[Stoves] Truth in stove reports Re: FW: REQUEST for complete sets of raw data of cookstove tests.

Thu May 2 00:06:00 CDT 2013

Crispin and list, Ranyee, Jim 

This should be viewed as part of my campaign to be sure that IWA and WBT testing treat char-making stoves fairly. Maybe there are ongoing discussions within GACC on that topic, but if not maybe this will be of some help. This is not complete. 
See below, skipping my short message from last nght, and starting with Crispin's reply to me, with new responses identified as [RWL3] 

----- Original Message -----
From: rongretlarson at comcast.net 
To: "Crispin Pemberton-Pigott" <crispinpigott at gmail.com>, "Discussion of biomass" <stoves at lists.bioenergylists.org> 
Sent: Tuesday, April 30, 2013 10:58:06 PM 
Subject: Re: [Stoves] Truth in stove reports Re: FW: REQUEST for complete sets of raw data of cookstove tests. 

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" <crispinpigott at gmail.com> 
To: rongretlarson at comcast.net 
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. 

[RWL3-1: I hope we can hear from others on how repeatable the tests are. I have not seen test results and would like to. 

I wonder if anyone has proposed a test trying to maximize the amount of water boiled away for a batch (char-making presumably) stove? I think this would be of interest to many stove purchasers and should be quite repeatable (not "crude"). The amount of energy required for vaporization seems to be relatively independent of pressure (altitude) and not very dependent on the boiling point temperature. I hope others can check those statements. I found (after quite a search) this at a New York School system webs ite (and couldn't f ind a way to give you a cite) : 

" The heat of vaporization of water at 70°C is 2.33 x 103 joules per gram, whereas at 100°C it is 2.26 x 103 joules per gram. 

This says that Denver area at 3-4 degrees lower boiling point, assuming linearity, would have a heat of vaporization of 
about 2.27x10^3 joules per gram - a difference of less than 1/2 % . . I had assumed until I went looking that there would be much greater variation. Of course there will be variation with different pot shapes and materials, but that s true for any WBT. If need be, it seems that even this 1/2% difference can be modified - as is the case for fuel moisture. 

So knowing weights lost should give an accurate measure of energy input - much closer than the differences you (Crispin) describe above. Or what am I missing? 

Different energy inputs will certainly give different speeds of vaporization, but that could be part of the testing procedure. It is not clear that the energy inputs per kg water evaporated will be wildly different. Has anyone experience on this? It doesn't matter too much if the procedures are the same for all stoves of a similar type. 

Wind should make a difference by removing water molecules above the surface, but these tests will mostly be in a lab. Also a wind/breeze will also cool the pot and probably reduce the flame efficiency, so a fan is not necesarily going to show better energy transfer. But will tests in the same lab space give repeatability? Can stoves be equitably compared? And can the placement of stoves into the IWA categories of 1-4 be done in a fair manner with this system? I see no reason why not. 

For those who haven't read in this area, all stovers will be striving to achieve >45% efficiency (category 4). Gas and liquid fossil fuel stoves are reported in these sort of tests (p 18, Figure 7 of above Berkeley report) to achieve 50%. According to that same figure, char-making and fan stoves have a chance. End RWL3-1 

[RWL 1 or 2?] 

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. 

[RWL3-2: Cr ispin may also be saying he trusts his, but not those from the current WBT4.2.1 (not saying anything about Jim Jetter's skills). So I have to ask Crispin i f that can be true? Reporting yes for both, believability no for J im, yes for you ? ] 

>>> [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. 

[RWL3-3: The other "8" metrics are not clear to me. Certai nly there is a safety ca tegory, but here is a list of 13 "metrics" from the above Berkeley report : 

Output metrics 

Row 1 Fuel use(4) Emissions(7) Time (2) 

Row 2 Thermal efficiency ; Species: CO2, CO, CH4, NMHC, PM, BC, OC; Time per test phase 

Ro w 3: Specific energy consumption ; Emissions per MJ delivered ; Time per task 

Row 4: Specific energy consumption rate; Emissions per kg and MJ fuel 

Row 5 Fuel use per capita; Emissions per minute 

Row 6: Emissions per task 

Row 7 Modified combustion efficiency 

Row 8 Combustion efficiency 

I note that there is nothing in this list about the efficiency of making char (which is why I am writing this, but that is a different story. As long as E!, E2, and E3 are reported, per Jim Jetter's note of a week ago. But this list can be for various tasks and I am suggesting that one that is fair to char-making stoves is the one above - boil away until the maximum amount of char has been produced . This is to ask if others think this might be a reasonable task to compare char-making stoves? If not, why not? 

The metrics that would be added to the above list or do this as one of the tasks (none removed) could be: 

Measured: weight of water(kg) evaporated, fuel, and char 

times to complete and operator time 

Calculated: E2 and E3 (percent energy in the boil-away and in the char) 

ratios of water weight evaporated to input fuel and char weights (and/or inverses - dimensionless figures of merit) 

Above for minimum time (max power) and maximum efficiency (minimum fuel use); these might require three- four tests ranging from barely bubbling to rolling boil. T his may be excessive - maybe doing just twice near the extremes of the turn-down ratio is enough. Mostly this test can be done unattended - for maybe an hour. End RWL 3- 3] 

CPP: 

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. 

[RWL3-4: I don't see that my above suggest ion falls into this "invalid" category. Everything is defined and repeatable in di fferent c ountr ies, altitudes, etc. The assumption is a normal full load of fuel and a normal (uncapped) pot of water - something many people do every day. How l ittle fuel and emis s ions and how much char for this ta sk ? ] 

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. 

[RWL3-5: I concur that simmering is d i ff icult. I am proposing something much easier to do (for char-making stoves only, probably , assuming th ey have primary air control). More specifically, your sentenc e above wh ich reads 

"... mass of water in the pot is not related to either the fuel consumed nor the emissions" 

can be changed to read: 

" ... mass of water evaporated in the pot is directly related to both the fuel consumed and the emissions "] 

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. 

[RW3-6: I t is too late for me and th is already too long. Maybe tomorrow. But as long as char is included in the E2, E3 manner (char weight and energy related to inputs), proba bly other def init ions are OK as well.] 

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. 

[RWL3-7: I concur this array of tests is horrible. As near as I can tell, the only test being considered by the GACC is the WBT4.21 - and it i s NOT (or has not been) des igned at all w ith char product ion in mind. I th ink the above is a fair way to com pare char-making stoves. Other stove types can or need not do the same. 

Anyone agree? End. Ron] 

Regards 

Crispin 

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