[Stoves] Crispin's testing procedures; was Tom Miles' Highlights from ETHOS 2007

Fri Feb 2 10:12:26 CST 2007

Crispin and all,

Your comments and numbers should make us all think about this important issue.

Are you proposing that both the PHU method and the SFC method be used for both
(or which one) of the bring-to-boil stage and the simmer stage, and do we use
the initial weight (volume) of water (5 liters) or use the final weight
(volume)?

Acutally, if we have 2 methods and 2 stages and 2 weights, there would 
be 8 sets
of data, being 4 for each stage.  Our task is to decide on which 
combination is
best.

Paul
-- 
Paul S. Anderson, Ph.D., Geography professor - Emeritus
Telephone:  USA-309-452-7072 (residence and office)
Internet site:  www.ilstu.edu/~psanders
For my gasifier stoves info, go to:
http://bioenergylists.org/contributors#Paul_Anderson

Quoting Crispin Pemberton-Pigott <crispinpigott at gmail.com>:

> Dear Kevin
>
> You wrote:
>
> "I would like to see the laboratory techniques for measuring performance and
> emissions become closer to what is experienced in the field so that the
> transition from lab to field is smoother. As Dean and others have emphasized
> the lab tests are nothing more than development tools, but we need to make
> them as close to the actual use as it is practical. Step by step maybe we
> can reduce the differences between them."
>
> On the serious side, I see that there is a problem aligning this 'it's only
> for the lab' idea with what happens in practise.  People who know little
> about stoves use the lab-based 'performance' tests for making decisions
> about what stove to build.  It is as simple as that.  While it is obvious
> that one should use a test of cooking in the homes using normal fuel (etc)
> when making such selections, these are 'early days' and there is precious
> little information available on stove performance anywhere.
>
> On the very serious side, a stove test that does not represent something
> close to reality is of questionable value when developing a product to be
> used in a real place.  I give as a single example the surface area to volume
> ratio (A/V) of the fuel.  The gassing rate of the fuel is a very important
> factor during the burning as it determines the need for primary and
> secondary air.  Preheating air that is headed toward high A/V fuel requires
> that the air supply be choked, or chokeable, and secondary air provided as
> appropriate.  A good example is grass.  Burning grass 'openly' would result
> in a huge, brief plume of white smoke in most stoves because the air supply
> is completely wrong.  Conversely, putting one fat piece of hardwood in a
> Rocket Stove with yield nothing because it will go out.
>
> On the really serious side, a stove test that purports to aid developers
> cannot report incorrectly or it will mislead the developer to think that an
> improvement is making things worse, and vise versa.  For example, it you are
> testing two stoves which are very similar in all respects save for a
> slightly different pot-skirt gap, you would expect a standard stove test to
> show whether or not the gap change produced a better or worse performance.
> Let us take the boiling phase of the test.  This test can be reported in
> tems of Percentage Heat Utilized (PHU) though now it is popular to use
> Specific Fuel Consumption (SFC).
>
> A PHU test reports how much water was boiled out during the boiling phase,
> the time, and the fuel required.  To report this a combination of the
> initial water mass, final mass, and fuel burned is used with calculations
> for heat absorbed in the water, evaporation of the missing water and fuel
> burned, adjusted for moisture in the fuel (the heat required to heat and
> evaporate that moisture).  Given identical initial water and fuel masses,
> the principle numbers are the final mass and the fuel burned and the portion
> of heat that got into the pot.
>
> An SFC test reports on how much wood was burned to boil a unit amount of
> water (i.e. per litre).  If the initial water mass is taken, the result is,
> say, 500 grams of wood burned divided by, say, 5 litres of water to give a
> rating of 100gm/litre.  If the final mass is 4.92 litres and is used for the
> calculation, it is given as x-many grams of wood burned divided by 4.92
> (litres) to give an SFC of 101.62.  Thus the same test, reporting on the
> same basis, gives different results depending on whether you use the initial
> or final mass of water in the pot.
>
> It would not matter how this is reported unless you are trying to improve
> the stove.  Suppose a second stove is 1% more thermally efficient than the
> first.  The fires are identical, the wood and water masses too.  The result
> will be that the second stove evaporates more water than the first for the
> same amount of wood burned during the boiling phase.  If you are reporting
> on an SFC basis, they have the same result when taking the initial water
> mass, showing no performance increase: 100 gm/litre so a comparison of SFC
> based on initial mass is literally of no value to a designer.
>
> Suppose the first stove evaporated nothing during the boil, and the second
> boiled 80 cc leaving 4.92 litres.  If you use the final water mass for an
> SFC calculation, the stove with the better performance shows a _higher_ fuel
> consumption, viz. 101.62 v.s. 100 gm/litre.  This gives the stove with the
> better thermal efficiency a worse performance.  Obviously this is going to
> mislead the designer.  The effect of calculating stove performance in this
> manner was shown in Bailis' talk at ETHOS.  He got a negative correlation
> between lab and field results, which is what one would expect if the manner
> of calculation were to rate the more efficient stove as having a higher
> specific fuel consumption.
>
> Thus it is shown that the SFC method is not useful for determining
> improvements to stove performance or comparing the thermal performance of
> different stoves during the boiling phase.  If the initial water mass is
> used, there may be no difference shown in the test result unless one fire is
> smaller than the other and still manages to boil the water in exactly the
> same time - something very difficult to achieve in practise.  During the
> boiling phase, only a PHU test can be guaranteed to show an increase or
> decrease in performance.
>
> However this is not the case with the simmering phase because the simmer has
> a clear definition: keep the water in a narrow range of temperature between
> the local boiling point and three degrees below that point.  The fuel used
> to do this might be given in SFC terms or PHU terms, and the initial or
> final water mass could be chosen, with quite different results.  This is not
> a calculation of work done, it is a calculation of how much fuel was used to
> accomplish the defined task so the SFC is attractive.
>
> Given two stoves of slightly different heat transfer efficiencies, what
> information do these approaches provide to the stove developer?
>
> SFC based on the final mass will tell the developer that the more efficient
> stove used a higher mass of fuel per litre to simmer the water, giving it a
> lower performance rating because more will have boiled away during simmering
> (presuming a similar fire size).
>
> Using a PHU calculation taking into account the work done boiling off excess
> amounts of water, will rate the more efficient stove higher as it did more
> work, but does this also presume the stoves used exactly the same amount of
> wood?  If they used different amounts of wood and boiled different amounts
> of water, it is not actually interesting to know how much work was done
> because the work in question is to maintain a temperature slightly below
> boiling.  The amount of water boiled off is of no consequence.  That being
> the case, only the initial amount of water can be used, together with the
> wood consumed to give a useful figure for simmering, and it will be an SFC
> rating.  Only reducing the amount of wood burned can give a figure of value
> to a designer, and it should be expressed in gm/litre simmered.  Using the
> final water mass gives an poorer rating to the better performing stove.
>
> In conclusion, it can be argued that if the purpose of a lab test is to
> assist the stove developer, then the test should give results that indicate
> an improvement when there is one, either in the boiling or simmering phase.
> The proposed Modified Water Boiling Test does not accomplish this, as ably
> demonstrated by Bailis for which I thank him.  He has not proposed what I
> have written above, but he did show in carefully run experiments that there
> is a negative correlation between lab tests done according to the modified
> WBT and field performance which can be explained by the analysis above.
>
> Best regards
> Crispin
>
>
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