From: kk881595@longs.LANCE.ColoState.Edu (kevin knappmiller)
Subject: Re: Renewable Energy - solar
Date: Thu, 19 Nov 1992 23:39:43 GMT
Reply-To: kk881595@longs.LANCE.ColoState.Edu
Organization: Colorado State U. Engineering College


|> > I wonder what you mean by efficient.  If one considers the
|> > solar energy hitting a leaf, photosynthesis converting that energy,
|> > the plant forming cellulose from that energy, that cellulose 
|> > being collected by humans, then burned to produce heat, that heat
|> > converted to work that is then converted to electricity as
|> > the system, do you really claim that this is more efficient
|> > than even the worst photovoltaic cell?  If one added the
|> > additional processes involved in converting the plant to
|> > fossil fuel then the "efficiency" would be even worse.
|> 
|> Now why would anyone wnat to make such a comparison?  Are you proposing that
|> we mow down all the forests and replace them with solar cells because
|> the forests are so inefficient? 

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.

|> 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.

|>  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.


|>   The only efficiency that counts is the efficiency of the
|> equipment we use to convert the available energy into useful work, it doesn't
|> matter how the energy got there in the first place.
|> 

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.

The reason I brought the point up in the first place is because
when we use terms like efficiency we should understand what we are
saying and how it will be interpreted or else our communication
is far less effective.  How can anybody but the person who posted
the claim of *best* efficiency really know what he meant. 
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.

Kevin Knappmiller
Solar Lab
Colorado State University
Fort Collins, CO  80523
(303)491-8215
kk881595@longs.lance.colostate.edu
Newsgroups: sci.energy