[Stoves] Conceptioal Errors and possible pitfals.

[Crispin Pemberton-Pigott]

Dear Paul

 

I think there are excellent questions. I think we have a responsibility to
'bring the group' along on this because there are a lot of people reading
the list who are able to do the calculations.

 

It is through these thought experiments that we find out whether or not the
metrics being used are valid (or not).

 

It would be more helpful to know the heat transfer efficiency at low power
than that turns out to be invalid. Getting the low power efficiency is easy
to find out, though not by simmering a pot.

 

To give you a brief answer regarding heat transfer, the quantity of heat
transferred during simmering is not affected by the volume of water in the
pot (unless there is nearly none left). Heat transfer is a relationship
between the pot and the stove, not what is in the pot. This fact invalidates
several WBT metrics.

 

Regards

Crispin

 

 

 

 

From: Paul Anderson [mailto:psanders at ilstu.edu] 
Sent: Tuesday, May 07, 2013 12:39 PM
To: Discussion of biomass cooking stoves
Cc: Crispin Pemberton-Pigott; Jim Jetter; Tami bond
Subject: Re: [Stoves] Conceptioal Errors and possible pitfals.

 

Dear all,

Further thoughts about just one of Crispin's examples below:

>    3.  How about giving us ONE paragraph in 4.2.2  to look at  - and with
especially new language on how you would change that paragraph?  Even if
that has to be a year or two from now.

Crispin wrote:



How is this:

I give you 5 litres of water in a pot and ask you to bring it to a boil.

You put it in a fire and bring it to what we agree is 'a boil' by which time
100 cc has evaporated.

I ask you how many litres of water you have just boiled.

What is your answer?

That is an example of a conceptual difference. The WBT says you boiled 4.9.
An engineering will say you boiled 5.0. 

Is this cognitive dissonance or conceptual error?

And take this to the simmer stage, which typically is for 45 minutes in the
WBT.   If the pot has not lid (and it does not have a lid in the official
testing), additional water will be boiled away.   Assume two cases, one with
a very low fire and one with very high fire (a common result of not having
much of a turn-down ratio on powerful stoves).  

In the low fire case, assume that there are still 4.5 liters in the pot.
In the high fire case, assume that there are only 3 liters in the pot.

In the final minutes, the difference in the amount of water in the two pots
is 1.5 liters, which is 33% of 4.5 liters and 50% of 3 liters.   So, are you
actually "simmering" 5 or 4.5 or 3 liters?   And how much actual ENERGY is
needed to accomplish that task if the amounts of water are so different?   

And which stove has the "advantage" (that is, looks the best in the reported
results) depending on what number(s) are used in the analyses?    (Will
someone please do the math using the formulae, just changing the number of
liters from 5 to 4.5 to 3 so that we can see the impact.)   This is
hypothetical, but educational.     

I think that this is the type of stuff that Erin was mentioning about issues
not well understood.  If you have the answer WITHOUT doing the 6
calculations, then you are quite special.   But I will still ask you to give
me the numbers.

           Topic                 Stove type
Results  5 L            4.5 L           3 L                    Comments:
                                    (L or H heat)                           

Results of      
A.   Efficiency???       for LOW heat stove:        ________
______      _______          _______..........

B.           dito                   HIGH
________           ______      _______          _______..........


C.  Result ???            for LOW heat stove:        ________
______      _______          _______..........

D.           dito                   HIGH
________           ______      _______          _______..........

And more???

Sorry, I do not know the answers, and I am not qualified to do the
calculations.  But I would like to know the results.   Maybe more than one
"result" is correct and should be reported in the results of the stove
testing?  If this is meaningless, I want to know why.

Note:   If the results of the above are NOT of interest to you, then perhaps
none of the discussion about testing protocols is of interest to you.   All
of us will be grateful for these types of questions being resolved.

Paul



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