Re: low power efficiency

[Tami Bond <tami.bond@noaa.gov>]

Dear Emma,

As nobody has yet taken this up (on-list at least) I will do so,
although I am not expert. You have hit on a good point and no, you are
not missing anything obvious-- other than the fact that sometimes
standardization misses the obvious!

Now I am going to go about repeating your mail, and/or stating the
obvious. Bear with me...

Balancing heat loss is an old and honorable tradition; after all, that's
all the furnace in your home is doing (assuming you have one), if you
keep the home at the same temperature always. Your
temperature-controlled oven is doing that too. This is what we want to
do with the pot after it has got up to the right temperature. Your
question, if I understand it right, is how one estimates the value of
this service and gives credit to the stove for performing it. 

Heat transferred to the pot goes to balance losses and to evaporate
water. In the water-boiling test, only heat that is used to boil off
water is counted as transferred to the pot. BUT, the evaporation of
water is incidental to the task of interest-- nobody really sets out to
evaporate water; it just happens because the water gets hot enough that
it has a high vapor pressure. So really, the WBT is measuring the wrong
part of the heat-in: the part that is irrelevant to what people want! To
make the point ridiculous, your user would not be very happy about a
system that gave 100% efficiency but just boiled off the water
instantly, leaving the food uncooked. 

It's temperature over time that matters, right? I think this raises
questions about how one would evaluate the efficiency of other
continuous low-power tasks. Bread-baking, for example.

Hmmmm.

I could think of a bonehead way to calculate a rough estimate of heat
loss from the system: quench the fire when the pot is hot, and monitor
the water temp. You should see an exponential decay, and the decay rate
would be proportional to the heat loss. But this will be in error for
several reasons: for example, the air flowing over the pot will not have
the same velocity field when there's no fire and when the pot is cooler.
Likewise the internal convection of the water would decrease. So the
heat transfer coeff might be completely unrepresentative.

> I think I'm going to give up, and just find the savings compared to a 3-stone
> fire instead. 

I guess this is the best option. Positive side: The true measure of
efficiency (IMHO) is not MJ delivered but service provided. What matters
to users (and the environment) is wood used per meal cooked and
emissions per meal cooked. (Also perhaps peak emissions but we'll leave
that aside for now.) Negative side: The test would not be comparable
with other efficiency tests that are widely reported. 

Tami

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