>> It mentions conversion of thermal energy into electrical energy so 
>> it's
>> almost certainly a thermo-electric device.  I doubt they would be 
>> able
>> to manufacture and sell a stirling engine/generator combination for 
>> that
>> price.

> Correct me if I'm wrong (and I may well be), but isn't this the kind 
> of
> situation for which a Stirling engine would be pretty good?  Far 
> simpler
> design, much more durable, and probably a higher efficiency?

A Stirling engine could indeed be used in that application. You can 
buy Stirling engine fans which run off a hot source which could 
include a stove - the ones I have seen are made in India. They are 
intended for moving cooling air and using, AFAIR, an oil lamp flame or 
similar. The maximum theoretical efficiency achievable is the "Carnot 
efficiency" = (Thot-Tcold)/Thot (temperatures in degrees absolute). In 
practice Stirling engine s typically achieve 20% to 50% of Carnot.

Peltier modules, being semiconductor based devices, generally don't 
like to exceed 200 C and some would prefer less. At 200C and say Tcold 
= 50C Carnot efficiency is (200+273 - (50+273) / (200+273) = 150/473 = 
32%. A Peltier module will typically get 2% to 6% when used as a 
thermoelectrixc generator or about 5% to 20% of Carnot. I used 50C as 
you have to cool the cold side and unless the heatsink has an 
excellently low C/Watt drop you get a large temperature differential 
between cold side and air temperature. In their case the "cold" side 
is cooled by the air above a wood stove so it won't be overly cool. 
Note that they specify an upper limit for the stove top. This is no 
doubt to keep the Peltier hot side below 200C. There will be some 
temperature drop between stove top and Peltier module. Fail to follow 
their instruction just once and your fan may die.

A reasonably done Stirling engine in the same environment could run at 
hotter temperatures and achieve a greater efficiency. Many small model 
Stirling engines are exceedingly inefficient. To get decent 
efficiencies you MUST have a regenerator (despite what some will tell 
you*) and to get decent power densities you need to pressurise and/or 
use a better working gas than air - typically Helium.  Hydrogen is 
even better but has a nasty habit of diffusing out through anything - 
even through steel walls given enough temperature and pressure.

* The regenerator may not LOOK like a normal regenerator as long as it 
functionally IS one. It's job is to store heat energy or "coolth" as 
gas transits between hot and cold sides. Without this the transferred 
energy is "destroyed" (dissipated) on each half cycle and wasted.

Compared to a Peltier a Stirling solution is liable to be more 
expensive to implement but potentially far more durable.


        Russell McMahon




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