[Stoves] Good enough stove?

Sun Nov 22 11:32:01 CST 2015

Dear Frank

Given the paper just referenced on this list from Dr Suresh Jain you make a crucial statement that is not addressed by Dr Jain:

>First we lay down the ground rules. Our clients are the poorest of the poor.  Poor quality and hard to get fuel. They cook with a minimum of tasks required. 

>If this is a reasonable client description? I go forward with we need to 1) complete a task 2) using the minimum quantity of fuel as our goal. So yes in this case thermal efficiency is relative to the amount of fuel used that was provided to the lab. 

(italics added)

The very issue I am repeatedly pointing out is that the ‘thermal efficiency’ is not representative of the fuel consumption. It is representative of the energy consumption.

The observation that recovered char, which is not all the char, just the recoverable portion according to the definition of the person doing the recovering, could be used in another stove in the same home. I have prepared an analysis of such a case and will circulate it shortly. It is quite possible that a family could have two stoves in which one produces char and another burns at least someone it. The relevant question is how to account for the performance of each stove, and the pair as a cooking system.

If you read the paper here <http://ac.els-cdn.com/S1364032115012216/1-s2.0-S1364032115012216-main.pdf?_tid=ddc9505c-9138-11e5-a09e-00000aab0f26&acdnat=1448211093_2bec190e59e80c0ff7fc6ecc9b8d7f99>  by Dr Jain, it will be clear that his analysis has only included the energy that could be released from the fuel completely consumed. Note that it does not mean the total energy released, because gases may not have been completely burned (the combustion efficiency is probably less than 100%). So if fuel is missing, then it is assumed to have been burned completely, according to the formula in the WBT. That is one ‘thermal efficiency’. If you were designing a stove you might what to know the heat transfer efficiency (considering the heat actually released) or the thermal efficiency (taking unburned CO and H2 to be a loss) or the energy efficiency, being the energy removed from the forest, all compared with the energy delivered into the pot.

Dr Jain does not address this matter directly, rather discussing in several sections, the mismatch between the lab tests (which report the dry fuel mass equivalent of the energy that could theoretically have been released) and the KPT which is an observation test of family behaviour relating to fuel consumption from the available resource.  The Berkeley CCT is also discussed but not the differences in the formulas used to calculate the thermal efficiency (the WBT and CCT gives different answers for the same input data because the formulae are not the same).

>And where fuel is in very short supply (as in this example) thermal efficiency value is very important when picking the stove that uses less of the type of fuel received to do the same task.

Well, the combustion efficiency is important, as is the completeness of combustion of the fuel. If a stove doesn’t burn the fuel if consumes, the ash will be full of unburned bits and pieces. Which measure constitutes ‘fuel consumption’? The fuel consumed, or the energy released from the fuel consumed?

This is a very practical question.

>Remember: This is a test method that is run in labs all over the World and results used in comparison. 

Which test is that? There is no test that is used all over the world. India and China, in full knowledge of the existence and content of the WBT, have not used it, preferring to develop their own protocols that quite frankly, avoid many of the obvious mathematical and conceptual errors in the WBT. It is interesting that Dr Jain’s paper does not cite to any of the published critical analysis of popular test methods in all these countries. 

>FIELD Fuel consumption cannot be determined in the lab. If the stove takes in sticks 3c m diameter and 10 cm long the rest of the biomass from the tree is wasted. And we need to get Box 1 through Box 6 taken care of and under control if we are ever going to have labs around the World testing stoves.  That so they can be tested at a reasonable cost and we can get lots of stoves in the list to compare. (our goal)

Field fuel consumption observations have many valuable facts to contribute such as co-firing, fire maintenance when there is no cooking going on, and many other things, and these can be used to inform lab tests so that what is done in the lab can reflect field conditions as closely as possible. This is done with everything from hair dryers to photocopiers – so why not stoves? 

>That is why there needs to be a collection and inventory of possible biofuels from an area. Then a request for stoves using the available fuels (already in a list).   

This is a very practical step. If you wanted to know how a stove will perform in a certain area with certain fuels, it has to be tested with relevant fuels. Reason? The fuel, size, preparation and moisture has a strong influence on performance. There is no ‘universal test’ that will tell you anything useful about how a stove will perform in a given situation. The strange thing is that people are testing in the lab in a manner completely different from use in the target area, then picking ‘winners’ and then going into the field to check the ‘real emissions’ and performance. What’s up with that?

The confusing thing is when the fuel saving is overstated because people report some proxy for the heat transfer efficiency instead of the energy consumption (represented by kg consumed). 

>Perhaps I am missing something here? 

>Stove testing in labs we can only do heat transfer efficiency HTE. 

That is not correct. By simply observing the quantity of raw fuel needed to complete each test you have the fuel consumption. That has an energy equivalence. That is the energy consumption. There are electrical meters that check the electricity consumption (kWh) of electrical stoves. This is not a new concept. Total energy in, energy delivered to the pot. 

>…It seems we no longer want labs around the World to test stoves and open the selection to many more stove designers? 

I certainly an open to all sorts of new stoves and test many of them including prototypes. The labs frequently give advice on how to improve the stoves – free – because we want successes, and many more of them. 

Suppose you have a car with a leaking fuel tank. Everywhere you drive, fuel leaks onto the road. Is the vehicle’s fuel consumption the amount that you put into the tank each week, or the amount that gets burned by the engine, discounting the leaks because it wasn’t burned?

In theory another guy could drive behind with a long pole and a bucket, catching the leaked fuel and use in in their car. Quite possible. But it would not reduce the amount of fuel you have to put into your car. Agreed?

Regards

Crispin

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