Piezoelectricity as a Future Energy Source

The Future Smart House is going to have a large number of sensors and microelectronic devices located throughout the house. These devices will need a clean, reliable source of energy that won’t need constant maintenance. The goal of this report is to get awareness about the piezoelectric energy sources to provide power to certain applications in the house. Initially the researchers were planning to create an energy scavenging floor that used piezoelectric transducers to harvest wasted energy in the foot strike of a human being. In consideration of the high cost and minimal power output of these piezo-sources, it seems more feasible to create small, localized energy sources rather than one large unified system. This idea has lead to several potential applications.

The first is to combine a piezoelectric power source with sensors such that there would be no need to ever change the batteries in these sensors. The next application is to use piezoelectric cable throughout the floors of the house as a means of tracking. The final application is to combine piezoelectrics with a device to eliminate vibrations in household appliances. Introduction So generally Piezoelectric Energy Harvesting is based upon the piezoelectric effect. The essence of the piezoelectric effect works as follows: by applying a mechanical stress to a crystal, one can generate a voltage or potential energy difference, and thus a current.

Also by applying a current to a piezoelectric one can stress or strain the material. The mechanical stress can be supplied by any source. But with so many people walking around from place to place, why not harness that kinetic energy to power stuff, right? Technical Overview Piezoelectric materials exhibit the unique property known as the piezoelectric effect. When these materials are subjected to a compressive or tensile stress, an electric field is generated across the material, creating a voltage gradient and a subsequent current flow. This effect stems from the asymmetric nature of their unit cell when a stress is applied.

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As seen in Figure 1, the unit cell contains a small positively charges particle in the center. When a stress is applied this particle becomes shifted in one direction which creates a charge distribution, and subsequent electric field. These materials come in several different forms. The most common is crystals, but they are also found as plastics and ceramics. There are several companies and research institutes throughout the world who are focusing on finding useful applications for piezoelectric energy sources.

Several years ago a project was done at MIT entitled, “Energy Scavenging with Shoe-Mounted Piezoelectrics. ” In this project the researchers lined the bottom of a shoe with piezoelectric transducers and saw what kind of power they got out of it. They eventually attached an RF-transmitter to the shoe that was powered by the piezoelectrics. The two materials they used were polyvinylidene fluoride (PVDF) and lead zirconate titanate (PZT). Their initial results were that the PVDF material produced 1. 3 mW per foot strike and the PZT produced around 8. 4 mW.

They went back and tried numerous other approaches but they were confined to working with the limitation of a shoe. In this report they hinted at the fact that much great power output could be achieved if they were not confined to working with a shoe. Another company that is looking into using piezoelectric sources to power networks of wireless sensors is MicroStrain Inc. This company setup an experiment where piezoelectric transducers were attached to the support beams in a structure. As the structure was constantly under strain, the voltage created by the piezoelectrics was stored up in a capacitor.

Once the capacitor voltage reached a certain level, the power was than transferred to a transmitter which sent a wireless signal to some receiver. It was reported that the cycle time was about 20 to 80 seconds to store up a charge of 9. 5 V on the capacitor given the size of the piezoelectric was 17 cm2. A German based company called EnOcean already has a commercially available product. The slogan for this company is “no batteries and no wires. ” They create products that use piezoelectric transducers to power RF transmitters.

One particular product is a light switch that requires no wiring at all. Behind the actual switch is a piezoelectric transducer. When the light switch is flipped, this motion is used by the transducer to power the RF transmitter, which signals the receiver on the actual light to turn on. This would be an interesting technology to demonstrate in the house because it enables you to have light switches on places you never could before such as on a window or outside in the middle of the yard. Two other companies, Ferro Solutions Inc. and Continuum Control Corp. make small ambient energy harvesting sources. Ferro Solutions makes a product called the Energy Harvester. This little device about the size of two AA batteries contains an electromagnetic generator inside. There are two magnets and in between them is a coil of wire. When vibrations cause the coil of wire to move around in the magnetic field, current is generated in the wire. This small energy source could be used in place of batteries or as a means to recharge batteries. The device supplies about 1 to 10 mW of power and the company is looking to license this echnology to wireless sensor companies. Continuum Control Corp. makes two different products called the PiezoFlex and the iPower Generator. The PiezoFlex is a new type of piezoelectric material that is both flexible and robust. At the same time it also cheaper to manufacture than most other piezoelectric materials. The second product, the iPower Generator contains a piezoelectric transducer that converts mechanical input into electrical output. This product was used was used as a backup energy source in an Antarctic Expedition several years ago.

The device has a crank and as you turn it, the mechanical energy is converted into electrical energy. At the MIT Media Lab, researchers in the Responsive Environments Group created a piezoelectric floor called the “Magic Carpet. ” This floor contained a grid of piezoelectric cables spaced 4” apart. The goal of this project was to create a floor that could track the movements of the person walking across it. This technology was then combined with lights and sound such that depending on where you walked a different sound would play. Today you can find examples of the magic carpet in some of the museums at MIT.

A similar project was done at Georgia Tech. At Georgia Tech they built a “Smart Floor” that used piezoelectrics as sensing devices to monitor and predict when people were walking across it. They could predict with about 90% accuracy what person was walking across the floor just given the way the piezoelectrics reacted to their foot strike. Application to Residential There are several ways to incorporate piezoelectric technology into a residential setting. The first would be to use small piezoelectric sources in the place of batteries.

These piezoelectric sources could be used to power sensors throughout the house so that the batteries would never need to be changed. Another application was proposed by MIT researchers. Their idea was to use piezoelectric materials to cancel out vibrations in certain household appliances. They envisioned a device that could autonomously adapt the amount of dampening based on the magnitude of the vibrations. They proposed that first the device be built with a battery power source and eventually switch to an energy source that converted the vibrations into energy using either piezo or electromagnetic generators.

Their second idea was the opposite of the first: rather than cancel the vibrations, you would enhance them. This idea originated from the fact that subwoofers only go down to about 20 Hz, yet the electrical signal coming from the actual media contains frequencies below 20 Hz. These signals are too low for us to hear, but they can be converted into vibrations. For example when there is an explosion in a movie, there are frequencies below 20 Hz, but the subwoofer can’t make a sound with that long of a wavelength.

If you were to install proof mass actuators into the floor of the media room, whenever there was a signal below 20 Hz, these actuators would respond accordingly and literally shake the room.. The next set of applications would involve piezoelectric cable. Piezoelectric cable behaves much the same way as flat piezoelectric transducers only the cable is much cheaper. The cable resembles the standard coaxial cable that plugs into the TV, but the piezo cable has a layer of piezoelectric polymer wrapped into it.

This cable could be used to create a grid across the floor that could generate power from people walking across it, or the more viable purpose would be for tracking. These cables come with varying degrees of sensitivity. The cable could be used to track both the location and the orientation of a person in the house. Based on the way they walk, the floor might also be able to identify who the person is. Design The major obstacle in designing your own piezoelectric circuit is finding a way to maximize the power output.

The major components involved in this circuit would be an AC/DC rectifier, a filter capacitor, and a DC-DC converter. The AC/DC rectifier converts the AC signal from the piezo-source into DC current. The filter capacitor smoothes electrical flow and the DC-DC converter is what allows the battery to store the energy. Most of the power conversion comes into play in the DC-DC converter. Fig. 2: Adaptive Energy Harvesting Circuit Recently in a paper published in Power Electronics entitled “Adaptive Piezoelectric Energy Harvesting Circuit for Wireless Remote Power Supply,” esearchers found a way to increase power output by roughly 400% compared to when a converter is not used. They used an adaptive control technique in the converter that adjusts itself to find the optimal power transfer options for every moment. Last year, MIT created a setup to demonstrate the power output you could achieve using the vibrations in a plane wing. They didn’t use piezoelectrics, but rather electromagnetic generators. Regardless, the circuit for capturing and storing the energy would still be the same. He still has the circuit and is willing to let us use it in designing a piezoelectric energy source.

Simulation Results From the study of a project report undertaken in MIT a circuit was modeled. The circuit used a piezoelectric transducer as the voltage source which than fed into an op amp. The purpose of the circuit was to provide a voltage gain of 100. The simulation results proved that the op amp did in fact multiply the voltage from the piezo-transducer by a factor of 100. Unfortunately, in the program they could not find an actual piezo-transducer so they had to model it with an ideal voltage source with an estimated equivalent Thevenin resistance.

They considered this project a success on the grounds that it proved that piezo-transducers can in fact act as voltage sources. It also showed that the voltage provided by the transducer can be multiplied by several orders of magnitude using certain circuits. This circuit essentially failed to answer the real question of what kind of maximum power one can get out of the piezo-transducer. A final word on piezoelectricity as a future energy source In the near future, the lights at your favorite coffee shop or the stereo system at your local bar could very well be powered by foot traffic.

If we want to maintain a certain lifestyle in an increasingly competitive, resource strapped world, we need to think outside the box when it comes to energy production. There are a million sources of clean, environmentally conscious power all around us, we just have to explore and utilize these sources in the most efficient manner possible. Ambient energy capture will no doubt play a large part in the future of renewable energy, and there's no time like the present to work on developing new, innovative means of electricity generation. I feel confident in saying that the world of tomorrow looks pretty bright, both literally and figuratively.

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