Heat Engines (Stirling & Nitinol)

[Tom Booth]

"The efficiency of elastocaloric materials is more than ten times greater than that of current air conditioning systems or refrigerators"

That seems pretty remarkable considering the current COP of existing systems.

https://cleantechnica.com/2024/07/22/ela...eat-pumps/

Since this is basically just moving heat, one location is cooled but another must also receive the heat, which also, I believe, the heat transported also includes heat that results from the work performed in stretching the Nitinol wire.

No mention is made in the article of utilizing this additional heating potential.

Current systems do well, or are considered efficient with a COP of 3 and many people wonder what would happen if you combine a heat pump with a heat engine.

Well, what happens when you multiply the efficiency of the heat pump by 10?

The problem I think would be somehow transferring the heat from the stretched Nitinol wires to the engine.

Probably that could most easily be achieved by stretching the wire while emersed in a fluid, then circulating the heated fluid through the engine and back to the heat pump.

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Now, imagine also recovering the mechanical work of stretching the Nitinol springs using the see-saw cam principle described earlier.

   

Sorry again about the poor drawing but I don't have time to redo it right now.

But I think this could be turned so the spring stretches under water, then the springs raised to absorb heat from the open air.

The sea-saw cam arrangement "recycles" the mechanical energy used to stretch the Nitinol as it tries to spring back

If this is really 10x more efficient than a conventional heat pump it seems like a Stirling engine + Nitinol heat pump combination should have more potential than a Stirling + vapor-compression type heat pump combination.

[Tom Booth]

Apparently the kind of mechanical recovery I imagined or thought possible above has actually been implemented but using some form of hydraulics, apparently.

Another interesting innovation mentioned in this article is the use of Nitinol tubes rather than solid wires.

The tubes are cooled by a fluid, though it is not entirely clear from the article if the fluid circulates inside or around the Nitinol tubes.

"How Does the New Elastocaloric Cooling Device Work?

"Dr. Takeuchi’s new cooling device is made of two cylinders. Each cylinder consists of bundled nitinol tubes surrounded by stainless-steel tubes. The nitinol tubes are compressed from the top and bottom of the cylinders. As shown in Figure 3, the internal temperature of the nitinol tubes increases upon compression. That heat then needs to be expelled from the system. For that purpose, the cylinders housing the nitinol tubes also contain an evenly dispersed heat exchange fluid, which transfers the generated heat away from the nitinol tubes. The compressed tubes then decompress, lowering the material’s temperature and allowing the tubes to absorb heat from the environment. As one set of nitinol tubes decompress, a set of hydraulic cylinders converts the energy coming from the decompression into a compressive force to be applied to the other set of nitinol tubes. The two sets of tubes are therefore coupled so that as one bundle is being compressed, the second is allowed to expand back to its original shape. "

Re-reading that, I'm thinking that perhaps the Nitinol tubes contain the hydraulic fluid.? So as one set of tubes is compressed the internal pressure is transfered to the other set of tubes helping them expand.

I'm providing a link to the article through the Wayback archive because the original Harvard site is generating a warning.

https://web.archive.org/web/202407250556...ng-device/

This is the original link provided by Google, if anyone is adventurous and chooses to ignore the warning. Apparently just an expired certificate. There is no CGI forms or login so not likely actually "dangerous" IMO.

https://sitn.hms.harvard.edu/flash/2024/...ng-device/

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This is apparently a video of the machine described in the above article.

The video description reads in part:

"Superelastic NiTi tube bundles are compressed using hydraulic actuators. In this configuration, there are two top bundles and two bottom bundles operated in concert in direct work recovery mode"

The video is 10X speed, so the machine actually operates very slowly.

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This infrared video of the "drinking Bird" ambient heat engine demonstrates the principle I have argued before, which actually got me banned from a popular physics forum.

What is apparent from the infrared image is that the glass of water is actually hotter than the birds head. Only slightly cooler perhaps than the ambient environment 

What this means IMO, is that the motion of the bird swinging its head about in the air greatly accelerates the cooling process.

So, contrary to what is claimed, that the bird operates purely on the basis of passive evaporative cooling is false.

Heat from the environment powers the bird. The heat is converted to motion or thermodynamic "work" output, and that work output is used to power a quite ACTIVE refrigeration system.

This is IMO, a clear example of a "perpetual" heat engine + heat pump combination. That it is supposedly only taking advantage of an existing environmental temperature difference is not true. The toy is using environmental heat to do work to drive its own active cooling system, as evidenced by the fact that the birds head is obviously much colder than the glass of water, due to its own mechanical action and not a pre-existing environmental temperature difference.

[dd_alf]

Very impressive concepts here. RE evaporation cooling I have noticed a wet cotton towel is always 8 degrees C colder than a dry one, measured indoors with an IR thermometer. I think any heat difference can be used to achieve a temperature fluctuation, which is half the rent to a motor of some kind. Very interesting observation about the ice you mentioned tho.

Years ago I made a paper motor that would pick up moisture on one side of the rotor/wheel, where there was a water evaporator (plaster of paris, sucks water up from the ground), so certain paper stripes, having one side covered in thin plastic, would bend and almost roll up, effectively transposing the center of gravity to the other side (lever), making the rotor turn. But as it turns, bent stripes reach dry area again, straighten, while new stripes start bending at the wet side, resulting in continuous rotation (at 2 RPM). It ran for months, until fatigue had deformed it and made it insensible to moisture. I was thinking, a material that lets moist air pass through, but is rather a 3D structure, than the 2D stripes used, would also develop much more force. Comparable to a wedge of dry wood between two heavy objects, when allowed to suck up water, it will expand with tremendous force, capable of splitting rocks.

However I was also thinking, it should be possible to do this with Bi-Metal stripes, rather than paper, and use evaporation cooling of the blades or spokes that just came out of the water in which the wheel is partially submerged, to achieve a similar effect. I'd imagine a gold-plated surface that has just the right texture so water will not run off (like on a smooth surface), but build a very thin water film that evaporates quickly.

In order to help air flow through a 3D structure, I'd then add a fan to the rotor. Also, rather than to just use dynamic imbalance of the wheel, it'd be more effective to use the deformation directly for a physical contact push / pull concept .

I'm getting a bit off topic here, but my thoughts just went ahead. I wish you the best for your work and I am looking forward to seeing more of it.

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BTW here's another interesting experiment, that kind of borderlines with Maxwells "demon": You fill half a cup of almost boioling water into a PET plastic bottle, then quickly screw the top on. First you're getting over-pressure, but soon the bottle gets underpressured as the air inside cools down, the bottle flattens itself, doing noises. Now you only have to shake the bottle to repeat this process, which works quite a few times. Unless in Maxwells idea however, where order leads to energy supply, here it seems as if chaos does the same, Which is counter-intuitive, seen from a 2nd law perspective, where chaos and distribution accelerates entropy.

[dd_alf]

(I meant "unlike in Maxwells idea") but can't see the edit button right now)

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Uhrm, BTW2, just asking, did anyone ever try to make Nitinol at home? As far as I see, it's just an alloy or Nickel and Titanium at a certain ratio. 1668 deg C would be required. A flame of oxygen and hydrogen reaches about 2000 degrees. Personally I have smelted some bronze, brass, silver etc. in a modified microwave, but if adequate materials are used (heat resistant insulation), it might go up to more than the 1200 I reached. Nickel is cheaply available, IDK about Titanium, but titanium.white wall-paint contains a lot of titanium-oxide, which probably could be extracted and reduced to metallic state, at an industrial price aka cheap, compared to Titanium samples from ebay or where-ever.

[Tom Booth]

Very impressive concepts here. RE evaporation cooling...

Evaporative cooling is not really the point.


Your other comments, while of some potential interest, as you point out yourself, are basically off topic.

[Tom Booth]

..., BTW2, just asking, did anyone ever try to make Nitinol at home?...

DIY Nitinol appears to be out of the question for an amateur metallurgist.

Apparently titanium reacts violently with oxygen and other common gases in the air and attempting to melt it will cause it to ignite and burn with a white hot heat that cannot be extinguished 

The processes involved create deadly toxic gases and the whole process needs to be carried out in an inert atmosphere of Argon gas...

Etc. etc.

[Tom Booth]

I was not really expecting this:

I had planned on doing this experiment years ago, and have done similar experiments, but I have not seen ice adhere to the bottom of a Stirling engine for three hours straight.

Mostly I just wanted to see if the ice would melt faster of slower, or at the same rate, if it was used to run a Stirling engine.

I will be putting another identical engine together soon to run additional tests.

What makes me think the ice is FREEZING to the engine rather than just adhering from moisture is the ice does not slide off, as long as the engine continues running.

As soon as the engine stopped, the ice, though still "stuck" to the bottom, slid off to the side when tilted.

To be sure, I'm going to try using food coloring and freeze the ice in colored layers.

Let's say the top thin layer of the ice  in contact with the engine is dyed yellow, the layer under that blue etc.

It should be possible to see if the layer in contact with the engine is actually melting or not, or if the ice is only melting from the bottom up.

Tesla wrote that because heat is a form of energy that is converted to mechanical "work" or motion by a heat engine when running on ice (or the ambient heat) the heat going into a heat engine should not reach the ice, or what Tesla called a "cold hole", but might run indefinitely.

Certainly, it appears as though the ice was melting from the bottom up and staying frozen on the side in contact with the engine. I wouldn't swear to it without more testing though.

After the engine stopped I drained off all the melt water from each container.

There was 7 tablespoons of melted water under the ice by itself and only 6 1/4 under the engine.

I rechecked over and over so I think that is about right, but it was difficult using kitchen measuring spoons, so I sent for some graduated cylinders to make it easier and more accurate, for next time.

[Tom Booth]

Well, I tried using food coloring to stain different layers of ice, but it just kept turning out a mess before I could even get the ice frozen. Trying to add one color layer on top of another just ended up bleeding together.

So, in the end I came up with another idea: Just separate the different layers with pieces of string.



For now, to start with,  I'm just making a string sandwich with two layers of ice I already had frozen.

That should provide a clear indication if the ice is melting on the top engine side above the string layer or if it is .melting from the bottom up from the surrounding ambient heat.

If you watch where I try to set the engine on the partly melted ice, it should be understandable why it seems so remarkable to me that while running the engine seems to stick or freeze to the ice.

Normally, without the engine running, the ice is so slippery it is nearly impossible to keep the engine from sliding around.

The contrast in having to literally break the running engine loose from the ice is very striking when doing these experiments first hand.

[Tom Booth]

I have this 3,000 watt electrical output Stirling engine manufactured around 2009.



My intention as seen in this video was to power it by blasting the solar receiver end with my 140,000 BTU "salamander", but that proved to be something like trying to boil a cup of water by. Breathing on it.

The engine came loaded with thermocouples so it was easy to monitor the temperature of the engine, but basically all I was able to accomplish was heating up the antifreeze in the cooling system.

It's actually a bit puzzling to me how this engine could ever be gotten running at all with the liquid cooling jacket virtually adjacent to and surrounding the heater head




In the animation the cooling jacket and coolant lines can be seen on the right hand side of the engine, hardly any distance at all from the heater head.

The engine in my shop hooked up to a radiator.



In tests, the water in the radiator got quite hot but the engine itself, according to the thermocouple readings never got anywhere near operating temperature.

The only way that could possibly work IMO is if the metal heater head is made from some kind of titanium alloy or some similar metal that does not conduct heat.

Here is a test I made of some titanium. Heated with a torch I could still hold onto the metal only an inch away from the flame:



The displacer in these engines vibrates at a high frequency and does not move the distance it "normally" would in most Stirling engines, so I assume that is what makes the close spacing between heater and cooler necessary.

So I haven't given up on it.