30-Minute Hydrogen Refuel Revisited
By Bill Moore
Posted: 07 Oct 2009
Recently, Terry Tamminen, the former 'green' advisor to California Governor Schwarzenegger, cited his recent drive to Palm Springs in the hydrogen fuel cell car he leases from Honda as proof that this technology is superior to battery electric cars. He explained that he was able to drive all the way out to the desert community and refuel the car at a public refueling station in around 7 minutes time. He contended that had he been driving an electric car like the Tesla Roadster, he'd be stuck somewhere back up the road, spending hours waiting to recharge the battery.
In my recent EV World Insider commentary, I challenged his assertion of a seven minute refill time, based on remarks I'd heard while on my recent trip to Montreal. Someone, and I cannot recall who, explained that it would take at least half a hour to completely refill a hydrogen fuel cell car's tank. So, I dutiful noted this in my commentary.
Then I got an email from Keith Wipke at NREL.
Keith informed that that my comment was, in effect "factually wrong," and he asked me to call him, which I did. It turns out that the information I had been given was out of date. Wipke is the focal point of NREL's hydrogen fueling station data collection network monitoring 20 stations and 140 fuel cell vehicles. To date, he's collected data on 21,000 refueling events, so he knows whereof he speaks.
He explained that Terry Tamminen could easily have refueled his car in the 7 minutes he describes, since the average flow rate of those 21,000 events is 0.8 kg/minute. The Honda FCX Clarity has a 5,000 psi tank holding approximately 4 kg of hydrogen. At 0.8 kg/ minute, you can refuel the Clarity in 5 minutes, he estimates. Even short times are possible, since he's also seen flow rates in excess of 1 kg/minute.
The comment about half filling the tank is based on a hydrogen refueling station that does not use cascade refueling. If only a single pressure tank is used, as you refuel the car, the two tanks tend of equalize their pressure at somewhere below the maximum for either. In order to completely refuel the car's tank, you have to turn on pumps and repressurize the system, which can take much longer.
In cascade refueling, typically three pressure tanks are used, starting with the lowest. A sensor in the system automatically switches from one tank to the next, each being at a higher pressure relative to the car, allowing its tank to be refueled back to its originally designed pressure.
Thanks to Keith for the clarification.
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