Abstract
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.