Here comes emissions-free air-conditioning

As temperatures continue to rise, people worldwide will come to turn increasingly to air-conditioners to keep cool. Yet air-conditioners themselves are driving climate change in two main ways: 1) they require plenty of electricity, which is often obtained by burning fossil fuels, and 2) they rely on hydrofluorocarbons (HFCs), which are chemical compounds used as refrigerants that also function as greenhouse gases.

And so, paradoxically, the more the climate warms, the more we will need air-conditioners, but the more air-conditioner we use, the more the climate will warm. But what if we could collect not only sunrays to power devices but also coldness from outer space to help cool us down at the same time? It may sound like science fiction, but scientists at Stanford University in the United States have done just that.

Most of the heat that objects radiate in the form of infrared light is bounced back to earth by particles in the atmosphere, but some of it manages to escape into space. This process allows surfaces that emit larger amounts of infrared radiation to become cooler than objects in their surroundings.

The newly designed “radiative cooling” technology takes advantage of this process by reflecting large amounts of infrared light to cool its surrounding area. The result is an air-conditioning apparatus that does not emit greenhouse gases. Not only that, but the technology can be used simultaneously with sunray-harvesting photovoltaic cells on the same rooftop.

“It is widely recognized that the sun is a perfect heat source nature offers human beings on Earth,” says Zhen Chen, a Chinese scientist who is the first author of a study published in the journal Joule. “It is less widely recognized that nature also offers human beings outer space as a perfect heat sink.”

Chen and his colleagues have designed a device that combines radiative cooling with solar absorption technology. The double layers of the apparatus consist of a germanium solar absorber that is placed atop a radiative cooler with silicon nitride, silicon, and aluminum layers, which are enclosed in a vacuum to minimize unwanted heat loss. The device’s photovoltaic cell can supply solar energy to a home while its built-in radiative cooler can keep the indoors of a house cool whenever needed.

“Both the solar absorber and the atmosphere are transparent in the mid-infrared range of 8-13 microns, offering a channel for infrared radiation from the radiative cooler to pass through to outer space,” the researchers explain. “The team demonstrated that the combined device can simultaneously provide 24°C in solar heating and 29°C in radiative cooling, with the solar absorber improving the radiative cooler’s performance by blocking heat from the sun.”

The trick is now to scale the device up for commercial use and also make it affordable. The latter is a matter of concern because some of its parts such as its infrared-transparent window, which is made from zinc selenide, remains costly. “I think this technology could potentially revolutionize the current solar cell technology,” Chen stresses. “If our concept is demonstrated and scaled up, the future solar cell will have two functions in one: electricity and cooling.”