Stack coolant flow

The models that examine only stacks focus mainly on the temperature distribution within the stack. As mentioned, there is a much higher temperature gradient in the stack than in a single cell, and it provides design information in terms of coolant flow rate, among other things. - - Also, as mentioned above, transient effects have also been examined. 

For liquid-cooled stacks, the coolant flow rate and the heat exchanger together control the coolant temperatures at the stack inlet and outlet. Figure 1.21 illustrates a coolant loop. A pump is the driving force that circulates the liquid coolant in the loop. It sends coolant from the coolant tank into the stack... 

Although all the methods mentioned above can be used to balance the water, the methods that are based on controlling temperatures are more practical and easier to achieve than the method based on the air flow rate in an actual fuel cell system. The stack inlet and outlet temperatures are mainly controlled by the heat exchanger and the coolant flow rate, and the amount of heat removed is mainly controlled by the fan mounted on the heat exchanger. 

Balancing water is the most difficult task for a PEMFC due to the high water production rate and the various water movement possibilities. Although a previous estimate is very helpful in determining the values of certain key parameters, such as the reactant humidification temperature, the stack inlet and outlet temperatures, the coolant flow rate, and the reactant stoichiometric ratio (mainly air), automatic adjustment by the fuel cell system itself is important in achieving the optimal operation conditions, especially if the fuel cell is in the load-following mode. 

Most of the evaluations on a stack are similar to those on a single cell. The V-I polarization curve is used to eshmate the stack s performance from lower to middle and to higher current densities. Parameters such as the humidi-ficahon temperature, temperatures of the coolant at the stack inlet and the outlet, the coolant flow rate, the reactant stoichiometric ratio, and the RH should be optimized through experiments. The compression force on the stack assembly should seal the stack but not cause damage to the GDM. 

For a liquid-cooled stack, a pump is needed to circulate the liquid coolant through the stack and the heat exchanger, which are the major sources that resist the coolant flow. If a flow meter is incorporated in the coolant flow loop, it can also generate significant resistance to the coolant flow. A pump must be able to meet the coolant flow rate and overcome the resistance in the entire flow loop. 

Part of the coolant loop has two separate paths one path does not go through the heat exchanger and is used from the startup of the fuel cell system until the coolant temperature increases to a preset value (e.g., 60°C) to accelerate the rise of the coolant temperature the other path goes through the heat exchanger and is used when the coolant temperature reaches the preset value. Based on the stack needs, the coolant flow rate is adjusted by the coolant pump, and the heat that is dissipated out of the system enclosure is controlled by the fan mounted on the heat exchanger. 

Fig. 20.9 Repeating unit of the basic stack setup (left), coolant flow Add on the rear of the cathode side bipolar plate half-shell (right)...

Systematic deviations during operation stem from the location of the cell in the stack (i.e., near inlets of air, fuel, or coolant or far from these). Because the media are fed in parallel to the cells and the cross sections of the manifolds are of limited size mainly differences in pressure drop across the cells lead to differences in reactant and coolant flow through cells in different locations. 

The pin-type elements consist of a number of cast, cylindrical, fuel pins stacked end pn end within a thin-waU, circular, metal tube with a thin sodiumbond between the fuel and the tube wall (Fig. 43). The tvibes are spaced in a hexagonal pattern within each subassembly by a slingle, helical rib welded on the outside of each tube. The only difference between the twQ designs of the pin-type is that the Modified Reference Design has smaller diametral dimensions. In the pin-type element the coolant flows on the outside of the tubes. 

The heat transfer from the bipolar stack plate to the coolant flow can be simply estimated by consideration of an energy balance on the flui ... 

Example 5.18 Determination of Coolant Flow Rate Required Consider a 100-plate, 10-kWe PEFC stack operating at 48% thermal efficiency, as determined from a stack voltage measurement. Each individual fuel cell is to be cooled by a flowing Uquid water coolant channel. In order to balance the water generated in the fuel cell and prevent flooding, it is... 

Figure 5.43 Stack plates manifolded with coolant flow channels.

Stack Manifold Flow One of the most difficult engineering challenges in fuel cell stack design is the proper manifold design for fuel, oxidizer, and coolant flow. The manifold design challenge centers around three main consttaints ... 

Temperature Distribution Along the fuel cell channel, the temperature distribution is directly controlled by the heat transfer boundary conditions. For small fuel cell stacks, with no active coolant flow, the external boundary conditions control the temperature distribution and at low current can be considered as uiriform in temperature. For larger stacks with active cooling, the temperature distribution can be engineered to match the desired humidity profile to control flooding and promote longevity by ehmination of dry- and hot-spot locations. In the in-plane direction, temperature variation exists, with more water accumulation under generally colder lands, as discussed. The temperature distribution in the stack can be fairly... 

AT is typically a design variable. Most typically, AT is below 5°C, and rarely above 10°C. Smaller AT results in more uniform stack temperature distribution, but it requires larger coolant flow rate, which in turn increases parasitic losses. Sometimes larger stack temperature variations are needed to maintain the water in the desired state [11],- in that case, the coolant AT is dictated by the stack temperature requirements. 

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