[Stoves] Efficiency of clean fuel

[IPC]

Yes - indeed!  The simple water-loss test is what we have standardized on to
compare fuels.  It is so easy to measure, and doesn't depend on the pot or
any one of a host of other variables.  You just get the pot boiling well,
then measure the rate at which the fuel is being used and the rate at which
steam is being lost from the pot. The rate at which fuel is being used gives
you the heat in, and the rate at which water is lost given you the heat
reaching the pot.  The ratio is the efficiency, directly.

However, we haven't tried charcoal, and I have a mental note to try some
when we start work again later this month.  Right now it is holidays in the
southern hemisphere! Sand, sun, surf, and little thought of stoves.

(Dr)Philip Lloyd
Energy Research Centre
University of Cape Town
Private Bag Rondebosch 7701
South Africa
Tel +27 (0)21 650 3896
Fax +27 (0)21 650 2830


 

-----Original Message-----
From: stoves-bounces at listserv.repp.org
[mailto:stoves-bounces at listserv.repp.org] On Behalf Of AJH
Sent: 02 January 2008 12:27
To: Discussion of biomass cooking stoves
Subject: Re: [Stoves] Efficiency of clean fuel

On Tue, 1 Jan 2008 21:45:05 +0200, Philip Lloyd wrote:

>I don't recall a proposition that "an electric hob transfers 40% of the 
>thermal power into the pot."

Thanks for pointing that out, it was my mistake, you did indeed say 80%
efficiency from hob to pot and the 40% was for a "thermal cook stove"
>
>What we have measured is an 80+/-1% efficiency of transferring 
>electrical energy received by the hob into thermal energy in the pot.  
>This efficiency was measured with a voltmeter and wattmeter on the 
>electrical feed to the stove, and the mass of water turned to steam 
>during boiling, at close to sea level.

When I first came to the list this type of simple water loss calculation is
what we used to get a figure of merit per kg of fuel burned, if all other
things remain the same then it should be a decent enough comparator between
and electric and a gas hob.

Did you start a rolling boil and then measure the electricity used from that
point? Perhaps someone with a gas hob can do something similar. I think I
have a butane stove that I could try.


> The hob was a simple resistance element coiled into a spiral, such as 
>is widely available as a cheap cooking device, and drew about 0.9kW at 
>peak power.  The pot was a simple, flat-bottomed aluminum pot.
>
>For a power station at 37% thermal efficiency, and losing 8% of the 
>electrical energy in transmission and distribution, that works out as 
>27.2% overall efficiency from heat energy supplied at the power station 
>to heat energy into the pot, which is better than most.

Yes and as I said to Sharon one of the reasons it is good is that there is
no mass flow to carry away sensible heat. Now gas won't be far behind as it
can be burned near stoichiometrically and the flue gas can be rejected at a
low temperature, I wonder whether a coal or charcoal stove will be better as
these will have less losses from discarding the latent heat of steam. Of
course the charcoal might be penalised if the heat losses during production
aren't recovered in some way.