Stove top Eco fan with a difference

[88v8]

I presume many of us with wood stoves have one of those heat-driven fans powered by a peltier.

Here's something different....
http://www.ebay.co.uk/itm/VULCAN-Heat-Powered-Wood-Burning-Stove-Top-Fan-Eco-Friendly-/271275651078?pt=UK_HG_FireplacesMantelpieces_RL&hash=item3f2949f006
one powered by .......
....a Stirling engine.
http://en.wikipedia.org/wiki/Stirling_engine

The video in the eBay listing shows an example running on a stove burning £5 notes.

Ivor

 

[jackpoint]

I want one so badly!

 

[Penners]

The following thoughts come from a complete ignoramus on this topic (and many others!).

I've always been a bit sceptical about the idea that stove fans cost nothing to run. That seems too much like perpetual motion to me. Yes, the fan runs without any electrical input, but surely it must use some of the stove's heat the power itself. And that heat is generated by burning fuel (wherein lies the invisible running cost).

I'm sure the counter-argument to this is that the small running cost is more than offset by the improved heat circulation around the room.. However, I once worked in an office heated by ducted warm air that emerged from wall vents and blew around the office. It was very uncomfortable for anyone who sat anywhere near one of the ducts - your throat, skin and eyes dried out horribly. I wonder whether there's some element of this effect with a stove fan.

Assuming that one still wanted to run a fan, I also wonder about the advisability of the Sterling engine job. A peltier driving a small electric motor is going to be nearly silent, but I can remember experimenting with a small Sterling engine at school and - given that it's a reciprocating device - it isn't silent. OK, a small one doesn't make a great deal of noise, but I think I would still grow tired of the constant whirring.

What a killjoy I am!

 

[Greg.Hobson]

As the proud owner of both Peltier and Stirling Engine fans, I feel able to comment.

The Stirling Engine is wonderful! As pointed out, it is not silent, but having an (albeit small) "ta-pocketa-pocketa-pocketa" machine, seeming to run on its own with no visible means of support is a joy and a talking point. It does move the warm air around as claimed but, alas, like so many mechanical toys, mine suffered from the incompetence of its owner and failed due to inadequate maintenance.

The Peltier version, purchased as a far cheaper alternative is, in comparison modern, almost silent and much more effective at moving the warm air around but lacks a Heath-Robinson magnetism (at least for the male observers).

I would recommend either or, better still, both!

 

[Flyfisher]

I've always been a bit sceptical about the idea that stove fans cost nothing to run. That seems too much like perpetual motion to me. Yes, the fan runs without any electrical input, but surely it must use some of the stove's heat the power itself. And that heat is generated by burning fuel (wherein lies the invisible running cost).

Thermodynamics tells us that there must be a cost involved in running the fan and this will be as a result of converting some of the heat of the stove into the motion of the fan. The stove therefore gives out less heat. However, the amount of energy required to turn the fan will be minuscule compared to the stove output so the reduction of heat will be tiny and, practically, immeasurable. So instead of the room being heated to 20C it may only be heated to 19.99C ( or something like that - I don't know the actual number)

We have a peltier stove fan and although it works very well, I wouldn't say it makes a dramatic difference and certainly doesn't create a gale of warm, throat-drying air.

For those avid DIY tinkerers among us, a peltier stove fan is not too difficult to make. Peltier modules can be bought on ebay for about a fiver and a small electric motor is no more than a couple of pounds. The peltier module generates a voltage when there is a temperature difference across it's two faces so it should be mounted between a piece of metal that stands on the stove and a heat sink that loses heat to the room. A CPU heat sink from an old PC can be used for this. Probably the trickiest piece to make is the fan blade. I used an aluminium drinks can - cut off one end then cut down the can and fold out and bend to create fan blades. Not very elegant but quite functional. And a pleasant diversion from doing the house painting!

 

[Penners]

instead of the room being heated to 20C it may only be heated to 19.99C

Ah yes - the difference being (in Mrs P terms) a reduction from "nice and warm" to "freezing".

In Mrs P's world, these are the only two temperatures.

 

[worms]

What I want is a Stirling engine to convert flue heat to electricity. You can get systems for this for gas boilers but there don't seem to be any available as a retro-fit or for oil or solid fuel flues. I guess this is down to the harsher environment of the exhaust gases in non-gas flues, but surely there must be ways around this?

I've heard that even the gas installations can be problematic, so perhaps it is just a technology that promises more than can be economically or practically delivered.

 

[worms]

instead of the room being heated to 20C it may only be heated to 19.99C

Ah yes - the difference being (in Mrs P terms) a reduction from "nice and warm" to "freezing".

In Mrs P's world, these are the only two temperatures.

I suspect that human biology may also play a part - living in the tropics we found we could very accurately tell when it dropped below 20C as that was the point where we went looking for jeans, fleeces or lit the fire. Of course, the opposite is the case back home in northern Scotland...with obvious economic consequences!

 

[plasticpigeon]

Thermodynamics tells us that there must be a cost involved in running the fan and this will be as a result of converting some of the heat of the stove into the motion of the fan. The stove therefore gives out less heat. However, the amount of energy required to turn the fan will be minuscule compared to the stove output so the reduction of heat will be tiny and, practically, immeasurable. So instead of the room being heated to 20C it may only be heated to 19.99C ( or something like that - I don't know the actual number)

All of the heat converted to work in the engine, will be seen as heat again shortly after due to the air slowing down and friction in the bearings.

 

[Flyfisher]

Not ALL of the heat though. Some of it certainly, probably most of it, but some of the energy has been used to do other work such as move air from one place to another and to rotate the fan blades.

 

[plasticpigeon]

What other form will that energy take? I believe it will all degrade to heat.

 

[Penners]

An interesting exchange of views between FF and PP - I look forward to the definitive outcome. My own understanding is extremely limited, but I think I remember a school physics statement that said something about how matter and energy could interchange but could not be created or destroyed. :?

Or am I thinking of the school lunch treacle pudding?

 

[Flyfisher]

What other form will that energy take? I believe it will all degrade to heat.

Not sure what you mean by "all degrade to heat" - a reference, perhaps to the 'heat death of the universe', though we'd need to expand our time horizons a little bit for that theory to come into play: http://en.wikipedia.org/wiki/Heat_death_of_the_universe

In the meantime, my reasoning is that 'work' requires energy. Thus, rotating a fan blade and physically moving air constitutes 'work', which will require some energy. If this energy is derived from the heat of a stove, then some of that heat must be 'lost' so that the 'work' can be done. I think.

 

[plasticpigeon]

Some of the heat from the stove will be transferred to the engine and converted to work in the form of a rotating shaft which turns the fan blades. Some of the work goes into overcoming friction in the bearings, and some is used to move the air, it accelerates the air in the vicinity of the fan blades in order to move it around the room. But the motion of the air in the room is retarded by shear forces within the air due to its viscosity and it slows down again. If it didn't the fan blades would be able to continue doing work on the air until there was a whirwind in the room. The kinetic energy that is lost by the moving air due to shear (or friction) heats up the air. The energy has to go somewhere and it all ends up a low grade heat energy that cannot be used to do more work.

 

[Penners]

Well, I'm certainly not having anything low grade in my house.

However, laying aside poor attempts at humour, would you scientists say that a stove fan is a worthwhile investment? I appreciate that it won't actually create any more heat but, given that Mrs P and I sit around 3m from our woodburner in the evenings, do you think the improvement in convection will mean that we are (a) warmer, and (b) able to run the stove at a lower setting?

I'm sure that's a piece of string question, but your musings would be welcome.

 

[Flyfisher]

The energy has to go somewhere and it all ends up a low grade heat energy that cannot be used to do more work.

Like a small-scale 'closed system' model of the universe 'heat death' model perhaps? So the heat 'lost' by moving air around the room is subsequently given up as the air slows down?

What if the electric motor was not used to power a fan but a crane that was used to raise a weight from the floor to the ceiling. Wouldn't the energy used to do the work of lifting the weight against gravity remain stored as potential energy that could be released at a later time by allowing the weight to fall back to the floor? If that stored energy was derived from the heat of the stove then that heat must have been 'lost' from the room - temporarily at least.

 

[Penners]

My head hurts.

 

[Flyfisher]

Mine too. It's too early for thought experiments!

Well, I'm certainly not having anything low grade in my house.

However, laying aside poor attempts at humour, would you scientists say that a stove fan is a worthwhile investment? I appreciate that it won't actually create any more heat but, given that Mrs P and I sit around 3m from our woodburner in the evenings, do you think the improvement in convection will mean that we are (a) warmer, and (b) able to run the stove at a lower setting?

I'm sure that's a piece of string question, but your musings would be welcome.

Based on our experience of two stoves and two fans, including using both fans on one stove, I would say the answers to (a) and (b) are 'no'.

The fans do not actually create much of a 'wind' and I suspect that walking around the room probably creates as much mixing of the air as the fan. Plus, there is presumably already a natural convection-powered flow of air around a room heated from a point source.

But, that does not take account of the fun factor and talking point of having such a fan in the first place or the possible placebo effect of simply 'feeling' warmer - in much the same way that an actual fire is more comforting than a steel radiator.

 

[Penners]

Thanks for those thoughts, FF. I can certainly see the fun/talking point aspect of the fan, although I think it would have to be locked away when the grandsons visit. The temptation to investigate it would be far more powerful than granddad's instructions not to go near the stove!

 

[88v8]

I was surprised at the difference in top-plate temperature between the Clearview, which will readily boil a kettle, and the Esse, which did not.
This translates into performance or not from the fan.
Another influencing factor is whether the stove is top or rear outlet. The fan derives its power from the stove top, but the air it moves comes from the back of the stove. I noticed a significant change in output when I removed a back-exit stove where of course the air at the back is very hot, and substituted a stove with a top outlet where the air from the back is merely at the temperature of the casing.

When the stove is burning at a good clip, our peltier fan makes such a draught that although it is blowing hot air, it feels like a draught and one does not wish to sit in the line of fire. This was less of an issue with the rear exit stove as the air being moved was hotter.

If the stove is set back into the fire opening, I am sure the fan will assist in moving hot air into the room. However, even with the cheapo Chinese peltier versions, I cannot imagine the fan actually paying its way, especially considering that the output is minimal unless the stove is burning quite hot.

Ivor