Other national labs were tasked to try to improve on the elephant in the room — the vapor condensing air conditioner first designed by Willis H. Carrier in 1909. That left Judkoff and his NREL colleagues free to look at alternatives to the dominant approach. A key in combining desiccant drying with evaporative cooling was finding a way to separate the desiccant from the air.
Eric Kozubal, now NREL’s principal investigator on the DEVAP cooling system, found a piece of the puzzle in a membrane that mimicked the properties of the semi-porous clay. The holes are so tiny that they’re referred to as micropores. The membrane allows the desiccant to pull moisture out of the air through the membrane while preventing any desiccant from coming in direct contact with the air.
A DEVAP air conditioner would typically have a heat and mass exchanger that has hundreds if not thousands of air passages, each lined with microporous membrane. A mixture of fresh air and building return air flows through these passages and water vapor gets absorbed into desiccant flowing behind the membrane. Because this water vapor travels through the membrane, it is imperative that it have sufficient permeability. Simultaneously, adjacent air passages are in thermal contact with the flowing desiccant. These air passages are wetted with water and a working air stream flows to evaporate this water film, and thus remove the heat of absorption from the desiccant. This method of integrating indirect evaporative cooling creates a very efficient method of dehumidifying the air.
Eric Kozubal said, “Essentially, we were able to design a heat and mass exchanger with four fluid streams coming into thermal and mass transfer contact. We did this in a manner such that none of these streams became mixed with another.” This was no simple task, and it was the ability to use membranes to contain the liquid desiccant that enabled such a design.
“It wasn’t until advances in membrane technology and careful thermodynamic modeling and design that Eric was able to come up with a method to cheaply and efficiently build such an air conditioner,” Judkoff said. Without the membranes, there is a ticklish problem called droplet carryover, in which some of the corrosive desiccant gets entrained in the air. That air gets into the duct work and corrodes it. It also can corrode metal fan blades, and in rare cases, structural steel.
Once the air is sufficiently dried out, clever indirect evaporative heat exchanger design allows it to be cooled down enough to cool a building. What comes out is air as dry and cool as the air in Colorado on a nice fall day. NREL enlisted two companies, AIL Research and Synapse, as partners to build prototypes. The final device incorporated ideas from each.
“We knew we couldn’t just slap on any indirect evaporative cooler off the shelf,” Kozubal said. “We needed an evaporative cooler that could reduce temperature below the wet-bulb temperature, minimize water usage and purge air.” And they needed to maintain a size and weight for the entire DEVAP package similar to conventional roof-top air conditioners.
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