Engineers at the National Renewable Energy Laboratory, Golden, Colo., recently provided an update on their progress with a combined evaporative-desiccant cooling device that could be far more efficient than ordinary A/C heat pumps. The NREL device, dubbed EVap, could end up using 50 to 90% less energy than conventional units when commercialized. NREL says it is working with commercial HVAC companies in this regard and expects to see the first commercial units using the technology come to market by 2015.
The new air conditioner can also be powered by a variety of sources that include natural gas or solar power. While evaporative cooling reduces the temperature of air flowing through the device, desiccants -- in this case, a lithium chloride liquid -- removes the humidity. The result: cold and dry air.
Of course, evaporative coolers have been around for a long time. They don't work well in humid settings where it is difficult to get much evaporation. That's where the desiccant comes in. According to Eric Kozubal, an NREL senior engineer and coinventor of the EVap air conditioner, the device might exhibit a COP (coefficient of performance) of 1.5 on a a humid day calculated with total source energy. This compares with a COP of about 1.2 for a standard A/C unit. But on hot dry day, the EVap might exhibit a source COP of 10 or 20. "Basically you are only using energy for fans in that case," says Kozubal. "Aggregated over a year, in a humid climate like that of Houston, we estimate the energy savings to be 40% over that of a high efficiency A/C; in Phoenix where it is dryer, energy savings is more like 80 to 85% over that of a high-efficiency A/C unit with a SEER of about 16," he says.
Key to the EVap's operation is a special membrane material that only became commercially available in the past year. The polypropylene microporus membrane looks a lot like an ordinary trash bag, says Kozubal. But under a microscope it looks more like a net, with about 70% open area, than a thin sheet of plastic. Its pores are about 0.1 micron in diameter. "It acts like a filter. You could almost breath through it. The surface tension of water does not allow it to pass through the small pores. It would take over 80 psi of pressure to push water through its pores. But we work at three or four orders of magnitude less pressure then that," Kozubal explains.
Precious attempts at combining desiccants and evaporative cooling have been thwarted by an inability to keep the hot and cold air streams separate from the flow of the liquid desiccant. "People have tried it but the water has mixed with the desiccant or the desiccant gets entrained in the air stream," says Kozubal. The new membrane material avoids such difficulties.
In the device, incoming air is dried as water vapor passes through the membrane separating the incoming air flow from the desiccant. Incoming air also cools as its heat is drawn through the membrane into a flow of coolant, most likely water. The coolant, in turn, gives its heat up to a stream of exhaust air -- actually cooled, dry air diverted from the unit's output.
Last year, NREL completed testing of two lab-scale prototypes built on breadboard-like platforms. But these are a far cry from anything that can be installed in a house, Kozubal says.
National Renewable Energy Laboratory's most recent report on the EVap was in April: http://www.nrel.gov/docs/fy12osti/54755.pdf