Recharging the Force

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Energy Harvesting and the Future of Warfighter Power

By George Jagels

The growing power demands for modern warfare, in which batteries for radios, GPS receivers, computers, and optics, among other devices, compete for rucksack space with water and ammunition, are forcing the U.S. military to rethink how it powers the warfighter. A reliable source of renewable energy could allow for fewer batteries clogging an already heavy rucksack. This would reduce both the numbers and variety of batteries carried, as rechargeable units could do most of the work. The result could be a more resilient force less dependent on complicated logistics and, consequently, engaging in fewer dangerous resupply operations.

Soldiers and Marines are already wasting a lot of energy—not by throwing away half-charged batteries or burning diesel on under-loaded generators but by walking. Around 10 watts (W) of mechanical power is lost in each human leg per step, according Professor Tom Krupenkin of the University of Wisconsin. Much of that power dissipates as heat in shoes, muscles, and joints, and the Marine Corps and Army want to recoup that loss.

Recognizing the greater power, improved testing, and lighter weights making energy harvesting more realistic, an Experimental Forward Operating Base (ExFOB) event was held at Camp Pendleton, CA, in May 2014. The USMC’s Expeditionary Energy Office brought together industry and military outfits in both kinetic and non-kinetic energy harvesting to demonstrate their products, causing one vendor to refer to it as a “watershed” event in the sector. Over a hundred Marines tested the devices, and their reviews may influence     which, if any, energy harvesting products the Corps pursues.

The Army, which sent representatives from Natick Soldier Research, Engineering, and Development Center (NSRDEC) to the event, is also pursuing an Energy Harvester Suite. Noel Soto, project officer for energy harvesting at NSRDEC, told DoD P&E that the Army’s efforts in kinetic energy harvesting were “headed for the dustbin” a few years ago; however, this fall, there will be “on the soldier” events at Fort Benning, GA, to test some  of the 15 types of systems the Army is considering.

Both NSRDEC and the USMC are examining kinetic and non-kinetic ways of recharging their critical batteries (such as the Conformal Battery and BB-2590), including wearable solar fabrics and chargers, energy-generating knee pads, and moving backpacks. Though generating less than 100W, these technologies might represent a way to obtain “cheap,” burden-free energy in the field.

From Pack to Battery

The long patrols in which energy harvesting can provide the most impact require warfighters to hump very heavy packs, and Larry Rome, founder of Lightning Packs and a University of Pennsylvania professor, saw this as an opportunity. Over the last decade, his company has built a system that can generate around 15W of electricity at a walk (35W at a run). Springs suspend the moving frame holding the payload from the fixed frame attached to the user. When walking, the fixed frame moves up and down with the hips while the moving frame lags behind the fixed frame. This linear movement creates energy captured by a generator that routes electricity through a power management system.

The ultimate goal is to make the Lightning Pack (pictured above) weigh just four pounds more than a current military backpack. According to Rome, Soto, and many Marines who tested the product at ExFOB, the Lightning Pack is more ergonomic than fixed packs. “Because the load lags behind, your hip doesn’t have to lift and drop the load as much, reducing the forces on your body,” Rome said. “The Marines noticed immediately … that the backpack feels lighter.” Rome said concerns raised by some about noise and discomfort are already being addressed.

The frame will have to be designed to fit each particular backpack, but Rome stressed that he does not foresee any problems arising from this. “The backpack that we demonstrated at Camp Pendleton can carry 80 to 100 pounds. We also have a version where the Army’s large MOLLE rucksack attaches right onto our system,” he said.

NSRDEC is working with Lightning Packs on a midsize MOLLE pack. Natick has already performed a parachute drop simulation, and Rome said the Lightning Pack passed the test. He expects future contracts will allow for larger production and field testing.


The nPower PEG can be placed on the warfighter or around the gear to create the up-and-down motion needed for energy harvesting.

Personal Energy

Tremont Electric, whose nPower technology was demonstrated at ExFOB, released its first generation consumer product, the  Personal Energy Generator (PEG), in 2009. Founder and CEO Aaron LeMieux, who conceived the idea while hiking the Appalachian Trail, was initially unable to interest the Pentagon, but he noticed that “lots of servicemembers getting ready for deployment wanted our product.”

Humans tend to locomote at a fairly constant frequency of two hertz, LeMieux said, so he decided to develop a vibration isolation device—a magnet spring system—that resonates at that frequency.To take power off the system, nPower uses a “standard generator configuration” of a moving magnet passing through a conductive loop. There is only one moving component involved in nPower.

The militarized version of the nPower PEG is essentially a 17-inch, five pound stick capable of generating up to 3.3W at a walk. To obtain maximum and continuous energy generation, the best place for the PEG is on the warfighter—in the rucksack or on the body armor, LeMieux said. In the spirit of minimizing burdens, the PEG tested at ExFOB used a Camelbak holder with which Marines were already familiar. The system can still generate energy if the user is mounted; a bumpy road, for example, creates the up-and-down motion needed for the device to harvest kinetic energy.

The military version of the second generation PEG is still a work in progress: LeMieux’s goal is to produce a three-pound device that can replace a BB-2590 over a 72-hour mission at a cost of between $2,000 and $3,000.

NEW solar

Bren-Tronics solar panels with the new light weight flex charger system showing two charge locations for PRC-148/152 batteries and one location for the BB-2590.

Wearable Solar

One of the non-kinetic energy harvesting options presented atExFOB was the Wearable Solar Charger by Bren-Tronics. Based on a currently fielded product, the charger sits on top of the BB-2590/U battery and receives power from a lightweight solar panel that generates up to 20W of power. Made commercially, this solar panel is inexpensive and is rated at just under 18 percent efficiency, according to Bren-Tronics.

“Right now a lot of people are playing with the most high tech solar panels using technology designed for NASA and expensive power managers,” said James Voulgarakis, an engineer at Bren-Tronics. “Those are wonderful solutions, but it will be a while until they are mass produced and affordable enough for widespread fielding.” He added that the Bren-Tronics batteries and chargers can be stowed in a MOLLE pouch worn anywhere the warfighter would choose. For example, it can be left secured in a backpack, with the solar panel hanging over the side.

The BB-2590s Bren-Tronics produces now hold about 290Wh of charge, meaning they could take fourteen hours to recharge. This time could be reduced significantly if multiple panels were connected to one charger. Voulgarakis said the company is considering the market for a basic kit and another with “a lot more bells and whistles.” The target price for the basic kit is around $450.

Kinetic Energy Harvesting knee braces prototype

Bionic Power’s PowerWalk M-Series generates power from the knees

Power Through the Knees 

Bionic Power’s PowerWalk M-Series began as a military-focused project in 2008 with financng from the Canadian Forces and later the U.S. Army. The knee-based power generator selectively engages when the knee is “braking or doing negative work,” according to Bionic Power CEO Yad Garcha. “It’s a bit like a hybrid car, which only generates electric power when it’s going downhill or braking.”

The system generates electricity (through a tiny three-phase magnetic rotary generator) with each stride; on a level grade, Garcha said the PowerWalk, which resembles a pair of knee braces, generates 8 to 12W of power, but more is produced when the user is going downhill. Additionally, the braking assists the user by reducing localized muscle fatigue and metabolic costs when on a steep grade. Enhancing this effect is the light weight of the system: Garcha claimed the body doesn’t notice anything less than 2.2 pounds on the knee, and the PowerWalk weighs only 1.9 pounds per leg, with expectations of lower weights in the future.

Marines responded fairly well to the device at ExFOB, according to a USMC press release which quoted a Marine who thought the system was a “joke” until he tried it on and came away impressed. Garcha told DoD P&E that Marines suggested improvements such as adding a pistol holster and removable ballistic protection and turning the upper belt into a tourniquet.

Currently, each component of the PowerWalk is custom made only a few at a time, and it is therefore extremely expensive. Garcha expects costs will decrease significantly if large-scale production is initiated. The Army has already conducted preliminary trials at Fort Devens, MA, with more planned for 2014 and beyond.

Energetic Soles

Recognizing the overall rise in electronics use on the battlefield, STC Footwear began a research program in 2009 aimed at harvesting the “free energy” that comes from walking—elevation and descent mass, or gravitational energy—while staying “as invisible as possible” with maximum comfort, according to Michel Bisson, president of STC. Unsurprisingly for a footwear company, STC, which has partnered with Lockheed Martin (LMCO) on the program, developed a generator in the heel section of the sole of the new Kinetic Boot.

Over the past two years, the energy to weight ratio of the Kinetic Boot has improved: 2-3W of energy at a walk to 1.5-3 ounces of weight. As with other kinetic energy harvesting technologies, larger weights and higher speeds tend to generate more energy. The system, however, works in other contexts as well. “The key harvesting piece that transfers momentum into electricity could also be used remotely from the boot and in other oscillating systems,” Bisson noted.

Marine response at ExFOB was generally positive, but LMCO’s Alex Moore noted that the product must get tougher. To ruggedize the Kinetic Boot, LMCO and STC plan on working with the military on longer exercises in order to obtain feedback from the troops on comfort and utility. “There will need to be a systematic approach to analyze the various work conditions and [tailor] the product according to that,” Bisson told DoD P&E. “Lockheed Martin [also] has some facilities available to test the biomechanics of the product … so that when it enters the field it is fully proven.”


A civilian shoe outfitted with InStep NanoPower technology.

A much smaller, but no less intriguing player in shoe-based kinetic energy harvesting is InStep NanoPower, which also demonstrated at ExFOB. Their device can generate between 2 to 4W for a pair of boots, depending on speed and weight, and is currently marketed to hiking and cross-training shoes consumers in association with Vibram, which also makes the soles of many military boots worldwide.

Built inside the sole is what Krupenkin, the company’s president, called an “energy generating tube” that is connected to two flexible chambers, at the heel and toe. When the heel presses down during the stride, liquid goes through the tube to the front of the shoe. The opposite occurs when the user pushes off from the toe. The electrical energy generated goes into a small cell phone-sized battery in the sole, which can then power a useful load. “You can use the energy stored in this battery for … connecting external devices through a mini-USB port or for internal, embedded in the shoe electronics [such as] GPS, gyros, and a Bluetooth connection,” Krupenkin said. It can also connect to charge a larger battery or power a Wi-Fi hotspot implanted in a shoe.

Yet one may wonder how well a gym shoe design would do in the field. InStep introduced its most recent device at ExFOB, so they haven’t yet fully tested it. Krupenkin noted that comprehensive testing is planned for 2015, and he is confident that the technology will move forward. “Conceptually, this product is reliable because it is simple: It is basically two hermetically sealed chambers with a tube connecting them [where] a benign, oil-like fluid runs,” he said.

Top photo caption: Cpl. Brandon O’Connell tests out the Lightning Pack on a treadmill during ExFOB. The pack converts normal walking movement into electrical power using a generator. (Lance Cpl. Kathy Nunez)

This article was originally published in the Fall 2014 issue of DoD Power & Energy magazine.

Correction: The print version of this article incorrectly stated that “The BB-2590s Bren-Tronics produces now hold about 290kWh of charge…” This has been corrected to “290Wh.”