[Stoves] A call to stop using the WBT

[Crispin Pemberton-Pigott]

Dear Andrew


>> Part of the problem with the WBT is its abuse of efficiency. It is fundamentally flawed to subtract the energy contained in any char from the energy in the fuel fed, in calculating the efficiency.


>[Doesn't] that just takes us back to the beginning, there may well be other problems with the WBT but this treatment of the energy in the char remaining from a TLUD burn is not one of them.

>Even Crispin has acknowledged that one of the protocols allows for char from one burn to be carried over to another test, this therefore acknowledges the energy remaining after the cooking task is finished has some value, allowing for the energy in char from a TLUD burn is just an extension of the same idea.

The energy value of the char has NEVER been in doubt. What has been in doubt is the assumption that ALL char produced will be carried forward as fuel to be burned in the same stove on the next replication of the test sequence. 

In other words, the WBT treats all char as if it is unburned fraw fuel energy that will be consumer by the same stove on the next burn. 

The test method you refer to that permits remnant fuel to be carried forward is the HTP as implemented in the CSI protocol. It applies not to char only but any remnant fuel. Where such fuel is left over, the first valid test of the stove is the second in a sequence because the first test doesn't have 'carried forward fuel'. 

The statement to the stove producer is basically: it you say the stove can burn the remnant fuel on the next replication of the test, show us, do it. Then we rate the stove based on that. 

Some char making TLUD's can indeed burn remnant char. We haven't discussed this on this list because of the obsession by some with cheating on the efficiency rating. There are stoves that can gasify the char, a good portion of it, if there is new fuel mixed into it. 

The CSI test has the calculation needed if the amount of residual fuel or char is less than 'all of it'. Some TLUD gasifiers can only handle, for example, 50% of the remnant char. From that second burn more char is collected to be used in a subsequent burn. 

When the stove burning the remnant fuel is NOT the same stove, either the first stove is rated alone or the second one is rated as a pair with the first as a 'cooking system'. 

The rules for doing that I have discussed previously on this list. 

Remnant char or fuel
>Okay but while that char has energy which the user is aware of they are similarly aware that the same energy cannot have been "fed" to the cooking pot.

The observant cook may be aware of it, but the programme manager is not if they are given a WBT result. That is why Xavier is calling for any WBT test result not to be used for making financial or product selection decisions. 

>> If you insist of subtracting the char from the fuel fed to calculate an efficiency, as in the WBT, you get a meaningless number.

>It is not meaningless, it tells the user what proportion of the
energy was "fed" to the stove and what energy is retained in the char.

If it told us what portion of the energy was NOT retained in the char we might, by subtraction, have the total energy released from the fuel by the fire, at least potentially. That could be used to calculate the heat transfer efficiency. The heat transfer efficient is a useful sub-system metric, but it does not tell us what the fuel efficiency is because nearly no wood stoves burns all the fuel completely. That is why solid fuel burners have different calculations from liquid and gas stoves. 

But it doesn't tell us that total. The recovered char is a opinion-driven fraction of the total, and the energy content is unknown. That cannot be resolved for a reasonable cost. Jim Jetter tried using a bomb calorimeter, but reported basically a better heat transfer efficiency, not the duel efficiency which was the question to be answered. Philip is correct: the WBT answer is meaningless. It is merely a poor approximation of the heat transfer efficiency which VITA thought, back in the day, meant the same thing as the fuel efficiency, which is why they termed the result of the calculation the 'fuel consumption'. 

>>It does not tell you how efficiently the cookstove cooks, and it also does not tell the char user whether the cookstove does a good job of turning some of the fuel into char.

>Of course it does but I acknowledge there may be some difficulty in measurement.

'Difficult in measurement' cannot be used in a national standard.

Further, measurements in test methods are made with a purpose. The request for char making stove producers is to rate the char production in some manner. They did ask for the stoves to be mis-rated and mis-characterised so as to fool programme managers into selecting say, Rocket Stoves over Rocketworks stoves, because the Rocket Stoves, though equal, were given 'a hand up' with a WBT rating. 

Frankly, this matter is treated as if it is academic and of little consequence. The single example of the disastrous USAID project in Uganda is sufficient evidence of the waste and disappointment cause directly by the 'incompetence' of the WBT and its proponents. 

It is not only a problem of concern for stoves deliberately making char. A very large number of stoves make at least come char and their specific fuel consumption must be reported correctly. This applies to heating as well as cooking. 

I must credit Harold Annegarn with drafting the fuel consumption definition used in the CSI protocol which was later adopted by the ISO working group. It captures everything correctly. The China-South Africa scientific cooperation is responsible for the underlying mathematics. 

Regards 
Crispin in balmy Beijing (-10 C)