[Stoves] The need to continue the discussion: simmer efficiency

[Jiddu Broersma]

Dear All,

First,
Paul, thank you for keeping the conversation going with great intentions.
Much appreciated.

I have been a quiet follower of the stovelist and I just caught up on the
simmering discussion. I believe it is a necessary discussion.

I would like to share my opinion. It is one from the perspective of a stove
manufacturer and a theoretical physicist.
I do this purely to contribute to the discussion. Although I will formulate
my opinion direct with strong language, I want to assure you all that I
value everyone's opinion equally.
Now, I will jump straight into the low power metrics.

*Simmering*
As has been explained, simmering is a 'zero work' task, it does not have an
efficiency. Evaporating water and overcoming heat losses require work, they
can have an efficiency. However, is anyone interested in knowing the
efficiency of evaporating water? Or is anyone calculating how much energy
is lost from the pot?

Either way, simmering is only a term that we will hopefully agree on. Most
important is to understand the meaning of the metrics that are calculated:
Low power specific fuel consumption, Low power CO and Low power PM.

*Low Power Specific Fuel Consumption*
>From the WBT sheet I find that (equation)
Low power specific fuel consumption = (weight fuel consumed * calorific
value of fuel ) / (weight of water remaining * minutes of simmering * 1000)
To explain in steps:

   1. Weight of fuel consumed divided by minutes simmering is the burn rate.
   2. If we multiply this by calorific value we get the energy released by
   the combustion per minute. Let's call this the 'energy rate'.
   3. If we divide this weight of water remaining we just divide it by a
   random number that has no meaning. Keeping more food/water hot does not
   require more energy. (I believe it does the contrary, because volume grows
   faster than surface when you increase quantities)

-> We have the energy released by the combustion divided by a random number.

If we wish to calculate some kind of efficiency number we require useful
energy (into pot) divided by used energy (from combustion). What we have
calculated is nothing close to this.

We could possibly calculate the energy into evaporation and heat loss from
the pot, but we have no interest in this.
Hence, a simmering task can't give us a useful efficiency type of number!

That was me as a physicist, as a employee of a manufacturer I am concerned
that stoves are unfairly compared. Because the system can be manipulated to
gain better results by using a pot with larger volume capacity and higher
insulating properties.
Also another big issue is that more efficient stoves can have lower rating
at the same power output. I'll explain by example (as others have done
before me):
*Two stoves are equal except that one has better heat transfer efficiency.
When they both run at the lowest power possible (which is the same power
for both), the one with the better heat transfer efficiency will have
likely evaporated more water because more energy went into the pot. Do to
more evaporation it will end up with a rating that is worse!*

*Low power Emissions*
Both PM and CO are given in
weight  / (minute * liters of water)

Similar breakdown:
Weight of emissions per minute is straightforward.
Dividing this by number of liters in the pot is simply dividing the
emissions by a number of your choice (the liters you fill in the pot). It
has no value to stove rating.

Generally I think that weight of emissions per minute is not a bad metric.
However, simmering is not a task that can be compared fairly between two
stoves because it is not a specific task! If we can't compare the way two
stoves simmer because we know nothing about the useful energy that went
into the pot we are not allowed to compare the results because it means
nothing without enough information.

*Wrap-up*
The list of complications that result from these invalid simmering metrics
goes on. Please read again Crispin's and Philip Lloyd's comments for a more
comprehensive list.

Worst is that many manufacturers are optimizing their products using the
WBT and that stoves are not actually improving in the field. The WBT can
result into the production of bad stoves for millions of already suffering
women!
I find myself in the position where I have to tell my company to produce
stoves with lower rating because it will be better for women that we build
them for.

It is not a matter of right or wrong, it is a matter of how we stimulate
the production of stoves that are really improving lives.
My opinion is that we need to review the meaning of metrics at fundamental
level. (Ie. Boiling and simmering are not scientific ways of describing a
state of cooking for stoves).

Due to the differences in opinion in the stove world, I believe an external
review (scientific: engineers, physicists, mathematicians) would be the
most suitable option. The review could simply explain the physical meaning
of all calculated results.

Best regards,
Jiddu

*Jiddu Broersma*
*Technology and Organization Officer*


www.praktidesign.com

Spirit Sense, Old Auroville Road
Bommiyarpalayam
605104 Tamil Nadu
INDIA
stoves at lists.bioenergylists.org
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.bioenergylists.org/pipermail/stoves_lists.bioenergylists.org/attachments/20150219/28bd1e36/attachment.html>
-------------- next part --------------
A non-text attachment was scrubbed...
Name: desconocido.png
Type: image/png
Size: 4837 bytes
Desc: not available
URL: <http://lists.bioenergylists.org/pipermail/stoves_lists.bioenergylists.org/attachments/20150219/28bd1e36/attachment.png>