[Stoves] stoves and credits again

[Xavier Brandao]

Dear Philip,

 

Thanks for sharing this experiment, this is edifying.

 

It shows how the Dice Roll Test (DRT) is a very serious contender for the WBT. Both in term of practicality and statistical reliability.

 

Best,


Xavier

 

 

De : Stoves [mailto:stoves-bounces at lists.bioenergylists.org] De la part de Philip Lloyd
Envoyé : lundi 25 septembre 2017 11:30
À : 'Discussion of biomass cooking stoves'
Objet : Re: [Stoves] stoves and credits again

 

I carried out a thought experiment.

 

In case 1, a cookstove burning wood boiled 5 litres of water at an energy efficiency of 30%.  The useful energy provided was 5*4.186*(100-15) = 1779kJ. This required 5*4.186*(100-15)/0.3 = 5930kJ.  If the as-fired wood had a LHV of 15MJ/kg, it would have needed 5930/15000 = 0.395kg wood

 

In case 2, the same stove was operated to produce charcoal while also boiling 5litres of water.  More wood would be needed, because not all the wood would be combusted – some would be left as char. If you fed 0.395kg wood to be turned into char at 29% efficiency, and the char had an LHV of 28MJ/kg, then the char would have an energy of 0.395*28000*0.29=3163kJ. The wood from which it was prepared had an energy content of 5930kJ, which was what was needed to boil the water in the absence of char production. So 5930-3163 = 2768kJ of additional energy* would be needed to boil the water if there was char production. At 15MJ/kg, this is 2768/15000 = 0.185kg extra wood, or an increase of 47% in the wood supply. The total energy supplied would then be 5930+2768 = 8698kJ.  The energy efficiency of cooking would therefore fall to 1779/8698*100 = 20.5%, while the efficiency of char production would have been 3163/8698*100 = 36.4%.  The system efficiency would have been 20.5+36.4 = 56.9%

 

If you used the WBT formula, the efficiency of boiling with char production would have been 1779/(8968-3163)*100 = 32.1%.  Given the measurement errors inherent in the WBT method, this would have been statistically indistinguishable from the efficiency with no char production – i.e. it would have shown no benefit to char production. 

 

Prof Philip Lloyd

Energy Institute, CPUT

PO Box 1906

Bellville 7535

Tel 021 959 4323

Cell 083 441 5247

PA Nadia 021 959 4330

 

*This assumes that there is no endotherm in the pyrolysis of wood in the presence of air, and that all the pyrolysis products except the char burn to provide heat. There is evidence in the literature of no endotherm in the presence of air.



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