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| Fig. 1: Human interaction with a resistive touchscreen. A voltage is produced when conducting layers (4) are pressed together. (1) is the glass panel, (2) is a resistive coating, and (3) shows micro-insulators. (Source: Wikimedia Commons) |
As energy needs rise with advances in technology and increases in the global population, alternative energy sources are more critical than ever. The most prominent alternative energy sources are solar, wind, and biomass because they produce the greatest amount of energy. However, the problems with these sources are that they require large start-up costs and are often limited by geography. Thus, it is important to invest in more accessible, small-scale sources of energy including those found at your fingertips.
Piezoelectric energy harvesting is one such alternative energy source. Energy harvesting refers to taking energy from some source and converting it into electricity. [1] In this case, the energy harnessed comes from the mechanical deformation of a piezoelectric material, which results in a voltage output that is converted to energy. A piezoelectric material must be non-centrosymmetric so that when a force is applied, electric dipoles are aligned unevenly, inducing a polarization. In this way, charge is generated in the material. This polarization leads to a voltage difference in the electrode layered on the piezoelectric; in this voltage difference, the charges will move accordingly to minimize their potential energy. This leads to a current that travels to the rest of the circuit. [2] This is known as the direct piezoelectric effect. But the inverse could also happen - an applied electric field can be used to polarize the stacks, which induces deformation in a piezoelectric. This is the converse piezoelectric effect. Because it goes both ways, the effect leads to a coupled system of equations
| Direct Effect: | D = d × σ + ε × E |
| Converse Effect: | S = s × σ + d × E |
Where D is the electric displacement, S is strain or the physical displacement, d is the piezoelectric coefficient, σ is stress, ε is the permittivity of the material, E is the applied electric field, and s is the mechanical compliance or stiffness. [3] All of these are tensor quantities which depend on directions and dimensions. Common piezoelectric materials include quartz, piezoceramics such as lead zirconate titanate (PZT), and polymers such as polyvinylidenefluoride (PVDF). [4] Each of these has a significant piezoelectric coefficient, meaning that they can produce a significant voltage when a force is applied. For PZT, the piezoelectric coefficient for a force applied perpendicular to the face, or d33, is reported to be 460 x 10-12 Coulomb/Newton. [2]
Piezoelectric energy harvesting from humans has primarily been implemented in pavements. The kinetic energy from human walking on a piezoelectric can produce 19 mW/cm3. [4] These pavements have been implemented in Japan, and have been proposed to be implemented in high traffic areas such as Magic Kingdom. [5] However, these energy values are small compared to the energy needed to power these places. One problem is that human walking has a very low frequency compared to the resonant frequencies of piezoelectrics, which allows for the greatest efficiency. [1] Piezoelectric ceramics have a resonant frequency of 100 Hz, whereas human walking only has a frequency of 1 Hz. [4]
Obviously, the solution to this is to harness mechanical energy from human activity with a greater frequency than walking which is teenagers using their phones! With the high frequency and duration of adolescent phone use, surely enough energy could be generated to turn piezoelectric energy harvesting into a prominent alternative energy source.
A touchscreen has multiple layers including a glass coating, insulating and conductive layers that send signals through currents when they are pressed together. Since touchscreens work through deformation, they are ideal candidates for piezoelectric energy usage. These components of a resistive touchscreen are pictured in Fig. 1. Some considerations to keep in mind: implementing energy storage is not feasible due to size constraints of the phone. The best way to make use of the energy harnessed from piezoelectricity is to send it right back into the battery, enacting a self-charging mechanism. Piezoelectrics work best when excited at their resonant frequencies, which implies a constant stimulus, so irregular phone usage already does not bode well for producing energy for this self-charging mechanism. Fully charging an iPhone takes about 1.5 x 10-3 kWh, so piezoelectric energy harvesting needs to meet this number. [6]
Let us consider a touchscreen comprised of PZT, ignoring the fact that it has lead which is toxic and increasingly being regulated. [2] A human touching their phone screen generates a force of 0.5 Newtons per tap and around 1 Newton for a slide. [7]
Since the amount of force a human touch applies is below the mechanical yield point of the screen, we can assume that we are in the elastic regime of the PZT and thus the touchscreen will not permanently deform or break upon a touch. This also allows us to define a time-independent force F(t) = F. Our simplified mechanical energy then comes from the equation for work:
Where d is the displacement. We can estimate the displacement from the strain induced by an interaction with the touchscreen. The relationship between stress σ and strain ε is quantized by Young's modulus Y as
| Y | = | σ ε |
= | (F/A) (d/L) |
Or the force applied per area A divided by the fractional change in length L of the material. A is the area of a fingertip on a screen. This is about π⋅ r2 for radius 1.5 centimeters. L is the thickness of the screen since the force is applied perpendicular to the screen, which is about 9 millimeters for an iPhone. [8] The Young's modulus of PZT has also been measured to be around 85 gigapascals at room temperature. [9] Plugging in numbers, the amount of mechanical energy generated per tap is
| Emech | = | F2 L A Y |
= | (0.5 N)2 × 0.009 m π × (0.015 m)2 × (85 × 109 N/m2) | = | 3.75 x 10-11 J |
where N is Newtons, m is meters, J is Joules, and we know 1 J = 1 N⋅m. For sliding, the force is twice as large, so Emech,slide = 4 × Emech,tap = 1.5 × 10-10 J.
These are very small numbers. We also need to account for the energy conversion efficiency from mechanical to electrical, which depends on the electromechanical coupling factor and the physical setup, it is a maximum of about 80% for a PZT stack. [10]
Let us say that our teenager spends half their time texting friends and half their time scrolling on TikTok. A teenager has an average screen time of 4 hours. [11] If 2 hours are spent texting, with an average of 5 taps per second (frequency of 5 Hz), then this will produce Ntap = 5 taps/s ⋅ 3600 s/hr ⋅ 2 hr = 36,000 taps/day. During their 2 hours on TikTok, they probably scroll after watching a video for 15 seconds. So Nslide = 1 slide / 15 s ⋅ 3600 s/hr ⋅ 2hr = 480 slides/day. So the total electrical energy produced by this teenager is:
| Eelec | = | 0.8 × [Ntap × Emech,tap + Nslide × Emech,slide] |
| = | 0.8 × [(36000 taps d-1 × 3.75 × 10-11 J tap-1) + (480 slides d-1 × 1.5 × 10-10 J slide-1)] | |
| = | 1.14 × 10-6 J/day |
or 3.16 × 10-13 kWh per day. This could be significant! Unfortunately, it is not. This is 10 orders of magnitude smaller than the required amount of energy to fully charge an iPhone.
The solution is simple; the teenager only needs to slide 1011 × Nslide times per day to produce enough electrical energy to self-charge their phone. This would only require the teenager to scroll past 1 billion TikToks per second for all 24 hours of the day... . Unfortunately, this might be a bit much even for the most screen-addicted teenager.
The problem with a piezoelectric touchscreen specifically is that the displacement of the screen is too small to produce much energy. The problem with piezoelectric devices as a whole is that they have low energy conversion efficiency. Mechanically, energy is limited by any damping effects in the structure of the phone or in the elastic modulus of the material; overstretching a material to harness more energy will only result in the material breaking. How much energy can be harnessed from the transducer is then determined by the piezoelectric coefficient of the material. Finally, transmitting the voltage produced by the material through the circuit to wherever else the electricity will be used can lead to losses although wires are quite conductive, so these losses will be minimal after impedance matching. [1]
In conclusion, there is both good and bad news. The bad news is that a self- charging phone using piezoelectric energy harvesting is not feasible. The good news is that you can scroll on TikTok without worrying too much about your energy consumption. Perhaps mental consumption becomes a bigger worry at that point!
© Diana Spulber. 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.
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[3] N. Sezer and M. Koç, "A Comprehensive Review on the State-of-the-Art of Piezoelectric Energy Narvesting," Nano Energy 80, 105567 (2021).
[4] E. M. Nia, N. A. W. A. Zawawi, and B. S. M. Singh, "Design of a Pavement Uing Piezoelectric Materials," Materialwiss. Werkstofftech. 50, 320 (2019).
[5] L. Jimenez, "Disneyland Electric Pavement," Physics 240, Stanford University, Fall 2022.
[6] S. Manoharan et al., "A Review on Smartphone Charger: Technologies and Challenges," IEEE 10511183, 5th Intl. Conf. on Intelligent Communicaion Technologies and Virtual Mobile Networks, 11 Mar 24.
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[8] J. Xiong and S. Muraki, "Effects of Age, Thumb Length and Screen Size on Thumb Movement Coverage on Smartphone Touchscreens," Int. J. Ind. Ergon. 53, 140 (2016).
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[10] J. Cho et al., "Efficiency of Energy Conversion by Piezoelectrics," Appl. Phys. Lett. 89, 104107 (2006).
[11] B. Zablotsky et al., "Daily Screen Time Among Teenagers: United States, July 2021-2023," U.S. National Center For HealthStatistics, Data Brief, No. 513, 2024.
