If thirst is crucial to knowledge, then one crucial step in the evolution of air conditioning was born in the 1970s, when Ron Judkoff was a hot, thirsty Peace Corp volunteer in Kedougou, Senegal, one of the warmest places on Earth.
“That’s where I really saw the effectiveness of evaporative cooling,” said Judkoff, director of Buildings and Thermal Systems at the U.S. Department of Energy’s National Renewable Energy Laboratory (NREL). Judkoff was talking about DEVAP, NREL’s Desiccant-Enhanced Evaporative (DEVAP) system that works in any climate and achieves comfortable cooling while saving 40% to 80% of the energy use of a conventional air conditioning (A/C) system.
“The Senegalese would make these clay pots to keep water in,” he recalled. “The pots didn’t feel wet on the outside, but they were semi-permeable. There was enough porousness in the clay that there was evaporation taking place. You could take a nice drink of cold water — and the water would stay cold in the pot.”
That semi-porous clay operated in a similar way to the high-tech membranes in NREL’s DEVAP system, which recently received a coveted R&D 100 award — often called the Oscars of Invention — from R&D Magazine.
Learning from Indigenous Peoples and the Ancients
During his Peace Corps tenure, Judkoff also noted how indigenous people in Saharan and sub-Saharan climates would effectively cool their buildings with clever use of the spray from fountains and transpiration from plants. He went on to study at Columbia University under James Marston Fitch, a pioneer in bio-climatic architecture, and gained a greater appreciation for the ways ancient peoples and modern indigenous people achieved cooling. They could even make ice in deserts using night sky radiation. They fabricated wind scoops to channel soothing natural ventilation into otherwise stifling buildings.
Later, Judkoff researched desiccants — materials with the capacity to dry out moist air, which are a must if air conditioning is to be comfortable in a hot, humid climate.
In the 1970s, “I had this notion that if we could only combine desiccant and evaporative cooling we might be able to come up with something really important,” Judkoff said. “But it was just a notion, because with the materials available at that time, the cost, the weight, the volume — it just didn’t look like it would pan out.”
Still, Judkoff never completely let go of the idea, and in his early days at NREL he oversaw the first full-scale leap into evaporative cooling at NREL’s second building, the Solar Energy Research Facility (SERF). In Colorado’s dry climate, evaporative cooling by itself can achieve comfortable indoor climates. But it doesn’t work in vast stretches of the United States and parts of the world where the air is too humid.
NREL Takes a Different Direction
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 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 mimicks the properties of the semi-porous clay. The holes are so tiny that they’re referred to as micro-pores. 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 typically has a heat and mass exchanger that has hundreds if not thousands of air passages, each lined with a micro-porous 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 the membrane 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 way to dehumidify the air.