Heat engines in practice [Archive]

Y. T.

Aug20-04, 02:18 PM

<jabberwocky><div class="vbmenu_control"><a href="jabberwocky:;" onClick="newWindow=window.open('','usenetCode','toolbar=no, location=no,scrollbars=yes,resizable=yes,status=no ,width=650,height=400'); newWindow.document.write('<HTML><HEAD><TITLE>Usenet ASCII</TITLE></HEAD><BODY topmargin=0 leftmargin=0 BGCOLOR=#F1F1F1><table border=0 width=625><td bgcolor=midnightblue><font color=#F1F1F1>This Usenet message\'s original ASCII form: </font></td></tr><tr><td width=449><br><br><font face=courier><UL><PRE>\n\nNot entirely sure if this should go here or somewhere into\nsci.engr.something but it strikes me as a research thing, so I\'ll send\nit here.\n\nWhat with the current global situation with regards to energy, it\nappears to me that venues of generating usable forms of energy that\nare currently deemed inefficient will come more to the forefront of\nthe human collective thinking.\n\nIn that context I was wondering what the smallest temperature\ndifference is across which anybody has ever actually managed to run a\nheat engine process in a sustained way. I understand that carnot\nefficiency goes to heck as the two heat baths approach each other in\ntemperature, but that is only a matter of having enough energy at hand\nto begin with.\n\nHere at the border of southern CA and AZ, for example, we get a whole\nlot of solar energy during the course of the day. If this was used to\nheat some tank full of oil and run a heat engine between the hot oil\nand the cold night-sky, it appears one could generate a decent amount\nof electricity from solar irradiation even during the night. Obviously\nsuch a system should be designed such as to use the minimum amount of\nstorage fluid possible (to get the maximum temperature for a given\namount of energy which will yield the best carnot efficiency upon\nback-conversion). However there\'s also the question of thermal losses\n- which will increase with increasing temperature (but also decrease\nwith decreasing volume (better:surface)) of the storage medium.\n\nIf I figure using 10000sq ft (i.e. 1000sq m) for gathering solar power\nat about 1kW per sq m that would be a megawatt of input - and if I\ncould get only 1/10 of 1 percent efficiency total out of this it would\nbe 1kW which is more than we\'re actually consuming right now in the\nmonth to month average.\n\nIn order to do even the simplest back-of-the envelope calculation,\nhowever, I need to know how cool my storage medium could get and still\nallow me to run a heat cycle from it at all - efficiency be darned for\nthe moment.\n\nIs anybody in the world doing research on this kind of thing? Trying\nto get electricity out of smallish temperature differences? What\'s the\nstate of the art?\n\nIf there\'s one thing we have in the desert here then it\'s large\nuseless areas with nothing but sun shining on them and as much as I\'m\nlooking forward to getting out of the desert this fall (college) it\nseems that there\'s this resource here that is completely unused...\n\n\n\n\n\n\n\ncordially\n\nY.T.\n\n--\nRemove YourClothes before you email me.\nhttp://p.zapto.org/yt\n</UL></PRE></font></td></tr></table></BODY><HTML>');"> <IMG SRC=/images/buttons/ip.gif BORDER=0 ALIGN=CENTER ALT="View this Usenet post in original ASCII form">  View this Usenet post in original ASCII form </a></div><P></jabberwocky>Not entirely sure if this should go here or somewhere into
sci.engr.something but it strikes me as a research thing, so I'll send
it here.

What with the current global situation with regards to energy, it
appears to me that venues of generating usable forms of energy that
are currently deemed inefficient will come more to the forefront of
the human collective thinking.

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. If this was used to
heat some tank full of oil and run a heat engine between the hot oil
and the cold night-sky, it appears one could generate a decent amount
of electricity from solar irradiation even during the night. Obviously
such a system should be designed such as to use the minimum amount of
storage fluid possible (to get the maximum temperature for a given
amount of energy which will yield the best carnot efficiency upon
back-conversion). However there's also the question of thermal losses
- which will increase with increasing temperature (but also decrease
with decreasing volume (better:surface)) of the storage medium.

If I figure using 10000sq ft (i.e. 1000sq m) for gathering solar power
at about 1kW per sq m that would be a megawatt of input - and if I
could get only 1/10 of 1 percent efficiency total out of this it would
be 1kW which is more than we're actually consuming right now in the
month to month average.

In order to do even the simplest back-of-the envelope calculation,
however, I need to know how cool my storage medium could get and still
allow me to run a heat cycle from it at all - efficiency be darned for
the moment.

Is anybody in the world doing research on this kind of thing? Trying
to get electricity out of smallish temperature differences? What's the
state of the art?

If there's one thing we have in the desert here then it's large
useless areas with nothing but sun shining on them and as much as I'm
looking forward to getting out of the desert this fall (college) it
seems that there's this resource here that is completely unused...

cordially

Y.T.

--
Remove YourClothes before you email me.
http://p.zapto.org/yt

Uncle Al

Aug20-04, 02:32 PM

<jabberwocky><div class="vbmenu_control"><a href="jabberwocky:;" onClick="newWindow=window.open('','usenetCode','toolbar=no, location=no,scrollbars=yes,resizable=yes,status=no ,width=650,height=400'); newWindow.document.write('<HTML><HEAD><TITLE>Usenet ASCII</TITLE></HEAD><BODY topmargin=0 leftmargin=0 BGCOLOR=#F1F1F1><table border=0 width=625><td bgcolor=midnightblue><font color=#F1F1F1>This Usenet message\'s original ASCII form: </font></td></tr><tr><td width=449><br><br><font face=courier><UL><PRE>\n"Y. T." wrote:\n>\n> Not entirely sure if this should go here or somewhere into\n> sci.engr.something but it strikes me as a research thing, so I\'ll send\n> it here.\n>\n> What with the current global situation with regards to energy, it\n> appears to me that venues of generating usable forms of energy that\n> are currently deemed inefficient will come more to the forefront of\n> the human collective thinking.\n>\n> In that context I was wondering what the smallest temperature\n> difference is across which anybody has ever actually managed to run a\n> heat engine process in a sustained way. I understand that carnot\n> efficiency goes to heck as the two heat baths approach each other in\n> temperature, but that is only a matter of having enough energy at hand\n> to begin with.\n[snip]\n\nConsider a thermocouple junction warmed above absolute zero (not\ntoo difficult, that). Heat is spontaneously converted into\nelectricity. One could take all the beta-decay waste from\nnuclear reactors and harvest its high voltage electron emission.\nOne could ring Lake Superior with giant Dippy Birds, tie lanyards\nto their butts, and pull on generators.\n\n<http://nicholnl.wcp.muohio.edu/DingosBreakfastClub/DippyBird/DrinkingBirdCarnot.html>\n<http://math.ucr.edu/home/baez/physics/General/dippy_bird.html>\n<http://science.howstuffworks.com/question608.htm>\n\nThe economics stink in all cases. You put more value in than you\ncould ever hope to get out. Ditto sewage batteries, hooking\ngenerators to gym exercise equipment (close to breakeven), and\nburning corn for fuel. That last one is a 70% net energy *loss,*\n\n<http://www.news.cornell.edu/releases/Aug01/corn-basedethanol.hrs.html>\n\n--\nUncle Al\nhttp://www.mazepath.com/uncleal/\n(Toxic URL! Unsafe for children and most mammals)\nhttp://www.mazepath.com/uncleal/qz.pdf\n</UL></PRE></font></td></tr></table></BODY><HTML>');"> <IMG SRC=/images/buttons/ip.gif BORDER=0 ALIGN=CENTER ALT="View this Usenet post in original ASCII form">  View this Usenet post in original ASCII form </a></div><P></jabberwocky>"Y. T." wrote:
>
> Not entirely sure if this should go here or somewhere into
> sci.engr.something but it strikes me as a research thing, so I'll send
> it here.
>
> What with the current global situation with regards to energy, it
> appears to me that venues of generating usable forms of energy that
> are currently deemed inefficient will come more to the forefront of
> the human collective thinking.
>
> 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.
[snip]

Consider a thermocouple junction warmed above absolute zero (not
too difficult, that). Heat is spontaneously converted into
electricity. One could take all the \beta-decay waste from
nuclear reactors and harvest its high voltage electron emission.
One could ring Lake Superior with giant Dippy Birds, tie lanyards
to their butts, and pull on generators.

<http://nicholnl.wcp.muohio.edu/DingosBreakfastClub/DippyBird/DrinkingBirdCarnot.html>
<http://math.ucr.edu/home/baez/physics/General/dippy_bird.html>
<http://science.howstuffworks.com/question608.htm>

The economics stink in all cases. You put more value in than you
could ever hope to get out. Ditto sewage batteries, hooking
generators to gym exercise equipment (close to breakeven), and
burning corn for fuel. That last one is a 70% net energy *loss,*

<http://www.news.cornell.edu/releases/Aug01/corn-basedethanol.hrs.html>

--
Uncle Al
http://www.mazepath.com/uncleal/
(Toxic URL! Unsafe for children and most mammals)
http://www.mazepath.com/uncleal/qz.pdf

Oz

Aug23-04, 04:06 AM

<jabberwocky><div class="vbmenu_control"><a href="jabberwocky:;" onClick="newWindow=window.open('','usenetCode','toolbar=no, location=no,scrollbars=yes,resizable=yes,status=no ,width=650,height=400'); newWindow.document.write('<HTML><HEAD><TITLE>Usenet ASCII</TITLE></HEAD><BODY topmargin=0 leftmargin=0 BGCOLOR=#F1F1F1><table border=0 width=625><td bgcolor=midnightblue><font color=#F1F1F1>This Usenet message\'s original ASCII form: </font></td></tr><tr><td width=449><br><br><font face=courier><UL><PRE>\n[Moderator\'s note: Replies should please make sure they have some genuine physics\ncontent or, otherwise, be taken to private email. -usc]\n\n\n\nUncle Al <UncleAl0@hate.spam.net> writes\n>The economics stink in all cases. You put more value in than you\n>could ever hope to get out. Ditto sewage batteries, hooking\n>generators to gym exercise equipment (close to breakeven), and\n>burning corn for fuel. That last one is a 70% net energy *loss,*\n\n[Note table should be viewed with a monopitched font and\ndue to the wide table width you may wish to widen your\nlinelength to view this post. Apologies here but\nalternatives degrade clarity significantly.]\n\nHmmm. Elsewhere someone posted a url that gave net energy costs for\nbiofuels. The point with biofuels is that they harvest sunlight, if\nvery inefficiently. In practice the efficiency depends on limiting\nfactors. In particular for most plants assimilatable nitrogen is\nlimiting and quite modest additions give a significant increase in\neffective photosynthetic efficiency (the amounts used below\neffectively double yield). Non-N energy includes pesticides, other\nfertilisers, mechanical costs etc etc, combined for clarity.\n\nI converted the maize study into GJ/Ha (1Ha = 2.5ac) and estimated\nfigs for canola. Note that whole maize grains and canola oil are\neasily used in automatic boilers from domestic to industrial.\nUnmodified Canola oil can be used in diesel engines.\n\nNote that canola is european yields where maize would be 10T/Ha,\nfor the same growing costs.\n\n======Something I posted (elsewhere) earlier. ====================\n======ref is http://www.usda.gov/oce/oepnu/aer-814.pdf ==========\n======gives some figures that can be used \'Maize study\'.==========\n> [ More results from www.usda.gov ]\n\n\nMaize OSR\nStudy Canola\n\nYield T/Ha 8.40 3.5 Of grains only.\nTot energy produced GJ/Ha 137 72.5 Canola: oil only @45% oil & 46GJ/T\nExcludes crop residue&byproducts.\n\nNitrogen energy GJ/Ha 6.18 7 160-170kgN/Ha for canola.\nNon-N Energy @1.5GJ/T GJ/Ha 12.6 13 Should be similar to maize\nNet Return on energy used % 731% 362% Excluding residue&byproducts.\n\nNo further processing required for burning grains.\n\nProcessing GJ/Ha 44.7 5 Canola unlikely to be this high.\nFermentation loss GJ/Ha 64.6 NIL\nTotal Energy Cost GJ/Ha 128.1 25.0\nUsable energy produced GJ/Ha 72.6 72.5 3727L ethanol from maize\n\nNet energy gain GJ/Ha 8.9 47.5\nReturn on energy used % 14% 190% Excluding fermentation loss.\n\nByproducts not costed but valuable none the less:\nResidue of feed T/Ha 1 1.9 high grade high protein animal feed\nPlant residue T/Ha 3 2 Combustible: worth 1/3 oil.\nEnergy value GJ/Ha 42 28 Plant residue (not feed).\n\nNote 1: Currently canola used in commercial diesel engines is\nmethoxlyated and purified. AIUI this is primarily to give\nequivalent viscosities, freezing points and to degum the oil.\nIf unmodified or limited modification canola become significantly\nused then there will have to be some modifications to vehicles\n(eg heated fuel tanks in cold climates) and additives included\nin the fuel (eg degumming additives).\n\nNote 2: This is an energy balance and not a financial one.\nThe dearer fuels become, the more competitive canola becomes\nbut then the dearer the fuel in your tank either way.\n\n\n\n--\nOz\nThis post is worth absolutely nothing and is probably fallacious.\n\nBTOPENWORLD address about to cease. DEMON address no longer in use.\n>>Use oz@farmeroz.port995.com<<\nozacoohdb@despammed.com still functions.\n\n</UL></PRE></font></td></tr></table></BODY><HTML>');"> <IMG SRC=/images/buttons/ip.gif BORDER=0 ALIGN=CENTER ALT="View this Usenet post in original ASCII form">  View this Usenet post in original ASCII form </a></div><P></jabberwocky>[Moderator's note: Replies should please make sure they have some genuine physics
content or, otherwise, be taken to private email. -usc]

Uncle Al <UncleAl0@hate.spam.net> writes
>The economics stink in all cases. You put more value in than you
>could ever hope to get out. Ditto sewage batteries, hooking
>generators to gym exercise equipment (close to breakeven), and
>burning corn for fuel. That last one is a 70% net energy *loss,*

[Note table should be viewed with a monopitched font and
due to the wide table width you may wish to widen your
linelength to view this post. Apologies here but
alternatives degrade clarity significantly.]

Hmmm. Elsewhere someone posted a url that gave net energy costs for
biofuels. The point with biofuels is that they harvest sunlight, if
very inefficiently. In practice the efficiency depends on limiting
factors. In particular for most plants assimilatable nitrogen is
limiting and quite modest additions give a significant increase in
effective photosynthetic efficiency (the amounts used below
effectively double yield). Non-N energy includes pesticides, other
fertilisers, mechanical costs etc etc, combined for clarity.

I converted the maize study into GJ/Ha (1Ha = 2.5ac) and estimated
figs for canola. Note that whole maize grains and canola oil are
easily used in automatic boilers from domestic to industrial.
Unmodified Canola oil can be used in diesel engines.

Note that canola is european yields where maize would be 10T/Ha,
for the same growing costs.

======Something I posted (elsewhere) earlier. ====================
======ref is http://www.usda.gov/oce/oepnu/aer-814.pdf ==========
======gives some figures that can be used 'Maize study'.==========
> [ More results from www.usda.gov ]

Maize OSR
Study Canola

Yield T/Ha 8.40 3.5 Of grains only.
Tot energy produced GJ/Ha 137 72.5 Canola: oil only @45% oil & 46GJ/T
Excludes crop residue&byproducts.

Nitrogen energy GJ/Ha 6.18 7 160-170kgN/Ha for canola.
Non-N Energy @1.5GJ/T GJ/Ha 12.6 13 Should be similar to maize
Net Return on energy used % 731% 362% Excluding residue&byproducts.

No further processing required for burning grains.

Processing GJ/Ha 44.7 5 Canola unlikely to be this high.
Fermentation loss GJ/Ha 64.6 NIL
Total Energy Cost GJ/Ha 128.1 25.
Usable energy produced GJ/Ha 72.6 72.5 3727L ethanol from maize

Net energy gain GJ/Ha 8.9 47.5
Return on energy used % 14% 190% Excluding fermentation loss.

Byproducts not costed but valuable none the less:
Residue of feed T/Ha 1 1.9 high grade high protein animal feed
Plant residue T/Ha 3 2 Combustible: worth 1/3 oil.
Energy value GJ/Ha 42 28 Plant residue (not feed).

Note 1: Currently canola used in commercial diesel engines is
methoxlyated and purified. AIUI this is primarily to give
equivalent viscosities, freezing points and to degum the oil.
If unmodified or limited modification canola become significantly
used then there will have to be some modifications to vehicles
(eg heated fuel tanks in cold climates) and additives included
in the fuel (eg degumming additives).

Note 2: This is an energy balance and not a financial one.
The dearer fuels become, the more competitive canola becomes
but then the dearer the fuel in your tank either way.

--
Oz
This post is worth absolutely nothing and is probably fallacious.

BTOPENWORLD address about to cease. DEMON address no longer in use.
>>Use oz@farmeroz.port995.com<<
ozacoohdb@despammed.com still functions.

Franz Heymann

Aug23-04, 10:22 AM

<jabberwocky><div class="vbmenu_control"><a href="jabberwocky:;" onClick="newWindow=window.open('','usenetCode','toolbar=no, location=no,scrollbars=yes,resizable=yes,status=no ,width=650,height=400'); newWindow.document.write('<HTML><HEAD><TITLE>Usenet ASCII</TITLE></HEAD><BODY topmargin=0 leftmargin=0 BGCOLOR=#F1F1F1><table border=0 width=625><td bgcolor=midnightblue><font color=#F1F1F1>This Usenet message\'s original ASCII form: </font></td></tr><tr><td width=449><br><br><font face=courier><UL><PRE>\n"Oz" <oz@farmeroz.port995.com> wrote in message\nnews:3yZi0JDJHxJBFwDF@farmeroz.port995.co m...\n>\n[snip]\n\nDo your calculations take into account the energy costs of\nSowing?\nWeeding?\nIrrigating?\nHarvesting?\nT ransporting?\nProcessing?\n\nFranz\n\n</UL></PRE></font></td></tr></table></BODY><HTML>');"> <IMG SRC=/images/buttons/ip.gif BORDER=0 ALIGN=CENTER ALT="View this Usenet post in original ASCII form">  View this Usenet post in original ASCII form </a></div><P></jabberwocky>"Oz" <oz@farmeroz.port995.com> wrote in message
news:3yZi0JDJHxJBFwDF@farmeroz.port995.com...
>
[snip]

Do your calculations take into account the energy costs of
Sowing?
Weeding?
Irrigating?
Harvesting?
Transporting?
Processing?

Franz

Joseph.D.Warner

Aug24-04, 04:55 AM

<jabberwocky><div class="vbmenu_control"><a href="jabberwocky:;" onClick="newWindow=window.open('','usenetCode','toolbar=no, location=no,scrollbars=yes,resizable=yes,status=no ,width=650,height=400'); newWindow.document.write('<HTML><HEAD><TITLE>Usenet ASCII</TITLE></HEAD><BODY topmargin=0 leftmargin=0 BGCOLOR=#F1F1F1><table border=0 width=625><td bgcolor=midnightblue><font color=#F1F1F1>This Usenet message\'s original ASCII form: </font></td></tr><tr><td width=449><br><br><font face=courier><UL><PRE>Y. T. wrote:\n> Not entirely sure if this should go here or somewhere into\n> sci.engr.something but it strikes me as a research thing, so I\'ll send\n> it here.\n>\n> What with the current global situation with regards to energy, it\n> appears to me that venues of generating usable forms of energy that\n> are currently deemed inefficient will come more to the forefront of\n> the human collective thinking.\n>\n> In that context I was wondering what the smallest temperature\n> difference is across which anybody has ever actually managed to run a\n> heat engine process in a sustained way. I understand that carnot\n> efficiency goes to heck as the two heat baths approach each other in\n> temperature, but that is only a matter of having enough energy at hand\n> to begin with.\n\n\nCheck on Solar Sea Power. It generates energy through the difference in\nthe temperature of the surface water and deep ocean water. I believe,\nDr. Zener was the original proposer of the idea and advocate in the late\n1970s. As I remember DOE built a demonstration plant off the coast of\nHawaii. Beyond that I don\'t know its fate.\n\nThis plant would use the surface water to boil ammonia and would pump\nthe deep water up to cool the ammonia and turn it back into liquid to\ncomplete the cycle. Of course the "hot" ammonia fumes turned the blades\nof a turbine.\n\nA side benefit from this plant was that nutrients from the bottom of the\nocean was brought to the surface and was thought to be able to increase\nthe number of fish around the facility.\n\nI hope that help. I am sure a search on Solar Sea Power and Dr. Zener\nwill give you the information you seek.\n\n</UL></PRE></font></td></tr></table></BODY><HTML>');"> <IMG SRC=/images/buttons/ip.gif BORDER=0 ALIGN=CENTER ALT="View this Usenet post in original ASCII form">  View this Usenet post in original ASCII form </a></div><P></jabberwocky>Y. T. wrote:
> Not entirely sure if this should go here or somewhere into
> sci.engr.something but it strikes me as a research thing, so I'll send
> it here.
>
> What with the current global situation with regards to energy, it
> appears to me that venues of generating usable forms of energy that
> are currently deemed inefficient will come more to the forefront of
> the human collective thinking.
>
> 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.

Check on Solar Sea Power. It generates energy through the difference in
the temperature of the surface water and deep ocean water. I believe,
Dr. Zener was the original proposer of the idea and advocate in the late
1970s. As I remember DOE built a demonstration plant off the coast of
Hawaii. Beyond that I don't know its fate.

This plant would use the surface water to boil ammonia and would pump
the deep water up to cool the ammonia and turn it back into liquid to
complete the cycle. Of course the "hot" ammonia fumes turned the blades
of a turbine.

A side benefit from this plant was that nutrients from the bottom of the
ocean was brought to the surface and was thought to be able to increase
the number of fish around the facility.

I hope that help. I am sure a search on Solar Sea Power and Dr. Zener
will give you the information you seek.

Gerard Westendorp

Aug26-04, 04:31 AM

<jabberwocky><div class="vbmenu_control"><a href="jabberwocky:;" onClick="newWindow=window.open('','usenetCode','toolbar=no, location=no,scrollbars=yes,resizable=yes,status=no ,width=650,height=400'); newWindow.document.write('<HTML><HEAD><TITLE>Usenet ASCII</TITLE></HEAD><BODY topmargin=0 leftmargin=0 BGCOLOR=#F1F1F1><table border=0 width=625><td bgcolor=midnightblue><font color=#F1F1F1>This Usenet message\'s original ASCII form: </font></td></tr><tr><td width=449><br><br><font face=courier><UL><PRE>Y. T. wrote:\n\n[..]\n\n> In that context I was wondering what the smallest temperature\n> difference is across which anybody has ever actually managed to run a\n> heat engine process in a sustained way. I understand that carnot\n> efficiency goes to heck as the two heat baths approach each other in\n> temperature, but that is only a matter of having enough energy at hand\n> to begin with.\n>\n> Here at the border of southern CA and AZ, for example, we get a whole\n> lot of solar energy during the course of the day.\n\n\nNature itself already makes quite good heat engines operating on\nsmall temperature differences: Wind. Even small temperature\ndifferences, say only 1K, can create significant pressure\ngradients if you have have a high "chimney":\n\ndelta_P = rho * g* h * delta_T /T\n\nJust make h big, like 1000 meter.\n\nSmall differences in the temperature of the atmosphere get converted\nto kinetic energy (wind). We can harvest this with a windmill.\n\nMaybe if you dug a tunnel(or lay down a big tube) from the top\nof a mountain in a desert to the bottom, you would get quite\na strong downward draft in the tunnel, because the air in it\nwill be colder than the hot air outside during the day.\n\nHmmm, making use of a mountain seems a lot better than those\nproposals for building 2 km high towers in the desert...\nThese are seriously being considered!\n\nGerard\n\n</UL></PRE></font></td></tr></table></BODY><HTML>');"> <IMG SRC=/images/buttons/ip.gif BORDER=0 ALIGN=CENTER ALT="View this Usenet post in original ASCII form">  View this Usenet post in original ASCII form </a></div><P></jabberwocky>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