[Stoves] How to convert thermal efficiency into fuelsavingsfigures
Kevin
kchisholm at ca.inter.net
Thu Oct 11 17:28:09 CDT 2012
Dear Marc
----- Original Message -----
From: Marc-Antoine Pare
To: Discussion of biomass cooking stoves
Cc: Discussion of biomass cooking stoves
Sent: Thursday, October 11, 2012 2:13 PM
Subject: Re: [Stoves] How to convert thermal efficiency into fuelsavingsfigures
Kirk Smith's "Short Primer on Stove Efficiencies" was the best explanation of "efficiency" as it relates to stoves that's have encountered. And only 2 pages!
http://stoves.bioenergylists.org/stovesdoc/Smith/Primer/Primer.html
# Paul is trying to relate stove efficiencies to savings. This paper does not help him much in that regard. This Paper basically seems to be saying is that the "...stove design and combustion efficiency are not as important as heat transfer to the pot... to make a better stove, find better ways to get the heat out of the hot gases and into the pot..."
# The definition of "Nominal Combustion Efficiency" is not a good one, in that it does not recognize the importance of "stack loss" resulting from excess air.
Careful in comparing stoves of different classes -- TLUD "efficiency" can mean something quite different from Rocket "efficiency". For example: what do you do with the leftover char? It probably depends on where in the world you're talking about (even with the same stove!)
# In the case where it is intended that the stove function primarily as a "Cooking or Space Heating" stove, then the production of charcoal as "unburned carbon in the ash" is a dead loss, and will result in a very low "Stove Efficiency." On the other hand, if it is desired for the stove to produce charcoal, (for use as fuel elsewhere, or for use as biochar), then it is quite fair and rational to deduct it's energy content from the energy content of the input biomass, when calculating "stove efficiency."
# Equally, if a "space Heating stove" was also used for cooking, its loss of heat to the living space is benefit, and not a loss, or an inefficiency.To calculate the efficiency of such a stove system (ie, cooking and heating), one could simply do a stack gas analysis, and knowing the temperature of the stack gases, one could easily calculate a meaningful efficiency. On the other hand, if shell loss from the stove was not desired for space heating, then this method would not be relevant, in that it cannot distinguish between the fraction of input energy doing "useful work" (cooking) compared to the fraction that is wasted through the stove walls.
# Stove efficiency, for stoves intended for use in the tropics where the "heat leakage" is not desired or beneficial, can be improved by reducing the "shell loss" with insulation.
Best wishes,
Kevin
-Marc
On Oct 11, 2012, at 9:13 PM, Kevin <kchisholm at ca.inter.net> wrote:
Dear Andrew
----- Original Message -----
From: <ajheggie at gmail.com>
To: "Discussion of biomass cooking stoves" <stoves at lists.bioenergylists.org>
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
> <psanders at ilstu.edu> 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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