[Stoves] Confirm stove efficiencies in Vietnam

Crispin Pemberton-Pigott crispinpigott at gmail.com
Mon Jul 2 12:33:48 CDT 2012


Dear Marc and Tuong

 

>http://ehs.sph.berkeley.edu/krsmith/?p=249 

>It would be good to cross reference the efficiencies you found. For
example, for an LPG stove, the report eff is only just over 50%.

Not all LPG stoves are the same of course. There is a test of an LPG stove
done using the Indian thermal efficiency test that reports a figure of about
62% and when using the same hardware, a reasonable similar gas composition,
but at a different altitude and quite different pot, we got a figure within
2%.

Note there is calculation error in the Indian efficiency test which is that
it does not credit as absorbed heat used to boil out water. As the range of
temperature is from 20 to 90 degrees, there is certainly some evaporation
before reaching 90 and that affects the final answer. I suggest you run such
tests in Vietnam at 30-70 degrees and then you can safely assume you will
start below 30, and not evaporate anything meaningful by 70.

In a series of tests we found that evaporation from 30-70 was within the
error of the scale readings (±0.5g) so could not be said to be detected
‘with confidence’.

The Indian efficiency test does correctly apply the heat capacity (Cp) of
the pot taking into consideration its mass and material. This corrects a
significant error in most WBT’s, significant meaning it makes a
statistically significant difference to the final result. If you want to
correctly state the thermal efficiency <±5% the pot material needs to be
considered. Ti is not difficult to add.

The approach taken in the Indian test calculation is to calculate the water
mass equivalent of the pot the add it to the mass of water heated. This is
simple and reliable and easily understood later in the calculations.

>Also, I'm not sure reporting a single efficiency number is all that useful.


Reporting the efficiency doing ‘one thing’ using one pot at one power level,
or two power levels summed so you can’t see the difference, is not very
useful, I agree. People want to know the performance profile of a stove, not
just ‘one number’ which contains little information. If that number is not
even accurate, there is almost no information contained in it. 

>The stove types you detail have much variation depending on construction
quality, operation, fuel type, pot size, etc.

The performance of a stove varies with pot size operation method, fuel and
to a small extend, construction quality. If you interpret ‘performance’ as
‘durability’ then that matters too and you have to report it. A stove might
fall to pieces quickly if it is operated at high power continuously, for
example.

Regards

Crispin 

 

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