Storing Sunlight for Soldiers

New technologies for capturing sunlight and storing it as energy are being pursued by the Air Force Research Laboratory (AFRL) to help soldiers on the ground and those operating unmanned aerial vehicles (UAVs).

Working with Plextronics Inc. and the Pennsylvania NanoMaterials Commercialization Center, both located in Pittsburgh, scientists at the AFRL Materials and Manufacturing Directorate (AFRL/RX) in Ohio developed a ready-to-use, cost-reducing technology that can capture sunlight and store it as energy to power global positioning system (GPS) components, portable communications, and other devices for US soldiers.

Plexcore® technology allows solar cells to form-fit soldiers' uniforms to power GPS components and communication devices, and could also be used to "print" solar panels onto thin films incorporated into military tents. Commercial applications include solar energy batteries for cell phones, radios, and other portable devices. (Photo: US Air Force)

"Military and commercial operations demand portable, highly efficient power sources. Using the power provided by natural sunlight via solar cells is an attractive option, yet has thus far been restricted by cost and size," said 2nd Lt. Christopher A. Vaiana of the directorate's Nonmetallic Materials Div.

AFRL/RX provided guidance and funding to develop a special "conductive ink" that can be used to make printed organic photovoltaic solar cell panels on very thin, flexible surfaces using ink-jet printing, Vaiana said. This new technology, called Plexcore®, made by Plextronics, allows solar cells to form-fit soldiers' uniforms. It can also be used to print solar panels onto thin films in military tents.

The commercialization center helped develop a roadmap identifying technologies AFRL/RX needs and is interested in funding. The center is responsible for reporting these needs to industry and requesting proposals, then reviewing proposals with AFRL/RX and selecting the most promising for funding. One result of the partnership is the program with Plextronics.

"Plextronics' new technology represents a significant step forward in printing inexpensive solar cells capable of powering a wide range of portable devices such as cell phones and radios. Key outcomes include lower costs and reduced logistical footprints for military operations across the battlefield environment," Vaiana said.

Plexcore is significantly less expensive than silicon-based solar cells, costing about $50 per square meter compared to $500 per square meter for silicon panels.

Dye-sensitized solar cells (DSSCs) are expected to power Air Force unmanned aerial vehicles (UAVs) in the future because they are an optimum energy harvesting source that may lead to longer flight times without refueling.

The University of Washington's Multidisciplinary University Research Initiative (MURI) project team, under lead researcher Dr. Minoru Taya, is working on airborne solar cells by using a flexible film and a thin glass coating with transparent conductive electrodes. He has found that DSSCs made from organic materials, which use dyes and moth-eye film, are able to catch photons and convert them into synthesized electrons that can harvest high photon energy.

Flexible, dye-sensitized solar cell. (Photo: Nagata and Taya, University of Washington)

A few years ago the team mounted dye-sensitized solar cells on the wings of a toy airplane. The propeller was effectively powered, but the plane was not able to become airborne because the glass-based solar cells they were using were too heavy. Upon experimentation, they decided to use film battery technology, which worked and in fact, enabled the plane to fly.

"These kinds of solar cells have more specific power convergence efficiency (PCE), very clean energy and easy scalability to a larger skin area of the craft, as well as low-temperature processing, which leads to lower costs overall," said Taya.

The team is currently working on DSSCs with higher PCEs using bioinspired dyes, which are installed in the wings of the UAV as airborne energy harvesters.

"Any airborne energy harvester must satisfy additional requirements, like weight and durability in airborne environments. If those are met, then there may even be longer UAV flight times," said Taya.

In the meantime, the engineers are researching the challenges of DSSCs' technology and are seeking to learn how durable they are and how well their technology may integrate with other Air Force vehicles. The team is also trying to determine how to build the solar cells in the wing surface of the aircraft and how to store energy harvested from them.

"Some of these challenges will be overcome by the researchers working under this AFOSR MURI within the next two years. In order to make the DSSCs' solar energy harvester transferable to the wings of a UAV, additional engineering tasks remain, which may require another project to be funded for five additional years," Taya said.

The team hopes to reach their goal of developing large, flexible DSSCs with higher energy conversion efficiency. Because solar cells that are larger usually have decreased efficiency, the team is using a metal grid that has high surface resistance and can accelerate electron transport for larger-sized flexible DSSCs while maintaining high efficiency.

For more information, visit: www.wpafb.af.mil/AFRL/afosr/