[Stoves] Advocacy action: ask the GACC to stop promoting the WBT

[Crispin Pemberton-Pigott]

Dear Paul

You are not trying to find the heat transfer efficiency. If you knew what it was, you will agree it will not help you.

Consider: you have hot gases passing the pot. You quantify (by subtraction) how much energy is in the hot gases. You determine how much heat is transferred to the pot. Divide the latter by the former and you get the heat transfer efficiency.

Now what do you do with the number? That is the metric stove designers chase to improve. But there are other considerations for the designer.

The Big Consideration is how much fuel is needed to achieve the cooking. The appropriate metric for that is the 'cooking efficiency', which considers the amount of fuel needed and the cooking accomplished. Not too surprisingly, fuel consumption is a popular metric to use when selecting a stove to buy, same as buying a car.

The popular metric name is 'fuel consumption'. ‎If you look at the bottom of the Quad II test report from CREEC Lab you will see that is the name of the metric. It says the stove used 636 g of fuel. But the stove used about 1200 g of fuel! That is not 636. Why? What's going on? Why isn't the dry fuel mass consumed per test reported as the dry fuel mass consumed? Bit of an obvious error, eh?

They have reported an imperfectly calculated heat transfer efficiency as the fuel efficiency. That is what a WBT does, and why the observed field performance is always worse than the WBT lab test, even if the cooking in the field exactly matches the test sequence of the WBT.

‎I have previously discussed what that 636 number actually is, but it certainly is not the mass of dry fuel consumed, which is what the number is called. Anyone who uses that number to calculate the relative fuel consumption of two or more stoves will be misled because it does not represent the fuel consumption.

Unfortunately the 'Stove Comparison Chart' uses that number to report the fuel efficiency. Obviously the ratings are in error, each by a different amount depending on how much char they happened to produce that day. This leads to misunderstandings and mis-belief about what people are getting and what they paying for.

Regards
Crispin



Crispin,

I have not yet had time to read your lengthy response to me (very busy today).   But I did read your response to Ron (below).

I find part of it to be astonishing.

What I cannot do is subtract anything from the denominator, unless I am trying to determine the heat transfer efficiency, which so far no one has claimed to be trying to do.

I certain have been terribly stupid.  All along I thought we were talking about the same thing.   Obviously not.  I accept the blame for my lack of precision in my use of terminology.    That is why I am not on the ISO or other technical committees.   Someday (when we have more time) we can put all of the terminolgy into a list and show how each one is different.

So, as long as we discuss the "heat transfer efficiency", I now believe that we three (you, Ron and me) are in complete agreement.

Thank you.

Paul


Doc  /  Dr TLUD  /  Prof. Paul S. Anderson, PhD
Email:  psanders at ilstu.edu<mailto:psanders at ilstu.edu>
Skype:   paultlud    Phone: +1-309-452-7072
Website:  www.drtlud.com<http://www.drtlud.com>

On 1/23/2017 1:58 AM, Crispin Pemberton-Pigott wrote:

Dear Ron

Nearly everything you ask is answered in the message to Paul

“This is a 2013 EPA webinar (achieved by GACC) devoted to batch stoves - principally char-making.”

The presentations makes a fundamental mistake of making two calculations and calling them both ‘thermal efficiency’. You can’t just go subtracting and adding things willy-nilly and assume that the result has meaning.

The presentation from the EPA takes the incomplete WBT heat transfer efficiency formula (defective because it does not include all the necessary variables) and extends the application of it to claim it is considering the benefit of the energy in the char. It is not.

It is also stated that the energy in the pot cannot be added to the energy in the char! Rubbish! They are both forms of energy, one sensible and the other stored chemical energy. I can added them without even using a calculator. They are both benefits of the operation of the stove, benefits derived from the consumption of the fuel. So is space heating, or operating a TEG to generate electricity. I can easily add all the energy in those benefits and arrive at an overall efficiency for the stove. Or, I can report them individually as if the other benefits were not needed or wanted.

What I cannot do is subtract anything from the denominator, unless I am trying to determine the heat transfer efficiency, which so far no one has claimed to be trying to do. Were I to subtract numbers correctly, I could get the heat transfer efficiency to the TEG, the heat exchange, or the pot. But no one asked for that. They asked for the fuel consumption, the cooking efficiency and Paul want the char energy retention efficiency. No problem. Please see the response to Paul.

Explain to me why you would want the cooking+char production efficiency of 37.5% reported as 16.66%. I look forward to that.

Regards
Crispin





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