Principles of Piezoelectric Energy Harvesting
What Is Energy Harvesting
Energy harvesting is approaching an interesting technological juncture wherein the power requirements for electronic devices have been reduced while at the same time the efficiency of energy harvesting devices has increased. Out of various possible energy harvesting technologies, piezoelectric vibration energy harvesting has emerged as a method of choice for powering meso-to-micro scale devices.1,2,3,4 Piezoelectric materials and transducers can be designed to handle a wide range of input frequencies and forces allowing for energy harvesting to occur.
Principles
Piezoelectricity is found in crystalline materials that possess non-centrosymmetry. This effect induces an electric polarization proportional to an applied mechanical stress (direct piezoelectric effect) or a mechanical strain proportional to an applied electric field (converse piezoelectric effect). During vibration energy harvesting, piezoelectric materials convert mechanical strain into an electrical charge or voltage via the direct piezoelectric effect. The power output of a particular piezoelectric energy harvester depends upon intrinsic and extrinsic factors. Intrinsic factors include the frequency constant of the piezoelectric element, piezoelectric and mechanical properties of the material, and the temperature and stress dependence of the physical properties. Extrinsic factors comprise of the input vibration frequency, acceleration of the base/host structure, and the amplitude of the excitation. Figure 1 illustrates different configurations of piezoelectric harvesters and their features. The combination of the mechanical architecture and material properties allows for variations in the frequency operating range and the power output.
Figure 1. Different configurations of piezoelectric energy harvesters and their features.
The efficiency and the power density of a piezoelectric vibration energy harvester are strongly frequency dependent, because, the piezoelectric material generates its maximum power at the electromechanical resonance frequency. The low frequency fundamental mode should be targeted in the design of the energy harvesting device since the potential output power is proportional to 1/f (f = the frequency of the fundamental vibration mode). That said, the lower the frequency of the vibration base, the more complex it becomes to design the energy harvesting structure, as the dimension and weight constraints limit the use of the ceramics to achieve the desired fundamental frequency.
Design
Cantilever geometry is one of the most widely used architectures in piezoelectric energy harvesters, especially for mechanical energy harvesting from vibrations, because, a large mechanical strain can be produced within the piezoelectric material during vibration. Additionally, the fabrication of piezoelectric cantilevers is relatively simple and inexpensive. More importantly, the fundamental bending mode of a cantilever is much lower than the other vibration modes of the piezoelectric element. The majority of the piezoelectric energy harvesting devices developed employ a unimorph (one layer of piezoelectric material bonded to a non-piezoelectric layer) or a bimorph (two layers of piezoelectric material bonded to a non-piezoelectric layer) with a cantilever design. Bimorph piezoelectric cantilevers are more commonly used in piezoelectric energy harvesting studies than unimorphs, because, the bimorph structure doubles the energy output without any significant increase in the device volume. To further lower the resonance frequency of the cantilever, a proof mass can be attached to the free end of the cantilever. The power output of a cantilever energy harvester has been found to be proportional to the proof mass. The zigzag cantilever having smaller stiffness can also be used to lower the resonance frequency of structure.
Off-resonance energy harvesters, such as cymbal transducers and piezoelectric stacks, represent a different architecture of energy harvesters. The cymbal transducers with a force amplifying structure, can be used to improve the power output of the energy harvester. In the stack architecture, piezoelectric materials utilize the d33 mode, which has a higher coefficient than d31. Due to the high stiffness of cymbal and stack structures, the natural frequency of piezoelectric stacks is usually over 1 kHz, which is beyond most of natural vibration in the environment (natural vibrations usually exist in the range of 10 Hz – 100 Hz). Further the cymbal and stack structures have low sensitivity to applied stress, limiting their potential use in energy harvesting from natural low amplitude ambient vibration sources. While most cymbal transducers and piezoelectric stacks are used off-resonance they can withstand larger mechanical load (especially piezoelectric stacks) than cantilever energy harvesters. This makes them suitable for achieving a higher energy output than cantilever energy harvesters.
Piezoelectric energy harvesting has been utilized at different scales ranging from several square meter piezoelectric floors to sub-micron nanowire arrays. Based on their size, piezoelectric energy harvesters can be classified into three groups: (i) macro- and mesoscale, (ii) MEMS scale, and (iii) nanoscale. The size of a piezoelectric energy harvester is dictated by a variety of parameters such as its weight, dimensions, fabrication method, power output level, and potential application areas. Energy harvesting devices involving manual assembly and bonding can be categorized as macro- and mesoscale. Devices fabricated using standard photolithography techniques can be categorized as MEMS scale. Nanoscale energy harvesters involve the use of piezoelectric nanowires.
Piezo Material Selection Considerations
Besides the structural design requirements, the selection of a certain piezoelectric material for a specific energy harvesting application is equally important. Because, the main focus in vibration energy harvesting is to convert the maximum fraction of input mechanical energy into electrical energy, a material with high electromechanical coupling factor, k, is preferred. The square of k defines the efficiency of the material at converting the input mechanical energy to the output electrical energy. A more precise figure of merit for piezoelectric energy harvesters can be derived by considering the power response of the piezoelectric transducer. Recently, Priya conducted detailed modeling of the piezoelectric cantilever and proposed a dimensionless figure of merit (DFOM) for the piezoelectric transducer material in an energy harvesting application as:5
where k31 is the transversal electromechanical coupling factor, Qm is the mechanical quality factor (inverse of the Qm represents the mechanical loss), s11E is the elastic compliance at the constant field condition, d31 is the transversal piezoelectric strain constant, g31 is the transversal piezoelectric voltage constant, tanδ is the dielectric loss factor. At the resonance condition, besides the high coupling factor k31, an important material parameter to consider is the mechanical quality factor, Qm. The latter defines sharpness of the resonance peak. Although a sharp resonance peak (high Qm) is beneficial to achieve superior output power, it leads to a narrower bandwidth. This suggests that the output power will fall off quickly if the input frequency of the vibration is slightly off the resonance.
Generally, an energy harvester extracts the maximum amount of power when operating at resonance. However in many cases, it becomes impractical to match the resonance frequency of the piezoelectric materials with the input frequency. In the off-resonance condition, the primary factor for the selection of piezoelectric materials is d ∙ g. The relationship between d ∙ g and k2 can be given by:
It is challenging to achieve a high d ∙ g coefficient through the composition modification in conventional piezoelectric ceramic, because, any increase in the piezoelectric constant (d) is generally accompanied by the large increase in the dielectric permittivity (ε), thus, high d usually results in low g. Recently, Yan et al at Virginia Tech found that the template grain growth (TGG) technique yields textured relaxor-PT/PZT piezoelectric ceramics with a large (d ∙ g) magnitude.6,7,8 Maurya et al. also recently demonstrated enhanced piezoelectric response in textured lead-free piezoelectric materials, which can be used for fabricating environment friendly energy harvesters.9
Piezo Material Options
Piezoelectric materials for use in energy harvesting applications can be divided into four different categories: ceramics, single crystals, polymers, and composites. Generally, piezoelectric ceramics are used as the piezoelectric material in energy harvesting devices due to their low cost, good piezoelectric properties, and ease of integration into energy harvesting devices. PZT-5H (APC 855) and PZT-5A (APC 850) have been widely used in fabricating energy harvesters for generating power of the order of milliwatts. PZT ceramic-based harvesters are generally used at 50 Hz or higher frequency range. In order to use these harvesters at lower frequencies, either a long or large PZT element is required, or a large excitation (acceleration or force) is needed.
Piezoelectric polymer-based energy harvesters are suitable for applications with lower input frequencies (<10Hz) or larger amplitude of excitations due to their increased flexibility. Unfortunately, piezoelectric polymer-based energy harvesters generally provide the smallest power output, on the order of microwatts or nanowatts, due to their small coupling coefficients. The incorporation of polymers into the structure allows PZT-polymer composites to achieve larger mechanical strain without breaking. However, the power output is similar to that of the PZT ceramics only with a slightly lower application frequency.
Use of piezoelectric single crystal-based energy harvesters is rare due to the high cost of piezo single crystals and their lower mechanical strength. Although they have shown better power density than other piezoelectric materials, prototype single crystal-based energy harvesters have been shown to only provide power outputs up to a few milli-watts as of early 2015.
Challenges
From the device design point of view, the biggest challenge in the vibration piezoelectric energy harvesting, is the low input frequency and acceleration of the mechanical energy sources. Most of the vibrational energy harvesters are designed to operate in resonance mode and the half-power bandwidth is usually small. As a result, matching such a low frequency and acceleration for a piezoelectric harvester architecture becomes challenging. Many studies have demonstrated that even a 5% mismatch may result in 100 times smaller power generation than the maximum value obtained around the resonance frequency. Therefore, this is one of the most important challenges of piezoelectric vibration energy harvesting as the frequency of ambient vibrations varies over a wide range. Although some intriguing progress (such as lowering fr towards fi, up-converting fi to fr, and broadening the bandwidth of the harvester) has been made, these techniques add to the intricacy of the harvesting systems with limited power output improvement. Moreover, from the materials side, the current-level of performance of the piezoelectric materials makes it quite challenging for developing an energy harvester that can truly replace batteries as a large power source. The future of piezoelectric energy harvesting is likely to be dependent upon continuous decreases in the energy consumption of electronic devices and developing/improving performance of the piezoelectric materials used in energy harvesting applications.
APC International
APC International supplies PZT based piezoelectric ceramic materials to a wide range of different customers. Typically we are asked by our energy harvesting customers for piezoelectric ceramics manufactured from APC 850 or APC 855. APC also supplies single crystal piezo elements but we have found that the high cost of single crystals limits their use in commercial energy harvesting applications. At this time APC does not supply polymer-based piezo materials.
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By: Yongke Yan & Deepam Maurya of Virginia Tech’s Center for Energy Harvesting Materials and Systems
References
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3. Huidong Li, Chuan Tian, and Z. Daniel Deng, Applied Physics Reviews 1 (4) (2014)
4. Alperen Toprak and Onur Tigli, Applied Physics Reviews 1 (3) (2014)
5. S. Priya, Ieee T Ultrason Ferr 57 (12), 2610 (2010)
6. Y. K. Yan, K. H. Cho, D. Maurya, A. Kumar, S. Kalinin, A. Khachaturyan, and S. Priya, Appl Phys Lett 102 (4) (2013)
7. Y. K. Yan, Y. U. Wang, and S. Priya, Appl Phys Lett 100 (19) (2012)
8. Yongke Yan, Anthony Marin, Yuan Zhou, and Shashank Priya, Energy Harvesting and Systems 1 (3-4), 189 (2014)
9. D. Maurya, Y. Zhou, Y. K. Yan, and S. Priya, Journal of Materials Chemistry C 1 (11), 2102 (2013)
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