External Humidification - Methods

In laboratory test systems the reactant gases of fuel cells are humidified by bubbling them through water, whose temperature is controlled. This process can be called sparging. 

It is generally assumed that the dew point of the air is the same as the temperature of the water it has bubbled through, which makes control straightforward. This is good for experimental work, but this will rarely be practical in the field. 

One of the easiest to control is the direct injection of water as a spray. This has the further advantage that it will cool the gas, which will be necessary if it has been compressed or if the fuel gas has been formed by reforming some other fuel and is still hot, as in Chapter 8. The method involves the use of pumps to pressurise the water, and also a solenoid valve to open and close the injector. It is therefore fairly expensive in terms of equipment and parasitic energy use. Nevertheless, it is a mature technology and is widely used, especially on larger fuel cell systems. 

Another method recently described (Floyd D.E., 2001) uses metal foam to make a kind of fine water spray to humidify the water. This system has the advantage that only a pump is needed to move the water - the water is finally introduced to the air in a passive way. 

All the systems described above require liquid water to operate. This means that the exit air must be treated so that a good deal of the water content is condensed out as liquid water, stored, and then pumped to where it is needed for the humidification process. This clearly adds to the system size, cost, and complexity. There are some systems that use the water in the exit gas to humidify the inlet air, but without actually condensing it out as liquid water. One way to do this is to use a rotating piece of water-absorbing material. It is put into the path of the exit air, and it absorbs water. It is then rotated so that it is in the path of the dry entry air, which will at least partly dry it out. If the piece is made circular this can be done on a continuous basis - constantly transferring water from exit to entry gases. This method has the disadvantage that it will still be fairly bulky and will require power and control to operate. 

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The different humidification approaches could be classified as internal or external methods. Internal humidification means that humidification procedure concerns exclusively the inner spaces of fuel cell stack, while external humidification involves modifications in feeding stream humidity ratio outside of the stack [1. 29]. 

A possibility is to saturate at different temperatures the reactants before they enter into the stack [33]. This approach can be accomplished by several procedures based on external dewpoint, external evaporation, steam injection with downstream condensers, or flash evaporation. High temperature values allow to absorb significant water amount in gas streams and then transport it inside the stack compensating the water losses due to internal fast evaporation. However, the main problem with external humidification is that the gas cools after the humidifier device, the excess of water could condense and enter the fuel cell in droplet form, which floods the electrodes near the inlet, thereby preventing the flow of reactants. On the other hand, internal liquid injection method appears preferable for example with respect to the steam injection approach because of the need of large energy requirement to generate the steam. 

External humidification uses additional equipment. Common methods include warm-up, dew point, permeable membrane, and liquid water spray. 

As discussed in Chapter 5, PEM fuel cells widely use PFSA membranes, whose proton conductivity strongly depends on their water content. To achieve high membrane proton conductivity and good PEM fuel cell performance, it is necessary to add water to fuel cell systems to maintain a sufficient membrane hydration level. Water is often added externally with the reactant gases at the anode and the cathode. So far, several humidification methods, such as bubble humidification and direct liquid water injection, have been developed for PEM fuel cells. 

Tetraaminobiphenyl and the dicarboxylic acids were used to produce PBI-PPA nanocomposite membrane by sol-gel method, where PPA acted as polycondensation agent and polymerization solvent [83]. Its fuel cell performance without external humidification revealed 0.2 A cm current density at 0.65 V and 160 °C. With oxygen as the oxidant, for the same current density, voltage was increased to 0.75 V. Inorganic proton conductors such as ZrP [84],... 

Methods of Active Humidification Active humidification requires a discrete, external humidification system. In a laboratory environment, a sparge-type humidifier, as illustrated in Figure 6.9, is often used. In this system, gas is sparged through a porous rock and into heated water to absorb moisture before entering the fuel cell. This system is not useful outside the laboratory because it is dependent on orientation and almost never 100% efficient. Care must be used to ensure proper humidification is achieved and careful calibration is neccessary. 

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. 

Benicewicz s group evaluated the durability of the fuel cell with the PBI membranes prepared by solgel method [55]. They also reported a very good steady-state long-term durability of greater than 14,000 h at 120°C at 0.2 A cm" without external humidification [55]. 

Fig. 16 Various types of humidification (six diagrams showing conventional fuel cell humidification methods used in research (a) the self-humidifying fuel cell with no active external humidification of the reactant streams, (b) the setup featuring liquid water injection into an inactive portion of the fuel cell, (c) a typical dew point humidifier, (d) the evaporation setup, (e) the steam injection system and (f) the flash evaporation method (from Evans, 2003).

Two pressure transducers are located upstream of the stack to monitor anode and cathode pressure during the experimental runs. A spiral heat exchanger, using external water at room temperature as second fluid, is used to control the temperature of the cooling water. The FCS humidification strategy is based on the deionized water injection method (see Sect. 4.5), activating the injection when the outlet air temperature is higher than 60°C. 

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