Passive humidification

In designing a microscale fuel cell, there are several considerations that need to be accounted for. These include the following is the fuel cell to be completely active or passive will it operate at room temperature or elevated temperatures will the fuel be at atmospheric pressure or elevated pressure will external humidification be required and finally, will the fabrication techniques... 

During start up, when the stack temperature and then performance (voltage) are low the passive BOP devices devoted to stream and stack humidification (enthalpy wheels membrane humidifiers) do not work as they are essentially based on the difference of temperature and then are practically excluded by the FCS management on the other hand, at low temperature the requirements for stack humidification are strongly reduced and membrane could self-hydrate if a sufficient water production (proportional to power requirement) occurs. 

Together with often complex aspects of DMFC system design (applicable to both passive and active DMFC system concepts), durable operation of the fuel cell stack and the ability to deliver the required power for several thousand hours are generally considered crucial to the ultimate large-scale commercial success of DMFCs. Other key requirements include cell (stack) operation at a highest achievable voltage and cathode operation at ambient pressure, low airflow, and natural air humidification. 

A new ventilation system will be built. The normal operation of the new system will be passive, the air flux being activated by the daily internal temperature variation (the container is metallic). The ventilation air, after filtration and de-humidification, will enter area 1, then area 2 and, after filtration, will be released outside. Special check valves, with extremely low pressure drop, have been designed, tested and qualified for the purpose. The overall air flow has been calculated as 600/700 Nm a day in summer and 300/400 Nm a day in winter, and ensures 4 complete air changes each year. An active extraction fan will also be available. 

Humidification This system is responsible for humidification of the flow of reactants, as described in the following section. Some systems (especially portable designs) are designed to be passively humidified and eliminate this subsystem completely at the expense of reduced performance. 

Humidification is accomplished by two main approaches, passive and direct humidification. In passive humidification, the water generated by reaction is used to maintain a proper moisture balance and humidify the incoming flow without external power. In active humidification, a separate humidifier is used to directly provide the humidification of the incoming flow with stored or recycled water. 

Figure 6,11 Passive humidification with external water recirculation. This system can be used to recycle water produced by fuel cell stack into incoming fuel or oxidizer stream, eliminating need for separate water storage container.

Figure 6.13 Passive humidification via internal membrane water generation from embedded platinum particles.

Despite the fact that the PEFC is a water generation reactor, some humidification of the reactants is usually necessary to enable high performance and longevity. A dry inlet feed results in poor local performance, hot spots, and internal stresses that can lead to short lifetimes. There are various active and passive humidification methods used to accomplish humidification, including membrane humidification, direct injection, and internal or external recirculation. 

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