We often take air conditioning for granted as we escape from the sweltering summer heat in our climate-controlled homes, but it is an expensive, energy-intensive technology. By some estimates, it accounts for about 14 percent of the electricity consumed by American households.
That current comes largely from coal- and gas-fired power plants — bad news as we look for ways to cut carbon emissions to soften the impact of global climate change.
Now, engineers at the National Renewable Energy Laboratory in Golden, Colo., have developed an innovative air conditioning concept that promises to cut electrical demand by up to 90 percent — and it works well in both Gulf Coast humidity and desert heat.
It goes by the not-very-mellifluous moniker of DEVap (for "desiccant-enhanced evaporative") cooling. But with major air conditioning manufacturers showing interest, it could soon be an attractive alternative to conventional technology.
"We've discovered what we think is a new concept in air conditioning," says Ron Judkoff, the principal program manager on the project. "We recognize its potential, but it has a ways to go before it's out of the lab and available to consumers."
The new, patented system abandons the power-hungry compressor-driven refrigeration process used in many domestic (and virtually all commercial) air conditioners in favor of a couple of high-efficiency pumps and fans. But it also uses water for evaporative cooling — a concept familiar to many people living in the arid West who have roof-mounted "swamp coolers."
Swamp coolers work well when the outside air is dry, as evaporating water carries away heat, cooling and moistening the air that is re-circulated into the house.
But Judkoff identifies a few shortcomings of traditional evaporative cooling. He says the evaporation happens on wetted pads that are susceptible to mineral buildup and even bacteria growth. And if the outside air is both hot and humid, evaporation alone can't lower the air temperature to comfortable levels.
He says that in Phoenix, where the annual monsoon and human activities like irrigation raise summertime humidity levels, there might be as many as six weeks each year where evaporative cooling won't work.
Evaporative cooling also introduces moistened air into the building, which might be an advantage in a very dry climate, but isn't always desirable, Judkoff says.
In a warm, humid climate, like Houston's, most people turn to vapor compression cooling because it can cool (albeit somewhat inefficiently) with moist air.
At NREL, engineers talk about "sensible" cooling loads - the dry bulb temperature shown by a thermometer — and "latent" loads, which reflect uncomfortable humidity. "The problem with normal vapor compression air conditioning is if most of your load is latent," Judkoff says, "you've got to accomplish a little bit of cooling to make people comfortable, but you've got to accomplish a lot of drying to make people comfortable."
Vapor compression machines run warm, humid air by a chilled coil. "The coil is cold enough that that humid air goes to the dew point and it drops moisture. But as soon as you satisfy the thermostat — let's say you've got the temperature down to 70 or 72 degrees — well, it stops. The humidity is still there."
This problem is usually solved by chilling the air to a much colder temperature, wringing out the moisture, he says. Then the air is warmed back up to a comfortable room temperature, a brute-force solution that wastes a lot of electricity.
Going back to the drawing board, NREL engineers turned to desiccants — water-absorbing compounds that dry the surrounding environment. Desiccants have been used on a commercial scale in manufacturing processes that required strict moisture control, Judkoff says. (They are also in the little sugar-packet-sized sachets — clearly marked "DO NOT EAT" — found in the packaging for many consumer electronics.)
In building a prototype, the NREL researchers used a calcium chloride salt solution as a desiccant and relied on a technical breakthrough from a company that had found a way to keep tiny, corrosive desiccant droplets from leaking into the metal ductwork of the device.
They turned to another vendor for a high-tech membrane that prevents liquid water from crossing from one side to the other but allows water vapor to freely move through it. The desiccant solution is contained on one side of the membrane, but as air is drawn through from the other side, it is cooled through evaporation and dried as the desiccant absorbs water, Judkoff says.
"We've designed all that into a single core, in which the drying and the cooling are accomplished sort of instantaneously as the air passes through," Judkoff says. "We got rid of all the disadvantages of evaporative cooling, but we kept all the advantages — evaporative cooling is a very efficient form of cooling."
In modeling how the DEVap system would perform in Phoenix, "we get on the order of 90 percent savings" in electrical demand, Judkoff says, when compared with a high-efficiency 18 SEER vapor compression air conditioner.
The system sees a 50 percent power savings even in swampy Houston-like conditions, Judkoff says.
While others have proposed somehow combining evaporative and desiccant cooling, Judkoff says NREL is the first to come up with a practical, cost-effective approach. "We've proved it out thermodynamically in some testing and have now written some more careful models to see how it actually behaves in some typical buildings and climates," he says.
One caveat is that DEVap air conditioners require a low-temperature heat source (in the range of 160 to 180 degrees Fahrenheit) to warm the desiccant so that it releases the water it has absorbed. Judkoff says that could be accomplished through solar heating in some configurations, reducing power requirements even more.
In addition to reducing the load on the electrical grid, which should translate into lower carbon emissions, DEVap cooling eliminates the need for ozone-depleting CFC and HCFC refrigerants. Judkoff notes these compounds are actually worse greenhouse gases than carbon dioxide. For each unit of mass, "they can be 10,000 times more debilitating to the atmosphere," he says.
It could be two to three years before DEVap coolers become commercially available, Judkoff says. "We've gotten calls from a lot of major players out there," he says. "The size of the potential market is very large and the size of the potential energy savings worldwide is very large."