[Stoves] Shields E450c as a way to test char-making stoves (attn: GACC testers)

[Crispin Pemberton-Pigott]

Dear Frank

 

This is not really a matter of only determining the heat released from the
fire, unfortunately, though I find you approach inspiring.

 

Before trying to find a novel method of working out the heat that was
extracted from the fuel by deducting what is left from the original, it is
wise to first consider what the point of the exercise is.

 

The whole purpose of working out the energy flow is to be able to say how
much fuel was used to perform a cooking task. If the final result does not
predict energy efficiency (i.e. a mass of fuel used turned into a quantity
of energy)  it has no real purpose for someone buying the stove.

 

In the same way, of the 'rating' of the stove does not tell a programme
manager how much fuel the stove will use, they will not be able to predict
the effect it will have on a forest if large numbers are disseminated.

 

The number you seek (we all seek) is the energy needed in the form of fuel
to be fed into the stove to accomplish a task - hopefully a task that
represents what people do with the stoves they would eventually use. 

 

Fuel put into the stove has a certain potential to generate heat. What comes
out of the stove might, as in the case of an open fire, be put into the next
fire the next day.  If the same task is performed, the fuel remaining from
day 1 goes into the fire on day 2. This is real life. Some of the fuel is
burned, some of the fuel is discarded in the ash, some may be used the next
day.

 

What matters is the energy requirement to complete a daily cycle. 

 

If you were to find out how much energy was released from the fire, it does
not tell you how much energy was consumed in the process of that cooking
cycle. The energy needed to be fed in each day is the real consumption.
Whether that is burned fully, burned partly to CO, roasted to charcoal,
burned to small pieces of unusable charcoal, does not really matter if there
is nothing the stove can use the next day. In that case, whatever went in
was 'consumed' for all intents and purposes. 

 

If it is an open fire, there will be some fuel remaining that is useful,
some that is lost. 

 

If we know the amount of heat transferred from the flame to the pot, it is
interesting, but it is not related directly to the fuel that will be drawn
from the forest each day, which is what buyers want to know. They have to
collect and transport it. 

 

As you know the WBT standard formula (which has since been edited)
calculates the energy used by the fire. Then it turns that into a dry mass
equivalent. But that is not the fuel consumption nor the dry fuel
consumption. It is just the energy from the fire turned into a dry mass of
fuel of a certain type.

 

As Jim Jetter tried to explain very carefully in his recent webinar, if the
fuel remaining is not burnable to the stove that created it, it is lost to
that device - even if it is useable for something else. 

 

The calculation should be: the amount of fuel arriving from a previous fire
(heat content unknown) plus any new fuel added, minus the amount of burned
but reusable fuel remaining, noting the additional mass of fuel that is not
usable (including ash). If you repeat the test, the leftover fuel
(presumably about the same because the task is identical) is put into the
next fire. The energy content does not have to be assessed. Just put it into
the next fire, and get more left from that one. The more times you repeat,
the less it matters what the heat content is because the energy put in and
coming out will balance.

 

The thing to know is the new fuel mass required for each replication, and
the mass of fuel disposed of as unusable.  You do not need to know the
energy value of that either, only the raw mass of fuel needed to create it.
Simple subtraction reduces this to a surprisingly simple solution for the
total energy needed per cycle. If you did 3 or 4 tests, just divide the
total new fuel by the energy content of new fuel. As the species and
moisture content will be known, that is also easy.  No fiddling with
charcoal and half-burned sticks at all. Why? because we are not, at any
point, trying to work out how much heat was created by the fire, only the
heat that got into the pot(s) which is very easy.

 

Next, the efficiency, meaning the overall thermal efficiency, is the work
done in the pot(s) divided by the energy put into the system each time the
task is replicated. It is the system efficiency from an energy point of
view, and it is predictive of energy use, meaning fuel use.

 

There is one fiddle required that deals with the usable fuel remaining.
Suppose the amount you have left is after 3 or 4 tests is not the same mass
as was put into the first measured run. It would be helpful to know the
difference, and the heat value of that material. This are several ways to
find that including the one you propose. Divide the number by 3 or 4 (for
the number of replications) and you have the correction to be applied to the
raw fuel mass using the ARV (As Received Heat Value of the raw fuel).

 

The final result is much easier to find than working with 'charcoal
remaining' and burned sticks, it is much more precise, and it is predictive
of future performance. 

 

Correctly predicting the overall thermal efficiency requires knowing how
much heat got into the pot. That means also considering the mass and
material the pot is made from. The British Standard, the Indian Standard,
the SeTAR and Indonesian Standard use this. I am sure many other do as well.
Again, the result is more precise and gives predictive results even if the
pot material or mass is changed.

 

The only thing left is to make sure that the task performed is culturally
relevant and representative of actual use, including fuels available in the
community.

 

Lots to think about.

 

Regards

Crispin

 

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