I stand before you, holding in my arms a weightless bottle containing one ounce, 25.4 grams, of hydrogen gas. It's compressed to 300 atmospheres, about 4400psi, so it fits in a one-liter container. I count eight ways, so far, in which this can be converted into energy.
1. Use its potential physical energy. Drop it on a paddlewheel, making it turn. The four-foot fall will convert its initial potential energy into kinetic, which can then become electricity, or can grind corn, or can spin a siren.
2. Use its pressure. Open the valve, releasing the pressure into the paddlewheel, making it turn.
3. Use its potential chemical energy. Burn it, creating heat.
4. Use its chemical energy again. Feed it into a fuel cell, creating electricity.
5. Partially convert its mass into energy. Fuse it into helium (if it is partially deuterium, since we need those neutrons).
6. Directly convert its mass into energy, with complete yield.
7. Use its potential economic energy. Sell it for money, purchase energy with the money.
8. Misdirection: use the potential economic energy that encloses it. If you can learn how to reproduce the technology that allows me, standing here at 1G but by adding no weight, to contain a liter of hydrogen in my arms at 4400 psi, you can ignore the hydrogen and get value from the storage technology.
Not all of these technologies are available today. Each has an associated inefficiency. But all assume that the hydrogen exists. It is easy to get excited about the potential of hydrogen, but unless the whole equation is included, from getting the hydrogen to using it, that potential is not being accurately evaluated. If hydrogen is to be burned, it is needed in large quantities; if those will be taken by converting coal or petroleum, the overall cost and efficiency, compared to using coal or petroleum directly, has to be considered. This is why hydrogen is considered only a transfer technology, not a primary one.
