[Stoves] Differences in stove testing

[Bond, Tami C]

Crispin,

I think it is worth my pointing out that when the WBT 'thermal efficiency' metric was created by VITA it was not called 'thermal efficiency’. …<snip>
It was called the 'Percentage Heat Utilized'. PHU was in common use until the 2000's and I used the term myself in discussions on this list.

I concur with your interpretation of Baldwin’s work. In his book, PHU was also presented in conjunction with the combustion efficiency. Together those represent the whole system efficiency. Baldwin pointed out how those two were separate aspects that could be addressed in design.

Quoting Baldwin:
"Improving the fuel efficiency of a stove thus requires attention to a number of different factors. Among these are:

Combustion Efficiency: so that as much of the energy stored in the combustible as possible is released as heat.

Heat Transfer Efficiency: so that as much of the heat generated as possible is actually transferred to the contents of the pot. This includes conductive, convective, and radiative heat transfer processes.”

Notes:
- Elsewhere, Baldwin equates the Heat Transfer efficiency with the PHU.
- The PHU does contain the “denominator subtraction.”
- Baldwin also recommends attention to the "Control Efficiency: so that only as much heat as is needed to cook the food is generated.” I wish we had more people working on that!


Now, Baldwin does give PHU as an output of performance testing and reports it in tables— but these performance tests were focused on improving heat transfer within the stove, as shown by the attention to stove geometry and correlations drawn. Baldwin’s tests and reports of PHU serve the same purpose as Mr. Breckenridge’s 400 boiler tests, with a similar metric.

Why is it approximately OK to do the “denominator subtraction” for PHU? Because you can draw a control volume around the heat transfer apparatus (pot, etc) and treat the (fuel in minus char energy) as an estimate of heat in.
Why do I (still) question the “denominator subtraction” for efficiency of the entire system? Because you cannot draw any control volume around the system in which the char or its energy is not included in the input.


'K gang I am falling off the wagon. No more posts.


Regards,

Tami
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