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| Fig. 1: A piezoelectric ceramic soldered with wires to measure the voltage produced by a mechanical stress. (Source: Wikimedia Commons) |
When certain types of crystals undergo mechanical stress - such as being compressed or squeezed - an electrical potential results, the intensity of which is proportional to the magnitude of the stress. [1] If the crystal is then allowed to decompress and return to its original form, it will produce further voltage should another mechanical stress be applied. Quartz is perhaps the most well-known example of a crystal that displays this property, called the piezoelectric effect.
It is key to note that so long as a piezoelectric material is intact - has not broken down, as is inevitable over time - a voltage will be produced every time a new mechanical stress occurs and is released. If that voltage can be connected to a circuit, resulting in a current, it is possible to utilize piezoelectric materials undergoing mechanical stress to generate electricity.
In a world necessarily shifting towards renewable and clean energy, piezoelectricity has a possible role by contributing a stream of electrons back to the electrical grid, relieving a need for some quantity of fossil fuel to be burnt in electricity production. A wealth of conceivable applications meeting that goal exist.
Imagine, for example, piezoelectric crystals embedded in the sidewalks of high-foot-traffic metropolitan areas like New York City of Tokyo, compressing and stressing tens of thousands of times daily beneath the active pedestrians. Or think about placing piezoelectric materials at the base of a mighty waterfall such as Dettifoss in Iceland, the most powerful in all of Europe by water flux, using the force produced by the large mass of water and gravitational acceleration to repeatedly stress the crystals. [2]
Examples of large-scale attempts at generating piezoelectric power are not common, but one particular well-documented example exists at Club Watt, a night club in Rotterdam. There in 2008, a 270 square foot, piezoelectric dance floor was installed by the management. According to Club Watts research, an average individual dancing produced roughly 20 watts of electricity. [3] All told, the club hopes to produce 10% of their energy needs from the floor. However, at a cost of $257,000 - nearly $1,000 per square foot - the floor was not cheap. [3] Yet, the company reports roughly an inquiry per day from other clubs interested in doing the same and attracts a large crowd from patrons who support their eco-friendly initiative.
Advances in piezoelectric materials have been made with ceramics (see Fig. 1), lead zirconate titanate, and polyvinylidene fluoride all promising for different applications. For example, lead zirconate titanate is cheap and easily obtainable, while polyvinylidene fluoride is more robust and better for rapid frequency stresses. [4] And while the efficiency of converting the initial mechanical stress into usable energy certainly depends on the material used, there is a lack of consensus about the upper limit of piezoelectric yield.
On the optimistic side, Kim et al. have conceptually claimed that yields can reach above 80%. [5] Shafer and Garcia, by contrast, mathematically put the absolute maximum efficiency of piezoelectrics at 44 percent. In trials, Akaydin et al. measured an efficiency of a meager 0.72%. [6,7]
However, take a 100 kg person who dances at an average pace of one step per second and compresses the piezoelectric disc by 0.5 cm each time. Given a perfectly efficient piezoelectric materials, the output per step would be 100 kg × 9.8 m/sec2 × .005 m = 4.9 joules. Dividing the joule amount by the time between jumps, we get 4.9 J / 1sec = 4.9 Watts. Think of a standard 100 Watt light bulb; you would need more than 20 people dancing to power it, an extreme amount. This only gets worse when an inefficient material is used.
Harnessing the piezoelectric effect to generate electricity in a clean, renewable manner is feasible from a physical perspective, as evidenced by Club Watt's success in a commercial setting and the genuine popular interest in its growth that resulted. And even though it takes a lot of dancers to light one bulb, the energy being expelled by the club participants is otherwise not being harnessed productively at all. However, piezoelectricity can only ever be useful if it is economically viable, which rests on its efficiency reliably increasing to levels in agreement with Kim et al.'s findings and the cost of generation equipment falling from the near $1000-per-square-foot price tag that Club Watt paid. [5]
© Jamie MacFarlane. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
[1] S. R. Anton and H. A. Sodano, "A Review of Power Harvesting Using Piezoelectric Materials (2003 - 2006)," Smart Mater. Struct. 16, R1 (2007).
[2] A. Brandenburg, "Disc Turbulence and Viscosity," in Theory of Black Hole Accretion Disks, ed. by M. A. Abramowicz, G. Björnsson and J. Pringle (Cambridge University Press, 1999), p. 61.
[3] E. Rosenthal, "Partying Helps Power a Dutch Nightclub," New York Times, 23 Oct 08.
[4] G. Shang et al., "On Piezoelectric Harvesting Technology," Adv. Mat. Res. 516-517, 1496 (2012).
[5] M. Kim, J. Dugundji, and B. L. Wardle, "Efficiency of Piezoelectric Mechanical Vibration Energy Harvesting," Smart Mater. Struct. 24, 055006 (2015).
[6] M. W. Shafer and E. Garcia, "The Power and Efficiency Limits of Piezoelectric Energy Harvesting," J. Vib. Acoust. 136, 021007 (2013).
[7] H. D. Akaydin, N. Elvin, and Y. Andreopoulos, "The Performance of a Self-Excited Fluidic Energy Harvester," Smart Mater. Struct. 21, 025007 (2012).
