Harvesting energy with piezoelectrics
Our roadways form an invaluable network. They make our modern life possible by connecting families, transporting goods and driving our economy by bringing millions to work, daily. Could we soon include “power plant” in their description?
Step outside and you can see and feel many energy sources – sun, wind, waves and tides. The energy supply is seemingly infinite, but that energy must be captured and converted to a usable form if we want to put it to good use.
It turns out that roadways can be pretty good collectors of energy. The five million kilometers of asphalt pavements in North America are excellent solar collectors, and many have seen the potential of harvesting that energy for electricity, using photovoltaic panels or thermoelectric devices, or heat by using hydronic pavements. Asphalt pavements also are collectors of mechanical energy, deflecting and then relaxing under the load of each passing vehicle. On their own, these pavement vibrations are not of much use, but if that energy could be converted into a usable form, such as electricity, new possibilities are opened. This is where piezoelectric devices come in.
Piezoelectric materials generate an electrical charge in response to mechanical deformation – this is known as the piezoelectric effect. Many materials exhibit the piezoelectric effect; some commonly used materials are quartz, lead zirconate titanate (PZT), zinc oxide, and polyvinylidene fluoride. Many piezoelectric materials can produce potential differences of thousands of volts. Nonetheless, the current (and thereby the power) produced by such materials is low and so multiple piezoelectric devices are often stacked in parallel to improve the energy output. Overall energy output increases with the magnitude and the frequency of the applied strain. Today piezoelectric devices are commonly used in sensors and actuators and have been used to harvest energy from vibrations in the floors of nightclubs and subway turnstiles.(1) Could they be used to harvest energy out of roads?
Several theoretical, laboratory, and field studies have been conducted to try to answer this question using several different device designs. Two frequently examined designs are compression-based systems and cantilever systems. In compressive systems, stacks of piezoelectric materials are arrayed within some sort of tile. As the array is compressed under each axle of a passing vehicle, a pulse of power is generated. In cantilever systems, passing vehicles induce vibrations in a beam of piezoelectric material. In such a system, power continues to be generated after the vehicle has passed while the vibrations decay. Depending on the design of the device, piezoelectric generators might be placed at the road surface or embedded in the pavement, beneath the surface course.
Energy output from a roadway with embedded piezoelectric generators will be dependent on the type of device, and the type and frequency of traffic. Heavier vehicles will increase the load on the piezoelectric device and generate more power. Higher speeds, higher traffic, and a higher number of axles per vehicle will increase the energy output by increasing the number of times each generating device is activated. Any energy generated by the roadway could be fed into the electrical grid or used to charge batteries to power streetlights or road signs.
Determining the potential magnitude of energy output from a piezoelectric roadway requires a number of assumptions about the traffic and efficiency to be made. Most field trials of this technology involve only small stretches of roadway, as short as a few meters, with results extrapolated to estimate energy output over longer lengths of roadway. In a report(2) prepared for the California Energy Commission (CEC) in 2013, power metrics from several device manufacturers and trials were tabulated – claimed energy outputs were in the range of 0.16 – 22.6 kWh per vehicle per kilometer. With an assumption of an average of 600 vehicles per hour, the daily energy generation could be in the range of 100 kWh to 13MWh per kilometer per day. For reference(3) the average American home uses 11 MWh per year or roughly 30 kWh per day. In a more recent study(4), data from a trial of a piezoelectric energy-generating roadway led to an estimated energy generation of 125 kWh per day per kilometer, assuming one vehicle (specifically a Nissan Altima in this study) passes on average every three seconds (1200 h-1). Obviously in any scenario increasing or decreasing the volume of traffic would have a corresponding impact on the amount of energy generated. Also important is that the passing vehicles actually pass over the embedded generators – consistent wheel paths are a must!
Modern LED streetlights might require only a couple of Watts to operate, but even assuming higher power 100 W incandescent bulbs, the total energy requirement to have lights lit every 25 m for 12 hours of the day would be only 48 kWh. Thus, it seems feasible that a piezoelectric roadway could power its own lighting while feeding excess energy to the grid.
Even so, is the juice worth the squeeze? The aforementioned CEC found the installed cost to be in the range of $2000 – 4000/kW, compared to ~ $1000/kW for solar panels or wind turbines.(5) The upfront cost is acceptable provided that the lifetime of the piezoelectric devices is sufficiently long. For example, if a 30-year lifetime is assumed, the levelized cost of energy could be as low as $0.01/kWh.2 However, researchers from Virginia State University found that the power outputs from six experimental devices installed at weigh stations were at or trending toward zero within twelve months.(6) Thus, it is paramount that device durability is measured and considered. Even if the piezoelectric generators do not fail, if the surrounding pavement needs repair or replacement, the investment could be lost. Attention should be given to the impact of embedded piezoelectric generators on pavement lifetime (especially as the device and surrounding pavement will have disparate stiffnesses and coefficients of thermal expansion) and the ability to conduct pavement maintenance.
One might also consider the law that has stumped many would-be billionaire inventors of perpetual motion machines – namely conservation of energy. If the piezoelectric generators are simply making use of deformations in the pavement that would occur with or without their presence and otherwise be dissipated as heat, there is no concern. Conversely, if compressing the piezoelectric devices robs passing vehicles of a portion of their forward kinetic energy beyond what is normally lost, their fuel efficiency would be reduced, leaving, in essence, streetlights powered by gasoline engines of the passing cars. Larger field studies might be needed to determine how much efficiency is stolen from passing traffic.
The technology is yet immature and pertinent questions need to be answered, but it could become one answer among many to improve our energy efficiency. Piezoelectric pavements might just be the way of the future. Alongside learning from audiobooks, think of it as another way to make your morning commute more productive – generate a kWh before your first meeting.
Noël is an Asphalt Technical Advisor for ExxonMobil and is based in Ontario, Canada.
References
1 Hurley, Billy. 2017. “A Piezoelectric Highway? Engineers Take Another Test Drive”. Tech Briefs.
2 Hill, Davion; Tong, Nellie, (DNV KEMA). 2013. “Assessment of Piezoelectric Materials for Roadway Energy Harvesting”. California Energy Commission. Publication Number: CEC-500-2013-007.
3 US Energy Information Administration. 2019. https://www.eia.gov/energyexplained/use-of-energy/electricity-use-in-homes.php
4 Sun, Jian-Qiao; Tian-Bing, Xu; Atousa, Yazdani. 2020. “Ultra-High Power Density Roadway Piezoelectric Energy Harvesting System”. California Energy Commission. Publication Number: CEC-500-2023-036.
5 Stehy, Tyler; Duffy, Patrick. 2020. “2020 Cost of Wind Energy Review”. National Renewable Energy Laboratory. Technical Report: NREL/TP-5000-81209.
6 Xiong, Haocheng. 2014. “Piezoelectric Energy Harvesting for Public Roadways”. PhD Thesis. Virginia State University.
