Every day the sun beams down on our planet, providing sunlight and heat. At night, the planet releases much of that heat back into our atmosphere. This, as we now know, is causing major problems.
Our atmosphere, clogged with greenhouse gases, now traps much of that heat, leading to climate change.
So far, we humans have started down a pretty good path of capturing at least some of the sunlight and converting it to renewable solar energy in an attempt to reduce the impacts on the atmosphere. But what about the heat?
Until recently, there was only a quantum theory that we could somehow harness that heat energy and convert it into electricity that we could use. The theory began in the 1940s and was advanced by CU Professor Garret Moddel of the Department of Electrical, Computer and Energy Engineering.
But it wasn’t until 2019 when Moddel’s PhD student, Amina Belkadi, was able to prove that microscopic rectennas (short for “rectifying antennas”) could harness earth’s expelled heat energy and turn it into energy.
OK, so that’s a lot of talk about theories and quantum thingies. But what could this all mean?
Well, in just a decade, it could be a future when you have solar panels on your roof that capture sunlight for energy during the day and then, at night, the panels would flip to a film (also developed by CU students) that would capture the heat coming from the planet and convert it to safe, renewable energy.
While Belkadi may have provided the first step to solving our greatest environmental issue, Belkadi is just happy to have physically proven a long-held theory. “I love my work and my electrons,” she said. “What I have done is prove a theory in a physical concept. Everything else is for others to do.”
The materials used now in the rectennae provide very little energy, but proving the theory will now allow continued progress in this field. “This innovation makes a significant step toward making rectennas more practical,” said Moddel in a CU press release. “Right now, the efficiency is really low, but it’s going to increase.”
Belkadi said there needs to be an improvement about 10 million times what she has found to be commercially viable, but she says that was about where solar panels were 40 years ago.
For everyone else, the experiment was relatively simple (after researching the correct materials for about a year and a half). Belkadi collected about 250,000 rectennas – which fit on a space about as big as your fingertip – and placed them on a hot plate. She then created a temperature differential between the room temperature using a beaker of liquid nitrogen, which has a boiling point of -320 degrees Fahrenheit. The extreme temperature difference was needed to showcase the energy created by the warmth of the earth due to limitations with the equipment.
And that’s when they saw the first energy produced from heat using rectennas. As this process is improved, extreme temperature differences will not be needed to capture energy, as this will simply happen between the cool of the night and the heat coming from the planet.
“I couldn’t believe it at first,” she said. “I called my husband from another lab to come see. And then I called others in and my advisor. My advisor said that now I could officially graduate.”
And that’s what happened … 35 days later she defended her thesis and earned her PhD.
The defense started with an infrared picture of Earth from space showing all of the heat that comes off the planet every day. She then moved onto showing the research the other CU students were doing on the film to collect the planet’s heat (See their Nature paper here.)
So what do you do after you solve global warming and the energy crisis? Well, for Belkadi, she had a baby nine months ago and is now planning a return to the lab to focus on materials to improve the efficiency of the rectennas.
As for Moddel, seeing his theory proven “satisfied his curiosity,” Belkadi said. Moddel retired in 2020 to work in the private sector. “I am very proud to say I was his last grad student,” she said.