The Edge of Energy

The Edge of EnergyOur thirst for energy comes at an environmental cost. Human beings have a profound effect on the planet, and the debate is no longer about whether we need to move away from carbon-based fuels, but when and how. Princeton researchers are looking for solutions at the edge of energy research.

“The move toward a sustainable future requires truly innovative approaches with an emphasis on a range of fundamental investigations and applications,” said Emily Carter, the Gerhard R. Andlinger Professor in Energy and the Environment and founding director of the Andlinger Center for Energy and the Environment, which supports a vibrant program of research in energy development, conservation and environmental protection.

“With Princeton’s mix of engineers, scientists and social scientists, we are uniquely poised to solve these complex energy problems,” she said.

Innovations from Princeton could radically change how we produce and consume sustainable energy. For example, one group is developing a solar energy-driven charging station that could recharge your cellphone anywhere. Another group is tilting windmills on their sides to increase their efficiency, and another is attempting to mitigate the waste of combustion by turning carbon dioxide and water back into fuel.

Solar cell-ophane

If you own a smartphone, chances are you’ve resorted to poaching electricity by recharging your phone at an outlet in a public place such as an airport, lecture hall, library or museum.

A new technology developed in Princeton’s School of Engineering and Applied Science could make it possible to charge your phone just by placing it on a surface covered with a special plastic lining. The flexible plastic lining is embedded with solar cells and electronic circuits that convert sunlight into a wireless power signal strong enough to charge a phone or laptop.

With this cheap, tough and flexible plastic sheet, any surface could become a charging station. You could charge your phone on the table while out to lunch, or by placing it on your desk or on your beach blanket. Entire walls or roofs could be covered with these large-area sheets.

Solar charging station

Solar charging station: Plastic sheets embedded with solar cells and flexible electronics are under development in the laboratory of Assistant Professor Naveen Verma and colleagues in Princeton’s School of Engineering and Applied Science. The sheets could have a range of applications including solar charging stations for electronic appliances. On the left, electronic components are sandwiched between two solar cells. On the right, a closeup view shows the structures needed for wireless transmission. (Image courtesy of Naveen Verma)

“Our prototype integrates the energy-harvesting device with power electronics,”said Naveen Verma, an assistant professor of electrical engineering who developed the technology with James Sturm, the William and Edna Macaleer Professor of Engineering and Applied Science, and Sigurd Wagner, a professor of electrical engineering. The project is funded by the National Science Foundation (NSF) and the U.S. Department of Energy (DOE).

The flexible sheets of solar cells, or photovoltaic cells, are already commercially available. What is new is the incorporation of the electronic components for wireless transmission into the same technology, creating a path to a full charging system on one flexible sheet. Before now, flexible photovoltaic sheets needed to be wired to hard and inflexible integrated-circuit devices.

Creating flexible electronics was a challenge because the devices are made from amorphous silicon, which is not nearly as efficient as the rigid crystalline silicon used in conventional electronics. Because amorphous silicon is inefficient at transmitting electricity, large plastic sheets will be needed to charge even small devices.

Additionally, the engineers had to invent new circuit designs, said Verma, referring to the contributions of graduate students Liechao Huang, Yingzhe Hu, Warren Rieutort-Louis and Josue Sanz- Robinson. “We figured out how to build power inverters and amplifiers, and control circuits, all integrated with inductors and capacitors; these are all needed for wireless transmission,” Verma said.

Tilting windmills

The arms of giant wind turbines in today’s commercial wind farms rotate around a horizontal axis. But the efficiency of wind farms could be greatly improved, Princeton researchers suggest, by redesigning the wind turbines so that they rotate on a vertical axis (though the blades themselves are horizontal). In a vertical axis turbine, the blades can be supported in two locations rather than radiating from a single hub, so they can be built larger than current designs.

“The larger the area swept by the blades, the more energy you can capture from a single turbine,” said Alexander Smits, the Eugene Higgins Professor of Mechanical and Aerospace Engineering, who is working on the project with Luigi Martinelli, associate professor of mechanical and aerospace engineering.

Wind turbine

Wind turbines that rotate around a vertical axis, as shown in this experimental turbine built by Hopewell Wind Power Ltd. in Yangjiang, Guangdong Province, China, have the potential to be more efficient than conventional wind turbines, which rotate around a horizontal axis atop a fixed pole. (Image courtesy of Alexander Smits)

Support for the project has been provided by Princeton’s Seibel Energy Challenge funded by the Thomas and Stacey Siebel Foundation, and Hopewell Wind Power Ltd., a subsidiary of Hopewell Holdings Ltd., a Chinese firm headed by Princeton alumnus Sir Gordon Wu, Class of 1958.

Another advantage is that vertical blades can turn regardless of wind direction. Conventional wind turbines are fixed and point primarily in one direction — the predominant wind direction — but they cannot reorient when the wind shifts. In contrast, wind coming from any direction can push the vertical axis blades. Because of their design, vertical axis turbines can be built taller than traditional designs and be more closely packed together.

Using computer simulations and experimental models, Smits and Martinelli are studying fundamental fluid-dynamics aspects of wind-power generation and are working to optimize the design of these vertical axis turbines. Smits is testing small-scale prototypes in a wind tunnel assisted by mechanical and aerospace engineering graduate student Tristen Hohman, while Martinelli and mechanical and aerospace engineering graduate student Mark Lohry are focused on the computational modeling of wind flow. Larger-scale testing will be conducted in Guangdong Province, China, using a prototype turbine with blades 26 meters long that was built by Hopewell Wind Power Ltd.

A number of challenges remain in the development of vertical axis turbines. First, winds travel more slowly near the ground versus high in the air, thereby pushing the blade unevenly. Smits and Hohman are working to replicate these conditions in their wind tunnel. In addition, the interaction of the wind flow around the support structure may interfere with the blades by creating vibrations that in the long term will weaken the structure of the blade. Finally, the blade can stall, resulting in uneven electricity generation. This last challenge may be overcome by optimizing the shape of the blade.

Reverse gear — running combustion backward

Although renewable resources such as solar power, wind energy and fusion are our future, society will continue to rely on the burning of fossil fuels for some time. But what if we could turn the resulting carbon dioxide back into fuel?

This reverse combustion is the goal of Professor of Chemistry Andrew Bocarsly. His team is exploring ways to use sunlight to convert carbon dioxide into fuels such as methanol, which can in turn be converted into gasoline. The technology is being commercialized by New Jersey-based Liquid Light, which was co-founded by Emily Cole, who earned her Ph.D. in 2009 in Bocarsly’s lab and now is exploring ways to scale up the technology.

Andrew Bocarsly and Emily Cole

Emily Cole (right), who earned her Ph.D. in 2009 in Andrew Bocarsly’s (left) lab, is director of chemistry at Liquid Light, a company she co-founded with Bocarsly to commercialize technology to convert carbon dioxide into fuels.

In the reverse combustion reaction, light drives the reaction of carbon dioxide and water in the presence of a catalyst and a semiconductor electrode to become methanol with the release of oxygen.

The project has received funding from the Air Force Office of Scientific Research (AFSOR), NSF and DOE. The collaboration between Liquid Light and the University was supported by the DOE Small Business Innovation Research program and the AFOSR Small Business Technology Transfer program.

One way to enhance the efficiency of this reaction is by improving the catalyst, but finding materials that efficiently drive the reaction is a challenge, said Princeton’s Emily Carter. She is carrying out theoretical calculations to identify new semiconductor electrodes that could improve the efficiency of reverse combustion. These electrodes are made from affordable elements such as iron and other metals that Carter, with funding from DOE and AFOSR, has already found have the potential to assist in the conversion of carbon dioxide to methanol.

“We aim to take the energy from sunlight, carbon dioxide and water and convert all three back into fuel,” Carter said. “It is really quite a trick to make that process run backwards.”

Further reading:

Cole, Emily B., Prasad S. Lakkaraju, David M. Rampulla, Amanda J. Morris, Esta Abelev and Andrew B. Bocarsly. 2010. “Using a One-electron Shuttle for the Multi-electron Reduction of CO2 to Methanol: Kinetic, Mechanistic, and Structural Insights.” J. Am. Chem. Soc., Vol. 132, no. 33: 11539-51.

Wagner, Sigurd, James C. Sturm and Naveen Verma. 2012. “Integrated All-silicon Thin-film Power Electronics on Flexible Sheets for Ubiquitous Wireless Charging Stations based on Solar-energy Harvesting.” Symposium on VLSI Technology, Paper C23-3.