How Beneficial is Turning Thermal Energy into Electricity?

Providing lightweight, portable electricity has gotten much more difficult with sensors and improved communication technologies. However, an Army-funded study revealed a novel technique to convert thermal energy into electricity to supply soldiers with compact and efficient power on future battlefields.

Photons are emitted by hot things into their surroundings. A photovoltaic cell can catch the released photons and convert them to usable electric energy. Far-field thermophotovoltaics, or FF-TPVs, is a method of energy conversion that has been in research for a long time; nevertheless, it has a poor power density and hence requires high emitter operating temperatures.

The study, done at the University of Michigan and published in Nature Communications, reveals a novel technique in which the distance between the emitter and the photovoltaic cell is decreased to the nanoscale, allowing for considerably higher power production than FF-TPVs at the same emitter temperature.

NF-TPV

Near-field thermophotovoltaics, or NF-TPV, is a method that allows energy to be captured that would otherwise be trapped in the emitter's near-field. It employs custom-built photovoltaic cells and emitter designs that are suited for near-field working circumstances.

According to Dr. Edgar Meyhofer, professor of mechanical engineering at the University of Michigan, this technique had a power density nearly an order of magnitude higher than the best-reported near-field-TPV systems while operating at a six-fold higher efficiency, paving the way for future near-field-TPV applications.

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The Army's Power Consumption

"The Army consumes much power during deployments and battlefield operations, and it has to be carried by the Soldier or a weight-constrained system," said Dr. Mike Waits of the Army Research Laboratory of the US Army Combat Capabilities Development Command. "If effective, near-field TPVs might serve as more compact and efficient power sources for Soldiers in the future, as these devices can operate at lower operating temperatures than traditional TPVs."

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 determines the efficiency of a TPV device. Thus, while increasing the temperature of the emitter increases the number of photons above the photovoltaic cell's band-gap, the number of photons below the band-gap might heat the photovoltaic cell must be reduced.

Dr. Stephen Forrest, professor of electrical and computer engineering at the University of Michigan, said, "This was done by manufacturing thin-film TPV cells with ultra-flat surfaces and a metal rear reflector." "Photons over the cell's band-gap are absorbed efficiently in the micron-thick semiconductor, while those below the band-gap are reflected the silicon emitter and regenerated."

Developing Photovoltaic Cells

The researchers developed thin-film indium gallium arsenide photovoltaic cells on thick semiconductor substrates, then peeled off the cell's active semiconductor layer and moved it to a silicon substrate.

All of these advancements in device design and experimental methodology culminated in developing a new near-field TPV system.

An Improved System Compared to its Predecessors

Dr. Pramod Reddy, professor of mechanical engineering at the University of Michigan, said, "The team has produced a record kW/m2 power production, which is an order of magnitude greater than systems previously reported in the literature."

Researchers also used advanced theoretical calculations to evaluate the performance of the photovoltaic cell at various temperatures and gap sizes and found that the tests and computer predictions were in good accord.

"This recent demonstration directly illustrates the possibility for creating future near-field TPV devices for Army applications in power and energy, communication, and sensors," stated Dr. Pani Varanasi, program manager, DEVCOM ARL, which sponsored the work.

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