Turning thermal energy into electricity

Date:
August 30, 2021
Source:
U.S. Army Research Laboratory
Summary:
With the addition of sensors and enhanced communication tools,
providing lightweight, portable power has become even more
challenging. New research demonstrated a new approach to turning
thermal energy into electricity that could provide compact and
efficient power.



FULL STORY ==========================================================================
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.

========================================================================== Story Source: Materials provided by U.S._Army_Research_Laboratory. Note: Content may be edited for style and length.


========================================================================== Journal Reference:
1. Rohith Mittapally, Byungjun Lee, Linxiao Zhu, Amin Reihani, Ju
Won Lim,
Dejiu Fan, Stephen R. Forrest, Pramod Reddy, Edgar
Meyhofer. Near-field thermophotovoltaics for efficient heat to
electricity conversion at high power density. Nature Communications,
2021; 12 (1) DOI: 10.1038/s41467- 021-24587-7 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2021/08/210830140229.htm

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