Stirling Engines Anyone

General Discussion
#21

Hi, Dan!
19.2.2018
I was just looking back at Yahoo groups “Hot Air Engines”; it has been a very vivid group on this subject, but sadly there is not much anymore than the “bold picture” of the founder sitting at his desk. No new messages in a couple of years…
Founded Oct 17, 2000.
Maximum messages per month:
707 in september 2007.

But now some low level reflections:
I gather, that home build equipment is excluding use of those “laboratory” high performace, high energy carrying gases, mentioned in laboratory reports. (for closed units)

Another exlusion seems to be very high temperatures with heating any gas, as it complicates the handling of inlet valve operation.

But first: Since 1907 the norm has been: A closed system with a displacement of air alternatively from the heater or the cooler side.

This way the enclosed air has been alternatively heated and cooled to get a driving alteration in air pressure to drive a piston in ratio to the surrounding atmosphere.

But now, if we can tolerate some friction losses, we can build a system with re-circulation and constant positive working pressure for a working cylinder (double action) combined with a following re-heter (for feeding the main heater) and then a constant flow cooler.

So, from the cooler the air is taken to a “compressor”, followed by the mentioned
re-heater after the working cylider, and then to the air-heater proper.

This should be the whole circulation loop.

Now, in which “thermal borders” would a home “plant” be able to work,
with air as the driving medium?

If starting with the feeding end: Modest air cooling (without water)
say 50 deg. Celcius = ~122F as the bottom temp in the cycle and

~300 deg. C = ~? F , Sorry, lost my scale. Returning when finding.

~300 deg. C on the top, as to manage inlet valve precaution…
This will result in a (free air) relative volume expansion of:

From abs. zero point: 273 + 50 = 323 as “volume reference”.

And the heated output has: 273 + 300 = 573

The difference is 573 : 323 = 1,77399.

This would be the (atmospheric) volume gain.

So the expansion available, or rather the filling factor
(with no friction or power output) would be:

1 : 1,7740 = 0,5637

Here is some room for a power outtake, or how should it be understood?
I have assumed, that the working cylinder volume is
twice the pumping cylinder volume, to make adjustable expansion possible.

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#22

Max very interesting. I haven’t looked at Stirling in a while. I had a design in mind for a Roberson hot air engine built into a wood stove. My plan was to draw the inlet air for the stove around the cool side pistion in a cooling heat exchanger or to use a fan powered off the engine to just move room air across it. I think if I remember correctly I could pull 1kW off a wood stove with the numbers i had worked out. I should dig out my notes it was a long time ago. In the end I decided o didn’t want a spinning generator in my living room. But if you look at the hours you run a wood stove it wouldn’t take much power to provide all your electricy. I was planing to run it through a rectifier and into a solar charge controller simple off the shielf solution.

That is the Roberson hot air engine you can see how it could be easily built into a wood stove.
My other concept I have been kicking around for years is a low temp hot air engine. Simular to the toy models. If the volume ratio is 50:1 in the two cylinders it should work with the hot side at about 80C and the cold side around 4C. The idea would be to run it off the delta between ground temp and a hot water boiler. I never came up with a design where I felt I had a stiff enough displacer pistion to handle the torque over a big pistion which would generate a usable force. I the best design I came up with was a 4 pistion engine 2 hot side pistion two cold side working 180 degrees out of phase so the mass would offset each other on the cam shaft. The catch is the hot side pistion needed to be about 1 meter square to have enough force to actually turn a 1kw or close to that generator. It was a long time ago I looked at it so those numbers might be way off. My plan was to build the engine into the top cover of a 700 gallon hot water tank which I could heat with a wood boiler. If I put the system in the basement I figured the cold side would stay about ground temp anyway. But you could always add a ground loop if needed. The idea being that it would just be a constant slow spinning engine running all the time charging a battery bank. You could heat the tank with solar in the summer easy enough. While you would need a small battery bank to handle in rush current loads the hot water would make a cheap big battery to store the bulk of your energy. That was my theory but I never did come up with the materials I wanted to use for the displacer.

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#23

MaxG this is an excellent presentation.
Let me rephrase into words/visuals if I may. Please do correct as needed if I muddy up your concepts.

For expansion cylinder heat engines it can be understood as the greater the difference between the hot-side and the cold-side: the greater your power/energy density; and potential fuel conversion efficiency.

As a source detailed let-out one modern woodgasifier told me that the modern Stirling engine developer who approced and worked with him wanted him to continuously heat-side supply at 2200 degrees (C?) continuously. It was the only way they could get effiiency numbers in the physical size of their powerplant.
There once was a related Net video up of an in-Asia effort showing a continuously heated to red-orange hot-side on their heat engine. Metals materials cannot stand this without deterioration and failures. And precision cast ceramics that can endure these levels of thermal; in-use fail from cold to warming up, and refueling thermal shock fracturing.
So currently you can have it BIG and low efficiency durable for a comparable power output. Or, small and much less power than a current production IC piston engines.

IC piston engines crankshaft driving . . . it is NOT just about thermal expansion! It is about in-cylinder rapid pressure rising matching to the connecting rod angles and the crank throw angles abilities to convert that pressure rise/peaking/ then rapidly falling into rotary shaft power.
Think bicycle and your legs. Too “high” a gear hurt! And develops low forward motion power transfer. Too “low” a gear hurts too over spinning, for again low forward power transfer. Get it right; feels good and is maximum YOU engine fuel to work, useful.

IC piston engines connecting rod to crankshaft driving can be crafted to match and harness the greatly variable pressure/peaking/falling energy releases of most any air-fuel combinations the best.
Even powdered fossil coal dust. The first IC compression engines fuel. Look it up. True. 1850/60’s.
Even fuel with fine powered wood charcoal dusts.
What? You mean no one else here has not yet even tried this?
Ha! Works. Just not really practical for from cold stating up, to hot running of a variably loaded IC piston engine using wood soots as an only fuel.
Now what clean no-ashed cored carbon chained soots does get woodgas supplied into an IC engine combustion chamber is well used as power making fuel.
No free lunches in life. It is the getting it in without delivery surfaces coatings flows restricting building ups that is the problem there.

So not that alternative engines/fueling combinations have been overlooked and undeveloped. 300 years of thousands of attempts so far.
Only the practical-use are what survive the shake outs into wide-use deployments. And only those designs that in all world around use prove: #1 Safe; #2 Durable; then and only then #3 efficient, are kept into service. Manufactured as replacements for themselves.
Just like all other practical in the real world use machines and systems.
You can have it safe, durable, useful.
Or: you can have it one-time stunting only ever to really make that fantastic number occasionally; or under artificial steady state geek/lad-rat conditions.

You can have all of the fantasy cakes in life that you want. Along with the teeth decay; spike blood sugars leading to soft pudding body obesity; and micro-nutritional deficiency diseases. I do cake at birthday and events at last once every tree months. Daily, I bread. Whole grains with seeds and nuts. The staffs of life.

Daily I do IC engine use almost daily. Fuels vary to what is appropriate. Practical; before fantasy.

Regards
tree-farmer Steve unruh

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#24

Hi, Dan!
21.2.2018
You are not bound to use a displacer “bumling”-size at all, if setting it up as a circulating loop system! Constant heat exchanging has a much higher efficiency than “heatpumping” in and out and in on an object with heat accumulative properties! One direction is never completed, before it is contradicted again! Half-breed efforts!

You need a “heater”, working cylinder, heatexchanger, cooler, and a pumpcylinder for feeding the heater via the heatexchanger!
That makes it easier, but at these “very low-difference” end-points, you will have trouble with friction; perhaps bellows work better than cylinders and pistons?

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#25

Hi, Steve!
21.2.2018
Now the low heat-end is up for discussion. How to come to turns with low source temps, not far from available cooling temps; how can a thin matt be depressed, would be a good anomaly?

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#26

Hi, Dan!
21.2.2018

One way to start the mechanical “composition” would be
to get an old boxer motorcycle motor. 2 cylinders.

From the crankcase outward, to both ends, the old cylinders and pistons (without compression-rings now, oil ring intact) are becomming guidings.
The cylinder tops are to be removed.

To these original piston tops are bolted piston rods for the new
“add on” double-action cylinders and pistons.
Air ventilation in and out for the original cylinders. (above piston tops, on the former gas-side, combustion room)

The working cylinder should have an area doubling the pump cylinder area.
Both pistons + their rods should have the same weight exactly!, otherwise you cannot maintain a balanced function!

Then, on both new (added) cylinders, you have to establish a new scavenging system; on the working cylinder that can be a system like on locomotives, Heusinger von Waldegg’s excellent linear methode of full regulation of the filling degree and linear compression.

The 180* or 0* (depends on inside or outside scavening slide) movement is taken from the piston rod, and the balance (coulisse), gives the 90* movement from an intermediate mechanism like in that old one you presented!
Or an excenter on the crankshaft, outside the crankcase.

The slide valve as a piston slide, is the easiest to do on a lathe.

The pump cylinder has always full filling “on its menu”, so it can work with simple inlet and outlet valves, needing no “exitation” or steering.

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#27

Yes MaxG, cold-end for year around differential IS a problem alright.
Air can be too much too high.
Free flowing surface waters “seems” ideal until it freezes. Mine here is currently frozen.
Deep ground sourced like in temperate latitudes is considered to be ~50F constant. True. Until you start adding or removing massive amounts of heats energy. Then you heat up or freeze a big “cone-plume” effected area.

Why I think so much the focus by modern heat-engine folks on just the hot-side. They feel that they can force/control that. Ignore cold-side variables.

Regards
tree-farmer Steve-unruh

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#28

Steve I am planing solar for the bulk of my power so a hot air engine would be for winter use when I am heating the house and need extra electricity because the winter solar is lower gain. That would help with the cold side issues. I probably won’t be doing anything any time soon. Too many other projects at the moment.

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#29

This is what I would like to deploy in house. I don’t know if it lives up to specs though or how durable it is. It’s efficiency is bad but no noise.
http://www.tegmart.com/water-cooled/

#30

I looked at those but here to be practical or should I say to meet the daily need at the hours it would actually run in the winter I would need one 1kW and that is expensive. Now to avoid using a generator when the solar comes up short many I wouldn’t need so much I didn’t run the numbers with that in mind to be honest. From the reading I did on those it seams there was also a lifespan issue but I don’t remember the details. For some reason I am thinking I expected 5 to 10 years max which made them expensive. I was wondering at the time why someone doesn’t cover the back side of a PV array with them as cooling the array would be a double win. But the answer is they have a shorter life and are much more expensive.

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#31

Check this out - this Japanese effort made an efficient low temperature differential Stirling. It looks like a lot of engineering went into low friction seals on the hot side displacer. The displacer they used was based on urethane foam set on a light cast aluminum base. Reinforced plywood should give similar results. Max may have a better idea with the bellows idea. It should be adaptable into the Japanese design.

http://www.bekkoame.ne.jp/~khirata/academic/kiriki/yama1/300strct.html

#32

Another quality of Stirlings interests me, that being refrigeration. My intuition tells me that using direct mechanical energy from perhaps a small windmill or watermill to turn a Stirling might compare fairly well in over all system efficiency. Although Peltier effect thermoelectric cooling is quite attractive too, but it is limited in the cooling temperatures it can achieve.

#33

Garry,

Glad you brought up Peltier devices. They can also be used as generators, thermoelectric generator (TEG). They aren’t usually cost effective, but in some cases they can be. Imagine a heating device with electronic controls that MUST not fail even in the event of a power failure. A TEG can sometimes be a more reliable and cost effective solution than a battery UPS system. Amazon.com : thermo electric generator

Rindert

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#34

As I understand it, the thermocouples aren’t very efficient (possibly an area ripe for a breakthrough with nanotechnology), but for certain purposes they work very well. The products you listed show very credible energy density, 7w for 4" sq is pretty attractive. I wonder what temps they can handle, and life expectancy.

The Voyager space probes used a hot radioactive material surrounded by a thermocouple array, the one had enough power a few years ago to answer back when it crossed into galactic space. With more radiation it would function even longer.

If the price and efficiency were better, some thermoelectric pads on a wood stove would be very hard to beat.

I have mentioned this elsewhere before, but years ago Sunpower had partnered with Woodmizer, and briefly offered a sealed linear Stirling, said to generate 1kw. They sold it as part of a sawdust burning woodstove. Coleman got involved also in producing a linear Stirling refrigeration system for a cooler.

The linear Stirlings promised long life and high reliability, as parts ride on gas bearings. They had been proven mission reliable for NASA.

But now I see nothing from that company, and nothing remotely like the consumer products they had been making. Very unfortunate, as 1 kw from the wood stove would address my energy needs, and would allow many people to diversify the electrical grid.

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#35

Check out this stirling company: https://www.inresol.se/technology

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#36

From everything I could find last winter the technology just isnt practical yet for those thermal generators. As I recall the life span was also pretty short. More battery storage looked more cost effective here with solar.

#37

Nonetheless, it is true that, with over 15,000 TEGs inthe field and a commercial operating history dating from 1975, failures are exceedingly rare. TEG Reliabilityreport | PDF | Reliability Engineering | Analysis

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#38

While I was rehabilitating and testing a “vintage” Acer Chromebook today, I came across a YouTube video with a Stirling engine twist which was so unique I had to say “cool.” Perhaps you will find it interesting as well.

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#39

I spent a lot of time looking at Stirling engines for off grid power generation in a CHP configuration. Two key advantages: (1) External combustion means you can use any fuel, including tarry wood gas and (2) they have very few moving parts leading theoretically to long life. But in my mind it’s a dead end:

(1) For starters… you can buy plenty of toy stirlings that spin an unloaded wheel but you can’t buy a 5-20kw Stirling for any reasonable amount of money if at all. (2) And if you try to build one the engineering constraints are a total bear. Which leads right back to why (1) isn’t going to change.

To expound:
The “hot side” requires a material that can take high gas pressure while super hot.
It wants to be spherically shaped for mechanical strength and low gas friction, cylindrical so the piston can get in and out for reduced dead space and multi thin tube arrayed for heat conduction.+

The heat exchanger wants to have high surface area and turbulent flow for rapid heat transfer with low gas volume to reduce “dead space” losses; but also high volume, low surface area and laminar flow for reduced gas velocity and piping losses. The trade offs here are super tough.

Also it wants to cycle fast for high power but slow for efficiency. High cycle rate and high gas pressure are basically required for an engine of practical size, generating practical power. Skimp on one and you have a truck sized engine putting out 15hp.

Oh and you want to use hydrogen as the working gas for best thermodynamic properties but hydrogen also embrittles the steel housing (boom) and leaks out over time. Helium is second best on physics, doesn’t embrittle but also leaks, can’t self source it and is expensive.

A cousin to the Stirling is much more promising, the Ericsson heat engine. You can’t buy one of those either, and it requires a hot valve where a stirling does not, but at least the engineering isn’t working against itself at every turn. Also uses external combustion.

In researching Ericsson engines I came across these folks:
http://www.proepowersystems.com/PROEHOME.HTM

I read their materials just nodding my head - it’s a good explanation of why stirling is mostly a dead-end. Sadly they don’t seem to have made a go of it which is too bad.

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#40

Great post AnthonyB.
“Pretty promising Rose Gardens have many thorns”, me.
S.U.

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