You are here

High-Efficiency Energy-Harvesting Battery Charger/Storage Unit


TECHNOLOGY AREAS: Ground/Sea Vehicles, Materials/Processes


OBJECTIVE: Develop an innovative lightweight, rugged, durable, high-efficiency battery charger/storage unit (BCSU), capable of continuous conversion of radiative environmental energy into usable direct current (d.c.), to charge Soldier batteries at a rate of 20 W in five hours (at night and in poor weather), while weighing much less than the  batteries it replaces.  The BCSU must involve novel materials approaches, not traditional photovoltaics (PVs), to harness radiative environmental energy (e.g., infrared, solar, THz) and provide on-demand recharging of the Soldier’s batteries in the field under battle conditions, while fitting into the dismounted Soldier’s equipment.

DESCRIPTION: The battery is as integral to the Soldier’s mission and equipment as his/her firearm, and must reliably power electronic functions.  To ensure that equipment is always energized, extra batteries are carried into remote locations, restricting Soldier mobility under dangerous conditions.  Because current portable solar-powered PV battery chargers [1] operate only under bright, sunny conditions (rare in Afghanistan), dismounted Soldiers use them only as a lightweight, emergency back-up.  We propose a lightweight, reliable, portable BCSU to go beyond PV cells (while building on their success) by using novel technologies to efficiently convert environmental energy (e.g., infrared/visible/THz energy from the Earth and other warm sources) into continuous (e.g., 24-hour, under all weather conditions, nighttime, etc.) d.c. for charging batteries.  The Army’s Land Warrior concept requires that Soldiers carry at least 12 2 lb. 100 W-h batteries over a 72-hour unsupplied mission, so the proposed BCSU must output at least 20 W in order to recharge a battery in a reasonable amount of time (~5 hours).  The BCSU (which must be flexible and easily rolled/folded) would, after development, replace ~ 9 of these batteries, requiring the Soldier to carry only 2-3 batteries (one always operating), considerably lessening the Soldier’s load.  The BCSU should weigh less than 4 lb. (the weight of one commercially-available PV cell and one battery that it replaces).

Innovative, perhaps nanomaterials-based approaches (not traditional PVs and infrared/THz technology) will be considered for this SBIR topic.  Higher efficiency conversion of radiative environmental energy, potentially a very disruptive technology (since it permits rapid battery recharging), could be enabled by nanomaterials such as plasmonic or dielectric nanoparticles for confining/scattering light within [2,3], or metallic nanopatterning for better contacting, a semiconductor, improved quantum dots (QDs), polymer-based PVs with variable bandgaps, thermophotovoltaics or thermoelectrics, nanorectennas, etc.

Commercial PV cells have efficiencies limited to 40% at AM1.5 [4], due to their intrinsic bandgap (tuned to only one photon wavelength).  QDs and “nanorectennas” are thought capable of much higher efficiencies in the vis/ir regime [5,6].  QDs have demonstrated multiple exciton generation, one route to very high efficiency, in the laboratory [5], and could convert incident vis/nir energy into an engineered infrared spectrum, which is then harvested by a tuned absorber. A “nanorectenna” consists of an antenna, coupled to a rectifying diode, working at the nanoscale to convert incident vis/ir light into direct current.  Rectenna arrays are very efficient in the radio frequency regime and can be designed to resonate over any desired wavelength range (no bandgap) [6].

Because rectenna efficiency scales with incident power [7], it is conceivable that power could be beamed to a small squad of Soldiers from nearby, if a new airborne platform and lightweight, portable receiver could be developed.  Efficiency, and therefore charge time, would be greater for this “power beaming”.

PHASE I: Research and propose an innovative technology for a high-efficiency, radiative energy-harvesting BCSU to generate 20 W of continuous power for 10 hours total under all relevant weather conditions and during both day and night, from radiative environmental energy and stored power.  For example, the BCSU may be similar to a commercially available PV cell [1], with an additional coating/electronics (negligible weight) to harvest infrared environmental energy.  The BCSU must weigh less than 4 lb., and must be conveniently carried in a rucksack (if a “roll-away”, like PV cells, that roll out into a flat area < 2 m2 in area like in Ref.[1]) or on a Soldier’s helmet (in which case the BCSU must be less than 100 cm2 in area, sufficiently flexible to mount on a helmet, and not produce a visible signal when illuminated), be reliable, and be realistically manufacturable.  Consider nanomaterials-based technologies, such as nanoparticles to enhance scattering, quantum dots, nanorectennas, thin film supercapacitors, etc.  Traditional PVs will not be considered. Mitigate risk by identifying and addressing the most challenging technical hurdles in order to establish viability of the technology (including proper thermal coupling of the BCSU to the environment to ensure continuous, efficient energy conversion from the environment).  Perform proof-of-principle experiments in a laboratory environment, and predict the BCSU’s efficiency at AM1.5.  Provide credible projections of performance, size, weight, energy requirements, and cost of a system suitable for fielding.  Power beaming is acceptable only if the target is large, lightweight (carried by one person), and can receive sufficient power from approximately ¼ mile away.


Physical specification                     Value

Weight                                               < 4 lb. (less than PV module + battery)

Cost                                                    Less than nine batteries planned for displacement

Power output                                     20 W continuous (all-weather, day/night) in the field, capable of supplying 200 W-h total before resupply

Area                                                    < 2 m2 (roll-away) or < 100 cm2

Fire retardant Toxicity                    In accordance with MIL-PRF-44103D

Toxicity/mildew Fire Retardant     In accordance with MIL-PRF-44103D

Temperature range                           In accordance with MIL-PRF-44103D

Durability                                          6 months’ operation in the field

PHASE II: Carry through the Phase I proposal by fabricating/developing a high-efficiency energy-harvesting, lightweight, portable BCSU harvesting energy under all environmental conditions, and demonstrate 20 W continuous output over a 10-hour period (minimum total 200 W-h stored energy) in the field. The BCSU must be sufficiently mature for technical and operational testing, limited field-testing, demonstration, and display. Determine the appropriate technology for the BCSU, and characterize its efficiency at AM1.5. 

Characterize and refine the performance of the BCSU in accordance with the goals in the description above.  Deliver a report documenting the theory, design, component specifications, performance characterization, and recommendations for optimizing the BCSU’s efficiency and power output.  Address manufacturability issues related to full-scale production for military and commercial utilization within applicable systems.  Predict and optimize the usage cycle of the BCSU; i.e., how many times these battery chargers can be used in the field, without resupply.  Provide user manuals and training to support government testing of this equipment. 

PHASE III: The BCSU developed in Phases I and II will be the first high-efficiency energy-harvesting battery charger that meets military standards (i.e., recharges at the rate of 20 W in 5 hours ten times before resupply). Phase III will develop, through technology transition and/or commercialization, the full battery charging and storage system required for the dismounted Soldier to recharge batteries.   This reliable, portable, low-weight flexible battery-charging and storage system must be manufacturable at a reasonable cost.  In general, efficient conversion of solar and infrared energy into electrical energy is one of society’s most important technological challenges, and solving this problem for the Soldier would enable commercialization of efficient, inexpensive solar and infrared energy for the general public. TRL 8

US Flag An Official Website of the United States Government