Revamping Energy Recovery: New Way To Efficiently Convert Waste Heat Into Electricity

Advancing Energy Recovery: A Breakthrough Approach to Efficiently Convert Waste Heat into Electricity

A groundbreaking solution has been developed by a collaborative team from the National Institute of Standards and Technology (NIST) and the University of Colorado Boulder. Their innovative device, utilizing gallium nitride nanopillars on a silicon platform, significantly enhances the conversion of heat into electricity. This breakthrough holds the potential to recover substantial amounts of wasted heat energy, benefitting various industries and power grids.

The NIST researchers, led by Kris Bertness, have successfully fabricated a novel device capable of greatly improving the conversion of heat into electricity. If perfected, this technology could help reclaim a significant portion of the approximately $100 billion worth of heat energy wasted annually in the United States.

The new fabrication technique, devised by Kris Bertness and her collaborators, involves depositing countless microscopic columns of gallium nitride onto a silicon wafer. Subsequently, layers of silicon are removed from the underside of the wafer until only a thin sheet of the material remains. The interaction between these nanopillars and the silicon sheet impedes the transfer of heat within the silicon, enabling a larger proportion of the heat to be converted into electric current. The findings of this study, conducted in collaboration with the University of Colorado Boulder, were recently published in the journal Advanced Materials.

Once the fabrication process is perfected, the silicon sheets could be wrapped around steam or exhaust pipes, allowing the conversion of heat emissions into electricity. This electricity could then be used to power nearby devices or fed into the power grid. Additionally, another potential application for this technology would be in cooling computer chips.

By implementing the growth of nanopillars above a silicon membrane, the NIST scientists and their colleagues have successfully reduced heat conduction by 21% while maintaining high electrical conductivity. This achievement has the potential to dramatically enhance the conversion of heat energy into electrical energy. Heat energy in solids is carried by phonons, which are periodic vibrations of atoms within a crystal lattice. Some of these phonons in the membrane resonate with those within the nanopillars, effectively slowing down the heat transfer process. Importantly, the nanopillars do not impede the movement of electrons, ensuring that electrical conductivity remains optimal, resulting in a superior thermoelectric material.

The foundation of the NIST-University of Colorado study is rooted in the Seebeck effect, initially discovered by German physicist Thomas Seebeck in the early 1820s. Seebeck noticed that when two metal wires, made of different materials, were joined at both ends to form a loop, a temperature difference between the junctions caused a nearby compass needle to deflect. Further investigations revealed that the temperature difference induced a voltage between the regions, causing an electric current to flow from the hotter region to the colder one. The resulting current generated a magnetic field that deflected the compass needle.

In theory, the Seebeck effect presents an ideal method for recycling heat energy that would otherwise be wasted. However, there has been a major obstacle to overcome. In order to convert a substantial amount of heat into electrical energy, a material must possess poor heat conductivity to maintain a temperature difference between two regions, while simultaneously exhibiting excellent electrical conductivity. Unfortunately, most substances have a direct correlation between heat conductivity and electrical conductivity, making it challenging to achieve both properties.

Through their study of thermoelectric conversion, Mahmoud Hussein, a theorist from the University of Colorado, discovered that these two properties could be decoupled in a thin membrane covered with nanopillars. These nanopillars are standing columns of material, each only a few millionths of a meter in length, roughly one-tenth the thickness of a human hair. Hussein’s discovery paved the way for the collaboration with Kris Bertness.

Utilizing the nanopillars, Bertness, Hussein, and their team successfully uncoupled heat conductivity from electrical conductivity in the silicon sheet, which is the first such achievement in any material. This breakthrough milestone enables efficient conversion of heat into electrical energy. The researchers managed to reduce the heat conductivity of the silicon sheet by 21% without affecting its electrical conductivity or altering the Seebeck effect.

In solids like silicon, atoms are constrained by bonds and cannot freely transmit heat. As a result, the transport of heat energy occurs through phonons, which are collective vibrations of atoms. Both the gallium nitride nanopillars and the silicon sheet carry phonons, but those within the nanopillars are fixed in place by the walls of the tiny columns, similar to how a vibrating guitar string is held at both ends.

The interaction between the phonons traveling through the silicon sheet and the vibrations in the nanopillars slows down the phonons, making it more challenging for heat to pass through the material. This reduction in thermal conductivity amplifies the temperature difference between the two ends of the material. Furthermore, this interaction maintains the electrical conductivity of the silicon sheet, which is crucial for efficient energy conversion.

Currently, the team is focused on developing structures made entirely of silicon with an improved geometry for thermoelectric heat recovery. The researchers anticipate demonstrating a high heat-to-electricity conversion rate, making their technique economically viable for various industries.

Reference: “Semiconductor Thermal and Electrical Properties Decoupled by Localized Phonon Resonances” by Bryan T. Spann, Joel C. Weber, Matt D. Brubaker, Todd E. Harvey, Lina Yang, Hossein Honarvar, Chia-Nien Tsai, Andrew C. Treglia, Minhyea Lee, Mahmoud I. Hussein, and Kris A. Bertness, 23 March 2023, Advanced Materials.
DOI: 10.1002/adma.202209779

This research received partial funding from the Department of Energy’s Advanced Research Projects Agency-Energy.

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More about energy conversion

• Nanopillars on Silicon for Efficient Heat-to-Electricity Conversion – NIST article providing insights into the research on nanopillars and silicon for efficient heat-to-electricity conversion.

• Advanced Materials Journal – The journal where the findings of this research were reported, titled “Semiconductor Thermal and Electrical Properties Decoupled by Localized Phonon Resonances.”

• Department of Energy’s Advanced Research Projects Agency-Energy (ARPA-E) – Information about the funding agency involved in supporting this research on energy conversion technologies.