Monday, December 7, 2015

The Science and Art of Line-drying

For most people, energy is a lot like art--they aren't sure what it is, but they know it when they see it.  People see it in sunlight, in a rock balanced on the edge of a cliff, in a runaway dump truck, in electrons coursing through wires.  But what is it?  Big Picture, that is.

One formal definition is that energy is the ability to do work.  That works okay until you wonder what "work" is.  Answer: work is done when energy is expended.  Kinda circular!  Just to show how mysterious the whole business really is, I quote a physics prof I knew a few years ago: "When you remove all matter from the universe, energy is what's left over."

My own favorite definition, gleaned from a beginning physics text (high school level): Energy is anything that can cause change.

Energy is my all-time favorite Unifying Theme, and a very useful one, too.

For example, as a new but rabid fan of line-drying the wash, I am keenly aware of how much energy is needed to dry a load of clothing (a LOT), how much I depend on the sun to get 'er done, and how much energy=money=greenhouse gas I am saving by doing it.  

Here's a quick look into the physics of drying clothing. 

We will count our energy in calories, since it is a familiar unit to many people, and particularly well-suited to this topic.  A calorie is defined as the amount of energy needed to raise the temperature of one gram of water (almost a thimble-full) by one Celsius degree.  (The "big C" Calories familiar on food labels are actually ten times this large, and technically called kilocalories.  One kilocalorie could raise the temperature of a kilogram (liter bottle) of water by one Celsius degree.) 

Here's the rub in drying clothes, and the reason it's so expensive to run our electric clothes dryer: although it only takes one kilocalorie to warm a liter of water a degree, it takes 540 kilocalories to evaporate that same amount of water!  In households that have them, the electric dryer alone typically accounts for more than a quarter of the electric bill!

A last few damp garments huddle in the little sun still available.

Needing to dry our clothes, what can we do to conserve our electricity?

First, avoid heavy materials that hold a lot of water.  Jeans, heavy cotton sweatshirts and the like.  The lighter the material, the less water you'll need to evaporate.

Second, use a washer that has a very high spin speed: the faster the spin, the less water will be left behind in clothing, needing to be dealt with.

Third, dry your wash on a clothesline in the sun.  Solar energy is free, and very effective at this job.  Darker clothing is best, since it absorbs more light.  Your clothes line must be in full sun for as long as possible at this season, and of course it will be time-consuming and require forethought to get your wash out at the right time and in the right weather. 

Dry air alone will do the job in absence of sunlight, but the process will be veeerry slow--especially at this time of year.  It is well worth it to take special care to get your clothes out at the right time and in the best place. 

Dark pair of PJs in direct sun on a December day: the PJs are warmed by the sun,
increasing evaporation.  The water vapor coming off the top makes its presence known
as fog condensing in the cooler air.

What does the sun actually do, here?  Wherever there is liquid water, it is always evaporating and condensing at the same time.  It evaporates much faster in the higher temperatures your clothes reach in full sun (especially if they're dark colored).  So with warmer clothes in fairly dry air, evaporation will rapidly win the race over condensation, and you will be able to haul in even your thick (and dark) towels bone-dry in only an hour or two. 

What to do when the weather is uncooperative?  Line drying is difficult in damp or overcast weather,and impossible in the rain.  But you can still hang your clothes indoors over a drying rack.  This is slower that line drying in the sun (leave the rack up overnight).  It is also not free: the water evaporating out of your clothes is still absorbing heat (that same 540 Calories per liter), but it is absorbing it from your warm house, which will fire up the heating system to compensate.  This is still better than using the dryer, though--at least at my house.  We have an oil furnace, which is a cheaper way to generate heat than electricity. 

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