[Stoves] Thermal efficiency

[Alex English]

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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