I'm impressed about how Hymotion designed the hydrogen tanks for fuel cell cars:
Volkswagen mounted this hydrogen intake in place of a gas filler. The HyMotion holds just over two and a half kilograms of hydrogen in tanks wrapped in carbon fiber and Kevlar. John Tillman, head of Fuel Cell Research, pointed out that jet fighter fuel tanks are made of similar materials, and can be jettisoned at high altitude to be recovered later. The HyMotion can go about 160 miles when its tanks are full, getting the gasoline equivalent of 42 mpg to 62 mpg.
Thanks for the info. Their site is down at the moment. Will check it out later.
Solid state storage is another approach to safety and higher packing. There are many approaches going on at the moment and some involve intermediate processes to release the hydrogen.
I've used Kevlar before, in fishing... So I know how strong and light it is. Other excellent candidates for making the tanks are materials used for spiderwire, ten times stronger than steel weight for weight, and others are even stronger and lighter.
One advantage of compressed hydrogen gas is that there is energy that can be utilized in compressed gas, but so far, I have not seen any concept cars out there that are first using the energy of compressed hydrogen, and then using the hydrogen in fuel cells. There are efficient compressed air engines such as those of www.engineair.com, mentioned by Jason in other posts, or those used by Motor Development International (MDI) Founder Guy Negre.