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Thermoacoustic Refrigeration

AndrewHolden Member Posts: 1
Hey guys,

This is my first post on this site, been lurking for quite awhile. I apologize in advance if a post like this is off topic.

I was browsing YouTube the other day, and came across an interesting video about using sound waves to transfer heat. I will do my best to explain how it works.

You take a tube that is closed on one end, and place a speaker on the other end of the tube. You need a tone generator to play the resonant frequency of the tube itself.(This just means the sound wave is the same length as the tube) You can actually test this by placing tiny Styrofoam balls into the tube and lay it on its side, when you play the resonant frequency, the balls will pile up directly in the middle of the tube. (They do this because sound is waves of compressed and decompressed air, this causes the balls to arrange themselves where the pressure is lowest, in the middle of the tube.)
The closed end of the tube and the end of the tube where the speaker is will pulsate into high pressure as each sound wave compresses at the nodes, and the middle of the tube will remain in a fairly constant low pressure. You can use this pressure differential to move heat energy around.

In the video I was watching, the guy did this by taking a piece of plastic stripping, he wound it into a spiral the same diameter as the tube, but prior to doing so he took a pin and poked holes up and down the sides of the entire plastic strip. The reason he did this is the holes create an air gap between each wrap of the spiral, this way the sound wave won't be impeded as it travels back and forth through the tube. He referred to it as a stack, and it is the medium he used basically like a heat sink. He placed the stack into the tube about a quarter of the way from the closed end of the tube. He played the resonant frequency of the tube, and was able to generate a 25° F difference across the stack. 

After searching this on google I was able to only find a handful of research papers on this topic. They all seemed to conclude that "design improvements would be necessary to achieve efficiency levels of modern compressor refrigeration."

That said, I have been pondering on this and have a few ideas to try to make these things a viable system (at least for comfort cooling), but I don't really have money, means, or space to try them. 

My first idea involves seeing what effect replacing the air inside the tube with a gas of a different density, maybe try a light gas like helium or a heavy gas like xenon. Take a vacuum pump and pull the tube down to remove atmosphere, and break the vacuum with the gas like we sometimes do with nitrogen on a normal unit. You would have to design a vacuum tight enclosure to fasten the speaker to the tube which would probably be pretty difficult. The speaker diaphragm itself may pose a challenge too. May not be able to reach a deep vacuum but I'm not sure.

Another idea is to make the stack out of a strip with 3 parts. Highly heat conductive materials on the outsides and an insulator in the middle. In the video, the guy used plastic because of its low heat conductivity. He did this to prevent the heat from migrating from the hot side to the cold side. If you use a highly conductive material on the outsides, you may be able to transfer heat away at a higher rate, without having to worry about it conducting through the middle so much.

It looks to me like much if the challenge with this type of system is getting the heat out of the tube. In the video, the guy tried to put rings of copper out the outsides of the stack, he folded them flat to reduce the restriction on the sound wave, he connected them to other pieces of copper tubing and ran them out of the end of the tube. This brings me to another idea.

On a big enough scale, you may be able to configure a big enough tube to allow you flow water through separate copper tubes that are in contact with the hot and cold ends of the stack. You could route the warm line to a cooling tower as we do in WSHPs, and maybe even route the cold end to cold water fan coils in the space. It may be also possible to daisy chain the water lines in series with multiple tubes to achieve greater capacity. You could also valve it in such a way that you can reverse flow for heating as well, but I'm not as knowledgeable in that area.

Let me know what you guys think, thanks for your time.