RETURN OF THE FREE-PISTON STIRLING ENGINE
Researchers at the University of New Mexico demonstrate that the elegant mechanical engine is well suited for nuclear microreactor applications.
Written by Kayt Sukel
Minghui Chen (right) and student Phat Doan (left) working on the high-temperature, high-pressure helium test facility for testing Stirling engines.
WHEN ROBERT STIRLING INVENTED his namesake engine in the early 1800s, he built it in hopes of replacing the steam engine. Ironically, its unique design, which converts heat energy to mechanical energy, was largely used to power low-energy freezers for much of its early life. The development of a free-piston Stirling engine (FPSE) in the 1960s, which used free floating pistons to support energy conversion, made it possible for the engine to do much more than previously thought. The National Aeronautics and Space Administration (NASA) understood its potential to help power space travel. One such free-piston Stirling power converter system has been in continuous (and maintenance-free) use at NASA’s Glenn Research Center for more than 14 years.
Researchers from the University of New Mexico’s Department of Nuclear Engineering see new potential for this type of power system in microreactor applications. UNM assistant professor Minghui Chen said its compact design and high efficiencies make it an ideal candidate to support small nuclear reactors that can be used in a variety of different energy applications, including working as part of the electrical grid.
The team reported its findings in the paper, “Design and Analysis of a Free-Piston Stirling Engine for Microreactor Applications,” published in the Journal of Nuclear Engineering and Radiation Science.
“It’s very small, but very reliable,” Chen said. “NASA is already using it in deep space. It provides enough power and can also be set up for autonomous control for use in microreactors. It makes it an excellent choice for these applications.”
To test its feasibility, Phat Doan, a doctoral student in the Nuclear Engineer program, analyzed the design of a 20-kilowatt (kWe) FPSE using MATLAB to determine its efficiency and output. He and Chen discovered that their proposed FPSE had 21.7 percent efficiency with a total output power of 20.7 kWe. Chen said Doan spent a great deal of time using different models to do the calculations to make sure the design was sound. They were pleased to show that it is, but Chen said they have more work ahead to scale up the model for actual use in a microreactor setting.
Schematic of an FPSE connected to the microreactor loop.
A free-piston Stirling engine is “very small, but very reliable. NASA is already using it in deep space. It provides enough power and can also be set up for autonomous control for use in microreactors.”
– Minghui Chen, assistant professor in the Department of Nuclear Engineering at the University of New Mexico
The research team, Left to Right Phat Doan, Anutam Bairagi, Braden Foderaro, Minghui Chen, Bao Nyugen, Alexandra Martinez, and Yuqi Liu.
“Now, [Doan] is working on putting in more accurate modeling capabilities for testing. We are working on high-temperature and high-pressure helium experiment testing. We are also doing shakedown testing to test the instrumentation,” he explained. “But our current results are for one unit, about 20 kWe. If we wanted to have an output of 1 megawatt electrical, we are talking about putting about 50 units together. We cannot do that right now—but it is possible, with some design changes, we can upgrade the power in a single unit to 500 kWe in the future.”
To that end, the team is also looking at ways to make targeted improvements to the FPSE system. While Doan’s calculations demonstrated its current design is efficient, compact, and reliable, the team is looking at potentially replacing the heat exchanger for heat flux and developing a more robust foil for the regenerator. The team is relying on simulation tools to help them “optimize the configuration,” Chen said.
“It could be serial or parallel configuration of different engines in our new microreactor—we need to see how we can get it to have very high efficiency when it is coupled with the real microreactor,” he said.
In the meantime, Chen hopes that others will see the strong potential of this mechanical workforce for future microreactor applications.
“It’s an excellent choice,” he said. “It can be plug and play, ready for microreactor applications. And for those interested in these potential applications, our testing code and models are open source. Any scholar or student can use this code to do their own research in this area.”
Kayt Sukel is a technology writer and author in Houston.
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