[Stoves] A wisdom of Rebecca's stove

Tue Sep 3 23:27:37 CDT 2013

Dear Ron

I am not sure why you use the word ‘rebuttal’. 

>    a.  I am going to wait for release of the 72 slides from the EPA
webinar to answer much of this

I think that is a good idea.

> - since I am mostly trying to defend the material there, which is at great
odds with Crispin's approach (which I still do not understand)

With respect to the issue of fuel remaining after a test, we are in
agreement. The slides may explain it to your satisfaction.

>    b.  I note that Crispin did not rebuttals one of my points, 

Rebut??

 [RWL4:   In the first sentence, assuming that Crispin's  "cannot" includes
"is not designed to", I would interchange the terms "under" and "over".
This is getting back to the issue of apples and oranges - where I believe
there is some validity in adding them - if/when one is consciously
attempting to maximize both, they are expressed in the same (energy) units,
and are calculated with equivalent formulae.   

Both what? The apples are stoves that can burn the remaining fuel. The
oranges are stoves that cannot. If someone has a stove that can use the fuel
remaining and chooses not to, that is a feature of the user, not the stove
and does not affect the rating given to the stove. We don’t rate what people
do.

>   To repeat -  there is nothing about the present EPA/GACC approach which
supports Crispin's claim that   "For stoves that cannot burn any of the fuel
remaining, the current methods over-report the system efficiency and
under-report the fuel consumption."

It is true that no specific mention was made of the implications of how the
fuel is considered, only that it matters. If you have a stove that uses 1 kg
per firing and claim that it only uses ½ a kg, then the fuel efficiency is
over-reported, not so?

>I feel it best to get at this, using the Jetter slides.  This is a key part
of the disagreement between Crispin and myself.  

At least there is no disagreement between me and Jim.

>
I have no idea what this proposed Indonesian stove test will look like,
except that charcoal production seems certain to be hard to find in the
final reports .

I have explained only certain portions of them. They are designed to work
out early in the assessment if the stoves are powerful enough to interest
the prospective users. That assured, we examine the fuel efficiency and the
emissions per MJ in the pot on high power, same as the IWA though we use a
three step power level cycle, not two like a WBT. 

That pretty much covers it. There are two classes of product: those that
cook and those that only heat water.

>>What happens outside 

>[RWL:  Char is intentionally being made inside some stoves.]  

We all aware of that. Jim was at pains to explain that this does not affect
the calculation of the fuel required to feed the stove.

>>I have said this before and there is not 

[RWL: fortunately, others disagree with you.] 

Which others? What other interpretation would there be? That the energy in
the char remaining is not really there? What else can you do in the
calculation of fuel consumption that would be affected by the amount of fuel
consumed? How do you and others who think different feel the energy in the
charcoal should be treated when rating the char making stove?

>What a stove does with the fuel fed into it has to be quantified separately
[RWL:  it can be separate, it can also be part of]  

Part of what? Other than reporting it (which we are) what else is there to
do?

>Even if there is char deliberately produced, not [RWLa:  why not?] all the
char is useable and it is in any case a sub-section of the remnants/outcome
of combustion. 

There is no one saying ‘do not produce char’. Go ahead and produce char to
your heart’s content. I am not worried by production of char. I am just
measuring it. I also see how much fuel is needed to feed the stove.

>The ash also has many purposes. If you wanted  [RWLb:  I do.]  , we could
report the mass that looks like char and the mass that looks like ash. 

Well we do too and so does Jim. Are you contented now?

That, as you say, lets people decide afterwards what the implications of
them are. I think the more important point as Jim made clear  [RWLc:  Jim is
reporting what char-makers want - you are not.] 

That is in error. We reporting exactly the same thing which is the mass of
char remaining. Would you like to have the heat value of that char reported
as well?

is that the energy value of those remnants is not important [RWLd:  I never
heard Jim say this.] 

The heat value of char that is left over is not important if that cannot be
burned in the same stove, and he said this with respect to the calculation
of fuel consumed, as did I also.

>>[RWL2:  Same issue.  This seems to imply that char will either have to be
consumed or only be considered a loss.  No third option?   Was this your
intent?

>It is not just the char. Anything left over that cannot be used in the
stove on the next replication is a loss, whether we dress up the language or
not. Some stoves cannot even use totally dried wood.  The intent is to
report the performance of the stove being tested. Nothing else.  Combustors
and heat exchangers have been tested for centuries and it is strange to see
the slow speed at which very ordinary methods are applied to domestic stoves
by those most intimately creating and evaluating them. Doesn’t that strike
you as odd? 

[RWLe:  You have to be more specific.  I see nothing missing from what Jim
reported.   You answered neither of my questions.]]

I see no third option for the question as to whether or not the stove can
burn the fuel remaining after a test – whether that be processed wood, char
or anything else. It either can or cannot.  I did answer your question, I
did not give a third alternative. Perhaps you can provide one.  Which
‘intent’ are you referring to? The intent is to know whether or not the
remaining fuel can be used in the next replication. If it cannot, it is not
allowable to consider its heat content as the equivalent of fuel not burned.
Jim agrees with this.

You have for more than two year tried to find a way to credit stoves that
produce char with special consideration in a way that the char heat content
somehow credits the stove. Have I got that right? When rating the stove we
want to know the fuel consumption – how much additional fuel it takes to
feed it for another cycle. It a stove produces a left over fuel that it can
burn in the next cycle, then it is treated as unburned fuel.  If a user then
instead chose to use it for something else (like fertilisation) that is
their choice. It is not a function of the stove. If the stove cannot burn
the fuel remaining then the initial fuel load will have to be considered
‘consumed’ because it is. It is as simple as that.

> 

It is of course a little more complicated than that. All stoves burning
biomass make some ‘char’. The problem was that the char was being
arbitrarily [RWLf:  Not so arbitrary.]  assigned a heat value 

I beg to differ. Heat value was assigned (originally through the WBT 3
series, a heat value of 1.5 times the LHV, whatever the actual heat value
was. Later it was assigned a value of 29.5 MJ/kg. Those are both arbitrary
heat values. Jim has been reporting the actual value of the char remaining
in his later reports and it varies quite a bit. Imagine the difference
between pine char and rich hull char, per kg. 

>
 and also the stove was being credited for char produced as if it was
unburned raw fuel when reporting the fuel consumption, which clearly it is
not  [RWLg:  The char was far from "wasted" in the minds of many stove
makers (and users)  It seems illogical to make believe it wasn't produced.].

I am pointing to the working of the heat transfer efficiency calculation.
The reason it  is considered is because it was produced. Of course it was
produced. We are discussion what to do with the heat value of it. If it is
not ‘fuel’ for the next cycle, it is ‘waste’ as far as that stove is
concerned. 

RWLh:  I am happy with what I saw in the Jetter slides -nothing needs
correction - and believe I speak for the char-making stove community.  

You speak for no one else, same as me.  There are many things that still
need correction and many of the things I have asked to be corrected have
been implemented and I am very happy about that.  If you did not know there
were things wrong you might not have noticed the corrections. 

Do you know anyone working with char-making stoves who likes your proposed
approach?]

I am not sure if anyone working with char making stoves has expressed an
opinion. I work with char making stoves and have for 10 years. I like it
just fine because it correctly assesses the drawn-down of fuel from the
available resource. 

>>The fact that some stoves are making huge amounts of char (relative to the
total energy available to start with) brought this to a head when stoves
were claiming a ‘fuel efficiency’ of 50% while consuming as much or even
more fuel than the baseline stoves which were 15% efficient. Clearly
something was wrong with the calculation method. There was. We are agreed on
how to correct it. Jim covered it very well. [RWLi:  But it seems you are
going to do something  very different.  

With respect the treatment of fuel remaining after a test, particularly char
which is now bring produced in large quantities relative to the available
raw fuel at the beginning of the test, we are doing exactly the same thing. 

I would like to see the exact numbers being used here.]I  t would be
difficult to claim that a stove that consumes 50 kg of fuel a week does not,
when it does.  [RWLj:   You are stuck on a stove doing only one thing - a
stove can be designed to both cook and make char.  And make money (not spend
money) while doing so.]

I am not stuck on one thing but it seems you are. You are trying to credit a
stove that makes char with some form of economic benefit that should be, in
your opinion, reflected in the performance evaluation. A broader discussion
of the economics of a stove and how it fits into the family economy is
warranted and wise. It is however not a function of a fuel consumption test
to determine the economics of family. That is an assessment of the family or
the family’s economy. Each portion of that economy should be examined for
its impact, costs, opportunity costs and benefits. 

Char making has an opportunity cost, quite a large one actually. Suppose you
have two stoves of the same fuel efficiency. Taking the remaining fuel and
selling it instead of using it generates some income and must be offset with
procurement of new fuel. Paul O has pointed out that in Vietnam there is a
positive gain in doing this (it makes a profit). Good. Do it. It is not part
of the fuel consumption analysis, it is part of a business idea that might
appeal to the user. The user could then look for stoves that use less fuel
to produce the same mass of char as an output. The would look that up on the
fuel consumption line and the mass of char produced line. Simple. Clear.

As to what happens to the char produced, that is outside the stove [RWLk:
Denied again.]  The char production is not outside.]

You are misreading. Try again. What happen to the char is outside the stove.
Making char happens inside the stove. After it is made, the used makes a
choice about what to do with it., The stove does not. 

>I have settled on ‘system efficiency’ as perhaps the best way to describe
the raw fuel in and the work performed out. That ratio is the system
efficiency.  [RWLl - all sounds great if your stove makes no char.]

It also makes a lot of sense when your stove does make char because we are
not rating the family or regional economy. We assessing exactly what the
stove consumes and delivers to the pots. If someone made charcoal and did no
cooking at all (a pretty good deal if there was a profit in it) the cooking
efficiency would be zero. The fuel consumption would be the mass of fuel
consumed per cycle. This is not complicated.

 I am still not understanding either what Crispin's new method is or what is
wrong with what Jim proposed (which also is "new"), and hope Crispin will
explain what is wrong with either/both.

As for the treatment of char remaining there is no difference.  Have I said
that enough times yet? As for the conducting of the stove tests, the
replications, the re-use of fuel and the determination of the heat flux
rate, those are new.

>[RWLm:  Many of us want this char to never see a stove again - to go into
the ground - for CO2 sequestration purposes.  

No problem. Take it out of the stove and place it in the ground and
sequester the CO2. The fuel needed to produce that char will be considered
‘unburned fuel’ if the stove could have consumed it in a later cycle, and
considered ‘consumed fuel’ if it cannot.

Why can't (shouldn't) a stove test accommodate that desire?]  

Stoves done have desires. People do. We can rate the people and we can rate
the stoves. What people choose to do is not a function of the stove. 

>>It still boils down to how much new fuel the stove needs from the
available supply, on average, per replication. [RWLn:  Which I understood
the 72 slides to do - for the char-making stove that the webinar was about.]

Correct.

If it is designed not to, then it cannot, right?  [RWLo:  No, not right.
Every char-making stove I have ever seen could burn the remaining char -
and do so badly.]  

We need to decide whether the char remaining is fuel or not. The test is not
of the opinion of the producer, but of the stove. Suppose a stove can burn
10% of the char produced in the next go-round, and 100% of all dried wood.
That is not an idle claim, it has to be demonstrated. One of the beauties of
the triplicate test with fuel remaining (which is new and which we will
apply) is that the emissions from the stove will be measured while burning
fuel it is claimed the stove can burn. It is up to the manufacturer to make
claims on this matter, and then demonstrate the stove can do it. 

If it turns that for all practical purposes the stove cannot burn the fuel,
or the emissions are dreadful when doing so, then it is not really
‘useable’. This is simple and fair. The amount of new fuel that has to be
added per cycle is the fuel consumption. 

‘Cannot’ is the right word.  We [RWLp:  You.  Not the char-making stove
community.]

You are entitled to your opinion and I will consider it.

>>I was intrigued by the idea that we might mix char from pellets into new
pellets and burn some of the char. Some stoves might be able to use less
fuel that way. We have to consider all design and operational possibilities.
[RWLq:  I doubt we will have any difficulty with this aspect.  The issue is
when we don't want to burn the produced char.]

That is not a measure of stove performance. That is a user choice with costs
and benefits.  The stove stands alone in this.

>>Regarding the heat transfer efficiency, to get the closest number you can,
the pot material and mass must be considered, the unburned CO too, and the
lid should be on the pot [RWLr:  I am with EPA on this - forget the lid -
much more error introduced with varying lid fits.  Many stoves are used to
evaporate water.]] 

This is quite incorrect and should not be repeated. The fit of the lid has
ZERO influence on the performance.  Removing the lid creates greater
unmeasured losses and causes the heat transfer efficiency to be
underreported. Stoves that evaporate water should have a lid on as it
increases the efficiency measurably. You can check this quite easily by
running a simple test with a lid on and off.

Let me state that again: leaving the lid off a pot does not increase the
evaporation rate – it decreases it, and it decreases it by approximately
0.37 watts per square cm of open water surface for any constant power input
rate. That figure is based on a 100 degree water temperature in a 20 degree
room. In many cases (rolling boil) the loss of evaporating power is greater
because the water surface is larger. The water cools by IR radiation when
the lid is off.

>I have another completely different approach I can explain at another time
[RWLs:  Sorry to hear this.  When will we hear?]. 

I brought it to this forum before and no one took any interest. It involves
calculating the heat transferred at two closely spaced power levels. It is
very accurate. Most people don’t care. 

>>Important: the heat transfer efficiency does not represent the fuel
consumption. Relative heat transfer efficiencies do not represent relative
fuel consumption.  Keep that in mind.  [RWLt:  I haven't heard anyone worry
about getting fuel consumption badly wrong.]  

Have you heard of the fuss that was generated by the test of Roger Samson’s
rice hull gasifier when its emission and fuel consumption were misstated so
severely there were threats implied?

There was the case of Paul A’s Quad 2 stove tested at CREEC which was
discussed at length here (thanks Paul) which reported that it ‘used 636 g of
dry fuel to complete a WBT when in fact it was 1350 g. That is ‘badly
wrong’.  It was in presenting that case that Jim agreed that we had to
change the method applied to char remaining in stoves that can’t burn it.

This refers to stoves that produce char and cannot [RWLu:  Or felt it better
to not.] 

You cannot anthropomorphise stove. They do not have feelings.

> [RWLw:  And I think you are missing everything about the beauty of being
able to make char ( to count "oranges") while you are counting "apples".  

There is perhaps a misunderstanding on your part what the apples and oranges
refer to.  Se above for clarification.

>As near as I can tell, you want to make believe there are no valid cases
where oranges (char) exists.]

Char exists in nearly all biomass stoves. I am not sure what the intention
of that remark is other than to guess what ‘my intentions’ are. My intention
is to end the misrepresentation of the fuel consumption of stoves. Period. 

>>I have given descriptions of a couple of the elements of the assessment
method. One is the heat flux rate – the rate in Watts that a stove delivers
heat into a pot’s heated surface. This will be assessed.  [RWLx:  And is
now.  Energy (joules)  is given and is more fundamental than Watts - the
product of power (which varies a lot during a "boil") and time.] 

Joules is energy. Watts is the rate of energy generation or absorption per
second. The heat flux rate is the rate of heat per second per unit area.
Because it is factored twice is it represented by a character with a ‘double
prime’ notation, in this case Q’’.

>[RWLy:  I don't detect an answer to my question - what does this new test
look like? (in detail)]

It consists of a careful measurement of the heat gained by a pot while
operation the stove at maximum power. The heated surface area has a
definition and the number of Joules absorbed is divided by the number of
seconds and the area in sq cm. It does not require a measurement to be made
of the fire itself. It is a way of expressing the cooking power for any size
of pot.

>Will a char-making stove have its char output reported in any sense?  

It will be reported as a mass of ‘what looks like char’.  The reason is that
there will be a small [[RWLz:  "small" means what?] bonus (as previously
reported) [RWLaa:  Reported where?] for stoves that make char while saving
fuel through a higher system efficiency.

The maximum benefit available for crediting a char making stove is 10% as
was explained in the paragraph below.

>>Suppose we have a minimum system efficiency of 45% for a particular
application, and suppose you have a device that is 65% efficient (about the
same as an LPG stove).  That is quite a bit of fuel saved. If you were to
turn that saving into a lower system efficiency (45%) plus produce some
char, the stove will be credited with a slightly high figure than the 45%.
It is a bit complicated to explain here (sliding scale) but it boils down to
a 10% credit if you can meet the system efficiency requirement and make char
at 25% (as defined in the text  [RWLab:  What text?]).  

The SeTAR document that has the definitions of terms used in their test
methods. I believe the document is SOP 0.01

>>This is a programmatic decision, incorporated into the rating system. It
does not make the stove more fuel efficient and it does not state that the
stove is more fuel efficient than it actually is. It is just a [RWLac:
Subjective?] 

It is not subjective, it is calculated and compared with the performance
benchmark.  The result is a pass or fail. It is a yes/no question.

>There are problems remaining with the WBT and my opinion on them is in the
document analysing it in detail (2008   [RWLad:  Can anyone point me to
this?]). 

I could send it to you if you want. It was submitted to the ETHOS discussion
group. They went through it carefully. Several of the suggestions were
incorporated in to the WBT 4.1.  I wrote another one before the IWA meeting
nearly all of which was ignored. Since then several suggestions have been
accepted. Several remain.

>>The use of the mass of water remaining when simmering is another and is
probably unresolvable. [RWLae:  I put emphasis on the amount boiled away - a
well known and meaningful number.] 

It is a meaningless number. Let me say that again: IT IS A MEANINGLESS
NUMBER because it tells us nothing about the performance of the stove. The
task being performed at the time is simmering.  Simmering is a task that
does not require evaporation of water. The evaporation of water does not
tell us ‘the efficiency of simmering’ which is why the IWA workshop group
voted to have it removed from all stove testing. Do you still see that
simmering is required by the WBT? Yes you do. It is still there and still
calculated and still reported, even though the minds convened at The Hague
agreed that it is not scientific, is an undefined terms, and is not useful
for rating performance. 

>>The mass of water in the pot has virtually no effect [RWLaf:   No effect?
hmm.]  on the fuel consumed or the emissions during simmering so it does not
matter what the mass is. 

I challenge you to show that a stove simmering two identical pots at
identical power, having 2.5 litres of hot water in one and 5 litres in the
other, requires twice as much fuel to summer the larger amount. Rani 1992
reported there is no effect if the pot is over half full. Other have
reported it for years. But we still see the WBT dividing the fuel by the
mass of water in the pot as if there was a meaningful link between these two
variables. There is not. 

>>Thus no ‘specific’ performance metrics should be calculated for simmering
(now re-named ‘low power’ though it is not low power either).   [RWLag:  I
have heard a lot of people say this was important - that cooks want to
control power levels.  A positive feature of many char-making stoves.

I have not seen many char making stoves that had the ability to control the
power to say, 25% of the high power level, then raise it back to 100% again.
I am very interested to see one working.

>>..Simmering does not have useful MJ in the pot because ideally, there is
no change in enthalpy so no work is done.  [RWLah:  Not the way I see it.
All simmering will do some evaporation.  Some work.  You are never going to
use the enthalpy of water vaporization

You are missing the point. The point is that the fuel consumed when
simmering has nothing to do with the mass of water in the pot, and trying to
work out what the heat transfer efficiency is from the evaporation of water
al low power with the lid off gives completely unrepresentative values. This
is easily checked by putting a pot of cold water on the stove while keeping
the power level the same and heating it to 70 deg C. Compare the two
efficiency numbers that result. It is not incorrect to say they are wildly
different. The one heating the cold pot is correct, the one using evaporated
water is incorrect. The WBT uses the latter. What else can I say? This is
old news.

>[RWLai:  Seems like extra work, if everyone uses the same pot.  It could
always be put back into the computations.   

Everyone should not have to use the same pot. Some stoves have special pots.
Some cook woks only. Some are huge. We cannot have people designing stoves
to fit the test method, the test method must accommodate the stoves.
Including the thermal mass of the pot gives more precise answers. Excluding
it gives slightly incorrect answers that decline in error with increases
test length. 

A good place to note that at least the Chinese and Indian tests, like you,
choose to only consider char as a "waste".   I'll bet a beer to anyone
thinking they will have that same view in 5 years. ] 

Char is waste it is cannot be burned in the same stove. It is not called
‘waste’ it is classes as a mechanical loss (because it is unburned).

>[RWLak:  It sure means something to me - it means water evaporated.  

Evaporating water is not the purpose of simmering, as noted above.

It also should be a measure of how close one can keep to a desired
temperature.].

That is the purpose of simmering and it is measured (in some protocols).

>>As mentioned before, the specific emissions and fuel consumption when
simmering are also not informative so we are not generating them. [RWLal:  I
still don't know if anything is replacing their demise/]  

We are not reporting any metrics that do not have a valid mathematical
basis.

[RWLam:  "Mechanical" meaning char - which Jim is handling (well) and you
are pretending doesn't exist  (at least in the final report) ]

You are not following the conversation. Perhaps by this point you will
understand and withdraw the remark.

>[RWLan:  Maybe - but not for char-making stoves where the desire is to not
further consume the char.]  . 

This is not contributing to the conversation. I understand that you do not
like it that char remaining in a stove that cannot burn it is a mechanical
loss. It is also considered a mechanical loss by ANSI for a hydronic heater.
And everyone else in the world who deals with burners and heat exchangers.

>>I say that because making additional replications of a WBT4 – Jim
mentioned some large numbers in the webinar – does not improve the precision
of the result.   [RWLao:  He was(appropriately)  resisting those - and not
for reasons of precision.  He was pointing out different types of tests
(example different fuels) that others have proposed.] 

He was pointing out that the number of replications was getting large in
order to satisfy various interests. We are conducting tests that include
three power levels in a single test. If the test method is accurate and
precise there is far less need for replication. For some fuels there is no
need – LPG for example. If the test method includes conceptual errors that
increase the variation between very similar tests, then the impression is
given that preforming more will compensate for it. A better approach is the
design better experiments.

>[RWLap:  I am hugely impressed by how small the error bars are.   I believe
they do not report results with large error bars - they first figure how to
get good  test-to-test agreement - and they throw away the outliers.   

What small error bars? When did you see error bars reported from WBT’s? What
you have normally seen is the test results plotted on a chart with a
vertical bar connecting them, without any error bars at all.

Second, how do you know the result is accurate? If systematic errors are
large, then the number spat out the end is not representative of actual
performance. Consider all the tests performed prior to date where the char
remaining has been considered unburned dry raw fuel. For stoves where the
char is thrown away, it create a large fuel consumption error. Every time.
Perhaps the test result is a cluster of reproducible errors, not facts. This
might explain some of the large differences between lab and field
measurements.

>[RWLaq:  I'd suggest we stick for now to the webinar topic - char-making
stoves - the only stoves I am asking about and that you are reporting tests
on in a manner highly detrimental to makers, and potential users, of
char-making stoves (and policy makers).]

You have provided no evidence to back that remark. You should withdraw it.

I think you are not listening carefully, or do not want to listen carefully,
and cling to the hope that the energy content of char-making stoves will
somehow be credited to the fuel consumption number even if the fuel is not
useable in that stove.

Unless you have something new to add this discussion will end here.

Crispin

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.bioenergylists.org/pipermail/stoves_lists.bioenergylists.org/attachments/20130904/4f587921/attachment.html>