[Stoves] Stirling to drive fan

[AJH]

On Fri, 22 Sep 2006 11:55:39 -0700, jason marshall wrote:

>I have a question on this subject that I'm curious about the answer to.
>
>I have a heat source (fire).  I'm trying to increase the draft to make
>it more efficient. 

OK but first let's define your "efficient" what do you think it means?

It's a term I try to avoid unless it's qualified. To my mind what we
are trying to achieve is a more complete combustion with (mostly) the
minimum mass flow, this is because massflow impacts on temperature and
temperature affects heat transfer. We're burning in order for the
energy to do some sort of work for us.

So the perceived wisdom is for a good burn we want high Temperature,
high Turbulence and high Time retention in the fire. A good draught
actually assists in all three of these thing. The turbulence is
obvious but it also can mean the gaseous species are whirled around in
the combustion zone for longer, hence it aids retention time plus
because of goo mixing there is more chance the various reagents will
meet up with each other, so it can mean less excess air. Again less
excess air means lower mass flow for a given energy release and thence
higher temperature.

So that's the efficiency to do with combustion but it's not the whole
story because natural draught is very nearly good enough. What tips
the scales in favour of forced draught is not the few percentage
points of more complete energy release but the fact that better
burning of those last few Products of Incomplete Combustion, PICs,
including Polycyclic Aromatic Compounds (PAH) and particulates (PM2.5
- PM10) has a substantial impact on reducing the dangers from
pollutants breathed in near the stove.


> I'm going to pull energy out of the fire to do
>this.

Yes there is no free lunch
>
>What is it I'm going to accomplish by 2 energy conversions
>(heat-electricity, electricity-air motion) that I couldn't achieve by
>tuning the convective forces directly?

To know that you have to know what parasitic loss there is in the
conversions (with the TEG and fan all the heat remains in the system
anyway) and the losses associated with the heat engine that causes
natural convective forces. If you do the sums for the heat you need to
power the small pressure differential of a chimney you'll see it is a
poor return on the heat wasted in the flue.

Crispin's son produced a crib sheet to make the answer easy, I expect
it is still available on the Tom's bioenergylists site in the sig
below.

As to the conversions to produce electricity for a fan, I hope we get
to an answer for this for a self contained device but the sum for the
grid connected fan is easy and it amounts to about an additional
1W(e) input per 1kW(t) of stove output, how we measure its worth in
human health I do not know.

AJH