A problem that is raised over and over again in the energy literature is the poor conversion ratio of green energy sources. Hydrogen, for example, may lose as much as 75% of its energy value between raw material and use as energy. Most projections assume that we never will do very much better than current efficiency levels.
I have no idea why this is. Perhaps there is a fundamental issue I am not aware of, but looking back at the history of, say, the steam engine, the improvements inefficiency are astonishing. I have read that Newcomen’s engine converted less than 1% of the available energy, but James Watt and Matthew Boulton’s improvement were only the first of many massive improvements up to and including the modern steam turbine.