重組式甲醇燃料電池水熱管理之研究

Fundamental studies on thermal and fluid management in reformed methanol fuel cell (RMFC)

劉宗麟(Tsung-lin Liu)

指導教授 ： 潘欽

國立清華大學/原子科學院/工程與系統科學系/博士(2013年)

https://doi.org/10.6843/NTHU.2013.00441

摘要

甲醇重組式燃料電池為一整合系統，主要元件包括了甲醇重組器及質子交換
膜燃料電池。其中甲醇重組器為燃料轉換器，目的是將液態甲醇經重組反應後轉
為氫氣提供給質子交換膜燃料電池使用。而質子交換膜燃料電池為發電元件，可
將氫氣燃料轉為電能。質子交換膜燃料電池在電化學發電的過程中，會在陰極產
生水，過多的水可能阻塞氣體擴散層或觸媒層及流道導致電池效率下降。因此，
了解水在擴散層中的傳輸機制，進而使用高效率或自然的方式進行水管理是一重
要的課題。另一方面，甲醇重組器的重組反應其反應物為氣態甲醇，且操作溫度
需要接近250 °C 的高溫以提高轉換效率，但質子交換膜超過120 °C 就可能會導
致薄膜受損，因此重組反應產生的氫氣需要降溫後才能給予質子交換膜燃料電池
使用。然而，若增加甲醇蒸發器及散熱器是必會增加系統的複雜性及降低整體系
統效率。因此，有效的熱管理及簡單的系統整合有其必要性。本研究由上述的兩個熱流工程問題出發，首先設計一套水在氣體擴散層輸送實驗環路系統，使用影像觀察水穿透擴散層形成液珠的過程，並同步量測背壓分析，以模擬探討質子交換膜燃料電池陰極水傳輸機制。本研究採用四種不同型式的商用氣體擴散層，包括碳布、碳紙、碳布結合單面微孔層及碳布結合雙面微孔層。研究結果顯示，碳布結合單面微孔層可觀察到間歇”噴射流”水移除機制，並可維持背壓於一定值，表示可快速移除過多的水並維持氣體擴散層一定的水含量。另外，本研究所提出的背壓模式可量化不同碳材的水排除速率及其含水飽和度，更深入了解冷凝水在不同氣體擴散層的傳輸機制。另一方面，為了有效降低甲醇重組器的產出氫氣的溫度並預熱甲醇至氣態，本研究提出使用微型熱交換器整合甲醇重組器的概念，利用甲醇重組反應產生高溫產物的廢熱來蒸發液態甲醇。為了解微型熱交換器其熱流特性，本研究設計第
二套實驗系統，使用高溫氦氣模擬氫氣來加熱液態甲醇於自製的矽質微流道熱交
換器。此研究探討不同型式(同向和逆向)的熱交換器在單相、雙相區的熱傳特性
及差異，包括沸騰雙相流譜、單相對流及沸騰雙相熱傳遞係數、入出口溫度震盪
分析和熱效率分析。研究結果顯示，在接近乾化區時，逆向式微熱交換器在出口
處可看到液膜已完全蒸發，然而同向式的出口仍可看到液膜存在，且此時逆向式
的熱端流體出口溫度較同向式熱交換器低。於熱效率的分析中，逆向式和同向式
的最高效率分別可達0.9 和0.85。顯示逆向式較同向式微熱交換器適合整合於甲
醇重組燃料電池系統。另外，本研究所探討的甲醇雙相熱傳遞係數於微型熱交換
器中，其趨勢會隨著甲醇平均乾度增加而增加，直到接近乾化區後會開始下降。
而逆向式微熱交換器較同向式微熱交換器具有較高的臨界熱通率值，應是由於流向的不同使逆向式微熱交換器有較高的入口次冷度所導致。最後，本研究根據實
驗數據所求得的單相熱對流與沸騰雙相熱傳遞係數與文獻做比對，並進一步發展
了同向及逆向微熱交換器雙相沸騰熱傳遞係數的經驗公式，其誤差分別為10.1
和5.82%。

關鍵字

氣體擴散層 ； 水管理 ； 甲醇重組式燃料電池 ； 微型熱交換器 ； 微流道沸騰熱傳

並列摘要

A reformed methanol fuel cell (RMFC) is an integrated system, which includes two main components, a methanol reformer and a proton exchange membrane fuel cell (PEMFC). The methanol reformer is served as a fuel convertor that changes the fuel from liquid methanol to hydrogen, and then feeds the hydrogen into the PEMFC. A PEMFC is an electricity generation system, which converts the chemical energy of the hydrogen fuel into electricity via electrochemical process. However, this process will generate amount of water in the cathode of PEMFC. If too much water accumulated inside the PEMFC cathode, the performance of a PEMFC may be deteriorated oxygen transport may be limited due to water blocking the pores in the catalyst layer (CL), gas diffusion layer (GDL), and flow fields. This phenomenon, known as “water flooding,” is a critical obstacle to get higher efficiency and power density for a PEMFC. Thus, in order to alleviate such a mass transport limitation on
fuel cell efficiency, it is important to understand the water transport in the cathode GDL. On the other hand, the reformed reaction of the methanol reformer needs methanol vapor as the fuel, and the reforming temperature requires a high temperature as high as 250 °C. However, the proton exchange membrane of PEMFC may be damaged if the fuel temperature is higher than 120 °C. Thus, the temperature of the hydrogen produced must be reduced before it enters a PEMFC. Adding a methanol evaporator and a heat sink may solve the issue. However, it may significantly increase
the complexity of the RMFC system and may jeopardize the overall efficiency. In order to explore the above-mentioned two engineering issues, the present
study design an experimental rig synchronizing the flow visualization of growth of liquid droplets on the surface of GDL and the measurement back pressure to
investigate the transport phenomena of liquid water through GDLs with different morphologies. Four commercial GDL media, including carbon paper, carbon cloth, carbon cloth with micro-porous layers (MPL) are employed. The experimental results demonstrate the “self-eruption transport” mechanism in the carbon cloth with single-sided MPL only. Such self-eruption mechanism may help controlling the water contained inside the GDL regardless of the water generation rate. This suggests that a
GDL with single-sided MPL treatment may help effectively the water management in a PEMFC. Moreover, through synchronizing flow visualization of the growth of the water droplet that emerges out of the GDL and the measurements of the dynamic back pressure, the flow rate of water drainage and water saturation inside the GDLs are analyzed. The methodology proposed in this study enables a deep understanding and
knowledge for the water transport mechanism in the GDLs. A concept of using a microchannel heat exchanger (MCHE) to integrate with the micro methanol reformer is proposed in the present study. The MCHE utilizes the exhaust heat of the products from the micro methanol reformer to vaporize the liquid methanol. In order to deeply understand the thermal and fluid characteristics of a MCHE, the second set of experiments is designed to use high temperature helium gas
to heat the liquid methanol with a home-made, silicon-based MCHE. The effects of the flow arrangement (co- and counter-flow) on the heat transfer characteristics in
single- and two-phase flow regions are studied. Two-phase flow patterns, single-phase and boiling two-phase heat transfer coefficient, inlet/outlet temperature oscillation,
and the thermal efficiency of MCHEs are also investigated. The experimental results show that for nearly dryout zone with the approximately same helium heat flux, the
counter-flow type demonstrates fully dryout of the liquid film in the cold-side outlet but for the co-flow type, the liquid film is still present in the outlet plenum. At the
same time, the hot-side fluid (helium) outlet temperature of counter-flow MCHE is lower than that for the co-flow one. It indicates that the counter-flow MCHE not only
provides a methanol vapor with higher quality but also effectively reduces the hot-side fluid outlet temperature. In terms of thermal efficiency, the highest efficiency
of co-current design is about 0.85 and is 0.9 for counter-flow design. It indicates the counter-flow type is a better candidate to be integrated in RMFC system.
The heat transfer characteristics of the boiling heat transfer in the MCHEs with gas heating condition is also of significant interest for academic research as well as
engineering applications. The boiling two-phase heat transfer coefficient is also carried out in the present study. The experimental data of the present study shows that heat transfer coefficient increases with an increase in the mean vapor quality until they reach a maximum, after which dryout takes place and the heat transfer coefficient
decreases. The counter-flow MCHE exhibits a higher critical heat flux (CHF) than that of the co-flow MCHE. It suggests that the counter-flow MCHE results in a higher
methanol inlet subcooling and therefore a higher CHF. The heat transfer coefficient data of single-phase and boiling two-phase region are compared with correlations
from literature and empirical correlations for the co- and counter-flow MCHEs are also developed. The mean absolute errors of the present correlations are 10.1 % and
5.82% for the co- and counter-flow MCHE, respectively.

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