In this thesis, a PEMFC stack, which can be self-humidification of passive portable hydrogen fuel cell, will be developed. The stack is developed for portable applications, so the structure of the stack is simplified as possible as we can. As the cathode directly exposed under the atmosphere, so in a long time, the membrane easily lead to excessive evaporation of water, so that performance degradation. The traditional humidifier is more complex applications are not suitable for portable, so this thesis stack developed by the use of cotton capillarity, the water from the tanks transferred to the membrane, and then by cotton and a good touch to the membrane humidifier effect, this structure without an increase in large equipment, in line with the principles of portability. The PEMFC stack is made with carbon fiber bunches for current collectors and two 8-cell banded-type MEAs, the stack can develop a high voltage by serially connecting outside of the reaction chamber. 16-cell in series when the current density is greater than 110mA/cm2, use humidification to avoid long time operation, due to water cause a voltage drop. Humidification is not only to help transfer of hydrogen ions, and the role of a cooling stack, the cells temperature is too high will not cause transpiration rate of speed. Current density greater than 250mA/cm2, although humidity can still be effective, but insufficient humidification single cotton, one hour after, the voltage drop of about 20% longer cell performance, such as humidity will be no more have a more significant decline phenomenon, more cotton or additional external humidification humidifier can maintain a long-time stable operation. Therefore, the performance of the cells in order to avoid a recession, should pay attention to in a long time when working conditions and operating range.

The improvement of performance and the maintenance of stability of a portable air-breathing PEMFC are studied in this thesis. The water content within proton exchange membrane affects strongly on the performance and stability of a PEMFC stack, in which water within membrane can form a conduction channel to provide hydrogen ion transferring from anode to cathode. The over-dried condition caused by a long time operation can also be avoided to prevent the membrane from damaging. Thus the proper humidification of a stack is important for a portable air-breathing PEMFC system. The traditional humidifier is too bulky to be suitable for portable fuel cells. A simple humidification system developed in this research is making use of the water stored in the stack bottom and the self-generating heat by chemical reaction to drive the passive humidification system of this stack. The water at the bottom of the tank can be sucked with cotton threads in cathode and a piece of cotton cloth by capillary phenomenon and transfer to the membrane of MEA. The cotton threads humidification in cathode is enough in low and middle current density in this study. It is not enough in high current density due to the large water vapor dissipation in the cathode surface, so a cotton cloth in anode is added to increase the evaporating surface to supply water to membrane. This passive humidification system does not need extra energy, and it only employs the heat generated by the cells. The system follows the simple principle, which is always obeyed in a portable fuel cell system. A 16-cell HFC stack developed in this research adopts carbon fibers as current collectors. Two pieces of 8-cell anodes is placed in the inner sides of the stack, and the 8-cell cathode is located on external sides, which is exposed directly to the ambient air. The 16-cell can connect in series or parallel. The experimental results show that it is helpful to add cotton threads in cathode and cotton cloth in anode to improve the stability of the 16-cell stack during a the long period operation. The 16-cell HFC stack has succeeded in the operation and charging for an IPhone, digital photo frame, and LED light. The experiments have proved that this type of the lightweight humidification system is helpful in the future portable hydrogen fuel cell applications.

A run-around membrane energy exchanger (RAMEE) is a novel energy exchanger that is capable of transferring both heat and moisture, which can significantly reduce the energy required to condition outdoor ventilation air. The RAMEE uses a liquid desiccant to transfer both heat and moisture between two remote air streams, making it appropriate for many applications, including building HVAC retro-fits. Both initial system start-up and changing outdoor conditions require time for the desiccant to undergo changes in both temperature and concentration, and can cause significant transient delays in system performance. Under some conditions, these transients may be beneficial by increasing the system performance. However under some conditions, the transient delays can cause a substantial decrease in performance.<p> This thesis focuses on the development of control strategies that can be used to reduce unwanted transient delays. In order to develop these control strategies, the performance of a RAMEE is first investigated using both experimental and numerical methods. The transient numerical and experimental effectiveness results show satisfactory agreement, with a maximum root mean squared error of 10%. Both the numerical and experimental data show that a long transient time of several hours, or even several days, can occur upon initial system start-up.<p> The numerical model is used to investigate several control strategies to reduce unwanted transient delays. The control strategies investigated are: solution and air flow control, air flow bypass, solution temperature control, and solution concentration control. The solution and air flow control are shown to reduced the start-up transient time by up to 11%, but require either a reduction in air flow or an increase in solution pumping costs. Air flow bypass proves to be a better option which provides a 16% reduction in transient time, and only requires that a bypass damper be provided for each exchanger. Solution temperature control is capable of essentially eliminating the thermal transient time (time required for the solution to reach operating temperature), but the thermal transient time is found to be a minor contributor to the overall transient time (time required for the solution to reach operating temperature and concentration) when the initial concentration of the solution is different than the steady-state concentration. When thermal and moisture transients exist, total transient times may be over 18 days. A practical temperature and concentration control strategy is developed, which can reduce transient delays by over 90% and increase performance during variable outdoor weather conditions.

¡@¡@In this thesis, we develop a self-humidifying PEMFC. The humidifying effects on the stability and impedance of the fuel cell are studied. A portable and passive PEMFC stack usually exposes in the ambient no matter that it works or not. However, the ambient is far from saturated. The water within MEA will diffuse to the membrane¡¦s surface and evaporate continuously. The membrane will be short in water without water supplying. Because the conductivity of H+ of the membrane is highly dependent on water content, the dehydration of the membrane will reduce the interconnected passageway of H+ and affect the performance of fuel cell directly. And because of the different expansion rate the electrode of MEA is also possible to separate from its membrane when it operates repeatedly. This separation will make the performance of fuel cell an unrecovered decay. ¡@¡@At first, the hydration status of the dry membrane is observed. We measure the addition weight of water into membrane by using cotton thread humidifying, and estimate the water permeation distances. The maximum water supply rate of cotton thread is 4.26mg/min, and the permeation rate of water through membrane where is 2.5cm from water surface is 0.15mg/cm¡Dmin. Then we design the self-humidifying devices of PEMFC stack. The humidifying effects on performance and stability of the fuel cell are studied. ¡@¡@When the active area is 0.7¡Ñ4.5cm2 and the cotton thread is 5mm from the center of electrode the supplying water can arrive at the reaction area under the electrode through the membrane in one minute. The difference of the supplying water between the bottom and top is 7% by using 6cm cotton thread. Therefore water can hydrate the membrane and the difference of the supplying water between bottom and top is not oversize. The higher current load, the voltage efficiency is lower. The increasing heat generation rate results in the water evaporation rate would be greater than the water generation rate. So the drop of voltage under higher current is greater than lower current. By comparing with the difference of high frequency impedance the change of humidifying is smaller between 1hr operating. It indicates that humidifying by cotton thread keeps the membrane hydration.