[Stoves] Thermal efficiency

Frank Shields frank at compostlab.com
Mon Oct 14 10:38:00 MDT 2013


Hi Dean,

 

Anything as close to real world I am in favor of. So I agree about using the
water boiling test as-is. 

 

But I do like the design of the method Alex described that included all the
necessary checks that make for a good, repeatable test. Well designed. I may
use it here for measuring heat increase in cubic meter size, self-heating
compost bins with compost-char experiments. Thinking a coil of water filled
tubing would work better getting a more representative reading than a
thermometer stuck in the center.   

 

And maybe someday I will actually get to test some stoves.

 

Regards

 

Frank

 

 

Frank Shields

Control Laboratories; Inc.

42 Hangar Way

Watsonville, CA  95076

(831) 724-5422 tel

(831) 724-3188 fax

frank at biocharlab.com

www.controllabs.com

 

 

 

 

 

 

 

 

 

 

 

From: Stoves [mailto:stoves-bounces at lists.bioenergylists.org] On Behalf Of
Dean Still
Sent: Sunday, October 13, 2013 3:38 PM
To: Discussion of biomass cooking stoves
Subject: Re: [Stoves] Thermal efficiency

 

Hi Frank and Alex,

 

Boiling 2.5 or 5 liters of water in a pot shows if the stove is capable of
doing one of the tasks which are often needed for cooking food. It's
important information. Boiling and simmering are both frequently needed when
cooking. The WBT published by VITA was patterned after actual cooking tasks
for good reasons.

 

Best,

 

Dean

 

On Sun, Oct 13, 2013 at 12:43 PM, Frank Shields <frank at compostlab.com>
wrote:

Dear Alex,

 

I like it. Easy to calibrate a flow meter with a stop watch and volumetric
flask and both temperature meters calibrated using the same temperature
water to make sure they read the same. So we can all be on the same page
using different equipment in different parts of the world. 

 

So one thermometer is set, say 30c higher than the other and we plot the
flow of water?  Perhaps bucket filling with water on a balance being
plotted? Or flow gauge? 

 

Thanks

 

Frank

 

 

Frank Shields

Control Laboratories; Inc.

42 Hangar Way

Watsonville, CA  95076

(831) 724-5422 <tel:%28831%29%20724-5422>  tel

(831) 724-3188 <tel:%28831%29%20724-3188>  fax

frank at biocharlab.com

www.controllabs.com

 

 

 

From: Stoves [mailto:stoves-bounces at lists.bioenergylists.org] On Behalf Of
Alex English
Sent: Sunday, October 13, 2013 10:54 AM
To: Discussion of biomass cooking stoves
Subject: Re: [Stoves] Thermal efficiency

 

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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