[Default] On Thu, 11 Oct 2012 11:13:09 -0300,"Kevin" <[email protected]> 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 _______________________________________________ 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/
