From: stead@skadi.CSS.GOV (Richard Stead)
Newsgroups: sci.energy
Subject: Re: Renewable Energy - solar
Date: 20 Nov 92 15:15:45 GMT

In article <Nov19.233943.21763@yuma.ACNS.ColoState.EDU>, kk881595@longs.LANCE.ColoState.Edu (kevin knappmiller) writes:
> No.  I took issue with the statement that such a system was more
> efficient than "direct" solar.  It isn't, not even remotely.
> Therefore if you have a certain amount of land that gets sunshine
> and you want to get energy out of the sunshine on this land it
> makes more sense to put solar collectors or cells on this land than
> to plant it with crops, at least if the issue is energy production.

If that's the argument, then we also need to consider the energy recovered
and the useful energy expended in this recovery.  For a biomass example,
very little useful energy is expended - in some areas pumps may be required
for irrigation and maybe some fertilizer would be required.  In addition,
there would be the fuel the tractors use to sow and harvest.  This totals
to a very small amount of useful energy.  Solar, on the other hand, can require
tremendous investment of useful energy - particularly if solar cells are used.

> |> The coal that is dug out of the ground
> |> did all the solar collection work for us already.  Thus our only recovery
> |> cost is digging the coal out of the ground.
> 
> That short sighted view is why we are in potentially so much trouble.
> We are using fossil fuels far faster than they are being produced.
> There is a limited supply.

Not short-sighted, I am very much against coal use, but for environmental
reasons, not energy production.  The point is that it is a readily accessible
and very inexpensive (in terms of how much useful energy must be expended
to recover it) to use.  And we get a large amount of the energy it contains
as useful energy.  Supply is a pointless argument - if there is an energy
source and we plan to use all of it at some point, why do we care if it
is used first instead of a competing resource?  Are you arguing that
someone in the future wants to use it for energy?  But who cares - if we
have alternate sources now, they will have the same alternatives then.
If our energy use can not be altered, then it does not matter which
energy source we use to satisfy our needs now.  Of course, I favor
improvements in energy efficiency and conservation where appropriate, but
there is a minimum amount of energy we require now, and it doesn't matter
where it comes from.

Or perhaps you are arguing that coal is valuable for other reasons.  I
cannot agree with that.  Coal has no other use than to produce energy.
Now, oil I can understand.  It is vitally important to the chemical
industry and we would not have a lot of our plastics, pharmecueticals,
etc. without it.  So I can say, no we shouldn't burn up all our oil.
But burning the coal for energy is not removing it from some other use.

So we're left with my argument against coal which is the environmental
problems (coal is by far the worst environmental threat of any energy
source).  In that case, we shouldn't get any of our energy from coal,
ever.  But that argument is independent of efficiency.

> |>  No collectors to manufacture,
> |> and we don't have to care how efficient the original collectors were.
> |> Same thing with tapping hydro power.  Sure the atmosphere and earth's
> |> surface acted as the solar collector there, and probably weren't all that
> |> efficient.  But who cares? 
> 
> The issue was the claim that they were efficient.  The hydro 
> system is renewable and the collector area is huge and free so 
> if we can use it, and we do, then great.  But the "hydro power
> solar collector" is not efficient.

That still does not matter, the energy is just presented to us.  The question
is how easy is it to convert to useful forms, and in particular how much
useful energy must we consume to do that, and how much do we get out.
Efficiency provides a big clue towards the answer to that question.

> If that is what Gary meant then he should have said so.  But then
> the "system" is not solar.  If you talk about a solar energy system
> and its efficiency you have to start at the sun and go to the final
> state at which the energy is used.

I took his comment on the ultimate source of the energy as just a side
comment, not the focus of his argument.

> The point was made that a dam's generators may
> be 50% efficient and the extension was made that this means that
> since solar energy originally evaporated the water then it
> was solar energy and still 50% efficient.  Well the evaporation
> and mass transport are much less than 100% efficient.
> When you have multiple components to a system you have to
> multiply the component efficiencies to get the system 
> efficiency.  He may not know this.  He actually seems not to.
> That is why I asked.

Well, let's go back further then - the source of solar energy is nuclear
fusion.  And the earth is in a pretty bad position to make any reasonable
attempt to efficiently recover all the energy the sun is releasing.
Therefore, all discussed energy sources are all so terribly inefficient
that the differences between them are pretty meaningless.  It would
also imply that the only sane power source on earth is direct nuclear fusion,
since then we don't need to worry about all these efficiency losses
from the collection of solar energy.  Nuclear fission is also pointless
and inefficient, since we're recovering so little of the energy that was
available in the supernova that billions of years ago originally created
all that uranium we use.


--
Richard Stead
Center for Seismic Studies
Arlington, VA
stead@seismo.css.gov