[Stoves] (stoves) Haitian cooking

[Crispin Pemberton-Pigott]

Dear Tom

 

> This is very helpful. Has similar testing been doen for wood stoves?

 

The simple answer is 'no', but the more complex one is 'partially'.

 

>From my earlier message:

 

"It is very interesting that Jim Jetter (PCIA) has been performing something
closer to a heterogeneous test (like the SeTAR HTP) in order to get two
performance points (different moisture levels, but still only one pot size).

Significantly, one stove was not able to work at all with wood that was
moist.... Clearly a heterogeneous test is needed to provide performance
curves."

 

If you have access to the original data from a series of WBT's you can
extract from it several useful pieces of information which are lost when
summing the two sections (high and simmering power) to give the final
results.

 

First, and repeated here for the umpteenth time, there is no such thing as a
"thermal efficiency when simmering". That meme should be buried with the
other misconceptions in the test. To repeat, there is no change in the
enthalpy (heat content) of a perfectly performed simmering of a pot so there
is no work done, therefore the efficiency of good simmering is zero %. I
don't care how many times the dedicated pair of Baldwin and Winiarsky
independently got it wrong, it is basic thermodynamics and it is not going
to go away. All WBT3 results are contaminated by this issue. 

 

So, get the raw data and separate the 'simmering efficiency' from the water
heating efficiency. This means you can get a high power net heating
efficiency with a fuel consumption rate (which also gives a power level) and
a fuel consumption rate when simmering (which also gives a power level
needed to simmer).

 

Remember that the simmering power with an open pot is not the simmering
power of a lidded pot, nor the Low Power level. The turn down ratio is the
difference between the highest and lowest power. The WBT3 does not determine
the lowest power level. In the case of open-door Rocket-type stoves it is
completely determined by the fuel and the operator and has nearly nothing to
do with the stove. In others, it is highly dependent on the stove so for
example, a batch stove test yields an assessment of the stove's turn-down.
In an open fire it is a test of the operator and fuel, not the stove at all.

 

So, to produce performance curves based on the raw data from WBT's one can
get some helpful information. The first and most important is the efficiency
on high power.

 

To assess the thermal efficiency on lower power levels, it is necessary to
use cold water, not hot. There is a SeTAR Centre protocol for this. As long
as the water is above ambient, the results are pretty accurate. The point is
to find out the heat absorbed into the pot at different power levels. If the
water is already boiling, errors are maximised. I won't go into details at
this time.

 

If you want to know the heat transfer efficiency, rather than the overall
thermal efficiency, I have previously given detailed procedures for getting
this number. It involves operating the stove at two different power levels
which are not far from one another. In some cases a WBT provides this,
sometimes not. The best thermal efficiency numbers on high power are
obtained by using the method advocated by Piet Visser which is to bring the
pot to a boil and keep it on high power for 10 to 20 minutes, ignoring the
first 2 or 3. This will give a pretty accurate system efficiency on high
power (when boiling). Doing the same thing with cold water across the 40-70
degree C range is very good for the 55 degree point. The latter is a SeTAR
Centre procedure.

 

If you can pull some of these numbers out of existing test raw data, then
one can get useful information about the stoves and their relative
performance.

 

What is needed is an HTP that uses different fuel moisture, different pots,
lids on and off, and at three or four power levels, with various
efficiencies and emissions recorded. The result is a set of performance
curves that characterise the stove in a meaningful way.

 

Someone selecting a stove finds out what the cooking requirement is, what
the fuel is, then looks up the performance of the stoves on that basis. Most
stoves only perform really well in a narrow range, LPG being a notable
exception. I have hope for others.

 

I have never seen the sort of formula-style output from a series of tests
like the one in the previous message. That approach was developed in the
field for ProBEC.

 

Regards

Crispin

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