Piezoelectric materials are remarkable substances that generate an electric current when subjected to mechanical stress. They can be found in various forms, such as crystals, bones, and proteins. These materials have permeated numerous aspects of our lives, often without us even realizing it.
By harnessing energy from the environment, including sources like light, heat, and motion, piezoelectric materials are being integrated into a wide range of applications. They have found their way into solar cells, wearable and implantable electronics, and even spacecraft. One of their key advantages is the ability to keep devices powered for extended periods, potentially eliminating the need for constant connection to a traditional power supply.
To fully exploit the potential of these energy harvesters, it is crucial to determine their precise energy output. Recently, our research team has made a significant breakthrough in this area. Through a straightforward signal processing technique, we have unveiled an intriguing aspect of electrical signals used to assess piezoelectric materials— the inclusion of electro-static, or phantom, energy.
Our findings, which have been published in the esteemed journal Nano Energy, demonstrate that motion-based energy harvesting yields more electricity than previously anticipated. This additional energy, often referred to as “phantom” energy, must be taken into account when designing the next generation of advanced electronics. Until now, there has been no reliable method to quantify the presence of phantom energy in motion-based energy harvesters.
Fortunately, our research team has identified a simple approach to detect the presence of this phantom energy. By analyzing the electrical signals produced by a material subjected to motion, we can determine if and how much phantom energy is present.
This groundbreaking discovery will undoubtedly have a profound impact on the development of future electronic devices, enabling designers to optimize energy harvesting systems and enhance overall efficiency. The integration of piezoelectric materials and the consideration of phantom energy will pave the way for increasingly advanced and self-sustaining electronics.
Measuring phantom energy
Piezoelectric materials have a long history of being utilized for energy harvesting and sensing purposes. They have found applications in various domains, ranging from simple contact-based energy harvesters to complex systems such as industrial vibration sensors, pacemakers, structural health monitoring devices, and micro-thrusters in space satellites.
Conventional motion-based energy harvesters employ different energy conversion principles, including electromagnetic induction (e.g., wind turbines), electrostatic induction (e.g., Van der Graaff generators), and piezoelectricity.
Recent advancements in materials science have significantly contributed to the design and development of functional materials that leverage the phenomenon of piezoelectricity.
Piezoelectricity enables the conversion of mechanical energy, obtained through deformation, into electrical energy in the form of voltage. For instance, flexible polymers can undergo temporary physical changes like bending or twisting, subsequently returning to their original shape. This movement induces the internal polymer chains to generate an electrical output in certain types of polymers.
The continuous electrical output capability of piezoelectric materials with minimal effort has attracted the attention of researchers and manufacturers across various fields.
In today’s context, piezoelectric materials, particularly polymers, are extensively employed in wearable devices like smart shoes, watches, or gloves, where they convert motion into electrical energy that can be stored and utilized.
However, the friction involved in generating electrical output from piezoelectric materials can lead to the accumulation of electrostatic charges on their surfaces.
Many of us are familiar with static electricity, experiencing it as electric shocks when walking on carpeted floors in socks or observing lightning bolts during thunderstorms.
This phenomenon is known as the “triboelectric” effect, which occurs when two materials come into contact. In practical applications such as energy harvesting from motion, understanding the additional effects introduced by friction, including triboelectricity, is crucial to prevent unexpected surges in energy output that may impact delicate electronic devices.
Unfortunately, distinguishing between intrinsic piezoelectric signals and those hindered by triboelectricity is extremely challenging. This difficulty arises due to the similarities between piezoelectricity and the enigmatic triboelectric signals.
To address this challenge, we implemented shielding measures for energy harvesters, enveloping the equipment in conductive adhesive like carbon tape. This shielding approach allowed us to differentiate the measurements obtained from piezoelectric materials accurately.
Our findings revealed that signals from shielded energy harvesters, free from triboelectric interference, exhibited a distinct frequency response compared to the signals obtained from unshielded energy harvesters.
Finding phantom energy
Through our research, we made a fascinating discovery regarding the presence of phantom energy in energy harvesting measurements. By employing a common signal processing technique known as the fast Fourier transform, we were able to convert the electrical output from an energy harvester into the frequency domain. This technique can be easily implemented using mathematical software such as MATLAB.
The fast Fourier transform allows us to analyze the repetition and frequency content within the signal, which is represented as voltage over time. In the case of motion-based energy harvesting, we expect to observe a straightforward frequency spectrum, resembling a single skyscraper. However, when we intentionally introduced phantom energy into the system, the frequency spectrum transformed into a complex city skyline.
These harmonic induced distortions, identified as phantom energy interferences, often amplify the source signal. By being aware of how to identify and account for phantom energy, engineers can ensure that energy harvesting materials, whether employed in outer space or implanted within the human body, produce the precise amount of energy required—neither more nor less.
Removing phantom energy
The Fourier transform method is a valuable tool commonly employed in data analysis to detect patterns and anomalies within signals. In the case of our piezoelectric measurements, we can utilize this technique to identify interferences and ensure accurate data analysis.
During testing, numerous small areas on energy harvesting devices experience friction, and these localized interactions can significantly impact the output. For instance, what was expected to be a 1-volt (V) output during benchmark testing may unexpectedly spike to 10 V or even 50 V.
While such a surge in power may initially appear advantageous, it poses a considerable challenge. The excess energy cannot be effectively harvested, akin to a blown fuse during a lightning strike, and the device would not be able to handle the excessive energy input. This is an undesirable situation, particularly in the context of outer space or internal body applications.
To address this issue, we conducted extensive testing on piezoelectric samples and demonstrated the efficacy of our simple fast Fourier transform technique in identifying phantom energy during benchmarking. By accurately detecting and measuring phantom energy, researchers can employ straightforward signal filters to isolate and eliminate any interference.
With this newfound ability to identify and manage phantom energy, manufacturers of piezoelectric energy harvesters can confidently integrate the technique into device construction. This ensures the creation of devices specifically tailored for precision applications, such as bionics or spacecraft, capable of generating the exact amount of energy required to extend the device’s lifetime. It paves the way for potentially perpetual piezoelectric lifetimes.
Source: University of Melbourne