[Stoves] Re Alexis Belonio article, Energy required to 'make' steam

[Crispin Pemberton-Pigott]

Dear Simon

Many thanks for your few thoughts!

>From the data given (5 litres of water turned to steam in 1 hr 30 mins) I
>calculate that steam generation requires on average 2.4 kW of power.

Once the steam is running, the heat from the old steam is present in the 
steam generating area so the net loss is the heat in the exhaust gas, 
presumably 250 degrees or less (if the pot were more suitably shaped, sized 
and skirted).  Unless the steam injection temperature is above the exhaust 
temperature, there is a net loss even counting the recycling of the heat.

>...but addition of steam is also effective as the specific heat
>capacity of steam is high compared to other combustion gases
>(double that of CO2).

I wondered if this might increase the heat transfer by a factor of two. 
With the specific heat of steam at about twice that of air, the moist gases 
carry more heat.

>We also have slightly more gas flow passing the pot at a
>lower temperature and further calcs are required to assess
>whether the increase in velocity compensates for the drop
>in temperature in terms of heat transfer to the pot!

Exactly.  It seems that it will not be difficult to show the maximum volume 
of steam one can vent into the fire before there is a net loss for any given 
stove-pot heat transfer efficiency.

>I would be interested to know how the stove performs when
>the steam runs out, the only rice husk stove I've ever seen
>burnt incredibly cleanly without any draft or mixing assistance.

Good point.  I would like to see whan happens when the volume of steam is 
reduced and the temperature increased to perhaps 700 C with a superheater. 
The numbers start to look really good.  There are Japanese superheated steam 
cookers but being electric, they don't have the advantage of the combustion 
heat being thrown in for free. 
http://sharp-world.com/corporate/news/060808.html

You have an interesting idea here:

>Another test may be to fill the water container but redirect the steam pipe
>outside of the combustion area...

One could use the steam (lower temperature) in some way to pull the air 
through the stove rather than pushing it in.  I haven't see that yet, but I 
guess it would look like a chimney stove with the steam venting into the 
chimney to pull air into the fire.  That way the chilling effect could be 
avoided, the mixing retained, the extra volume of gases would be after the 
pot not before it and the benefit could be significant.

The chimney would get wet of course...

>Not sure that I've clarified anything, but it seemed that at least some
>comment on the energy required for steam generation was needed.

Much appreciated!  What is the volume of 1 gm of steam at 800 C?  Let's 
calculate the additional volume of gases, and to start, assume the excess 
air is 300%.

Regards
Crispin