[Stoves] Simmering

[Crispin Pemberton-Pigott]

Dear Friends

It is rare that Philip and I disagree on anything and this is no exception
as you will see. My answer was deliberately provocative. :) 

The problem is not that one cannot generate 'a number' by measuring the
water evaporated when simmering. The problem is that it not a number that is
worthy of the trust invested in it by stove project administrators.

There are several ways for heat that got into a pot to get out. One of them
is evaporation of water. This is the standard measure of thermal efficiency
but all real testers know it is not accurate to the nth degree.

Heat is radiated from all round the pot, lost by convection from the sides
and top, particularly if there is no lid, and radiated from the water
surface in the latter case. Water is a better emitter of heat than the pot
in most cases!

Determining the efficiency means stating a claim that relates to the heat
that went into the pot as a percentage of the heat that was offered to it by
the fire. That is the useful interpretation and the one most people think
of. 

How much of the heat offered to the pot was absorbed by the pot. As you can
see, the accuracy of 'missing water' as the sole measure of this heat
transfer depends to a great extent on how much of the total heat is lost
that way.

Suppose the stove is on high and the water is already boiling. Philip likes
this as a test of thermal efficiency on high. Piet Visser (BDG) does as
well. So do I. It is an easy way to get a thermal efficiency rating that is
reasonably accurate.

I am making up these numbers for demonstration:
Heat generated by the fire = 3900 watts
Heat loss from the pot to radiation and convection = 130 watts
Heat lost to evaporation of water = 1300 watts.

Total heat absorbed by the pot = 1430 watts.

Using a standard water boiling test (WBT) formula, the thermal efficiency is
1300/3900 = 33.3%.

This is close but not true. The real efficiency is 36.66%, 10% higher than
the simple WBT calculation shows.

Now turn down the stove and start simmering, everything else being the same:

Heat generated by the fire = 1300 watts
Heat loss from the pot to radiation and convection = 130 watts
Heat lost to evaporation of water = 65 watts.

Total heat absorbed by the pot = 195 watts.

The WBT formula says the thermal efficiency simmering is 5%. In fact it is
15%. The WBT-calculated average is 19.17%. The real average is 26.0%, a
value 35% higher.

Now, turn up the stove so that more fuel is used to accomplish the same
task:

Heat generated by the fire = 2600 watts
Heat loss from the pot to radiation and convection = 130 watts
Heat lost to evaporation of water = 1000 watts.

Total heat absorbed by the pot = 1130 watts.

WBT reports 38.46% efficiency when simmering. When wasting fuel, the stove
is given a higher rating!
The actual efficiency is 43.46%. The WBT average for the test would be
35.9%. The real figure is 40.1%, about a 10% error.

Unfortunately all the versions of the WBT I have seen so far proposed
(except 1) average the thermal efficiency during boiling and the 'number'
reported during simmering (not being its efficiency) and report the
aggregate as the average thermal efficiency for the cooking session, which
clearly it is not.

The question is, how close it is to the truth?

Well, for boiling it is reasonably close, 10% in the first example. For the
first simmering example it is not close at all and varies strongly with the
applied heat, back to 10% in the second example. With a covered pot and a
highly controllable fire (like an ethanol stove) the WBT reported efficiency
can be close to zero. Remember the discussion about the heat that is
released from the top of the pot lid by convection as the steam condenses on
it underneath? No lost water! Lots of lost heat.

>From a strictly thermodynamic point of view, keeping a mass of water at a
constant temperature for a period of time - which can just as easily be
accomplished with at retained heat cooker or a cover as some stovers have
shown (Lanny I am referring to you!) - does not accomplish any work. Keeping
water hot is not a question of thermal efficiency, it is a task. The fuel
needed to accomplish a delta-zero enthalpy task cannot legitimately be
expressed as a thermal efficiency, particularly as the thermal efficiency is
not being measured, only missing water which is a portion of it. It can have
a fuel efficiency of course.

That is why I said the thermal efficiency of simmering is 0%. Boiling water
off a simmering pot is not simmering, it is boiling that takes place while
completing the (separate) task of simmering. The simmering has no thermal
work accomplished if the temperature at the end is the same or lower than in
the beginning. Boiling off water (evaporating) can be measured but is by
definition, not simmering (maintaining a temperature). Cooking something
'down' to thicken a sauce is 'boiling' and could have a thermal efficiency
but if done at low power, the number obtained from the missing water method
would by a far cry from the actual thermal efficiency when view from an
engineering perspective.

There are two more parts to this message: a simple method of determining the
actual heat transfer efficiency, and what to do about the WBT's errors.

Operate the stove at any stable power. Determine the power level (with a
scale and a calculator). Measure the rate of heat loss as determined by
evaporation. Increase the power a little - say 10%. Measure the rate of heat
loss again by the same method. The heat lost by the increased loss of water,
the difference between the two rates, is compared with the increase in
power. Delta water loss heat divided by Delta power heat.  That figure (an
efficiency) is probably the actual heat transfer efficiency from the fire to
the pot at the power level midway between the two power levels used.
Subtracting the apparent thermal efficiency as indicated by the missing
water method from the actual thermal efficiency allows one to calculate the
actual heat loss from the pot alone. This works at any power level.

Dean Still, paraphrasing Sam Baldwin, often says that the way the
calculation is done when simmering rewards the production of excessive
amounts of steam. What is he talking about is that as shown in examples 2
and 3 above, simmering at higher power rewards the stove with a higher
average efficiency rating. In fact, it should be penalising the rating for
wasting energy. A stove that can't be turned down uses more fuel and gets a
really high simmering efficiency number! That is ridiculous. The more fuel
used, the higher the rated efficiency!  This is a serious WBT3 problem.
Stoves with a good turn down ratio are penalised by the present WBT math.
That is a Bad Thing. There should be no figure reported for simmering
efficiency. Tasks should be separated from genuine efficiency statements.

The thermal efficiency on low power can also be tested by putting on a new
pot of cold water and running the stove at low power, recording the water
temperature change with time. That gives a meaningful number. The SeTAR
Centre's heterogeneous testing protocol runs the stove at three or four
power levels with cold pots and plots the efficiency on a chart. That is a
thermal efficiency test with meaning. The missing water method is not.

Regards
Crispin

-----Original Message-----
From: stoves-bounces at lists.bioenergylists.org
[mailto:stoves-bounces at lists.bioenergylists.org] On Behalf Of Philip Lloyd
Sent: 14 October 2010 02:35
To: stoves at lists.bioenergylists.org
Subject: [Stoves] Simmering

It is rarely that I have to take issue with Crispin, but his recent
statement "Please note that one cannot easily state the thermal efficiency
of 'simmering' because no work is done. If the pot is as hot at the end as
in the beginning of the simmer, the efficiency is zero%" is, I believe,
wrong.  

During simmering, some heat is lost from the pot by evaporation of water.
The object of simmering is to keep the contents as close to local boiling
temperature as possible while minimising the water loss, but you cannot do
both unless you completely seal the pot or use quite sophisticated control
technology - in the real world of cookstoves, the latter is not possible.  

The loss of water can quite readily be measured, and converted to an energy
output via the heat of evaporation.  So you can measure the heat out as
steam, and measure the heat in by the usual methods, and so come up with an
efficiency.

In the same way we measure the efficiency of heating from the rate of loss
of steam from a boiling pot.  That tells you just how much heat is actually
going from the fuel into the water.  Because the water is boiling, the
temperature is constant, and radiant, convective and conductive heat losses
are constant. The efficiency is slightly different from that determined by
the time to boil, but in the time to boil test, radiant, convective and
conductive heat losses vary, so it is significantly less reproducible than
efficiency determined by measuring mass loss once boiling.

Philip Lloyd