Heat Generation in a Fuel Cell

For a given fuel supply rate (hf), a fraction (( )f) of the fuel takes part in the electrochemical reaction, producing electric power and the by-products water and heat. The rest of it may either react chemically to produce the product water or perform a side chemical reaction to produce other products and hence produce additional heat energy. We can classify the following three consumption rates of fuel molecules  

Each one of these reaction terms will generate heat in the fuel cell. For simplicity, the consumption of fuel molecules by the side reactions that produce other products may be assumed as negligible as a first approximation. 

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Heat generated in a fuel cell stack is then ... 

In other words, the heat generated in the fuel cell, plus the heat brought into the cell with reactant gases, is taken away from the cell by heat dissipation from the cell surface to the surrounding, by the reactant gases leaving the cell, and by the coolant. The temperature inside a fuel cell may not be uniform it varies from inlet to outlet, from inside out, or from cathode to anode. The cell temperature may be approximated by the following temperatures, which are much easier to measure than the cell temperature ... 

Equation 1.3 indicates that, for an electrochemical reaction, part of the reaction energy (AH) is used to generate electrical energy (AG), and the other part is used to produce heat (TAS). In a fuel cell system, the most useful energy is electrical energy, while the heat produced is sometimes not desired. Therefore, electrical efficiency (or the reversible or thermod5mamic efficiency), can be defined as the ratio of the maximum electrical energy from the cell reaction to the reaction enthalpy. This represents the theoretical upper limit for fuel cell electrical efficiency. 

For all but the smallest fuel cells a cooling system will be needed. In the case of CHP systems, this will usually be called a heat exchanger, as the idea is not to lose the heat but to use it somewhere else. Sometimes, in the case of the higher-temperature cells, some of the heat generated in the fuel cell will be used in fuel and/or air pre-heaters. In the case of the PEM fuel cell, to be described in detail in Chapter 4, there is often the need to humidify one or both of the reactant gases. 

In Appendix 2, Section A2.6, it is shown that the heat generated by a fuel cell, if the water exits as a vapour, is... 

As we have mentioned before, a bipolar plate serves multiple purposes in a fuel cell sfack. If serves as a current collector, dissipates the heat generated in the fuel cell by conduction through the solid material and by convection through gas channels and cooling channels, and provides the structural strength and support for the thin MEAs in the stack. Selection of a material that satisfies all these requirements is to some extent challenging and requires extensive analysis depending on the applications. 

The exact amount of water may be dosed by a metering pump. It is important that water is injected in the form of fine mist, so that a large contact area between water and air facilitates evaporation. However, simple injection of liquid water in the gas stream may not be sufficient to actually humidify the gas, because humidification also requires heat for evaporation. The enthalpy of water, even if the water is hot, is usually not sufficient and additional heat is required. The sources of heat may be the air compressor (obviously applicable only in pressurized systems) and the fuel cell stack itself. In most of the operating conditions, there is sufficient amount of heat generated in the fuel cell stack. It is the duty of the system to transfer a portion of that heat to the humidification process (Figure 9-20b). 

Electrical and thermal conductivity are important diffusion layer properties that affect the fuel cell s overall performance. The maferial chosen to be the DL in a fuel cell must have a good electrical conductivity in order for the electron flow from the FF plates to the CLs (and vice versa) to have the least possible resistance. Similarly, the DL material must have good thermal properties so that heat generated in the active zones can be removed efficiently. Therefore, in order to choose an optimal material it is critical to be able to measure the electrical and thermal conductivity. In this section, a number of procedures used fo measure fhese paramefers will be discussed. 

Hydrogen is a nonpolluting fuel which bums in air to produce water again releasing large amount of heat. Alternatively, H2 and 02 may be made to recombine in a fuel cell generating electrical energy. 

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