Recent university and scientific lab research from around the world has focused on the potential of kinetic energy harvesting methods, but it may be too soon to see wide-spread commercially viable production.
“We set out to answer the question of whether you can get useful amounts of electrical power out of something that looks like a plant. The answer is ‘possibly,’ but the idea will require further development,” says Michael McCloskey, associate professor in genetics at Iowa State University. McCloskey commented on a university research project to create a kinetic energy harvesting device that mimics cottonwood tree leaves with the goal of turning wind energy into electricity, as reported this month in the Energy Harvesting Journal online.
McClosky’s reluctance to predict reproducible outputs of the technology are typical of many recent announcements of new kinetic energy harvesting developments: it’s too soon to expect commercial rollouts of these concepts. But everyone feels it is only a matter of time before they become practical.
In the last year alone, several announcements have reported on a variety of approaches to the challenge of harvesting energy from natural sources. Much of the research is based in a process that has attracted scientists searching for more efficient technologies that can capture the energy that is found everywhere in the world: micro amounts of ambient energy produced by the wind; natural movements by animals, plants or people; and an infinite array of natural processes found in nature. These biomimetic processes are the focus of research and experimentation in fields as diverse as cell biology (e.g., McClosky’s ISU group), sports and exercise equipment design, and even architecture.
Powers of three
Many of these new energy harvesting efforts tend to focus on the fundamentals of kinetic energy and fall into three basic approaches: electrostatic effects, piezoelectric effects (mechanical energy conversion to electrical energy), or triboelectric energy approach (where energy is produced when two different materials come into moving contact with each other). All of them require some mechanical (kinetic) motion that induces a process to convert that motion into electrical charges. The majority of the research has concentrated on piezoelectric processes that engage with specific human activities (e.g., walking, exercising or cycling), to capture the associated energy available from regular human motions— in other words, regular repetitive motions versus irregular or occasional activity.
Feet, keep walking
In a paper published in the journal Scientific Reports, researchers at the University of Wisconsin-Madison put forward a proposal that captures energy from human motion using footwear-embedded energy harvesters. “Theoretical estimates show that it can produce up to 10 watts per shoe,” says Tom Krupenkin, professor of mechanical engineering. “A total of 20 watts from walking is not a small thing, especially compared to the power requirements of the majority of modern mobile devices.”
However, Krupenkin and co-author J. Ashley Tayor, senior scientist in UW-Madison’s Mechanical Engineering Dept., found the need to develop a new method of directly converting mechanical motion into electrical energy using a “reverse electro-wetting” process that Krupenkin and Taylor pioneered in 2011. This process uses a conductive liquid sandwiched between nanofilm-coated surfaces, and is combined with a bubbler method that uses reverse electro-wetting and the energy released from the growth and collapse of gas bubbles.
The device uses two flat plates separated by a small gap filled with a conductive fluid, the bottom plate perforated with tiny holes through which pressurized gas bubbles emerge. “The bubbles grow until they are large enough to touch the top plate, causing the bubbles to collapse. The speedy, repetitive growth and collapse of bubbles pushes the conductive fluid back and forth [between the layers of nanofilm], generating electrical charge(s).”
Another system has appeared that seeks to capture energy from foot power. The U.K.-based company, Pavegen, has developed a commercial kinetic energy harvesting system based on electro-magnetic devices within a flooring system that captures energy from people walking or dancing on the flooring, and stores the energy captured to power nearby lighting fixtures. (“Pavegen harvests power from footsteps in Washington, D.C.” and “Energy harvesting tiles to be installed for the French National Railway“)
However, some of the recent research, such as the ISU lab research, focuses on harvesting regular motions found in nature, an approach called biomimetics, which uses artificial means to mimic natural processes. McCloskey and team members Curtis Mosher, an associate scientist at Iowa State, along with Eric Henderson, professor of genetics, were inspired by the regular pattern of motion produced by cottonwood leaves, whose flattened leaf stalks produce an oscillating motion that optimizes energy generation in flexible polymer piezoelectric strips. McCloskey, Mosher and Henderson published their paper this month in the peer-reviewed academic journal Plos One.
Another paper reports on the potential of harvesting mechanical energy from a variety of textiles under compressive and bending forces, again a piezoelectric means of capturing energy. The research, conducted primarily at the University of Southhampton, U.K., and in collaboration with the U.K.’s Energy Harvesting Network, suggests that low-temperature screen-printable piezoelectric nano-composite films placed on flexible polymer and textile substrates can produce potentially useful amounts of energy when the textiles are bent.
“The potential for this material to be used to harvest mechanical energy from a variety of textiles under compressive and bending forces has been evaluated theoretically and experimentally,” the Southhampton researchers report. “The maximum energy density of the enhanced piezoelectric material under 800 N compressive force was found to be 34 J/m3 on a Kermel [meta aramid] textile. The maximum energy density of the enhanced piezoelectric material under bending was found to be 14.3 J/m3 on a cotton textile. These results agree very favorably with the theoretical predictions. For a 10 x 10 cm piezoelectric element 100 µm thick this equates to 38 μJ and 14.3 μJ of energy generated per mechanical action respectively which is a potentially useful amount of energy.”
Rubbing it in
Finally, a promising direction was recently reported here in Advanced Textiles Source, and features a triboelectric approach. “Tribo” stems from the Greek root word tribein, meaning to rub, hence triboelectric is a process that involves rubbing two different materials against each other to produce electrical charges. Georgia Institute of Technology researchers, led by Dr. Zhong Lin Wang, propose using a triboelectric effect combined with electrostatic induction to generate small amounts of electrical power from kinetic action, such as wind or human motion.
The application of these kinetic energy harvesting methods predictably will be part of the Internet of Things (IoT) as well as a growing market for wearable technology. Many of these kinetic approaches can power the low-voltage requirements of the increasingly efficient electronic devices that are expected in the future.