[Stoves] Analysis of a two-stove cooking system

Sat Nov 28 14:14:29 CST 2015

Dear Crispin, 

oh - this is perfect and I think I will learn a lot.  thanks

First: Lets define:

A ) (Fuel Efficiency%) = ((task 1 + Task 2) / (dry wt biomass fuel) ) X 100
This is the dry fuel taken from the forest required to complete task 1 and task 2. Doesn’t matter the energy or time it takes to do it. As long as the stove(s) can complete the task(s) the stove(s) can be tested. There must be a very sharp endpoint telling when the task(s) are completed. Excess fuel left in the stove is wasted. 

B ) (thermal efficiency%) = (((task 1 + task 2) / (total energy released)) X 100
This is the total energy in the dry fuel added to the combustion chamber(s) and used to complete task 1 and task 2. Doesn’t mater the energy going into the food or time it takes. Just that the two tasks can be completed. Anything left is wasted.

Note: this is  the values I suggest we use to ’normalize’ fuels we use in comparing stoves then calculate the fuel likely will be used from site location. Not sure a good suggestion so will need be tested. But we need to normalize some way. 

C ) (energy efficiency%) = ((energy going into water for stove 1) + (energy going into water for stove 2) / (total energy from dry fuel released)) X 100

This is more a test for stove research than for deciding the best stove from a list. Only useful for water boiling until there are ways to determine the energy at the surface of a frying pan that can now go into a food  material or the heat going into a box oven used for baking. Only useful for finding ways for improving a stove. Energy left over is subtracted from the total energy going into the combustion chamber. 

Are we on the same page?

Thats all for now:

Thanks

Frank

Frank Shields

> On Nov 28, 2015, at 11:34 AM, Crispin Pemberton-Pigott <crispinpigott at outlook.com> wrote:
> 
> Dear Paul 
> 
> >Typographical errors:    
> 
> Noted. Thanks. 
> 
> >Content:
> >Good exercise.   Well done.   Helpful for further discussion.  Yes, there can be cooking systems with 2 stoves such as you describe.
> 
> >But there is one issue concerning first paragraph of Item 2.5 (page 2):  
> 
> >The first 2 lines are fine.   But it seems that the final two lines are trying to say that fuel efficiency is the same as energy efficiency.   I read this at least 6 times.   How about some re-wording to clarify what you are saying.   
> 
> I was attempting to describe the efficiency with which the energy supplied to the stove is delivered to the pot up to the time the flame goes out. 
> 
> >You and I have agreed in the past that fuel efficiency and energy efficiency are not the same.   
> 
> I cannot think of a better term at the moment. Yes it is a change in my choice of words. While I could have chosen 'cooking efficiency' the heat might not be applied to cooking, it could be something else. I am open to suggestions. One might consider ‎the space heating efficiency. The main point of the exercise it to show that one cannot count some portion of the energy twice. Nothing can be more than 100% efficient without invoking magic. 
> 
> >For Stove1 the fuel efficiency is based on wood fuel.   Then for Stove2 you switch to have the fuel efficiency based on charcoal.
> 
> That is correct. My major point has been communicated it appears. The fact that the appearance of the fuel changed does not change the energy ‎content. 
> 
> >But you are not giving any credit to Stove1 for having produced the fuel for Stove2. 
> 
> A very important comment! Stove1 did not "produce fuel". That is a misconception. The fuel was produced by the forest. Stove1 extracted some of the energy available from the fuel and changed its appearance. 
> 
> While doing so it did some cooking. At flame-out the remaining matter, charcoal, was no longer useful to Stove1. It only extracted 1/2 the energy available to it and only 1/4 of the fuel's energy was delivered to the pot. 
> 
> No "fuel" was "produced". It was was always "fuel" and some of the energy was extracted - leaving half un-released. There is nothing to "credit". ‎ If Stove1 was the only product you had, 1/2 the energy was left at the end in an unusable form (for that stove) so the next meal requires more fuel from the forest. It is that requirement that is consuming the forest, often unsustainably. Buying a second stove to burn the charcoal does not increase the performance rating of Stove1. 
> 
> >And that is where Ron and I and others have been disagreeing with you.   
> 
> So this is your chance to show how the energy in the fuel consumed should be counted. 
> 
> It is important that you demonstrate how the two are assessed together, as I did, and then to analyse them separately using the same approach. Follow the energy. If the cooking efficiency is correctly assessed, then the combination of two stoves will be correct as well. 
> 
> If a different, single stove that has the fuel input of Stove1 and the remnant ash content of Stove2, and further, delivers the same cooking energy into a pot as the combined performance of Stoves 1+2, then the performance rating of that single stove will be exactly the same rating as the two stove combination, as it should be, because it did the same amount of work with the same quantity of fuel. 
> 
> ‎A chain of three stoves can be imagined. No correct method of analysis can have as a result an efficiency that is greater than all the work done divided by all the energy available. The term 'energy efficiency' can correctly be applied to any part of that system, or the sum of all parts. It's a pretty good name. 
> 
> I hope a clear distinction between the energy efficiency (or cooking efficiency) and the thermal efficiency ‎has been demonstrated. The thermal efficiency does not take cognisance of the total energy available in the fuel consumed, just the energy in the missing fuel. Thus it can't be used as a proxy for fuel consumption as has been done for years by the WBT. Notably, the Indian, Chinese and SeTAR test methods do not make that mistake. There was a pre-WBT 1 test method from 'Feu de Bois' and another from the University of Eindhoven that reported the fuel consumption correctly. 
> 
> While it would be nice to validate the WBT results as 'fuel consumption', it instead reports the thermal efficiency which is a different metric. That difference is the main reason why WBT lab tests do not match field observations of fuel use. ‎A second reason for the mismatch in results is the differences in testing context but we can leave that for another discussion. 
> 
> Regards 
> Crispin 
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