Re: Heat engines in practice
From: Gerard Westendorp (westy31_at_xs4all.nl)
Date: 08/26/04
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Date: Thu, 26 Aug 2004 09:31:26 +0000 (UTC)
Y. T. wrote:
[..]
> In that context I was wondering what the smallest temperature
> difference is across which anybody has ever actually managed to run a
> heat engine process in a sustained way. I understand that carnot
> efficiency goes to heck as the two heat baths approach each other in
> temperature, but that is only a matter of having enough energy at hand
> to begin with.
>
> Here at the border of southern CA and AZ, for example, we get a whole
> lot of solar energy during the course of the day.
Nature itself already makes quite good heat engines operating on
small temperature differences: Wind. Even small temperature
differences, say only 1K, can create significant pressure
gradients if you have have a high "chimney":
delta_P = rho * g* h * delta_T /T
Just make h big, like 1000 meter.
Small differences in the temperature of the atmosphere get converted
to kinetic energy (wind). We can harvest this with a windmill.
Maybe if you dug a tunnel(or lay down a big tube) from the top
of a mountain in a desert to the bottom, you would get quite
a strong downward draft in the tunnel, because the air in it
will be colder than the hot air outside during the day.
Hmmm, making use of a mountain seems a lot better than those
proposals for building 2 km high towers in the desert...
These are seriously being considered!
Gerard
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