Engineers at CU Boulder have debuted a new method for harvesting energy from waste heat such as that produced from factory smokestacks, bakery ovens, and AI Data Centers (which have become the largest producers of waste heat).
What are Rectennas
Rectennas are tools used for harvesting energy (other methods include mechanical vibration, thermal, magnetic, wind, solar, and radio frequency).
They were first developed by an engineer named William C. Brown in 1964 when he used microwaves to power a small helicopter. His rectennas were relatively simple tools made up of an antenna (which absorbed the radiation), and a diode (which converted that energy into DC currents).
"Rectenna" is short for rectifying antennas. They work somewhat like car radio antennas, but instead of picking up radio waves to produce tunes, optical rectennas absorb light and heat and convert them into power.
In order to capture thermal radiation today, however, and not just microwaves, rectennnas need to be dramatically smaller than Brown's creation. The smaller the device, though, the higher its resistance becomes, which can shrink the power output.
"You need a device that has very low resistance, but is also really responsive to light," says Amina Belkadi, lead author of a study published in the journal Nature Communications.
Optical Rectennas
New "optical" rectennas created by scientists at CU Boulder may be the answer. They are microscopic devices, thinner than a human hair, yet 100 times more efficient than the other harvesting methods noted above.
Optical rectennas use a process called "resonant tunneling" where electrons can easily pass through solid matter without spending any energy.
Under a microscope, optical rectennas look like the ends of two sharpened pencils (insulators) touching to form a bow. The electrons active in the heat travel down the edges of the bow to the center point where they are captured to produce energy. Previous rectennas were thicker and had only one point (insulator) which added a lot of resistance and reduced the amount of electricity that engineers could get out.
Scientists have long predicted that optical rectennas would one day sit on top of factory smokestacks or on pipes on the roofs of AI data centers, trapping the energy from heat produced that would normally go to waste.
Some researchers have even proposed putting these rectennas on high-altitude balloons or airships that fly high above earth's surface so they can capture the energy radiation from the planet into space.
Future Home Use
Optical rectennas are transparent and flexible because of their size. In the future, some engineers believe they could be used to power homes when applied to windows, walls, roofs, and other surfaces. They could also be applied to smartphones, IoT devices, and other gadgets to provide a supplementary power source. Many scientists believe rectennas will one day supplant solar energy as a preferred clean energy source.
Sources: Nature Communications, CU Boulder, ECE.CMU.Edu, Spectrum.ieee.org
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