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.
1. 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
2. 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.
3. 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.
4. 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.
5. 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.
6. 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
7. 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.
Georgia Tech Associate Professor Baratunde Cola measures the power produced by converting green laser illumination to electricity using a carbon nanotube optical rectenna.
Photos: Rob Felt
8. Pulp Energy
Although fossil-fuel emissions may be the poster child for global warming, there is also growing concern over environmental harm from discarded electronics.
Researchers at Georgia Tech’s Center for Organic Photonics and Electronics (COPE) and Renewable Bioproducts Institute are developing paper-based electronics — organic solar cells, organic light-emitting diodes (OLEDs), and organic field-effect transistors (OFETs) — fabricated on cellulose-based substrates that can be recycled easily.
Use of paper for substrates has generated considerable buzz among researchers, but its high porosity and surface roughness pose challenges. Today’s organic electronic components use very thin carbon-based semiconductor layers — about 1,000 times thinner than the average human hair. “Because they are so thin, you need nearly atomically flat substrates where the surface is down to a nanometer,” explained Bernard Kippelen, director of COPE and a professor in Georgia Tech’s School of Electrical and Computer Engineering.
To address this, Kippelen’s team is using cellulose nanocrystals (CNCs), a type of wooden wunderkind material, to develop new semiconductor devices, demonstrating that CNCs are a viable alternative to traditional plastic substrates — while offering new environmental benefits. Devices made on these substrates can be easily dissolved in water, allowing semiconducting materials and metal layers to be filtered and recycled.
Applications will depend on economics and performance. For CNC-based solar cells, the researchers have achieved power conversion efficiencies of 4 percent. Efficiencies could be increased to 10 percent but would require more expensive materials, Kippelen said. So instead of paper-based solar farms becoming the norm, he predicts low-power applications, such as computer covers and mousepads, for CNC-based solar cells.
Cellulose-based OLEDs, which have performance comparable with current devices, show greater potential for market adoption. “The trend in flat-panel displays is larger size and higher resolution,” Kippelen said. “Glass substrates, however, pose manufacturing and transportation problems because of their rigidity and breakability. And plastic has problems at the end-of-product lifecycle.”
Yet with the low cost and flexibility of paper-based OLEDs, flat panel displays could be the size of a wall.
9. Fuel from the Sky
In another intriguing project, researchers led by Peter Loutzenhiser are leveraging solar energy to reverse the combustion process and produce synthesis gas (mixtures of hydrogen, carbon monoxide, and small amounts of carbon dioxide), which can be converted into fuels such as kerosene and gasoline.
“Instead of using fossil resources to create fuel, we are using the byproducts of combustion (water and carbon dioxide) to re-energize the system with the sun,” explained Loutzenhiser, an assistant professor at Georgia Tech’s School of Mechanical Engineering.
The researchers are studying a two-step process using metal oxides that can split water and carbon dioxide. The first step, which occurs between 1100 and 1800 degrees Celsius, thermally reduces or “pulls off” oxygen from the metal oxide material. Then at temperatures of about 300 to 900 degrees Celsius, either water or carbon dioxide is introduced in the second step. These lower temperatures are favorable for re-oxidation, which enables the metal oxide to take back oxygen from either the water or carbon dioxide, resulting in hydrogen or carbon monoxide. “The two steps are important — otherwise the oxygen would recombine with either the carbon monoxide or hydrogen, resulting in the release of heat that would then be lost,” Loutzenhiser said.
The researchers have demonstrated that the technology works with zinc oxide, but they are searching for materials that can speed up the reactions and reduce the temperature of the first step. “You want something that can reduce at the lowest possible temperature in the high-temp stage and is capable of taking the oxygen from the carbon dioxide or the water vapor in the second step,” Loutzenhiser explained.
Recently, the group achieved promising results with mixed ionic electronic conducting materials. Now they are trying to tune these materials to break apart either the CO2 molecules or the water vapor molecules at lower temperatures.
If commercialized, the technology could transform desert areas into fuel farms, Loutzenhiser said: “Instead of pulling fuel out of the ground, we could pull carbon dioxide from the air and use the sun to convert it with water into a long-term storage medium that could be shipped and used around the world without changes to transportation infrastructure.”
10. Hello Graphene Supercaps, Good-bye Batteries?
Used in everything from military applications to elevators and cars, supercapacitors are attractive sources for clean energy because they quickly charge and discharge and have long cycling lives. But there’s one big drawback: low energy density.
“Today’s supercapacitors have only one-tenth the energy density of lithium-ion batteries,” pointed out Meilin Liu, a Regents Professor in Georgia Tech’s School of Materials Science and Engineering. “For the device to give you the same electrical energy, the device would have to be much bigger.”
Working with C.P. Wong, another Regents Professor, Liu is developing graphene-based supercapacitors that offer significantly increased energy density while maintaining high power and long operational life. The research is funded by ARPA-E.
Graphene is a two-dimensional material that conducts electricity better than copper and is both lighter than steel and 100 times stronger. Yet graphene has a tendency to stack together and form graphite. To prevent this, the researchers place molecular spacers between the graphene sheets, creating a 3-D porous structure that demonstrates a capacitance of 400 Faradays per gram — four times higher than current supercaps.
The researchers have also improved capacitance by dispersing transition metal compounds into the graphene-based structure.
Graphene alone can only produce a capacitance of about 400 Faradays per gram of material. In contrast, transition metal compounds have higher energy density (2,000 to 3,000 Faradays per gram), but poor electronic connectivity, which slows down the flow of electrons required for charging and discharging. Yet by combining the metal compounds with the 3-D porous graphene, which scores high marks for connectivity, the researchers have achieved capacitance of about 1,500 Faradays per gram while maintaining superior cycling.
The researchers are also improving energy density by broadening voltage using two different electrode materials (one positive and one negative). “Each redox material has its own operating window of potential, and we optimize the nanostructure to achieve their highest energy density,” Liu explained.
With these new developments, the researchers are approaching supercaps that can be as small as batteries, but charged and discharged faster and cycled for much longer, Liu said.
“This new breed of supercaps could replace batteries, providing cleaner, safer, and more robust power for many applications, from portable electronics to electric vehicles and smart grids.”
“This new breed of supercaps could replace batteries,” said Meilin Liu, Regents Professor, School of Materials Science and Engineering.
Photo: Fitrah Hamid
11. Monolithic Microscale Heat Pumps
Proving that good things come in small packages, researchers led by Srinivas Garimella have developed a novel textbook-sized cooling system that operates on waste heat rather than electricity.
The underlying technology has been used in very large-scale installations, such as hospitals and university campuses, explained Garimella, a professor in Georgia Tech’s School of Mechanical Engineering. Yet his team takes the science to a new level by working at the micro scale and creating a self-contained unit.
How it works: Extremely small passages are etched into thin sheets of metal with different areas representing different components. Working fluids flow in the same order as they would in a larger system, albeit in one space. The minimization of plumbing inlets and outlets translates into greater compactness — and lower price tags.
- No synthetic refrigerants are used, and less fluid is required, which further lowers costs and increases safety.
- No compressor is needed and there are few moving parts, decreasing noise and increasing reliability.
- Modular design allows units to be configured to generate anywhere from a few watts to tens of kilowatts of cooling or heating.
Since unveiling a proof-of-concept unit in 2009, the researchers have developed heat pumps with cooling capacities of one and two refrigerant tons. (Capacity of current residential units ranges from one to four refrigerant tons.) Efficiency has been substantially improved, and fabrication techniques have also been improved to enable mass production.
“Although initial cost to consumers might be higher than traditional heat pumps, lifecycle costs should be comparable because of lower operating costs,” Garimella said, noting that field tests are slated for late this year, and the technology might be ready for commercialization by 2017.
The researchers have also adapted the technology to provide cooling using waste heat from diesel-driven generators at military bases, where ambient temperatures are extremely high. “Not only is diesel fuel very expensive to transport, there are also risks to humans in delivering the fuel,” Garimella said. “Using the energy in the diesel fuel to the fullest extent by providing power as well as cooling through these units, without consuming additional prime energy, will lower overall costs and increase personnel safety.”
The research has been supported by ARPA-E, Department of Energy, U.S. Army, Naval Facilities Engineering Command, Georgia Research Alliance, and Atlanta Gas Light.
12. Next-gen Power Plants
Researchers in Georgia Tech’s School of Mechanical Engineering are working on major makeovers for power plants, introducing innovations that range from revamped power cycles to new infrastructure materials.
In one project, steam is being replaced with supercritical carbon dioxide (SCCO2) as the working fluid to operate turbines and produce electricity.
SCCO2 results when carbon dioxide is subjected to pressure above 7.4 megapascals and temperatures above 31 degrees Celsius. This magical state, somewhere between a liquid and a gas, provides high fluid density, thermal conductivity, and heat capacity.
SCCO2 is currently used in environmentally friendly dry cleaning and coffee decaffeination. In energy applications, its high density and compressibility would enable generators to extract more power from turbines, explained Devesh Ranjan, an associate professor of fluid mechanics. “Equipment could be made from top-notch materials, yet dramatically smaller, which would reduce production costs.”
“Because the heat transfer coefficient is very high with SCCO2, you can do dry cooling in an arid environment,” said Devesh Ranjan, Associate Professor, George W. Woodruff School of Mechanical Engineering.
Photo: Gary Meek
Another plus: the unique cooling properties of SCCO2. “Most power plants are near a lake or river because they need lots of water to cool them,” Ranjan said. “Because the heat transfer coefficient is very high with SCCO2, you can do dry cooling in an arid environment such as the desert, which is best for solar collection.”
Using SCCO2 in concentrated solar plants could push thermal efficiencies from 45 to 60 percent, enough to be competitive with fossil fuel, said Asegun Henry, an assistant professor of heat transfer, combustion, and energy systems. “Yet this requires higher operating temperatures — 800 degrees Celsius compared to current temperatures below 600 degrees — and current heat exchangers literally can’t take the pressure.”
To resolve this, Henry and Ranjan are working with Purdue University researchers to develop a new breed of heat exchanger that can withstand extremely high temperatures and pressures, a project supported by DOE SunShot funding.
Ken Sandhage, a former Georgia Tech professor now at Purdue’s School of Material Engineering, has developed a process for inexpensively fabricating a high-temperature composite material into complicated 3-D shapes.
In addition to making solar power more competitive, the heat exchangers could also be used with SCCO2 to boost efficiency in fossil fuel power plants. “More efficiency means less carbon dioxide emissions per kilowatt produced,” Henry said.
T.J. Becker is a freelance writer based in Michigan. She writes about business and technology issues.