Dear Ron
I am so pleased to read that you understand the core of my problem relating to the char calculations. I know there are a number of others who are following the chain of calculations. >The present GACC handling of char weight is to compute its energy content and >subtract this from the input biomass energy in the denominator of an energy >efficiency computation. I am not so sure it is the GACA that is doing this, but it certainly is the calculation done in the WBT 4.1.2 and the CCT 2.0 if I recall correctly. That is the correct way to get the heat transfer efficiency as measured between the fire (and its combustion efficiency, it you are precise about it) and the contents of the pot (including the pot mass if you are precise about it). The char is unburned so it was not generating heat. The heat value of that char is usually guessed at which is where a quite large error may be introduced as it could be between 12 and 32 MJ/kg. So the number resulting is a proxy for the heat transfer efficiency, and that is what people take it to be. It is thought of as the heat transfer efficiency which is really the efficiency as measured from the hot gases to the surface of the pot – something difficult to quantify. >This bothers some on this list, but I think is relatively OK if the char is >eventually productively used. I am not sure who that would be because everyone so far has agreed that the correct measure of thermal efficiency is done exactly as described. What is invalid is to divide the energy number (a number Joules) by the heat content of dry fuel and then claim that the resulting fuel mass equivalent of that number of Joules is equal to the fuel used by the stove to perform the test. I case that is not totally clear: the energy efficiency is being correctly calculated by deducting for char not burned. If that char is never going to be burned in that same stove, then the fuel that was loaded into the stove to create it has to be counted as part of the raw fuel that stove consumes each time it performs that same task. With the advent and popularity of char-making stoves, this error – the difference between the two methods of determining ‘fuel consumed’ – is so large as to give very misleading comparisons between stoves of different types. What happens to the char after the stove finishes producing it is not a function of the stove and the stove cannot be credited with ‘not burning it’ if each time the stove is used new raw fuel has to be taken from the environment to load it. What is happening now is that the ‘test result’ is claiming that the necessary fuel is not really being taken from the environment because of mathematical trickery. It is being taken – just watch a char-making stove and see how much fuel goes into it each time it is loaded. >…The source of the char should be emphasized when you report on the efficiency >of jikos. The maker of the Jiko has no control over the production method or raw materials used to make the char. The char might be from another stove. The jiko can only be evaluated for fuel consumption on the basis of how much fuel it consumes. It is a metric applicable to that product. Its thermal efficiency is (relatively easily) calculated from the fuel consumed to perform a thermal task. Its fuel efficiency can be measured doing any chosen task. None of these related to how the charcoal was made or what happens to waste char from the Jiko. For a systems perspective, the whole environment, there are many combinations of products that can intersect to create less CO2, or not waste fuel, or create more heat total by burning fuel in stages on different devices. Those are all legitimate calculations that one might want to do for the whole system. For example a TLUD gasifier usually has a high heat transfer efficiency. When viewed from the point of fuel consumption, the overall efficiency is not so good because of all the char, but that char could be used in a second stove to raise the combined efficiency of the pair of stoves. Great. They do that in Indonesia in the ‘improved kitchens’. The system efficiency is higher than any of ites parts. However when rating the components of the system they are still examined and rated individually. >We absolutely need two efficiency numbers we can add together - and I do not >now see them. I too see a need for two efficiency numbers – the heat transfer efficiency (of interest to designers) and the system efficiency (of interest to policy managers). You can’t add them though because one is a sub-set of the other. >But the production of biochar is necessarily accompanied by a reduction in >energy. I think that is a step too far. Are you assuming that the stove producing the char is more efficient than stoves that do not? If is a reduction in the efficiency of the use of energy available in the raw fuel, yes. But to accomplish any task, the energy required is not a function of the fact it makes char, it is a function of the heat transfer efficiency which might be high or low depending on the stove. I have tested a TLUD char-making stove recently with a thermal efficiency of about 19%. >Biochar enthusiasts recognize that we can't have both high energy output and >high char output. Totally agree. There is to be double-counting. Regards Crispin
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