Attendees at the recent ARPA-E Energy Innovation Summit, co-hosted by the Dept. of Energy's Advanced Research Projects Agency - Energy (ARPA-E) and the Clean Technology and Sustainable Industries Organization got a look at water-based air conditioning/refrigeration developed by Dais Analytic Corp. that uses neither refrigerant gas nor a conventional compressor. Last year, Dais got an ARPA-E grant to develop and bring the process up to commercial scale.

Called NanoAir, the system is built around a special membrane material that is permeable to water but not air. Air conditioners built with this membrane material, referred to as a high-charge-density electrolyte material, separate dehumidification and cooling into two separate steps.

"Our membrane can discriminate between moisture and the constituents of air such as oxygen, nitrogen, and argon," explains Dais Analytic Corp. founder and CTO Scott Ehrenberg. "This allows us to devise a dehumidification system where we draw a vacuum behind the membrane and flow air on the opposite side. To moisture, the membrane looks like a big open door so moisture expands into the space we've created behind the membrane. There is no net heat transfer through the membrane other than the mass of the moisture itself."

To cool air, the system first places water against the membrane. As the water transits through the membrane it becomes a gas. This process pulls off the water's heat of vaporization to produce cold water. The moisture that comes from the dehumidification step and the water that comes from the water-cooling step get sent to a vapor pump which pushes it to the outside the building to a third set of membranes for disposal, all at low pressure.

This process can be much more efficient than a conventional refrigeration cycle, Ehrenberg says. "Our overall efficiency is so high as to be unbelievable. What we like to say is that we use 50% less energy than what's required for an equivalent amount of conventional cooling and dehumidification."

Such efficiencies arise partly because conventional air conditioners must set the temperature of their cooling coils to be lower than the dew point of the ambient air, typically between 45 and 50° F. But because the NanoAir system dehumidifies before it cools, it can be set to cool just to the desired room temperature rather than to a dew point. "Not having to go down to 45 or 50° is one of the major efficiencies, says Ehrenberg. "If you want a 75° interior you are probably operating at 68 to 69° on the basic water temperature that is cooling the air."

The special membrane that makes the NanoAir system possible is a a nano-structured, high-charge-density electrolyte polymer. Water transfers across the material when there is a concentration difference between the two faces of the membrane. For example, if the water vapor concentration on one side of the material is at a higher concentration than the other, the membrane allows the water to permeate across until the densities are equal.

"The discrimination mechanism for the membrane is two fold," explains Ehrenberg. "First, things have to be darned small to fit in among the material that is already there, typically in the single-digit nanometer range. Second, the membrane discriminates based on the electrical characteristics of the molecule in question. The material has no electrical charge but instead contains numerous pared charges. Molecules that make it through have a high dielectric constant or, in other words, have an uneven charge distribution. Water fits that bill because electrons tend to hang around the oxygen molecule and not so much around the hydrogen molecule. Water is also an electrostatic chameleon. Whenever it encounters a charge, it flips its opposite polarity side to that charge and is attracted. Other molecules such as those of oxygen that has a low dielectric constant cannot do that, hence they are not pushed through the membrane."

The discrimination factor between the molecules of water and air is quite high, says Ehrenberg, much greater than 10,000. That means for every molecule of air that gets through, 100,000 molecules of water would pass. This high discrimination factor makes it possible to pull a vacuum in the system to move water vapor.

"The mass flows of water necessary to cool and dehumidify are fairly small. so with low mass and low work per mass, and you have a very efficient process," Ehrenberg says. Moreover, the permeation of water does not appear to wear out the membrane. Ehrenberg says the company has been shipping a related product employing the membrane material for six years and has never had one returned because of failure. Accelerated life tests indicate the material should last beyond 20 years.

Physically, the NanoAir system consists of a small vacuum pump to depressurize the system, a specialized vapor pump to move the moisture from the gathering points to the exit point, and a transfer area: basically a series of flat plates with a membrane on either side through which air passes.This equipment takes up about as much space as a conventional A/C system with the same capacity.

But someone watching the NanoAir system work would likely be struck by one aspect of its operation in particular: "It's dead silent," says Ehrenberg. "We have air-moving devices of various sorts but they tend to be low noise."

Ehrenberg also says the technique can cool well enough to handle refrigeration. Refrigeration equipment based on the process should be no more expensive that conventional gear, he maintains. It can even work in reverse, acting as a humidifier or as a heater.

The NanoAir membranes also provide a way of capturing energy from air that is exhausted from buildings. The company makes a separate product called an enthalpy exchanger that basically puts air being exhausted from a building on one side of the membrane and air brought into the building on the other. The membrane tends to equilibrate the two. "This conditioning will get you 70% of the way back to your interior conditions," says Ehrenberg. "If the exterior is 100° and the interior is 70°, the system can bring temperature down to 79° . You get same percentage for moisture removal. Exhaust air becomes a sink for the air coming in so you exhaust it back out."

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