[Stoves] How other tests calculate with remaining charcoal ... was Re: Advocacy action: ask the GACC to stop promoting the WBT

Crispin Pemberton-Pigott crispinpigott at outlook.com
Mon Jan 23 01:57:59 CST 2017


Dear Paul

I think I can answer the question you pose but let’s take this sequentially because if we are making progress towards a common understanding about how to create useful metrics, I do not want to lose momentum.

>The char that is produced in a TLUD stove is removed and is NOT used in any burning that relates to the TLUD stove that made it.   The char comes out.  Where it goes is not an issue here.

Good start. We are agreed that downstream applications have no effect on the rating of performance of the stove in question. A stove hoes not get a better rating because of something you promise to do with the char. The ‘benefit’ of char has to consider the char itself, not what might be done with it. External activities might be considered together with the stove, but that refers to a system of which the stove is a part.

>However, your reply still does not address my question about char values (weight or energy) being used in all those other stove testing methods.

None of the stove test methods which report the performance of the stove’s fuel efficiency, emissions, and power, for example, have anything to say about char produced. There is a very good reason for this and you will have to admit that it is legitimate: no one asked about this matter before (char deliberately produced) so there are no metrics describing the ‘benefit’ of the char in any stove test method.

>Your comments about big systems was interesting:
>>As far as I know, in general all tests of thermal performance such as power stations, fixed boilers, heating stoves and cooking stoves in the formal sector treat solid resides with energy content remaining as a 'mechanical loss'. That is the definition of a mechanical: unburned fuel that could in theory have been burned ‎but was not, and is left at the end.
>But, really, in the situation of cookstoves, what the formal sector with billions of dollars does is only of passing interest.

Engineers will disagree. All these systems can be described using the same toolbox of standard terms. Size had nothing to do with it, conceptually. Stoves can be just as complicated as steam engines.

>"Mechanical loss" is something like the smear of food remaining on a dinner plate after a hearty meal.  Just wash it off, dry the plate, and life goes on.

Not so in all cases, it can be a very large loss and we have to quantify it. High CO is a large chemical loss – many %. Carbon in the ash of a locomotive is an important fuel input to the brick making industry. Locomotives are optimised for size and power, not fuel efficiency. There is a lot of carbon in the ash. People buy it as a cheap fuel.

>However, in the cookstove world, where char is INTENTIONALLY CREATED AND SAVED, it is not a mechanical loss.

Well here we have to accept that there are standard definitions and they are accepted as correct. Anything that a stove cannot burn and appears in the form of a gas is a chemical loss for the purposes of rating the performance of that stove. Maybe the stove is designed to produce a high CO gas as an output, perhaps it is also cooking on heat that is available anyway. The CO constitutes a chemical loss if rating the cooking efficiency only.

If the loss appears in solid form, it is by definition ‘mechanical’ as far as rating the cooking. I understand that the word ‘loss’ is painful when you are trying to create char, but for cooking purposes is it not helping the cooking efficiency if the fuel is not combusted. You cannot expect that the cooking efficiency will be rated higher if there is a mechanical loss that is small, but the efficiency will be higher if the loss is large.

It is important, nay critical, to remember what the metric is reporting: the performance as a cooker. Full stop. If you want another metric to explore the char and its nature, no problem! Create one, but the cooking efficiency will not be improve just because the mechanical losses are recovered for other uses. We can go over this again and again, but the cooking efficiency will not be improved.

Now, there are those who say the WBT or char energy number is the ‘thermal efficiency’. That is a bad choice when we are talking about energy that has not been thermalised. Usually, when looking at the system as a whole, the potential thermal energy in the fuel is compared with the delivered thermal energy. Some called that the thermal efficiency even though if you look closely there are mechanical and chemical losses. The problem with the term is the rather obvious fact that some of the energy was not thermalised. “Energy Efficiency” is better because the term includes all forms of energy. The energy efficiency of a solar cooker consider radiation and thermal energy. No problem.

>It is a purposeful gain, something that is desired.

That is why I asked of you considered it a ‘benefit’. It is a benefit because it is something you want to create. No problem. That does not affect the cooking efficiency, right? The stove consumed the fuel, cooked a bit, and delivered some wanted char. No one is worried by its being wanted. They are worried about claiming that the stove didn’t used the fuel that was consumed to make that char, because it did.  They are also worried about claiming that if the stove didn’t release the energy in the fuel it consumed,  the energy efficiency should be rated higher. Why? Because it misleads people into thinking the stove used less fuel than it actually did. Numerous examples from the field confirm this to be true of WBT ratings – about 100% of them.

You are looking for a way to fairly report the energy needed for cooking and the energy contained in the char. You can do that, but you can’t call it the cooking efficiency because it is not. Are we agreed? It has to be called something else because the cooking efficiency is energy delivered to the pot over energy available in the fuel consumed. There is fuel consumed that is turned into char. We all agree with that. You even gave carbon contents as an example.  That char energy was not delivered to the pot, and it is not available as fuel in that stove, as you pointed out.

Because char is a benefit, and cooking is a benefit, they both have to appear in a numerator if the efficiency of ‘doing it’ is to be reported. Let’s call the fuel energy A, the cooking energy B and the char energy C. Space heating energy I will call D. There are three benefits to be reported.

Cooking efficiency is benefit 1 and it is
B/A        [1]
Char energy is benefit 2 and it is
C/A        [2]
Space heating energy is benefit 3 and it is
D/A       [3]

What you are asking for is some rating of the stove that combines B and C as benefits. No problem. The formula will be
(B+C)/A   [4]

Others may want the combined cooking and heating efficiency. That is
(B+D)/A  [5]

Others may want all delivered energy benefits:
(B+C+D)/A   [6]

What you do not have is a name for the sum of the cooking and char benefits. One thing is obvious: you are not going to find the correct answer by subtracting the char energy from the fuel energy. Why? Because it gives the wrong answer!

We need some numbers now for the demonstration:

Cooking energy 1 MJ
Char energy 2 MJ
Heating energy 3 MJ
Fuel energy 8 MJ

The efficiency if retaining the fuel energy as char energy is a legitimate calculation – formula [2] from above:
2/8 = 25% char energy retention in the recoverable portion

I have to add ‘in the recoverable portion’ because the total in the solids is more than 2 MJ. Of course what is a ‘recoverable portion’ has to be defined in the protocol. Let’s move on.

Cooking is 1/8 = 12.5%
Char is 2/8 = 25%
Heating is 3/8 = 37.5%

Cooking benefit + Char benefit is (B+C)/A
(1+2)/8 = 37.5%

This is the same as the sum of the two efficiencies calculated separately. This is the metric you are seeking. Give it a name. You cannot call it the cooking efficiency. The cooking efficiency is 12.5% and doesn’t change just because you made some char (or not).

Let’s look at the WBT formula:

Cooking benefit + Char benefit is not (B/(A-C)
1/(8-2) = 16.7%

Yet, this is the formula Ron is clamouring for. It grossly under-represents the sum of the cooking work performed and the energy content of the char produced. Why would you want to misrepresent the value of the stove? It has delivered as co-benefits 37.5% of the fuel energy. Why would you want it reported as 16.7%? It is not true! Why is it not true? Because the char energy was deducted from the denominator instead of being added to the benefits in the numerator.

Remember that the WBT formula was intended as a proxy for the heat transfer efficiency. It was never intended to report the benefits of char making, which needs an ‘energy retention efficiency’ metric.

Let’s move on:

Cooking benefit + char benefit + heating benefit is (B+C+D)/A
                (1+2+3)/8 = 75%

Cooking benefit + char benefit + heating benefit is not (B/(A-C-D)
                1/(8-2-3) = 33.33%

>Or at least not to be used as a penalty, as a way to make the stove appear less desirable.

It is not a penalty. It is a benefit and can be reported as such. But is it not deducted from the denominator because the denominator represents the energy in the fuel needed each time to run the test. There is no getting away from that. You want the correct answer, not an incorrect one that does not represent what is in the energy paths.

>BUT TO GIVE CREDIT FOR CHARCOAL AS "UN-USED ENERGY" IS NOT AN ERROR

It is if you do not calculate it properly! There has never been a problem to work out the energy, mass and usefulness, properties and acidity and density of char produced. But the energy in the char cannot be deducted from the energy in the fuel needed to create the char in such a way that it gives the impression that the fuel was not needed, or, more importantly, that it under-reports the combined efficiency of the co-benefits which in the example is 37.5%. You don’t want that reported as 16.7% (half its real value).

>Unless better evidence is provided, please do not use the calculations involving charcoal (subtraction in the denominator) as a major arguement against the WBT.

Char making is not a major argument against the WBT. The mere existence of the formula and the way the resulting number is presented as a proxy for fuel consumption is condemnation enough. But there are literally scores of other issues with the WBT which also have no bearing on char. The biggest of all is the high uncertainty of the result. It can’t be used to place stoves on tiers when the uncertainty is considered, which is has to be if it is to stand up to analysis as a test method. Don’t worry about the WBT. It is discredited without char production issues.

You need to worry about a proper descriptive name for a cooking+char efficiency metric. It is an efficiency because it is a ratio of energy benefits to energy cost.

>I would greatly appreciate some knowledgeable comments about how THOSE OTHER TESTS handle the issue of charcoal that is left in the stoves.   Is there some agreement between those many tests?

I think I have addressed that above. Virtually all test methods ignore char remaining. They do that because they treat all solid residue as a mechanical loss.

Agree on a name for your cooking+char efficiency number. We will use it. Don’t waste your time calling it the thermal efficiency. That term is allocated several times over. Remember it doesn’t cover all the energy in the solid residue, only a portion of it, the wanted and retained and recovered bit(s).

Regards
Crispin


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