The Alcohol Stove “Efficiency Percentage” Test Thread

[Delmar O’Donnell]

There have been a number of alcohol stove time-to-boil and efficiency percentage tests posted recently, but they are scattered into several threads, some of which have subject lines that no longer jibe with the content. Due to my obsessive/compulsive nature, this thread is an attempt to consolidate efficiency tests of alcohol stoves into an easy-to-find thread. (If it doesn’t work, it’ll fall to the bottom and be forgotten–so be it.)

Testing your own complete alcohol stove setup (ie, stove, screen, pot, lid…) for efficiency is pretty darned easy now, thanks to a sort of crowd-sourced project that’s been simmering here during March and April of 2014. Here's a link that will take you to several advanced spreadsheets that have been developed:

Ben's Improved Stove Efficiency Calculator/Spreadsheet

Some of the players are: Ben H, thermo dynamics engineer extraordinaire, who supplied the needed formulas and constants to make stove efficiency calculations; Glenn S, who compiled the calculations into several ever-improving and easy-to-use spreadsheets; David Gardner, who has of recent been evolving his stove kit with careful multi-burn experiments and detailed records he publishes for all to review; Richard Cullip, professional spreadsheet slinger, and of course Bob Grouse, who has proofread everything–as BPL’s unofficial editor, it is safe for you to assume that any errors you may find in this entire project are his and his alone. (Kidding.)

For the back-story, the recent genesis of the testing paradigm (and links to spreadsheets) can be found at these two threads:

More Fosters Fun

…the above contains Glenn's easy-to-use spreadsheet; and here is David's thread…

Welding Blanket Under Alcohol Stove for Winter Conditions

So you are invited to join the fun and calculate the efficiency of your own alcohol stove setup–compare it to others–can you join the Sixty Percent club? The exclusive Seventy Percent club?

Here are the basics you need:
– stove,
– windscreen,
– pot (wider is generally more efficient)
– lid,
– fuel (K-S SLX rumored 50/50 eth/meth; K-S Green 90/10 eth/meth)
– ml-graduated measuring cup for fuel (testing 15 or 20 mL is traditional)
– water & measuring cup for it (most folks test either 500 mL or 2 cups)
– cooking thermometer for measuring water temp,
– a timer, and
– Glenn’s or Ben's or Richard's spreadsheet found at the links above (they yield nearly identical results). You can of course add and subtract from the basics as your experiments dictate.

Also helpful to know your altitude, and if you're so inclined, the barometric pressure.

Couple of ground rules, in the interest of science:
1) Please describe your stove setup as tested, including its weight. (Photos are nice.)
2) In the interest of science, please publish your full data (not just the result of the efficiency percentage calculation) so others can review it. Be sure to post what you are varying from test to test, if anything.
3) Repeat measures of the same setup are ideal. For example, David will often perform 5 boils with a particular setup and average the results. This is superior to single-test data.
4) Find your boil target temperature (calculated by the spreadsheets), and measure time to achieve it. At sea level, with a normal barometer reading, this would be a temperature reading of 212ºF aka 100ºC. The spreadsheets allow you to compensate your targeted boil temperature by entering either barometric pressure or elevation or both. Ben's even allows you to enter the top temp you reached, if you missed your boil.
5) Start your timer when (or very shortly after) your stove is lit.
6) Please include info on anything else that might be affecting your results, such as wind, ambient temp, etc. For example, Glenn S specifies he uses SmartWater for his boil tests and Sunnyside brand denatured alcohol.

Anything else–what did I forget? Do I need to modify this post? If so, sing out.

[Delmar O’Donnell]

[EDIT: Much of the following discussion refers to creating spreadsheets that have been significantly updated and improved. To find the most recent versions, follow the links in the first post of this thread, or you can find Glenn's latest spreadsheet via the direct hyperlink in his post, just two posts following this one.]

[Greg Mihalik]

Delmar,
In english (versus the equation), what is the Efficiency Percentage number?

[Glenn S]

Until we get a few more players, I'll keep posting my results in the original spot back on my Foster's thread, then if we get more involved, I can easily move them directly onto this post. But for now, here's a direct link to my test results post (not just the general thread) for a quick hyperlink reference.

Greg:
I know you posed the question at Delmar, but here's my take on it…
I see it as a quantifiable number that reflects the amount of energy absorbed by the water in relation to the output energy of the fuel used. It's somewhat arbitrary, being a rather crude assessment of our limited parameters, but lacking a better system, it's something that provides a somewhat level playing field for testing different systems, rather than a single facet such as boil time alone. I see it as being used similarly to a furnace, or woodstove, in terms of "Energy Efficiency Ratings".

[Delmar O’Donnell]

^ That's my understanding also. Ben explained the Efficiency Percentage to me as:

Efficiency = Energy Absorbed (by the water) ÷ Energy Released (by the fuel).

Ergo, a more efficient stove setup (it tests the whole system, not just the stove) uses less fuel to heat the water.

Much of the guts of the formula concerns itself with joules (units of energy) but Ben H. converted a lot of the variables into a constant specific to His estimate of the energy in obtainable denatured alcohol (the 25kJ/g in the formula). The formula only works for denatured alcohol, not other fuels. (If you had access to pure ethanol, that number would go up to 26.8 kJ/g.) To really grok the formula in detail, you'd have to contact Ben H, and possibly take some classes on thermodynamics.

What this gives us (for the first time that I'm aware of) is a communal way we can compare stove setups that goes beyond just "I got MM:ss to boil" or "I used XX ml ethanol to achieve boil." It "boils down" comparisons to one easy-to-understand number.

[Bob Gross]

I see your initial water temperature. Where is the final water temperature?

The final water temperature will vary for altitude. I've seen water boil at 175*F.

–B.G.–

[Delmar O’Donnell]

> 6) Tell us your altitude (altitude at this point is not factored into the efficiency formula, but might be later) and anything else that might be affecting your results.

If you (or anyone reading) knows how to factor altitude into the formula, please do so! I would be most gratified to see that variable added. As I understand the formula, it is calibrated to sea level.

[Bob Gross]

There are charts which show the boiling point of water for different altitudes.

I mean, if you are going to make this, then make it right. Or, put a disclaimer up front that it only applies to sea level pressure.

–B.G.–

[Delmar O’Donnell]

I will beef up the disclaimer. I should not be the person "making this" (ie, the formulas — it's not my field of expertise) but perhaps someone out there can modify the formula to account for altitude. Kudos if they can.

[Glenn S]

The final temperature is the "100" in the first equation.

I believe it would be fairly straightforward putting the final water temperature back into the formula. It was there initially, but has been calculated as a constant in the condensed version.

Since the formula seems to simply factor the amount of water temperature rise with the amount of fuel used, simply changing the final temperature via a variable entry would still result in usable efficiency ratings that could be cross compared among users at any elevation I believe.

Wouldn't be much point adding a whole bunch of cross referencing to utilize an elevation entry, and would be much simpler just providing a chart with the corresponding boil temperatures that the end user could then enter. I eluded to my elevation issue in a post a while back somewhere.

Adding one more row for final temp would be simple enough, if that's what the mob rules. Cutoff points should be drawn out beforehand though, rounding off to the nearest degree or something, or this whole process will just get lost in details and never get anything useful accomplished.

Also, it could just be noted in the initial test description as well. If someone wants to use the above spreadsheet information for themselves, then just altering the "100" section of the formula to reflect a users altitude, since it would stay constant for that particular user, unless they changed locations. Conversely, a user could even take the fuel and water measurements and plug them into the formulas in place of the cell location, further simplifying the spreadsheet into even less lines, if they use the same quantities for every test. It's pretty easy to work with that way.

As Delmar noted, as long as we post the quantities and time information, and not just our own independent end results, then peer reviews would keep out any individual formula errors.

Delmar,
On a sidenote. Seeing how even the most straightaway thread can get derailed and convoluted into pages of off thread side discussions, if you wish to "host" something like this, I might suggest utilizing your first post for a compilation of links, and allow independent testers to form their own threads, that would of course fade into obscurity, but that they could continually update. Much like I'm doing with my Fosters thread. I fully expect that thread to vanish many, many pages down, but I have a link directly to the test results post for future referrance. You could collect links like that on the first post of this thread, and perhaps monitor them occasionally, or when users post on this thread about a new breakthrough.

Maybe something like a users link, along with their best recorded time or any noteworthy accomplishments. Or, just a list of links alone would keep the initial post of this thread useable for a quick referrance guide to test results, sort of like an index? Just thinking outloud I guess.

[Delmar O’Donnell]

I did a quick regression, looks like we could substitute "100" with "100+(-0.00104 * feet)".

So, at 10,000 feet, it would be 100 + (-.00104 * 10,000) = 89.6 C, which is what the table says here:

http://www.engineeringtoolbox.com/boiling-points-water-altitude-d_1344.html

[Glenn S]

That would be my understanding, correct.

Look at row 24 on Ben's original worksheet, it's "Final Temp".

I didn't bother with it in my testing, since I'm at 1k feet and from Florida to Denver I guess I wasn't getting too worked up over being that picky lol.

I don't know how many people will be performing these tests at 14,000 feet.

But I wouldn't mind using it, I would get even better results then ;)

[Delmar O’Donnell]

Removed for clarity.

[Delmar O’Donnell]

Just read this:

> I might suggest utilizing your first post for a compilation of links, and allow independent testers to form their own threads, that would of course fade into obscurity, but that they could continually update. Much like I'm doing with my Fosters thread. I fully expect that thread to vanish many, many pages down, but I have a link directly to the test results post for future referrance. You could collect links like that on the first post of this thread, and perhaps monitor them occasionally, or when users post on this thread about a new breakthrough.

Clever. Maybe that's what this thread will become. Good "Plan B". Since I've got the first two posts, that's doable.

[Glenn S]

Well as that paper states at the top, it's a "Nonlinear FEA White Paper", so barometric pressure (altitude) doesn't effect boil times in a linear fashion, but it's pretty dang close.

So for your worksheet example, you're adding a negative, won't affect the result, but I might change it from:
(100+(-.00104*C9))
to:
(100-(.00104*C9))

but otherwise, ya, looks right from what I can tell.

[Delmar O’Donnell]

Did I mention I just barely passed high school algebra? See, there was this girl who sat in front of me who…well, that's off topic, actually. So how's this:

.

If you are new to making spreadsheets in Excel, remember when writing a formula, there must be an "=" sign at the beginning. So C8 would actually start:

=C6*0.0042*((100… and so on. I couldn't put the '=' in for the screen shot, or Excel would think it was a real formula, and hide it from view.

I'm not the guy who should be doing math in public, so someone check me please. If I have made egregious flaws here, then obviously they are Bob's fault.

[Glenn S]

Seems right. I see you've changed the 5/9 to 1.8 too. That works. Same double negative concept lol.

Too bad there isn't a way to retro calculate my first 3 tests, since I don't know the earlier time that the water reached the upper temp with the elevation factored in.

Actually, now that I think about it, I don't think I'll get that much of a different result with the modifications. Sure, the boil time will be quicker with a degree lower temp requisite, but the formula modification will be stricter with the lower window of absorbed energy. So I expect it to be pretty much a wash as far as an efficiency rating is concerned.

To summarize, as Bob stated, water can boil at 175 degrees, but that would also mean it took less fuel to do so, having very little effect on overall efficiency. After all, this example only demonstrates why we're NOT using boil times as a defining characteristic. I'm sure there's some kind of impact, but I wonder just how extreme the conditions need to be before we have "high altitude" cooking directions. Again, someone schooled much further than myself would have to answer this, I'm just posing the conundrum. I've also never tried to boil water at high elevation. We just don't have that around here.

[Bob Gross]

"…then obviously they are Bob's fault."

Hey, I don't have a dog in that fight.

–B.G.–

[Rog Tallbloke]

My kind of thread.
The best I achieved with my beer can kelly kettles was 61%
.

The real issue is ambient wind. Doesn't bother the kettle design. Big issue for normal stove/pan combos. I'm assuming people are testing indoors? How about a column for total weight including windshields?

[Glenn S]

Well, ambient wind would still pull heat out of the water from the external sides of the can. It may not affect the operation of the stove, but I'm pretty sure it would effect overall efficiency.

Hard to see what's going on in that boiler design. I'm assuming internal exhaust venting from the Kelly Kettle reference? I 've never played with that design, but always thought it looked promising from the "efficiency" perspective. Just not very replicateable though IMO. I'm more into the "teaching a man to fish" outcome than running a kickstarter project, but that's just me of course.

So anyway, since we've got a little wind today and the temps are finally above freezing, I ran a single (failed) test out on my screen porch with all my current parameters (500ml water, 15ml fuel, 65-70* materials). It's 43 degrees out for air and surface temps (concrete patio block) with 64% humidity. Breezy enough to make my wind chimes play, but no idea the gust speeds, etc. I only was able to get 190 degrees out of the water, so I'd have to increase fuel to get a good and proper efficiency rating, but to extrapolate from the formula by altering the upper temp goal accordingly, it comes to about 47% efficiency.

Since it's dark out, I could see that the flame burned quite well and I even had an extended burn of almost 9 and a half minutes, probably due to the high humidity, so it's good to see my windscreen working effectively, which, after all, is what started me on this whole thing in the first place lol. It's pretty amazing to see how much heat gets drawn out from the external sides of the can due to exposure though. A taller windscreen would certainly be beneficial, but then I'd be back to square one on packability. As I mentioned over in my thread, I've kind of reached a compromise (at least for now) of geometry for my needs.

Update: Ran some more tests today with 17 ml fuel, 32 degrees outside, all else the same. Getting around 53%. Fuel gets cold before I get a chance to light it, so some variability in the whole thing. Tried a cylindrical chimney from the top of the cone to the top if the can, to act as a double wall insulator in the wind, but it surprisingly didn't seem to effect performance any. Then again, without a steady wind and only gusty, it's hard to get reliable test conditions. So anyway, in the real world, even 190 with 15ml is enough to have a hot meal. Also, I now attribute the longer burn times to a cooler running stove, as the concrete substrate was still cool to the touch after a burn. I think additional insulation is unnecessary, but a baseplate is for multiple reasons. Conductive insulator, and thermal reflection for functionality, as well as ground protection against burn scarring (shelters, picnic tables) or unwanted fires in dry conditions.

This is the chimney I mentined. (10g)
Oddly it had no meaningful effect.

As far as weight for just the boil system alone, I'm at 69 grams with stove, baseplate, windscreen, pot and Fosters lid. I could shave 5 grams by replacing the lid with one made from flashing, but it's not as cool looking lol

[Rog Tallbloke]

Glenn, yes, the flue runs up the middle. The removeable top section vents the exhaust and conducts heat back into the water. That 2 cup design uses a stove design which has flame jets both up the inside flue and the outside of the can. Best time with 9C starting temp was 2mins 45s to rolling boil with around 17g of fuel. Not as efficient as this one cup model, but real fast.
This one uses the fosters can as a windshield to prevent conduction loss in wind, and the smaller kettle is inside.

[James Marco]

We did a similar analysis on the old BackPackingLight Yahoo list about 12-13 years ago. I used an old Heinekin Keg/Can for the pot with a rolled aluminum center JB welded in. It turns out that this was fairly efficient (I was getting close to 75% but this was not using your formulas) but wasn't worth the trouble (mostly volume loss.) A heat exchanger on the bottom supplies roughly what the internal chimney supplied, in terms of efficiency increases. A cone on the outside provided about the same wind protection/heat trap. Your set up is a bit smaller but it looks much lighter.

The trouble with an internal chimney is the nature of the design is removing surface area from it as it gets smaller. For small 1 cup boilers this means the internal chimney is about 1/3 to 1/5 the surface area of the exterior. Anyway, after two tries I pretty much gave up with this because: 1) The JB weld leaked after each trial. 2)The efficiency gain was less than I'd hoped for (~15%.)

The 1) above can be ignored. I was thinking this could be resolved with better manufacturing. But, the water was a bit scummy…I was afraid to drink it. Even after 5 tests it still had a bit of scum on it.

2) above involves more thought. 2a) was the surface area as described. 2b) The center chimney was funneling heat at an increased rate. This meant less heat to the outside, or increased baffling to slow the exhaust gas. (Starts looking like a heat exchanger.) This is possible with a reversed cone inside, also. This lets you slow the exhaust gas (heat, if you will) and maintain good efficiency by increasing the surface area as the cone shaped chimney increases in diameter.

Anyway, this pretty much parallels my own experiments. With increases only around 15%, I decided to move to a wider pot with a heat exchanger rather than continue the internal chimney trials. The 15% was about the same as I get with a good HE pot but maintains the internal volume and was a whole lot easier to make. I'm glad to see this, you are doing some great work!

[Rog Tallbloke]

It took me months to perfect the art of soldering the very thin aluminium. But that's a tale for another thread, I don't want to derail this one.

[David Gardner]

Rog, would love to see you start a thread on soldering aluminum!

[David Gardner]

Delmar,

Thanks for starting this thread. Contemplated making efficiency tests as scientifically as possible, and have the following thoughts to add to your OP:

For the sake of precision, we might want to consider measuring the amount of fuel in grams. With a digital scale that displays to the nearest .01 gram, we can measure to .005 gram accuracy. I'm not sure there is any volumetric measuring device that accurate or as readily available. So we can weigh 11.84 grams of alcohol, but express it as 15.00 ml. (I mistakenly used 12.84 = 15.00 ml in some of my previous tests, skewing the efficiency results.)

Same thing with water. We can weigh 500.0 grams of water, and know that it is 500.0 ml.

Of course 15 ml and 500 ml have more practical application because they can easily be measured in the field vs. taking a scale and measuring 11.84 grams and 500 grams. But when we are in the lab trying to measure efficiency to within .05 percent, it can make a difference.

"Boil" is problematic as to its accuracy, since the temperature at which water boils depends on barometric pressure. "Boil" only equals 212* F/100* C if you're at 1 bar of pressure. At higher altitudes or when when a storm is coming and barometric pressure drops, the water will boil at less than 212*F/100* C and never reach that temperature. Conversely, if you are at sea level but a high pressure weather system moves in, water will not boil until it exceeds 212* F. So rather than elevation, we may want to quantify this factor by stating the barometric pressure at which tests were conducted.

Also, rather than visually observing "boil" (which is somewhat subjective), we may want to consider measuring the actual maximum temperature reached by the water and the time it takes to reach that temperature. I think (but will defer to those with more education and experience in this area) that this will account for any differences in barometric pressure.

Another factor to be considered is the effect of humidity. When I do boil tests with cold water in high humidity, there is a considerable amount of condensation on the outside of the cans, which sucks up a lot of heat before it can get into the can. Not sure how to factor this into efficiency ratings, except by only doing tests with water at a starting temperature high enough, or humidity low enough, to avoid condensation.

My two cents.

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