Heat pump uses a loudspeaker and wet strips of paper to cool air

A prototype heat pump that uses water and sound to cool is three times as efficient as previous comparable designs

A heat pump that uses sound to cool is three times as efficient as previous designs.

Heat pumps cool buildings by removing heat from the inside and pushing it outside, like a refrigerator. But unlike a refrigerator, they can also heat an enclosed area by reversing the process. To top it off, heat pumps are typically more efficient than conventional heating and cooling devices.

Guy Ramon at Technion-Israel Institute of Technology in Haifa and his colleagues wanted to see if they could make an even more efficient heat pump by using sound waves to change temperature instead of mechanical parts, which take more energy to move. Such thermoacoustic heat pumps have been built before, but except for a refrigerator on the space shuttle Discovery in 1992, they have not been efficient or powerful enough to be useful.

The team’s thermoacoustic heat pump consists of a metal tube filled with nitrogen connected on one end to a loudspeaker that plays a sound roughly a hundred times more powerful than the noise from a chainsaw. The sound waves cause the nitrogen to compress and expand. When the gas is allowed to expand toward the loudspeaker end of the tube, it gets cool, much like how perfume sprayed from a mister cools as it dissipates.

“Inside of the tube it’s as loud as rock and roll. Outside, it is dead silent,” says Ramon.

Other thermostatic heat pumps work similarly, but the new one also uses water via a stack of wet paper strips placed at the other end of the tube. When the nitrogen condenses and expands, it evaporates some of this water and the turns it into vapour. This process releases energy and cools the gas further.

The whole process can also be run in reverse to generate heat rather than remove it.

The new device is a proof-of-concept model that is about a hundred times less powerful than conventional heat pumps, but it works efficiently, says Ramon. For every unit of power it consumes, it can remove up to four times that much heat, comparable to household air conditioners and an approximately threefold improvement on past thermoacoustic devices.

Team member Nathan Blanc presented the device on 20 November at the annual meeting of the American Physical Society’s Division of Fluid Dynamics in Indianapolis, Indiana,

Simone Hochgreb at the University of Cambridge, who was not involved in the project, says that this kind of thermoacoustic device is promising because it may be efficient enough to run entirely off solar power. However, there is a trade-off. One benefit of thermoacoustic devices is that they are relatively simple to assemble because the only moving part is a loudspeaker. Adding the stack of wet strips makes the pump more efficient but also more complicated to manufacture, she says.

Though the device is comparable to other non-thermoacoustic heat pumps in terms of energy efficiency, Ramon says he and his colleagues have run calculations that suggest it could be made more efficient. They now plan to try to optimise every detail, he says.