In addition to the use of flywheels, energy efficiency can also be improved by using short-term compressed hydrogen storage over energy intensive storage as a cryogenic liquid. Additional short term storage capacity is potentially available within the transportation sector itself. All the scenarios modeled (Figure 6.6) in this chapter essentially take no credit for hydrogen storage onboard vehicles, using instead the conservative assumption that a future hydrogen transportation sector would have patterns of refueling identical to present fossil fueled transportation. This may not necessarily be the case. One intriguing possibility is the potential for energy consumers in a hydrogen transportation sector to adjust refueling and travel patterns, and perhaps end-use efficiency (speed), in response to near term and seasonal hydrogen supply levels. A responsive transportation sector could significantly impact renewable utilities. Fueling hydrogen transportation can account for 20-50% of electricity generation, and the hydrogen onboard vehicles at any given time is 2-4 times the amount in short term utility storage. If transportation demand turns out to be flexible, this could then buffer utility storage, improving hydrogen storage efficiency, perhaps ameliorating seasonal requirements for long-term cryogenic storage. Going one step further, some have proposed using fuel cell passenger vehicles as backup utility storage, noting that the generation capacity of parked vehicles in the United States would be on the order of 20 times the projected US generating capacity.
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