[Stoves] Request for technology proposals - Clean Stove Initiative, Indonesia
Ronal W. Larson
rongretlarson at comcast.net
Mon Feb 24 15:37:34 CST 2014
Crispin:
I am going to confine myself only to the last few of your paragraphs, which was the only topic in last night’s message. You said (from below):
> Regarding the 85% efficient stoves:
>
> A stove that has a very high combustion efficiency, burns all the fuel placed in it, and having a high heat transfer efficiency can put 85% of the energy theoretically available in the fuel consumed into the pot in the form of sensible heat. I have a water heater in my house that is 88% efficient calculated on this basis. I have a furnace that is 92% calculated on this basis. It is therefore possible to accomplish. [RWL: I said last time that I agreed it is possible. But we are starting at hundreds of dollars, I guess, for any wood burner with that possibility.
>
> If the challenge is to provide a pellet burning, char-making stove with a rated fuel efficiency of 45% (also called the overall thermal efficiency), it would be a requirement to have a heat transfer efficiency that is above 45% in order to have char remaining as unburned fuel. If the heat transfer efficiency is 85% and the rated fuel efficiency is 45%, the difference of 40% can be delivered in the form of char remaining from the fuel consumed
RWL: I am sorry that you have changed the problem from Tier two (55%) to Tier one (45%) - because now I have to do more work. Clearly you were not happy with the conclusion I gave - that your numbers didn’t hold together. I wish you would have acknowledged that they didn’t in any reasonable test, (you can’t keep at either 85% reported efficiency or keep the same amount of wood). Glad that you are now willing to report a 45% stove, because that gets closer to reality, but I am going to also keep the 55% Tier 2 results, because I feel a need to also correct what you did earlier.
You earlier made a mistake, after you divided by 0.55 in the previous Tier-2 55% case in your lines that I repeated yesterday:
2 Stars means using up to 7/.55 = 12.727 MJ of heat
Difference is 4.49 <J or 155 g of char
If done correctly by including the losses in the 45% that doesn’t get reported for this Tier 1 example, the 4.492 MJ and 155gm for char would have been smaller at 3.257 MJ and 112 gm (by subtracting the loss of 15% X 8.235 = 1.235 MJ). The char weight percentage would have only been 112/749 gm = 14.95%. This would not be considered successful for a TLUD.
> I don’t think you need to me put numbers on it to show this.
[RWL: Wrong. I hope you see why from looking below at my putting numbers in play. Because of time pressures, I am going to just give results and not show equations - I will do for 85% (no char), 55% (Tier 2, your last example, some numbers above, after correction for handling lost energy in what I consider a proper manner) and 45% (Tier 1, the latest example)
These are the
useful energy (MJ): 7, 7, 7
Tier: 3, 2, 1
Desired “useful" %: 85, 55, 45
input wood energies (MJ): 8.235, 12.727, 15.555
Input fuels (gms): 484, 749, 914
Lost energy (MJ): 1.235, 1.235, 1.235
Lost energy %: 15, 9.7, 7.9
Char energy(MJ): 0, 3.257, 6.095
Char energy (%): 0, 25.6, 39.18
Char weights (gm): 0, 112, 210
Char weights (%): 0, 15.0, 23.0
Note the third row from bottom is not what you predicted (because of your assumption on lost energies that I think is not likely - overly simplistic. The conclusion is that any TLUD exceeding 23% char by weight will not achieve Tier 1 standing. More than 23% is fairly likely. There is no way most TLUDs could ever achieve Tier 2, since they would have to produce less than 15% char - which is hard to do, We find the perverse result that the better the char-making characteristics, the worse ranking the stove receives. Exactly why I hope to see more than a single number for any stove test that involves char.
> Of the original energy in the fuel consumed, 45% would get into the pot as sensible heat, and 40% would emerge as unburned char.
[RWL: I have made a different assumption above - that it is not the % efficiency that stays the same, as one adds char-making - rather it is the uncaptured energy. My approach is more optimistic. Since char-making stoves are usually reporting higher efficiencies (less smoke, etc) than rockets, why should energy losses go up as one adds char-making. The same total energy is arriving at the pot, with or without char-making
> To achieve this would require a fire heat (factored for combustion efficiency) to pot heat (net heat gained) efficiency of 85%.
[RWL: That is what I think I am doing - and think you are not. You are keeping the pot heat at 7MJ, but the lost energy (starting at 15%) keeps going up as one makes more and more char. This assumption needs justification.
> The system efficiency is 45% (because for each unit of fuel consumed, 45% of the energy available is doing something useful in the pot).
[RWL: Ditto - I think I am doing this, but the energy in the gas flow is the same for me in all three cases - not in yours.
> The char production efficiency would be the mass of char yielded divided by the mass of char potentially yieldable (I suppose) or the mass of char divided by the dry mass of fuel used.
[RWL: I don’t think we should introduce “potentially yieldable”; sounds like a bag of worms that won’t help stove users or sellers. Yes, we should report weights, but to really know what is going on, we also need a ratio of energies - what I called eff2.
I look forward to hearing whether you have changed your mind on a) wanting to keep the input wood quantity and wood energy the same as you test char-making stoves, b) whether lost energy percentage stays the same as one adds a char-making function, and c) whether you are providing appropriate information for the char-making stove community (and the country of Indonesia).
Ron
On Feb 24, 2014, at 9:10 AM, Crispin Pembert-Pigott <crispinpigott at outlook.com> wrote:
> Dear Ron
>
> This is getting complicated and the definitions are not completely in alignment so rather than bore people with endlessly inserting them into statements, I will try to re-organise everything for plainer reading. If any misunderstandings are cleared up at het same time, so much the better.
>
> Last point first:
>
> Sustainablility of biomass is defined by the UNFCCC and the current rating for all of Indonesia is that biomass is sustainable. I don’t create ratings, I just read them. There is no CDM funding available for projects ‘conserving biomass’ in Indonesia because the supply is considered to be sustainable. It happens that in Central Java there is a fully sustainable supply of biomass available for domestic cooking and industries. In fact large quantities of biomass are burned (Kaliandra for example) just to get rid of it.
>
> Energy equations: It will be helpful for the casual reader to know ‘what you are up to’ so to speak with this char making stove thing so from my perspective, if I may, I will state it as follows: There is a desire to have the energy remaining of the form of unburned char resulting from a fire credited in some manner so as to reduce the rated fuel consumption of the stove.
>
> There is no dispute between us whatsoever as to the energy consumption: the energy remaining in the char represents energy not liberated from the fuel consumed.
>
> The important question is not what we want, but what the customer of the test result wants. They are not asking how much energy was used when cooking, they asked how much fuel was consumed. The answer is of course different if there is char remaining and that char is not ‘fuel’ to the same stove for the next fire.
>
> With a stick burning stove, sticks left over can be used in the next fire and they are counted as ‘not consumed’. No problem, and they are fuel sticks that can be used ‘next time’. Char is not, when it comes to TLUD-char making gasifiers or pyrolysers.
>
> Energy balance with respect to the fuel and the fire:
>
> A - Energy in the fuel put into the stove
> B - Energy liberated by the fire
> C - Energy lost due to incomplete combustion of gases and particles
> D - Energy remaining in char that is recoverable
> E - Energy remaining in char and ash that is not recoverable.
> F - Energy in fuel remaining that can be burned in the same stove during the next fire.
>
> A is calculated on the basis of the fuel ‘as received (AR) which means considering the heat content factored for moisture.
> B is calculated by deduction as it is not directly measurable. In a clean burning stove it will be more than 99% of the heat available from the combusted material.
> C is determined by gas mass and composition and particle mass and its composition). In a clean stove the CO will dominate the value and the total value will be small.
> D is determined by mass and bomb calorimeter or guessing a value. The heat content of char is highly variable and guessing has not been shown to give meaningfully accurate numbers.
>
> E can be determined by bomb calorimeter.
> F can be determined by bomb calorimeter though the CSI-WBT avoids needing to measure it each time by recycling the leftover fuel into the next test. All reported tests are started using fuel remaining from a previous identical test, where remaining fuel is usable in that stove. For some stoves this is not possible and the matter does not arise. Stoves in this category (not able to re-use the fuel remaining) include most TLUD char-making pyrolysers and some gasifiers, though the matter remains less than fully explored.
>
> The fuel consumption (the measurement we are trying to provide to the programme manager) is determined by measuring the fuel mass consumed per replication of a task. It is similar in principle to a KPT in that one observes fuel disappearing, but instead of disappearing in to a homestead, it is measured disappearing into the stove each time it is used. It is a very non-negotiable measurement. Either the fuel is going into the stove or it is not. Credit is given for fuel remaining that can be used next time:
>
> Energy consumption in the form of raw fuel per cycle = A-F [Joules]
>
> The result can be expressed in Joules or in unit of mass. Programme managers want to hear the ‘savings’ of a stove in % terms. Thus it is possible to use the calculated energy-equivalent of the mass of fuel needed for each replication, and the reduction in that mass with a new candidate stove.
>
> Thus results can be presented as “This stoves uses 3 kg of fuel and that one uses 2 kg to complete the assigned task.” Or it can be presented as the second stoves saves 1/3 of the fuel. It is not desirably to present this as an ‘efficiency’ unless the efficiency in question is carefully explained as an efficiency is always a ratio.
>
> The major misunderstanding that has dogged the stove community for so many years, embedded as it was in the UCB-WBT, is the idea that the heat transfer efficiency is the same as the fuel efficiency. This is a hangover from the days of power station measurements and when the calculation was first adopted by the stove community, power station calculations were employed. They were quickly adapted to be more realistic (as power stations are not measured when starting and stopping). By 1984 the Eindhoven Group had a pretty good handle on this calculation and the term ‘percentage heat utilized’ was widely adopted. The problem with PHU was the defective understanding of the heat transferred during simmering so results are inconsistent, that is to say, highly variable, even if the stove itself was not. This was dealt with (by Eindhoven) by assuming fuel remaining was thrown away. That is, char remaining was not useful for the next fire.
>
> Efficiencies:
>
> Here is a slide from a presentation on measuring stove performance:
>
> <image003.jpg>
>
> The calculation of the “heat transfer efficiency” has been used in place of the “overall thermal efficiency” in many calculations. Heat energy potentially available in fuel consumed minus the heat energy theoretically available in the char remaining is not the heat energy delivered by the fire. There are so many things that are not the ratio we want I can’t list them all.
>
> Upon looking closely, you will see from the above slide that the heat theoretically released by the fire based on the missing mass of fuel is not really the heat available to the pot – it has to be factored for combustion efficiency and mechanical losses like dropped fuel and unusable fuel remaining.
>
> It is not the heat released by the fire either. That is why I call that number (as you have calculated it and as the UCB-WBT calculated it) a ‘proxy for the heat transfer efficiency’. It does not use the heat actually available from the fire nor does it use the heat transferred to the pot, nor does it even use the heat that changes the temperature of the pot and what is in it (sensible heat). It is an ‘efficiency’ because it is a ratio but it is not a useful number for rating stove performance, or even designing a stove.
>
> Let’s call the energy that could have been released from the burned fuel ‘Fuel Energy”. Let’s call the energy in the char remaining, “Char Energy”. Let’s call the difference between them, ‘Fire Energy”.
>
> Fuel energy – char energy = fire energy
>
> Agreed?
>
> Fuel energy can be expressed as a mass of fuel.
> Char energy can be expressed as a mass of fuel equivalent (which is how the GACC-WBT works)
> Fire energy can also be expressed as a mass of fuel equivalent.
>
> Let’s call the heat getting into the pot, “Pot Energy”.
> Let’s call the heat accumulated in the pot, “Net heat gained by the pot” which is the same as ‘sensible heat’ or ‘detectable heat’ as the termsensible implies.
>
> “Pot energy” and “Net heat gained by the pot” are not equal because some heat that gets into the pot is lost from the sides, especially by convection and the amount as a % of the total, varies (a lot) with firepower.
>
> Back to Efficiency:
>
> “Net heat gained by the pot”
> “Fuel energy – char energy”
>
> Is not the energy efficiency, nor is it the fuel efficiency, nor is it the heat transfer efficiency, not is it the mass equivalent ratio of any efficiency. It is just a number. It is used as a proxy for the heat transfer efficiency. It is not the heat transfer efficiency, but it is close enough for government work, apparently, because the UNFCCC uses a variation of it to determine ‘fuel savings’ which clearly cannot be the case if the stove produces a lot of char. I believe it is the same as your Eff 3.
>
> It is not the answer programme managers are asking for. They are asking what the reduction in fuel consumption is, which is easily determined by measuring the fuel demand of the stoves during each replication of the cooking task. It does not involve any jiggery-pokery with energy in some fuel fractions and not others.
>
>
> Regarding the 85% efficient stoves:
>
> A stove that has a very high combustion efficiency, burns all the fuel placed in it, and having a high heat transfer efficiency can put 85% of the energy theoretically available in the fuel consumed into the pot in the form of sensible heat. I have a water heater in my house that is 88% efficient calculated on this basis. I have a furnace that is 92% calculated on this basis. It is therefore possible to accomplish.
>
> If the challenge is to provide a pellet burning, char-making stove with a rated fuel efficiency of 45% (also called the overall thermal efficiency), it would be a requirement to have a heat transfer efficiency that is above 45% in order to have char remaining as unburned fuel. If the heat transfer efficiency is 85% and the rated fuel efficiency is 45%, the difference of 40% can be delivered in the form of char remaining from the fuel consumed.
>
> I don’t think you need to me put numbers on it to show this.
>
> Of the original energy in the fuel consumed, 45% would get into the pot as sensible heat, and 40% would emerge as unburned char. To achieve this would require a fire heat (factored for combustion efficiency) to pot heat (net heat gained) efficiency of 85%. The system efficiency is 45% (because for each unit of fuel consumed, 45% of the energy available is doing something useful in the pot). The char production efficiency would be the mass of char yielded divided by the mass of char potentially yieldable (I suppose) or the mass of char divided by the dry mass of fuel used.
>
> The question most often posed by the programme managers is the relative fuel consumption of candidate stoves relative to the baseline products in common use. So far no one has asked for a char production ratio or a heat transfer efficiency or a heat transfer efficiency proxy.
>
> Regards
> Crispin
>
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