The comfort of air conditioning on a sweltering summer day often comes at the cost of steep energy bills. A new air conditioning process needing 50 to 90 percent less energy than today's top-notch units, however, could offer a cool new solution.
The new A/C system melds membranes, evaporative cooling and so-called liquid desiccants in a way that has never been done before in the centuries-old science of removing heat from the air. As a bonus, the process also cuts climate-warming carbon emissions by requiring less energy.
"The idea is to revolutionize cooling, while removing millions of metric tons of carbon from the air," said Eric Kozubal, a mechanical engineer at the National Renewable Energy Laboratory (NREL) and co-inventor of the Desiccant-Enhanced eVaporative air conditioner (DEVap).
From hot and humid to cool and dry
For most of the country, refrigeration-based air conditioning is the preferred way of keeping cool. A lower-cost alternative is evaporative coolers that work well in dry climates that do not get too hot or humid – Denver versus Miami, say. In evaporative coolers, water flows over a mesh and a fan blows air through the wet mesh to create humid, cool air.
But in humid climes, adding this water to the air creates a hot and sticky building environment. Furthermore, the air cannot absorb enough water to become cold.
The DEVap solves this problem. It relies on materials called desiccants to create dry air using heat and evaporative coolers 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."
Dedicated desiccants
Most people know of desiccants as the pebble-sized handfuls that are included with new shoes to keep them dry.
The kind 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 rather than commercial and residential cooling markets, where easy installation and maintenance is desired.
To get around this issue, DEVap uses thin membranes that simplify the process of integrating air flow, desiccants, and evaporative cooling. The membranes are hydrophobic, which means water tends to bead up rather than soak through the membranes (imagine rain falling on a freshly waxed car).
That property allows the membranes to control the liquid flows within the DEVap cooling core and "keeps the water and the desiccant separated from the air stream," Kozubal said.
The air is cooled and dried from a hot-humid condition to a cold and dry condition all in one step. "The desiccant and evaporative cooling effect work together to create cold-dry air," said Kozubal.
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.
Cool green
Traditional air conditioners use a lot of electricity to run the refrigeration cycle, but DEVap replaces that 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.
This means that DEVap could become the most energy efficient way to cool your house whether you live in Phoenix, New York, or Houston.
Because DEVap uses salt solutions rather than refrigerants, there are no harmful chlorofluorocarbons (CFCs) or hydrochlorofluorocarbons (HCFCs) to worry about. A pound of CFC or HCFC in refrigerant-based A/Cs contributes as much to global warming as a ton of carbon dioxide.
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, as well as 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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