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Keeping It Cool to Create Power

By Sarah Williams, Photos by Jeremy Jackson

Published Apr 1, 2020 8:00 AM

Aaswath Raman engineers new technologies to cool buildings and generate electricity using a little-studied natural phenomenon.


“[Radiative cooling] seemed too good to be true. So why wasn’t it being used? Why did so few people know about it?” Aaswath Raman says.

Aaswath Raman, a UCLA assistant professor of materials science and engineering, was a graduate student when he stumbled across a handful of papers on radiative cooling — the process by which heat radiates upward from objects on Earth all the way to the cold depths of outer space. It captured his interest, and he now studies how to harness this movement of heat for technological gain. Raman’s efforts have earned him a Sloan Research Fellowship, an MRS Nelson “Buck” Robinson Science and Technology Award for Renewable Energy and a spot on MIT Technology Review’s annual list of 35 Innovators Under 35. Raman’s 2018 TED Talk about his research has nearly 2 million views.

Why did you become an engineer?

I was always interested in science and research. I grew up doing a lot of science fair projects, and they often revolved around energy and sustainability. I’m from Alberta, Canada, and my dad worked in the oil industry, so I was always very aware of the importance of energy issues. As I grew older, I became particularly passionate about climate change.

I ended up studying physics and computer science, and then I worked at Microsoft for a little while. I almost had a very different career path in the tech industry. But I decided to go back to school and get a Ph.D. because I really felt that basic, fundamental scientific research was needed to tackle energy challenges and climate change. When I came across this concept of radiative cooling, it led me on what’s been a very unexpected and interesting path.

What exactly is radiative cooling?

Radiative cooling is a natural phenomenon that happens all the time. All materials emit light and heat corresponding to their temperature. You can’t see this light with your naked eye, but when you look [at something] through a thermal infrared camera, that [light] is what you’re seeing. At night, objects facing the sky send this infrared light and heat upward. Some of it bounces off greenhouse gases in the atmosphere and comes back down, but some of it escapes out to space. Because some of the heat is lost, objects facing a clear night sky can actually cool down to below the air temperature.

Seeing radiative cooling in action is as simple as going outside on a cloudless night and measuring the temperature of your roof or observing frost on the ground, even when the air temperature hasn’t quite dipped below freezing.

Why did this concept intrigue you?

It seemed too good to be true — it’s free cooling, and all you have to do is put something outside [that faces] the sky. So why wasn’t it being used? Why did so few people know about it? It seemed like an interesting opportunity to ask questions about whether this could be used to drive cooling technologies. The first big puzzle was figuring out how you can make use of this effect when we need cooling the most — during the daytime. Historically, radiative cooling was called “night sky cooling,” because that’s when we notice the effect. During the day, the sun tends to heat up almost every natural material enough to completely counteract radiative cooling.

How did you solve that problem?

That’s where materials engineering comes in. I realized that we could engineer an artificial material that has a special set of optical properties. I designed a material that reflects sunlight, so it doesn’t heat up during the day, and at the same time, it also very effectively emits heat through radiative cooling. This means it’s able to stay cooled to below the air temperature, even in the middle of the day when the sun is shining on it.

What is that cooling ability useful for?

The immediate application is to cool the air for air-conditioning and refrigeration systems. I launched a company, SkyCool Systems, to pursue this commercially. They’ve already launched three pilot studies in Northern California, and it’s moving forward quite well. I’m a chief scientific officer now and not involved in the day-to-day operations, but it’s been really neat to watch it grow. Just a few years ago, this was a lab experiment, and now it’s out there as an actual technology.

More recently, you’ve actually generated energy. How does that work?

We wanted to do something else with radiative cooling, and the other big thing that came to my mind was that we should be able to generate some electricity at night, when we can’t generate solar power. This time, we didn’t need to engineer any new kind of material. We used a device called a thermoelectric generator — which converts differences in heat to electricity — and paired it with a black aluminum disc. As the disc radiated heat, the generator captured that temperature difference to produce energy.

How much energy does it generate?

Not very much yet! We showed that we could keep one LED lightbulb on. But I think it could be useful in any scenario where you don’t have access to the sun for solar power. In polar regions, for instance, you could power sensors or small lights for decades without worrying about having to replace the battery. But we’re also looking at ways of improving and scaling up the technology to generate more energy.

If it’s creating such a small amount of energy, why is work like this important?

In general, we need more investment in both basic and applied research when it comes to energy and climate-related challenges. In reality, no one solution is going to solve every aspect of the energy challenges we face. People focus a lot — rightly so — on renewables and solar, and I’m a huge advocate of them. But I think talking about [how to] efficiently use electricity — especially when running appliances — is equally valuable, because that reduces the demand for electricity to begin with. Air conditioning and refrigeration are a sizable chunk of our energy use, and therefore our carbon emissions. If radiative cooling could help reduce the amount of energy used by these systems, that could make a huge difference.

What do you hope people learn from your work?

I hope what gets conveyed by all this is a sense of optimism. People tend to get pessimistic and concerned by everything they hear about climate change and the perceived lack of progress on the energy front. And while the stuff I work on is by no means going to solve it, it’s a great example of how there are still possibilities for unexpected and creative solutions out there.