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Harvest of Motion PUBLIC ACCESS

A Small-Scale Generator Uses a Catch and Release Strategy that Can Turn a Casual Stroll Into Useful Electric Energy.

[+] Author Notes

Brian S. Hendrickson is a design engineer and Stuart B. Brown is managing partner at Veryst Engineering LLC, an engineering firm in Needham, Mass.

Mechanical Engineering 130(09), 56-58 (Sep 01, 2008) (3 pages) doi:10.1115/1.2008-SEP-8

This article discusses that a small-scale generator uses a catch-and-release strategy that can turn a casual stroll into useful electric energy. Many devices now require fractions of a watt continuously, often with occasional bursts of 1 to 10 W during peak activity. However, batteries occupy device volume and have limited life. Even rechargeable batteries can withstand only a finite number of charge cycles and, perhaps most important, recharging them can be inconvenient or expensive. Engineers must develop strategies to harness the abundant energy in low-frequency, time-varying motion before energy harvesting can achieve its greatest potential. Water waves, swaying and bouncing structures, and biomechanics are potential environmental energy sources that are largely out of the reach of the current vibration-inspired, motion harvesting technologies. Being able to economically convert low-speed motion to electricity will be a key to realizing practical long-term power generation for distributed devices. The Veryst energy-harvesting concept is one approach that intends to do just that. As with other energy harvesting projects, much work remains, but initial research and development suggest strong potential.

Throughout its existence, mankind has found ingenious ways to harness natural motion and put it to work, wind and water being classic examples. The earliest engineers built rafts and water wheels that took advantage of currents, and built sailing ships and mills that ran on wind. The self-winding watch captures energy from the motion of the human wrist.

More recently, researchers have developed devices that can take advantage of irregular periodic motion to generate electricity. A project at Clarkson University in Potsdam, N.Y., has built prototype wireless sensing devices to monitor bridges. Their electricity comes from generators actuated by the vibration of passing traffic.

Georgia Institute of Technology in Atlanta is experimenting with zinc oxide nanowires that can generate electricity to power implants in the human body, or perhaps be placed in a boot heel to power the electronic equipment of a soldier of the future.

Unconventional power sources have particular appeal as the world increasingly takes delivery of its energy in the form of electricity. Many electrical devices, including pacemakers, GPS locators, and machine sensors, are multiplying and straying farther from established sources of power as applications continue to miniaturize and proliferate.

These new devices have increasingly relied on batteries that, by many accounts, have not kept pace with the demands placed on them by the latest gadgets. Advances in efficient, low-power electronics have worked in the favor of batteries. Many devices now require fractions of a watt continuously, often with occasional bursts of 1 to 10 W during peak activity. However, batteries occupy device volume and have linuted life. Even rechargeable batteries can withstand only a finite number of charge cydes and, perhaps most important, recharging them can be inconvenient or expensive.

Devices can draw useful power from motions that surround them, but converting those motions to electricity is challenging because of the low frequency and variability of many of the sources.

Fully sealed consumer devices, remote sensor and communication nodes, and implanted medical devices are a handful of the many .instances in which the costs of replacing or recharging a battery far outweigh the costs of the battery itself Moreover, the environmental impact of-producing and disposing of chemical cells is an increasingly pressing concern.

In light of such issues, a number of groups have turned to harvesting energy from. natural sources as an alternative or supplement to batteries. Familiar energy harvesters include solar cells, wind turbines, and thermal energy converters. More recently, however, interest in energy harvesting has expanded to smaller-scale power generation for small mobile or distributed devices, like those in development at Clarkson and Georgia Tech. Converting mechanical vibration into electricity has been a particular focus for many researchers and a handful of small companies.

Although no efforts have yet produced widespread adoption, the vibration harvesters have demonstrated at least one important point. They have shown that low levels of energy can be efficiently accumulated as electricity over time and then dispensed as needed to attached devices. For example, an energy harvester producing just 20 mW of continuous power from its environment would exceed the energy content of a typical 9V alkaline battery in approximately five days of continuous operation. In fact, commercial vibration harvesters to date typically generate for less power. But, they have already delivered significant value when combined with low-power, wireless condition monitors placed on vibrating machinery.

However, vibration energy harvesters have also demonstrated a key shortcoming inherent in their strategy. The harvesters achieve their efficiency by resonantly coupling to their vibrating sources. As coupled resonant systems, they are sensitive to the frequency of vibration, and their already modest energy outputs drop if their inputs stray from their tuned points. Of course, most environmental sources of motion can and do vary over time. A further problem is that tuning these devices to resonate at low frequencies can require larger masses, which limits the scalability of vibration-type harvesters.

A prototype low-frequency energy harvester, approximately the size of a D cell battery, has a system efficiency of about 17 percent.

Grahic Jump LocationA prototype low-frequency energy harvester, approximately the size of a D cell battery, has a system efficiency of about 17 percent.

Engineers must develop strategies to harness the abundant energy in low-frequency, time-varying motion before energy harvesting can achieve its greatest potential. Water waves, swaying and bouncing structures, and biomechanics are potential environmental energy sources that are largely out of the reach of the current vibrationinspired, motion harvesting technologies.

Walking, running, and even breathing are natural repetitive motions where energy harvesting would provide useful power for various devices, but their frequencies are generally below 5 Hz. Past attelflpts to harness them have produced levels of electricity that are too small for many low-power devices. A recurring issue is the task of converting the low speeds of countless natural motions to the high speeds that allow methods such as electromagnetic induction and piezoelectricity to efficiently generate electricity.

In the case of induction, Faraday's Law states that electrical power is proportional to the square of the velocity of a magnetic field moving relative to a coil. Gearboxes paired to conventional generators are the status quo in booting that velocity, but gearboxes add cost, weight, volume, and opportunities for increased wear and decreased efficiency. In fact, the trend in wind turbines and other low-speed energy harvesters has been a shift away from geared systems.

Veryst Engineering has been developing an energy harvesting concept to achieve the speed up-conversion required for efficient power generation. The method exploits the energy accumulation concept from vibration harvesters, but achieves conversion to electrical power in a new way. Whereas resonant vibration harvesters use an elastic structure to couple with an input motion, the Veryst concept intentionally de couples an elastic structure from the input motion, and instead uses it as a mechanical energy accumulator.

The strategy operates by having a periodic input motion act on an elastic body, perhaps a coil spring. Work done by the input motion is therefore stored in the spring, and once the input motion reaches a maximum displacement, the elastic body is momentarily stopped by a latching mechanism. When the input motion has reversed direction, the elastic body is released to oscillate without interference:: from the input excitation. Energy is pulled from the oscillating structure during this stage using a method such as electromagnetic induction. The converted electrical energy can then have stored in a capacitor or battery. Because a system would have an onboard generating device, its battery bank could be much smaller than without the harvester. Overall system mass and volume could be greatly reduced, and the maintenance- free operating period could be extended.

A generator that can use low-frequency motion. (A): A periodic input motion acts on an elastic body, represented as a coil spring. Work done by the input motion is therefore stored in the spring. (8): Once the input motion reaches a maximum displacement, the elastic body is momentarily latched. (C): The input motion has reversed direction, and the elastic body is released to oscillate without interference from the input excitation. Energy is pulled from the oscillating structure during this stage using a method such as electromagnetic induction, as represented by the magnet and coil. Converted electrical energy can be stored in a capacitor or battery.

Grahic Jump LocationA generator that can use low-frequency motion. (A): A periodic input motion acts on an elastic body, represented as a coil spring. Work done by the input motion is therefore stored in the spring. (8): Once the input motion reaches a maximum displacement, the elastic body is momentarily latched. (C): The input motion has reversed direction, and the elastic body is released to oscillate without interference from the input excitation. Energy is pulled from the oscillating structure during this stage using a method such as electromagnetic induction, as represented by the magnet and coil. Converted electrical energy can be stored in a capacitor or battery.

The elastic structure allows the conversion stage to operate at a frequency many times greater than that of the original excitation. Moreover, because the oscilla-: tion frequency is determined by adjustable factors such as the stiffness of the elastic body and the damping due to energy extraction, the harvester can be set to operate a specific energy-conversion method near its optimal speed, irrespective of input motion speeds or frequencies. The Veryst harvester converts energy discontinuously, absorbing energy during the first half of an input cycle, and then converting it to electricity during the second half. In some instances, the harvester may absorb only half of input motion energy, a tradeoffthat increases device simplicity and converter efficiency.

The intermittence of the harvester may actually be helpful. Researchers, for example, have shown that an intermittent, "regenerative braking" approach can successfully produce electricity from walking with minimal additional effort by the person. The electricity, once de-: livered to a capacitor or battery, can be conditioned using the same efficient electronics developed for vibration harvesters and low-power systems.

Veryst Engineering has been investigating the viability of the catch-and-release concept using several proof-ofconcept prototypes and numerical modeling. The test systems and model have been based on stainless steel torsion springs as the elastic structure and a brushless dc motor as the energy conversion device. The motor provides a prepackaged, high-efficiency magnet!coil set and gives the volumetric efficiency of a rotational system. Veryst has built several energy harvester prototypes, including one with a form factor similar to that of a D cell battery. A catch-and-release mechanism that triggers at fixed displacement positions regulates the loading and release of the torsion spring.

At its heart, the new harvester concept is a classic damped harmonic system. However, system response is complicated by a number of factors, including electrical resistances, friction, rectification, and coupled nonlinear electromagnetic behavior. Such considerations were included in a computational model, which was verified by comparison with the outputs of the prototype systems. The model allows tracking of dynamic quantities and the flow of energy through the system.

Prototypes assembled using commercial, off-the-shelf components have already shown competitive performance and have confirmed the accuracy of computationalmodels. The D cell prototype has a system efficiency of about 17 percent. However, because the energy contained in typical low-speed natural motions is significant, even that preliminary efficiency can produce power output per volume that is five times greater than many leading vibration energy harvesters. Power output per unit mass is four times better. System modeling also indicates that harvester performance could be improved two- to three-fold using purpose-built components, which would yield efficiencies that are highly competitive across a broad range of input frequencies and forces. Veryst has looked at designs suitable to various applications. The harvesters could be used to generate energy for medical implants, including pacemakers and insulin pumps, and for sensors on offshore buoys.

Of course, no energy harvester can generate energy on its own. An energy harvester is a converter that requires usable work as input in order to produce electricity. That said, variable, low-speed environmental motions remain untapped sources of energy that are both substantial and commonplace, increasing the number of potential applications. Even at initial prototype efficiencies and with modest energy input of.O.5 to 1.5 joules each second, the Veryst harvester can convert useful amounts of energy over reasonable periods of time. The D battery-size prototype exceeds the energy content per mass and per cost of primary batteries in about a month. The prototype surpasses rechargeable batteries similarly quickly, especially when the additional costs of replenishing batteries are considered.

Being able to economically convert low-speed motion to electricity, even as that motion changes in frequency and amplitude over time, will be key to realizing practical long-term power generation for distributed devices. The Veryst energy harvesting concept is one approach that intends to do just that. As with other energy harvesting projects, much work remains, but initial research and development suggest strong potential.

Copyright © 2008 by ASME
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