Saturday, December 12, 2015

I saved 3 kilowatt-hours of electricity and kept 6 pounds of CO2 out of the air today! (And much more by staying off the road!)

How much energy did I save today by line-drying, I wondered, 

as I hauled a full basket of boy-clothes (including three heavy sweat jackets and a pair of jeans) indoors after dark.  This particular basket weighed* 27lb out of the washer, 23lb after it came off the line, and (because it was still damp with no prospect of improvement in the near future) 20lb coming warm from the dryer. 

Because the dryer is electric, this is easier to figure out than you might imagine.  Key fact: evaporating water takes about 2265 kJ/kg.  (A kiloJoule is about a quarter of a Calorie--a more familiar measure of energy--while a kilogram of water weighs 2.2lb and works out to one liter.)  In other words, the two kilograms of water that evaporated from the clothes while on the line absorbed about 4500 kiloJoules or roughly a thousand Calories of energy in the form of (free!) sunlight. 

I also benefited from my time outside on a sunny, breezy day.

Now for the easiest part: we can directly convert the energy needed to evaporate that water into energy units familiar on our electric bill.  (Stay with me here.)  The familiar Watt of energy is actually a metric unit of power equal to 1J/s (one Joule per second).  And the nearly-as-familiar kilowatt-hour we see on our electric bills is 1000 Watts (1kJ/s) of power expended for one hour's time; in other words, 1 kW-hr equals 3600kJ.  The 4500kJ needed to evaporate the 5lb of water in my son's clothes needed 1.25kW-Hours of electricity just for the evaporation.  If done in the dryer, more electricity would have gone to the motor that keeps the clothes tumbling, and a bit more to see to it that the moist air coming out of the vent was warmer than the dry air that went in. 

That was just the one load.  Before that, I had line-dried a big load of towels and other heavy, absorbent cotton things, and also a smallish load of whites.  I'm guessing the sun evaporated about 5kg of water in all, saving 11,000kJ of energy, equal to at least 3 kilowatt-hours. 

That might not seem like much in monetary terms, but consider that the electric clothes dryer is typically the biggest energy hog in the home, and alone accounts for over one-quarter of the electric bill.  Consider also that--in most areas of the US--generating that energy produces planet-warming carbon dioxide.  Just how much?  Well, that turns out to depend on what fossil fuel is being burned.  Coal runs a little over 2lb of CO2 per kW-hr, while natural gas produces only 1.2lb.  That compares to about 18lb of CO2 produced for every gallon of gasoline your car uses. 

An eye-opening comparison.

My Ford Focus goes about 30 miles on a gallon of gas, which produces about 18lb of carbon dioxide.  So the amount of carbon dioxide I save by a month's worth of line-drying is cancelled out by one day's 30-mile-round-trip commute to work.  (That result surprised me: cars are an even bigger problem than I thought!)**

If you have a gas dryer, by the way, the numbers are much better.  Only a half-pound or so of CO2 is given off by enough natural gas to produce 1 kilowatt of heat--making it four times more efficient.  This is partly because natural gas is a better fossil fuel than coal, but more because a lot of energy is saved by burning the gas directly, and not generating electricity as an intermediate step.  

*And no, if I had any friends I suppose I wouldn't be doing geeky things like weighing my wash and calculating energy use!

**If you own a big car, the best and easiest gift you can give the environment is to switch to a smaller one!  (That makes an even bigger difference than going from a small car to a Prius!)

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