Anywhere heat becomes trapped—say in a large data center or even your house—it can pose threats to people and infrastructure. Too often, we respond by burning more fossil fuels to power cooling systems, which only adds more carbon to the atmosphere.
Indeed, the burning of fossil fuels is an unsustainable practice that must be stopped entirely if the world hopes to maintain no more than a global 1.5-degree temperature change, says United Nations (UN) Secretary General António Guterres. But inaction would mean that the current trajectory is just under three degrees of global warming, a level that equates to “mass human suffering,” according to UN Climate Chief Simon Stiell.
Now photonics researchers are exploring new solutions to the heat problem through radiative cooling—reflective surfaces, coatings, and other materials that use physics to passively remove excess heat from Earth by radiating it into space. Radiative cooling could also channel excess thermal energy and convert it into useful work that would drive motors or perform other tasks. Eventually, these insights might enable us to harvest thermal energy at night and convert it to electricity in much the same way sunlight can be converted to electricity by photovoltaics. Of the UN’s 17 Sustainable Development Goals, work on radiative cooling technologies would meet goal 7 that addresses affordable, reliable, and sustainable energy; goal 9 for industry, innovation, and infrastructure; and goal 13 for addressing climate action.
Jeremy Munday, a professor of electrical and computer engineering at University of California, Davis, says his approach has been to address global warming as a photonics problem. “You would like to be able to reduce the warming, so that we’re not continuing to have these elevated temperatures and increased natural disasters every year. We’d also like to be able to generate power. And so, we’re talking about optical technologies that can try to do both things.”
But first, the physics. Earth and the sun are blackbodies, meaning they are radiating heat out to space—especially the sun. Earth absorbs heat from the sun, and by re-reflecting it to space, a kind of equilibrium between the two bodies happens.
Except it doesn’t, or at least not as much these days.
“If you look at how much [heat] the Earth is absorbing,” Munday says, “it’s absorbing about one watt-per-meter-squared more than it’s emitting. That imbalance, which is due to heat trapped from greenhouse gases, is effectively why the Earth is warming.”
Delicate energy flux balance between Earth and sun. Earth is absorbing about 1 W per-meter-squared more than it is emitting. Photo credit: Jeremy Munday.
Various geo-engineering solutions have been put forth that would block or reduce the amount of sunlight reaching Earth, but their practicality and expense have been prohibitive.
What Munday proposes instead are reflective-emissive materials like various paints and polymers. “The idea is to make a material that is reflective to the sunlight, yet emissive to the wavelength range of 8–20 µm,” Munday wrote in a 2019 Joule article. “In doing so, no solar radiation is absorbed, and only radiative cooling occurs (during both day and night).”
He says such materials can be printed roll-to-roll at a cost of about 25 cents per square meter. With only 1–2% of the Earth’s surface covered by such material, he adds, the total heat fluxes into and away from Earth would be balanced and warming would cease.
Other photonics solutions have been proposed to achieve similar results and, along with Munday’s work, were presented at SPIE Optics + Photonics in August. For example, Shanhui Fan, a professor of electrical engineering at Stanford University, described work on a multilayer thin-film structure made of silicon dioxide, hafnium oxide, and silver that can achieve passive radiative cooling under direct sunlight. It has a strong emissivity in the transparency window of 8 to 13 µm. What this means is that the researchers’ material can actually get colder than freezing even when it’s sitting in direct sunlight—without using any electricity. Since that experiment, there have been a large number of material systems in which sub-ambient daytime radiative cooling has been observed.
One potential application of radiative cooling, Fan says, is to improve the efficiency of building air conditioning systems. That would require coupling the radiative cooler with a standard air conditioning system. In subsequent work, his group built radiative cooling panels with a circulating water loop underneath that reduced the temperature of the water a few degrees Celsius below ambient air temperature. “When you feed this kind of radiatively cooled water into a standard water-based cooling tower, one can improve the system efficiency by 10 to 15 percent. Air conditioning is a major energy consumer in the US,” Fan says.
Munday has proposed additional applications of the heat exchange between the sun and Earth. In one scenario, he envisions flipping the script on solar-cell photovoltaics, which are cold bodies that receive heat from the sun and then convert it to electricity.
“At night, you don’t have the sun,” Munday explains, “but we still have a thermally warm Earth, which is around 300 Kelvin. And we have the night sky, which is around 3 Kelvin, and so very, very cold by comparison.” What if, he asks, “we try to put some sort of engine or a photovoltaic device that could extract the power as it goes from the Earth to space and perform work using that?”
Solar photovoltaics wouldn’t work, but Munday thinks it might come down to using different materials. The problem is the bandgap. For a silicon solar cell, the bandgap is about 1.1 electron volts (eV), well suited for absorbing solar light because the sun is very hot. The emission peak of the blackbody spectrum is somewhere in the visible range and silicon absorbs that well.
“Now, if you think about Earth [at 300 Kelvin] being the emitter, its blackbody emission peak is at a much, much longer wavelength. It turns out what you want is a very low-bandgap material, something less than 0.1 eV,” Munday says. The amount of power such a nighttime photovoltaic system could generate would not be as much as in daylight, but it would not be insignificant.
“This is really all about optics,” Munday says. “You can use optics to cool things down. And with optics, you can use a cool surface to generate electrical or mechanical power.” Either way, keeping the planet cool is the endgame.
William G. Schulz is the Managing Editor of Photonics Focus.
