A group of US and Chinese researchers have developed a generator that can convert footsteps into electrical power, which may pave the way for large scale energy harvesting in urban areas.
We are surrounded by enormous amounts of wasted mechanical energy – our footsteps might propel us forward, but a lot of their kinetic energy is also lost as vibrations into the floor. Moving cars lose energy into the road, and the power of ocean waves causes walls (and coastlines) to crumble. There have been a number of efforts to harvest this type of ambient mechanical energy, as it’s known, but it has generally been focused on a much smaller scale – harvesting vibrations to power small sensors for example. But the team led by Zhong Lin Wang at the Georgia Institute of Technology, Atlanta, have used nanotechnology to produce a triboelectric nanogenerator, capable of producing large electrical power outputs, and with impressive efficiencies.
The nanogenerator comprises a polymer and gold sandwich. The top layer consists of two polymers, PMMA [Poly(methyl methacrylate)] and PDMS [polydimethylsiloxane], with a thin layer of gold in between.
The bottom layer consists of a block of PMMA coated with gold nanoparticles. When you walk or step on the generator, these two layers come into contact. Electrons from the gold nanoparticles flow into the PDMS layer, resulting in surface charges on each layer (negative on the gold, positive on the PDMS). As you lift your foot off the generator, the two layers separate and a potential difference (voltage) is produced, which drives electrons through the system; these can be tapped off as an electrical current. The nanoparticles are necessary to provide a large surface area than a flat gold surface would, dramatically increasing the number of charges that can accumulate. And each time the nanogenerator is stepped on, this cycle of charge build-up – separation – flow is repeated, scavenging energy as you walk.
To demonstrate the power of this effect, Wang and his team produced a nanogenerator smaller than a human palm which generated an instantaneous output power of 1.2 W. While this may not sound like much, it is enough power to light a bank of 600 coloured LEDs with a single human step!
The generator has an efficiency of 14.9% - i.e. it converts almost fifteen percent of the kinetic energy in a footstep into electrical energy. This compares favourably to other harvesting technologies like turbines and photovoltaics, but has the added benefit of producing power instantly, and the nanogenerator is not weather-dependent.
So the potential for this technology is vast. Maybe soon, we’ll be powering our own disco dance floor!
But the amount of energy you get out is pretty miniscule compared to the time, effort and energy required to make, install, and maintain any such system. And of course this will make it a little bit more tiresome for the individuals to walk about. There is never any real energy-payback with such systems. "Energy harvesting" is rarely ever really about "energy saving" - it's only real value is when no other source of energy is viable (eg in the middle of nowhere, impractical to have someone changing batteries on a regular basis etc etc).