低溫多效蒸發海水淡化系統之最適化

Abstract

近年來，台灣南部的缺水壓力因氣候變化和水庫淤積變得更加險 峻。海水淡化技術在必要時能夠緩解這個即將到來的問題並能夠持續 供應民生及工業用淡水，其中，多效蒸發技術已經廣泛應用於海水淡 化，並具有低電力消耗的特點。在這篇論文中，吾人首先建構多效蒸 發系統的數學模型，並討論如何增加使用低溫顯熱之效率。該模型主 要根據質量和能量平衡，為一個高度非線性的規劃問題。同時也對新 的架構－增強型多效蒸發系統，基於廢熱性能比進行研究。廢熱性能 比定義為餾出物的焓與熱源的可利用能量之比。對於傳統多效蒸發系 統，廢熱的離開溫度仍高導致廢熱無法被有效利用。在討論廢熱性能 比中，增強型多效蒸發系統優於傳統多效蒸發系統，同時更能降低其 熱交換面積。增強型多效蒸發系統能夠在指定的低廢熱出口溫度下充 分利用顯熱廢熱。此外，廢熱的出口溫度是選擇不同系統的關鍵要 素。當指定的廢熱出口溫度高於傳統多效蒸發系統的最低可行溫度 時，使用傳統多效蒸發系統將在產水效率更有優勢。

In recent years, water stress becomes more severe in southern Taiwan due to climate change and reservoir siltation. Desalination technology can mit- igate this upcoming issue and supply fresh water persistently. Multi-Effect Evaporation (MEE) system is one of thermal desalination technologies which has features of high efficiency in power consumption. A mathematical model ofMEEsystemisdevelopedtoeffectivelyutilizethesensiblelowgradewaste heat. The model is based on mass and energy balances and it is highly non- linear. Besides conventional configuration, an advanced process, namely the Boosted MEE (BMEE) is also investigated on the basis of waste heat perfor- mance ratio. Waste heat performance ratio is defined as the ratio of the en- thalpy of the distillate to the maximum exploitable energy of the heat source. For MEE system, the leaving temperature of waste heat is quite high and waste heat can not be efficiently exploited. To improve the operating effi- ciency, BMEE system is studied and the results show that the BMEE system is superior to the conventional MEE system in both waste heat performance ratio (up to 8%) and heat transfer area (up to 14%). The BMEE system has shown the capability to fully utilize the sensible waste heat at specified low waste heat outlet temperature. Moreover, the outlet temperature of waste heat is the key to choose either MEE system or BMEE system in terms of fresh- water production. While the specified waste heat outlet temperature is higher than the lowest possible temperature of the MEE system, it becomes more appropriate to use the MEE system rather than the BMEE system.