When designing a fuel cell many nuances must be considered. The proper selection depends on the application, type of fuel cell, component design (MEAs, flow fields, etc.), and a host of other factors.
Most often, the user starts with a power requirement, such as “I need a 10kW fuel cell stack.” Even this statement must often be inspected. The actual size of the fuel cell may be able to be much smaller since fuel cells are typically operated in conjunction with a battery to help take peak loads. This means the fuel cell would only need to be sized for the average power use of the application.
As an example, if an aircraft needs 10 kW to take off, but only 3 kW when in normal, level flight.
The battery can be sized to deliver power during take-off and maneuvering. This means the fuel cell only needs to be big enough to provide cruising and maneuvering power and possibly to recharge the batteries for the next maneuver. With this setup, a 3 - 5 kW fuel cell would be more practical than a 10 kW fuel cell. This is can be a big cost and space savings over a 10 kW stack. Sizing a fuel cell like this can be very complex, this is further complicated by the fact that a fuel cell can be made to operate at different efficiencies. For example, a 10 kW fuel cell operating at 1 kW will be more efficient than a 1 kW fuel cell operating at maximum power. This efficiency usually plays a factor only in applications with longer operating times where the amount of H2 stored can affect the overall system (i.e. long run time aircraft where total mass is critical).
After deciding on the proper size fuel cell, the next thing to decide is what active area, and how many cells are practical for your application. As with everything fuel cell related, there are a lot of factors to keep in mind:
1. The current a fuel cell can produce is decided by the active area. Current is completely independent of the number of cells in your stack The current produced is the current density (A/cm²) the stack will operate at multiplied by the active area (cm²) of the cells.
2. The voltage is dependent on the number of cells in the fuel cell. This is determined by looking at the voltage each cell will produce when operating at that current density (from the IV curve of the MEA) then multiplying that by the number of cells.
3. The total power of the fuel cell at that operating point is the current from (1) multiplied by the voltage from (2).
4. Cost, weight, etc. is usually dependent upon the number of cells since each cell will consist of both an MEA and a bipolar plate.
5. The voltage of the MEA (and thus the fuel cell) will vary according to the amount of power being drawn from it. Therefore, you will usually need some sort of power conditioning to regulate the power to work with the rest of the system.
It is much easier to design a stack that operates at either a lower or higher voltage than your system needs, than trying to design the stack to always operate at maximum voltage. If a stack is designed to operate at a lower voltage, then using a DC/DC to boost the voltage to the required voltage will lead to a more stable and clean voltage than allowing the stack to fluctuate. If the stack is allowed to fluctuate and a boost/buck DC/DC is used to meet the requirements this is likely to increase cost and weight since it is doing the job of two devices.
Thanks to our handy Hydrogen Air Calculator Sheet, you can take the IV curve of any membrane electrode assembly (MEA), assign an active area, current density, and desired power output and the calculator will determine the number of MEAs needed along with the voltage and current of the fuel cell operating at that point.
Download the spreadsheet here.
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