Walking

It is no secret that the automobile has shaped and defined cities for the past seventy years. No matter how many highways slice through cities or parking lots get built, cars will never define cities as much as people do. Walking is still the lifeblood of the city. Some people drive, but everyone walks. The average American walks around 5,000 steps per day, but this figure globally is closer to 7,000 [1]. Additionally, we can safely intuit that urban populations walk more than others, simply because it is denser and contains better walking infrastructure. Of course, though, walking is really just expending energy, in the same way a car or bus does. So, if we could capture and harvest this energy, we could use the natural movement of people in cities to generate electricity.

    A surprising amount of development has already occurred in footstep-harvesting pavements, and most use piezoelectric crystals to harness the energy. As mentioned previously in ‘Sound’, piezoelectric materials generate electricity when a stress is applied to them. A footstep is a perfect example of such a stress. The Japanese company Soundpower has been producing piezoelectric floor tiles for over ten years now, and has installed them in high-traffic areas like Fujisawa City Hall (population 429k), Shibuya central station in Tokyo, and in ticket gates throughout the Tokyo subway system [2]. Each 50 cm^2 tile produces about 0.1W per footstep, although the power may increase or decrease based on the weight of the individual taking the step. This corresponds to a power density of 20 W/m^2, which is comparable to desert-based, high-intensity photovoltaic plants. Of course, manufacturing these tiles is quite a bit more expensive, but further research should see costs decrease, just like solar over the past decade. There are also some concerns for the extremely environmentally-conscious, namely that the piezoelectric used in these tiles is PVDF [3]. While PVDF avoids the use of lead that other piezoelectrics use, the manufacturing of PVDF requires toxic chemicals, like perfluorooctanoic acid. However, proper manufacturing procedures should avoid any risk of environmental damage, and PVDF is an environmentally-safe chemical by itself.

    Another method of converting footsteps into energy is the novel use of triboelectric nanogenerators (TENGs). Triboelectricity is a phenomenon similar to piezoelectricity in that it produces electric energy from a mechanical deformation, but while piezoelectricity occurs within one material, triboelectricity occurs through the forced contact of two different materials. When these two materials have different natural charges and are forced together by a stress (from resting position slightly apart), electrons physically transfer materials that otherwise would not. As the stress releases, the electrons flow back to their original state, creating a current. Research published in October 2020 used TENGs mounted to floorboards to produce electricity. The materials used in the TENG were two widely available materials, copper and teflon, and they found a single footstep generated an average voltage of 86V and a current of 6.2 μA [4]. This is enough power to light up 110 LED bulbs for a few seconds. In places with relatively uninterrupted traffic flow, it is easy to see why TENGs are so promising in energy harvesting. Moreover, each nanogenerator is smaller than a shoe sole, so many of them could be implemented in a limited stretch of pavement.

    A significant benefit to these footstep-harnessing technologies is that they are robust to heavy traffic in a way that similar experiments in roadways have not been [5]. Therefore, the implementation of these devices is highly geared towards incentivizing walking - either through improving existing pavements or by developing future plans to prioritize pedestrian traffic over car traffic. In the developing world especially, where urbanization is set to skyrocket over the coming decades (and the vast majority will not own a car), these technologies can play an important role in increasing the viability of healthy, sustainable neighborhood development. Moreover, expanding walking infrastructure anywhere in the world will not only reduce the need for cities to consume external power (either renewable or not), it will also reduce CO2 and GHG emissions from cars, making the planet and cities healthier places to live. 

Citations [Movement]

[1] Althoff, T., Sosic, R., Hicks, J., King, A., Delp, S., and Leskovec, J. “Large-scale physical activity data worldwide reveal activity inequality,” Letter, 2017. doi:10.1038/nature23018.
[2] “The Power Beneath Your Feet,” Web Japan, 2017. https://web-japan.org/trends/09_sci-tech/sci100107.html. Accessed 29 Nov, 2020.
[3] Nia, EM., Zawawi, NAWA., Singh, BSM. “A review of walking energy harvesting using piezoelectric materials,” IOP Conf. Ser.: Mater. Sci. Eng, 2017.
[4] Yao, M., Xie G., Gong, Q., and Su, Y. “Walking energy harvesting and self-powered tracking system based on triboelectric nanogenerators,” Beilstein Journal of Nanotechnology, 2020. doi:10.3762/bjnano.11.141
[5] Li, C. “Road Performance of Common Piezoelectric Transducer for Asphalt Pavement Energy Harvesting,” Applied Mechanics and Materials, 2015.