[Stoves] (no subject)

Crispin Pemberton-Pigott crispinpigott at gmail.com
Thu Nov 21 06:08:42 CST 2013


Dear Marc

 

Firs I congratulate you on trying to make the engineering aspects of stove
evaluation more accessible to the tinkerers that are genuinely trying to do
something new and useful. Accessible science makes the world a better place.

 

>I've just published an online tool for simple water boiling tests:

http://smallredtile.com/stove/wbt-buddy/

 

>Is it useful?

 

It is because many people have no idea what to calculate when doing a simple
test like bringing water to a boil (which is a useful task even if boiling
is arbitrarily defined. The advantage is that if a person thinks a 'rolling
boil' is needed each time, then they have a meaningful comparison.

 

>What would make it more useful?

 

It would be good to have some clear explanations about the implications of
saying 'overall thermal efficiency' because as I read it, that is not the
outcome. If all remaining fuel is considered to be 'unburned raw fuel' then
the calculated result is not the overall thermal efficiency but the work
done divided by the heat probably liberated by the fire, even if the fire
does not burn the fuel completely and can't do so alter on. The difference
is quite small for stoves with a fine grate, and quite large for stoves with
no grate, or which can't burn charcoal produced. As the core problem with
the mis-characterisation of stoves over the past 13 years has been this
issue, it is worth trying to get the user of the site to put in numbers that
will deliver a predictive result in the field.

 

It is very likely that anyone who knows how much fuel mass they burned will
also know the mass of the pot(s) and what they are made of. As demonstrated
with the discussion about finned pots, the correct answer and comparison has
to consider the heat absorbed by the pot. If you put a cell for pot mass and
another for material (could be from a simple choice of stainless steel,
aluminum and cast iron (called 1, 2 or 3) it would allow for a more accurate
answer.

 

>Did I make any mistakes in the model?

 


 

"Overall Thermal Efficiency (OTE) is the ratio of energy absorbed by the pot
of boiling water (Ewater) to the energy released by the fuel (Efuel)"

 

This definition is not correct. This is the fire-to-pot efficiency. The
overall thermal efficiency (the exact term adopted by the Indonesian
National Standards Committee at the last meeting) is the energy gained (net)
by the pot divided by the energy available from the raw fuel consumed (as
defined above) considering the material and mass of the pot as well as the
wood type, moisture content and the usability of the fuel remaining.

 

>Is it possible to make the Water Boiling Test calculation simpler and
easier to follow?

 

Definitely! But more to the point, how about using a burn cycle that is,
overall, more representative of actual cooking. The Hot Start is
particularly a problem because no one operates a stove like that. Prof
Annegarn won't let the students perform if because he says it is plainly
unsafe.

 

>Can we make the steps of the calculation more visible? (to help newcomers,
to help identify problems with the calculation)

 

I think that is clear enough. Most people not used to formulas have a
problem with representational math so you could put it into words, or have
the word formula pop up if the mouse hovers over it. That is pretty easy to
code.

 

>I find that, even with an engineering background, the WBT Excel sheet is
not easy to use. The equation for finding thermal efficiency is buried in
one cell and looks like this:

 

=IF(W26=0,0,(4.186*SUM((W12-$N15)*(AA8-W8),(W13-$N16)*(AA9-W9),(W14-$N17)*(A
A10-W10),(W15-$N18)*(AA11-W11))+2260*W27)/(W26*$E20))

 

This can be made much easier to understand if the cells are Named. It also
helps trouble-shoot it so that errors like having $E22 instead of $E20
(which was there for several years giving strongly misleading results) are
easily spotted.

 

A major problem with the WBT (well, the versions up to 4.2.1) is that it
oscillates between using energy and fuel mass. This is rooted in the old
method of using fuel mass to discuss energy - very much and EPA thing.
Emissions per kg of fuel and all that. The whole ting should not use mass at
all once the fuel is burned.

 

>Also, some of the steps of the calculation (moisture content, char energy
content) aren't necessary for many use-cases of the WBT. For example, if I'm
trying out a new design, all I want to know if the general range of
efficiency: 10-20%, 40-50%, etc. 

 

That is OK but over time it is really hard to try to get fuel that is the
same - seasonal changes are significant so it is important to let us put in
the moisture content - an initial and final mass of a sample from the pile.
Again, it only takes a scale to get that and the sheet presumes the user has
a scale.

 

>I think there may be some benefit to emphasizing that the WBT is an
estimate. Wasn't it originally designed as a quick-and-dirty test for the
field?

 

Yes it was. As time passed, VITA settled the fuel remaining question in 1985
by recommending that anything left be ignored (as consumed). The University
of Eindhoven earlier came to the same conclusion.  Importantly, Nate Johnson
found in his work in Mali that if he ignored char remaining (as if burned)
the CoV was cut in half. 

 

>So, I simplified the engineering model, tried to build a user friendly
interface, and laid out the step of the calculation as clearly as I could. 

 

It captures a lot of it.

 

I think you have to provide two outputs on the efficiency side: a best guess
at the heat transfer efficiency and an overall thermal efficiency that
represents fuel consumption.

 

Regards

Crispin

 

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