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Re: Fuel Cells and "green" energy

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.

Availability of solar energy coincides pretty well with peak electricity
demand, especially in places like southern California. Also, solar and
wind combined will be much more "available" than either individually.
Whether in the future we'll have renewables supplementing fossil fuels
or fossil fuels supplementing renewables is a question of semantics,
I think.

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.

Not at all true. Even if wind and solar were to provide 100% of total
energy needs, only a PORTION of the energy would have to be stored for
later delivery. We can argue about how large that portion is--obviously
it depends on location. In any case, 25-50% is a long way from 100%.

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.

I think your factor of 4 is overly pessimistic. Again, remember that
the conversion losses will apply only to a portion of the energy, not
to all of it. Besides immediate use of electricity, there's also solar
space heating and water heating which don't even suffer from the
low efficiency of photovoltaics. Some energy can be stored in
pumped hydropower facilities, which are much more than 25% efficient.
Even when we need to use hydrogen for energy storage, it is
quite possible to convert that hydrogen to electricity at better
than 50% efficiency. Combined-cycle gas-fired plants already do
better than 50%, and fuel cells may do better still, as you say.
For transportation, we currently live with engines that are only
about 25% efficient. Replace those with 50%-efficient fuel cells
and you can probably live with the energy cost of producing the
hydrogen in the first place.

Currently, many utility customers pay higher rates for electricity
at times of peak demand, because it costs more to generate the
extra electricity at those times. In the western U.S., peak
electricity prices paid by utilities have been skyrocketing, as
I'm sure you've read in the news. In a future economy based
mostly on renewables, electricity prices will probably be much
higher at times when the sun isn't shining and/or the wind isn't
blowing. This will encourage customers to use less electricity
at those times, just as many large industries already use less
electricity when they have to pay peak rates. It isn't obvious
to me that the overall inconveniences will be any greater than
the ones we currently live with (especially when you factor in
the environmental costs of current energy sources).