GOLDEN, Colo. - The U.S. Department of Energy’s (DOE’s) National Renewable Energy Laboratory (NREL) announced that it has invented a new air conditioning process with the potential of using 50 percent to 90 percent less energy than today’s top-of-the-line units. The DEVap, which stands for desiccant-enhanced evaporative air conditioner, uses membrane technology to combine the efficiency of evaporative cooling and the drying potential of liquid desiccant salt solutions.
“The idea is to revolutionize cooling, while removing millions of metric tons of carbon from the air,” said NREL mechanical engineer Eric Kozubal, co-inventor of the DEVap. “We’d been working with membranes, evaporative coolers, and desiccants. We saw an opportunity to combine them into a single device for a product with unique capabilities.”
Evaporative coolers are a lower-cost alternative to a/c in dry climates that don’t get too hot or humid - say Denver, but not Phoenix or Miami. Water flows over a mesh, and a fan blows air through the wet mesh to create humid, cool air.
In humid climes, adding water to the air creates a hot and sticky building environment. Furthermore, the air cannot absorb enough water to become cold.
In Phoenix or Tucson, the evaporative cooler can bring down the temperature, but not enough to make it pleasant inside on a 100°F day or during the four to eight week moist period known as monsoon season. The cooling bumps up against the wet bulb temperature, the lowest temperature to which air can be cooled by evaporating without changing the pressure. The wet bulb temperature could be 75° or 80° on a mid-summer Tucson day. Typically, evaporative coolers only can bring the temperatures about 85 percent of the way to the wet bulb level.
So, for most of the country, refrigeration-based air conditioning is the preferred way of keeping cool.
The DEVap solves that problem by relying on the desiccants’ capacity to create dry air using heat and evaporative coolers’ capacity to take dry air and make cold air.
“By no means is the concept novel, the idea of combining the two,” Kozubal said. “But no one has been able to come up with a practical and cost-effective way to do it.”
The kind of desiccants that NREL uses are syrupy liquids - highly concentrated aqueous salt solutions of lithium chloride or calcium chloride. They have a high affinity for water vapor, and can thus create very dry air.
Because of the complexity of desiccant cooling systems, they have traditionally only been used in industrial drying processes. Inventing a device simple enough for easy installation and maintenance is what has impaired desiccant cooling from entering into commercial and residential cooling markets.
To solve that problem, the NREL device uses thin membranes that simplify the process of integrating airflow, desiccants, and evaporative cooling. These result in an air conditioning system that is said to provide superior comfort and humidity control.
The membranes in the DEVap air conditioner are hydrophobic, which means water tends to bead up rather than soak through the membranes. That property allows the membranes to control the liquid flows within the cooling core. “It’s that property that keeps the water and the desiccant separated from the air stream,” Kozubal said.
“We bring the water and liquid desiccant into DEVap’s heat-mass exchanger core,” Kozubal said. “The desiccant and evaporative cooling effect work together to create cold-dry air.”
The air is cooled and dried from a hot-humid condition to a cold and dry condition all in one step. This all happens in a fraction of a second as air flows through the DEVap air conditioner. The result is an air conditioner that controls both thermal and humidity loads.
Because the DEVap uses salt solutions rather than refrigerants, it eliminates greenhouse gas concerns. Also, traditional air conditioners use a significant amount of electricity to run the refrigeration cycle, but DEVap replaces the refrigeration cycle with an absorption cycle that is thermally activated. It can be powered by natural gas or solar energy and uses very little electricity.
NREL has patented the DEVap concept, and Kozubal expects that over the next couple of years he will be working on making the device smaller and simpler and perfecting the heat transfer to make DEVap more cost effective.
NREL said it will eventually license the technology to industry. “We’re never going to be in the air conditioner manufacturing business,” said Ron Judkoff, principle program manager for Building Energy Research at NREL. “But we’d like to work with manufacturers to bring DEVap to market and create a more efficient and environmentally benign air conditioning product.”
Publication date: 06/21/2010
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