利用空氣冷卻的加濕除濕(HDH)海水淡化系統的實驗與模擬研究

Experiment and Simulation Study of Air Cooling Humidification Dehumidification (HDH) Desalination System

陳彥平(Yan-Ping Chen)

指導教授 ： 許麗

國立臺灣大學/工學院/機械工程學研究所/碩士(2019年)

https://doi.org/10.6342/NTU201902673

摘要

加濕除濕 (Humidification Dehumidification, HDH) 系統是可用於進行海水淡化的系統之一。藉由模仿自然界降雨的方式，利用露點溫度差的方法來產出純水。本論文將呈現利用空氣冷卻的HDH系統的實驗與模擬研究。傳統的HDH系統利用水進行冷卻，其弊端是冷凝器的成本較高、換熱面積小，同時還有鏽蝕的問題。在HDH系統中將空氣作為冷卻流體為一個新穎的概念。雖然空氣相對於水，熱容較小，但所需要的冷凝器結構簡單，且可以克服水冷的諸多弊端。因此如何設計良好冷卻能力的空氣冷卻HDH系統，為此研究的動機。
本研究的實驗設計主要有兩個特點。一是使用水簾片作為加濕器主體，水簾片是一種交錯結構的纖維板材料，其在單位體積上具有大量的表面積，促使熱水與空氣能夠達到有效的質傳與熱傳；另一個則是採用堆疊中空的聚碳酸酯 (Polycarbonate, PC) 板作為除濕器主體，透過密集的堆疊，使除濕器在單位體積中也具有大量的表面積，促使冷卻空氣與高溫濕空氣達到有效的熱傳。
本論文闡述了對此HDH系統的實驗和模擬的研究。整個系統在熱水水量與熱水溫度越高的情況下，無論是產量或者GOR都有較佳的表現。實驗和模擬都表明除濕器的冷卻能力對於HDH系統的效能至關重要，若冷卻能力偏低，純水產量以及GOR將大幅降低。在實驗上，為了提高除濕器的冷卻能力，我們在濕空氣的風道出口添加擋風板，使得濕空氣流量降低。在冷卻空氣的流量不變下，相對的冷卻能力上升，促使整體產水量與GOR皆上升了約兩倍。與傳統水冷的HDH系統進行比較，性能更為優異。透過實驗得到本系統在15 ℃ 的環境溫度與50 ℃ 的熱水溫度下，最高產水量約為每天129公升，同時GOR (Gained output ratio) 約為0.54。
另外，此系統未來將配合光伏熱系統 (Photovoltaic thermal, PVT) 所產生的廢熱去進行海水淡化，因此本論文也探討了關於電子元件最佳化以及環境溫度的影響。兩個部分同時指出冷卻能力對於HDH系統非常重要，也就是冷卻風扇的功率不能太低，以及冷卻空氣的溫度，也就是環境溫度對冷卻能力的影響很大，當環境溫度較低時，能產生較多的純水，同時也能得到較佳的GOR與COP。
本論文在模擬的部分分為兩大系統進行研究，分別為水封閉循環-空氣開放循環 (Closed-water open-air, CWOA)以及水封閉循環-空氣開放循環 (Closed-water closed-air, CWCA) 系統。兩者主要差異在用於乘載蒸氣的空氣是否為封閉循環。由熱力學分析的模擬結果，封閉循環的產水量和GOR都能夠得到提升。
本研究後續也進行了上述的實驗驗證。實驗數據表明CWCA系統與CWOA系統相比，確實有較佳的表現。源自於濕熱空氣不會被排放到大氣中，而是回到了加濕器的入口處，減少了整個系統的熱損失；同時，加濕器入口空氣溫度與濕度的提高也導致加濕器空氣出口溫度與濕度的提高，亦即攜帶了更多的水蒸氣進入到除濕器中，從而得到了更多的純水產量和更高的GOR。

關鍵字

海水淡化 ； 加濕除濕 ； 空氣冷卻

並列摘要

Humidification Dehumidification (HDH) is a method of desalination system. It produces fresh water according to using concept of dew point, which is the method of raining in the nature. This paper present experimental and simulative research in HDH air-cooling system. The tradition HDH desalination system commonly used water as the cooling fluid in the condenser, there are many bad effect such as higher cost of system design, lower heat transfer area and problem of corrosion. It is a novel concept by using air as cooling fluid. Even though its heat capability is not better than water-cooling, but its structure of condenser is relatively simple, and it can eliminate some bad effect that occurred in water-cooling system. Hence, the motivation of this research was how to design an air-cooling system that can provide comparable or even better cooling capability than water-cooling system.
There are two major points in the experimental design of this research. First, we used the cellulose pad as the majority of the humidifier. The cellulose pad is complex structure and fiber material, which has large surface area per volume which makes mass transfer and heat transfer more efficient between hot water and air. Second, we used closely packed hollow polycarbonate (PC) boards as of the majority of the dehumidifier. By efficient stacking the PC boards, it also has large surface area per volume to make heat transfer efficient between hot humid air and cooling air, and solve the problem of pipe corrosion from salted water and reduce cost of system at the same time.
This paper presented experimental and simulative result of the HDH system. It was found that the performance of the system was better in productivity and GOR as the HDH had higher mass rate or higher temperature of hot water. Experimental result and simulation all showed that the cooling capability was very important for the system performance. If the cooling capability was poor, the productivity and GOR would be significantly decreased. In the experiment the cooling capability was modified by addition of wind guard at the exhaust exit of the dehumidifier. The wind guard made the mass rate of air lower, and the cooling capability was relatively improved under the mass rate of cooling air unchanged, and eventually the productivity and GOR increased about two times. Hence, this air-cooling HDH system could provide better performance than water-cooling system by using the wind guard and efficient stacking of PC boards. In our experiments the maximum productivity was as high as 129 kg/day, operating at hot temperature of 50 oC and ambient temperature of 15 oC, and GOR was 0.54 at the same time.
The HDH system will be combined with photovoltaic thermal (PVT) system to achieve waste recovery for desalination, so we also studied how to maximize coefficient of performance (COP) by tuning power of electricity device and the effect of ambient temperature. Two parts also found that the cooling capability was very important for HDH system. The power of the cooling fan should be high for higher mass rate of cooling air, and ambient temperature have to be lower for the better cooling capability because the temperature of cooling air was ambient temperature, then the HDH system can produced much more distilled water under lower ambient temperature, and obtained better GOR and COP.
In the simulation part, the work was also divided into two systems, one was closed-water and opened air (CWOA), and the other was closed-water and closed-air (CWCA). The main difference between the two systems is whether the air is closed or not. By the thermodynamic analysis, productivity and GOR of CWCA all got improved.
We also assessed CWCA system and made comparison between CWCA and CWOA after the simulation. The experimental data showed that the performance of CWCA was really better than CWOA, because there was no exhausted air to the surrounding in CWCA system, which reduced the heat loss of the system, and it also made the temperature and humidity at the outlet of humidifier was higher than those in CWOA system, caused more water vapor entered the dehumidifier, and then obtain more productivity and higher GOR.

並列關鍵字

desalination ； humidification dehumidification ； HDH ； air cooling

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