# Reverse Stirling Cycle - it's a Stirling, but run with a motor.... I've a question please :)



## MERLIncMan (Apr 14, 2021)

My friends,

     In the mad lab there is something brewing! Seriously, I'm getting rather ridiculously excited as she nears functionality.... been working on it for a few years.... MWAHAHAHAHAH!!!!!!   

You'll see soon!

Anyways, it is likely that there are many here who have been-there-done-that with the Stirling Engine. I've made one, but it never worked  - alas, the past and whatnot.

SO! You likely know that if you take a Stirling and run it with a motor, the "hot spot" actually gets stupid cold - used as a heat-pump in cryocooling; there was even a Coleman cooler made this way, but they're discontinued and now psychotically expensive. Surplus Stirling cryocoolers are indefensibly expensive on Ebay.

Too bad I don't have a new machine shop.... OH, WAIT! (It's even paid off now!!!!!)

From what I gather, the easiest method for me would be a "Gamma-Alpha" type - it's Gamma because it uses a displacer (rather than two pistons), but it's an Alpha because it gets its 90 degree phase differential by virtue of the cylinders being at right angles, but both rods being pinned at the same place on the crank.

Making the Hot/Cold cylinder of aluminum or copper (with a thermal gasket between sides), then wrapping them with soft-copper tubing would allow a pretty good heat exchange between both the hot and the cold sides - heat being exchanged to a brine solution.

From what I understand, the regenerator can be cotton, or likely rayon - as again, it is a heat-pump rather than an engine and cotton/rayon acts better at low temperature than the common steel-wool.

Here is my question: I have an Airpot (graphite-ceramic piston in Borosilicate glass cylinder) that I intend to use as the power piston (though for a heat-pump, I don't know that the name is appropriate). What volume ratio ought I use when making my Hot/Cold cylinder in which the displacer runs? 

Anyone know? Given the swept volume of the power piston, what ought to be the swept volume of the other cylinder? Bigger I think, but how much bigger? I've got some copper pipe, and some much larger aluminum seamless so....

This gizmo would then be the 12volt powered beating heart of the Mark V version of the super secret awesomeness that I hope to be showing quite soon!!!

Thanks for reading


----------



## Aukai (Apr 14, 2021)

I have absolutely no idea what all of that is going to do, but your excited about it, so I'm happy for you


----------



## hman (Apr 14, 2021)

@Aukai - The little "toy" Stirling engines you can find for sale work by converting relatively small temperature to mechanical motion.  Example:





						Amazon.com: Sunnytech Low Temperature Stirling Engine Motor Steam Heat Education Model Toy Kit (LT001) : Toys & Games
					

Buy Sunnytech Low Temperature Stirling Engine Motor Steam Heat Education Model Toy Kit (LT001): Science Kits & Toys - Amazon.com ✓ FREE DELIVERY possible on eligible purchases



					www.amazon.com
				



But the cycle is reversible ... power the same engine fron an external source, and you can create a temperature differential across the larger piston.  I recall that some years ago I saw a sensitive infrared camera that had a built-in Stirling cooler to chill the image detector.


----------



## Aukai (Apr 14, 2021)

One of the toys I didn't get growing up


----------



## Hidyn (Apr 14, 2021)

I actually didn't know that you could use a Stirling engine like that!


----------



## MERLIncMan (Apr 15, 2021)

Hidyn said:


> I actually didn't know that you could use a Stirling engine like that!


SO!

     In response to the previous guys - the toy stirlings are neat, but I've decided against getting one because I've not found one that could be easily fitted with the required heat exchangers (coils of copper tubing). I never did get that toy either... 

     As for using a Stirling as a refrigerator, that is actually the only real arena where they have found any commercial success (aside from toys). Shame really, as they need to be huge to compete with steam engines (and turbines) for power generation, that doesn't matter because they are simpler - ie. last longer and cheaper maintenance - and power-plants don't need to move around.
     Some things about the methods that get commercial success really irritate me! Many (too many) people are convinced that Stirlings are a novelty toy and so tend to get all derisive if you discuss their capabilities (like the fact that you can run them on solar, then reverse them to make cold or heat to charge a thermal battery, then reverse them again to make power.... gee, I wonder if that solves a big issue in "renewables"? but noooo! Can't do the simple things!)

     Anyways, the cooling capacity of the Stirling Cycle is far more fascinating than their work as a hot-air engine. Like I said, to be good generators they have to be huge to compete with diesel or steam, but as a heat pump? There, the sizes don't have to be so huge (not that stationary engines need to be small - they don't move!)

     Imagine one device in your home that heats, cools, and powers both your refrigerator and freezer. This device has 4 (count it) 4 moving parts - and can even be done without bearings (sealed floating piston types). You could maintain and repair it with a tool-kit from Harbor Freight, learning how it works in the space of an hour...

     HVAC and a very nice refrigerator with one machine, one motor, and can easily be run with a couple of solar panels and a battery....

(Starting to see the source of my irritation? How much cash do we waste because some markets decided to do things a certain way years before we were born?)

Here is an old video (



)

You watch it, think of what these gizmos can do - where you could use them - and you'll understand why the toys are neat, but sad that they are toys.... If I had the resources, I'd put some machines on the market that would show just how silly we've been in insisting on doing things the same way for so long.... 

Seriously, watch the video and it will begin to dawn on you how very neglected this SIMPLE AND OLD process is...

I ANY CASE! I found the equations!

Funny, of course it's all about the threes....

     Displacement space is 1.5x the volume of the power piston. (.5 x 3 obviously....)
     Hot space is 2/3 of the displacement space <--- in a cooler this is the cold part
     Cold space is 1/3 the displacement space  <----- in a cooler this is the hot part
     Displacer is 2/3 the volume of the displacement space.
     Rhombic drive is best for balance, but has lots of moving parts and bearing surfaces
     Displacer is 90 degrees ahead of the piston.

     Displacement space can be larger (10 to 1 even) to change the thermal gradient (for an engine, it would run on lower delta T, for a cooler I'm not sure...)

Thank you for reading!

Now, let's all make Stirling Cycle Hot-Cold power gizmos!

Tell your friends!


----------

