[Stoves] How to convert thermal efficiency into fuel savingsfigures

[ajheggie at gmail.com]

[Default] On Thu, 11 Oct 2012 11:13:09 -0300,"Kevin"
<kchisholm at ca.inter.net> wrote:


>
>> 
>> If we take the efficiency figures as being correct and that they are a
>> measure  of how much of the fuel energy liberated is delivered into
>> the pot then a 15% efficient 3 stone fire will burn 3.7 times as much
>> fuel to do the job compared with a very good stove with 55%
>> efficiency.
>
># That is only true in the limited cases where the stated efficiencies are correct

Kevin I did define my assumptions in my post and that was that the
stated efficiency figure was a ratio of the energy in the fuel to the
heat energy delivered into the pot.

>> At these higher levels of efficiency the heat exchange interface with
>> the pot will be more significant then the completeness of combustion.
>
># How are these "levels of efficiency" actually attained? Are the real benefits of "heat loss to the room" included, when relevant? 

Again this was not part of my assumptions. I acknowledge the
difficulty and failure to reach agreement on measuring how well a
stove delivers heat for cooking.
>> 
>> Given that the mass flow through  the stove and above the pot should
>> be the same then a quick calculation using the temperature just above
>> the flame and at the exit from the pot will give a relative figure for
>> heat exchange; the ratio of heat supplied to heat rejected. Note at
>> higher efficiencies this will decrease more noticeably as the pot gets
>> hot because heat transfers to the pot depends on delta T between the
>> hot gas and the pot contents.
>
># This is not really "stove efficiency", but rather, it is more like a determination of "efficiency of coupling between a heat source and a heat sink." It is like drawing a graph with a suppressed zero... a picture is painted but it is not necessarily the correct, meaningful, or important picture. 

Of course it is part of stove efficiency because it deals with the
difference between heat released by the combustion and that rejected
from the pot. I agree that efficiency of fuel use is not likely the
most important attribute of a cook stove. We have a good parallel in
the electricity generation field. A diesel generator will reach over
40% efficiency and a steam turbine only 33% but the running costs of
the diesel will be five times those of the turbine and use a cheaper
fuel.
>
># My fundamental concern is that stated stove "Efficiencies" and "Savings" can be very misleading and are prone to being used out of context. The only way to get a fair and true comparison of efficiencies is to do a "Mass and Energy Balance" on situations that do indeed have a true basis for comparison. "Good figures never lie, but Good Liars often figure. ;-)


Yes but I was trying to provide a simple explanation for Paul's
question, I realise the intent might be for some sort of peeing
contest between the Canadian and American contingents but that was not
what I responded to.

Anyway you may try but the hind leg is staying on this donkey :-)

AJH