[Stoves] A review of chronological development in cookstove assessment methods: Challenges and way forward

Thu Nov 26 18:24:45 CST 2015

Dear Ron

I will remind you that I clearly labelled it the ‘Cooking efficiency”. There are many efficiencies one can calculate about a stove.  The historical problem is that the thermal efficiency has been (by the WBT) used to report the fuel efficiency, which is another matter entirely. I refer you to the WBT protocol document where the energy that could theoretically by released by the mass of fuel missing is re-calculated into an equivalent ‘dry fuel equivalent’ and reported to be ‘the fuel consumption’.

            Apparently you believe that the inefficiency of this stove  (which I would call a wonderful stove) is 1- .334 = .666 (66.6%).   I disagree- that is not the inefficiency.

You disagree, but you want to calculate something other than the cooking efficiency. I am interested in the cooking efficiency. That is why I called it the ‘cooking efficiency’. You could report the char making efficiency, the efficiency at making CO, or the efficiency at reducing the fuel to ash. It depends on what the customer asks. The photo of the results shows the recoverable mass of char produced in the bottom cell. It is reported in case someone wants to know the char production efficiency. The pilot project does not. They are interested in the energy efficiency: heat delivered to the pot divided by the energy in the fuel needed per replication of the cooking task. 

If someone sells a stove as a cooking device, then it will be rated as a cooking device, which means its use of fuel related to cooking. In your example the energy efficiency as a cooker is 33%.

Efficiency is a ratio: energy delivered to energy available. There are various inefficiencies. This is a well-known science and details can be found in any textbook on the analysis of, for example, a boiler or a power station. There is a very comprehensive chart of heat flow provided in Ch 3 of the CSI China document reported on this list and described by Prof Lloyd as ‘excellent’.

The stove was not “trying to produce” char. It is a cooking stove that has been optimised to produce minimum emissions of CO and PM, which it does really well. You may note that the PM and CO performance is very good. It was claimed loudly at the Accra ISO meeting that such a performance was ‘impossible’. Yet here it is. And this is not the cleanest stove tested for this project.

>…But to get your 12.81 MJ/kg (four significant figures !!) in the fuel consumed, you must have the char energy content to the same accuracy.

>How can I find how you got the numbers of 12.81 MJ/kg.  

I am not sure you understand what the energy in the fuel consumed is. It is the calorific heating value of the fuel, compensated for moisture, multiplied by the mass used to conduct the cooking event.  A stove may be using recycled fuel from a previous test (carried forward) and new raw fuel. The net calorific value of the fuel required for a test is often a combination of two fuels – the raw fuel and some leftovers from a previous test. The energy content can be obtained at will using a bomb calorimeter. 

>Do you really believe you know that to 4 significant figures? 

Yes.

>What was the fuel moisture content and how recently measured?

The fuel moisture is indicated in the picture. 7.98%. It is determined by a standard lab protocol. The actual precision of that number is greater and has been rounded to 7.98%.

>          Why is 2.1 kW a “Fail”?  Sounds pretty good to me.   

That cell/test is an SNI (Indonesian National Standard) requirement to provide a minimum firepower. This stove does not meet that requirement. The ‘Fail’ is automatically generated by testing the firepower against the Standard Requirement. Firepower is not a very helpful metric for rating a stove. The cooking power of ten stoves all having the same firepower will be different. The only thing of interest to the cook is the cooking power. This stove has a very high thermal efficiency (49.5% if calculated using the WBT formula). Another stove with the same 2.1 kW might only have 500 watts of cooking power. That is useful information; the 2.1 kW is not.

>Is this due to the same reasoning that causes a CSI rating of only two stars?  Would the EPA methodology give this two stars?

Which EPA method?   The EPA doesn’t have a stove test method.  Jim’s lab uses a customised system for determining performance and usually report performance using the GACC’s WBT metrics. As you will already be aware from earlier discussions here, three of the metrics have no physical validity (low power), others are inappropriate and or misleading. This is going to be corrected by the ISO experts.  

The firepower has nothing to do with the number of Stars. There is no requirement to have a minimum firepower. The report below shows the results from two different protocols simultaneously (differentiated by colour).  There is a suggested minimum heat flux into the pot (2.7 W/cm2) but it serves only as a guide for manufacturers. Stars are awarded for CO, PM and energy efficiency. To get 3 stars for efficiency it has to exceed 40%. The WBT method over-reports the energy efficiency by a factor of 1.5-fold. That is why the ‘la test results’ are not matched by field performance. It is no mystery – the mismatch is rooted in the improper calculation of fuel consumption.

>          Whew!  I still have to “commend” you for trying so hard to convince the world that biochar and char-making stoves make no sense.  

I think you have more understanding of what I am trying to do than your sly comment reveals. Let me remind you: You are advocating that the energy released by a fire should be interpreted as the fuel mass required to perform a cooking task. That is cheating and I am working hard to stop it this years-long nonsense because the Stove Community (sic) has refused to do so.  The WBT does not report the fuel consumption of a stove. This test method does. Taking your comments at face value indicates you have some homework ahead to catch up with developments in this field.

A further reminder for others who may not have followed this years-long argument against the continuing misrepresentation stove performance: I have no problem at all with char making stoves, but they will not be ‘rewarded’ with a performance rating that misrepresents the amount of fuel they consume.  I can see your main mission remains unchanged: to have the energy released from the fuel, or some number close to it, to be interpreted by various means to be the fuel taken from the available supply. 

Because certain favoured stoves have benefitted from the calculation errors in the WBT, numerous stove programmes have failed because of the misrepresentation of the fuel mass needed to complete a task. This is a serious matter. I encourage you to take it more seriously. 

I have provided you numerous suggestions about how to correctly report performance. This has included a method for directing research so as to be able to produce char and still meet the performance requirements of fuel saving programmes. Fortunately one of the stove manufacturers has listened closely and the performance report below is the result. An acceptable cooking performance with superb emissions ratings and, in addition, a significant quantity of char.

I hope you will not the very high % char produced by this stove. The fuel mass * (1-moisture) = 993.8 g of dry fuel. The char mass is just over 30% of that number.

Regards

Crispin

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