<html><head></head><body bgcolor="#FFFFFF"><div><br></div><div>Kirk Smith's "Short Primer on Stove Efficiencies" was the best explanation of "efficiency" as it relates to stoves that's have encountered. And only 2 pages!</div><div><a href="http://stoves.bioenergylists.org/stovesdoc/Smith/Primer/Primer.html">http://stoves.bioenergylists.org/stovesdoc/Smith/Primer/Primer.html</a><br><br><div>Careful in comparing stoves of different classes -- TLUD "efficiency" can mean something quite different from Rocket "efficiency". For example: what do you do with the leftover char? It probably depends on where in the world you're talking about (even with the same stove!)</div></div><div><br></div><div>-Marc </div><div><br>On Oct 11, 2012, at 9:13 PM, Kevin <<a href="mailto:kchisholm@ca.inter.net">kchisholm@ca.inter.net</a>> wrote:<br><br></div><div></div><blockquote type="cite"><div>
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<div><font face="Arial">Dear Andrew</font></div>
<div><font face="Arial"></font> </div>
<div><font face="Arial">----- Original Message ----- </font>
<div><font face="Arial">From: <</font><a href="mailto:ajheggie@gmail.com"><font face="Arial">ajheggie@gmail.com</font></a><font face="Arial">></font></div>
<div><font face="Arial">To: "Discussion of biomass cooking stoves" <</font><a href="mailto:stoves@lists.bioenergylists.org"><font face="Arial">stoves@lists.bioenergylists.org</font></a><font face="Arial">></font></div>
<div><font face="Arial">Sent: Thursday, October 11, 2012 5:02 AM</font></div>
<div><font face="Arial">Subject: Re: [Stoves] How to convert thermal efficiency
into fuel savingsfigures</font></div></div>
<div><font face="Arial"><br></font></div>
<div><font face="Arial">> [Default] On Wed, 10 Oct 2012 12:02:41 -0500,Paul
Anderson<br>> <</font><a href="mailto:psanders@ilstu.edu"><font face="Arial">psanders@ilstu.edu</font></a><font face="Arial">> wrote:<br>>
<br>>>Dear Stovers,<br>>><br>>>I do not know how to convert
thermal efficiency into fuel savings figures.<br>>><br>>>I hope it
Is a linear transformation. 10% TE wastes 90% of fuel,
<br>>>while 40% TE wastes only 60%? But I suspect it is
more complicated <br>>>than that.<br>>><br>> <br>> I hope
Kevin's answer didn't confuse you as much as it confused me!</font></div>
<div><font face="Arial"></font> </div>
<div><font size="4" face="Arial"># My apologies for a confusing explanation. If you
could point out where my explanation was confusing, I will attempt to
clarify.</font></div><font size="4" face="Arial"></font>
<div><br><font face="Arial">> <br>> If we take the efficiency figures as
being correct and that they are a<br>> measure of how much of the fuel
energy liberated is delivered into<br>> the pot then a 15% efficient 3 stone
fire will burn 3.7 times as much<br>> fuel to do the job compared with a very
good stove with 55%<br>> efficiency.</font></div>
<div><font face="Arial"></font> </div>
<div><font face="Arial"><font size="4"># That is only true in the limited cases
where the stated efficiencies are correct, and where there is no benefit from
the heat that does not actually enter the pot. If I burn wood in a stove to heat
my cabin, what is the stove efficiency, if I do not have a pot on the stove
making stew?...</font></font></div>
<div><font face="Arial"><font size="4">---> Is it 0% Efficient because I
have no pot to measure its heat gain?...</font></font></div>
<div><font face="Arial"><font size="4">---> Does it suddenly attain
some degree of efficiency if I set a pot of something on it to
cook?</font></font></div>
<div><font face="Arial"><font size="4"></font></font> </div>
<div><font face="Arial"><font size="4"># Consider the example of Rocket Stoves, and
TLUD's venting into the living space, and a box stove with a chimney that
vented the products outside the living space, that are used in different
applications:</font></font></div>
<div><font face="Arial"><font size="4">1: a thatched hut in a Tropical Jungle
Setting, </font></font></div>
<div><font face="Arial"><font size="4">versus</font></font></div>
<div><font face="Arial"><font size="4">2: an application (eg, in Mongolia or
Northern Canada requiring both space heating and cooking capabilities), where
the same stove is used for both space heating and cooking. </font></font></div>
<div><font face="Arial"><font size="4">compared to</font></font></div>
<div><font face="Arial"><font size="4">3: A box stove, intended for both space
heating and cooking, having a chimney that vented outside the living
space.</font></font></div>
<div><font face="Arial"><font size="4"></font></font> </div>
<div><font size="4" face="Arial"># In comparison of Case2 and Case
3, the Rocket stove would probably show much better pot boiling rates than
the box stove, but total system efficiency and annual fuel savings would
probably be far greater with the box stove. We don't see Rocket Stoves or TLUD's
that vent their products of combustion into the living space being used for
space heating applications in temperate or cold climates, because the need for
excess ventilation will require much greater fuel quantities to heat the cold
fresh replacement air required for health and safety. In Case 1, a TLUD may
win over a Rocket, in the instance where the person wanted to make a quick
pot of coffee, but the Rocket may win, in the instance of where the person
wanted to simmer a pot of stew for 1 hour.</font><font face="Arial"><br>>
<br>> At these higher levels of efficiency the heat exchange interface
with<br>> the pot will be more significant then the completeness of
combustion.</font></div>
<div><font face="Arial"></font> </div>
<div><font face="Arial"><font size="4"># How are these "levels of efficiency"
actually attained? Are the real benefits of "heat loss to the room" included,
when relevant? </font><br>> <br>> Given that the mass flow through
the stove and above the pot should<br>> be the same then a quick calculation
using the temperature just above<br>> the flame and at the exit from the pot
will give a relative figure for<br>> heat exchange; the ratio of heat
supplied to heat rejected. Note at<br>> higher efficiencies this will
decrease more noticeably as the pot gets<br>> hot because heat transfers to
the pot depends on delta T between the<br>> hot gas and the pot
contents.</font></div>
<div><font face="Arial"></font> </div>
<div><font face="Arial"><font size="4"># 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. </font></font></div>
<div><font size="4" face="Arial"></font> </div>
<div><font size="4" face="Arial"># 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. ;-)</font></div>
<div><font size="4" face="Arial"></font> </div>
<div><font size="4" face="Arial">Best wishes,</font></div>
<div><font size="4" face="Arial"></font> </div>
<div><font size="4" face="Arial">Kevin</font></div>
<div><font face="Arial"><font size="4"></font></font> </div>
<div><font face="Arial"><font size="4"></font> </font></div><font face="Arial">
</font><div><font face="Arial"><br>> <br>> AJH<br>> <br>>
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