Amidst last week’s savage heat wave in much of North America, brownouts and even deaths were the norm. As temperatures rose–here in New York City, the heat index reached 112 degrees Fahrenheit on Friday; utter misery–people responded reasonably by any standard. They cranked up the AC. For most Americans, living in a hot place means having constant, almost uninterrupted access to air-conditioned spaces. Ever since Willis Haviland Carrier built the first AC unit in 1902, places long renowned for miserable summer conditions have become hospitable to more and more human beings year round as buildings and transportation were equipped with mechanical climate control. But even though air conditioning made comfortable living possible in even the hottest climates, it comes at a cost. AC units have become more efficient over the years, but energy consumption during hot summer months can increase significantly, boosting both the amount of money spent on electricity and the volume of greenhouse gasses emitted in the energy production process. Sometimes, as happened in New York last week, energy consumption outstrips supply. That’s when power failures begin.
Air conditioning efficiency improvements are always possible, but in the dry American West, scientists and engineers have a wider range of technology available. Researchers at the University of California Davis Western Cooling Efficiency Center used evaporative cooling to make drastic reductions in cooling energy consumption. Unlike standard vapor compression cooling, a process which uses a high load motor-driven compressor to compress gaseous refrigerants, evaporative cooling uses evaporating water and a fan to cool air. The Center enlisted the help of a number of AC unit manufacturers for an ongoing commercial cooling project called the Western Cooling Challenge, with each company approaching efficiency upgrades in a different way. Field tests are performed on campus at UC Davis, and at Target and Walmart stores in Davis. Program director Jonathan Woolley said that the challenge has brought forth a few drastically more efficient designs, but it also encourages commercialization of technologies to stimulate large scale manufacturing and lower production costs.
“In many respects, the Western Cooling Challenge doesn’t go far enough and call for better building design,” said Woolley, pointing out that because it uses water in the cooling process, evaporative cooling is dependent upon a reliable water supply in order to work.
According to U.S. Department of Energy statistics, 40 percent of U.S. energy is used for buildings. In both commercial and residential buildings, 11 percent of that energy goes toward cooling. Nearly 80 percent of the country’s commercial buildings are cooled by self-contained rooftop cooling units, known in HVAC circles as RTUs. Unlike the super efficient building design-incorporated air conditioning systems often used in very large structures, RTUs are the industrial equivalent of a window-mounted air conditioner–which is to say, not terribly efficient in most cases. The Western Cooling Efficiency Center partnered with Walmart and Target to test a few of the new designs, which include a number of hybrid vapor compression/evaporative RTUs. While the specific approach differs from manufacturer to manufacturer, the basic idea is that the evaporative part reduces the load placed on the vapor compression part’s motor, increasing its coefficient of performance, or COP. One design uses a nozzle to spray mist on the compression system’s hot condenser coils. Another uses a more conventional setup–a fan draws air through a pad continuously bathed in water–to lower the temperature of the air used to cool the compression cooler’s condenser.
Innovations made by Western Cooling Challenge participants have resulted in drastic efficiency increases. The Center reported a 38 percent COP increase at a Davis Target store participating in the study. Woolley said that the challenge’s goal is to encourage boosts in efficiency similar to or greater than that, rather than the incremental increases required by federal regulation. But although the new technology is a vast improvement over existing RTUs, it’s not without its faults. For starters, evaporative coolers don’t work in hot, humid climates (because the air is already saturated and water can’t evaporate), and their COP decreases with each degree the temperature goes up. Also, because they’re not widely produced, the most efficient efficient designs can be expensive to build.
Evaporative coolers also use water, and in the West, where large scale water supply faces an uncertain future, water use is definitely an issue. Currently, water costs 60 cents per cubic meter in Davis, so water consumption eats into less than ten percent of the energy savings brought on by new hybrid RTUs. The price of Davis’ water is scheduled to increase fourfold sometime next year (to accommodate expensive infrastructure upgrades), but the new technology’s economic benefit will still be high enough to justify its use. In Los Angeles, water delivery is energy intensive. Roughly a third of the city’s water comes from the State Water Project, a massive water delivery system that accounts for about 20 percent of the water consumed in California (that’s what happens when you pump millions of gallons of water over mountain range every day). Changing water allocation agreements means that L.A. may have to rely more upon desalination for drinking water in the future, further increasing its cost. Woolley said it might just be cheaper to use standard, albeit less efficient, vapor compression systems in Los Angeles, or to place more emphasis on better building design. But he also pointed out that water is consumed during electric power production as well–water evaporates from hydroelectric reservoirs and escapes as steam from the cooling units of coal, natural gas, nuclear, and other heat-based plants.
Regardless of efficiency upgrades acheived by new technologies, they won’t likely amount to much if they can’t be commercialized. If COP numbers vary too much from region to region, low demand will quash hybrid RTU manufacturing. “We recently took a stab at quantifying these relationships in a paper on water use efficiency of evaporative coolers,” said Woolley in reference to a study co-authored by Theresa Pistochini and Western Cooling Efficiency Center Director Mark Modera. He said the Center is also investigating regional climate characteristics to find out which areas, other than Davis, can benefit from evaporative cooling. “I don’t think this is the answer, but it’s a start.”