Cheaper Chilling

June 21, 2010

Summer is now officially with us, and here in the metro Washington DC area, it’s been here unofficially for a while.  We are supposed to be getting some of our 95² days later this week: 95° F, 95% relative humidity.  Going outside from an air-conditioned building feels like being swallowed.  Legend has it that at one time, presumably before air conditioning, British diplomats in Washington got extra pay for a tropical assignment.

So we are pretty glad to have air conditioning around here, but it can get expensive.  However, a new air conditioning process developed by the National Renewable Energy Laboratory [NREL] may help lessen the heat on our bank accounts.   The process, described in a press release from the NREL, uses a combination of evaporative cooling to lower air temperature, and a desiccant solution to lower humidity, to produce conditioned air at a significant energy savings:

The U.S. Department of Energy’s National Renewable Energy Laboratory 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. It uses 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.

Evaporative cooling is based on the fact that the phase change of water from liquid to vapor requires a great deal of heat.  It is the principle that your body uses to cool you by sweating.  I have visited a number of places, in hot but very dry climates (for example, around Palm Springs CA), where evaporative cooling is used even  in outdoor spaces.  A fine mist of water is produced, and the cooling produced can make it quite pleasant, at least in the shade, even when the air temperature is 100° F.    Of course, it does not work terribly well in a humid climate like ours here.

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.

The NREL’s invention, called the Desiccant-Enhanced eVaporative air conditioner (DEVap), addresses the humidity problem by using a syrupy liquid desiccant, typically a concentrated solution of calcium or lithium chloride, to absorb water from the air.  The device also uses a special hydrophobic membrane to keep the water separated from the cooled air stream; the membrane has pores big enough to allow the passage of water vapor, but not of the liquid solution.    The water can be evaporated out of the desiccant solution by heat, possibly from solar energy.

In addition to its energy efficiency, the DEVap has the additional advantage of not requiring any CFC or HCFC refrigerant gases.  These are of environmental concern, because they are much more potent greenhouse gases than carbon dioxide.  (In terms of global warming impact, one pound of CFC refrigerant is approximately equivalent to 1 ton, or 2000 pounds, of carbon dioxide.)

The NREL is still working on improving the efficiency of its device, but it plans to license the technology for industrial use.

The Technology Review also has an article on this development.

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