Re: low power efficiency

["Dean Still" <dstill@epud.net>]

Dear Emma and Tami,

As I figure it, the low power efficiency part of a stove test can show
differences between stoves. Can the stove/pot combo supply the right amount
of heat to keep the water gently simmering without loosing heat into the
stove body or into the air around the pot? A great stove would get a higher
score because it is most efficiently changing water to steam. Lots of stoves
loose on both scores.

Using this test we top out at around 40% if two pots are efficiently
submerged into the heat stream. Around 34% if one pot is in there. But if
the heat flow path is uninsulated and the heat only hits the bottom of the
pot we're usually around 8% to 15%.

A 3 stone fire can be made very well and can score above 20% (Tami may hold
the world's record for BEST THREE STONE FIRE) because no stove body absorbs
heat while heat transfer to the pot is equal to most stoves. Making stoves
that score better than a laboratory test of the 3 stone fire means the stove
designer can't use earth or heavy ceramic or a griddle, etc. that lowers
scores, especially if stoves are tested from a cold start.

In real life, people don't make such great fires and average efficiency is
much lower. Complicated subject especially for folks interested in funding
stove projects.

As you point out, changing water into steam ain't what we start stoves to do
but it seems a related method to get at differences. Food cooks as it
simmers with the water at boiling temperatures. How much does a lid help
cooking? When steam condenses on the lid the heat is transferred in some
part to the lid. This hot lid is on top of the food however and seems out of
the way, not positioned to help cooking very much. A lot of older lids let
the steam out anyway so the difference between lid/no lid might not be all
that big unless its tight?

A high mass stove has traditionally been considered to improve in efficiency
the longer its running. Folks have measured that less heat is lost into the
stove body once it's hot. It's just darn hard to do stove tests that take
that long. Lorena stoves don't establish a stable loss pattern until 5 or 6
hours in my last test. And it takes a full blown continual fire to get there
without rests in between meals...

I'd like to try this test: give the household a weighed amount of wood in a
open, nice looking container. Come back every 24 hours to weigh the
remaining and replenish the supply. Do that for a couple of weeks. Do the
same thing in 10 houses using the improved cook stove and 10 houses using
the open fire.

Do you like this idea, Emma? I'm thinking about doing this in our projects.
I'd really prefer something more anthropologically correct, no interference,
like counting wood sold to houses with and without stoves, etc. but it seems
too hard, possibly confounded by using free supplies.

Best,

Dean



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