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There is a major problem here in that wind and solar are not energy on
demand resources. Wind is a little more reliable but the sun sure doesn't
shine at night! Therefore, to be used directly they both are best used to
supplement current power sources--to provide extra energy at peak times.
However, if you want to project wind and solar as eventually providing
25-50% of the TOTAL energy needs then these must be stand alone sources.
You might try to do that by using the wind and solar to make hydrogen (where
this thread began) and then use the hydrogen to power conventional style
power plants--now very clean burning. The problem with that is that you now
have the thermodynamic efficiency losses plus losses in the hydrogen
production such that you must now produce
4 kWh of wind and solar energy for every 1 kWh you can deliver to consumers.
I don't know what the overall efficiency would be if we used giant fuel cell
complexes running off the hydrogen--maybe that is better. Anyway, to get
large amounts of solar and wind into the system as 'energy on demand'
sources may require up to 4x the generators and solar collection areas as
would be needed if these were used directly. Such a system would then
require tens of thousands of square miles of photovoltaics and 10s of
millions of large (megawatt) wind generators.