Low Temp Stirling Design - Low Weight?

Discussion on Stirling or "hot air" engines (all types)
toomey
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Low Temp Stirling Design - Low Weight?

Post by toomey » Mon Oct 05, 2015 11:27 am

Hi all,

I haven't found any threads talking about this yet, but feel free to refer me there if I missed anything.

I'm a third-year engineering student building a stirling engine with a team.
The plan is to drain some small ponds with a stirling-powered pump.

The trick is that it has to weigh less than 25 lbs, and occupy less than 2x2 ft area. Do we want to build multiple small engines on a crankshaft, or would we get better power to weight from one big engine?
My thought was to start with a small gamma type engine and optimize it, then bolt a bunch of them in a row...

Also, are there any thoughts about the pump itself - should we design one to integrate with the engine or just buy one and bolt it on?

Thanks for your thoughts and your time,
Matt

Ian S C
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Re: Low Temp Stirling Design - Low Weight?

Post by Ian S C » Tue Oct 06, 2015 2:30 am

You would need a very small pond, and a lot of patience, the Stirling Engine that I have with a water pump will pump 2L per minute (a bit over .5 gal US).
You would be best to build the motor first, and find out what sort of power you have, then have a look around at pumps, maybe a diaphragm type. Either buy or make, it may depend on what's available.
By low temperature, what do you mean, if it's 212*F or less, its going to take a bigger motor than 2' x 2' to pump b****r all. If you mean solar heat, that's not too low temp if well designed.
Ian S C

StirlingDragon
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Re: Low Temp Stirling Design - Low Weight?

Post by StirlingDragon » Tue Oct 06, 2015 8:26 am

Hey, toomey's lab partner here.

We are designing this engine to pump out an artificial pond that often floods and has other debris (i.e. grass clippings and mulch). It's outdoors, so using solar energy for the heat source would be really impressive, but probably inconsistent given our track record with overcast days.

We have two ponds to choose from. One is essentially a cylindrical well, 15 inches in diameter and about 34 inches deep, but in a very shaded area. The other is an irregularly shaped, roughly 9ft by 11ft ellipse and 2ft deep. Both are often flooded after strong rains. I don't suppose the quantity of water really matters (unless we use the water itself for a heat sink), but rather we should focus on the rate of our design and even more importantly, the height we need to raise the water.

For the small pond, if we are going to drain it, we would need to raise the water more than 34 inches, let's call it 40. If we are going to recycle water through the artificial waterfall, then (34 inches plus ~2 ft more = 58 inches) but let's call it 60 inches. For the large pond, draining it would need maybe 3ft and recycling water through the artificial waterfall would need 5 ft to raise the water.

I'll be back after my circuit exam.

toomey
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Joined: Mon Oct 05, 2015 8:04 am
Location: Moline, Illinois

Re: Low Temp Stirling Design - Low Weight?

Post by toomey » Tue Oct 06, 2015 8:34 am

Thanks, Ian S C. Very insightful.

I guess the biggest question is in regards to power per weight - does one big engine create less power than many small ones, or are there too many variables to answer that in a meaningful way?
Honestly, I would be happy to get an engine running by December. It's just a bonus if it happens to move some water around :laugh:

-Matt

P.S. Thanks for the background, StirlingDragon

Ian S C
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Re: Low Temp Stirling Design - Low Weight?

Post by Ian S C » Wed Oct 07, 2015 1:48 am

On a unpressurised Stirling Engine power to weight ratio is not good, in the old days, at the peak of hot air engine use, about a ton per horse power was considered about average. with a small motor about 1 Watt per cc is good.
It would be very good to draw the water through a water jacket for cooling the cold end, if you'r going to pump it, you might as well use it.
What method of heating have thought of using?

StirlingDragon
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Re: Low Temp Stirling Design - Low Weight?

Post by StirlingDragon » Wed Oct 07, 2015 1:29 pm

We are strongly considering kerosene, but other possibilities (however realistic or not) include sunlight, warm water, warm atmosphere (assuming we only need this to function during warm summer months), and maaaaaaybe geothermal (Impractical? It sounds so cool! Can we at least look at it later?). I include most of those just as options, realizing that they each have downsides and can be easily ruled out.

Ian S C
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Re: Low Temp Stirling Design - Low Weight?

Post by Ian S C » Thu Oct 08, 2015 1:21 am

Kerosene would be my choice, with a proper pressure burner. Another idea for fuel is waste engine oil, that could be cheap. A well designed solar collector would do well (as you say, if the sun shines). Warm, boiling water, to do any work you need massive, but fairly lightly built. Geothermal, depends where you live, could be just the thing if you lived in parts of New Zealand where we have active geothermal areas, one place has a large geothermal power station (Mega Watt big) that supplies power to the national grid from generators powered by steam turbines.
Ian S C

StirlingDragon
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Re: Low Temp Stirling Design - Low Weight?

Post by StirlingDragon » Thu Oct 08, 2015 2:49 pm

Geothermal does sound cool, but given our class project size and time limitations, that's probably best to leave aside.

For the small size artificial pond pump, let's go with kerosene. From Wikipedia (and welcoming an alternative source), the lower hearing value is 43.1 MJ/kg, and the higher heating value is 46.2 MJ/kg, and the flash point is up to 65 degrees C. We'll need to design a chamber that can handle that combustion, and a way to control the fluid consumption rate (mass per time) so that we can control the power generated.

I'll be posting things on here throughout the night as I come up with them. Feel free to correct me on anything I misunderstand.

Ian S C
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Re: Low Temp Stirling Design - Low Weight?

Post by Ian S C » Fri Oct 09, 2015 2:29 am

Use stainless steel for the hot cap, and displacer.
One of my next projects is a diaphragm pump that will work on about 5 Watts of crankshaft power.
Ian S C

StirlingDragon
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Re: Low Temp Stirling Design - Low Weight?

Post by StirlingDragon » Fri Oct 09, 2015 9:10 pm

I was looking at this model on Youtube:

The displacer piston needs to have some clearance within the gas exchange cylinder. How much clearance do you suggest? And since it will have clearance, then, the only thing holding it up is the rod going through the power piston to the flywheel, do I have that correct?

And for the power piston, it needs to form a tight seal with the gas exchange cylinder at changing temperatures. Would a rubber gasket work okay here? I realize that may involve some loss of energy through friction.

And the cooling fins: Those are to increase surface area to cool things down more quickly, right?

Ian S C
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Re: Low Temp Stirling Design - Low Weight?

Post by Ian S C » Sat Oct 10, 2015 1:52 am

Will see the video at the beginning of the week. Yes the fins are to increase the cooling area, but as you want to pump water, replace the finned area with a water jacket, and pump the pond water through that.
About 1/16" would do for a gap, the displacer should be about three times it's diameter in length, and 1.5 : 1the swept volume of the power piston. The long displacer acts as a regenerator. The bush that the displacer rod passes through should be quite long to give support without the displacer rocking too much.
When a low temperature is used, the ratio between the two increases until with a low temp motor(boiling water & less) the ratio could be 20:1.
Ian S C

StirlingDragon
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Re: Low Temp Stirling Design - Low Weight?

Post by StirlingDragon » Sat Oct 10, 2015 1:24 pm

Ok, how feasible do you think it would be if we got rid of the water jacket and had the heat sink end submerged in the water itself? I'm thinking that the structure would be built-in along the side of the pond. Cooling fins might be used again in that situation, especially when the water drains out the pond enough that the water level drops under the cool side, switching over to air cooling. Does that sound reasonable?

Another advantage of that would be reduced weight and reduced cost of construction (both of which are factors in our project).

Lastly, can we discuss the calculations of energy/power output by the power piston? If the lower heating value of kerosene is used (mentioned above as 43.1 MJ/kg), how much of that will be transmitted out of the power piston? If the volume of water pumped in a cycle is very small, we might get away with having the power piston pump the water itself (rather than transmit energy to a flywheel, then a shaft, etc., losing efficiency with each step). This would also reduce the weight and cost of materials. Unless I'm leaving something out, I really like this idea.

I have some paper and pencil math to back this up, and am open to corrections.

Ian S C
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Re: Low Temp Stirling Design - Low Weight?

Post by Ian S C » Sun Oct 11, 2015 1:59 am

It is possible to have a free piston motor, where the displacer is run as in a Ringbom motor, and the power piston has some form of spring return, this type of motor is often made with a linear alternator, but a pump is another thing it could drive, and no crankshaft. The best orientation for these motors is vertical, hot end at the top. easiest built in the same form as a GAMMA motor, my first attempt was as a BETA format, but I had problems with it, so the one I have is as a GAMMA type with an alternator giving me 6V at a few milli watts, enough to light a few LEDs, or run a small transistor radio.
Ian S C

StirlingDragon
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Re: Low Temp Stirling Design - Low Weight?

Post by StirlingDragon » Sun Oct 11, 2015 1:20 pm

Miliwatts? What is the size and heat source of your engine in that case? And what problems did you have with the beta style engine (so we know what to look for)?

Bookmarking this for myself for later:

Ian S C
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Re: Low Temp Stirling Design - Low Weight?

Post by Ian S C » Mon Oct 12, 2015 2:09 am

The free piston motor is heated with LPG, and is one reason I recommend stainless steel for the displacer, it did take three aluminium displacers melting down to convince me(slow learner).
For a pumping engine look up "Fluidine Engine" by Dr Colin West. This pump/ motor was devised at the AERE*., Harwell in England, there is a book "Liquid-piston Stirling engines, published by Van Nostrand Reinhold CO of New York, USA.
*Atomic Energy Research Establishment.
Ian S C

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