Crispin and list. Thanks
I see now I have not been paying enough attention to the IWA methodology. For others, you also may want to look at a report out of Berkeley, discussing the new IWA ranking/comparison rules, which include a WBT (probably 4.2.1??) found at: http://www.docstoc.com/docs/154033344/Stove-Performance-Inventory-Report---Global-Alliance-for-Clean I want to make sure that the IWA rules (5 % is an important efficiency difference number!) are handlng char production in a manner fair to char-producing stoves. I'm not yet sure of anything. More tomorrow. Ron ----- Original Message ----- From: "Crispin Pemberton-Pigott" <[email protected]> To: [email protected] Sent: Tuesday, April 30, 2013 6:45:39 PM Subject: RE: [Stoves] Truth in stove reports Re: FW: REQUEST for complete sets of raw data of cookstove tests. Dear Ron I can add: > RWL1b: Is the current WBT4.2.1 a "regular"? "blunt"? I would have guessed > (not looked) that 5% difference resolution is being claimed. WBT 4.2.1 has a resolution that is dependent on several things. Because it uses the final mass of water in the pot for boiling and simmering, and these are quite variable from one test to the next, then you cannot expect the resolution even for very simple direct measurements to be very precise. If you look at three replications of a test and see what the variation is, you can get a feeling for the precision. None of that determines the accuracy which is another matter altogether. Yes it is a blunt instrument and cannot provide, for example, the heat transfer efficiency with a resolution of 5%. To determine the heat transfer efficiency with good precision you have to avoid crossing the boiling point. This is easily demonstrated by calculating the efficiency between 40-85 degrees (remember to account for evaporation) and then between 55-100. The 40-85 degree range will consistently give the same result but the 55-100 will differ from test to test, and will differ from the 40-85 degree figure even though the stove is operating in pretty much the same conditions all the time. Changing the pot also gives a different answer because the heat transfer efficiency is a matter of the relationship between the stove and the pot, not what is in it. I think that Jim is (using WBT4.2.1) testing for and reporting on heat transfer efficiency. Not true? Jim and I both report the heat transfer efficiency and the fuel efficiency. >>> [RWL2a In next to last sentence, you say (emphasis added): << "There are particular metrics which provide valuable information about performance." > [RWL2b. Are there some particular metrics that could be, should be, and are > not now supplied through the WBT 4.2.1 procedures? There are 9 metrics in the IWA. Only one is provided by the WBT 4.1.2 which is referenced in the document (a proxy for heat transfer efficiency). In order to overcome that shortfall a small team is working on updating the calculated outputs from WBT tests. Version 4.2.1 has a new section added to each of the Test1-3 tabs which calculates some of the metrics needed for the IWA. There may be problems with some of those calculations. If so, they will come out in an independent review. If there is no review, we are at risk, as before, of adopting a method that has defects that matter. There are still remaining problems which is that three of the metrics in the IWA are not really valid. All relate to the low power phase. This has been brought to the attention of the relevant parties. The root problem is that the heat transfer efficiency during low power and the fuel consumed to run a ‘simmering test now called a low power test’ is not related to the mass of water inside the pot. As has been point out many times here in the past, the WBT rewards, with a higher performance rating, the evaporation of water during simmering. Anytime the mass of water in the simmered pot is divided into something, an invalid number results. Simmering (which is not a scientifically defined term) was discussed at the IWA meeting and it was agreed to dispense with all references to simmering (which were duly removed). However the metrics requested still require simmering to be obtained which is a contradiction. You cannot, for the reason mentioned in the preceding paragraph, have a ‘specific’ performance number from a simmering phase (which is why it was dropped). The meaning is that you might divide the fuel, or emissions, by the mass of water in the pot at the time. Well, the mass of water in the pot is not related to either the fuel consumed nor the emissions from the fire so we still have a conceptual problem. If you double the amount of water in a pot, it does not use more fuel to simmer it. The YDD Lab has been conducting accurate experiments showing this. We are not discussing conceptual problems as a group and I have raised that omissions with the relevant parties. No doubt the WBT (which is one of several tests that can be done) will be further refined and we will eventually agree on what valid measurements are for it. There are still problems with definitions so I have recently made some suggestions in that regard. I posted some definitions of efficiencies here a few days ago. In many cases there is no need to invent new terms or definitions. Engineers have been measuring and describing heat transfer for many years and there are many books on the subject but they are not the Book of the Month Club list. I try to make noise about the most important ones and in each case provide alternative calculations, definitions or alternative metrics which may be valid. As you know there are several versions of WBT spreadsheets still in use: UCB-WBT 3.0 CCT 2.0 UCB-WBT 3.1 (actually there are 3 or 3 versions of this one) PEMS Hood v 7.1.2 which appears to be based on UCB-WBT 3.1 ETHOS WBT 4.1.2 (there are 2 or 3 versions of this one) GACC 4.2.1 (current version Feb 2013) PEMS Hood v 4.1.2 which appears to be based on UCB-WBT 3.1 but it has elements of the last version of 3.1 and also elements of the first (see calculation of the Dry Fuel Equivalent) There is another version of the PEMS Hood spreadsheet (or program) but I have not seen it yet. As far as I know it is the same spreadsheet as the 7.1.2 version. It is being updated by Ryan. If you enter the same test data in each sheet, you will get a different answer from each for the thermal efficiency. Regards Crispin
_______________________________________________ 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://stoves.bioenergylists.org/
