Tapping Energy Out of Human Movement

Consider wearing a skirt, shirt, or jacket that can power personal electronic devices such as fitness tracker, mobile phone, and the like while walking, waving, or just sitting down.

This can be achieved by an innovative, ultrathin energy harvesting system created at the Nanomaterials and Energy Devices Laboratory at Vanderbilt University. Fabricated using black phosphorus layers just a few atoms thick by means of battery technology, the new device produces electricity in lesser amounts upon being bent or pressed even at very low frequencies typical to human motion.

In the future, I expect that we will all become charging depots for our personal devices by pulling energy directly from our motions and the environment.

Cary Pint, Assistant Professor of Mechanical Engineering, who led the study

The innovative energy harvesting system has been reported in a paper titled, “Ultralow Frequency Electrochemical Mechanical Strain Energy Harvester using 2D Black Phosphorus Nanosheets” which was published online in the ACS Energy Letters on 21^st July 2017.

“This is timely and exciting research given the growth of wearable devices such as exoskeletons and smart clothing, which could potentially benefit from Dr. Pint’s advances in materials and energy harvesting,” explained Karl Zelik, Assistant Professor of Mechanical and Biomedical Engineering at Vanderbilt, who is a Specialist in the biomechanics of locomotion who did not take part in developing the device.

At present, there is extensive research being carried out into finding out more efficient methods for tapping ambient energy sources, including radiant energy devices that can extract energy from radio waves, light and other types of radiation; mechanical devices that can extract energy from deformations and vibrations; electrochemical devices that extract energy out of biochemical reactions and thermal devices with the ability to tap energy from variations in temperature.

Compared to the other approaches designed to harvest energy from human motion, our method has two fundamental advantages. The materials are atomically thin and small enough to be impregnated into textiles without affecting the fabric’s look or feel and it can extract energy from movements that are slower than 10 Hertz—10 cycles per second—over the whole low-frequency window of movements corresponding to human motion.

Cary Pint, Assistant Professor of Mechanical Engineering, who led the study

Nitin Muralidharan and Mengya Li, Doctoral Students, co-headed the attempt to develop and investigate the devices. “When you look at Usain Bolt, you see the fastest man on Earth. When I look at him, I see a machine working at 5 Hertz,” stated Muralidharan.

Tapping usable energy from low frequency motion has been found to be highly difficult. For instance, many research teams have been creating energy harvesters using piezoelectric materials that transform mechanical strain into electricity. Yet, such materials often function optimally at frequencies greater than 100 Hertz, which means that they cannot function for more than a miniscule fraction of any human motion, thereby accomplishing only lesser efficiencies of below 5-10% even at most favorable conditions.

“Our harvester is calculated to operate at over 25 percent efficiency in an ideal device configuration, and most importantly harvest energy through the whole duration of even slow human motions, such as sitting or standing,” stated Pint.

The ultrathin energy harvester created at Vanderbilt lab is dependent on the team’s study on advanced battery systems. In the last 3 years, the Researchers have investigated the basic response of battery materials to stretching and bending. They were the pioneers in experimentally showing that the operating voltage gets altered when battery materials are under stress. Upon applying tension, the voltage increases and during compression, it is decreased.

The Researchers worked in co-operation with Greg Walker, Associate Professor of Mechanical Engineering, who applied computer models to corroborate the observations in the case of lithium battery materials. The outcomes of the research have been reported in an article titled “The MechanoChemistry of Lithium Battery Electrodes” published in ACS Nano on 27^th June 2017.

These observations guided Pint and his colleagues into rebuilding the battery with negative as well as positive electrodes made of the same material. While this blocks storage of energy in the device, it enables the device to completely make use of the changes in voltage caused due to twisting and bending, thereby producing notable amounts of electricity as a result of human movement.

The initial research works conducted at the lab were published in 2016. They were influenced more by a parallel advancement achieved by a research team at Massachusetts Institute of Technology who developed a postage-stamp-sized device from lithium and silicon, which had the ability to harvest energy by making use of the effect studied by Pint and his colleagues.

Responding to this, Researchers at Vanderbilt were determined to go very thin and used black phosphorus nanosheets—a material that has attracted the attention of the 2D materials Researchers due to its stunning optical, electrical and electrochemical characteristics.

As the thickness of the harvester’s fundamental building blocks are nearly 1/5000th of that of human hair, the Researchers can develop their devices with the required thickness for particular applications. They discovered that when their prototype devices are bent, nearly 40 microWatts per square foot of electricity is produced and the generation of current can be sustained over the entire duration of movements that are as slow as 0.01 Hertz, one cycle per 100 seconds.

The research team admits that one main difficulty faced by them is the comparatively low voltage generated by their device, which is in the millivolt range. Yet, they are putting to use their basic understanding of the process to increase the voltage. They are investigating the design of electrical components—such as LCD displays—functioning at lower than normal voltages, as well.

One of the peer reviewers for our paper raised the question of safety. That isn’t a problem here. Batteries usually catch on fire when the positive and negative electrodes are shorted, which ignites the electrolyte. Because our harvester has two identical electrodes, shorting it will do nothing more than inhibit the device from harvesting energy. It is true that our prototype will catch on fire if you put it under a blowtorch but we can eliminate even this concern by using a solid-state electrolyte.

Cary Pint, Assistant Professor of Mechanical Engineering, who led the study

One much advanced application of this technique may be electrified clothing, wherein it can power up clothes infused with liquid crystal displays that enable wearers to change patterns and colors just by swiping on their smartphone. “We are already measuring performance within the ballpark for the power requirement for a medium-sized low-power LCD display when scaling the performance to thickness and areas of the clothes we wear” noted Pint.

Pint also considers that there are prospective applications for their device other than power systems. “When incorporated into clothing, our device can translate human motion into an electrical signal with high sensitivity that could provide a historical record of our movements. Or clothes that track our motions in three dimensions could be integrated with virtual reality technology. There are many directions that this could go.”

Rachel Carter, a Postdoctoral Researcher at the Naval Research Laboratory at present and Nicholas Galioto, an Undergraduate Mechanical Engineering Student, both of whom are Vanderbilt Doctoral Students, also contributed to the study. National Science Foundation grant CMMI 1400424 and Vanderbilt University’s discovery grant program supported the study.