Hydrogen can be stored on board a vehicle as a gas, liquid, or solid. All of these techniques have been demonstrated in HFCVs in recent years. All continue to be developed through government research and industry programs. At this point there is no consensus as to which storage state will be the best long-term solution. However, there is a growing consensus that in the short term, high-pressure gaseous storage is the most practical solution.
Solid hydrogen storage systems that are made up of a low-pressure tank filled with a solid storage material with a thermal management system have the potential to be small and lightweight, which aids in overall weight savings and improved fuel mileage. They may present the best long-term storage solution but at present are still mainly in the research stage (to identify the best solid medium for storing hydrogen). Thus, it is important to develop solid hydrogen storage systems as well as to integrate them into the vehicle in an energy-efficient manner. For further discussion of solid storage technical readiness, see NRC (2008) and Walsh et al. (2007). Based on the committee’s current knowledge that no solid storage medium has yet met all of the developmental targets, it is unlikely that solid hydrogen storage systems will be production ready in 2015.
Liquid hydrogen currently provides the densest form of storage, which means that the most fuel can be stored on board. BMW has recently demonstrated a liquid storage system. Two formidable problems must be overcome to make liquid storage practical for widespread use. Liquid hydrogen must be kept at about −252°C (about 20°C abve absolute zero). The BMW system has impressive insulation, but some heat still gets in and gradually boils off the hydrogen. This can result in serious safety issues. The other issue is cost, currently on the order of $500/kWh, with a goal of approximately $100/kWh in the “next generation” (Brunner, 2006). The eventual goal is $15/kWh. To put these numbers in perspective, the FreedomCar targets for 2010 and 2015 are $4/kWh and $2/kWh, respectively. As shown in Figure 3.2, today’s system suffers from performance, durability, and maintenance issues in addition to the noted major cost issues. The figure shows just inside the rectangle the areas in which R&D is being performed, and the goals are on the perimeter of the 10-sided figure (e.g., keeping evaporation losses to less than 25 percent per month for the infrequent driver). Inside this is the current status of progress toward the goal (e.g., less than 50 percent of the way for evaporation loss). Because of these concerns, the committee does not believe liquid storage systems will be commercially viable in the 2015-2020 time frame without unexpected breakthroughs in liquefaction and insulation.
As noted by Walsh et al. (2007), “With the exception of BMW, every other OEM [original equipment manufacturer] contacted indicated that this (compressed gas) was the only realistic short term (5-10 years) choice available and only Honda indicated that they intend to limit the storage pressure to 350 bar.5 All the other OEMs preferred 700 bar, which will provide storage of over 50% more fuel in the