Re-Imagining Energy: Generating Energy

As the land-grant university for the energy-rich state of Pennsylvania, it isn’t surprising that Penn State counts among its core strengths a broad and deep expertise in energy-related research. Today, in areas from materials science to policy, from environmental chemistry to architectural and electrical engineering, the range and quality of our research make Penn State a world leader in energy research.

We've produced a package of five stories that capture just a sliver of that expertise, briefly sampling some of the more innovative ideas of Penn State researchers working together to solve key questions of making and using energy.

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Storing energy—revolutions in materials to make batteries that charge faster, last longer, and are safer than conventional batteries

Catching carbon—new technology to capture CO2 before it gets into the atmosphere and either sequester it or use it to create new products

The built environment—how new inventions and design principles are making our buildings and appliances more energy-efficient

Pulling it all together—integrating new sources of energy with the traditional electric grid to provide reliable, sustainable power for homes and businesses

And for an inside look at how Penn State students are making a mark in the field of wind energy, see A Shift in the Wind.

Fast-growing global demand, combined with rising environmental challenges, requires new and cleaner sources of energy. Penn State researchers are working on a broad range of innovative technologies to efficiently harvest the sustainable energy of natural processes and power our future. Here are a few outstanding examples.

WHERE THE RIVERS MEET THE SEA

At the places where fresh and saltwater meet, there are vast amounts of potential energy to be tapped—enough to supply up to 40 percent of global electricity needs.

Mixing takes energy, Chris Gorski explains. “Normally, where a river meets the sea, that energy gets dissipated as heat. But there are ways to try to capture it.”

For Gorski, and assistant professor of environmental engineering, the best way is through electrochemistry. With Penn State colleagues Bruce Logan and Taeyoung Kim, he has devised a flow cell that exploits differences in salt concentration to create an electric current. “It’s really similar to the way a battery operates,” Gorski says.

Already, they have demonstrated twice the power density achieved by previous technologies. Now, with an NSF Early Career Award, Gorski with focus on maximizing that efficiency even further.

But will this idea ever get out of the lab? Practically speaking, “Solar and wind are getting so cheap that it’s going to be tough to compete with them,” Gorski admits. “But there are locations where you can’t do solar or wind. These places might be good candidates for salinity gradient energy.”

—David Pacchioli

How It Works

Stage 1  Saltwater and freshwater are flowed through opposite sides of a cell, separated by an anion exchange membrane that only allows negatively charged ions to pass through it. Each side contains a battery electrode made of copper hexacyanoferrate, a metal-based material that can efficiently incorporate sodium into its crystal structure.

The difference in salinity creates an electrochemical voltage. In the freshwater channel, sodium present in the electrode is released into the water, causing an electron to pass through the circuit.

On the saltwater side, the electrode takes up sodium ions from the salty water, which is balanced by receiving the electron released by the other electrode. The flow of electrons from one side to the other creates an electrical current.

Stage 2  When the system reaches a state of equilibrium—the saltwater electrode is saturated with sodium ions, and the freshwater electrode has given up all the sodium ions it can—the flow of water through the cell is switched, with the saltwater side now receiving freshwater and vice versa. The switching perpetuates the reaction, maintaining the current.

FOCUSED ON A SOLUTION

A concentrating photovoltaic system with embedded microtracking can produce over 50 percent more energy per day than standard silicon solar cells, according to the engineers who recently field-tested a prototype unit.

In contrast to silicon solar panels, which currently dominate the market at 15 to 20 percent efficiency, concentrating photovoltaics (CPV) focus sunlight onto smaller but much more efficient solar cells like those used on satellites, says Chris Giebink, Charles K. Etner Assistant Professor of Electrical Engineering. Current CPV systems enable overall efficiencies of 35 to 40 percent. But these systems tend to run large—the size of billboards—and have to rotate to track the sun during the day.

"What we're trying to do is create a high-efficiency CPV system in the form factor of a traditional silicon solar panel," Giebink says.

To do this, his team embedded tiny multi-junction solar cells, roughly half a millimeter square, into a sheet of glass that slides between a pair of plastic lenslet arrays. The whole arrangement is about two centimeters thick and tracking is done by sliding the sheet of solar cells laterally between the lenslet array while the panel remains fixed on the roof. An entire day's worth of tracking requires only one centimeter of movement.

Over the course of a day’s testing, Giebink reports, their prototype reached 30 percent efficiency, in contrast to 17 percent for a commercial silicon cell. The CPV system produced 54 percent more energy than the silicon.

Major challenges lie ahead in scaling the system and proving long-term reliability, but with the right engineering, Giebink says, it “could be useful in applications ranging from rooftops to electric vehicles—really anywhere it's important to generate a lot of solar power from a limited area."

—A'ndrea Elyse Messer

A BIGGER HARVEST OF ENERGY

A wearable energy-harvesting device developed by researchers from Penn State and the University of Utah could generate energy from the swing of an arm while walking or jogging. The device, about the size of a wristwatch, produces enough power to run a personal health monitoring system.

"The devices we make using our optimized materials run somewhere between five and 50 times better than anything else that's been reported," says Susan Trolier-McKinstry, the Steward S. Flaschen Professor of Materials Science and Engineering and Electrical Engineering at Penn State.

Energy-harvesting devices are in high demand to power the millions of devices that make up the internet of things. Many take advantage of the so-called piezoelectric effect, whereby certain crystals can produce an electric current when compressed or change shape when an electric charge is applied.

In this work, funded by the National Science Foundation, Trolier-McKinstry and her former doctoral student, Hong Goo Yeo, took a well-known piezoelectric material, PZT, and coated it on both sides of a flexible metal foil to a thickness four or five times greater than in previous devices. Greater volume of the active material equates to generation of more power, Trolier-McKinstry explains. By orienting the film's crystal structure to optimize polarization, they were able to markedly increase energy-harvesting performance.

The researachers then designed a wristwatch-like device that incorporates the PZT/metal foil materials for maximum efficiency.

In future work, the team believes they can double the power output already achieved by using cold sintering, a low-temperature synthesis technology developed at Penn State. In addition, the researchers are working on adding a magnetic component to the current mechanical harvester to scavenge energy over a larger portion of the day when there is no physical activity.

—Walt Mills

Chris Gorski is assistant professor of environmental engineering. Bruce Logan is Evan Pugh Professor in Engineering and the Kappe Professor of Environmental Engineering.  Taeyoung Kim was a postdoc in Civil and Environmental Engineering. Chris Giebink is the Charles K. Etner Assistant Professor of Electrical Engineering. Susan Trolier-McKinstry is the Steward S. Flaschen Professor of Materials Science and Engineering and Electrical Engineering. Hong Goo Yeo earned his doctorate in Materials Science and Engineering in 2017.

This story first appeared in the Fall 2018 issue of Research/Penn State magazine.