ABERDEEN PROVING GROUND, Md. — Army researchers are exploring new ways to keep Soldiers’ electronic devices powered during extended missions by using wearable fuel cells for on-the-move battery charging.

As the Army continues to modernize the force with high-tech Soldier-worn and handheld equipment like radios, GPS, night-vision devices and weapons, the energy demand is continually increasing.

Engineers are working on optimizing the power density and efficiency of emerging fuel-cell based power generation technology when operated with packaged fuels as well as commonly available substitutes such as windshield washer fluid.

Army Futures Command (AFC) is leading work on the Soldier Wearable Power Generator (SWPG) that enables on-the-move charging, thus reducing the number of batteries required to be carried.

“We’re aiming to deliver a simple, easy-to-use way for Soldiers to extend battery life and keep moving in the field by developing wearable fuel cells,” said Shailesh Shah, a chemical engineer with the Command, Control, Communications, Computers, Cyber, Intelligence, Surveillance and Reconnaissance (C5ISR) Center — a component of AFC’s Combat Capabilities Development Command (DEVCOM). “Enabling fuel cell operation on windshield washer fluid already in the Army’s supply chain avoids the need to set up a logistics re-supply of custom fuels. The SWPG simultaneously reduces dependence on logistics for battery re-supply.”

The C5ISR Center’s research aligns with the Army’s renewed emphasis and interest in fuel-cell power generation for supplying power to dismounted Soldiers, according to Shah. Technology in the industry has improved significantly in the past 10 years in terms of size, weight, noise, thermal signature reduction, improved modularity and mounting systems.

Adding wearable fuel cells to the Soldier system enables users to charge the currently fielded thin, flexible Conformal Wearable Battery (CWB) worn on vests as a central power source for electronic devices. The fuel-cell research complements the C5ISR Center’s concurrent work to modernize the CWB with advanced materials.

Researchers are continuing SWPG design modifications to improve performance of the prototypes with an emphasis on size and weight reductions, according to Christopher Hurley, chief of the Center’s Tactical Power Branch. Soldiers on 72-hour missions could save 12 pounds in battery weight under normal power draws with current prototypes.

Additional organizations providing support are the Army’s Project Manager Integrated Visual Augmentation System, DEVCOM Soldier Center, the FBI and the National Defense Center for Energy and Environment.

“A key to enhancing our fuel-cell development is placing different prototypes in the hands of Soldiers during field exercises,” Hurley said. “C5ISR Center engineers have been side-by-side with Soldiers to gain feedback during the Army Expeditionary Warrior Experiment in 2020 and 2021 with plans for 2022. The Soldier touch points are an invaluable resource in our development process so we can immediately turn around and incorporate their evaluations into our hardware systems.”

By Dan Lafontaine, DEVCOM C5ISR Center Public Affairs

RESEARCH TRIANGLE PARK, N.C. — With the addition of sensors and enhanced communication tools, providing lightweight, portable power has become even more challenging. Army-funded research demonstrated a new approach to turning thermal energy into electricity that could provide compact and efficient power for Soldiers on future battlefields.

Hot objects radiate light in the form of photons into their surroundings. The emitted photons can be captured by a photovoltaic cell and converted to useful electric energy. This approach to energy conversion is called far-field thermophotovoltaics, or FF-TPVs, and has been under development for many years; however, it suffers from low power density and therefore requires high operating temperatures of the emitter.

The research, conducted at the University of Michigan and published in Nature Communications, demonstrates a new approach, where the separation between the emitter and the photovoltaic cell is reduced to the nanoscale, enabling much greater power output than what is possible with FF-TPVs for the same emitter temperature.

This approach, which enables capture of energy that is otherwise trapped in the near-field of the emitter is called near-field thermophotovoltaics or NF-TPV and uses custom-built photovoltaic cells and emitter designs ideal for near-field operating conditions.

This technique exhibited a power density almost an order of magnitude higher than that for the best-reported near-field-TPV systems, while also operating at six-times higher efficiency, paving the way for future near-field-TPV applications, according to Dr. Edgar Meyhofer, professor of mechanical engineering, University of Michigan.

“The Army uses large amounts of power during deployments and battlefield operations and must be carried by the Soldier or a weight constrained system,” said Dr. Mike Waits, U.S. Army Combat Capabilities Development Command’s Army Research Laboratory. “If successful, in the future near-field-TPVs could serve as more compact and higher efficiency power sources for Soldiers as these devices can function at lower operating temperatures than conventional TPVs.”

The efficiency of a TPV device is characterized by how much of the total energy transfer between the emitter and the photovoltaic cell is used to excite the electron-hole pairs in the photovoltaic cell. While increasing the temperature of the emitter increases the number of photons above the band-gap of the cell, the number of sub band-gap photons that can heat up the photovoltaic cell need to be minimized.

“This was achieved by fabricating thin-film TPV cells with ultra-flat surfaces, and with a metal back reflector,” said Dr. Stephen Forrest, professor of electrical and computer engineering, University of Michigan. “The photons above the band-gap of the cell are efficiently absorbed in the micron-thick semiconductor, while those below the band-gap are reflected back to the silicon emitter and recycled.”

The team grew thin-film indium gallium arsenide photovoltaic cells on thick semiconductor substrates, and then peeled off the very thin semiconductor active region of the cell and transferred it to a silicon substrate.

All these innovations in device design and experimental approach resulted in a novel near-field TPV system.

“The team has achieved a record ~5 kW/m2 power output, which is an order of magnitude larger than systems previously reported in the literature,” said Dr. Pramod Reddy, professor of mechanical engineering, University of Michigan.

Researchers also performed state-of-the-art theoretical calculations to estimate the performance of the photovoltaic cell at each temperature and gap size and showed good agreement between the experiments and computational predictions.

“This current demonstration meets theoretical predictions of radiative heat transfer at the nanoscale, and directly shows the potential for developing future near-field TPV devices for Army applications in power and energy, communication and sensors,” said Dr. Pani Varanasi, program manager, DEVCOM ARL that funded this work.

By U.S. Army DEVCOM Army Research Laboratory Public Affairs