Hydrogen Storage Systems

Storing sufficient hydrogen on a vehicle to power it for adequate distance, safely, and at reasonable cost, without an excessive weight penalty has been and remains a serious challenge for the automobile industry and its suppliers. Hydrogen storage is among the two or three areas of greatest concern, including all of the other cost and technology challenges associated with developing fuel cell systems for consumer vehicles.

Unlike other major technologies being pursued in support of ZEVs, hydrogen storage technologies have advanced relatively little in recent years. However, in the last 3-4 years, as it became apparent that on vehicle fuel reformers for generation of hydrogen from carbon based liquid fuels were not a viable option, many alternative storage concepts have begun to receive significant research attention.

A few concepts (e.g., metal hydrides and carbon nanotubes) that have been investigated at relatively low levels of effort for many years are now receiving increased attention. However, these efforts are fairly young and it is still too early to determine if they will result in technically and economically realistic hydrogen storage system alternatives. Near Term Outlook

In the near term, the dominant form of storing hydrogen onboard light vehicles will continue to be compressed hydrogen gas. Most OEMs preferred 700 bars, which will provide storage of over 50% more fuel in the same space envelope and correspondingly provide almost 50% more range than 350 bars. Using 700 bar storage pressure is not, however, without problems. The volumetric density (kWh/L) will be higher but unit energy cost ($/kWh) is also expected to be higher and the gravimetric energy density (kWh/kg) about the same. It may also require either reduced fill rates or pre-cooling of the hydrogen prior to transferring into the vehicle tank to avoid overheating the tank structural materials.

Liquid hydrogen storage is being demonstrated as workable but with limitations. It provides both higher gravimetric and volumetric density advantages over compressed gas storage but has issues with boil off and dealing with cryogenic liquids. It is not likely to be widely accepted by automobile OEMs.

An important issue with any of the short-term hydrogen storage options is the need for widely accepted codes and standards for permanent storage, onboard storage, and all aspects of transferring and transporting hydrogen.

Cost is another important issue, especially for the short term since none of the storage systems are produced in sufficient volumes to allow significant production economies of scale. Current or near-term costs for the essentially one-of-a-kind hydrogen storage systems are approximately $10,000 or more each for both liquid and compressed gas storage. Longer Term Outlook

For the longer term, some of the alternative storage technologies being researched may prove to be effective. Both solid and liquid carriers are being researched with hydrogen "recharging" being carried out both onboard and off of the vehicle. There don't appear to be any clear winners at the present among these alternatives. It appears to be too early to make reasonably accurate projections. Conclusions

On-board hydrogen storage is a major challenge for hydrogen fuel cell vehicles. At present, the only technology being demonstrated by the OEMs, with the exception of BMW, is compressed hydrogen gas storage which has problems providing sufficient vehicle range without excessive volume, weight, and cost.

The volume issue can be partially resolved by using 700 bar storage (thus a smaller required volume) and by innovative vehicle design or design modification. Such innovations might include utilization of a long, small-diameter tank running longitudinally where the center "tunnel" is located and/or replacing rear coil springs with leaf springs to increase space available for hydrogen tanks. Thus, depending on the type of vehicle and system efficiency, it seems likely that sufficient compressed hydrogen could be stored on a vehicle to provide a range in excess of 200 miles, perhaps reaching 300 miles or more.

Liquid hydrogen storage technology appears to have advanced sufficiently that, within certain constraints, it could be utilized. The advantages of liquid hydrogen, higher storage density and low pressure, suggest that it also could provide an adequate range.

However, it seems unlikely that either compressed or liquid hydrogen storage systems can meet weight or cost targets, especially for 2015. Using the TIAX estimates for mass-manufactured tanks, the system cost would be about $10-$12 per kWh for 350 bar systems and $13-$15 per kWh for 700 bar systems compared to DOE targets of $4 per kWh for 2010 and $2 per kWh for 2015. Assuming that at least 5 kg (165 KWh) of hydrogen will be needed to provide sufficient vehicle range, the cost would be $1,650 even with the lowest TIAX tank cost estimate. For liquid storage, the cost would be even higher. There is little expectation that the cost of either of these systems will go much lower even with higher volumes.

The weight outlook is better than the cost outlook. The TIAX projections for weight fraction are slightly over 6% for both 350 bar and 700 bar systems, compared to the DOE targets of 6% for 2010 and 9% for 2015. The pressure tank manufacturers have also indicated that 6%, and perhaps a bit higher weight fraction is within reach. For a 6% weight fraction system to contain 5 kg of hydrogen, the system would weigh about 83 kg (about 183 lb). Neither TIAX nor the tank manufacturers project that the 2015 target of 9% can be met with pressurized hydrogen tanks.

There are many alternative hydrogen storage systems under investigation. Some of the absorption materials being investigated are relatively inexpensive and have shown, at least in the research phases, the capacity to contain well over 6% hydrogen. However, the remainder of the support system could have a huge effect on both cost and weight fraction.

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