[Stoves] Thermal efficiency

Crispin Pemberton-Pigott crispinpigott at gmail.com
Sun Oct 13 17:50:03 MDT 2013


Dear Alex

 

I have very seriously considered this option and have offered it as a good
method of determining the effective heat gain. I am first going to try it
using with condenser because there is less to build and I can perform the
test as the boiling point.

 

One of the guys here in Beijing is going to have a stab at modelling the
heat loss from the pot with and without a lid because we don't have an
example of that. I would like to know precisely how much load is placed on a
stove when the lid is removed and what the influence of the radiation from
the rolling water is compared with a shiny aluminum lid. We know it taxes
the stove but it may be in the form of evaporation (100%) rather than an
additional undocumented loss. I would like to put that puppy to bed.

 

I have been looking at the formulas used for calculating LHV together with
two students. This was a very interesting exercise. The EPA method for
calculating the LHV of biomass co-fired with other fuels is exactly the same
(save for units) as the Chinese National Standard but not like the method
used by the WBT (discussed in a footnote in the document). I think that is
worth exploring because we should start off our theoretical work using the
right fuel heat content.

 

(Note to Tom M - we figured it out!)

 

The advantage of using the boiling and condenser is that we can separate
fuel mass burned from water evaporated in real time allowing us to plot the
change in efficiency with power i.e. a standard engineering performance
curve. Using a pressure cooker allows us to push the steam (if necessary) a
little into a heat radiator or one of those cool glass curly things you see
in the movies. We are trying not to make it look like a still for making
Newfie Screech or Witblitz.

 

Regards

Crispin having fun with Harold and Lodoysamba and Tseyen-Oidov and Hao and
Dong and all the others

 

 

Crispin,

On the heat transfer side there are clearly a lot of variables. Get rid of
them like we do with boilers. Use a sealed pot with its contents (water)
kept at a constant temperature (example; either high ~90C in-95C out, with
no condensation or low ~55C in-60C out, with some condensation or both) and
a flow meter to measure the thermal work done. A modern automated three way
valve controlling the flow through a heat exchanger can regulate the return
temperature precisely.

The pot can be any pot with a non standards lid disc with in/out plumbing
connections, glued with silicone to the top of the pot. It should withstand
a few inches of water pressure with a stand pipe open to the atmosphere
after the outflow thermocouple. The vapour losses then are outside of the
measurement frame. The pot could be filled with water or be part filled and
have a vapour/air head space. Flow rates and velocities could be high enough
to eliminate biases between pots or to mimic natural convective patterns
within the pot.

All you need is a whole bunch of disc lids from 20 to ??cm in diameter in
1cm increments, or custom make them as the required.

Or you can see how many different efficiencies you can fit on the head of a
pin.

Heuristically yours,
Alex

On 13/10/2013 11:21 AM, Crispin Pemberton-Pigott wrote:

Dear Friends

 

As we are, at the CAU stoves conference, talking about thermal efficiency
tomorrow, here is something to think about.

 

++++++++

Efficiency is a ratio, but of what to what? Let us follow the heat and
decide which 'efficiency' we want to report. 

 

1.	Heat available in the raw fuel if it was to be burned completely
2.	Heat available in the dry portion of the raw fuel
3.	Heat available from the fire considering incomplete combustion
4.	Heat available to the pot, at the pot in the hot gas stream passing
by

 

5.	Heat transferred to the pot - all of it
6.	Heat transferred to the pot and subsequently lost from the pot into
the surrounding environment
7.	Heat absorbed the pot material changing its temperature
8.	Heat absorbed by the water - all of it
9.	Heat absorbed by the water changing its temperature 
10.	Heat absorbed by the water and evaporating water (whether the water
is hot or not)
11.	Heat absorbed by the water and lost from the water (by radiation,
not by evaporation)
12.	Heat absorbed into the food and being absorbed chemically
(transforming it into cooked food)

 

System efficiency [Overall thermal efficiency] is (7+9+10+12)/1.   [When
boiling water only #12=0]

 

Heat transfer efficiency is .. ?  Which one is the one you were thinking of
when asked about 'thermal efficiency'?

 

+++++++++

Regards

Crispin

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