Powering your cell phone as you walk

Dr. Neil Canter, Contributing Editor | TLT Tech Beat November 2011

New technology converts human motion into electrical energy to power portable electronic devices.

KEY CONCEPTS
• A new energy-harvesting technology called reverse electrowetting has been developed to convert mechanical energy into electrical energy.
• Reverse electrowetting has the potential to use the energy from walking to power mobile electronic devices.
• Work is underway with footwear manufacturers to determine the best way to place a device in the bottom of a shoe to achieve this goal.

As the efforts to improve energy efficiency continue, steps are being taken literally to see what can be done to utilize energy that has been wasted in the past. A prominent example is the utilization of the large amount of heat produced in an internal combustion engine.

In a previous TLT article, a new thermoelectric material known as a binary skutterudite was described that can convert automotive exhaust heat to electricity (1). This material has the potential to reduce fuel energy lost as heat by approximately 40%.

This process is related to the concept of energy harvesting. Tom Krupenkin, associate professor of mechanical engineering at the University of Wisconsin in Madison, Wis., and president of InStep NanoPower, LLC, also located in Madison, terms energy harvesting as a very old technology. He says, “Energy harvesting is an attempt to utilize energy available as part of a natural process. Two of the best known examples are windmills using the power of the wind and solar cells using light energy. Both are converting free energy available in the environment to electrical energy.”

Current emphasis is on either the high-power applications such as windmills or on very low-power applications such as calculators, watches and sensors. Krupenkin says, “There has been little work done in the intermediate-power applications that involves the harvesting of several watts of power. This amount of energy can be used to power such devices as cell phones, smartphones, computer laptops and other mobile electronic devices.”

One type of energy harvesting that has not been looked at very much is the amount of energy generated while humans walk. Krupenkin says, “In normal walking, approximately 300 watts of total metabolic power is generated by the average person. Up to 20 watts per foot of power is lost as heat during this process.”

Krupenkin believes that if a technology could be created to capture at least a fraction of the heat lost during walking, it could be used to help power portable electronic devices. Such a technology is now in the process of being developed.

REVERSE ELECTROWETTING
Krupenkin and his associate, J. Ashley Taylor, have developed a new energy-harvesting technology that can be used to convert human motion into sufficient electrical energy to power portable electronics. The technology is known as reverse electrowetting.

Krupenkin says, “We have developed a way to convert mechanical energy derived from the motion of a liquid droplet when it is interacting with a dielectric-film-coated electrode into an electrical current. Reverse electrowetting uses external motion to move the droplet so that it decreases its overlap with the electrode. This reduces the total charge that can be maintained at the droplet-electrode interface, causing excess charge to flow through the electrical circuit. The resulting electric current then can be used to power portable electronic devices.”

Three common approaches to achieve reverse electrowetting involve the placing of droplets between oscillating plates, having droplets between sliding plates and using droplets in a microchannel. The first technique uses vibrations as the source of mechanical energy. In the second approach, in-plane shearing creates the mechanical energy, while in the third technique in-channel motion of the droplets is the source of the mechanical energy.

A good deal of work was done to determine the right type of materials to use in reverse electrowetting. The researchers determined that the best droplets were prepared from the liquid metal mercury and the liquid-metal alloy galinstan, which is prepared from gallium, indium and tin. Krupenkin says, “We found that galinstan displays better performance than mercury. Galinstan has superior conductivity, surface tension and is also lighter than mercury. This alloy also has a favorable melting point that can go as low as -20 C.”

One other important factor is that galinstan has a much more favorable environmental profile than mercury. The only drawback of this alloy is that it oxidizes fairly quickly. But Krupenkin points out, “In the reverse electrowetting process, the device will be kept in an air- and water-free compartment, similar to what is used for lithium-based batteries.”

Krupenkin also indicates that galinstan has replaced mercury in such devices as thermometers.

The favored dielectric film is tantalum-pentoxide-coated with a thin film of a fluoropolymer. This combination provides superior resistance to voltage breakdown and reduces charge trapping.

All three approaches for doing reverse electrowetting were successful. The researchers were able to scale-up this type of device by placing 150 droplets between sliding plates. A maximum of 0.4 milliwatts was generated. Krupenkin says, “We believe that a scale-up to several watts of power can be done by utilizing about 1,000 millimeter-size droplets.”

From the standpoint of a shoe, this device can take up a small area which is probably about one-quarter of a footprint. Krupenkin says, “The droplets also can be placed into a three-dimensional pattern and are all quite small with diameters less than a millimeter.” Figure 1 shows a schematic of how the device may be placed in the shoe.


Figure 1. A new process known as reverse electrowetting has been developed that may enable mechanical energy generated during walking to be converted into electrical energy to run mobile electronic devices. (Courtesy of InStep NanoPower, LLC)

The researchers believe that the maximum power this device can generate will be one watt per shoe if the individual takes one step per second. They also indicate that this will provide sufficient electricity to power a cell phone.

Now that the technology for reverse electrowetting has been developed, work is underway to develop a device that can be incorporated into footwear. Krupenkin says, “We are in the process of arranging for collaboration with footwear manufacturers to transition this technology from the research to the development phase.”

Krupenkin believes that this process will take a few years. Additional information about reverse electrowetting can be found in a recent paper (2), by accessing www.instepnanopower.com or by contacting Krupenkin at tnk@engr.wisc.edu. 

REFERENCES
1. Canter, N. (2011), “Electricity from Exhaust Heat,” TLT, 67 (3), pp. 10-11.
2. Krupenkin, T. and Taylor, J. (2011), “Reverse Electrowetting as a New Approach to High-Power Energy Harvesting,” Nature Communications, 2 (448), doi:10.1038/ncomms1454.

Neil Canter heads his own consulting company, Chemical Solutions, in Willow Grove, Pa. Ideas for Tech Beat items can be sent to him at neilcanter@comcast.net.