[Stoves] How to convert thermal efficiency into fuel savings figures

[Paul Anderson]

Stovers,

Thank you Kevin and Andrew for your replies.

Using Andrew's example below, 55% divided by 15% = 3.7 times the amount 
of fuel for the 3-stone fire versus the better one at 55% efficiency.

Or     15 divided by 55 =  .27 or only 27% of the fuel is used for the 
55% TE stove compared to the referenced 15% TE 3-stove fire.

These are reciprocals.      3.7 x .27 = 1.00       (precisely, it is    
3.66666...   x    0.2727... = 1.000000 )

So the better stove saves 73% of the fuel in this example.

FIRST question:    Is this correct?   Anyone else able to confirm or 
refute this?

SECOND question:    If it is correct, then two small tables (A: TE from 
0% to 100% in 5% increments on each axis -- one for each of the stoves 
being compared;   and B:   Fuel Savings from 0 to 100%  on each axis) 
and or an applicable formula or both could be provided for everyone to 
use.    And would that table / formula be useful?      It would be 
useful to me and to the person who originally asked me about this.  The 
user could quickly look up to see values that can be compared.

REQUEST:   If yes to both questions, would someone (or a small group) 
please volunteer to prepare a one-page explanation plus table plus formula?

Thanks,

Paul

Paul S. Anderson, PhD  aka "Dr TLUD"
Email:  psanders at ilstu.edu   Skype: paultlud  Phone: +1-309-452-7072
Website:  www.drtlud.com

On 10/11/2012 3:02 AM, ajheggie at gmail.com wrote:
> [Default] On Wed, 10 Oct 2012 12:02:41 -0500,Paul Anderson
> <psanders at ilstu.edu> wrote:
>
>> Dear Stovers,
>>
>> I do not know how to convert thermal efficiency into fuel savings figures.
>>
>> I hope it Is a linear transformation.     10% TE wastes 90% of fuel,
>> while 40% TE wastes only 60%?    But I suspect it is more complicated
>> than that.
>>
> I hope Kevin's answer didn't confuse you as much as it confused me!
>
> 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.
>
> At these higher levels of efficiency the heat exchange interface with
> the pot will be more significant then the completeness of combustion.
>
> 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.
>
> AJH
>
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