[Stoves] Testing Cookstoves: Autocorrelation and White Swans

[Paul Anderson]

Julien,

Good observations for us to consider.

I do raise one issue:   If a stove is SPECIFIC for use with pellets (as 
an example), the proof that the stove does not work well with other 
fuels is not the needed scientific testing.

In fact, there might be some pellets that are not acceptable as fuel for 
the stove.  For example, years ago I tried pelletized pet feeds and they 
did not pyrolyze well at all.

But in fact the customer for the stove wants the stove to be reliable 
for the known and available type of pellets.   Period. Full stop.   How 
would you envision the testing for such as situation?   At what point in 
the testing cycles is it reasonable to declare some solid findings?

Paul

Doc  /  Dr TLUD  /  Prof. Paul S. Anderson, PhD
Email:  psanders at ilstu.edu
Skype: paultlud      Phone: +1-309-452-7072
Website:  www.drtlud.com

On 2/18/2015 11:42 AM, Julien Winter wrote:
> 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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