[Stoves] Examples of results of simmer efficiency Re: [Ethos] Additional presentations at ETHOS 2015

[Crispin Pemberton-Pigott]

Dear Paul

 

Thanks for that, and I will add a correction to the part you highlight:

 

>So the difference in the TEMPERATURE OF THE WATER is of little consequence,
being about 3 deg C.   

 

The energy required to evaporate water at a temperature other than 100
degrees (at sea level pressure) is higher. The difference is 2.44 Joules per
degree under the boiling point.

 

Thus the idea that it takes a different amount of energy to lose water at a
different temperature is not correct.

 

To calculate the energy gained by a pot that is evaporating water (and
ignore the losses from the pot itself, which are fixed) the calculation is
not evaporated mass x 2257 Joules/gram.

 

It is (the boiling temperature minus temperature at which the water
evaporated x 2.44) + mass evaporated x 2257 = Joules.

 

If you don't know the temperature at which the water went missing, and you
only have the final temperature (which is the case in the WBT) the
calculation is:

 

Missing water mass x 2257 + missing water mass x (boiling temperature minus
final temperature) x 2.44. That is the heat that was gained by the pot plus
the heat that was lost be a reduction of temperature (and that actually heat
turns into evaporated water).

 

The correct accounting for this energy is straight forward and not novel.
The WBT formula for the heat gained by the pot is almost correct. The HTP
does it correctly. Other methods do it incorrectly (Indian and Chinese
methods).  All these sorts of things are picked up during external review.
If the methods were reviewed by experts, these mistakes would have been
eliminated long ago (we can assume).

 

There is a more fundamental problem with this 'heat gained' problem however.
Knowing how much heat (net) is gained by the pot does not mean we can
calculate 'the efficiency of simmering'.  There is no 'efficiency of
simmering' that can be expressed on a thermal basis. This fundamental fact
means the metric is wrong to start with. As a measurement, it is not in the
IWA because by then the EPA and others had accepted the fact it was not
useful. Continuing to talk about the thermal efficiency of simmering only
indicates that one does not understand what it means.  Simmering is an
arbitrary task that does not have a thermal efficiency component because it
does not require 'work' to be performed, and 'work' has a definition.

 

If you hold a bucket of water at a constant distance above the floor for 30
minutes, you have not accomplished any work, but you will have expended a
lot of energy to hold it there. You can measure that energy used, not you
cannot express that energy in terms of having accomplished the task of
'doing nothing' that qualifies as 'work'. You could just as well hang it on
a nail. The nail isn't doing and 'work' either and the 'energy efficiency'
of hanging a bucket on a nail is mathematical nonsense.

 

Hanging a bucket of hot water on the nail of 97 degrees is not 'work'.
Therefore there is no 'thermal efficiency of simmering'. It is an oxymoron.
The metric is invalid. Changing the calculation method doesn't make it
valid. It fails at the conceptual stage.

 

Overcoming losses of heat from the pot by convection and radiation is
'work', but no one is measuring that and you can't report the efficiency of
doing something you have not measured.

 

I am glad we are getting onto the same page, which is normal science and
engineering.

 

Regards

Crispin 

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