R&D Targets Lightweight Power-Generation Devices
Dec 5, 2008
By Bettina H. Chavanne
Battery power, rather than firepower, is rapidly becoming a major factor in military planning, as the high-tech electronics that are powering systems become more reliant on portable battery power.
The need for electrical power on the battlefield is high and continues to grow. The energy capacity of batteries, consequently, means a great deal to ground forces. Heavy, expensive, nonrechargeable military batteries slow warfighting efforts and weigh down soldiers in critical combat situations.
According to the U.S. Defense Dept., the average power load of a nine-man rifle platoon is 10.3 watts per person, with an average use of 12.3 watts per soldier. Within a decade or so, soldiers will need about 50 watts of power. To put the numbers in perspective, the average 72-hr. mission requires 885 watt-hr. of energy, or 5.9 kg. (13 lb.) of batteries. A 96-hr. mission requires 1,181 watt-hr. of energy, or 7.9 kg. of batteries.
As the need for power increases, so does the weight of power sources—a growing component of the soldier’s load is primary and backup power sources for individual systems: communications, navigation, imaging, computation and sensors. The weight has become such a burden that some American platoons designate individual soldiers as “battery mules.”
Bullets and batteries: Army soldiers in Iraq are loaded down with ammunition, weapons, sensors, radios and up to 13 lb. of batteries.Credit: U.S. ARMY
The logistics burden of supplying power to the battlefield is also growing and becoming more complex due to a lack of battery standardization. When Operation Iraqi Freedom began, for example, batteries were in short supply, as troops in the south of Iraq used half of the projected total war requirements in only a few days. Supplies to combatants in the north were unavailable. Forward stocks of batteries drained in the first days of the war, and the U.S. Army’s entire supply would have been used up in two months if not replenished under an emergency program. Batteries were airlifted from U.S. depots to Iraq and 24/7 production of new inventories was initiated with six manufacturers worldwide.
Unless sources of power evolve with the systems that use them, they will create a logistical and tactical burden for soldiers. Because new power-consuming systems quickly become an integral part of how the U.S. and other industrial militaries fight, better ways must be found to support power requirements.
Initiatives are underway to save money on batteries and to enable combat and materiel developers to reduce expenses early in a product’s life-cycle. Power sources must be an important consideration in all materiel developments and combat operations. As soldiers expect more out of the equipment they use, they also expect more from their power sources.
The French army’s Felin infantry combat suite uses several power sources to support the core system, radios and wearable sensors. The kit uses two batteries—a high-capacity pack and a standard pack providing the minimum power required for mission-critical systems.
The high-capacity pack is worn in the combat vest. This flat, waterproof, lithium-ion (Li-ion) rechargeable battery weighs about 600 grams (21 oz.) and supplies 75 watt-hr. to support the core system. A smaller and more compact Li-ion battery weighing 180 grams in the standard pack delivers 18.5 watt-hr., powering peripheral units and sensors (helmet and weapon). Both batteries are designed to sustain the Felin system for 72 hr. The batteries use a built-in microprocessor and data bus to monitor power levels while the system is running and to optimize charging cycles.
The U.S. Defense Dept. sponsored a competition for wearable power technologies. UltraCell demonstrated its XX25 fuel cell.
A different power system was designed by ABSL for the British Army’s FIST system. The company developed its own version of a modern soldier system’s power source. The core of the system is a 4.3-kg. (9.5-lb.) four-channel “smart charger,” which can be connected to various d.c. sources like vehicle batteries, fuel cells and other batteries, to tap any power source in the field. It provides a fast recharge cycle, delivering power under 80% of a charge in less than 60 min.
The charger supports a smaller portable charger, which charges any secondary smart battery. This lightweight (340 grams) charger is carried by the soldier, enabling the warfighter to tap any power resource in the field. It supports a charging rate of 7.4 ampere-hr., enough to provide 80% of the charge in less than 60 min.
Medis Technologies in Israel is introducing a fuel-cell technology called Power Knight. Contained in a small, flat backpack, it is designed for up to 72 hr. of continuous operation, delivering 20 watts to the individual soldier. A prototype system has been developed for General Dynamics for evaluation in its Future Soldier Systems program gear. The company began producing a commercial version in March.
Primary batteries, particularly those made from lithium, deliver up to eight times the watt-hr. capacity of conventional rechargeable batteries. New rechargeable batteries using lithium anode will also have higher capacity than conventional rechargeable batteries. Although lower than those of the primary sources, they will provide a choice between freedom from charging and longer shelf life of the primary, or potential cost savings over rechargeable batteries.
The logistics of primary batteries are straightforward—portable energy can be made available at remote distribution points that are unmanned and have no electricity. Disposal is safe because little toxic material is used. Because of one-time use, though, the cost of the primary battery is about 30 times higher than that of rechargeable cells.
Primary batteries are simple to store, as they require no maintenance, and have a shelf life of 10 years. In contrast, lithium batteries are good for only 2-3 years, whether used or not, although cool storage at a 40% charge prolongs longevity. Nickel-based batteries are good for five years or longer, but require priming to regain performance after long storage.
Stocking rechargeable batteries requires significant maintenance, keeping track of the battery’s state of health, cycle count and age. Due to high self-discharge, nickel-based batteries exhibit a high self-discharge rate of 10-20% per month. This compares with 5-10% for lithium and lead-based batteries. Self-discharge rates increase at higher temperatures.
For this reason, secondary batteries are not an effective medium for long-term energy storage and must be charged before each activity. Maintenance procedures must be followed with each type of chemistry, operational use and environmental condition.
Smart monitoring of effective battery power is a significant aspect for military use, but is rarely available with primary batteries. The use of state-of-charge indicators and smart bus-type communications enables users to monitor the charge state of batteries. For rechargeable batteries, a combat device also requires an additional state-of-health indication that depicts the anticipated life of the fully charged device.
Advanced Materials’ fuel cell is designed to be worn as a backpack.Credit: BETTINA H. CHAVANNE/DEFENSE TECHNOLOGY INTERNATIONAL PHOTOS
While such measures add to the cost of each battery, they reduce the life-cycle cost of battery inventories by decreasing unnecessary replacement of batteries and maximizing the use of available power.
In a network-centric environment, where high-capacity transmission of data is required, power requirements for portable electronics are outgrowing power-source capacity, leading to shorter service per battery. Regenerative power is thus critical for missions.
In seeking to solve power issues facing soldiers, the U.S. took an old-fashioned approach: In early 2008, it launched the Wearable Power Prize (WPP) competition.
William Rees, deputy undersecretary for defense for laboratories and basic sciences, calls the competition “motivation for innovative people.” His goal with the WPP was to invite innovators into the Pentagon who wouldn’t have access to national security projects. The WPP’s sponsor, the office of the Director of Defense Research and Engineering (DDRE), evaluated hundreds of ideas before settling on a competition.
“The criteria were simple,” Rees says. “It had to have impact if it worked and had to be something that experts felt was not making progress through traditional methods.”
Among applications for a power pack: lightweight laser rangefinder designators, VHF and UHF radios, portable satellite communications terminals and night-vision equipment.
The WPP’s parameters called for an autonomous wearable power system that provides 20 watts of average power for 96 hr., has 14- and 28-volt outputs, and weighs less than 4 kg.
Power pack finalists ran their inventions through a gauntlet that started with a 92-hr. bench test, during which the power pack was subjected to loads of less than 20 watts and periods with peak loads ranging up to 200 watts for 5 min.
Once teams passed the bench test, conducted in the heat of the U.S. Marine Corps Air Ground Combat Center in Twentynine Palms, Calif., they moved on to a 4-hr. field test, during which 80 watt-hr. of energy were drained from their systems. Ten stations were set up at which additional energy draining tasks were performed including inflating a small boat, powering a personal cooling vest and being jostled about for minutes at a time while strapped to the back of a soldier marching in place.
Winners were announced on Oct. 4, the day of the finals.
The top three teams proved what the Defense Dept. suspected: fuel cells are the most efficient power-generation system. The winning team, DuPont and Smart Fuel Cell (SFC), took home $1 million for the M-25 portable fuel cell. DuPont used a methanol fuel cell and combined it with a commercial fuel system from SFC. The U.S. Army is now testing the M-25.
Adaptive Materials took second place for its fuel cell, which was only 28 grams heavier than DuPont’s M-25, and the reason it placed second.
SFC also shared the third place prize for the Jenny fuel cell, which it developed with Capitol Connections. NATO is testing it. Both the Jenny and the M-25 use a fuel cell, fuel cartridge, voltage converter and rechargeable Li-ion battery.
One finalist who attracted attention was an inventor from Utah named Steven Middleton. He arrived with a battery that relies on electrostatics to generate energy. Middleton compares his power system to an everyday occurrence—rubbing feet on a carpet and getting a shock.
“That’s the basic part of my theory,” he says. “That electron is moving. All I have to do is control the movement and move more of them.”
Interested parties included the U.S. Army Research Laboratory, which invited him to discuss his research at its headquarters in Adelphi, Md.
With David Eshel in Tel Aviv.
