I continued the process of Getting Rid of Stuff.
Today I started in on my lifetime accumulation of camping equipment. I was an avid backpacker as a teenager, and continued hiking and camping well into adulthood. The resulting equipment runs the gamut from ancient to merely old.
I stopped when I ran across a couple of packages of nine-hour candles. These are made to be burned in a backpacker’s candle lantern. That’s a cheesy, lightweight, spring-loaded contraption that pushes the candle up as it burns, keeping the flame in the same place as the candle is consumed.
I couldn’t put my hands on my old candle lantern, so I looked on-line to see what was available now. To my surprise, candle lanterns have all but disappeared from the camping/hiking market, even in stores catering to backpackers (e.g., REI, Campmor). The sole option is one high-end candle lantern from one manufacturer — the same one who made my candles.
After about a minute of thought, it finally dawned on my why nobody uses candle lanterns any more.
Once upon a time, this was a reasonable way to have a bit of light when you were backpacking. If your only electrical alternative was a flashlight with carbon-zinc batteries and an incandescent bulb, candles arguably provided more light per unit of weight than a standard flashlight could.
But as technology changed –– first with alkaline batteries, then with LEDs — candles became obsolete as a lightweight source of light. A quick internet query shows that the inefficiency of candles as a light source is common knowledge in the backpacking community.
But just how obsolete are they, really? That is, what’s the illumination-to-weight ratio for candles, compared to AA batteries running an LED flashlight? Does electricity now have a slight edge, or is it more the case that you’d have to be an idiot to take a candle lantern backpacking these days?
Inquiring minds want to know.
Candles: Not so great, as a source of light.
The first hurdle is getting an estimate of the light output of a nine-hour candle. Virtually everyone assumes that, well, it’s a candle, so it must produce one candlepower. But candles vary widely in terms of combustible material, wick size, and the resulting light output.
The original definition of one candlepower was based on a candle that burned at a rate of 7.8 grams of combustible material per hour. (Reference). The actual material used to define candlepower was wax from a sperm whale. That said, it’s likely that the energy density of that animal-based hydrocarbon is similar to the energy density of modern petroleum-based hydrocarbons, including paraffin wax.
Effectively, then, one candlepower is what you’d see from a candle that burns 7.8 grams of wax per hour. The definition changed somewhat over time, but not enough to matter for this rough calculation.
Unsurprisingly, these candles — meant to burn as slowly as possible — produce less than one candlepower. These nine-hour candles weigh almost exactly 50 grams, meaning that they burn at a rate of just 5.6 grams per hour. Based on the standard definition above, we’d expect these slow-burning nine-hour candles to produce (5.6 / 7.8 =) about 0.7 candlepower.
But how does that compare to the light output of a flashlight? When applied to candles, candlepower really is a measure of total light output. By contrast, when that term is applied to flashlights, by convention, candlepower is used to describe only the brightest part of the beam. If you want total light output, for a flashlight, you need to get it in lumens.
And, just as a matter of faith, I see several internet sources that all derive the same conversion factor. One candlepower = 12.57 lumens (reference).
And so, 50 gram nine-hour candles will produce a total of (0.7 candlepower x 12.57 lumens per candlepower x 9 hours) about 80 lumen-hours of light.
Coincidentally, two alkaline AA batteries or four AAA batteries weigh just about exactly the same as one nine-hour candle. Two AAs weigh just shy of 50 grams (reference). Or, four alkaline AAAs would weigh in at around 46 grams (reference). Close enough.
So, how many lumen-hours can I get out of two AA or four AAA alkaline cells?
West Marine advertises one nautical safety light getting 11 hours’ run time, at 25 lumens, using a single AAA cell (reference). They advertise another with 30 hours’ run time, at 20 lumens, from two AA cells (reference).
Respectively, four of the AAA lights would provide (11 hours x 25 lumens x 4 =) 1100 lumen-hours. One of the AAA lights would provide (20 x 30 =) 600 lumen-hours.
And I haven’t even tried to look for the most efficient flashlight available. These are just off-the-shelf marine safety products.
In short, candles aren’t even close to competitive with flashlights these days, on an illumination-to-weight basis. An off-the-shelf LED flashlight, with standard alkaline batteries, provides roughly 10 times as much light as a candle, per unit of weight.
Sure, I’ve ignored the weight of the devices themselves, and only concentrated on the fuel (or batteries) consumed. And, in theory, you might have to carry some dead batteries around for a while, if you were backpacking with them. And so on.
But it’s no wonder backpacking candle lanterns have all but disappeared. They’re a really dumb idea in the era of alkaline batteries and LEDs.
Candles: Much better as a heater.
That said, the actual energy density of candle wax is far higher than the energy density of alkaline batteries. In the same way that (say) ten gallons of gasoline stores vastly more energy than the equivalent weight of charged lithium batteries. It’s really merely the case that candles are incredibly inefficient at converting that energy to light. For that efficiency, you see estimates that are all over the map, but if I had to guess, I’d guess that vastly less than 1 percent of the energy of the burning candle is actually released as light. The rest is released as heat.
So, as as heat source, candles stack up pretty well against any battery-powered device. But just how well?
A typical high-end AA alkaline cell holds about 4 watt-hours of energy (reference). So two of them would be able to release about 8 watt-hours.
Candle wax contains about 46 kilojoules per gram (reference), and a watt-hour equals 3600 Joules (reference). Together, all of that means that one 50-gram candle contains about (50 grams x 46,000 Joules/gram / 3600 Joules/watt-hour =) 640 watt-hours of energy. Restated, one nine-hour candle contains about 80 times as much total energy as two AA alkaline cells.
This shouldn’t be a surprise. In fact, it’s probably conservative, given that gasoline is cited as being somewhere between 50 and 100 times as energy-dense as lithium batteries, depending on the metric and the source of the comparison.
So, candles as heaters, great concept. Candles as a light source, not so much.
Finally, we can roughly infer just how inefficient candles are at converting chemical energy into light. A typical figure for modern LED efficiency is 30%. That is, 30% of the electrical energy ends up as light, the rest ends up as heat. All told, that nine-hour candle has 80 times as much energy, and produces one-tenth the light. Which means that if an LED is 30% efficient, then a candle is about 0.04% efficient. Which, surprisingly, is quite close to a quoted figure of 0.05% (reference). In other words, 99.9+% of the energy in the candle wax is converted to heat, not light.
That said, per unit of weight, as a heat source, candles are no better than any other hydrocarbon. Other than an ability to burn them quite slowly, there doesn’t seem to be any advantage to using candles as a heat source rather than (e.g.) gasoline, kerosene, propane, butane, etc.
Addendum: Can’t I just add a mantle?
Traditional kerosene lanterns are also incredibly inefficient at converting fuel to light.
But more than a century ago, the dismal inefficiency of a traditional flat-wick kerosene lamp was improved by the addition of an incandescent mantle. A modern kerosene lamp works not by producing light directly, but by heating the mantle until it glows. Aladdin kerosene lamps, for example, are unpressurized kerosene lamps that use a mantle. They produce about four times as much light as traditional flat-wick kerosene lamps, per gallon of fuel (reference).
The use of an incandescent mantle is standard for all modern lights powered by burning hydrocarbons. Next time you see a decorative natural gas light, look closely, and you’ll see a mantle over the flame. Burning natural gas, by itself, produces almost no light. The light you see from a gas lamp is the light of the glowing mantle, heated by the burning natural gas.
So, why not stick a mantle in a candle lantern? Near as I can tell, the flame from burning wax is just as hot as the flame from burning kerosene.
I’m pretty sure that the issue isn’t so much the temperature of the flame, as it is the total amount of heat produced. Both Coleman (pressurized) and Aladdin (unpressurized) lamps burn about three ounces of fuel, per burner, per hour. That works out to roughly 65 grams of hydrocarbons burned, per hour, or roughly 10 times the burn rate for my nine-hour candles. Plausibly, if I put a Coleman lantern mantle over my candle, I could get a little spot to glow. But there’s no way I’m going to get that entire mantle to glow with just the energy input of a nine-hour candle.
Addendum to addendum: Well, I couldn’t just leave it like that, so I bought a Coleman lantern mantle and tried it. Turns out that the only part of the candle flame hot enough to make the mantle glow is a tiny bit of it, right in the heart of the flame. With that, you can get a piece of the mantle about the size of a match head to glow.
The upshot is that a mantle is totally impractical for a candle lantern. You have to suspend a tiny piece of the fragile burnt mantle literally inside the candle flame. And then, you get a modest increase in light output. Even if you could set that up and make it work, that fragile piece of mantle would never survive even the slightest bump.
