How Long Will My Fuel Cell Run?

One common question among those using fuel cells for off-grid power applications is, how long will my fuel cell last? How do I determine the storage capacity of my fuel cell? In many cases, after talking to a fuel cell supplier, you receive a cryptic answer suggesting that the run time of their fuel cell depends upon the load and duty cycle of the device, commercial electronics, or collection of devices you are looking to power. A number in Amp-hours (Ah) representing the storage capacity of the fuel cell is quoted, leaving the novice with little recourse. But figuring out exactly how long your fuel cell will last is actually very easy.

For example, let’s assume we’re tailgating at an NFL football game and decide to use a hydrogen powered PEM fuel cell to power our tailgating party. The device is a 5kW hydrogen powered fuel cell rated at 420Ah with an output power of 12VDC. During the tailgating party, we intend to use the following items.

• (1) 60 watt light used for 2.5 hours
• (1) compact refrigerator rated at 60 watts with a 25% duty cycle (a cooler would be much more efficient, but for argument sake, let’s say we want a fridge)
• (1) 13” TV rated at 100 watts used for 4 hours

We will assume that the required power for each of these devices is 120VAC. The total power required by these devices is 220W. Therefore a 300W power inverter should be large enough to convert the 12VDC output of the fuel cell to 120VAC. The first step is to calculate a total power required in Watt-hours at the rated voltage. This can be calculated by multiplying the device’s power, by the time in hours, by the duty cycle.

Power (W) x Time (hrs) x Duty Cycle (%) = AC load (Wh)

The total AC load can be calculated in this way:

• 60W Light: 60W x 2.5hrs x 100% = 150 Wh @ 120VAC
• Refrigerator: 60W x 10hrs x 25% = 150 Wh @ 120VAC
• TV: 100W x 5hrs x 100% = 500 Wh @ 120VAC

Total Daily AC Load = 800Wh @ 120VAC

Determine the DC current rating by dividing the Total Power by the DC voltage and multiply by a correction factor of 1.1 representing the typical efficiency of a power inverter. In order to correctly obtain the DC current rating for these devices, the efficiency of the power inverter, generally close to 90%, must be considered.

800Wh / 12VDC x 1.1 (Efficiency) = 73A

At 420Ah, the 5kW fuel cell should be able to sustain these loads for about 5 hours 45 minutes on a single tank of hydrogen. Or 420Ah / 73A = 5.75 hours. Therefore to keep the party going, a second hydrogen tank will be required.

If our power source were a battery instead of a fuel cell, things would be different. Typically, batteries are only discharged to 50% of their rated storage capacity in Ah, as discharging below 50% will damage the battery. Therefore, in our example above the 420Ah associated with the fuel cell must immediately be cut in half to make sure that the battery is recharged when a mark of 50% of the battery storage capacity is reached.