The researchers discussed thermoacoustic heat pumps, a decades-old technology they believe could emit fewer greenhouse gases than conventional cooling technologies.
As global populations grow and the climate warms, humanity will need increasingly effective ways to cool itself off — on an enormous scale. Experts predict that, globally, the number of cooling systems, like air conditioners, could double or even quadruple by 2050.
One research team is working on a unique solution known as phase-change thermoacoustic cooling. Doctoral student Nathan H. Blanc of Technion, the Israel Institute of Technology, says thermoacoustic cooling has big advantages over conventional air conditioning. He says, “With a longer technology lifecycle, it’s much more sustainable.” Blanc presented recently published research at the November 2022 APS Division of Fluid Dynamics (DFD) meeting in Indianapolis, Indiana.
Despite new interest in thermoacoustic cooling, the technology itself was pioneered in the 1980s. Greg Swift, a now-retired research scientist, says, “We started with simple experimental tests of the most basic thermoacoustic math”, and the field was born.
How does thermoacoustic cooling work? First, an inert gas is sealed inside a tube that’s lined with heat exchanging materials. Then a sound wave is fired into the tube. In low-pressure areas of the wave, the gas expands and absorbs heat; in high-pressure areas, the gas is compressed and expels the heat. Controlling the sound wave controls where heat is absorbed and released. Although a wave only moves each bit of gas a short distance, the cumulative effect amounts to a “bucket brigade,” creating a reliable heat-pumping system with few or no moving mechanical parts, Blanc said at the DFD meeting. This reduces the chance that the system breaks and lengthens its lifetime.
The technology is so reliable that a variation of it was recently launched into space aboard NASA’s James Webb Space Telescope to cool its mid-infrared light spectrograph instrument. “You would only send a technology to space, where no one is there to fix it, that you can count on,” says Blanc.
Swift said, “Our flagship attempt to do something practical (with the technology) was a combustion powered cryogenic liquefier of natural gas. We worked out the physics, made great progress on the engineering — and then the flagship sank.” By the mid-2000s, he says Swift’s work with thermoacoustics had ended, and most American innovation with the technology shuttered soon after.
But Blanc’s team isn’t simply picking up where Swift left off. Instead of relying solely on an inert gas for heat transfer, Blanc’s team is adding a phase-change fluid. The phase-change fluid is cycled between its liquid and gas phases to store and release even more heat. So far, the team has experimented with water, ethanol, and isopropanol.
Although the phase-change fluid restricts the system’s temperature range, their prototype is still twice as powerful as traditional thermoacoustic cooling when pumping heat between temperature differences of 10°F, according to the team’s most recent data. Calculations suggest that full-scale models could be more powerful and outperform conventional air-conditioning technologies.
Swift said, “Getting most of the heat to happen with phase change instead of adiabatic pressure changes in an inert gas might be a game-changer.”
That’s exactly what Blanc, Ramon, and Yang are hoping for. The efficiency boost “provides an opportunity for thermoacoustic cooling to compete with classical technologies,” says Yang. The trio are optimistic. “I hope our research can make the large-scale application of thermoacoustic cooling into reality,” says Yang. Today, the team’s heat pump prototype is powered by grid electricity, but Blanc says the next step is to design the pump so that its power comes entirely from heat — either solar or waste heat.
Today, 3 billion refrigerators and heat pumps are in service, and air conditioners and fans consume 20 percent of electricity in buildings. As more of these technologies are used, waste heat, power consumption, and greenhouse gas emissions will grow, too. This is crucial to resolve a world trying to rein in global warming.
Content courtesy : Liz Boatman: aps.org
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