[Stoves] A wisdom of Rebecca's stove

[Ronal W. Larson]

List,  cc Crispin and Jock (with thanks for his later comment not shown below)

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

    b.  I note that Crispin did not rebuttals one of my points, which I considered quite important and so repeat.  This is the underlined last part of my "4":
 [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.   
   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."    
   I feel it best to get at this, using the Jetter slides.  This is a key part of the disagreement between Crispin and myself.  

   c.  I insert a few rebuttal remarks below.  (These are all underlined, preceded by the usual "RWL")  But I have to continue to repeat, 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 .

On Sep 2, 2013, at 9:33 PM, Crispin Pemberton-Pigott <crispinpigott at gmail.com> wrote:

> Dear Ron
>  
> >The following is an attempt to be sure that char-making stoves get a fair shake in your proposed (but still unknown) new method of stove testing.  
>  
> I am pretty sure you will defend the interests of the char making stoves. To do that you are going to have to stay tuned to the purpose of testing an individual stove. What happens outside [RWL:  Char is intentionally being made inside some stoves.]  that stove is not part of that stove’s emissions or efficiency rating. Such external considerations are happily included in programmatic analysis. I have said this before and there is not [RWL: fortunately, others disagree with you.] going to be another version of it: 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]  from the other things that come before and after. That applies to fuel preparation and fuel remnants. All that external information is real and can be considered by the person making a rational choice about how a stove fits into a broader system.
>  
> >>‘Wasted’ is not a term of criticism, it is used to mean ‘not used’ and is a ‘loss’. Losses classically are divided into ‘chemical losses’ (in the form of un-oxidized gases) and ‘mechanical losses’, meaning unburned fuel that falls to the ground from a conveyor, so partially burned fuel in the ash and char remaining is in the latter category of mechanical losses.
>    RWL1:   When reporting on stoves intentionally designed primarily for making char, how about replacing "wasted" with "char produced"?   Since the char may be used again for many purposes, what is wrong with always reporting char production and let later users of the test report decide how to use the data?
>  
> That would perhaps mislead many people and create room for baffle-gab and misdirection. 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. As noted before there is moist fuel, dry fuel, torrefied fuel, partially burned fuel, charred fuel and ash. There is always some carbon in the ash. It is difficult to find out exactly how much is in each category; it is usually not necessary to report more than the fuel consumption or the system efficiency which is what the great majority of questions are about. We are adding the heat flux rate because it is so valuable as a screening method.
>  
> 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. 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.] is that the energy value of those remnants is not important [RWLd:  I never heard Jim say this.] if the remnant is not fuel for the stove being rated. I believe that was specifically directed to this and several similar questions.
> 
> >>The question that arises, as highlighted by Jim, is whether or not the fuel can be burned in the same stove during the next replication. If it can, then it is unburned fuel that will go to the next round and could therefore be deducted from ‘fuel consumed’. If not, then it falls into the category of ‘mechanical losses’ and has been ‘consumed’. That means for fuel consumption purposes it should not be deducted from the mass of fuel consumed per replication.
>     >[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.]]
>  
> >    [RWL3.   For others,  Jim introduced a new term of "apples" for non-char-making stoves (and I thought was correctly done - really no difference from the old method) and "oranges" for char-making stoves (the only subject of the webinar).  
>  
> 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 that was probably not correct much of the time 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.]. That needed to be corrected.[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.  Do you know anyone working with char-making stoves who likes your proposed approach?]

>  
> 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.  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.]
>  
> As to what happens to the char produced, that is outside the stove [RWLk:  Denied again.]  The char production is not outside.], but is part of the total energy system. It is quite reasonable to examine what happens to the leftovers. It is also correct to examine the energy it takes to prepare or transport the fuel in the first place. Fuel is part of a larger system.
>  
> >I take this to mean that Crispin's "new method" is not going to be implemented by Jim.
>  
> He was in fact explaining the new method to you – that is not new.  I was referring to a new method of generating three representative replications. I have already discussed it with Jim. I have also discussed it with others. It is a great simplification for the tester and provides the unskilled tester or those with only a scale and a thermometer an opportunity to make a pretty accurate determination of a stove’s performance. The replication idea is from the SeTAR Centre. There is a definition of ‘fuel consumption’ that goes with it, clarifying what has been murky – by which I mean the claims for fuel consumption that were based on heat transfer efficiency and energy efficiency, rather than the need for more fuel. 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.]
>  
>  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.
>  
> I am extending the changes past the agreed reinterpretation of the fuel consumption number. What Jim and I agree already (he wrote about it some time ago you may recall and it was discussed here as well) is that we have to examine what happens to fuel remaining in order to assess the fuel consumption. If the stove can’t burn it in its normal operation how can it be considered fuel for that stove?  [RWLm:  Many of us want this char to never see a stove again - to go into the ground - for CO2 sequestration purposes.  Why can't (shouldn't) a stove test accommodate that desire?]  If it is fuel for a different stove, test that on too and made a programmatic decision.
>  
> There is of course the complication of stoves that can generate char in one mode and burn it in another. That has not been resolved yet. It could be that for three replications we should have to perform several iterations that used the fuel in various ways to create a realistic performance assessment. 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.]
>  
> >>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 because of this that we are implementing a completely new approach to the determination of fuel consumption. 
>    The heat transfer efficiency question is also being ‘spruced up’.
>     [RWL4:   In the first sentence, assuming that Crispin's  "cannot" includes "is not designed to",   [RWL4':  Note large section missing here - important to me.]
>  
> 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.]  ‘Cannot’ is the right word.  We [RWLp:  You.  Not the char-making stove community.] are concerned with the question of whether or not the stove can use the mass remaining – whatever it is. In a stick fed stove like an open fire one can argue that the sticks (now dried and blackened) can be used in the subsequent fire. No problem. That is what people do to save fuel. At the end there will be some more of the same.  Perhaps we will find that some TLUD’s can burn anything that is not pure char at a loading of 10% or 40%.  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.]
>  
> 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.]] It should not have a floating insulator on the water because that insulates the pot and falsifies its actual performance. That is what I meant by sprucing up the method. The purpose of the HTEff number is to assist designers of heat exchangers, so it should be a close to the actual number as possible. I have another completely different approach I can explain at another time [RWLs:  Sorry to hear this.  When will we hear?]. 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.]  
>  
> >I do not understand why the present formulae used by Jim/EPA/GACC in any sense of this phrase, does "over-report the system efficiency and under-report the fuel consumption"   
>  
> This refers to stoves that produce char and cannot [RWLu:  Or felt it better to not.] burn it.  If you pretend the char is unburned fuel, you understand [RWLv:  Typo?] the fuel mass burned.  The two phrases amount to the same thing. If you under report the system efficiency by saying is used more fuel than it did, the follow-on is that the fuel consumption is reported to be higher than it actually is. This inverse is also true. If you deduct the char remaining from the fuel consumed, it pretends that the char remaining is unburned fuel that could be used on the next replication. If that is true, no problem. If it can’t burn it, then it is not true. That was the entire point of the apples and oranges analogy.  [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".  As near as I can tell, you want to make believe there are no valid cases where oranges (char) exists.]
>  
> >I believe Jim, GACC, and the whole stove community would greatly appreciate hearing what is going to be done by Crispin in the new Indonesian tests.  
>  
> 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.] If it does not meet the minimum requirement, it means people are unlikely to want to buy it because it will be viewed as underpowered. We don’t want to waste time assessing stoves we are pretty sure no one in the target community will want. You might make a wonderfully efficient stove that has no emissions but which is so low in its maximum cooking power that it will be rejected. We can tell what the user wants by determining the heat flux rate for stoves they say are acceptable (not even ‘good’, which would be a higher number).  [RWLy:  I don't detect an answer to my question - what does this new test look like? (in detail)]
>  
> 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.
>  
> 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?]).  I believe this is the first time such a credit has been offered. The purpose is to encourage the use (probably as fuel, but at least for some economic benefit) of the char produced. 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?] way of rating the ‘pass/fail’ question about fuel efficiency and char making.
>  
> >What is the new formula to correct this major claim on inaccuracy?  What is the "spruce up"?   Can all of Jim's present set of numbers be inferred from the proposed new test method output?]
>  
> 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?]). Some issues remain uncorrected. The use of the mass of water remaining when boiling is one. It causes needless variability that should not be there.  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.] 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. 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.
>  
> The main numbers to report for any stove are the fuel consumption and emissions per useful MJ in the pot. 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
>  (2.26 MJ/kg?]   Ergo, no useful MJ.  The consideration of the mass of the pot and its material needs to be incorporated. The British, Indian, SeTAR, Chinese and Indonesian tests already consider it. The Indian version of how to do so is the best and the SeTAR method is copied from it. It is in turn descended from an old British standard. The Chinese method was developed independently I think.  [RWLai:  Seems like extra work, if everyone uses the same pot.  It could always be put back into the computations.   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. ] 
>  
> Because of the interest that is emerging in the char making ability of some stoves, we will provide a space to report it but will not make its characterisation part of the stove performance report which is confined to fuel consumption and emissions per MJ in the pot(s). I am sure others will take an interest in it later.   [RWLaj:  Understood.  Treat char only and always as waste.  (speaking with an attempt at humor.)]
>  
> I think all of Jim’s numbers could be inferred but he is generating numbers based on the old WBT 3.1 data sheet some of which are agreed not to be valid. For some reason they are still there. One is the efficiency of simmering which you can still see on the new spreadsheets copied from the old ones. It is misleading and untrue, as the WBT method does not determine the heat transfer efficiency when simmering. It just reports a number based on the missing water which may or may not indicate something [RWLak:  It sure means something to me - it means water evaporated.  It also should be a measure of how close one can keep to a desired temperature.]. Having the lid off makes it worse because of a pretty large IR radiation error (on the order of 200 Watts).  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/]  The heat transfer efficiency is compensated for chemical losses as per standard boiler analysis. The fuel consumption figure is compensated for mechanical losses (as Jim agrees is necessary).  [RWLam:  "Mechanical" meaning char - which Jim is handling (well) and you are pretending doesn't exist  (at least in the final report) ]
> 
> So we are pretty much on the same page, or getting there bit by bit. There are a lot of resolution issues (precision and accuracy) which the new triplicate replication method goes some way to improving. It happens to be easier and simpler, but if it was not we would still do it because it gives a more precise result [RWLan:  Maybe - but not for char-making stoves where the desire is to not further consume the char.]  . 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.] It only increases the confidence of where the middle of the error bars are. The size of the error bars is inherent in the protocol and the instruments.  We have to develop methods with smaller error bars. [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.   Biomass stoves are notoriously difficult to characterise well. Probably the most difficult. Anything we do for them will automatically improve the results for other fuel-burners.  [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).]

   [RWLar:  I still do not know what the new formula(s)  (the last question) are.   End   Ron
>  
> Regards
> Crispin
>  
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