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

[Kevin]

Dear Paul

That is a very good question!

On the one hand, Efficiency, Waste, and Fuel Savings are very simple....  If 
a system is say 40% efficient, then it has 60% waste. However, a "Savings 
Claim" is meaningless without a comparative figure. If a stove of 40% 
efficiency was being compared with a stove of 30% Efficiency, then the 
"Improved Stove" is 25% more efficient:
(40%-30%)/40% = 10/40 = 25%

On the other hand, they can get very complicated. In the case of stoves, 
their stated Efficiency can range from totally meaningless, to "ballpark" to 
"helpful" to " correct." To make a "Meaningful Statement of Efficiency", one 
must clearly define what heat is useful and what heat is truly wasted.

For example, one can have a that stove "cooks" at virtually 100% Efficiency. 
Consider a case where I have a heating stove, on which I place a pot of 
water and make a pot of tea and makes a pan of biscuits... the incremental 
addition of wood that I would have to burn to keep my cabin at a constant 
temperature, whether I cooked or not, would be virtually identical. More 
specifically, while the cooking process would absorb some heat from the 
stove, it would release it back into the living space when the tea and 
biscuits cooled.

Here is another example.... Say I have a pound of wood containing 8,600 BTU, 
and I put it in a TLUD Stove, because I want to make a batch of biochar, 
having an expected yield of 20% biochar with a heating value of say 13,000 
BTU/Lb, or with "contained energy" of 2,600 BTU. From the pyrolysis gases, I 
would have roughly 6,000 BTU of "free" energy, in that if I didn't use it, 
it would be lost as warm air. I could thus "cook for nothing" if the fuel 
cost was written off against the cost of the charcoal intended for biochar. 
Or, I could "share the fuel cost with the energy contained in the pyrolysis 
gases. I could also decide to have the biochar carry the "process losses" 
associated with reactor shell losses, or perhaps I could decide to share the 
shell losses with the "free energy" in the pyrolysis gases.

To make a "Defendable Savings Claim", one must really do a "comparative 
energy balance" and clearly show what fraction of the input energy is truly 
helpful and useful, compared to that which is truly wasted and lost.

Best wishes,

Kevin

----- Original Message ----- 
From: "Paul Anderson" <psanders at ilstu.edu>
To: "Discussion of biomass cooking stoves" <stoves at lists.bioenergylists.org>
Sent: Wednesday, October 10, 2012 2:02 PM
Subject: [Stoves] How to convert thermal efficiency into fuel savings 
figures


> 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 we hear soon from the number-crunchers and quantitative testers of 
> stoves.    Theory and real numbers are welcome.
>
> Paul
>
> -- 
> Paul S. Anderson, PhD  aka "Dr TLUD"
> Email:  psanders at ilstu.edu   Skype: paultlud  Phone: +1-309-452-7072
> Website:  www.drtlud.com
>
>
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