NREL Develops Promising Cooling Technology

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NREL senior engineer Eric Kozubal examines a prototype air flow channel of the DEVap air conditioner, which he co-invented. DEVap, which stands for desiccant-enhanced evaporative air conditioner, is a novel concept that uses membrane technology to combine the efficiency of evaporative cooling and the drying potential of liquid desiccant salt solutions. The graph superimposed on the photo shows shows how hot humid air, in red, changes to cool dry air, in blue, as the air passes through the DEVap core.

The U.S. Department of Energy's National Renewable Energy Laboratory (NREL) says it has developed 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. It combines membranes, evaporative cooling and liquid desiccants in a way that has never been done before in the centuries-old science of removing heat from the air.

Efficiency gains and CO2 reduced

"The idea is to revolutionize cooling, while removing millions of metric tons of carbon from the air," NREL mechanical engineer Eric Kozubal, co-inventor of the Desiccant-Enhanced eVaporative air conditioner (DEVap), said.   "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."  While the concept of combining them is not a new idea, doing it in a practical and cost-effice way is.

Improving on evaporative cooling

Denver based company Coolerado uses the thermodynamic cycle called the M-Cyle (Maisotsenko), brings in fresh air, filters it and then uses their Heat and Mass Exchanger (HMX) which consists of several plates of a special plastic that is designed to wick water evenly on one side and transfer heat through the other side.  It differs from a typical evaporative cooler by never increasing the moisture content of the supply air. It provides cool air through indirect, or closed circuit evaporative cooling.  The 5 ton H80 hybrid commercial rooftop air-conditioning unit was the first winner of the University of California-Davis Western Cooling Challenge in August 2009.

"It's a big improvement on evaporative cooling because it doesn't add moisture and still gives you cold air," Kozubal said. However, in a humid climate, it still does not provide cold air or humidity control.  The DEVap solves that problem. It relies on the desiccants' capacity to create dry air using heat and evaporative coolers' capacity to take dry air and make cold air.

How it is achieved

Previously desiccant cooling was impaired by the lack of a device simple enough for easy installation and maintenance .  To solve that problem, the NREL device uses thin membranes that simplify the process of integrating air flow, desiccants, and evaporative cooling. These result in an air conditioning system that provides superior comfort and humidity control. The membranes in the DEVap A/C are hydrophobic, which means water tends to bead up rather than soak through the membranes.

"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.

Helping the environment

The DEVap uses 50 percent to 90 percent less energy than typical air conditioning by using salt solutions rather than refrigerants, thus eliminating harmful chlorofluorocarbons (CFCs) or hydrochlorofluorocarbons (HCFCs). A pound of CFC or HCFC in refrigerant-based system contributes as much to global warming as 2,000 pounds of carbon dioxide.

The future

With the DEVap concept patented by NREL 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.

Eventually, NREL will 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."

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This illustration shows how the DEVap cooling core uses water and liquid desiccant to draw in outside air, exhaust some of that air and return cool, dry air to the area being cooled. DEVap's integrated evaporative component and its desiccant drying process offer improved dehumidification and, thus lower costs and much lower energy usage.