Engineers have designed a system that can help cool buildings in crowded metropolitan areas without consuming electricity — an important innovation at a time when cities are working to adapt to climate change.
The system consists of a special material — an inexpensive polymer/aluminum film — installed in a box at the bottom of a specially designed solar “shelter.” The film helps keep its surroundings cool by absorbing heat from the air inside the box and transmitting the energy through the Earth’s atmosphere into outer space. The shelter serves a dual purpose: helping to block incoming sunlight while also beaming thermal radiation emitted from the film into the sky. The polymer stays cool as it dissipates heat through thermal radiation and can then cool down the environment. This radiative or passive cooling does not consume electricity or battery power.
The system purposefully directs thermal emissions toward the sky. Normally, thermal emissions travel in all directions; the new system beams the emissions in a narrow direction. This enables the system to be more effective in urban environments where there are tall buildings on all sides. The shelter-and-box system measures 18 × 10 × 10". To cool a building, numerous units of the system would need to be installed to cover a roof.
The new passive cooling system addresses an important problem: How radiative cooling can work during the day and in crowded urban areas. When placed outside during the day, the heat-emanating film and solar shelter helped reduce the temperature of a small, enclosed space by a maximum of about 6 °C; at night, that figure rose to about 11 °C.
The radiative cooling system incorporates a number of optically interesting design features. One of the central components is the polymer/metal film made from a sheet of aluminum coated with a clear polymer called polydimethylsiloxane. The aluminum reflects sunlight, while the polymer absorbs and dissipates heat from the surrounding air.
Engineers placed the material at the bottom of a foam box and erected the solar shelter atop the box using a solar energy-absorbing material to construct four outward-slanting walls along with an inverted square cone within those walls. This architecture serves a dual purpose: First, it helps to soak up sunlight; second, the shape of the walls and cone direct heat emitted by the film toward the sky.