No Free Thermodynamic Lunch


Just an addendum to the last post, with regard to comments suggesting air-drying clothes as an eco-friendly alternative to using the electric dryer:

Air-drying is great for outdoors. And maybe even indoors in the summer. But is it really “free”/super-eco-friendly to dry things indoors in winter? My data says, no. It says that drying clothes in your house cools your house down, and not insubstantially.

Consider: turning liquid water to water vapo(u)r takes energy, and a lot of it, no matter how it’s done. If you’ve ever tended the fire in a maple-syrup evaporator you know how much firewood it takes to boil 10 gallons of sap down to a one gallon of syrup. Similarly, if you’ve ever stood outside on a cold day getting hypothermic in wet cotton clothing you have a sense much energy water evaporation sucks out of you. And, just as in melting ice to water, the vast majority of that energy is not to make the water warmer, but to change its phase from liquid to gas.

So– maybe you want numbers. Well, the first question is: how much residual water is left in a big load of wet laundry, when it comes out of the washer? I am going to determine this when next I do laundry, using a bathroom scale– but for now let’s guess maybe 1.5gal = 6litres? Which doesn’t seem like that much… until you do the thermodynamic math*, and find that it takes 3.76 kWh = 3,234 kcal to dry up that much water.

Perhaps you don’t have a sense of how much heat 3.76kWh is. It’s about 3.4lbs / 1.5 kg of nice, dry maple firewood, if you have an efficient woodstove. It’s about 1/2 a liter of gasoline, burned efficiently. It’s 125 sixty-watt lightbulbs, left on for half an hour. It’s the energy my particular electric dryer uses if you run it for 40 minutes.

And unfortunately, when you rack-dry clothes in a house that you’re trying to keep warm, that energy doesn’t come from the kcal-fairy. It has to come from your furnace, or woodstove, or space-heater, or whatever fuel you’re using to keep your house warm, running longer or hotter. Really. Disbelieve? Close off one room & try covering the radiator there with wet towels… see what happens.

But I have no contest with the argument that line-drying is better for your clothes.

(* Johanna: I will send you the math, if you insist.)

Johanna said…
Any farmer can tell you that it’s not just temperature that dries out the soil, it’s also wind and humidity. If you are heating with wood or have central heating, chances are, your home is fairly dry (and hence the humidifier in the furnace kicks in, and why my mother always has a pot of water sitting on the wood stove). Assuming evaporation is entirely based on temperature is a bit too simple – even your energy pig clothes dryer uses the moving air bit + temperature + low humidity. And *then* it sends the by-product out the vent, and even if you were to capture it, you wouldn’t get all of it.

I’ve line-dried clothes in the polar desert and attempted to do so in humid tropics. In the tropics, they would have grown mold if I hadn’t managed to get direct sun on them. In the High Arctic, it took a couple of hours on a windy day colder than inside your house right now.

And even if it *were* *just* temperature, many homes have spaces that are not really used but heated by default. My furnace room is one such place – all the ducts converge on the beast, and it’s the warmest room in my house. Even when three loads of laundry are strung out…

Of course, you are welcome to set up shop down there. It can be your bunker. I promise it’s toasty warm.

11/19/08 9:00 PM
Turboglacier said…
No, for reals, it *is* just heat. Wind and low humidity just help the drying happen faster; they don’t change the total amount of energy transfered into a unit of water to turn it into vapor. The do accelerate the process, though– hence the “wind chill” concept, and the usefulness of evaporative coolers in deserts, but not in tropical jungles…

11/19/08 10:33 PM
Weeble said…
Dig it. Based on your recent posts, I would like to propose a new name for your blog: “May Hypothesize and Compute”

11/19/08 10:57 PM
s said…
Yeah, I have a rack in a room no one ever goes in anyway so it doesn’t really matter if it’s cold in there.

11/20/08 12:29 PM
Johanna said…
This is great!
Now that I have been enlightened on the amazing room-cooling properties of wet laundry, next summer will no longer have me suffering through hot nights because I refuse to turn on the AC. I’ll just fill my house with laundry racks. Should cool it right down, no?

11/20/08 3:50 PM
Turboglacier said…
Yes, it definitely will cool it down!

However, if the ambient humidity is high, the clothes won’t dry very fast, so the cooling may be spread over a long period of time, and completely unnoticeable.

Also, it’s possible that the further increase in humidity FROM the drying laundry will cause your sweat to evaporate more slowly off YOU, resulting in reduced ability to cool yourself, and causing you to FEEL hotter, even if the room is a little cooler.

But, you know, give it a try 🙂

11/20/08 3:55 PM
Johanna said…
Ah, but I know how to get around that, thanks to you – wind will speed up the evaporation. Thus, my bedroom will be filled with laundry racks and an ordinary fan pointing at them to speed up the cooling.

Or, you know, I could ask you to come up with some complex equations which have the ultimate outcome of scientifically proving that turning on the AC is actually the sensible thing. Given that you can justify a 5600W dryer, should be a piece of cake.

11/20/08 4:00 PM
Turboglacier said…
Oh, that will work J.– but it might be more efficient just to point the fan at yourself, to speed up the sweat evaporation! That’s a little trick I use here in the summer!

As for the AC… nope, can’t argue for it. The dryer and AC are both net producers of heat. It’s just that heat is something desirable in the winter, but not in the summer….

11/20/08 4:05 PM
Anonymous said…
I thought that the low humidity *did* matter. something to do with the water going from high concentration to low concentration. When I lived on the steppe the snow would sublimate at minus 20.

11/21/08 8:33 PM
FrozenLaundry said…
My mother used to dry her clothes in winter OUTSIDE, in Buffalo, NY. She’d wait for a sunny, no snow day, then hang the clothes out. The air is very dry in winter. There’s wind. Some got dry and some came in stiff as a board to be hung until completely dry in the basement (which was unheated and separated from the rest of the house by a door.

11/22/08 6:41 PM

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