Dear Andrew
----- Original Message -----
From: <[email protected]>
To: "Discussion of biomass cooking stoves" <[email protected]>
Sent: Thursday, October 11, 2012 5:02 AM
Subject: Re: [Stoves] How to convert thermal efficiency into fuel savingsfigures
> [Default] On Wed, 10 Oct 2012 12:02:41 -0500,Paul Anderson
> <[email protected]> wrote:
>
>>Dear Stovers,
>>
>>I do not know how to convert thermal efficiency into fuel savings figures.
>>
>>I hope it Is a linear transformation. 10% TE wastes 90% of fuel,
>>while 40% TE wastes only 60%? But I suspect it is more complicated
>>than that.
>>
>
> I hope Kevin's answer didn't confuse you as much as it confused me!
# My apologies for a confusing explanation. If you could point out where my
explanation was confusing, I will attempt to clarify.
>
> If we take the efficiency figures as being correct and that they are a
> measure of how much of the fuel energy liberated is delivered into
> the pot then a 15% efficient 3 stone fire will burn 3.7 times as much
> fuel to do the job compared with a very good stove with 55%
> efficiency.
# That is only true in the limited cases where the stated efficiencies are
correct, and where there is no benefit from the heat that does not actually
enter the pot. If I burn wood in a stove to heat my cabin, what is the stove
efficiency, if I do not have a pot on the stove making stew?...
---> Is it 0% Efficient because I have no pot to measure its heat gain?...
---> Does it suddenly attain some degree of efficiency if I set a pot of
something on it to cook?
# Consider the example of Rocket Stoves, and TLUD's venting into the living
space, and a box stove with a chimney that vented the products outside the
living space, that are used in different applications:
1: a thatched hut in a Tropical Jungle Setting,
versus
2: an application (eg, in Mongolia or Northern Canada requiring both space
heating and cooking capabilities), where the same stove is used for both space
heating and cooking.
compared to
3: A box stove, intended for both space heating and cooking, having a chimney
that vented outside the living space.
# In comparison of Case2 and Case 3, the Rocket stove would probably show much
better pot boiling rates than the box stove, but total system efficiency and
annual fuel savings would probably be far greater with the box stove. We don't
see Rocket Stoves or TLUD's that vent their products of combustion into the
living space being used for space heating applications in temperate or cold
climates, because the need for excess ventilation will require much greater
fuel quantities to heat the cold fresh replacement air required for health and
safety. In Case 1, a TLUD may win over a Rocket, in the instance where the
person wanted to make a quick pot of coffee, but the Rocket may win, in the
instance of where the person wanted to simmer a pot of stew for 1 hour.
>
> At these higher levels of efficiency the heat exchange interface with
> the pot will be more significant then the completeness of combustion.
# How are these "levels of efficiency" actually attained? Are the real benefits
of "heat loss to the room" included, when relevant?
>
> Given that the mass flow through the stove and above the pot should
> be the same then a quick calculation using the temperature just above
> the flame and at the exit from the pot will give a relative figure for
> heat exchange; the ratio of heat supplied to heat rejected. Note at
> higher efficiencies this will decrease more noticeably as the pot gets
> hot because heat transfers to the pot depends on delta T between the
> hot gas and the pot contents.
# This is not really "stove efficiency", but rather, it is more like a
determination of "efficiency of coupling between a heat source and a heat
sink." It is like drawing a graph with a suppressed zero... a picture is
painted but it is not necessarily the correct, meaningful, or important
picture.
# My fundamental concern is that stated stove "Efficiencies" and "Savings" can
be very misleading and are prone to being used out of context. The only way to
get a fair and true comparison of efficiencies is to do a "Mass and Energy
Balance" on situations that do indeed have a true basis for comparison. "Good
figures never lie, but Good Liars often figure. ;-)
Best wishes,
Kevin
>
> AJH
>
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