What it is:
As Greg Allen, sustainable design strategist at HOK, Toronto, explains, night-sky cooling systems are almost the inverse of solar collectors. "Instead of collecting sunshine, they're losing heat back out to the outer atmosphere and beyond."
It's a technology that takes advantage of low night-sky temperatures to cool a building. "It's a nice tie-in between thermal energy storage and a chilled beam system. It's like the perfect marriage of those things," says David Callan, senior vice president, director of sustainable design and high-performance building technology, Syska Hennessy Group, Chicago. (See Chilled Beam System and Thermal Energy Storage)
How it works:
As Ted van der Linden, director of sustainability for Redwood City, CA-based DPR Construction Inc., explains, a night-sky cooling system is essentially a rooftop sprinkler system. "Water is sprayed over a flat or sloping metal panel roof at night. Cooled by the night's temperature, the water is funneled through roof gutters and rainwater leaders, and then stored in a thermal storage tank to be used by the building's radiant cooling system, with tubing located in the floor slabs and ceiling panels."
By cooling the roof, you cool the entire structure, explains Erik Ring, director of MEP services at Irvine, CA-based LPA Inc. "It's possible to store that water for use the next day to cool the building. By spraying water on the roof at night, you're able to take advantage of the cold sky and let both the building and the sprayed water cool down in advance of the next day's cooling needs."
"Essentially, the mechanism is pretty simple. You spray the water on the roof, it exchanges energy with the night sky, you store it, and then it's available for use during the day," says Rob Bolin, senior vice president, Syska Hennessy Group, Chicago.
These systems don't require the substantial infrastructure of larger HVAC systems (ductwork, chillers, etc.).
Costs are "negligible" when compared to traditional systems, including the necessary radiant tubing or panels, according to van der Linden.
Virtually no energy is needed to run night-sky cooling systems.
Night-sky cooling relies heavily on the climate. High moisture content in the air, or cloudy weather, decreases system performance and reliability.
"Water is still, in terms of the cost, competitive with a gallon of gasoline," says Bolin. Translation: Water conservation is an issue (although the sprayed water can be harvested and reused).
Water quality can pose setbacks. "Building owners have a difficult enough time keeping scaling and calcium deposits out of their cooling towers. Turning your entire roof into a big cooling tower presents challenges; at some level, you may not really want to do water treatment on the water, or certainly not chemical water treatment, if you're spraying it all over your roof," says Ring.
The roof needs to be properly designed and constructed, says Richard Williams, VP, architecture and sustainability, at HOK, Toronto. "If it's designed properly, the amount of water you put on it is nothing more than what a storm would leave."
Regulating temperature on demand is difficult, says van der Linden.
Night-sky cooling systems probably can't provide 100 percent of cooling needs. "The building has to have a really low load," says Callan.
Where it works:
Most experts agree that areas with nighttime temperatures ranging from 45 to 55 degrees F. are the best contenders. "It would work best in an area with cold, clear nights—a dry, desert-type climate. If nighttime lows are 80 degrees F., a system like this is not going to be effective," says Ring. "And, certainly, you wouldn't be able to use the system if there's any risk of a freeze." Buildings in climates with periods of dry and/or humid weather could still use these systems, but they would only be available for a portion of the cooling, explains Allen. "You'd have to have auxiliary cooling capacity in those climates."
Your chance of success with this system increases if you have minimal levels of pollution, lots of roof area, and a small building with a small load. "A night-sky radiant cooling system can work well in buildings that have less demand for cooling systems, like corporate offices, non-traditional lab spaces, and congregation spaces (conference areas, cafeterias, large open-plan office environments, etc.)," says van der Linden. Allen indicates that malls, industrial buildings, and schools also make good candidates, considering their likelihood of having high roof-to-floor-area ratios.