Energy Harvesting Developments Spring from Georgia Tech Research Projects

April 25, 2007 by Jeff Shepard

Recent announcements from the research community affiliated with Georgia Institute of Technology focus on the continuing developments in the field of energy harvesting. A nanogenerator that provides continuous power by harvesting energy from the environment has been unveiled, as have three-dimensional solar cells that capture nearly all of the light that they come into contact with.

Dr. Zhong Lin Wang, Regents’ Professor in the School of Materials Science and Engineering at the Georgia Institute of Technology (who is on the Advisory Committee of the upcoming nanoPower Forum), announced that researchers have demonstrated a prototype nanometer-scale generator that produces continuous direct-current electricity by harvesting mechanical energy from such environmental sources as ultrasonic waves, mechanical vibration or blood flow. Based on arrays of vertically-aligned zinc oxide nanowires that move inside a novel "zig-zag" plate electrode, the nanogenerators could provide a new way to power nanoscale devices without batteries or other external power sources.

The nanogenerators take advantage of the unique coupled piezoelectric and semiconducting properties of zinc oxide nanostructures, which produce small electrical charges when they are flexed. Fabrication begins with growing an array of vertically-aligned nanowires approximately a half-micron apart on gallium arsenide, sapphire or a flexible polymer substrate. A layer of zinc oxide is grown on top of substrate to collect the current. The researchers also fabricate silicon "zig-zag" electrodes, which contain thousands of nanometer-scale tips made conductive by a platinum coating.

The electrode is then lowered on top of the nanowire array, leaving just enough space so that a significant number of the nanowires are free to flex within the gaps created by the tips. Moved by mechanical energy such as waves or vibration, the nanowires periodically contact the tips, transferring their electrical charges. By capturing the tiny amounts of current produced by hundreds of nanowires kept in motion, the generators produce a direct current output in the nano-Ampere range.

Dr. Wang expects that with optimization, the nanogenerator could produce as much as 4 watts per cubic centimeter – based on a calculation for a single nanowire. That would be enough to power a broad range of nanometer-scale defense, environmental and biomedical applications, including biosensors implanted in the body, environmental monitors – and even nanoscale robots.

The Electro-Optical Systems Laboratory at the Georgia Tech Research Institute (GTRI) announced that it has developed three-dimensional (3D) solar cells that could boost the efficiency of photovoltaic (PV) systems while reducing their size, weight and mechanical complexity.

The new 3D solar cells capture photons from sunlight using an array of miniature "tower" structures that resemble high-rise buildings in a city street grid. The cells could find near-term applications for powering spacecraft, and by enabling efficiency improvements in PV coating materials, could also change the way solar cells are designed for a broad range of applications. The GTRI photovoltaic cells trap light between their tower structures, which are about 100 microns tall, 40 microns by 40 microns square, 10 microns apart – and built from arrays containing millions of vertically aligned carbon nanotubes. Conventional flat solar cells reflect a significant portion of the light that strikes them, reducing the amount of energy they absorb.

The researchers chose to make their prototypes cells from the cadmium materials because they were familiar with them from other research. However, a broad range of other photovoltaic materials could also be used, and selecting the best material for specific applications will be a goal of future research.

The new cells face several hurdles before they can be commercially produced. Testing must verify their ability to survive launch and operation in space, for instance. And production techniques will have to be scaled up from the current two-inch laboratory prototypes. Intellectual Property Partners of Atlanta holds the rights to the 3D solar cell design and is seeking partners to commercialize the technology.