I’ve been working on the problem of alcohol stove efficiency in my spare time, as a hobby, to relax from more pressing regular work matters, and I finally realized, the information on the internet is not very accurate, so I’m going to work out here the basic chemistry of alcohol fuel efficiency. My starting point was the very fine article on the Ion Stove by Sgt. Rock, which set me on my quest to get the actual data as closely as I could. If I make any errors in my calculations, do please post a comment of correction and I’ll update this posting.
While I might appear to get lost in the details, let me remind you of just why alcohol stoves are so lovely: they are silent. You can use any fuel, from toxic methonal to good quality ethanol in them, and they will still work, though of course you’ll need to carry about 20% more weight in fuel if you use a primarily methanol based fuel, as you will see as we do the calculations below. See appendix 1 for fuel types and how to get them. See appendix 2 for a table of fuel use / water starting temperature for ethanol and methanol.
The key point is that there is no such thing as ‘alcohol’, there are types of alcohol, and when you learn how to use your stove, and read how people get certain efficiencies, you need to be aware of significant differences in efficiencies, for example, ethanol, at 21.1 kjoules per ml, contains a whopping 33% more energy per ml than methanol, at 15.8 kjoules per ml. And, turning it around, methanol contains only 75% of the energy of ethanol. A blend like SLX probably contains around 18-19 kjoules / ml, though it could be less.
The basics – Joules to heat 1 pound of water
To make things easier, we are going to work from 2 cups of water, 16 oz, which weighs one pound. As always, we need to be clear on the units, we’ll be using 1 pound of water because 2 cups conveniently weighs 1 pound. wikipedia tells us that:
The British thermal unit (symbol Btu or sometimes BTU) is a traditional unit of energy equal to about 1055 joules, 1.054kj. It is the amount of energy needed to heat one pound of water by one degree Fahrenheit. In scientific contexts the Btu has largely been replaced by the SI unit of energy, the joule.
Appendix 4 shows some more conversion information for energy units.
For our purposes this is convenient. So 1 kj, actually 1.054 kj, heats one pound of water 1 degree F. 1 kj is about the energy released by one wooden match burning (pdf has energy content of many things, conversion units etc). However, it’s not quite that simple, since 2 cups of water weighs actually about 16.7 ounces by weight, ie, a bit more than a pound. Since 2 cups of water actually weighs 16.7 ounces, or 1.04375 pounds, this will throw off the following numbers for 2 cups slightly, but not a whole lot. However, all we need to do is multiply the total by 1.04375 to get the amount of fuel needed for the volume 2 cups, which is easy, so that’s what we will do below.
Energy content of ethanol
Sgt Rock lists the energy content per pound of ethanol as 12,550 btus per pound ie, 12,550btusx1.054=13,227 kj, and the energy content of methanol as 10,200 btus per pound (10,750 kjoules). First let’s check this. This source gives the energy content of ethanol as 23.4 MJ per kg. This comes from the IEA. Keep in mind that you can basically never find or buy 100% pure ethanol, all ethanols you will buy contain about 5% water, so that confused the matter. This pdf from ipst.gatech.edu for example lists it at 11,500 btus per pound. So clearly the first thing to do is find out what the actual energy content of 95% pure ethanol is. The question is made more difficult by the mix of units, ie, joules/btus per gallon (a volume measurement) vs joules/btus per pound, by weight, that is. So during this process we will need to find standard units to get things consisten.
The “heat content” of a gallon of ethanol is 76,000 British Thermal Units (BTU). But 1 BTU = 1054 joules, so the energy density (by volume) of ethanol is 21.2 kilojoules per cubic centimeter (aka, 1 ml).
Since the density of ethanol is 0.789 grams per cubic centimeter, the energy density of ethanol is 26.8 megajoules per kilogram. src