Energy Harvesting for Future UAVs Published Sept. 18, 2009 By Maria Callier Office of Scientific Research ARLINGTON, Virginia -- Dye-sensitized solar cells constitute an emergent technology expected to power Air Force unmanned air vehicles of the future. The basis of this expectation lies in the cells' capacity as optimum energy harvesters, power sources that may ultimately enable longer flight times without additional refueling. The Air Force Research Laboratory is funding the work of a University of Washington Multidisciplinary University Research Initiative team towards developing these important airborne solar cells. The MURI researchers are using a flexible film and a thin glass coating with transparent conductive electrodes to conduct their experimentation, an approach based on lead researcher Dr. Minoru Taya's discovery that DSSCs made from organic materials--specifically, (dyes and) moth-eye film--are able to catch photons and convert them into synthesized electrons that can harvest high photon energy. Early in its efforts, the team mounted DSSCs on the wings of a toy airplane. Though the cells effectively powered the propeller, the plane itself was unable to become airborne because the glass-based cells were too heavy. Upon further experimentation, the researchers decided to use film battery technology for the cells, a tactic that enabled the plane to fly. This type of solar cell has more specific power convergence efficiency. Further, these cells produce very clean energy, are easily scaled to a larger skin area of the craft, and tolerate low-temperature processing, qualities leading to lower overall costs. The MURI team is currently working to develop higher-PCE DSSCs using bioinspired dyes. Like any airborne energy harvester, DSSCs must satisfy certain requirements unique to the airborne environment in order to facilitate extended flight times. Accordingly, the team is addressing challenges related to DSSC weight, durability, and integration with other AF vehicles. The researchers are also devising a way to build the technology directly into the wing surface (i.e., its ideal installation), as well as a means for storing harvested energy. Ultimately, the AFRL-sponsored team hopes to reach its goal of developing large, flexible DSSCs offering maximized PCE. Generally, the larger a solar cell, the less efficiency. To circumvent this problem, the researchers are employing a metal grid, which maintains high efficiency while also facilitating the accelerated electron transport needed for larger-sized flexible DSSCs.