<html><head><meta http-equiv="Content-Type" content="text/html; charset=utf-8"></head><body style="word-wrap: break-word; -webkit-nbsp-mode: space; line-break: after-white-space;" class="">The thermoacousktic one is interesting, and surprises me a bit.<div class=""><br class=""></div><div class="">I worked on these systems a bit in the mid-1990s, when in a kind of purgatory in a navy research lab that mostly did acoustics.</div><div class=""><br class=""></div><div class="">Broadly, there are two limiting cases for a thermoacoutic engine. One uses a standing wave and is simple and robust to design and run. The other uses a traveling wave and is much harder to tune and keep tuned.</div><div class=""><br class=""></div><div class="">A difference is that the SW version, which we might say runs on a \u201cthermoacousktic cycle\u201d, makes intrinsic use of the phase lag for diffusion of heat through a boundary layer. As such, it has no nontrivial reversible limit, and has severe limits on the efficiency (or coefficient of performance, if you are running it as a refrigerator). So hearing that they get COPs comparable to existing mechanical systems would make me suspicious of they were using SW.</div><div class=""><br class=""></div><div class="">The TW version runs on, effectively, the Stirling cycle, and in principle it does have a reversible, Carnot-efficient limit. However, it has parasitic losses from viscous boundary layers. The engineering limit you need to approach ideal thermal transfer efficiency is one that chokes off the flow of the working fluid, and makes the viscous drag explode. Using an ideal gas like He reduces the viscosity, though also the heat capacity and diffusion rate through the fluid.</div><div class=""><br class=""></div><div class="">On their website, they have a little advertising graphic of a sound wave, which shows a traveling wave (or a mixed wave with large TW component). It would be reasonable, if they are scientists or engineers, for them to make their public graphics true representations of at least qualitatively what their system does. </div><div class=""><br class=""></div><div class="">In view of the fact that there is very little conceptual to do with a thermoacousktic engine, and it is all materials science and tweaking engineering details, I really wonder what would have taken 27 years to figure out, or to get around to doing.</div><div class=""><br class=""></div><div class=""><br class=""></div><div class="">For geeks who like this stuff, there is a fun continuum:</div><div class=""><br class=""></div><div class="">1. When I was a little kid, I got an ultra-simple Stirling engine from a mail advertisement (back when those weren\u2019t all scams), and was delighted by it.</div><div class=""><br class=""></div><div class="">2. In reading more about Stirling cycles etc., I learned about \u201cfree-piston\u201d Stirling engines, which have the same compartments and barriers, but use the compression-bounce of the gas to move the displacer piston rather than a mechanical linkage.</div><div class=""><br class=""></div><div class="">3. The TW thermoacousktic engine is just a free-piston Stirling without the piston: the shuttle of gas becomes the displacer.</div><div class=""><br class=""></div><div class="">4. Some years later, having been thrown out of String Theory for being too stupid to understand it, I was interested in the way adiabatic transformations look like mere coordinate deformations in state spaces, which means that one should be able to make Carnot-efficient reversible movement identical to equilibrium by use of a conformal field (the String Theorist\u2019s universal symmetry transformation, back in those days). So we can do thermoacousktic engines using String Theory (Horray!):</div><div class=""><a href="https://journals.aps.org/pre/abstract/10.1103/PhysRevE.58.2818" class="">https://journals.aps.org/pre/abstract/10.1103/PhysRevE.58.2818</a></div><div class=""><a href="http://www.santafe.edu/~desmith/PDF_pubs/Carnot_1.pdf" class="">http://www.santafe.edu/~desmith/PDF_pubs/Carnot_1.pdf</a></div><div class="">and then </div><div class=""><a href="https://journals.aps.org/pre/abstract/10.1103/PhysRevE.60.3633" class="">https://journals.aps.org/pre/abstract/10.1103/PhysRevE.60.3633</a></div><div class=""><a href="http://www.santafe.edu/~desmith/PDF_pubs/Carnot_2.pdf" class="">http://www.santafe.edu/~desmith/PDF_pubs/Carnot_2.pdf</a></div><div class="">Papers I know no-one has ever had any interest in, and very possibly no-one has ever read.</div><div class=""><br class=""></div><div class="">I thought it was very fun to be able to derive Carnot\u2019s theorem directly from a symmetry transformation, so entropy flux behaves like any other conserved quantity, rather than having to make arguments about limits to thermodynamic efficiency by daisy-chain proofs-by-contradiction (If you could do such-and-such, then by running an exemplar Carnot engine in reverse, you could make a perpetual-motion machine of type-XYZ). But I never did anything with it that yielded a new calculation, as opposed to just a restatement of common knowledge.</div><div class=""><br class=""></div><div class="">Anyway\u2026</div><div class=""><br class=""></div><div class="">Eric</div><div class=""><br class=""></div><div class=""><br class=""></div><div class=""><br class=""></div><div class=""><br class=""><div><br class=""><blockquote type="cite" class=""><div class="">On Jan 6, 2023, at 8:27 AM, Roger Critchlow <<a href="mailto:rec@elf.org" class="">rec@elf.org</a>> wrote:</div><br class="Apple-interchange-newline"><div class=""><div dir="ltr" class="">I was amused to see an announcement of a thermoacoustic heat pump the other day:<div class=""><br class=""></div><div class=""> <a href="https://linkprotect.cudasvc.com/url?a=https%3a%2f%2fwww.pv-magazine.com%2f2023%2f01%2f02%2fresidential-thermo-acoustic-heat-pump-produces-water-up-to-80-c%2f&c=E,1,OiK5I3jqVzs0YmBcsTYEvneGYZ1FEG28fiRx3ORcJqyfO1RYvaNtVheIXHOQn5kEDLKn-6-EP20t-76MRRk3ELbJ6W-Bs3A2-bQupekjrWftCWx_4KE1&typo=1" class="">https://www.pv-magazine.com/2023/01/02/residential-thermo-acoustic-heat-pump-produces-water-up-to-80-c/</a></div><div class=""><br class=""></div><div class="">then an ionocaloric refrigerator announcement turns up this morning</div><div class=""><br class=""></div><div class=""> <a href="https://linkprotect.cudasvc.com/url?a=https%3a%2f%2fnewscenter.lbl.gov%2f2023%2f01%2f03%2fcool-new-method-of-refrigeration%2f&c=E,1,iAUwwTChDuxb9xOXSzMsHYIRxOSXe0tMdebJdWYF2_mAo6ayMqaAT3VacwosXksSM2F4bAvq53cQQusX66IRJmWrzqfU0BdHdbD4hor2Sd1emyC_O8I_CJY,&typo=1" class="">https://newscenter.lbl.gov/2023/01/03/cool-new-method-of-refrigeration/</a></div><div class=""><br class=""></div><div class="">It seems that you won't recognize your air conditioner in a few years.</div><div class=""><br class=""></div><div class="">-- rec --</div></div>
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