Researchers are repurposing an ancient technology for the modern age.
By Marlene Cimons
Scientist Aaswath Raman long has been keen on discovering new sources of clean energy by creating novel materials that can make use of heat and light.
And lately, he has focused on developing better cooling systems, perhaps inspired by childhood summer visits to his grandparents in Mumbai, where the temperature can hover at 100 degrees F with killer humidity — and where his family refused to add an air conditioner.
“It’s not unusually different from Miami, I suppose — just a bit challenging without air conditioning,” he said.
His interests in clean energy and cooling led him to learn about a phenomenon called “radiative cooling,” which is when objects facing up shed heat into the sky after dark, cooling the surrounding area. This “aha” moment also recently led to his developing a pollution-free source of electricity.
“I was blown away by how this natural, passive cooling mechanism was ubiquitous, but something I’d never heard of,” he said.
Even more surprising was learning that the concept was centuries old. Ancient Middle Eastern civilizations–especially the Persians–used radiative cooling to make ice, pouring water into a pool as the sun set, collecting the frozen chunks the next morning. Even though the ambient temperature stayed above freezing, the pool would grow more cold than the surrounding air as the water radiated heat into the sky. During the day, heat from the sun would have kept the water warm, but at night, it grew colder and colder until it froze over.
Raman, an assistant professor of materials science at engineering at UCLA, wanted to take advantage of this phenomenon to produce clean energy. So he and his colleagues designed a device that can harness nighttime cooling to generate a small amount of power.
For now, the device is too costly and generates too little electricity to compete with other forms of clean energy. To power a 3-watt LED lightbulb, the generator would need to be 1300 square feet. Raman said he believes scientists could get that number down 60 square feet, and they could also lower costs enough to make it useful in remote areas disconnected from the power grid. A future iteration of the device could allow people without access to electricity to turn on a lightbulb, charge a cell phone, or power some other small device at night.
“There are also low-power sensors: think monitoring oil pipelines, or weather and climate monitoring in the Arctic,” he added. “The combination of remoteness and low-power needs make them a great fit for this device, especially in polar, northern regions where you have limited or no sunlight for a large fraction of the year.”
His invention is deceptively simple. It looks like a small disc propped up on four legs. The side facing up radiates heat into the night sky, while the side facing down is warmed by the surrounding air. Sandwiched in between these two surfaces is a thermoelectric generator, which uses the difference in temperature to produce power.
The top side of the black aluminum disc radiates heat into the sky, while the bottom side is warmed by the surrounding air. A thermoelectric generator inside the disc takes advantage of the temperature difference to produce electricity. Source: Aaswath Raman
A description of the research, co-authored with frequent collaborator Shanhui Fan, a professor of electrical engineering at Stanford University, was published in the journal Joule.
“This is an excellent demonstration of how the night sky can be used as a thermodynamic resource to do something useful,” said Jeremy N. Munday, professor of electrical and computer engineering at the University of California Davis, who has proposed using radiative cooling to combat climate change. He said that more research is needed to know if the technology could someday provide a workable alternative to wind or solar power, for instance.
Raman has also looked for ways to take advantage of radiative cooling during the day. For a long time, most experts had dismissed this as improbable, insisting that the cooling effect only worked at night, but Raman was convinced he could create substances that could take the concept of radiative cooling and make it work during the day.
Ultimately, he and his colleagues created ultra-thin, hair-like materials — known as metamaterials — that successfully produced daytime cooling by acting as a mirror, reflecting the sun and heat back into the sky during the daytime hours, essentially removing the heat. It’s the same principle at work as natural nighttime radiative cooling, but with a little boost from materials created by the scientists.
Raman and his colleagues designed these cooling panels, which use a special material that radiates heat into the sky during the day. Water is cooled as it runs through the panels, and they are integrating such panels into air conditioning systems to help them run more efficiently. Source: Aaswath Raman
The scientists have since incorporated this material into panels that use radiative cooling to improve the efficiency of existing air conditioning and refrigeration systems, helping to save energy. This is an important development as climate change will fuel more stifling and unrelenting heat waves, so the need for cooling–especially during the day–is going to increase, upping demand for electricity.
Initially, Raman did not spend a lot of time thinking about how to use radiative cooling to re-engineer existing air conditioners and refrigeration systems — and then, always up for a challenge, he changed his mind.
“It always seemed like such a mature and well-developed field,” he said. But, then, Raman asked himself: “Was there anything we could do to make it better? Turns out, yes!”Share This