Dear Marc
Firs I congratulate you on trying to make the engineering aspects of stove evaluation more accessible to the tinkerers that are genuinely trying to do something new and useful. Accessible science makes the world a better place. >I've just published an online tool for simple water boiling tests: http://smallredtile.com/stove/wbt-buddy/ >Is it useful? It is because many people have no idea what to calculate when doing a simple test like bringing water to a boil (which is a useful task even if boiling is arbitrarily defined. The advantage is that if a person thinks a 'rolling boil' is needed each time, then they have a meaningful comparison. >What would make it more useful? It would be good to have some clear explanations about the implications of saying 'overall thermal efficiency' because as I read it, that is not the outcome. If all remaining fuel is considered to be 'unburned raw fuel' then the calculated result is not the overall thermal efficiency but the work done divided by the heat probably liberated by the fire, even if the fire does not burn the fuel completely and can't do so alter on. The difference is quite small for stoves with a fine grate, and quite large for stoves with no grate, or which can't burn charcoal produced. As the core problem with the mis-characterisation of stoves over the past 13 years has been this issue, it is worth trying to get the user of the site to put in numbers that will deliver a predictive result in the field. It is very likely that anyone who knows how much fuel mass they burned will also know the mass of the pot(s) and what they are made of. As demonstrated with the discussion about finned pots, the correct answer and comparison has to consider the heat absorbed by the pot. If you put a cell for pot mass and another for material (could be from a simple choice of stainless steel, aluminum and cast iron (called 1, 2 or 3) it would allow for a more accurate answer. >Did I make any mistakes in the model? "Overall Thermal Efficiency (OTE) is the ratio of energy absorbed by the pot of boiling water (Ewater) to the energy released by the fuel (Efuel)" This definition is not correct. This is the fire-to-pot efficiency. The overall thermal efficiency (the exact term adopted by the Indonesian National Standards Committee at the last meeting) is the energy gained (net) by the pot divided by the energy available from the raw fuel consumed (as defined above) considering the material and mass of the pot as well as the wood type, moisture content and the usability of the fuel remaining. >Is it possible to make the Water Boiling Test calculation simpler and easier to follow? Definitely! But more to the point, how about using a burn cycle that is, overall, more representative of actual cooking. The Hot Start is particularly a problem because no one operates a stove like that. Prof Annegarn won't let the students perform if because he says it is plainly unsafe. >Can we make the steps of the calculation more visible? (to help newcomers, to help identify problems with the calculation) I think that is clear enough. Most people not used to formulas have a problem with representational math so you could put it into words, or have the word formula pop up if the mouse hovers over it. That is pretty easy to code. >I find that, even with an engineering background, the WBT Excel sheet is not easy to use. The equation for finding thermal efficiency is buried in one cell and looks like this: =IF(W26=0,0,(4.186*SUM((W12-$N15)*(AA8-W8),(W13-$N16)*(AA9-W9),(W14-$N17)*(A A10-W10),(W15-$N18)*(AA11-W11))+2260*W27)/(W26*$E20)) This can be made much easier to understand if the cells are Named. It also helps trouble-shoot it so that errors like having $E22 instead of $E20 (which was there for several years giving strongly misleading results) are easily spotted. A major problem with the WBT (well, the versions up to 4.2.1) is that it oscillates between using energy and fuel mass. This is rooted in the old method of using fuel mass to discuss energy - very much and EPA thing. Emissions per kg of fuel and all that. The whole ting should not use mass at all once the fuel is burned. >Also, some of the steps of the calculation (moisture content, char energy content) aren't necessary for many use-cases of the WBT. For example, if I'm trying out a new design, all I want to know if the general range of efficiency: 10-20%, 40-50%, etc. That is OK but over time it is really hard to try to get fuel that is the same - seasonal changes are significant so it is important to let us put in the moisture content - an initial and final mass of a sample from the pile. Again, it only takes a scale to get that and the sheet presumes the user has a scale. >I think there may be some benefit to emphasizing that the WBT is an estimate. Wasn't it originally designed as a quick-and-dirty test for the field? Yes it was. As time passed, VITA settled the fuel remaining question in 1985 by recommending that anything left be ignored (as consumed). The University of Eindhoven earlier came to the same conclusion. Importantly, Nate Johnson found in his work in Mali that if he ignored char remaining (as if burned) the CoV was cut in half. >So, I simplified the engineering model, tried to build a user friendly interface, and laid out the step of the calculation as clearly as I could. It captures a lot of it. I think you have to provide two outputs on the efficiency side: a best guess at the heat transfer efficiency and an overall thermal efficiency that represents fuel consumption. Regards Crispin
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