- Nanogenerators to Enable Battery-Free Handhelds
- Forget super-sizing. Nano-sizing is the future of energy harvesters, according to researchers who claim to have mathematical proof that battery-free electronic devices can be powered by mechanical energy from vehicle vibrations to body movements.
Energy harvesting today is confined to a few niche applications, such as powering wireless sensor networks in remote areas where it is too expensive to send out crews to constantly be replacing batteries. However, by nano-sizing the working element in these energy harvesters, Northwestern University researchers claim their efficiency can be improved enough to enable battery-free mobile electronics.
The most advanced semiconductor materials today are enabling advances in electronics that impact every segment of society—from consumer to IT. Gallium nitride (GaN), for instance, has enabled ultra-bright light-emitting diodes (LEDs) to replace automotive headlight filaments and Blu-ray drives to pack 50GB on a CD-sized disk. Now Northwestern University researchers claim that by nano-sizing gallium nitride and other piezoelectric semiconductors like zinc oxide (ZnO), their efficiency can be boosted by 20 to 100 times, thereby enabling battery-free electronic devices to enter the mainstream.
Piezoelectric materials, like GaN and ZnO, are structured as crystalline lattices of highly polarized molecules called dipoles where one end is positively charged and the other negatively charged. Whenever such a piezoelectric material is bent, or otherwise stressed, the distribution of these dipoles is reorganized, resulting in a momentary excess of charge that can be harvested as a current to drive electronic circuits. Since the deformation is usually cyclic—for instance, bending back and forth in sync with vibrations—the resulting voltage induces an alternating current. (This is in contrast to the direct current that comes from a battery.)
Northwestern University researchers fabricated a micro-electro-mechanical system (MEMS) to test their piezoelectric nanowires. (Source: Northwestern University)
Until now, energy harvesters using piezoelectric semiconductors could only generate nanoamps of power, which was only useful to electronic circuits with very low power requirements. For instance, piezoelectric remote controls have been demonstrated by Arveni SAS (Grenoble, France), which are powered by the stress induced on a piezoelectric transducer when you push one of its buttons.
To move beyond such ultra-low-power applications to power handheld electronic devices like smartphones, piezoelectric materials need to vastly increase their power output. And now Northwestern University professor Horacio Espinosa claims that by nano-sizing the piezoelectric materials into bundles of tiny nanowires, their output can be vastly increased. This would open the door to powering handheld electronic devices from environmental motion, such as that supplied by the flexing of the bottom of your shoe as you walk.
Using a computational method called Density Functional Theory (DFT) to model nanowires, Espinosa and his graduate student Ravi Agrawal claim to have proved that nano-sizing them to 6 angstroms in diameter (1 nanometer equals 10 angstroms) the energy harvesting capabilities of GaN and ZnO can be boosted by 20 to 100 times, respectively, clearing the way for a new generation of battery-free electronic devices.