DEVap: An Energy-Saving Air Conditioning Design
Unless it suddenly becomes socially acceptable for the thermally blessed to wear tank tops and shorts to the office, I think it’s safe to assume that air conditioning will continue to enjoy widespread use – despite the fact that it accounts for 5 percent of the 40 quadrillion British Thermal Units expended annually in American buildings (according to the U.S. Energy Information Administration). Those of us who enjoy high surface area to volume ratios and poor circulation often point out the high cost in resources and dollars of setting the temperature low in hot weather, and so it’s with a bit of a pained smile that I will relate the following very good news, which completely undermines my argument:
Researchers working at the U.S. National Renewable Energy Laboratory (NREL) in Golden, CO, have developed ”a new air-conditioner design that they say will dramatically increase efficiency and eliminate gases that contribute to global warming” (Savage). The air conditioner utilizes the principles of indirect evaporative cooling paired with a desiccant to further absorb moisture.
Image credit: Pat Corkery
Here’s how the desiccant-enhanced evaporative, or DEVap, air conditioner works. A polymer membrane coated with both a teflon-like substance that repels liquid water and a desiccant divides the air flowing through the system into two streams. The membrane has pores about 1 micrometer to 3 micrometers in diameter; these are large enough for water vapor to pass through but too small for the desiccant to sneak across. The desiccant draws moisture from the airstream, leaving dry but warm air. Indirect evaporative cooling takes place in a secondary chamber, chilling the other half of the divided airstream. As the air in the second chamber grows cooler and wetter it cools the dividing membrane, which in turn cools the first airstream, and out of the machine comes cool, dry air. The process uses up to 90 percent less energy, depending upon the humidity of the air that goes into the system at the start (Source: NREL).
Image Credit: NREL
NREL’s liquid desiccant takes the form of a 44% salt by volume solution of lithium chloride or calcium chloride (aka road salt). The corrosiveness of the salt “requires that metal be eliminated from the hardware. What’s particularly attractive is that it replaces the chlorofluorocarbons that are used as the refrigerant in traditional air conditioners. Those CFCs can easily leak, and every kilogram of them provides the same greenhouse gas effect as about 2,000 kilograms of carbon dioxide” (Savage). When the desiccant has absorbed too much water it can be heated to boil off the excess moisture. The system could take advantage of waste heat from industrial processes, or gather heat from solar energy that might otherwise go to waste.
NREL plans to hand off the design to industry for commercialization in about five years, which sounds like a long time – but you might just think of it as until the next World Cup plus one year. Another bonus is that the system has been designed to replace existing systems without many changes, meaning that it would be an air conditioner upgrade rather than an infrastructural overhaul. Either way, it looks like I’ll be wearing sweaters in June for a long time…
This post is all about water.
“Energy Saving A/C Conquers All Climates.” NREL. 06/11/10. Accessed 06/17/10. URL.
Savage, Neil. “An Energy-Saving Air Conditioner.” Technology Review. 06/17/10. Accessed 06/17/10. URL.