Dear Rebecca
I can’t respond in detail at the moment to the other questions but there is an important element of your test reporting that should be reconsidered. What follows is in line with the current thinking about how to treat leftover fuel and how to report ‘efficiency’. This is from your document: RESULTS: TIME TO BOIL: 41 minutes WEIGHT OF FIREWOOD USED: 1.75kg (3.00 – 0.10kg unburned wood – 1.00kg unused wood – 0.15kg charcoal) There are two things of interest to the stovers: the heat transfer efficiency and the fuel consumption. It is important to keep in mind these are not the same thing and one does not translate into the other. Jim Jetter in his webinar mentioned this last week. The implications are significant. Jim and I and many others have discussed this for a while and we are settling on some clarifying remarks, you could say, about these two metrics. People building stoves want to know if the heat transfer efficiency has changed, and there are standard engineering methods to work it out. The popular WBT does are reasonable though not completely accurate calculation of the heat transfer efficiency. It is not necessary in this message to go through that in detail, but it is important to understand that the WBT calculates the heat transfer efficiency, not the fuel consumption. Your method falls somewhere in between. The WBT does this by working out how much heat was available from the fuel, though does not include in that calculation the loss of energy from unburned particles and gases. The quantity of energy available from the fuel is compensated for moisture and the remaining charcoal which is given an arbitrary or measured heat value. This (theoretical) energy yielded total is expressed in Joules. The energy in Joules is then divided by the heat energy available in the raw fuel were it to have been dried completely. It is called the ‘dry fuel equivalent’. It really is the dry fuel equivalent, no doubt about it, but it is not the dry fuel that was consumed. It is the dry fuel equivalent of the energy that was calculated to have been available, and that is used to determine the heat transfer efficiency (or a reasonable proxy of it). As for the fuel efficiency, one can report the fuel consumption. Your list above (which is a very good way to present the information, by the way) has this: WEIGHT OF FIREWOOD USED: 3.00 minus 0.10kg unburned wood minus 1.00kg unused wood minus 0.15kg charcoal = 1.75kg wood consumed This means you are trying to calculate the mass of fuel consumed when performing the test, in this case a water boiling test. The consumption of fuel can be taken to mean the mass of new fuel that you need each time the test is ‘replicated’. The reason this is different from the dry mass equivalent of the energy used to calculate the heat transfer efficiency is that there is always some wasted fuel from burn cycle to burn cycle. ‘Wasted’ is not a term of criticism, it is used to mean ‘not used’ and is a ‘loss’. Losses classically are divided into ‘chemical losses’ (in the form of un-oxidized gases) and ‘mechanical losses’, meaning unburned fuel that falls to the ground from a conveyor, so partially burned fuel in the ash and char remaining is in the latter category of mechanical losses. The question that arises, as highlighted by Jim, is whether or not the fuel can be burned in the same stove during the next replication. If it can, then it is unburned fuel that will go to the next round and could therefore be deducted from ‘fuel consumed’. If not, then it falls into the category of ‘mechanical losses’ and has been ‘consumed’. That means for fuel consumption purposes it should not be deducted from the mass of fuel consumed per replication. I have been working on a new method of determining the fuel consumption in a realistic setting, though still using a lab test. I discussed this with Jim some time ago though it was not presented as a separate method last week. It will be used in the upcoming round of testing in Indonesia’s CSI Pilot so some stovers will get to see what the influence is on their claimed fuel consumption rates. I am hoping your stoves will be included in that testing. The intention is to get a more accurate consumption figure. The method is to ask the manufacturer what fuel from a previous cycle can be used in the next one. This claim is tested. The reusable fuel remaining is then included in the following replication, meaning the tests do not start off with new raw fuel each time and end with having some partially burned wood and char. Tests are started with old fuel in the mix. If that leftover fuel is ‘fuel’ for that particular stove, it goes into the next replication as part of the fuel to be consumed and the raw fuel that has to be added is monitored separately. When that second replication ends, there will again be some unused/unburned/partially burned fuel left over. That becomes part of the fuel for the third burn. And so on. The point is to determine how much additional raw fuel has to be added per replication so three are performed in a row with the old fuel going into the next round (if it can burn it). The calculation of the raw fuel consumed is surprisingly easy. Perform a burn cycle first, then using the leftovers, perform three more and divide the raw fuel needed to do all of them by three. It is quite accurate and represents use in the field. It also avoids trying to judge the fuel remaining from each test individually and having as a final answer, the sum of all the errors. This new method divides the errors by three. If a stove can’t use any of the fuel remaining, there is no error at all. What is consumed is what went in, 100%. WEIGHT OF FIREWOOD USED: 3.00 = wood set aside to burn 0.10kg unburned wood = wood that is still in its original form 1.00kg unused wood = partially burned but left over? If it is ‘processed’ in some manner, meaning dried out, partially charred and so on, obviously it has a different heat content. That matters, and it is hard to check. 0.15kg charcoal = some dry fuel equivalent, or some moist fuel equivalent, or some number of Joules. If those ‘remainders’ can or cannot be used as fuel in the next fire (or is unlikely to be used in the target community) then the formula for mass of wood consumed must be changed to reflect reality. = 1.75kg wood consumed = not really. It depends. Well, this kinda highlights my beef. The 1.75 value is not necessarily the same as the wood consumed, and if it was, it would (according to the WBT) be the mass of dry wood that is equivalent to the energy that was theoretically available to be released, based on the mass of fuel burned, factored for the As Received heat content and the energy in the charcoal remaining (which may or may not have been determined correctly). The first step to cleaning up this calculation is to ask if the fuel remaining can be used in the next replication. If it cannot, it is consumed and you do not subtract it from the total. You also don’t have to test it. Let’s assume the charcoal cannot be burned (based on the stove’s construction) but the ‘burned wood’ remaining can. We do not know the heat content of that burned wood, but per kg it will be quite a bit higher than the original moist fuel. Because nearly none of us have the ability to check on its heat content per kg, we should have started the test using such fuel left over from a previous test to cancel the error. That way the errors at the beginning and end will reasonably offset each other. As it is, the heat energy in the fuel remaining is ‘higher than the formula thinks’ which means the performance is under-reported. If the char remaining is 1.6 times the dry fuel heat value, then the fuel mass equivalent is 0.15 x 1.6 = 0.24 kg of fuel that should not be deducted. I think you are using the moist fuel mass for the 3 kg so it is not exactly a WBT method. If the unused fuel can be deducted, the partially burned fuel can (because we can’t go back to change the way the test was done) we end up with: 3.0 – 0.1 – 1.0 = 1.9 moist fuel kg consumed. To know the dry fuel equivalent we would have to know the moisture content and the dry fuel mass equivalents for each element of the formula. If the stove can burn the charcoal in the next round, then one could legitimately deduct (0.15 x 1.6 = 0.24) kg from the dry fuel mass burned of those masses are in ‘dry fuel’ values. If the numbers are all for moist fuel, then a different conversion would be required because char is dry, the partially burned wood is drier and the unused wood is moist. In short, try starting the stove with the fuel remaining from a previous test, and record the mass and condition. Try to do identical tests in a series so you can take them as a group. The new fuel you have to add each test is the consumption. Leave out the difficult and inaccurate estimates of the heat content of processed fuels. The end result will be a much accurate assessment and in your case, slightly better performance once the charcoal question is addressed. For any stove that can burn all fuel remaining, the current methods under-report the system efficiency and over-report the fuel consumption. For stoves that cannot burn any of the fuel remaining, the current methods over-report the system efficiency and under-report the fuel consumption. It is because of this that we are implementing a completely new approach to the determination of fuel consumption. The heat transfer efficiency question is also being ‘spruced up’. Regards Crispin From: reb-kees [mailto:[email protected]] Sent: Monday, September 02, 2013 3:53 AM Dear Crispin, Larry and All Stovers, In the attached trials, I attempted to separate the primary and secondary air flows in the Eco-Kalan -C under the following binding constraints: 1. that the Eco-Kalan-C stove remains 100% clay; and 2. that any change in the design of the Eco-Kalan-C has little impact on its production methods. I offer 2 methods of introducing the secondary air supply -- through 3 holes in the combustion chamber above the firewood (Trial #2); and through the same sized 3 holes in the Kalan at the height where the fire comes out of the vertical part (chimney) of the combustion chamber (Trial #4). The links will take you to a series of photos of the BURNS and WBT (my version) runs to determine the time it takes to boil 18.75 L of water in a 33L pot and the amount of firewood (star apple) consumed. At Felipa Beach, Philippines, we use this volume of water and size of pot to cook 11 kg of rice for a luncheon of 100 + people.--still being done on a 6-year old Eco-Kalan-C that is insulated with ash. Please take a look at the fires --can you tell if the separation of the primary and secondary airflows is contributing to a reduction in PM and CO? How can I get to boiling faster without increasing the PM and CO? I also reduced the height of the front opening of the combustion chamber for the 3 variations from the regular insulated Eco-Kalan-C in oder to lessen excess air. Without instruments and expert help, I wont really know how beneficial this modification has been. However, I have observed the following: 1. The separation in airflows could not cool down the combustion chamber and the outer Kalan sufficiently to be able to touch it and not get burned. With the insulated Eco-Kalan-C, the Kalan gets hot but wont cause a burn. 2. It takes longer to boil the same volume of water with the separated airflow models than with the regular insulated Eco-Kalan-C. I wonder if this is due to the reduction in the height of the pot supports from 11/16 inch insulated Eco-Kalan-C model to 8-9/16 inch in the separated airflow models. 3. The reduction in the height of the front opening of the combustion chamber and the resulting reduction in the size of the opening for the Kalan led to a wider neck around the most vulnerable part of the Kalan. In its present form, the Kalan can hold 43kg of weight when we cook pork & beans using the Eco-Kalan-C. With a wider neck, I expect it to carry a heavier load. Have you seen stoves in Indonesia that can carry and cook the same weight of food & pot or wok? 4. It is true what Crispin said about the combustion chamber being the weakest part of the stove. But in the insulated Eco-Kalan -C model, a cracked combustion chamber can still function effectively due to the insulating ash that holds it together. Proof of that is the reject (cracked during firing) Eco-Kalan -C stove we have used for 6 years in daily and special event cooking at Felipa Beach. 5. During the BURN runs (NO POT on the stove), the flames were coming out forcefully, but when the pot was placed on the stove, the fire in Trial 4 had a hard time rising. It seemed that the air flow through the Kalan was keeping the fire down. 6. I love the colours of the flames from the separated airflows. Can you tell from the photos if these are hotter fires than the one produced in the insulated Eco-Kalan-C? I would appreciate very much your comments and suggestions on how to improve the Eco-Kalan-C. Thank you. Rebecca _____ The principle involved is that of putting a good combustor into a supporting frame. Viewed dispassionately, most stovers build combustors and place a pot on top. Lots of people do not think of that as a ‘stove’. Paul Anderson has been pretty upfront about saying that is what he often avoids. There is a stove and there is a burner in it. In order to test for safety I want to see a tilt test used where the large diameter pots sitting on three tiny supports will fall off (= fail). Regards Crispin
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