Imagine your body as a human battery. When you walk or run — or do something even slower, like type — you create energy which keeps your smartphone, smartwatch, medical sensor, or other low-power device going. The faster you move, the more electricity you generate. Sound far-fetched? It may be closer than you think.
Such an innovation — which scientists predict could become reality within three years — comes from a device known as a triboelectric nanogenerator, or TENG, which is capable of capturing kinetic energy from human movements, as well as from other non-human energy sources, such as wind, wave, and machine vibrations. A TENG relies on the contact between two or more materials to produce electricity.
“We all have experienced that a balloon rubbed on our hair can [stick] to a wall or attract small pieces of papers,” said Ishara Dharmasena, a doctoral student at the University of Surrey, and lead scientist on the project. “Our clothing tends to stick to our skin on a dry day. We all have experienced lightening. All these are due to the triboelectric effect, or static charging. TENGs use the triboelectric effect to transform the movements…into electricity.”
In a real-life application, TENGs are connected to a motion source, such as a vibrating machine, clothing, and the contact/rubbing action of TENG layers results in the charging of triboelectric surfaces, he explained. When the charged triboelectric surfaces are moved relative to each other — contacting and separating or sliding — it creates a current.
“The power output is obviously dependent on the activity level,” Dharmasena said. “The faster your movements are, the higher the power generated. This technology can work with low frequency movements, that is, slower movements such as walking, typing, arm movements….[those] associated with typical daily movements for an average person.”
The effects of climate change have inspired a wealth of research seeking to find inventive clean and renewable ways of creating energy as an alternative to greenhouse gas emitting fossil fuels that intensify global warming. The Surrey researchers are among a number of scientists working toward the goal of using the human body as an energy source. Research similar to that of the University of Surrey, for example, also is underway at the State University of New York at Buffalo and the Institute of Semiconductors (IoP) at Chinese Academy of Science (CAS), while other scientists are experimenting with special outdoor sidewalks and indoor floors that can collect energy, and convert it to electricity, when people walk or run on them.
“The world urgently needs new forms of affordable and sustainable energy sources,” said Ravi Silva, director of the university’s Advanced Technology Institute (ATI). “TENGs could play a major role in making this dream a reality. TENGs not only present a wonderful opportunity for the consumer electronics industry, but they are an incredibly exciting material group that could be used in all countries and remote locations where the national grid does not extend, particularly for radios.”
Dharmasena agreed. “The future of global energy mix will depend on renewable sources, and TENGs allow us to capture energy from our surroundings,” he said. “This work redefines the way we understand energy harvesting, and allows researchers all over the world to exploit the true capabilities of triboelectric nanogenerators. We now expect this technology in household and industrial electronic products, catering to a new generation of mobile and autonomous energy requirements.”
The Surrey scientists, all from ATI, recently published a step-by-step guide on how to construct the most efficient TENGS, with the aim of improving their design and power output, and encouraging other scientists to make them. Their study appears in the journal Advanced Energy Materials.
“A TENG can be constructed very easily, simply by putting two plastic sheets together, and coating a metal on their back side,” Dharmasena said. “The contact and separation or sliding of these sheets result in the plastics being triboelectrically charged, and their movement producing electricity. An electric current can be obtained by connecting two wires to the metal layers. Any different TENG types can be designed and developed using various plastics, metals, fabrics etc. They can be made with fabrics such as cotton or silk, which are good triboelectric materials. They can also be made in different colors, or transparent, thicknesses and shapes. Therefore, they can be integrated to clothing providing excellent wearable and comfort properties.”
TENGs can be produced in sizes small enough to fit inside a bracelet or a shoe insole, while large TENGs can be designed to fit onto a pavement or smart floor, he said. “We can cover a machine, or a part of a vehicle with TENGs,” he said. “Some research groups have shown the capability of TENGs to be developed to mimic the skin — artificial skin or secondary skin — or TENGs that can be implanted into human [or] animal bodies.”
The invention can be used two ways, either incorporated into clothing made from the type of fabric that serves as a power unit, or attached as modules close to electronic devices, he said. “In both cases, TENGs can extract motion energy from walking, running, arm and leg movements etc., and can help powering the electronic devices,” he said.
Currently, energy harvesting requires a combination of technologies. “At present, TENGs can produce intermittent outputs, that is, the power/current is not continuous, and changes with movement,” he said. “Therefore, they need to be combined with capacitors etc., to ensure an effective power supply.”
Nevertheless, he added: “TENGs are ideal for powering remote sensors, for example, an electrical sensor placed outside a house, which can operate by capturing wind, and devices which require high level of mobility, such as medical or entertainment devices attached to a person. For instance, a TENG can be used to charge the battery of your mobile phone, fully or partially, during the day, using your body movements, or to operate the wearable sensors to provide health information.”
He said some estimates have the use of TENGs costing less than 3 cents per kWh, “although the actual cost for these devices is still to be evaluated, especially when it comes to large scale manufacturing,” he said. “However, TENGs can be made with common low-cost materials, with easy methods, therefore they can be cheap to make. Hence, scaling up TENGs is definitely a possibility in the near future.”