The Walden Effect: Farming, simple living, permaculture, and invention.

Thermoelectric power generation

making electricity from fire

I saw this interesting gadget the other day and have been wondering how it works and if it could be scaled up for use with a wood stove.

It uses something called thermoelectric technology to convert heat from the fire into electricity which powers a small fan to make the fire more efficient. Extra energy gets used to charge small gadgets.

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About us: Anna Hess and Mark Hamilton spent over a decade living self-sufficiently in the mountains of Virginia before moving north to start over from scratch in the foothills of Ohio. They've experimented with permaculture, no-till gardening, trailersteading, home-based microbusinesses and much more, writing about their adventures in both blogs and books.

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It's a thermoelectric generator. Deep space probes that venture too far from the sun to use solar power use these, with a nuclear fission heat source.

This one uses the heat from the fire, but not very well. Accorging to the manufacturer's specifications you get a continuous 2 W of electrical power output from a fire that puts out 3.4-5.5 kW of heat. That's not even 0.1% efficient. For reference, the generator in the wheel hub of my bicycle puts out 3 W when I'm just cruising.

You might use it to charge your cellphone, but it won't charge a laptop in a reasonable amount of time. A modern laptop battery can be around 4.4 Ah at 10 V. That is 158400 J. Divide that by 4 J/s gives 39600 seconds. So after stoking that fire for around 11 hours you've charged your laptop. :-)

Comment by Roland_Smith Fri Jul 20 16:22:30 2012
That thing isn't efficient at ALL. Mark, have you seen any of the wood gasifiers?
Comment by Heath Fri Jul 20 17:01:52 2012

+1 on Rolands comment. It's a TED (thermo electric device). Although not currently very efficient at all (like most green tech) they are becoming more common and more efficient as the technology matures.

Basically as long as there is a temperature differential from one side of the device to the other the device produces a current. Conversely, if you input a current the device will create a temperature differential on either side. The greater the temperature differential is the more power they will produce. This particular application seems very inefficient, much more so than other applications that I have seen.

The first time I saw one of these is when I was working for Ford. Some models have heated and cooled seats, this is accomplished with a TED.

I've seen other applications for woodstoves that power a fan to help circulate the warm air coming from the stove as well.

Personally I'm a huge fan of this technology. I think it may one day be more efficient than solar power when applied in the correct way (like under a solar reflector and fed with nice cold spring water). Maybe I'll invent that for my own microbusiness...

Comment by Andrew Fri Jul 20 17:03:27 2012

Roland --- Thanks for all that useful info! I suspect the reason this device is so inefficient is because it's dual purpose --- presumably, you're cooking on top of the flame, and the electricity is just a slight bonus.

Heath --- I'll let Mark chime in there. :-)

Andrew --- Excellent points (and I'd love to see your microbusiness product in action!) I like the idea of using one to move hot air away from a woodstove and into the rest of the house. Since a fan would create a temperature differential, that sounds like exactly the right use for the technology.

Comment by anna Fri Jul 20 18:09:42 2012

@Andrew: If you have concentrated solar and cold water, you could get much more output (because of the greater efficiency) by using a heat engine (like e.g. a Ericsson or Stirling engine) coupled with a generator. Free piston stirling engines are mechanically extremely simple.

Comment by Roland_Smith Fri Jul 20 18:49:42 2012
Heath-The only gasifier technology I'm familiar with is ones I've seen in WW2 era films powering small trucks due to the shortage of gasoline. I'm thinking there was one in "The English Patient" displayed briefly.
Comment by mark Fri Jul 20 20:20:39 2012

@Mark: you can get ready-made gasifiers complete with engine/generator combos now. There are also DIY kits and plans.

Most seem to use wood chips, so you'd need a chipper as well, but the first one I linked to it has its own chip dryer. In other designs drying the chips seems to be done in the process.

However, for 10 kW electrical output you'll need up to 12 kg (26 lb) of biomass per hour according to The energy content of wood is around 15 MJ/kg, so the energy input is around 50 kW, so the total electrical efficiency is around 20%. A large (600 MW) power-plant can get up to 40%. But drying the wood takes a significant piece of the input power. Of course you also can use the waste heat for warming the house, making hot water or drying stuff.

Comment by Roland_Smith Sat Jul 21 05:43:14 2012
Roland --- I think this is the same as the Jean Pain Method we discussed a few years ago?
Comment by anna Sat Jul 21 09:41:00 2012
The Dynamo bicycle hub would be an interesting thing to rig up to something. First I was thinking making a wheel into a wind turbine, but I wonder if you could use it to tap into the creeks flow of water.
Comment by Brian Sat Jul 21 11:20:46 2012

Not really. The Pain method is anearobic digestion, while gasification is basically a couple of chemical reactions at high-temperature in a relatively oxygen-poor environment, powered by incomplete combustion. I'll refer you to the linked wikipedia page for the detail.

As Mark mentioned, one of the most well-known applications is for the generated syngas to power an internal combustion engine.

Comment by Roland_Smith Sat Jul 21 11:44:55 2012
Brian-I like the idea of using one of those new Hubs with the dynamo built in for some micro-hydro power generation from the creek. I guess the first hurdle would be to see how water proof they are. Wouldn't have to be totally submerged, but there would be plenty of splashing on the hub.
Comment by mark Sat Jul 21 14:52:49 2012

Splashing doesn't look like a problem to me. I've been using a Schmidt hub generator for some years now in all weather without problems. It has never let me down, and it makes my cycling in the dark safer and more enjoyable. Even though they're quite expensive, I would recommend them.

But they're not really fit for this purpose. They're designed to power bicycle lights (mostly LED or halogen these days, which require little power) and to give not much drag when not in use. They are quite efficient for their size, around 65% (at 15 kph in a 28" wheel). There are adapters available to use these generators to charge gadgets like a cellphone or GPS, but that's about the best you can do.

They'll give about the same power as the TEG in the article, but you don't have to stoke the fire. :-)

Comment by Roland_Smith Sat Jul 21 17:30:22 2012

It's inefficient because of the relatively low temperature differential in this small stove.... I looked into designing something very similar to this quite a few years ago when I got hold of a Sierra zip stove and wanted to run the fan without having a battery and charge my phone to boot but stopped there because of the temp diff needed. On a proper wood stove it becomes more viable and people have been using such devices in the wilds of alaska for many years before. You can buy the generators to fit to your stove from here... Still not hugely efficient but better than a few years ago.

Comment by Jason Wed Aug 8 06:40:06 2012
Jason --- I like that unit a lot better! I could see it on our wood stove to supplement electricity in the winter when solar panels provide much less juice.
Comment by anna Wed Aug 8 09:09:55 2012

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