Applying heat to a liquid or gas will cause the spontaneous generation of sound waves – a thermoacoustic power that has long supported machines like engines and refrigerators.

At the 175th Meeting of the Acoustical Society of America , researchers from Purdue University demonstrated that thermoacoustics properties could theoretically occur in solids too.

Temperature gradients, when applied to liquids, produce waste heat or mechanical vibrations that be converted into other useful forms of energy.

Material properties of solids are much more controllable and “tailorable” than fluids. — Prof. Fabio Semperlotti

Refrigerators, for example, use vibrating motion to make cold areas colder and warm areas warmer. Waste heat generates the necessary mechanical vibrations for engines.

The thermoacoustics machines, however, have been fluid-based.

The Purdue researchers developed a theoretical model – a solid one – demonstrating that a thin metal rod exhibits self-sustained mechanical vibrations when a temperature gradient is periodically applied.

According to the team, the solids can be engineered to achieve necessary thermoacoustics performance.

“Fluids do not allow us to do this,” said lead researcher and Purdue assistant professor of mechanical engineering Fabio Semperlotti .

Like fluids, the solids were shown to contract when cooled down, and expand when heated. If the solid contracts less when cooled and expands more when heated, the resulting motion will increase over time.

Semperlotti spoke with Tech Briefs about why his concept is a solid one, perhaps especially for applications in space.

Tech Briefs: Regarding an engine, what are the advantages of solids vs. fluids?

Semperlotti: Material properties of solids are much more controllable and “tailorable” than fluids: one can think of engineering the structure of a solid to meet certain design constraints (particularly, sound and thermal transport properties) – doing the same with a fluid is much more difficult, if not unfeasible most of the times.

Tech Briefs: In what settings do you envision this kind of solid-based design being most valuable?

Semperlotti: We envision using these systems for low-power niche applications. This is mostly a system for the use of waste (or low-grade) thermal energy. Ideal applications for this technology involve scenarios where large temperature gradients are available and where the reliability of the energy conversion system is critical. As an example, this technology could be a viable candidate to build low-power on-board electric generators for space systems, such as satellites or orbiting stations. The large thermal gradients available in space together with the need for long-lasting and ultra-reliable systems could make this technology very competitive in this market.

Researchers envision thermoacoustics in solids eventually harnessing the extreme temperature gradient of outer space for electricity on satellites. (Image Credit: Purdue University/Mo Lifton)

Tech Briefs: What inspired this idea?

Semperlotti: Fluid-based thermoacoustic engines have been studied and developed for several decades. The concept of a solid-state engine operating on a similar principle appeared a reasonable extension given that waves in solids are governed by physical laws mathematically similar to the ones of fluids dynamics.

Tech Briefs: How do you envision this technology progressing?

Semperlotti: We envision the future of this technology going hand in hand with materials specifically designed to achieve optimal properties for thermoacoustic energy conversion. The rapid development of additive manufacturing capabilities suggests that this scenario could be in reach in the very near future.

There are also some other advantages that could be useful for specific applications. A solid-state thermoacoustic engine not only does not have any part in motion, but does not need any fluid that could eventually leak and reduce the operating life of the device.

See NASA's Take on Thermoelectrics NASA’s Glenn Research Center developed a Double-acting Extremely Light Thermo-Acoustic (DELTA) Converter.

Tech Briefs: Why do you think this idea hasn’t been tried before?

Semperlotti: In general, it is not natural to think of a solid as the ideal medium to obtain motion from static energy sources, like a thermal gradient. In addition, there are some drawbacks of solids compared to fluids. The response of fluids to thermal gradients is stronger than in solids (i.e., their thermoelastic response) and the mechanical energy dissipation is lower. Also, the experimental verification in solids is still a challenging task because it requires the implementation of simultaneous thermal and mechanical boundary conditions that are not easy to achieve.

Tech Briefs: What’s most exciting to you about this research and the possibilities?

Semperlotti: The idea that – after more than a century of thermoacoustics – there are still new treasures to be discovered is very exciting. At the same time, we believe that this idea can open a complete new range of applications, and we are excited to see what kind of applications the community will explore. In fact, we do not think of this technology as a replacement of fluid-based thermoacoustics, but instead as something that will complement it.

Tech Briefs: What’s next regarding your research?

Semperlotti: Time will tell. The idea was well received at the recent ASA conference in Minneapolis, MN. After all, fluid-based thermoacoustic devices are sufficiently mature technologies, so envisioning a fully solid-state implementation is quite reasonable.

At this stage, our study did not produce an experimental validation yet. This is the most important next step in order to bring the community on board with this concept. Once the concept is experimentally validated, we will work on integrating this technology in practical solid-state devices and – why not? – maybe testing this technology in space.

What do you think? Will solid-based thermoacoustics be used to someday power spacecraft? Share your questions and comments below.

Semperlotti developed the research with Professor Mihir Sen from the University of Notre Dame; Carlo Scalo, an assistant professor of mechanical engineering at Purdue; and Purdue graduate research assistant Haitian Hao.