The world human population is already more than 7 billion — a number that could exceed 11 billion by 2100, according to projections from the United Nations. This rising populace, coupled with environmental challenges, puts even greater pressure on already strained energy resources. Granted, there’s no silver bullet, but Georgia Tech researchers are developing a broad range of technologies to make power more abundant, efficient, and eco-friendly.

This feature provides a quick look at a dozen unusual projects that could go beyond traditional energy technologies to help power everything from tiny sensors to homes and businesses.



Na-TECC: Worth Its Salt

Shannon Yee, an assistant professor in Georgia Tech’s George W. Woodruff School of Mechanical Engineering, is developing a technology that leverages the isothermal expansion of sodium and solar heat to directly generate electricity. Affectionately known as “Na-TECC” (an acronym that combines the chemical symbol for sodium with initials from “Thermo-Electro-Chemical Converter” and also rhymes with “GaTech”), this unique conversion engine has no moving parts.

A quick rundown in geek speak: Electricity is generated from solar heat by thermally driving a sodium redox reaction on opposite sides of a solid electrolyte. The resulting positive electrical charges pass through the solid electrolyte due to an electrochemical potential produced by a pressure gradient, while the electrons travel through an external load where electric power is extracted. Bottom line, this new process results in improved efficiency and less heat leaking out, explained Yee.

The goal is to reach heat-to-electricity conversion efficiency of more than 45 percent — a substantial increase when compared to 20 percent efficiency for a car engine and 30 percent for most sources on the electric grid.



The technology could be used for distributed energy applications. “A Na-TECC engine could sit in your backyard and use heat from the sun to power an entire house,” Yee said. “It can also be used with other heat sources such as natural gas, biomass, and nuclear to directly produce electricity without boiling water and spinning turbines.”



Funded by the Department of Energy’s (DOE) SunShot Program, the research is being conducted in collaboration with Ceramatec Inc.



“A Na-TECC engine could sit in your backyard and use heat from the sun to power an entire house,” said Shannon Yee, Assistant Professor, George W. Woodruff School of Mechanical Engineering.

Photo: Fitrah Hamid



New Breed of Betavoltaics

In another project, Yee’s group is using nuclear waste to produce electricity — minus the reactor and sans moving parts.



Funded by the Defense Advanced Research Projects Agency (DARPA) and working in collaboration with Stanford University, the researchers have developed a technology that is similar to photovoltaic devices with one big exception: Instead of using photons from the sun, it uses high-energy electrons emitted from nuclear byproducts.



Betavoltaic technology has been around since the 1950s, but researchers have focused on tritium or nickel-63 as beta emitters. “Our idea was to revisit the technology from a radiation transport perspective and use strontium-90, a prevalent isotope in nuclear waste,” Yee said.



Strontium-90 is unique because it emits two high-energy electrons during its decay process. What’s more, strontium-90’s energy spectrum aligns well with design architecture already used in crystalline silicon solar cells, so it could yield highly efficient conversion devices.

In lab-scale tests with electron beam sources, the researchers have been achieving power conversion efficiencies of between 4 and 18 percent. With continued improvements, Yee believes the betavoltaic devices could ultimately generate about one watt of power continuously for 30 years — which would be 40,000 times more energy dense than current lithium ion batteries. Initial applications include military equipment that requires low-power energy for long periods of time or powering devices in remote locations where changing batteries is problematic.



Flexible Generators

Yee’s group is also pioneering the use of polymers in thermoelectric generators (TEGs).



Solid-state devices that directly convert heat to electricity without moving parts, TEGs are typically made from inorganic semiconductors. Yet polymers are attractive materials due to their flexibility and low thermal conductivity. These qualities enable clever designs for high-performance devices that can operate without active cooling, which would dramatically reduce production costs.



The researchers have developed P- and N-type semiconducting polymers with high performing ZT values (an efficiency metric for thermoelectric materials). “We’d like to get to ZT values of 0.5, and we’re currently around 0.1, so we’re not far off,” Yee said.



In one project funded by the Air Force Office of Scientific Research, the team has developed a radial TEG that can be wrapped around any hot water pipe to generate electricity from waste heat. Such generators could be used to power light sources or wireless sensor networks that monitor environmental or physical conditions, including temperature and air quality.



“Thermoelectrics are still limited to niche applications, but they could displace batteries in some situations,” Yee said. “And the great thing about polymers, we can literally paint or spray material that will generate electricity.”



This opens opportunities in wearable devices, including clothing or jewelry that could act as a personal thermostat and send a hot or cold pulse to your body. Granted, this can be done now with inorganic thermoelectrics, but this technology results in bulky ceramic shapes, Yee said. “Plastics and polymers would enable more comfortable, stylish options.”



Although not suitable for grid-scale application, such devices could provide significant savings, he added.



Recycling Radio Waves

Researchers led by Manos Tentzeris have developed an electromagnetic energy harvester that can collect enough ambient energy from the radio frequency (RF) spectrum to operate devices for the Internet of Things (IoT), smart skin and smart city sensors, and wearable electronics.



Harvesting radio waves is not brand new, but previous efforts have been limited to short-range systems located within meters of the energy source, explained Tentzeris, a professor in Georgia Tech’s School of Electrical and Computer Engineering. His team is the first to demonstrate long-range energy harvesting as far as seven miles from a source.

The researchers unveiled their technology in 2012, harvesting tens of microwatts from a single UHF television channel. Since then, they’ve dramatically increased capabilities to collect energy from multiple TV channels, Wi-Fi, cellular, and handheld electronic devices, enabling the system to harvest power in the order of milliwatts. Hallmarks of the technology include:

Ultra-wideband antennas that can receive a variety of signals in different frequency ranges.

Unique charge pumps that optimize charging for arbitrary loads and ambient RF power levels.

Antennas and circuitry, 3-D inkjet-printed on paper, plastic, fabric, or organic materials, that are flexible enough to wrap around any surface. (The technology uses principles from origami paper-folding to create “smart” shape-changing complex structures that reconfigure themselves in response to incoming electromagnetic signals.)

The researchers have recently adapted the harvester to work with other energy-harvesting devices, creating an intelligent system that probes the environment and chooses the best source of ambient energy to collect. What’s more, it combines different forms of energy, such as kinetic and solar, or electromagnetic and vibration.



Although some work remains to scale the printing process, commercialization of the National Science Foundation-supported research could happen within two years.



Pickin’ Up Good Vibrations

In another energy harvesting approach, researchers in Georgia Tech’s School of Mechanical Engineering are making advances with piezoelectric energy — converting mechanical strain from ambient vibrations into electricity.



Scientists have been exploring this field for more than a decade, but technologies haven’t been widely commercialized because piezoelectric harvesting is very case and application dependent, explained Alper Erturk, an assistant professor of acoustics and dynamics who leads Georgia Tech’s Smart Structures and Dynamical Systems Laboratory.



Current piezoelectric energy harvesters rely on linear resonance behavior, and to maximize electrical power, the excitation frequency of ambient sources must match the resonance frequency of the harvester. “Even a slight mismatch results in drastically reduced power output, and there are numerous scenarios where that happens,” Erturk said.



In response, Erturk’s group has been pioneering nonlinear dynamic designs and sophisticated computations to develop wideband piezoelectric energy harvesters that operate over a broad range of frequencies. In fact, one of their recent designs, an M-shaped harvester, can achieve milliwatt level output even for tiny milli-g level vibration inputs — a 660 percent increase in frequency bandwidth compared to linear counterparts. “The nonlinear harvesters also have secondary resonance behavior,” Erturk said, “which could enable frequency up-conversion in MEMS harvesters that suffer from device resonance being higher than ambient vibration frequencies.”



Although electrical output from vibration energy harvesters is small, it is still enough to power wireless sensors for structural health monitoring in bridges or aircraft, wearable electronics, or even medical implants. “Piezoelectric harvesting could eliminate the hassle of replacing batteries in many low-power devices — providing cleaner power, greater convenience, and meaningful savings over time,” Erturk said.



Power Rubbed the Right Way

Triboelectricity enables production of an electrical charge from friction caused by two different materials coming into contact. Although known for centuries, the phenomenon has been largely ignored as an energy source because of its unpredictability.



Yet researchers led by Zhong Lin Wang, a Regents Professor in Georgia Tech’s School of Materials Science and Engineering, have created novel triboelectric nanogenerators (TENGs) that combine the triboelectric effect and electrostatic induction. By harvesting random mechanical energy, these generators can continuously operate small electronic devices.



The first TENG debuted in 2012. Powered by foot tapping, it generated enough alternating current to power banks of LEDs. Since then the researchers have been pushing the envelope on their technology and have developed a self-charging system that not only converts alternating current to direct current but also features a power management unit that adapts to the variability in human movement.

Behind these recent milestones is a two-stage design: First the TENG charges a small capacitor. Then energy is transferred to a final storage device (a larger capacitor or battery) that matches the impedance of the generator’s output and provides appropriate voltage and constant output. Five seconds of palm tapping generates enough current to operate a wireless car door lock.



“The power management circuit is key to boosting efficiency,” said Simiao Niu, a graduate student and lead author on a paper recently published in the journal Nature Communications. “Without the circuit, charging efficiency is below 1 percent, but with it we’ve been able to demonstrate efficiencies of 60 percent.”



“This really broadens the number of possible applications,” Wang said, pointing to temperature sensors, heart rate monitors, pedometers, watches, scientific calculators, and RF wireless transmitters.



Although the self-powered system was initially developed to capture human biomechanical energy, the researchers have created four different modes to convert other ambient sources of mechanical energy, such as ocean waves, wind blowing, keyboard strokes, and tire rotation.

"The triboelectric system really broadens the number of possible applications,” said Zhong Lin Wang, Regents Professor, School of Materials Science and Engineering.

Photo: Rob Felt



Optical Rectenna

Researchers led by Baratunde Cola, an associate professor in Georgia Tech’s School of Mechanical Engineering, have developed the first known optical rectenna — a technology that could be more efficient than today’s solar cells and less expensive.



Rectennas, which are part antenna and part rectifier, convert electromagnetic energy into direct electrical current. The basic idea has been around since the 1960s, but Cola’s team makes it possible with nanoscale fabrication techniques and different physics. “Instead of converting particles of light, which is what solar cells do, we’re converting waves of light,” he explained.



Key to this technology are antennas small enough to match the wavelength of light (about one micron) and a super-fast diode — achieved in part by building the antenna on one of the metals in the diode. Cola describes the process:

Carbon nanotubes are grown vertically off a substrate.

Using atomic layer deposition, the nanotubes are coated with aluminum oxide to serve as an insulator.

Extremely thin layers of calcium and aluminum metals are placed on top to act as an anode.

As light hits the carbon nanotubes, a charge moves through the rectifier, which switches on and off to create a small direct current. The metal-insulator-metal-diode structure is fast enough to open and close at a rate of 1 quadrillion times per second.



From a performance perspective, the devices currently operate just under 1 percent efficiency. Yet because theory matches lab experiments, Cola hopes to increase broad-spectrum efficiency to 40 percent (which compares to 20 percent efficiency for silicon solar cells). Other important benefits: The optical rectenna works at high temperatures, and mass production should be inexpensive. The technology also can be tuned to different frequencies, so the rectenna can be used as a detector or in energy harvesting.



The researchers are now focused on lowering contact resistance and growing the nanotubes on flexible substrates for applications that require bending. The work has been supported by DARPA, the Space and Naval Warfare Systems Center, and the Army Research Office.