[Stoves] Differences in stove testing ---- was Re: ETHOS 2017 agenda and logistics

[Crispin Pemberton-Pigott]

Dear Andrew

Thanks for the quick response. 

>> Paul's objection to this is calling the char 'waste' as far as the char making stove is concerned. ‎Yet from a fuel consumption point of view it is.

>I disagree because the TLUD is co generating wood gas for cooking and ‎a char residue, the cooking effort is from the (already hot) gases.

I do not see how that is directly relevant. What the rejected fuel, the leftovers, the solid residue, the char, the remnant, is called is immaterial. They are objecting to the name, which is not helpful to addressing how to correctly report the fuel consumption. This type of analysis is absolutely routine in the boiler industry and has a well established lexicon. 

>> Prof Lloyd has just explained, clearly, twice, that to rate these additional 'benefits' all of which involve energy that originates from the fuel fed, is done by putting them in the numerator, over the total energy in the fuel fed, the 10 kg.

>But he also said that efficiencies from co generation of products are additive.

That is correct. I felt it unnecessary to repeat it. Suppose you cook in two pots simultaneously. Is the energy getting into pot 2 subtracted from the total fuel energy to find the efficiency of cooking in pot 1? Of course not. The total energy in each application of heat is summed and divided by the total energy in the fuel fed. 

<>> It would be helpful is the char making stoves were evaluated as part of a larger system...

>Agreed they are not simple cook stoves unless they continue through the char burning phase...

The analysis is chained then summed. I don't think of it as a difficult problem. 


>> Here are some ideas:

> Energy needed to convert the food into cooked food is an energy gain

>This is a dubious proposition

Before VITA joined the party hosted in those days by Eindhoven, it was standard procedure to cook food and calculate the energy literally absorbed in the transformation of veggies. They produced a chart with the energy needed for different food types. The reason was to account for energy that was not detected in water temperature and missing water. 

>> Cooling the char is an energy loss

>Yes but is that attributed to the cooking or the char making

I am not clear on what you mean. I don't think it is attributed to either. It is a loss and not credited to anything. It is part of the energy in the fuel fed and not delivered into any benefit. 

>»^<> If you want to know how much of the heat released gets transferred to the pot, you are asking for the heat transfer efficiency. Perfectly reasonable request. You have to determine all the energy in the residual solids, not just 'a major portion' of the char or the answer won't be accurate.

>On that basis I am probably interested in the heat transfer efficiency then as I am talking about the energy released from the fuel rather than the raw energy of the fuel. Smoke and char in the ash are the unreleased energy losses in my usage.

Designers are always interested in the heat transfer efficiency. The problem comes when you imagine that is the same as the fuel efficiency. For gas and liquid fuels the numbers are the same. For solid fuels they are usually not, with the exception of fluidised bed burners like power stations. For small solid fueled stoves, there can be a large difference between the fuel efficiency and the heat transfer efficiency, which is always higher. Using the higher value to calculate relative fuel performance, as is done with CDM projects, over-predicts fuel and emission reductions, which is what is being traded. 

Regards 
Crispin