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Lethargic Power Generator

Generate a bit of electric power from diurnal temperature changes
  (+2, -1)
(+2, -1)
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A given volume of methanol expands about .12% for each degree (Celsius) of temperature increase. If one put 500 liters of methanol in a tank designed to maximize heat transfer with the surrounding air and the temperature rose 20 degrees, the volume would increase by 12 liters. A piston 10 cm in diameter would be moved about 1.5 meters by the expanding liquid. If a toothed rod attached to the piston drove a small generator (through a gear train so that a small movement of the rod resulted in several thousand revolutions of the generator) then one could expect a bit of power each day. As the liquid cools and contracts the piston would return to its starting position, albeit with less force (the area of a 10-cm diameter piston is about 78 square cm, and since atmospheric pressure would move the cylinder back the force available on the return stroke would be about 76 kilograms).

This is a sort of Babbage electrical generator, a large clunky device that produces a small trickle of power (the piston cycles at a blazing .00069 rpm, hence "lethargic"). It doesn't need any large reflectors, like a solar Stirling engine, and is otherwise pretty technologically simple, but I can't think of any reason one would build one of these instead of installing photovoltaics. It will probably never exist except on the Halfbakery.

Tech considerations:

There are materials with a negative coefficient of expansion--they shrink as they get hotter--so I am assuming the tank holding the methanol can be engineered for minimal expansion.

Methanol expands much more than any common metal (aluminum expands roughly .0075% per volume per degree Celsius or .0025% per linear unit), and although some liquids (acetone, methyl ethyl keytone) have a somewhat higher coefficient of expansion methanol is relatively inexpensive and safe to handle--and unlike water one doesn't run the risk of damage during freezing weather.

I'm not sure how much pressure would be generated by thermal expansion of methanol but the expansion of liquid trapped between valves in a line can be enough to burst pipes, I believe (I'm not talking about freezing water, although that also bursts pipes).

Dog Ed, May 17 2001

Tech info http://strategis.ic...a/SSG/lm01433e.html
Coefficients of thermal expansion for selected materials [Dog Ed, May 17 2001, last modified Oct 05 2004]


       UnaBubba: Burning the methanol is of course an excellent idea, and toasted to a buttery brown. This is much sillier, er, I mean less polluting...you don't use up the methanol, it just sits there. I was thinking maybe because the mechanism could be completely enclosed and would be quite sturdy it could be useful for charging batteries at remote telemetry stations. But of course photovoltaics are already de rigeur (sp?) for that application.
Dog Ed, May 17 2001

       now that's _really_ half-baked!!! :o)
mihali, May 17 2001

       This idea is theoretically brillian, although most people here seem to have missed the point that the gas does not actually get used - this device could run until the sun runs out without ever needing any fuel.   

       You see, the gas isn't the fuel it is effectly a catylst - it collects the suns energy and turns it into kenetic energy.   

       The only major downside is that this would probably use more energy to create than it would ever produce.   

       The cow or animal concept wouldn't work as the cow is high maintenence and would die eventually and need replacing.   

       This concept could, however be viable if you could find something that is a liquid at 10c and a gas at 20c. This would result in much more movement of the piston. (Or more importantly, less gas needed per piston.) Not sure if such a chemical exists - will leave that upto the chem guys on here...?
CasaLoco, May 17 2001

       cover the tank in valves which are built to open up in cold weather to let the ice expand. perhaps even design the tank to come apart at said freezing, and then pull itself back together with elastic action as the ice melts. Then you would loose water.. so you have it collect and filter rain to refill the tank itself.
ironfroggy, May 19 2001

       The economics of solar power systems are all about the power generated and the capital cost of installation depreciated over the lifetime of the device.   

       [CasaLoco], you might consider freon.   

       This is a heat engine, and not even a terribly inefficient one. The piston moves very, very slowly, but with tremendous force. The way to extract a nontrivial amount of work from this device would be to hook it up to a generation system with a tremendous resistance -- possibly just an ordinary generator with a lot of gears in front of it (so that moving the piston a small distance spins the generator shaft many many times).   

       Once this is done, there will be a tremendous pressure on the piston, and therefore on the whole tank. Building a tank to withstand this pressure is expensive. Building a generation system with the requisite characteristics is also expensive. The result would probably cost much more than the equivalent photovoltaic installation or steam engine.
egnor, May 19 2001

       egnor: Yeah, I agree--the payback time on capital cost for building the requisite tank, gear train, etc, probably far exceeds the lifespan of the moving parts. And photovoltaics are getting cheaper and more efficient, so I really can't think why one would build a generator like this. Another idea doomed to languish unrealized in the files, I think.
Dog Ed, May 19 2001

       could the piston mech be replaced with those little electron producing deforming crystal thingys? or better still why not invest higher levels of thought to thermocouples. (I can sense oil infra-structure managers readying their reject stamps as this is typed )
wjt, Apr 26 2003


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