract

有機朗肯循環系統使用有機流體來擷取低溫廢熱並將其熱能轉為電能輸出，以達到節能減碳效果。本文利用熱力學定律分析一50kW有機朗肯循環系統，比較R123、R134a、R236fa、R245fa、R600、n-Pentane及HFE7000七種工作流體對系統的性能，並提出一得以判斷工作流體熱效率大小的優值(FOM)，此優值結合了傑寇數、蒸發溫度及冷凝溫度；由18種工作流體在不同條件下分析結果指出，熱效率越高其優值就越低，且與現有文獻數據比較亦符合此趨勢。 本研究使用的冷凝器與蒸發器分別為殼管式熱交換器與板式熱交換器，在熱阻抗分析結果指出冷凝器熱阻皆集中在殼側，可以使用熱傳增強管來改善熱阻值差異；在蒸發器中熱阻集中在工作流體側，尤其是在預熱區中阻抗值差異幅度明顯大於蒸發區，較好的做法為使預熱區與蒸發區分離成兩獨立熱交換器，再降低預熱區中工作流體的熱阻抗。

This study analyzes the system performance of a 50kW ORC system subject to influence of various working fluids. This study proposes a dimensionless “figure of merit” which combines the Jakob number, condensing temperature, and evaporation temperature that is quite helpful for screening of working fluid as far as thermal efficiency is concerned. The thermal efficiency normally decreases with the rise of figure of merit, and the proposed figure of merit is not only applicable for the present eighteen working fluids but is also in line with some existing literatures. Analysis of the typical ORC heat exchangers indicates that the dominant thermal resistance in the shell-and-tube condenser is on the shell side. Similarly, the dominant resistance is also on the refrigerant side for the plate evaporator. However, there is a huge difference of thermal resistance in the preheating zone whereas only minor difference in the evaporation region. A better way to resolve the extremely uneven resistance distribution is to use a separate preheater incorporating significant augmentation in the working fluid side and connected it to a much smaller plate heat exchanger.