[FRIAM] new thermal tech

David Eric Smith desmith at santafe.edu
Sun Jan 8 06:34:21 EST 2023

The thermoacousktic one is interesting, and surprises me a bit.

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.

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.

A difference is that the SW version, which we might say runs on a “thermoacousktic cycle”, 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.

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.

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.  

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.

For geeks who like this stuff, there is a fun continuum:

1. When I was a little kid, I got an ultra-simple Stirling engine from a mail advertisement (back when those weren’t all scams), and was delighted by it.

2. In reading more about Stirling cycles etc., I learned about “free-piston” 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.

3. The TW thermoacousktic engine is just a free-piston Stirling without the piston: the shuttle of gas becomes the displacer.

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’s universal symmetry transformation, back in those days).  So we can do thermoacousktic engines using String Theory (Horray!):
https://journals.aps.org/pre/abstract/10.1103/PhysRevE.58.2818 <https://journals.aps.org/pre/abstract/10.1103/PhysRevE.58.2818>
http://www.santafe.edu/~desmith/PDF_pubs/Carnot_1.pdf <http://www.santafe.edu/~desmith/PDF_pubs/Carnot_1.pdf>
and then 
https://journals.aps.org/pre/abstract/10.1103/PhysRevE.60.3633 <https://journals.aps.org/pre/abstract/10.1103/PhysRevE.60.3633>
http://www.santafe.edu/~desmith/PDF_pubs/Carnot_2.pdf <http://www.santafe.edu/~desmith/PDF_pubs/Carnot_2.pdf>
Papers I know no-one has ever had any interest in, and very possibly no-one has ever read.

I thought it was very fun to be able to derive Carnot’s 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.



> On Jan 6, 2023, at 8:27 AM, Roger Critchlow <rec at elf.org> wrote:
> I was amused to see an announcement of a thermoacoustic heat pump  the other day:
>   https://www.pv-magazine.com/2023/01/02/residential-thermo-acoustic-heat-pump-produces-water-up-to-80-c/ <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>
> then an ionocaloric refrigerator announcement turns up this morning
>   https://newscenter.lbl.gov/2023/01/03/cool-new-method-of-refrigeration/ <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>
> It seems that you won't recognize your air conditioner in a few years.
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