First, start with a cheap plentiful supply of biomass (algae, duckweed, switchgrass, etc.).
Second, build a solar concentrator.
Third, use the solar concentrator to cook the biomass to create charcoal (capture the vapors and condense them; no matter what kind biomass you use, they'll surely be
useful).
Fourth, (after all of the volatiles have been driven off) add water or steam, and keep adding heat via the solar concentrator. The steam will react with the carbon to create "water gas" (a mixture of carbonmonoxide and hydrogen, both of which are flamable).
Fifth, add more water to the "water gas", and more heat from the solar collector. At the right temperatures, a chemical reaction (called a "water gas shift") will occur, where the CO and H2O change into CO2 and H2. This hydrogen gas is the end product.
So one atom of carbon, and two molecule of water, have been changed to one molecule of carbon dioxide, and two molecules of hydrogen.
If the biomass was produced by algae or duckweed, we can extract the CO2 from the hydrogen, and send it back into the farm by bubbling it through the water that the greenery was grown in. (There might be better ways of dealing with the carbon dioxide, but I can't think of any offhand).
This was inspired by bungston's idea, the home solar charcoal distiller [link].