Stoves have long been a popular topic in backpacking circles. Alcohol stoves have seen a great deal of popularity in the past decade but recently integrated canister stoves (i.e. Jetboil) have been increasing in prominence. Alcohol stoves won converts from canister stoves by reducing the weight of the components (stove, fuel container) while offering decent fuel economy and a simple operating experience. Since then canister stoves have been on a weight diet and more importantly, integrated canisters systems now offer a less tip-prone cooking experience with remarkable fuel economy while retaining fast cooking speeds. Accordingly, alcohol stoves have been pushed into smaller niches like short trips where the weight of a canister is highly disproportionate to the fuel needed.
The Energy Density Theory
The generally accepted wisdom about canister stoves is they use less fuel because their fuel is more energy dense. These claims are all over backpacking websites. Example: http://www.pmags.com/stove-comparison-real-world-use
A quick look at the chemistry seems to support this. Canisters use a variety of fuels but all of them offer very close to 46 megajoules per kilogram. Conversely, the commonly used alcohols (methanol, ethanol) lag behind at 20 and 29 megajoules per kilogram. Big difference? No, because this advantage is entirely offset by the canister weight, which is mandatory because these fuels would be gases without huge pressure present. A 4oz fuel canister isn’t much more than 50% fuel, while a 16oz canister maxes at 70% fuel by weight. So when we look at energy density in terms of energy provided from the combined weight of the fuel + container, canister systems drop from 46 megajoules per kilogram to 25-32 depending on the canister size (4/8/16oz = 25/29/32 MJ/kg). This is the best case scenario as it assumes your trip requires burning the entire canister, yet results are still similar to ethanol, which drops by about 1 megajoule per kilogram when you include the weight of a 1oz pop bottle. Bottom line: Packing ethanol in pop bottles provides nearly the same energy (28 vs 29 MJ/kg) as packing 8oz fuel canisters. Even on long trips, there’s no substantial energy density advantage to a canister vs ethanol, and only a small advantage over methanol. Thus based on energy density alone a canister stove can’t offset the extra weight of the stove, nor come out substantially ahead on the fuel load.
The Energy Transfer Theory
Based on the above, it seems that we should be able to do the same amount of cooking on a canister as a similarly heavy bottle of alcohol. This is far from true. Typically you can boil about 50% more water with a similarly heavy canister.
Why? It’s due to increased efficiency at energy transfer, not energy density. Using the BackpackingLight article on integrated canister stoves and applying some additional math (see bottom of this post), we see that canister stoves can be 70% efficient at transferring energy from fuel to water in unrealistic indoor conditions. It’s amazing stuff. Conversely, alcohol stoves are lucky to hit 50% and most hover around 40% (personal data) in ideal conditions and much less in the outdoors (~30%). The best alcohol system I’ve ever tested (1.3L wide pot + caldera cone + Starlyte stove) was 53% efficient indoors for a pint. Canister stoves – and particularly integrated canister stoves – have several efficiency advantages over alcohol. The ones that come to mind are (1) pressured fuel is better mixed with air, resulting in a more complete burn, (2) integration between canister and pot (i.e. Jetboil) reduces heat losses and (3) insulated pots reduce heat losses (again, see Jetboil). This large difference in efficiency is why integrated canister stoves can theoretically more than offset the additional weight of the integrated stove on long trips where fuel demand closely coincides with the fixed amounts of fuel commercially available. Actually achieving this is tougher than most realize, as any scenario where the canister wins requires (1) starting with a close to full canister and (2) using the vast majority of the fuel present. Taking a half full canister will always be heavier.
This is interesting partly because several of these efficiency advantages aren’t intrinsic to canisters. Someone could easily design an elaborate integrated stove/pot system for alcohol with fins everywhere to increase the efficiency. Doing so would increase the system weight, but decrease the fuel used. You will never get quite as efficient or heavy as a canister system (unless you used a pressurized canister, which you could) but the weight-efficiency relationships are similar. For a short trip, a lighter but less efficient system wins, while for longer trips weight invested in efficiency pays off. Some alcohol manufacturers have attempted to add complexity to improve efficiency but nothing exists that’s nearly as elaborate as an integrated canister stoves. Perhaps this is for good reason, as an alcohol system that occupies the same place on the weight-fuel efficiency relationship isn’t that appealing. If similar efficiency was achieved, the weight would be similar so the alcohol system would be quieter, but slower and unable to simmer. Not a clear winner.
The core appeal of alcohol is the potential to strip away many of these features and occupy an extreme position on the weight-efficiency spectrum (very light but low efficiency) that isn’t possible with a canister because of the mandatory container mass. Thus unless something big changes, integrated canisters will continue to make sense on long trips, while a few ounces can be saved on short-medium trips with alcohol. I actually do think something big is going to change, but it’s a work in progress and it’ll be spring before I really know if it works.
With that said, there’s going to continue to be a contingent that prefers alcohol because it’s much cheaper to operate than canister stoves and it doesn’t entail the waste of spent canisters. Include me in this group. I can buy a summers worth of methanol for the price of an 8oz canister. I also have little need for speed or simmering and I like how easy it is to monitor my fuel situation. Thus I use an alcohol system for all 3 season trips, knowing that is lighter on most trips, and for longer trips I accept the initial weight penalty for the low cost, low waste and easy fuel monitoring of my alcohol system.
The super geeks out there might be interested in calculating the energy efficiency of their own cooking system. It’s a simple thing to do if you have a decently accurate way of determining your fuel usage (either by weight or volume):
Geek Section – Determining Efficiency
1) Determine Energy Absorbed by the Water
A gram of water requires 4.184 joules to rise one degree Celsius. So the total energy gained is 4.184 x water weight (g) x degrees gained.
Ex: Heating a pint (473ml = 473g) from 40F (4C) to a boil (100C) is 4.184 x 473 x 96 joules or 190,000 joules or .19 megajoules.
2) Calculate Theoretical Fuel Needed
Methanol provides 20 megajoules per 1000g. Ethanol is 29. Canister fuel is 46. Divide the energy gained (step 1) by energy density (i.e. 46) and multiply by 1000 to convert to grams.
Ex: .19 megajoules of heating theoretically requires 9.5 grams of methanol (.19 / 20 * 1000)
3) Compare to Actual Fuel Usage
Weigh stove before and after to find actual weight of fuel used or measure the volume used and adjust for density to find the weight. Your efficiency is the theoretical heat needed divided by the actual fuel used.
Ex. 9.5 g / 20g actual fuel used = 47.5% efficient
My typical efficiency in ideal conditions (inside) with a cone and Starlyte is 50%. The best I’ve been able to achieve is with a wide bottom pot (Evernew 1.3L) + cone + Starlyte which resulted in 55%. Typical alcohol stoves are around 40-45% in ideal conditions, and obviously lower in the wild.