Wood burner powered Fan.

[gordon_s1942]

I am in the process of replacing our well burnt out woodburner (Turbo10) to one with a built in FAN (at last) and I saw on Ebay a listing for a fan that is powered by the heat of the stove.
According to the specs, it just sits on top of the heater and generates electricity to drive the fan as it absorbs heat.
Seeing this ranges in price from $170 to $220 I aint exactly gunna race out and buy one but I was curious how they worked.
One thing I found over the years how much a ceiling fan set on reverse can move the warm air around the room and that can raise the temperature by up to 3 C when running.

So, experts in matters scientific, how does heating the frame generate electricity to drive the Fan?

[shred]

It uses the Peltier Effect. If you make one side of a semiconductor junction hotter than the other, it generates a tiny amount of electricity. Put a bunch of them in series on a thumping great heatsink, where one side is stinking hot and the other is at roughly room temperature and you can run a fan. The Peltier Effect works the other way too: put power on a semiconductor junction and one side will cool slightly, while the other side heats up. This is how the little 12v refrigerators that you use in the car work. In the mid 1990s, Apple used peltier effect CPU coolers in some of their PowerPC computers.

I'd be interested to hear how effective (or not) the fans for the wood heaters are. Are they just a gimmick? Are they great, but so noisy they drive you up the wall?

[Uncle Fester]

They don't always run with a peltier there are also Stirling motor versions that are more expensive and can take higher heat:
Peltiers can handle only a limited temperature maybe 200˚C or less.

Here is a video to see both types in action:

You can make one yourself with an old CPU heat sink and a $5 peltier element

[gordon_s1942]

I doubt most stove tops would pass 200 C when the fire is lit although you can boil water (100C) although it takes sometime and the water isnt as agitated as it is when over a Gas jet or Electric element.

I have the idea these are popular in both Europe and North America/Canada in areas where electricity isnt always available 24/7 or not at all which to me is the big selling point.

From what I read one time, its not the heat or cold in the air you feel so much as the Movement of the air and thats where a fan like this might useful.

[bazzle01]

I had one on top of my wood heater.
It needed the top to be over 120c to start working well.
Was better as it got hotter but didnt move a lot of air.
I reverted to the ceiling fan on reverse. It works a lot better as it pushes air down past the heater . Room is cosier too with no drafts.

[shred]

Since my original post in this thread, SWMBO went and bought one online as a surprise. My first impression was that it was very well constructed, but a quick look at the rear of it showed one of those cheap and short lived model motors is being used to drive the fan.

In use, it gets going pretty quickly after the fire is lit and it seems to make a little bit of difference to the temperature in the room - but I think it's much more effective to turn the air conditioner on, set to "fan only". My other half thinks the Valiant fan is wonderful though, so can't say this.

After a few months of use, it seems to be getting noisier.

This is the fan here:
(Fa fa fa.... she spent how much on that thing !!! ??? I know it's in NZ dollars, but still... it's insanely expensive for what it is)

[Tiny]

Well thanks for that shred.

Since this thread was started I replaced our wood burner heater in the lounge room with a Canadian made unit which has a thermostatically activated fan; so it will turn itself on or off as the box reaches the set temp.

It only draws 20w at lowest setting & 30w at highest.
Best thing since sliced bread for our room as we never had a fan assist before & have no sealing fan.
It is well constructed, very efficient, consumes way less wood than our old Saxon, yet delivers the heat & burns over night with no problems.

Heater is an Osburn Matrix.