Dear Frank
You are on the right track but there are good reasons to consider the actual heat used when claiming an efficiency, that than the HHV. It may surprise you to know there is a metric in our definition set that is the PHV (potential heating value). That is the potential heat that could be derived from a fuel if you dried it completely first. In your village level analyses that should be applied in some cases of assessment. If the fuel was dried first it is a very different amount of net energy than if it was used as fresh-cut wet fuel. The difference would pre-treatment by drying it first. The HHV is misleading and that is who people do not use it for applications where the heat level is about 100C. In the USA the heating and air conditioning guys do not use 100C they use 150 C on the basis that the 50 degree difference is pretty useless for doing work. 1) How much forest does a village use per year? To reduce this we need to know…. 2) How much does each household use (equals sum). To reduce this we need to know the difference in use for the ones using more and the ones using less. 3) Of this list (leave doors open, pre-dry wood, stove types, cooking more of a type of food …. , to reduce we need to zero in on each and optimize the process. Lets take stoves. So that is a macro look at the village energy use. Did you know that is the Nat Johnson did – in detail? 4) For stove efficiency we need only two numbers; a) wood used per b) task completed. We convert ‘wood’ to energy units’ for calculating purposes. That has value but to compare one stove with another before getting to the field we need metrics that can be applied to multiple fuels and different stoves that may have 1 or more pots. That is what makes for an interesting challenge. A good example is what happens if we change stoves and the new one is more fuel efficient per task but takes twice as long and four times as much time and attention? We need metrics to say that ‘those people will not spend money on that device as it is too slow and inconvenient. Similarly some stoves are not very safe. Nat Johnson rated the open fore for safety and it is nothing like the worst stove out there. For one reason, it can’t fall down. 1) …A calorimeter or ‘look-up’ is used for the HHV and moisture for dry weight. Using the LHV means we need to add hydrogen (another test variable) and not needed for the purpose of comparing stoves. We have to use the LHV because that is the available heat. We should not use heat that is not available to do an efficiency calculation. >…Say ‘fraction of the total energy’ (FTV) used. This gives us FTV energy >efficiency values (FTVEV). That is the ‘system efficiency’; or the overall efficiency comparing what goes in and what gets done. That is how you do it if you want to predict the draw of fuel from the available supply (fuel consumption). That is the metric people who read stoves tests think they are getting when they are given the heat transfer efficiency. And that is a problem. It is an ‘efficiency’ but there is more than one. The UNFCCC has the problem built into the GEF/CDM mechanism. Regards Crispin
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