Sep
22
2010

The air of the future will be conditioned, efficient

NREL's membraine: There's so much science in his head it's projecting colorfully out into the air as a graph.
NREL's membraine: There's so much science in his head it's projecting colorfully out into the air as a graph.Courtesy NREL
You’re worried about the future again, aren’t you? You’re afraid that everything will taste like cardboard, and that most people will be robots, and that the robots will be too cool to hang with you, and that our trips to the bathroom with be confusing and abrasive, and something about bats, and that you will be hot all the time, even in your own homes.

And I wish I could tell you otherwise. But I can’t. I just don’t know enough about the future. Except on that last point—it looks like air conditioning may yet be an option in a necessarily energy efficient future.

Air conditioning can use up a lot of energy. An air conditioning unit typically cools air by blowing it over a coiled metal tube full of a cold refrigerant chemical. The refrigerant absorbs heat from the air in your house, and then it passes through a compressor, which squishes the refrigerant down, making it hot so that it releases heat outside your house. And then the refrigerant expands, and cycles back into the cool tube. (Here’s the explanation with some illustrations.)

Other cooling systems rely on evaporation. So called “swamp coolers” pull hot, dry air from outside, and blow it over water (or through wet fabric pads). The water evaporates to pull heat out of the air, so what is blown into your house is cool, humid air. Swamp coolers are more efficient, but they only work in very dry environments.

And then there’s another way to control your indoor climate: desiccant cooling. A lot of what makes warm air uncomfortable is the amount of moisture it can contain. Normal AC units remove moisture from the air, but they use a lot of energy in doing it. Another way is to use chemicals called desiccants. Desiccants suck up water. The little packs of “silica gel” crystals you might find in a new pair of shoes are full of desiccants. Blowing humid air over desiccants will result in the chemicals sucking the moisture out of the air, making it more comfortable.

Figuring out how to use the desiccants has been a challenge, however; desiccant chemicals can be corrosive to building materials, so they, and any dripping water, need to be contained. With this in mind, US government researchers at the National Renewable Energy Laboratory have developed a membrane for desiccant cooling systems that allows the water vapor in humid air to pass through it one way, but does not allow the liquid water removed from the air to pass back.

The researchers claim that this air conditioning process is up to 90% more energy efficient than standard AC. Every so often, the desiccant chemicals need to be “recharged” by heating them up so they release the trapped water (outside), a job that can be done by electric heating elements, or with a solar thermal collector. The University of Minnesota used a desiccant cooling system for their entry into the Solar Decathlon competition. Their system didn’t rely on a membrane—rather, humid air was pumped up through a drum of liquid desiccant—but they did recharge the desiccant using heat from solar thermal panels (which are basically big, flat, black boxes that collect heat from sunlight).

It’s reassuring to know that in the future, even as we’re covered in flesh eating bacteria, and spam advertisements for Spam are being beamed directly into our brains, we’ll at least be able to relax in pleasantly dry, cool air, without worrying too much about the energy we’re using to do it.

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