Breakthrough in passive mirror cooling can save 15% of the energy used by buildings in the USA

Stanford engineers have invented a revolutionary coating material that can help cool buildings, even on sunny days, by radiating heat away from the buildings and sending it directly into space.

Previous systems could not also reflect sunlight, so they didn’t cool off during the day. Covering an entire roof with the material should eliminate the need for air conditioning. The group plans to leverage manufacturing technology that’s used to make coated windows, and it might be possible to make the material, which is only about two micrometers thick, on lightweight plastic films for easier installation. But the next step is a modest one: they’ll to go from the eight-inch demo to a square-meter tile of the material. They made the material by layering thin films of alternating layers of silicon dioxide (glass) and hafnium oxide deposited on an eight-inch silicon wafer.

Invisible light in the form of infrared radiation is one of the ways that all objects and living things throw off heat. When we stand in front of a closed oven without touching it, the heat we feel is infrared light. This invisible, heat-bearing light is what the Stanford invention shunts away from buildings and sends into space. Fan’s material becomes cooler than its surroundings by reflecting light and emitting heat at carefully tuned frequencies. The material emits heat at frequencies that match the planet’s “thermal window”—from eight to 13 micrometers—which lets it pass through the atmosphere and into space. It effectively cools down by using outer space as a heat sink.

Of course, sunshine also warms buildings. The new material, in addition to dealing with infrared light, is also a stunningly efficient mirror that reflects virtually all of the incoming sunlight that strikes it.

The result is what the Stanford team calls photonic radiative cooling – a one-two punch that offloads infrared heat from within a building while also reflecting the sunlight that would otherwise warm it up. The result is cooler buildings that require less air conditioning.

“This is very novel and an extraordinarily simple idea,” said Eli Yablonovitch, a professor of engineering at the University of California, Berkeley, and a pioneer of photonics who directs the Center for Energy Efficient Electronics Science. “As a result of professor Fan’s work, we can now [use radiative cooling], not only at night but counter-intuitively in the daytime as well.”

Stanford Professor Shanhui Fan, center, gazes into the pizza- size prototype with co-authors Linxiao Zhu, left, and Aaswath Raman, right. The high-tech mirror reflecting their faces beams heat directly into space. (Photo: Norbert von der Groeben)

Nature – Passive radiative cooling below ambient air temperature under direct sunlight

The researchers say they designed the material to be cost-effective for large-scale deployment on building rooftops. Though it’s still a young technology, they believe it could one day reduce demand for electricity. As much as 15 percent of the energy used in buildings in the United States is spent powering air conditioning systems.

In practice the researchers think the coating might be sprayed on a more solid material to make it suitable for withstanding the elements.

Stanford engineers have invented a material designed to help cool buildings. The material reflects incoming sunlight and sends heat from inside the structure directly into space as infrared radiation – represented by reddish rays. (Illustration: Nicolle R. Fuller, Sayo-Art LLC)

After commercial and residential buildings have this technology it might be scaled as part of a geoengineering option. This system would have a larger effect than previous studies of white roofs Besides the reflective effect there is also the reduced air conditioning energy demand. There is 30 billion square feet of commercial roof space in the United States. There was debate about the overall climate effects of white roofs which were 80% reflective instead of darker roofs that reflect only 15%.

A climate model revealed that increasing albedo by 0.1 only in GRUMP-designated urban areas would produce long-term cooling of 0.07 °C, equivalent to 130–150 billion tonnes of carbon. Using the MODIS data for urban areas, in contrast, would cool the Earth by 0.01 °C, equivalent to 25–30 billion tonnes of carbon. According to Akbari, albedo increases could lead to air-conditioning savings of about 20% for space under roofs. “This is a saving of about $50bn per year and carbon-dioxide savings of about 0.4 billion tonnes per year; over the next 100 years; that is an emission reduction of 40 billion tonnes, “he says. “The direct cooling of the Earth by reflecting radiation back into space is an added bonus that actually counters global warming while putting dollars in our pocket.”

Environmental Research Letters -The long-term effect of increasing the albedo of urban areas


Cooling is a significant end-use of energy globally and a major driver of peak electricity demand. Air conditioning, for example, accounts for nearly fifteen per cent of the primary energy used by buildings in the United States. A passive cooling strategy that cools without any electricity input could therefore have a significant impact on global energy consumption. To achieve cooling one needs to be able to reach and maintain a temperature below that of the ambient air. At night, passive cooling below ambient air temperature has been demonstrated using a technique known as radiative cooling, in which a device exposed to the sky is used to radiate heat to outer space through a transparency window in the atmosphere between 8 and 13 micrometres. Peak cooling demand, however, occurs during the daytime. Daytime radiative cooling to a temperature below ambient of a surface under direct sunlight has not been achieved because sky access during the day results in heating of the radiative cooler by the Sun. Here, we experimentally demonstrate radiative cooling to nearly 5 degrees Celsius below the ambient air temperature under direct sunlight. Using a thermal photonic approach we introduce an integrated photonic solar reflector and thermal emitter consisting of seven layers of HfO2 and SiO2 that reflects 97 per cent of incident sunlight while emitting strongly and selectively in the atmospheric transparency window. When exposed to direct sunlight exceeding 850 watts per square metre on a rooftop, the photonic radiative cooler cools to 4.9 degrees Celsius below ambient air temperature, and has a cooling power of 40.1 watts per square metre at ambient air temperature. These results demonstrate that a tailored, photonic approach can fundamentally enable new technological possibilities for energy efficiency. Further, the cold darkness of the Universe can be used as a renewable thermodynamic resource, even during the hottest hours of the day.

SOURCES – Stanford University, Nature, MIT Technology Review