Heat Management in Fuel Cells

For a fuel cell to run efficiently, there needs to be proper control of its temperature and heat generation. Some fuel cells work well in room temperature, but others require temperatures as high as 1000 ºC, and any value outside of the accepted range results in lowered efficiency of the device. Higher temperatures lead to faster kinetics and voltage, and lower temperatures cause shorter warm-up times, lower thermodynamical stresses and retardation of corrosion and other temperature-dependent processes. For fuel cells, higher temperatures also mean greater vaporization of the liquid water and, as a result, more of the waste heat becomes the latent vaporization heat.

The temperature profile in a fuel cell is ever-changing, even when the flow rate of the gases is constant. That happens because of the transfer of heat and phase change of some reactants. The accurate prediction of the temperature and heat distribution is essential to determine temperature-dependent parameters and rates of reaction and species transport. Between the solid surface and the gas flow, there is the occurrence of convective heat transfer, and conduction heat exchange takes place in the solid and porous materials.

The amount of produced power, the fuel cell reactions and heat loss are important variables to be considered in the total energy balance. The balance differs according to each fuel cell type due to the different reactions that occur in the designs. The overall energy balance, however, can be summarized to "enthalpy of the input gases equals the enthalpy of the products leaving the fuel cell plus the generated heat and power, and the heat lost to the surroundings".

One particular problem is that for small stacks of fuel cells or single stacks, the relatively large surface area means that heat dissipation to the surroundings through convection and radiation plays a more relevant role to the energy balance than in other settings.

Since heat is generated in fuel cells, another important topic to consider is cooling. This process can achieved in many ways. For example, passive cooling can be achieved with fins and heat sinks, dynamic cooling can be obtained with turbine reheaters and metal hydride containers. Fuel cell stacks usually need cooling systems to keep the homogeneity of the temperature distribution across the cells. On the other hand, small fuel cells may not need any cooling strategy.

Source: http://www.fuelcellstore.com/blog-section/fuel-cell-heat-transfer-management

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