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! http://stoves.bioenergylists.org/stovesdoc/Smith/Primer/Primer.html
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!) -Marc On Oct 11, 2012, at 9:13 PM, Kevin <[email protected]> wrote: > Dear Andrew > > ----- Original Message ----- > From: <[email protected]> > To: "Discussion of biomass cooking stoves" <[email protected]> > Sent: Thursday, October 11, 2012 5:02 AM > Subject: Re: [Stoves] How to convert thermal efficiency into fuel > savingsfigures > > > [Default] On Wed, 10 Oct 2012 12:02:41 -0500,Paul Anderson > > <[email protected]> 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! > > # My apologies for a confusing explanation. If you could point out where my > explanation was confusing, I will attempt to clarify. > > > > > 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, 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?... > ---> Is it 0% Efficient because I have no pot to measure its heat gain?... > ---> Does it suddenly attain some degree of efficiency if I set a pot of > something on it to cook? > > # 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: > 1: a thatched hut in a Tropical Jungle Setting, > versus > 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. > compared to > 3: A box stove, intended for both space heating and cooking, having a chimney > that vented outside the living space. > > # 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. > > > > 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? > > > > 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. > > # 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. ;-) > > Best wishes, > > Kevin > > > > > > > AJH > > > > _______________________________________________ > > Stoves mailing list > > > > to Send a Message to the list, use the email address > > [email protected] > > > > to UNSUBSCRIBE or Change your List Settings use the web page > > http://lists.bioenergylists.org/mailman/listinfo/stoves_lists.bioenergylists.org > > > > for more Biomass Cooking Stoves, News and Information see our web site: > > http://www.bioenergylists.org/ > > > _______________________________________________ > Stoves mailing list > > to Send a Message to the list, use the email address > [email protected] > > to UNSUBSCRIBE or Change your List Settings use the web page > http://lists.bioenergylists.org/mailman/listinfo/stoves_lists.bioenergylists.org > > for more Biomass Cooking Stoves, News and Information see our web site: > http://www.bioenergylists.org/ >
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