> But these aren't the only ways to use the field to collect 
> solar energy, nor is it the only way to utilize electrical 
> energy if that's what you have the field produce.

Very true. Perhaps I should have been more generic in my description.

> For example it's not clear to me at all that the best 
> artificial way to collect solar energy over a large field is 
> with solar panels.  At efficiencies of 10-18%, there is a lot 
> of room to do better.  A bunch of mirrors for concentrating 
> the radiation and a custom designed heat engine for the task 
> should be able to do considerably better.  The theoretical 
> maximum high temperature for the heat engine is the 
> temperature at the surface of the sun, and the low is ambient 
> air.  The carnot efficiency should be pretty good.  Even if 
> only half of that is actually realized, it should still beat 
> solar cells handily.

Now you have a few choices for heat engine. Stirlings have NOT caught on
well have they? Could it be the pricing? Or is it that to approach high
efficiencies they need working fluids that are caustic and expensive to
contain?

Is there a LOW cost heat engine? One that can be made from existing hardware
for example? 

Steam engines are a possibility, but the boilers are unsafe and valving
modifications can be difficult. I personally have seen a 5hp Brigs and
Stratton gas engine that was modified by replacing the cam shaft to run as a
steam engine. 

And there are other ideas.... <sly grin> Such as /internal/ boiler steam
engines:
http://www.massmind.org/techref/idea/mc-heat-inject.htm a version of this
with a much more expensive injector and an external (not quite) boiler has
been done
http://www.flashsteam.com/Steam_Engine_Project.htm

> Now what can you do with that energy?  A heat engine could 
> run a generator and then you can compare apples to apples 
> with the solar cells.  However there may be other ways to at 
> least partially utilize the heat or the mechanical energy 
> that would have turned the generator.  I don't understand all 
> the processes, but there must be some way to produce chemical 
> fuel, like separating hydrogen and oxygen from water.  Carbon 
> based fuels are more convenient to our current 
> infrastructure.  I've always been intrigued by the idea of 
> taking water and grabbing CO2 out of the air to make a carbon 
> based fuel, like methane or methanol for example.  I think 
> the real problem is that there is so little carbon in the 
> air, that concentrating it will be inefficient.  Still, there 
> is nothing theoretical against this that I can see, and the 
> low fraction of carbon in the air makes it difficult but does 
> not require it to be theoretically inefficient, although you 
> have to harness the energy represented in the "waste" oxygen 
> that will be produced.  I know this is totally impractical 
> today, but I wonder how feasible it could be if there was a 
> concerted effort to make it so.

Wow! Now that is interesting... Where does CO2 (as in fire extinguishers,
dry ice, etc... Come from now?) Pulling it out of the air could help to
reverse global warming as well, right?

> I think you need to separate what is theoretically feasible 
> from what is practical with today's existing infrastructure.  
> I think in this discussion we are talking about what could be 
> long term, so today's infrastructure limitations aren't relevant.

The cost of development is a cost, so that has to be considered as a startup
cost in the system. I would love to know more about what could be done
today, and is perhaps not being done because it is not considered efficient
enough or is to costly to start. Then you can look for ways to get around
those problems.

> 
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