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

Fri Feb 2 13:16:20 CST 2007

Dear Paul

Well I am proposing that it will take a careful think before accepting 
anything.  As I showed, SFC is pointless when assessing an improvement a 
stove in the boiling phase.  It either communicate nothing or it lies, or it 
tells the truth depending on the conditions.

A PHU test during boiling gives a thermal efficiency which is very useful to 
a stove developer but can be misleading to a novice buyer if reported.  It 
is likely to be misunderstood to represent stove performance overall.

When using an SFC rating for the simmer, it has to be the initial water mass 
if you are a stove developer, however I also look at the PHU during the 
simmer to see how it drops over time, comparing it with the burn rate (which 
I record on a continuous basis).

A stove run fast or slow in the boiling phase of the test can give very 
different results.  If the stove is highly variable and a great deal depends 
on the operator's skill, performance figures can be very misleading. 
Similarly, if stove performance varies a lot with the moisture content of 
the fuel, superb performance figures are at best little white lies.

The claim presented for the standard VITA-rooted WBT is that it is for stove 
development.  Clearly the test is misleading in that regard.  Thus all test 
done to date using that methodology are misleading and probably useless. 
Stoves 'perfected' using them have been adjusted to give a high performance 
figure, not a high performance.  This has to be changed. A legitimate 
complaint about the 'missing water method' of determining stove efficiency 
rewarded boiling excessive amounts of water, unless the burn rate was fixed, 
which is very difficult to do with wood.  Not so with paraffin or alcohol 
because the heat production rate can be governed accurately.  A PHU test is 
quite valid for a paraffin stove in the fuel consumption rate is known.

A Golden Rule is: If the stove is better at performing its task, the lab 
test should show it.

----- Original Message ----- 
From: "Paul S. Anderson" <psanders at ilstu.edu>
To: "Crispin Pemberton-Pigott" <crispin at newdawn.sz>; "Discussion of 
biomasscooking stoves" <stoves at listserv.repp.org>; "Crispin 
Pemberton-Pigott" <crispinpigott at gmail.com>
Cc: "Stoves" <stoves at listserv.repp.org>
Sent: Friday, February 02, 2007 11:12 AM
Subject: Crispin's testing procedures; was Tom Miles' Highlights fromETHOS 
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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