Energy harvesting, sometimes termed power harvesting or energy scavenging, refers to any process that captures and stores energy from external sources. Some of these processes are quite common already, such as photovoltaic panels or wind turbines. There are many methods, however, that have been largely overlooked until now. According to the Department of Energy, more than 50 percent of energy generated annually in the United States is lost as waste heat. Thermoelectric generators can be used to cool computers components and industrial equipment while simultaneously generating electricity by taking advantage of the thermal gradients that are created. Other devices are designed to harness the kinetic energy, or energy possessed due to motion, of vibrating engines and motors, dancers and night clubs, or even good ol’ fashioned walking!
Students rushing to class may never need to worry about their cell phone dying again.
Photo Credit: Nick LepakKinetic energy harvesting in particular is one of the most highly investigated areas in this field. While this may sounds like cutting-edge technology, the idea has actually been around since the 18th century. In 1770, a Swiss watchmaker pioneered the concept by inventing the first self-winding pocket watch. It essentially functioned the same as a modern-day pedometer; it wound the owner’s watch through the use of an oscillating weight. Today, almost all mechanical watches on the market employ some form of this technology.
Until the last few decades, kinetic energy harvesters relied only on linear dynamic systems. According to Matt Allen, assistant professor in engineering physics here at UW–Madison, current research is based on non-linear equations of motion. Allen says, “The mathematical models used to describe the systems allow for much more complicated systems which have the potential to harvest energy over a wider range of conditions.”
Much of the present-day research in this field is geared towards creating self-powered microsystems and personal electronic devices through the use of kinetic energy harvesters. “We all have something that’s burning power constantly in our pockets,” Allen says. “It’s not totally outrageous to think you could have cell phones that you never have to charge… as long as you’re not too lazy.” Allen is referring to the idea of using an energy harvester powered by the motion of walking to charge a cell phone’s battery.
There are many types of kinetic energy harvesters that could be used for such an application. The primary difference among these vibration-powered generators is the transduction mechanism employed. these mechanisms can be electromagnetic, electrostatic, or piezoelectric based, each with its own unique advantages and disadvantages. Current research here at UW–Madison is looking into using the last of these by having the device installed in the soles of your shoes!
Matt Allen, Assistant Professor in the engineering physics department is working to use the kinetic energy of walking to charge a cell phone.
Photo Credit: Nick LepakSo next time you are out running errands and that dreaded “Low Battery” warning rears its ugly head, there’s no need to rush home and find your charger—just make sure to walk where you need to go and your problem is solved.
Types of Vibration-Powered Generators
Electromagnetic based harvesters make use of basic electromagnetic induction, which is the generation of electric current in a conductor subjected to a magnetic flux. To achieve this, one can either have the conductor and magnet move relative to one another, or produce a time-variant magnetic field. There are several examples of this technology already on the market, such as self-charging flashlights. This approach is the most well-established and there are a variety of configurations and materials available, making this method fairly adaptable. On small scales, equipped with very powerful magnets.
Electrostatic generators involve displacing opposite plates of a capacitor. The capacitor is charged by a battery, which creates equal but opposite charges on the plates. Holding either the charge on the plates or the voltage between them constant, the plates are moved relative to one another by some external force. The work done against the electrostatic force between them produces energy that can then be harvested. Since this family of devices requires some energy input to establish the initial polarization, their application is the most limited.
Piezoelectric based devices rely on…wait for it…the piezoelectric effect! !ese devices use piezoelectric materials, such as quartz, which accumulate charge when subjected to mechanical strain. This charge accumulation leads to a polarization of the material, which can then be used to charge a capacitor or battery. This class of device offers the simplest and most direct approach, since the vibrations or displacements are converted directly into a voltage output. The limiting factor here is the piezoelectric materials themselves, because they tend to deteriorate quickly. There are few geometric requirements, however, and they do not require lots of additional components. This makes them the most effective per volume of the three.