July 20,2002

Use of air conditioning actually makes the world hotter

National Public Radio

By David Kestenbaum

Scott Simon, host;

The US Senate yesterday passed a resolution honoring the 100th anniversary of the air conditioner, that marvelous device that moves your heat away from you and closer to somebody else. NPR's David Kestenbaum has an assessment of this imperfect invention.

DAVID KESTENBAUM reporting:

I'm on my bike heading home. It's a hundred degrees out. And next to me in a car is a man. I can see through the closed window that he is not sweating. He has air conditioning. Is this fair? Christopher Grayce, a physical chemist who has taught thermodynamics at the University of California Irvine, says that, in principle, the world is no worse off. Mr. CHRISTOPHER GRAYCE (Physical Chemist): It used to be there was a certain amount of heat outside the car bugging you, and a certain amount of heat inside the car bugging him. And he has, indeed, shifted his burden on to you. So he's all cool now and you're twice as hot. And so you could argue about the inequitable distribution of heat, but nevertheless, he's not making more of it.

KESTENBAUM: Except that this is not the whole story. Grayce says there's a little something called the second law of thermodynamics. Basically, heat wants to go from somewhere hot to somewhere cold. Open the freezer and the heat rushes in. The second law says that, in order to refrigerate, to push the heat back out, you have to use a machine, and that machine always generates heat, in this case, it's the car.

Mr. GRAYCE: And so he's converting a certain amount of the chemical energy and gasoline into waste heat, so he really is heating up the world.

KESTENBAUM: Jim MacKenzie, a scientist at the World Resources Institute, estimates that for every three units of heat you shove out of your house with an air conditioner, some power plant has added another two to the world. Air conditioning warms the world. You cannot, it turns out, make a perfect air conditioner that just moves heat for free. Physicists have tried to dream them up. A hot room is just one where the average air molecule is moving fast. In a cold room, the average speed is slower.

In 1867, the physicist James Clerk Maxwell imagined the perfect refrigerating device, a small demon that could see those molecules. Christopher Grayce, envisions Maxwell's demon sitting on the threshold of his office.

Mr. GRAYCE: So it's a little guy with a fly swatter, and every time a fast molecule comes from the office and heads out the door, he lets it go, and every time a slow molecule comes from the office, he puts his little fly swatter up and it bounces off and goes back into the room. So he keeps all the slow ones in, lets all the fast ones out.

KESTENBAUM: The fly swatter is very light, so the demon uses essentially no energy. And Christopher Grayce's office gets cool for free. Unfortunately, as Maxwell realized in the 1800s, his demon has a hidden cost. In order for the demon to see the air molecules, there needs to be light. If there's light, there's power being used somewhere, making the world a little hotter. David Kestenbaum, NPR News, Washington.

SIMON: And it's 22 minutes before the hour.