Humidifiers and Cooling Plates

A fuel cell stack may contain other components. The most prominent ones are cooling plates or other devices and techniques for removing reaction heat and, possibly, humidifiers.

Cooling is vital to maintain the required point of operation for a given fuel cell stack. This may be an isothermal condition, or perhaps a temperature gradient may be deliberately superimposed in order to help water removal. Relatively simple calculations show that for very high power densities such as those

FIGURE 4.18 Sketch of a three-layer metal "mirrored" bipolar plate. The top and bottom plates are used as anode and cathode plates in two adjacent cells within a stack. After the three metal sheets are brazed together, the inner section allows turbulent cross-flow of cooling water inside the plate — General Motors (Rock, 2000).

FIGURE 4.18 Sketch of a three-layer metal "mirrored" bipolar plate. The top and bottom plates are used as anode and cathode plates in two adjacent cells within a stack. After the three metal sheets are brazed together, the inner section allows turbulent cross-flow of cooling water inside the plate — General Motors (Rock, 2000).

FIGURE 4.19 Two flow field plates for a 10-cm2 laboratory fuel cell: (a) machined graphite, and (b) low-cost, highvolume production metal flow field — Umwelt-Campus Birkenfeld and industrial partners.

attained in automotive stacks3, liquid cooling is mandatory. This is traditionally done by introducing dedicated cooling plates into the stack through which water is circulated, by using internally cooled bipolar plates such as those shown in Fig. 4.18, or — in very short stacks — by cooling the stack end plates only.

In less demanding applications, such as portable systems, where the system has to be reduced to a bare minimum of components, air cooling is sometimes applied. In the simplest case, the cathode flow fields are open to ambient, and reactant air is supplied by a fan, at the same time providing cooling. No long-term performance data have been reported for this type of air-cooled stack.

A second function sometimes integrated into the stack is reactant humidification. It is currently unknown whether innovative membrane concepts (see Section 4.3.1.3) will ever allow unhumidified operation in high-performance fuel cell stacks. In most automotive stacks to date both fuel gas and air are most likely humidified because maximum power is required and can only be achieved with the lowest possible membrane resistance.

3 Ballard and GM have reported electric power densities close to 2 kWdm-3.

The literature reflects several types of humidifiers, bubblers, membrane or fiber bundle humidifiers (Strasser, 1993), and water evaporators. The simplest humidifier is the well-known "bubbler," essentially the wash bottle design with gas directly passing through the liquid. Clearly, this approach allows only poor control of humidification, is less suited within a complex fuel cell system, and may cause potential safety hazards due to the direct contact of the fluids. Another approach is using a membrane humidifier. A semi-permeable membrane separates a compartment filled with water from a compartment with the reactant gas. Ideally, the gas is conducted along the membrane and continually increases its humidity up to or close to saturation as it passes from the gas inlet to the gas outlet. A counterflow arrangement is helpful (Strasser, 1993). Also, the membrane may be tubular (Strasser, 1993) or may even consist of tube bundles (such as hollow fiber devices), which can also be switched in and out in order to control the level of humidification (Katagiri et al., 2001). The use of activated carbon in the conduits for the humidification water has been suggested for water purification in situ (Kabasawa, 2001). Instead of liquid water, the use of the water-saturated cathode off-gas has been suggested for cathode (Strasser, 1993) or cathode and anode humidification (Ikegami et al., 2001).

At least three companies, Plug Power, Sanyo Electric, and Tanaka Kikizoku Kogyo (Yanagihara, 1997), have patented concepts that combine humidification and cooling. The Plug Power design (Vitale and Jones, 2000) consists of a cooler-humidifier plate with a flow of water to remove heat on one side and wicks that moisten the reactant gases on the other side. Evaporation of water at the humidification side provides additional cooling. Evaporative cooling on its own has also been suggested by researchers from Sanyo (Hamada et al., 1996) but is admittedly not sufficient for fuel cells that are large or have high output power densities.

This concludes the list of the most important functional components of a fuel cell stack. The fuel cell system contains a large number of other components for fuel generation (see Chapter 5), pumping, compression, etc., which are usually just summarized under the term balance-of-plant, BOP. A number of examples of automotive, stationary, and portable fuel cell systems will be presented in the second part of the book, in Chapters 8-10.

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