Hydrogen combustion and hydrogen fuel cells, on the other hand, use hydrogen directly. Therefore, they are a direct hydrogen usage approach. Water and oxygen are the only byproducts of this approach. Electrolysis of water (via solar, wind, hydro, human/animal power, etc.) yields hydrogen, so the byproduct can convert directly back into the fuel. Microbes can even be harnessed to produce hydrogen. Fuel renewing for direct hydrogen usage can occur in less than a second, as opposed to a growing season for crop-derived fuels and millions of years for fossil fuels. Furthermore, hydrogen is the most abundant element on earth and can be extracted from crops, livestock operations and even fossil fuels.
Read The Phoenix Project: Shifting from Oil to Hydrogen by Harry Braun