Cool strategies for cooling buildings
Cool strategies for cooling buildings
Researchers at the National Renewable Energy Laboratory credit ancient architecture and developing world cooling strategies for their outside-the-box thinking that led to an air cooler that just might revolutionize air conditioning. NREL’s Desiccant Enhanced Evaporative, or DEVAP, system won an R&D 100 Award this year.
The idea was born when Ron Judkoff was a young Peace Corps volunteer in Kedougou, Senegal, one of the warmest places on Earth. “That’s where I really saw the effectiveness of evaporative cooling,” said Judkoff, principal program manager for building technologies at NREL.
“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 as the high-tech membranes operate in NREL’s DEVAP system. DEVAP works in any climate and achieves comfortable cooling while saving 40 to 80 percent of the energy use of a conventional air conditioning system.
During his Peace Corps tenure, Judkoff also noted how indigenous people in Saharan and sub-Saharan climes would effectively cool their buildings with the clever use of spray from fountains and transpiration from plants. He went on to study architecture under James Marston Fitch, a pioneer in bio-climatic architecture at Columbia University, 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 he 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.
“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. 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 the world where the air is too humid.
Next page: Improving on the elephant in the room
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.
To do this Kozubal developed a counterflow indirect design wherein a small amount of supply air is bled off and fed through evaporative channels adjacent to the supply air channels. In this way the supply air is cooled via conduction to the evaporative channels, without adding any moisture to the supply air.
To complete the cycle, the liquid desiccant must be regenerated to remove the water it absorbed. To accomplish this, heat is added to the desiccant to raise the absorbed water’s vapor pressure. Blow air past this hot desiccant and water vapor is carried away, which is done in another specially designed heat and mass exchanger.
“This air conditioner works by adding heat,” Eric said, “We can use natural gas, solar heat or waste heat from many industrial processes to drive an air conditioner.”
“I can foresee a time when this approach replaces most air conditioning in the world,” said Judkoff.
Kozubal points out that aside from large energy savings, DEVAP has several other advantages over conventional cooling including:
- No need for environmentally damaging working fluids used in vapor compression systems.
- The working fluids in DEVAP are environmentally benign — water and a strong salt solution for the desiccant.
- DEVAP allows independent control of temperature and humidity, something that is not possible with conventional air conditioning unless an expensive overcooling and re-heating process is employed.
- No need for a compressor, and large amounts of expensive copper coils.
- DEVAP contains fewer moving parts in the form of simple low pressure pumps and fans.
- As efficient as DEVAP already is, there is lots of “thermodynamic room” for cost effective efficiency improvements.
A typical direct-exchange air conditioner cools the air and dehumidifies it all at once, but not in a controlled way. The limit to how much drying can be achieved is dependent on how much water condenses on the evaporator coils as the air passes through.
The wet-bulb limit is the reason typical evaporative coolers either can’t cool things down enough or can’t create a truly comfortable space when there is a lot of heat and humidity in the air.
By contrast, DEVAP can provide cooling in any climate. The first stage wrings out all the moisture in the air. In doing so, it lowers the effective temperature limit by which the indirect evaporative cooler can achieve. It has a wet-bulb effectiveness of 125 percent — a huge boon compared to most current technology that has tried to get as close as it can to 100 percent.
The other huge advantage of DEVAP is that it is an energy miser. A direct-exchange air-conditioner uses 25 percent of its energy removing humidity and 75 percent dropping the temperature. By contrast, DEVAP only uses energy for that first step, removing humidity. The second step is achieved simply by adding a little water.
“As a cooling process, evaporative cooling is incredibly efficient,” Judkoff said. “The fact that we have to put a little energy into drying the air is more than made up for out of the efficiency of evaporative cooling. Especially compared to typical A/C where you have to use electricity to compress the working fluid.
“Developing a brand new thermodynamic cycle and an apparatus to accomplish this process was difficult.” Kozubal said. Conceptualizing this new device required a lot of ingenuity and breakthroughs in materials to become a reality. It took a lot of pondering. I spent a lot of time looking up at the ceiling.”
Originally published in Innovation: America's Journal of Technology Commercialization. Reprinted with permission.
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