[Stoves] FW: REQUEST for complete sets of raw data of cookstove tests.

Wed Apr 24 09:25:05 CDT 2013

Dear Ron

I think you will have to look carefully at what I am trying to achieve because I find your restatements of my analyses different from my explanations.

Jim and I agree, as for Paul A and many others, that the performance rating of a stove has to be for that product, not a programme in which other issues are included. The reason for this segregation is that claims made for a stove have to scientifically valid. Claims made for projects that include additional considerations also have to valid. I have no problem with projects doing all sorts of complicated things like having a chain of stoves that use by products of each other, with calculating carbon emitted or not. But when it comes to dividing the portions of such projects into their (valid) segments, the rating of what happens in each ‘box’ as Frank calls them, must stand on its own.

One of the items on my ‘to do’ list is to share a perspective on the use of terms like fuel efficiency and energy efficiency and heat transfer efficiency. These terms are different and cannot be mixed together. A lot of the misdirection on stove performance and fuel consumption has its roots in the difference between energy efficiency (how much was used to cook) and the fuel consumption (where remaining fuel is tossed or goes to some other purposes).

>Thermal efficiency is calculated per the WBT protocol as: 3 / (10 – 2) = 0.375
    [RWL2:  For later use,  lets call this E1.  See also a later note about possibly needing to subtract energy in unconverted wood.]

That is the energy efficiency – the amount of energy that was liberated and the % of it that got into the pot (usually measured incorrectly, but bear with me).

>If the char is “discarded,” then thermal efficiency can be calculated as: 3 / 10 = 0.3    [RWL3   E2]

That is the fuel efficiency because it describes the consumption of fuel, some the energy was liberated, some was not. Conventionally, these are called mechanical and chemical losses (solid v.s. gaseous losses).

>The thermal efficiency for char production can be calculated as: 2 / 10 = 0.2      [RWL4:  E3]

That is not a ‘thermal efficiency’. An efficiency is a ratio. The char produced is not a ratio save expressed as a mass of char per dry mass of input fuel, or an energy contained (potentially) v.s. the energy contained (potentially) in the raw fuel. Neither is an energy efficiency because in both cases no energy is released and no work is done. It is just a way of expressing the mass or energy contained within some fuels.

>I think you are proposing to add the thermal efficiencies for cooking and char production: ( 3 / 10 ) + ( 2 / 10 ) = 0.5
      [RWL5:   Right -   E4= E2+E3]

This is not possible because the units are not compatible. Making char is not an ‘energy efficiency’. It is the processing of a raw biomass into a processed biomass with a different energy density per unit mass.

I’m not in favor of doing this, because, while there is a common denominator, I think the numerators are like apples and oranges – cooking (useful) energy and fuel (stored) energy.
   [RWL6:   I agree they are apples and oranges.  But sometimes the question is asked  -  how much "fruit" do you have  and in this example the answer is certainly E4 = 0.5.   

They are like comparing two different things which is why they have different units. The analogy breaks down at the ‘fruit’ question because they are not both ‘fruit’. One is an energy transfer question and the other is a fuel processing question.

>As long as the number "E3=0.2" is given a little prominence, I don't care if the number  E4 = 0.5 is also given.  I expect promotional char-makers will be using both E3 and E4, of course.

Understanding why they cannot be ‘added’ is very important to this discussion. They are different. Char making is a reportable feature of a stove, if you wish. Jim and I agree on how to report the difference between fuel consumption and energy consumption.  The % of char produced from the available carbon (for instance) is a third metric. It is not part of ‘thermal performance’, it is one of the variables used in the calculation of energy efficiency. It cannot be added to something from which it has just been subtracted.  Clearing up the prevailing confusion (if any remains) is an important step in getting test results of comparable value.

Regards

Crispin

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