Creating textiles that harvest energy lighten the warfighter’s load.
U.S. Army studies show that a fully equipped soldier carries between 50 and 120 pounds of equipment, depending on the mission. Efforts to reduce the amount of weight soldiers need to carry in the field are in full swing—and scientists and researchers are turning to advanced textiles as part of the answer.
“Lightweight, wearable power is very desirable since it will replace some batteries, which add weight to the Soldier’s load,” says Dr. Eugene Wilusz, senior nuclear, biological, chemical scientist in the Warfighter Directorate, U.S. Army Natick Soldier Research, Development and Engineering Center (NSRDEC). “The key to future advancements is the development of electronic circuits that are entirely made of fabric.”
Solar array blankets
Solar array blankets based on photovoltaics are already commercially available. However, these blankets are based on thin-film technology and are generally low in efficiency, Wilusz says. The U.S. Naval Research Laboratory has recently developed solar array blankets that combine high power output with light weight and flexibility. The technology, known as inverted metamorphic (IMM) triple junction (3J) solar cells, allows a thin membrane material to be placed on a flexible substrate. The resulting solar blanket has a higher power output while being lightweight.
“The portability of this blanket is an especially advantageous feature when considering the significant load a Soldier has to carry,” Wilusz says. “The blanket can be used to power equipment in the field, and therefore Soldiers will be able to shed a few batteries from their load.”
Wearable renewable power
Wilusz says the items being developed for integration into the clothing system include flexible antennas, power and data networks, soft keypads, and sensors. Existing systems are a combination of fabric-based and traditional electronics, drawbacks of which include a lack of flexibility and durability. “Current power and data distribution systems have cables attached at various points on the body of the garment,” says Kailash Shukla, project manager for the Army’s Soldier and Small Unit Operational Energy program. “We are working to eliminate these cables and connectors and bury them within the textiles.”
Shukla and his team are concentrating efforts on creating several wearable, renewable power generation technologies that will enable the Soldier to generate power while on missions, as opposed to relying solely on energy stored in batteries. Among the approaches to wearable renewable power generation is harvesting kinetic energy—capturing energy that is generated from the Soldier’s movements and converting that energy into electrical power. The team is working on two kinetic energy harvesting devices: one converts the movement of the Soldier’s knees during walking and the other converts the movement of the backpack as it moves up and down on the Soldier’s back.
The other approach to energy harvesting uses photovoltaics. High efficiency gallium arsenide—the latest high efficiency photovoltaic flexible solar cells—are mounted on a fabric substrate either on the helmet cover or the top of the backpack. The photovoltaic and kinetic devices are designed to be worn either one at a time or in any combination. The power generated from the devices is transferred through the smart textile to a central power and data manager, which is a rigid ½-inch thick box the Soldier carries in a pouch on the back of his or her vest. Field testing of the devices is scheduled to begin in the near future.
“We started funding these technologies about two years ago and the systems will be delivered to us within the next couple of months,” Shukla says. “At that point we can begin testing the prototypes—Soldiers conducting typical missions will wear and use the devices.”
Each of the kinetic and photovoltaic devices produces between 5 and 15 watts. “That is a fairly high level of power output from a wearable energy harvesting device and, if successfully deployed, can reduce the need for the Soldier to carry spare batteries or be resupplied with batteries on long missions,” Shukla says. “The long-term vision is to achieve a net-zero energy solution for the Soldier, meaning the mission’s duration will not be limited by lack of access to power.”
The data and power bus
“We’ve always looked at the uniform as a potential data and power bus. It could transport data and power throughout the Soldier system,” says Carole Winterhalter, textile technologist at the NSRDEC. Winterhalter is working on developing uniforms for that purpose under the Soldier-borne Carbon Nanotube Electrotextile Power and Data Distribution Networks program, which operates under the Soldier in Small Unit Operational Energy program.
The networks are currently made from copper-wrapped yarn and can be integrated into any type of base fabric. “It wasn’t difficult to incorporate the conductive materials within the clothing system,” Winterhalter says. “The difficulty came in joining the conductive materials at the seams to be sure there was electrical conductivity within the system.”
Infoscitex Corp., Waltham, Mass., developed SEWit (Selectively Enabled Wiring in Textiles) in 2007, the technology Winterhalter is using to integrate the network into garments. “We developed a system that allows the conductive media to be included in the fabric at the yarn level,” says Jeremiah Slade, principal engineer, Special Programs of Infoscitex. “The challenge for us was that the system had to allow the finished fabric to be cut it into pattern pieces, sewn into a garment using conventional processes, and still provide a simple and robust means for establishing network connectivity across seams.” Slade and his team developed a series of e-yarn designs, welding processes and connecting techniques that allowed them to do that.
The Infoscitex team is currently working to expand the technology developing a software design tool that can predict where electro-mechanical connections will be formed at the seam of the garment. According to Slade, this capability can be used to determine where along a seam a weld needs to be made in order to connect two given wires, how large the welding horn needs to be in order to maximize the chance of forming a successful weld, and what other wires might be unintentionally connected. This information can in turn be used to design welding templates that will greatly simplify the garment assembly and welding process and make it accessible to conventional garment assembly facilities and personnel.
“However, the most compelling application of this tool will likely come when it is used to guide the early stages of the design process,” Slade says. “This information can in turn be used to design a suitable e-textile fabric that contains e-yarns only in those locations where they will be utilized in the final garment. In this manner the quantity of e-textile material present in the garment can be minimized and located where most needed, thereby reducing material costs and minimizing system weight.”
Infoscitex is considering several types of garments into which they will integrate the power and data networks, including an ACU (Army combat uniform) jacket and the IOTV (improved outer tactical vest). “Of the possibilities, those are the two that are the most relevant pieces of the Soldier ensemble based on feedback we’ve received from Natick,” Slade says. Slade points out that the IOTV is an especially logical application since that is the garment to which the majority of the power-consuming devices or storage power-distribution devices are attached.
Researchers are also beginning to explore using carbon nanotubes in addition to copper to build the networks. “Carbon nanotube conductors hold up much better to repeated flexing than do copper wires,” Slade says. “They’re also more flexible and much lighter. They may not be able to completely replace copper conductors in all instances but it’s something we have a lot of interest in for certain applications and expect to explore in the next year.”
Durability and comfort
One of the concerns with integrating power and data management networks into textiles is the durability of the system. “Standard military cables are heavy, bulky and typically over-engineered,” Slade says. “There is a lot of additional plastic in the jacketing and over-molding of these cable assemblies. It makes them very rugged but a lot of that material doesn’t do anything else but protect those cables. Part of what we’re doing here is allowing the fabric itself to provide some of the protection for the conductive materials.”
Certain performance characteristics are required for all combat uniform fabrics, Winterhalter points out. “If you think about it, Soldiers live in those uniforms for days and weeks on end, and the garments are usually worn against the skin,” she says. “So when we integrate these conductive materials into that fabric we have to be especially concerned about how those materials are going to feel, what the hand of that fabric is going to be like in terms of stiffness, moisture wicking, and how durable that material is going to be.” Because copper behaves very differently than cotton and nylon Winterhalter and her team take that into consideration as they develop the materials. “When we combine the wire with yarn, we do it in a way that doesn’t significantly affect comfort,” she says.
“From a comfort standpoint we’ve had plenty of people handle swatches of these fabric networks, including full garments, and for the most part they’ve been indistinguishable from conventional textiles, as long as you don’t get too carried away with the quantity of e-yarns that you use,” Slade says. “But it’s very tailorable. We’ve looked at a variety of e-yarn designs, material compositions and fabric structures but because of our client’s needs, we’re primarily dealing with nylon-cotton ripstops.
The bottom line
If these projects are successful they will route power and data from energy harvesters to a central power hub that provides energy to various power-consuming devices on the Soldier, such as a radio, personal computer, phone, imaging devices and night vision goggles—all of those peripheral devices that Soldiers would normally have on them—with comfort and durability.