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So You Want to Build a Stirling Engine

04/14/2009 12:12 PM

The following arrived in my email this AM.

"Subject: Seeking your advice

Hi there. I understand that you are a Stirling engine expert.

I'm trying to create a Stirling engine powered generator to be manufactured using recycled material in developing nations as part of a microenterprise development program I am working on. If you don't mind, I have a few questions I need your help with:

  1. How do you size the engine?
  2. How do you determine its power output?
  3. How do you determine the number of RPMs it makes from its design parameters?
  4. How do you determine its power input requirements?

I hope I'm not imposing on you. If I am, I want to apologize for it in advance and hope you can help me with my problem.

Thanks & best regards."

My Reply follows:

"I offer you a one word piece of advice. D O N ' T . Not flippant but serious advice.

Designing a power producing Stirling engine from the starting line is a long and arduous task frought with a steep learning curve and much time devoted to preparation and knowledge acquisition.

Even the experts (very few and far between) have to build a prototype and then modify it to reach a really good engine. This requires more R&D financing than is likely to be available.

To find more information go the < stirlingengine.com > and look at some of bottom of list topics on the Power Producing, Waste Heat, & Model Engine Forums.

A CD with items of interest is available for the cost of mailing & a blank CD.

Also many links are to be found at < NotSCar.wik.is >

Engineering Thermodynamics < http://www.ent.ohou.edu/~thermo/ > Is required understanding of the subject of Stirling engines. Dr. Izzy published this jr-sr level web site for his engineering students to make it available to anyone interested. It is a quite complete treatment of the subject as well as providing the necessary background for any attempt at construction of a real Stirling engine. Note that is is quite long and has many more pages linked to one another.

There are may sources on the web with plans and instructions to build Model Stirling engines.

This has been presented to provide the best advice and information that I can think of at present. While not a designer but having followed the progress or lack thereof of the topic of Stirling engines the last 15 or so years and being a retired mechanical engineer I have a well rounded knowledge of what is involved in the design and production of Stirling engines. Anyone with even modest skills can build a low temperature (LTD) engine from plans and/or design one from observing what others have done. When you want real power output you have a different "Kettle of Fish."

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Guru

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#1

Re: So You Want to Build a Stirling Engine

04/14/2009 3:10 PM

Here is an additional email message from another person's email to me.

"...have to build a prototype and then modify it to reach a really good engine. This requires more R&D financing than is likely to be available."

There's the answer you might reconsider giving this individual. Ask him/her if they have the money required to complete the above process. If so, recommend that they spend it on an already experienced "expert" who's in the R&D stages already. If, by some strange chance, the reply comes back that the money can be spent in this manner, then inform them that they must offer the money in the same manner as if they were designing it themselves. In other words, high risk, no guarantees, no set goal timeframes or deadlines, no outside (or entrenched) 'expert' advice and no bean-counters.

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#19
In reply to #1

Re: So You Want to Build a Stirling Engine

08/02/2009 2:50 PM

Stan contact me at earth.visions@yahoo.com, I am looking for 1.5-2hp stirling, Mac, I live in Ktown part time

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Guru

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#2

Re: So You Want to Build a Stirling Engine

04/15/2009 3:09 AM

NotSCar.wik.is

http://www.ent.ohou.edu/~thermo/

Dear Stirling Stan,

the link to stirlingengine.com is working but

the two links above not. ???

What to do?

Thanks

RHABE

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#3

Re: So You Want to Build a Stirling Engine

04/15/2009 3:28 AM

Thank you Stan,

What you might never forget is that it is an engine and you need to be very cautious with the design, it contains less moving parts but it is a complete engine.

There are so many things to work out and calculate that it is impossible to do this overnight.

And to frighten everyone who is not satisfied yet: just be aware that a power unit is working at high pressures (20 till 100 bar) and filled with Hydrogen.

Take a 2 years time frame for a decent team of engineers and make sure you have the money ready before promising anything.

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Guru

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#6
In reply to #3

Re: So You Want to Build a Stirling Engine

04/15/2009 5:06 AM

Hi Gwen S.

is there existing a limiting factor of pressure else then leaks and stress?

The energy per cycle should be better at even higher pressure.

So 200bar hydrogen is not a thing to just play around with.

What lubrication?

I assume that the lubricants are partially sprayed and evaporated into the gas at the hot side, maybe converted to carbon there and limiting lifetime or temperature?

RHABE

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#7
In reply to #6

Re: So You Want to Build a Stirling Engine

04/15/2009 5:51 AM

Just try to do it without lubrication. certainly at the high temp side.

Hydrogen migrates in the metal and makes it brittle. The higher the pressure the faster this goes.

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#8
In reply to #7

Re: So You Want to Build a Stirling Engine

04/15/2009 8:09 AM

Thank you.

Is hydrogen embrittlement a problem in all alloys?

Helium would be next but tightness is a still bigger problem.

I cannot think without lubrication as the time to failure in internal combustion machines is not at all acceptable. So some solution has to be found.

RHABE

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#4

Re: So You Want to Build a Stirling Engine

04/15/2009 3:45 AM

Interesting reading.

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#5

Re: So You Want to Build a Stirling Engine

04/15/2009 4:02 AM

Thanks, Stan. This is truly informative.

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#9

Re: So You Want to Build a Stirling Engine

04/15/2009 11:41 AM

Hello Stan,

Do you know of any reason why Nitrogen might not be good? I know you mentioned air but air is a mixture of several gases, and I am wondering if using "pure" Nitrogen would be just as good based on your recollection of Stirling engine designs, you may have seen.

Nitrogen seems interesting only because the threat of explosions and other issues that comes with hydrogen is then averted.

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#10
In reply to #9

Re: So You Want to Build a Stirling Engine

04/15/2009 11:57 AM

Just play with the calculator on this website.

http://www.bekkoame.ne.jp/~khirata/academic/simple/simplee.htm

The problem is the heat conduction of the gasses: air is insulating and as N2 is the main part of Air it is in fact the N2 which insulates.

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#16
In reply to #10

Re: So You Want to Build a Stirling Engine

04/16/2009 11:37 AM

Hello Gwen,

You above response got me thinking for a while about the significance of the Thermal conductivity differences between the gases and I came away with various design considerations which occurred to me:

  • First that the gases with only s-orbital electron configurations seem to have higher thermal conductivity, meaning that the more the energy levels of the gas the lower the thermal conductivity, hence the sp and sp-hybridized molecules are very insulating and therefore the molecules by the Wall-Gas Interface will keep absorbing the thermal energy and not much of that energy will permeate to the molecules farther from the wall [the periodic table should be quite helpful here (?)];
  • Second, for insulating gases, radiative heat transfer design must be adopted meaning that choice of the material at the heating zone must be such as to have higher emissivity at the internal wall as to enable more radiative heat transfer, such as would permit more penetration of the thermal energy into the bulk-gas;
  • Third, for high pressure operating engines, the choice fabricating material - steel, molybdenum, etc, - must be such that the lattice structure does support the "gassisity" or "gasity" [as in message#7] of the gas being used else the design will have to have a means of periodically pumping gas into the engine to maintain the design pressure.

These are just my initial thoughts come back on them upon reflection if you may.

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#11
In reply to #9

Re: So You Want to Build a Stirling Engine

04/15/2009 2:23 PM

Nitrogen is used to inflate tires because it has low expansion / contraction with temperature changes. Would that not suggest nitrogen is not a good choice for a Stirling engine?

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#12
In reply to #11

Re: So You Want to Build a Stirling Engine

04/15/2009 2:39 PM

I did not realize that Nitrogen deviated so significantly from the other gases with respect to the general gas laws. Would you attribute this to any property of Nitrogen besides thermal conductivity? The suggestion by Gwen of Nitrogen conductivity makes more sense that the implied implicit deviation from the gas laws. Very curious I must say.

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#13
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Re: So You Want to Build a Stirling Engine

04/15/2009 5:26 PM

Sorry I do not know the exact physics of these gases. But race cars have used nitrogen in tires for years and more recently COSTCO is filling new tires with nitrogen. Reason given being the low change in expansion / as tires heat up from use. It also minimizes deflated tires in the morning after a cold snap dropped overnigh temps. Extreme deflation would cause a tire to be ruined before it heated up from flexing.

I spent more than I care to admit paying a tire shop to look for a non existent leak in deflated tires that only showed up in the morning.

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#14
In reply to #13

Re: So You Want to Build a Stirling Engine

04/15/2009 5:49 PM

Relevant to the original post, ther are two companies that I know of that produce commercial qualtity of Stirling engines. Whispergen in New Zealand and a new company in switzerland. (The name escapes me at the moment)

Whispergen was marketing their product in North America but withdrew the product as a result of service longevity issues. They are now field testing a new designed unit but will not allow the product out of country. The Swiss design is a similar story. They told me it would be a couple of years before they would consider North American sales.

The point being both these companies have spent many years and lots of research money developing their product to this point. Even so they are having design issues.

I am wondering how much of a product could be made from recycled materials. Hydrogen and helium are the favored gases and both require extremely close tolerance machining not to mention extremely high quality control. A california company also developed a product but now this has been diverted into a solar power complex and small individual units are not for same. The one thing all of them have in common is extremely high cost to make. This does not seem compatible with a low cost solution from salvaged / recycled material to be sold in a third world country.

I have read of several other approaches to creating lower cost energy in third world countries that appear to be much more cost effective. All of these were CR4 threads detailing power generation in Africa India and Asia. Some involved low tech limited power generation from flowing water, another involved using a very oil rich natural plant to create vegetabl oil fuel that could run a converted conventional ICE.

Regrettably such projects appear doomed for development in North America because here the emphasis seem to be on mega watt projects. No one except a few of the off-grid users are interested in power generation under 10kW and more often around 3 kW outputs. Yet that is all that is really required to power even energy wastful North American households. A frugal third world household would probably be self sufficient with 1- 2 kW generation capacity.

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#15
In reply to #13

Re: So You Want to Build a Stirling Engine

04/15/2009 6:19 PM

Any gas that is far above boiling temperature is near an ideal gas, so p.v=R.T, or ideal gas equation is a very good approximation.

so density ρ = 1/v = p/(R x T).

Quite a different situation is with thermal conductivity - needed in heating, cooling and heat-regeneration.

Thermal conductivity is very bad in any gas. I don't know details, is this linked to viscosity of the gas?

Viscosity is given by impulse exchange of gas molecules. So mean free path length of molecules is important, collisions of molecules, energy exchange at collisions.

Thermal conductivity to be high and viscosity to be low is best with hydrogen, second best with Helium.

For more practical tests I would start with air, then go to Argon, then Hydrogen if embrittlement can be solved else Helium.

RHABE

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#17
In reply to #11

Re: So You Want to Build a Stirling Engine

04/17/2009 9:53 AM

I would assume that using nitrogen in the tire avoids the presence of oxigen, thus lower oxidation/aging of the tire from inside out. The effect of oxigen is going up when the partial pressure goes up.

This is also a problem with pressurising the stirling with normal air, in the hot section corrosion is going to be a serious problem.

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#18
In reply to #17

Re: So You Want to Build a Stirling Engine

04/17/2009 11:30 AM

That would make sense because as I had noted at high pressures with pressure differential between dividing walls, such as with tires, and metal walls, gases on the high pressure side dissolves into the wall-materials to be "conducted" through the wall to the other side - termed "gassisity or gasity". I believe that during the conduction process, some of the oxygen would react with the wall-materials either the tires rubber or the metal of the engine, hence corrosion.

I am curious though for which metals, used in the stirling engine, has the corrosion been determined, do you know? I should like to perform some analysis comparing the atomic diameter of oxygen against the diameter of the inscribed circle of the metal's lattice face-holes.

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#20
In reply to #11

Re: So You Want to Build a Stirling Engine

08/26/2010 12:08 AM

I don't think that nitrogen is put in tyres for this reason. It does not leak out of tyres like oxygen does so the pressure remains constant.

Remember PV=nRT. n and R are constants for a filled tyre (and V to a greater extent) so the pressure rises with the temperature.

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