[Stoves] Testing Cookstoves: Autocorrelation and White Swans

Philip Lloyd plloyd at mweb.co.za
Thu Feb 19 02:42:11 CST 2015


Julien

 

You conclude “before a cookstove is promoted as an exemplar, or sold by the thousands, there is a lot of testing to be done to characterize stove performance over a range of conditions.  One water-boiling test just doesn't make the grade.” Yes, but there is one huge BUT. Before a cookstove is promoted it must be put into the hands of the people who are going to use it, and they must test it in day-to-day use. My laboratory tests proved conclusively (to me!) that ethanol gel was a poor fuel; yet users did not worry about the deficiencies I had found, merely that it was easy to use, easy to light, and cooked the food without imparting any additional flavour – “It tastes like it was cooked on an electric stove!”.  The last of these was my personal “black swan”.

 

Prof Philip Lloyd

Energy Institute

Cape Peninsula University of Technology

PO Box 652, Cape Town 8000

Tel:021 460 4216

Fax:021 460 3828

Cell: 083 441 5247

 

From: Stoves [mailto:stoves-bounces at lists.bioenergylists.org] On Behalf Of Julien Winter
Sent: 18 February 2015 07:42
To: Discussion of biomass cooking stoves
Subject: [Stoves] Testing Cookstoves: Autocorrelation and White Swans

 

I haven't got as far as water-boiling tests yet, because I have been working on fundamental issues of burner design, and that takes a while.

 

However I can see a some problems with the way stoves are being tested.

1)  Measurements made over the course of a run are autocorrelated.

2)  Proper testing of a stove involves a range of fuels.

3)  Test to find the boundaries of failure, not success.

In summary, proper testing of a stove, prior to manufacturing thousands, or making it an exemplar for millions can't be done with one test.  It takes many tests; tests that try to find out not only where the stove succeeds, but most importantly, where it fails.

 

1)  AUTOCORRELATION and Correlation vs. Independent Observations.

 

Autocorrelation, or serial correlation, is a statistical term use to say that two observations, for example, of temperature, are not independent, because they are related in space or time.  Autocorrelation can be a good thing to study if you are looking at spatial patterns in soils, but it can be a problem if you are trying to measure properties of a stove.

 

If we are trying to measure energy transfer during boiling, followed by energy transfer during simmering, all in the same run, then these two measurements of energy transfer will be autocorrelated.  They are autocorrelated, because there history to the combustion reaction, especially in a TLUD.  In a TLUD, the depth of char increases over time, and changes in temperature.  This change can alter the chemical composition of the pyrogas.  In TLUDs and other stoves burning thick pieces of fuel, char combustion can increase over time.

 

Now it may be that boiling followed by simmering is so common that energy transfer from fire to pot over this the sequence should be measured.  However, if we a primarily interested in how efficient energy is transferred at different power levels, then having a separate run for each power level would make the observations at different power levels independent of each other.

 

There are actually two different turndowns to measure:

a) the turndown of fuel consumption rate

b) the turndown of energy transfer rate to a pot

 

2) A RANGE OF FUELS

 

It is important to test stoves over a range of fuels, because they behave quite differently depending on moisture content, volatile content, particle thickness and shape.  In a ND-TLUD the fire in wood chips invariably, channels; with thick fuel (e.g., sticks) there is char combustion on the surface while the interior pyrolysis; and, if air spaces are vertical then a very strong draft develops in the fuel bed.   Channeling of the ignition can increase as primary air is cut back, especially in wood chips.   

 

Across all these fuels there is >5 fold change in ND-TLUD gasification rate.  In other words, turndown is not properly represented by a single fuel.

 

3) TEST FOR FAILURE NOT SUCCESS

 

Critical testing of stove should try to find where it fails.  Although it is useful to see where a stove succeeds, repeated observations of success is not critical testing.  

 

Scientist are encouraged to design critical experiments that reject hypotheses, not confirm them.  If, under critical test, our hypothesis is not rejected then it is probably true.  We owe this line of reasoning to the philosopher, Karl Popper.  Queen Elizabeth (of England) gave him a knighthood, therefore, he must the right!!

 

In the case of cookstoves, we should cover a range of conditions (fuels and turndowns) to see where they fail.  Using a single fuel is not subjecting a stove to critical testing.

 

To use a more simplistic example, let us say that a European looks in the sky and sees only white swans, and comes up with the hypothesis that "all swans are white".   To confirm the hypothesis is not the way to go.  We can count thousands of white swans in the skies of Europe, and we still haven't put our hypothesis to a critical test.  We have to devise a circumstance or an experiment, where our hypothesis could fail.  So lets search all corners of the Planet to see if we can find a non-white swan.  Lo, in Australia, the swans are black.  The crucial point here is that in our critical test, we only had to see one black swan for us to reject our hypothesis that "all swans are white"; all the thousands of white swans, previously seen, now count for naught.  

 

 

To test cookstoves (i.e., ND-TLUDs) on wood pellets is to count white swans.

 

 

To conclude:  before a cookstove is promoted as an exemplar, or sold by the thousands, there is a lot of testing to be done to characterize stove performance over a range of conditions.  One water-boiling test just doesn't make the grade.

 

Cheers,

Julien.

 

P.S.:  Scientists must really get inventors and engineers pissed-off.  Not only do the want to try to break the stove, they want to do it four times so that they can say, "Yup, I am 95% sure that your stove is broken."

 


-- 

Julien Winter
Cobourg, ON, CANADA

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