Double-acting solar Gamma, twin rotary displacers

[spinningmagnets]

I've been reading about hot air engines for years, they are quite fascinating. So far, I've never taken that final step into building one yet. Although...I am closing in on a design that I feel might be worth actually trying out. I scan google and youtube about once a year to see if anything new has come up, and I'll try to credit those builders that have inspired parts of this.

Most proposals seem to focus on a large single engine, rather than two smaller ones in tandem, perhaps for the cost savings. However, I always liked the ability of tandem engines to have a more balanced pulse on the flywheel. Part of my interest in a double-acting power cylinder is that we can use a piston if we like (rather than a diaphragm), because any gas that leaks out past one side of the piston simply migrates some of the gas to the other side. With leakage being roughly equal in either direction, gas volumes on both sides would remain fairly equal over time. This allows the option of upgrading to Helium or Hydrogen later, although there is nothing wrong with building with air, especially in the prototyping stage.

Although I would construct the rotary displacers to be fairly balanced, I would prefer to plan on them being vertical, so the power piston is not fighting the weight of moving a displacer. For a conventional reciprocating-displacer Gamma, I would mount the displacer vertically, but...rotary displacers have grabbed my attention for now (partially because the displacer can be any weight, allowing it to act as the flywheel too.

VAWT auto-start...

Since the rotary displacers are vertical, its an easy arrangement to mount a Vertical Axis Wind Turbine above it (Google: Savonius). The problem with any kind of wind turbine is that...they only work if there is wind, plus...if you will be depending on them, you really need a 60-ft pole (or higher) to make the amount of energy harvested worth the effort...

After pondering a dozen half-baked self-starting schemes for a solar-heat Stirling, it occurred to me that a small VAWT near the ground has decent torque, but low RPMs...making it a weak option to generate 12V, BUT!...all it takes is an occasional breeze to make it spin a couple times. Within an hour of the sun rising, a solar Stirling with an attached VAWT would be spinning. If they were attached through a one-way clutch bearing (which I have used), once the VAWT has gotten the Stirling spinning after sunrise, the Stirling will continue spinning when there is no more air to spin the VAWT. Of course, any small breeze would allow the VAWT to help the Stirling spin a small generator, so it definitely sounds worthwhile to me.

Of course, having an auto-start system means that a large 'single' engine is an even more useful option, but I still like the idea of balanced forces on a double-acting power piston (or diaphragm).

Sooo...two vertical displacer cylinders side-by-side, rotary displacers in each, with a power cylinder on top using horizontal connecting rods. Here is a link to an example that is similar to what I envision (crank and piston located inside the sealed gas area):

http://www.bekkoame.ne.jp/~khirata/english/rot_se.htm

Here's a video of a working example of a rotary displacer, and this guy also adds two magnets to create a pneumatically-sealed rotary actuator to turn the external crank (a waste when used here, since the piston can leak, but...he could swap to a diaphragm in the future).

https://youtu.be/iZZ70nwU77c?t=36

If anyone is good at graphics or animations, I might be persuaded to pay for an example of this idea so I can better show what I mean?

[Bumpkin]

Nearly 40 years off-grid here myself, and a big Stirling fan, but it's hard to compete with modern P.V. system prices, even including batteries wires controllers and inverters. A solar Stirling combination with a VAWT is intriguing. The big problem with Stirling rotary displacers is that they move air from the hot to cold sides and back without forcing any rapid/efficient thermal exchange or regeneration. Like thermal lag engines, they're seductively simple but frustratingly elusive for useful performance. But I still believe there's a place for Stirling's in cold climate biomass cogeneration and wish you happiness in your work. Bumpkin

[spinningmagnets]

Thanks for the reply. I do agree, that a more traditional Gamma (to be comparable, perhaps a twin Gamma with vertical displacers?) would be more powerful. Plus, I have an emergency back-up solar PV panel stored a just a few feet from me at this very moment. Rotary displacers seem like they would be easier to encapsulate in order to use helium or H2...

There's nothing I can do to improve solar PV panels, simply upgrade to a better set when performance improves, or prices drop even more...however, I think it might be fun to put something together that actually works and can be replicated.

The solar-heat collector would work best when under glass to reduce emmissivity losses. Therefore, part of the reason I am leaning towards a matched twin set is...the actual build size is limited to parts that are obtainable (nothing custom-ordered), so...if the biggest aluminum cylinder I can buy is a cooking pot, I can double the power of the engine by pairing two large cooking pots, so...why not?

I'm reading that copper is a better heat transference material, but...finding a large copper cylinder is hard, and will be expensive. Therefore, I can find a reasonably large aluminum cylinder for a much more affordable price. Sooo...I am looking for a large aluminum cylinder (pot?) and a glass cylinder that is a slightly larger diameter (perhaps cut with a Dremel and abrasive wheel?).

Unless I find a better material, the hot and cold sides (both aluminum and glass) might be separated lengthwise by wood or treated particle-board. I don't mind the glass cylinder being significantly larger than the aluminum cylinder, so the search for aluminum will likely determine the final size of the entire unit. The top and bottom are likely to be particle-board, or maybe polycarbonate on the piston end...

Here's an example of a small rotary displacer cylinder that has hot and cold-side isolation to reduce heat-creep...https://www.youtube.com/watch?v=MrjTVoE8qYs

edit, one actual reason this can be truly useful is that...at night and in the winter...many locations don't have much sun (if any) and those are the times when you are using some method to heat your home. As long as you are burning wood or propane...why not generate a couple watts of electricity? A portable Stirling that can run off of the sun is productive enough that...other common heat sources would work too.

In other words, instead of developing a Stirling that works off of wood-heat, and then trying to adapt it to also run off of the sun...to me it's better to develop a solar Stirling that can also run off of any other heat...

[spinningmagnets]

I'm using this discussion thread as a place to park ideas during my research.

Since the VAWT "auto start" device should be on the top, then the power diaphragm should be on the bottom, in-between the twin displacement cylinders.

The VAWT should be located above one of the displacement cylinders, and some type of small generator will eventually be on the other one. Both displacers will turn in the same direction so that they can be connected with a set of pulleys and a timing belt (mounted between the displacer cylinders and the VAWT). Of course the two displacers will be 180-degrees out of phase. Each displacer will have two magnets embedded in them near the edge, and I will attach an aluminum bar with two matching magnets outside the cylinder...this will provide a hermetically-sealed connection between the inside and outside.

If the interior heat get very high (which is a "good" problem), I may need to upgrade to hi-temp samarium cobalt magnets on the inside, or at the very least a steel insert inside, which the external neodymium magnets will track. Each displacer cylinder will have one of these "magnetic rotary actuators", even though the pair will both be connected through a timing belt. http://www.sdp-si.com/products/Timing-B ... /index.php

I am thinking the difficult parts will be hard enough to source, so...I now think It would be best for the outer glass shell to be made from a faceted set of flat glass panes (on the solar side, of course). The chances of me finding a large glass cylinder of a size that matches the smaller aluminum cylinders gives me a headache...That leaves the thick-walled aluminum cylinder as the only problem. Once that is found and purchased, all the other component sizes can be determined. I'm thinking the length of the cylinders should be twice the diameter, unless someone can persuade me otherwise?...

[Ian S C]

DON"T use aluminium, it will work at first, but it conducts the heat from the hot side to the cold side too fast. The metal to use is stainless steel, probably thinner than stainless cooking pots, but kitchen storage containers are about right.
It might be a good idea to build a conventional motor , and get it going first.
Ian S C