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Energy Harvesters: Generating Electricity During Walking with Minimal User Effort

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By:

Martin Keung

This is a summary of a journal article published in Science. Dr. Andy Hoffer, a principal investigator at ICORD, was one of the researchers involved with this project.

J.M. Donelan, et al. (2008). Biomechanical energy harvesting: generating electricity during walking with minimal user effort. Science. 319(5864): 807-10. You can find the original article here.
 

INTRODUCTION:

A biomechanical energy harvester is a device that generates electricity from activities such as walking. The main focus of the study was to determine if a custom-made energy harvester would be able to generate electricity during walking with minimal user effort. If the energy harvester was successful at efficiently generating electricity with minimal effort from walking, the device can potentially assist those who wear prosthetic limbs or other portable medical devices.

To test if the invention would be efficient at generating electricity, the COH (cost of harvesting) of the device was calculated. COH is a “dimensionless quantity defined as the additional metabolic power in watts required to generate 1 Watt of electrical power.” For those of you who like mathematics, the formula is:

Cost Of Harvesting

The Δ represents “the difference between walking while harvesting energy and walking while carrying the device but without harvesting energy.”

Coupling a generator to leg motion would generate electricity during the motion of walking by increasing the load on the muscles during acceleration and assisting the muscles during deceleration.

The energy harvester created by the researchers was specifically for the knee. This was done because the knee mostly performs negative work during walking. A perfect situation where “negative work” is performed is when a braking motion occurs. Normally, energy from the braking will be released and dispersed as heat energy; in other words, energy lost. Instead, the energy harvester takes the energy from braking and uses it to drive a generator inside the harvester. The energy harvester was programmed to only engage during the end of the swing phase of walking. This would generate electricity and assist the knee in deceleration at the same time.

THE EXPERIMENT:

The device was tested on 6 male subjects with no artificial limbs. Each subject wore the device on each leg while walking on a treadmill at 1.5 m/s. The metabolic cost was estimated using a respirometry system, a system that measures an individual’s metabolism based off of their carbon dioxide production and oxygen production. In addition, electrical power output was also measured. The participants partook in 3 different trials. The control, cyclic motion, and cyclic motion with continuous generation or with generative braking. Continuous generation means the generator will resist acceleration and assists with deceleration. Generative braking means the generator will only assist during the deceleration of the limb.

MAIN RESULTS:

Astonishingly, the energy harvester was extremely efficient at harvesting energy during continuous generation and also during generative braking. Subjects during the generative braking part of the study were able to generate an average of 4.8 ± 0.8 Watts of electricity with an insignificant increase in COH, meaning minimal effort was required. To put into context, the electricity generated from generative braking could power up to 10 cell phones at the same time!

THINGS TO CONSIDER:

Although this energy harvester study was not done with individuals who have artificial limbs, it definitely is a step forward in the field of prosthetics. To emphasize, the main goal of the study was to find out if the energy harvester created, would be efficient, at generating electricity with minimal user effort. From the results of the study, it definitely proved it was efficient at generating electricity. However, there were refinements that could be made to the energy harvester. For instance, the energy harvester could be made more efficient by changing the material of the harvester to a lighter one such as carbon fiber. To conclude, we could one day see prosthetic limbs and other portable medical devices be powered solely by daily activities such as walking with minimal effort.

 

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