internal fragmentation

a personal journal of hacking, science, and technology

Candle Power

Fri, 6 Mar 2009 01:49 by mpm in Uncategorized (link)

My local green-oriented coffee shop has started putting out candles at dinner time. It struck me as odd that they still thought burning things for light was green, so I’ve decided to do the math on their power efficiency and carbon impact.

A typical votive candle is about 2 ounces of paraffin and burns for about 15 hours. Its brightness is approximately 1 candela or 12 lumens and consumes power (in the form of paraffin fuel) at a rate of about 40W. That’s one or two (or 15!) laptops’ worth of power per candle.

Paraffin (like most fuels) is nearly all carbon by weight and when burned it generates more than its weight in carbon dioxide (12 atomic units of carbon from the candle + 32 atomic units of oxygen from the air per CO2 molecule). About .2kg total or about 13g per hour. Not much, but put a couple dozen of these out every night for a year, and you’re beginning to talk about a substantial amount of carbon.

Now let’s compare this to a 14W compact fluorescent bulb (as bright as a 75W conventional bulb). This generates about 840 lumens of light. That’s equivalent to 70 candles. If we hooked this up to a dirty old coal-fired power plant (generating 1050g of CO2 per kWh), we’d generate about 14.7g of CO2 per hour – almost the same as our candle! If instead we used wind power (readily available in Minnesota) and include the amortized impact of building the wind turbines (something like 22g/kWh worst case), that drops to less than .3 grams per hour.

In other words, the global warming impact of candles is on the order of 60 to way more than 3000 times worse than fluorescent lighting for the same amount of light.

This doesn’t consider heat though. In the winter, a candle is quite an efficient heat source as almost all of its energy is converted to heat. So we end up with 13g of CO2 per hour per candle to get 40 watts of heat. That same heat from an electric heater powered by our coal worst-case would result in 42 grams of CO2/hour. But with wind power, it’d be less than 1 gram of CO2 per hour in the worst case.

In the summer, there’s no upside: candle heat battles against air conditioning. A single candle produces enough heat to cancel out more than 1% of a so-called “ton” of refrigeration (12000 BTUs / hour).

Lastly, we should look at production cost and impact. Light bulbs of any variety almost certainly win, simply because they have lifespans of thousands of hours and are so much brighter. It would take something like 45000 candles to match both the brightness and lifespan of our typical CFL bulb.

So wish for a greener tomorrow and blow out those candles.

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