There are many opportunities to conserve power and reduce costly battery dependency in the electronics of military munitions. Energy harvesting can play an important role in addressing this need by converting energy from previously unemployed, environmentally-derived sources into usable power.

One example of harvestable energy is excess heat, which can be converted into electrical energy by thermoelectric materials. Another example is energy derived from mechanical vibrations and shocks, which can be converted to usable energy by piezoelectric materials (Figure 5-1).
Many issues remain in optimizing the performance of thermoelectric materials such as thermophotovoltaics (TPVs). In this area, the EMPF can address issues of packaging, miniaturization, energy conversion efficiency, and manufacturing methods.

For military munitions, energy harvesting can reduce dependency on large batteries for long-range missions. It can also address military needs to withstand a wide range of temperatures, maintain a long shelf life (up to 20 years) and usable life, and withstand high accelerations, shock, and spin.

Currently, oxyhalide liquid reserve batteries are used, but present formulations are difficult to produce in cylindrical battery designs. Liquid reserve systems also require time to increase their voltage to a usable level.

The main intention for an energy harvesting system is to replace or supplement current battery systems and create a more efficient form of power. Systems that require power to determine initial velocity and position at launch would still require a storage battery but would not need a long-term battery system.

Methods in development, such as TPV power generation, are leading to energy harvesting advances. Positive attributes of TPVs include improved safety (no initial power) and long shelf life. Their compact size allows conformable integration into munitions.

A combination of energy systems can be utilized to form a hybrid energy system, creating a best match for electronic requirements. Hybrid energy systems have the advantages of solid state electronics, including durability, compact size, and lack of wet chemicals. Many of these systems are being developed for a 50,000g HEAT (High Explosive Anti-Tank) round.

An energy management system is necessary to retain the energy produced and to distribute it in an efficient manner. The U.S. Army is developing a reconfigurable central energy management system (Figure 5-2). In this system, an ASIC component will harvest energy from several input source methods, such as photovoltaics, RF charging, and piezoelectrics. The component will produce control energy from small lithium ion batteries, supercapacitors (qualified to high g), and thermal batteries.

The U.S. Army has developed a roadmap of methods to harvest energy for Future Combat Systems (FCS) that includes and utilizes the following technologies:

• Super-capacitor
• Lithium polymer
• Thermal
• Conformal battery

The Advanced Precision Concepts Branch at the Army Research Development Engineering Center (ARDEC) in Dover, New Jersey is developing energy harvesting systems for use in hybrid energy systems. The objective is to bring a systems approach to the management of power distribution throughout the mission profile of selected munitions. This would be achieved by developing devices that are conformable to the munition structure or to the available space allocated to the power source. In addition, it is desirable to reduce the need for multiple batteries to power long-range munitions and to supplement current batteries in new or upgraded systems. This research supports an Army effort to improve the reliability, safety, producibility, performance, and cost of munition energy systems.

Technologies focused on harvesting energy from gun-fired munitions are also under development at ARDEC. One method stores mechanical energy in piezoelectric springs and transforms it over time to electrical energy. This method has been demonstrated to produce electrical power much greater than could be generated by a piezo element in its traditional role as an impact sensing device.

Another method uses TPV cells, integrated into the nose of supersonic projectiles and their leading edges. In flight, friction in the atmosphere heats the nose to as much as 1400 K. The harvesting of this energy significantly reduces the
required volume assigned to batteries or other sources of power. As a result, more space is freed to house future sensors, actuators, other guidance and control components, and new, advanced lethal mechanisms.

Energy harvesting is an emerging technology that strives to reduce battery dependency through improved energy conversion from previously untapped sources and improved storage of converted energy. Size, shelf life, and the ability to integrate into hybrid systems are the driving factors for continued improvements that the ARDEC and the EMPF can make to energy harvesting for current and future munitions.

For further information regarding ARDEC, please contact Mr. Chris Janow or Mr. Carlos M. Pereira (cpereira@pica.army.mil, 973-724-1542).