Air Conditioning That's Out of This World
New materials enhance the cooling effects from outer space to reduce energy use in small- to medium-sized buildings
Reducing air conditioning energy use by using new materials to enhance the cooling effects from outer space may not be such a far-out idea. In a simulation study using DOE's Energy Plus software and new selective emittance material technology developed at Stanford University, PNNL researchers found that daytime radiative cooling—the physical process by which an object loses heat to another object of lower temperature—could reduce energy consumption of an office building by 30 to 50 percent.
All objects emit thermal radiation, simply known as heat. In the case of buildings, radiative cooling results from the heat transfer between building surfaces and the colder atmospheric layers up above. In their research paper, PNNL advanced building controls engineers Nick Fernandez, Srinivas Katipamula, and Weimin Wang (former PNNL) reported that radiative cooling was sufficient to meet the cooling load of a medium-sized building (25,000 to 100,000 square feet) for five months of the year. Findings also revealed that, thanks to its selective emittance material, the surface temperature of the rooftop-mounted radiator remained well below the ambient air temperature even on warm days in Las Vegas.
This research demonstrates the potential to design buildings that meet a substantial portion of their cooling needs by tapping into the "coldness of space" through the use of selective emittance materials in radiative cooling systems. As a result of this new material technology, building energy consumption is reduced. Most buildings in the United States today, which consume 40 percent of the country’s total annual energy use, do not have this type of cooling system.
TYPES OF RADIATION
Until now, it was thought that radiative cooling was ineffective during the day because the sun’s radiation offsets any cooling effect. However, Stanford University recently demonstrated the potential to achieve radiative cooling in the daytime by manipulating light at the nanoscale through “selective” (also known as photonic) emittance materials. These materials show great potential for daytime radiative cooling applications.
As Fernandez explains it, “Selective emittance materials are designed to release and absorb radiation only at beneficial wavelengths. They reflect a large portion of shortwave radiation from the sun, while increasing longwave radiative exchange with the relatively cool sky. This enables higher radiative heat transfer compared to a surface that radiates strongly at all infrared wavelengths.”
Basically, selective emittance materials allow radiative cooling to work during the day—not just at night—by holding onto the coolness of the sky while releasing the warmth from the sun. The result is a radiator surface cold enough that you can run water through it and chill the interior of a building.
For their study, Katipamula, Wang, and Fernandez simulated an outdoor temperature of 85 degrees (29 degrees Celsius) and a radiative cooling temperature of 60-65 (15-18 degrees Celsius) degrees to model the reduction in energy consumption in an office building. The PNNL team received a Best Paper award for their research. The award-winning paper, titled Modeling and Simulation of a Photonic Radiative Cooling System, was presented at the 2016 International Building Performance Simulation Association (IBPSA) Conference, known as “SimBuild.”