A research group at Technion – Israel Institute of Technology has developed a novel phase-change thermoacoustic heat pump (PTHP) concept for residential and industrial heat pumping.
“Phase-change acoustic conversion offers great potential for a clean and efficient cooling technology, that may be powered by a hybrid source of either electricity or, more importantly, heat,” the research's lead author, Guy Ramon, told pv magazine. “Use of acoustics greatly simplifies device construction, reducing the need for moving parts, and the presence of phase change dramatically increases the power density of these devices, while utilizing sustainable materials.”
The acoustic-driven system utilizes a binary mixture of an inert and a reactive component as the working fluid that undergoes condensation and evaporation during the thermoacoustic cycle. Atmospheric air is used as the inert gas, while water or isopropanol was utilized as the reactive component.
The heat pump consists of a loudspeaker, a resonator, and a thermoacoustic core placed inside the resonator. The core includes a cold heat exchanger, a stack, and an ambient heat exchanger.
“The length and diameter of the resonator are 0.88 m and 47 mm, respectively, and the stack is a piece of 600 cells per square inch ceramic honeycomb with a length of 95 mm,” the researchers explained. “The ambient heat exchanger is a plate-fin type, with a spacing of 2 mm and fin width of 0.7 mm.”
The system incorporates a cold heat exchanger made of 0.5 mm-thick nickel-chrome wire and is embedded within a 3D-printed skeleton. It is used to provide the heat load into the system, which is in turn driven by a loudspeaker at the resonant frequency of the system, which is 86 Hz.
The system works in four steps. Initially, the mixture of the reactive component and the inert gas moves toward the low-pressure region. Then the lowered partial pressure drives the evaporation of the reactive component carrying latent heat with it. In the third step, the mixture undergoes the reverse processes of the first two steps and the reactive component condenses on the solid, transferring the latent heat. Finally, the acoustic power is consumed in order to pump heat against the temperature gradient.
The research group compared the performance of the heat pump with that of a reference device with no phase change. It found that the heat pump may achieve a coefficient of performance (COP) of over 40% with an air-isopropanol mixture at a concentration of 0.8.
“Our experimental results show that a larger cooling power and a higher COP can be obtained with phase change when the temperature difference is below a critical value, which verifies that the phase change enhances thermoacoustic heat pumping,” it stated. “However, when the temperature difference is increased to exceed the critical value, the performance becomes worse than the equivalent classical thermoacoustic heat pump, because the time-averaged mass flux reverts its direction and carries heat against the heat pumping direction.”
The novel heat pump concept was presented in the paper “Environmentally-sound: An acoustic-driven heat pump based on phase change,” published in Energy Conversion and Management. “The patented technology is now entering the initial stages of commercialization,” Ramon stated, adding that further investigation is still required for the proper realization of its full potential.
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