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Small device to get voltage from fire

by bassplayer142
Tags: device, voltage
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bassplayer142
#1
Jul5-10, 01:13 AM
P: 422
Does there exist a device that can be put in a fire or other hot heat source to generate energy? I'm looking more into if a portable or smaller device fits this category.
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Dadface
#2
Jul5-10, 05:12 AM
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Quote Quote by bassplayer142 View Post
Does there exist a device that can be put in a fire or other hot heat source to generate energy? I'm looking more into if a portable or smaller device fits this category.
Try googling thermoelectricity.
blimkie.k
#3
Jul5-10, 08:41 AM
P: 129
Google peltier cells. You can order them in a wide variety of sizes I have seen them about 1" x 1" on certain websites. There used for cooling cpu's. When you apply voltage to them one side gets cold and the other side gets hot. The interesting thing about them is you can apply heat to them and they generate voltage. There is a youtube video on this as well where the guy applies a blowtorch and gets a couple volts.

Topher925
#4
Jul5-10, 09:33 AM
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Small device to get voltage from fire

How about a thermocouple/thermopile?
DaveC426913
#5
Jul5-10, 09:43 AM
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I saw the awesomest, simplest device evar sitting on my friend's wood-burning stove. It's a convection fan powered entirely by heat.



Bottom gets hot, top stays cool. Difference causes electricity to flow, powers fan.
dlgoff
#6
Jul5-10, 06:57 PM
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Quote Quote by DaveC426913 View Post
I saw the awesomest, simplest device evar sitting on my friend's wood-burning stove. It's a convection fan powered entirely by heat.



Bottom gets hot, top stays cool. Difference causes electricity to flow, powers fan.
Hey Dave. I went to their web site wondering what the temperature difference needs to be for normal operation and couldn't find anything. Do you know any of its specs?
DaveC426913
#7
Jul5-10, 08:46 PM
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I dunno. I do know that it works sitting on top of a wood-burning fireplace.
andrewr
#8
Jul7-10, 07:52 PM
P: 263
Quote Quote by bassplayer142 View Post
Does there exist a device that can be put in a fire or other hot heat source to generate energy? I'm looking more into if a portable or smaller device fits this category.
Several; but none that simply are in the fire altogether -- generators run off of a difference in heat, not based on temperature alone. In order to generate electricity you need both a source of heat -- and a place to get rid of heat used. The larger the temperature *difference* the more efficient the generator can be.

For low temperature operation, eg: from a lighter, or ambient sunlight, or fermenting animal dung, the champion electricity generator is called a Stirling cycle engine. These extract around 10-30% of the theoretical energy available from moderate to low temperature difference sources. Some novelty engines will even run off the heat from your hand. They generally use freon/HCFC's as a working fluid to drive a piston back and forth similar to a steam engine -- but with the fluid in a closed cycle and not being exhausted to the atmosphere. These engines are extremely safe, although they don't generate the most power/fuel $ possible. Their low maintainance, however, is making them very attractive in solar power generator plants as opposed to solar steam boilers.

The thermocouple junction, whether metallic or semiconductor, is a very, very, low power generating system. A metallic junction makes on the order of a few microvolts (6 or so) due to heat -- so that many junctions need to be hooked up in series in order to arrive at any significant amount of voltage. With only two metals, one set of junctions will be of opposite polarity and cancel the voltage of the neighboring junction -- so to make it work in practice -- every other junction needs to be hot and all remaining junctions need to be kept cold. In essence, dissimilar metals make diodes just like semiconductor junctions do -- but with lower voltages. A Peltier device is nothing more than thermocouple junctions made out of silicon or (theoretically, but not in production) other semiconductors.
The problem with these devices is that one wants the heat across a junction -- but not leaking down a wire or semiconductor from the hot side to the cold side -- which would ideally happen because of electricity and not thermal conductivity. Unfortunately, in practice, on can only make semiconductor junction pairs so thin when they are mechanically cut and soldered into a ceramic housing -- so what happens is that only around 1% of the heat is transferred due to electric charges and 99% leaks out without doing anything useful. A 120Watt peltier device hooked up to a 45watt soldering Iron (200C) and the other side held at 20C gave me an output of only around 50milliwatts. (0.050Watt generated power). So, unless you only need a tiny bit of power -- buying a CPU cooling device is not worthwhile to generate electricity from. Cheap solar cells are also around 1% efficient, can be found on calculators for $1 at a dollar store -- or one can get big and fairly efficient cells for lawn lights at around $4 each (4 inch square cells...) >>>>> 1% efficient!!!!. These can generate about as much electricity off the light emitted by a dull fire light or ceiling light that a peltier device could do with all the heat going through it not just the light.... Also, if you intend to make some kind of generator -- you could use a ceramic cavity with a fuel like butane or propane to produce large amounts of infra-red light by building something similar to a gas kiln. (handheld size is also quite easy to do). The better the insulation -- the brighter will be the light output by such a cavity and the less fuel will be required. Simple Raku clay, vermiculite, and the use of a glaze to make the insulation water tight can produce a *VERY* effective light source for solar collection; black-body radiation at 950C inside is quite bright orange/yellow and the outside is only warm to the touch. However, silicon diode voltages drop as they get hotter -- so one still has to have a cooling source to keep the solar cell efficient.

I have some researched designs for peltier devices which are far more efficient than those available on the market -- and they would be quite cheap and robust compared to silicon versions -- but that would require someone with factory facilities to complete the R&D on; in theory it ought to work -- but one doesn't really know until all the pieces are put together. The key problem in getting the efficiency up is reducing the size of the junctions -- massively increasing the number of them -- and simultaneously inserting a very thermally insulating material between junctions to prevent most of the leakage. Strangely enough, these goals all can be achieved in one process which solves the problems simultaneously -- but the technology isn't in production yet. For now, one can buy optimized generating peltier devices -- but the price is prohibitive.

Hope this overview is of some use.
--Andrew.
aprillove20
#9
Jul14-10, 02:09 PM
P: 14
Well, that engines are extremely safe, although they don't generate the most power/fuel $ possible.
bassplayer142
#10
Jul15-10, 12:33 AM
P: 422
Thanks for the replies. Not really looking to build anything out of it, but had some ideas...


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