How Many Cells do I Need in my Fuel Cell? (Simple Fuel Cell Design Guide)

 

As with Electrolyzers on my previous post, there are many nuances in the optimal design of a Fuel Cell.  The proper selection depends on the application, type of fuel cell, component selection and design (MEAs, flow fields, etc.) and a host of other factors.

 

However, I will start with a simple tool that will help determine some general characteristics of a typical H2/Air fuel cell.  Thanks to our handy simple Fuel Cell Design Guide, you can take an IV curve of any MEA, choose an active area, operating point (current density) and desired power output and we will tell you how many MEAs you will need and the voltage and current of the fuel cell operating at that point. If you want to skip to the excel sheet, here it is, otherwise I will  show you how this sheet works and why it’s useful.

 

Most often, the user starts with a Power requirement: “I need a 10 kW fuel cell.” Even this statement must often be evaluated.  The actual size of fuel cell may be able to be much smaller since fuel cells are typically operated in a hybrid mode with a battery to help take peak loads.  This means that the fuel cell only needs to be sized for the average power consumption for the application.

 

As an example, maybe you’re designing an aircraft that needs 10 kW to take off, but only 3 kW when in normal level flight.  The battery can be sized to provide the power during take-off and maneuvering, so the fuel cell only needs to be big enough to provide cruising and maneuvering power and maybe recharge the batteries some for the next maneuver.  Now you only need to have a fuel cell that’s 3-5 kW, not 10 kW.  

 

This is can be a big cost and space savings (and can even be a weight savings).  Of course, doing the work to size a fuel cell like this can range from back of envelope, educated guesses to complex simulations and evaluations of load profiles, etc.  And 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 fuel cell designed for a maximum power of 1 kW.  This efficiency usually plays a factor only in applications with longer operating times where the amount of H2 stored or supplied can affect the overall system (i.e. long run time aircraft where total mass is critical).

 

Let’s assume you have already done this and you have concluded that you need a 5 kW fuel cell.  That’s well and good and we can actually sell you a complete 5 kW fuel cell (email me for more information if you just need a complete fuel cell), but you’ve decided you either want or need to design & build a fuel cell yourself.

 

One of the first things you will need to decide is what size (Active Area) and how many cells & MEAs you need to buy from yours truly (after all, who else would you turn to for your custom fuel cell components, right?).

 

As with everything fuel cell related, there are lots factors to keep in mind:

 

 

1. The current a fuel cell is able to produce is determined by the active area and is independent of the number of cells.  The current produced is the current density (A/cm²) you will operate at multiplied by the active area (cm²).

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) and 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 on the number of cells since each cell will consist of both an MEA and a Bi-Polar plate (twice the savings by reducing the number of cells)

5. The voltage of the MEA (and thus the fuel cell) will vary according to the amount of power you are drawing from it.  Therefore you will usually need some sort of power conditioning to regulate the power to work with the rest of the system.

 

I’ll keep it mostly at the system level:  You may say you want your Fuel Cell to output 12V, but the reality is that the fuel cell output will vary as the power demand from it varies.  When you are drawing less power, the fuel cell will operate more efficiently and actually produce power at a higher voltage and lower current.  

 

As you try to draw more power from it, the voltage will begin to decrease and the current will increase (batteries do something similar, but I believe the phenomenon is more pronounced in fuel cells).  This means that you may need a DC/DC converter to keep the voltage output at your required voltage to operate well with your load.  This isn’t always the case and depends on how clean the load requires the power to be.  Simply charging batteries can take a pretty wide voltage range, but many electronics may require much tighter and cleaner voltages.

 

If you do need a clean voltage output, rather than trying to design a fuel cell that operates at 12V (but in reality may output voltages either higher or lower), you can design a fuel cell that can only output a maximum of 10V and then use a DC/DC to boost it to a stable and clean 12V.  A note on the system design:  

 

It is usually much easier to design the FC to always be providing either less voltage than the final desired amount, or more, but not operating where it sometimes provides less and sometimes more.  Finding a buck/boost DC/DC that meets your requirements and can handle both increasing and decreasing the voltage is more expensive and usually heavier since it is basically two devices in one.

 

And all of this doesn’t address the finer points like flow field design, operating parameters, etc.  But not to worry, we’ll do what we can to help you with all of that, as well!