PHU - Percent Heat Utilized
From a message: Testing in Vereeniging Fri Mar 21, 2003
Crispin Pemberton-Pigott, New Dawn Engineering, 2003
Example test conditions:
The ambient temperatire was 25 C.
The mass of the pot was 7 kg approx
The mass of water was 2000 gm exactly
The fuel burned (at maximum possible rate) including lighting up and stove
heating losses etc was 253 gm of wood which everyone agreed was not very
close to air dry but I used 16MJ/Kg anyway. The avg was 26.6 gm/minute of
fuel. The fuel might have been 20% WMC.
"Standard" PHU calculation:
Fuel used 0.253 Kg x 16MJ/kg = 4,048,000 Joules
2kg water x 95-25 deg temp rise x 4.2 J/Cal =
2000 * 70 * 4.2 = 588,000 Joules
PHU "ignoring the pot" = 588,000/4,048,000 = 14.53%
The cast iron pot weighed 3.5 times as much as the water and was obviously
heated in the process. Cast iron is not a very good conductor of heat so I
estimate the external skin temperature to be 130 and the internal skin
temperature to be 100. The part of the pot above the waterline is much
hotter but I will leave that out. The effect when the pot is fuller varies.
Average pot temperature (final) = 115 C (at least)
Delta T = 115-25 = 90 C
Mass of pot = 7 kg
Heat in cast iron at 300 deg Kelvin = 420 J/Kg/Deg K
Heat in the pot collected from the fire and not passed on to the water =
7*420*90= 264,600 Joules
PHU "including the pot" = (588,000+264,600)/4,048,000 = 21.06%
That is an apparent increase of 45% in PHU value! It is unbelievable that
any PHU figures from rapid boiling tests have meaning unless the pot has
been included in the calculation. Suppose you are cooperating loosely with
someone on the other side of the world and you don't have simlar pots?
Suppose the other person uses a 500gm aluminum pot and does the same test
using a different stove and also takes 9.5 minutes:
PHU "including the aluminum pot" = (588,000+(905*0.5 kg*approx 75
deg))/4,048,000 = 15.36%
Arguably the test in the first stove would have been concluded much faster
using the aluminum pot (just under 7 minutes, using some 184 gm of wood = a
PHU of (588,000+(905*0.5 kg*75 deg))/2,953,000 = 21.06%, the same as before.
Disregarding this theoretical aluminum pot would give a PHU of
588,000/2,953,000 = 19.91%.
POINT 4
Calculating for any water boiled out during the power test would have added
to the PHU value as well. Having the lid off only confounds things. I
didn't weight the water that boiled and drifted off the surface. I was
trying to make a point about the PHU and a pot weighing more than 3 times
the water.
I can't care what people are 'all doing' if it gives meaningless or highly
misleading PHU results, and those results are later cited off-handedly to
'rate' approaches and compare stove configurations.
THE SIMPLEST MEANINGFUL TEST
- Fuel burned, water content known and correct heat value used
- Mass of water boiled in a closed pot so evaporation is negligible and loss
does not need to be measured
- Water temperature rise during the test
- Mass of pot heated using some 'standard' temperature value given for heat
conduction through the wall, 420 for cast iron, 903 for sheet aluminum, 450
for steel (Joules/Kg/Deg C).
This comes close to a meaningful application like making tea or preparing
porridge in a real pot on a real stove.
With regards to all
Crispin