Why build a Stirling Engine?

[KWILParticipant @kwil]

Risky but here goes, [I did not like to butt in to the existing thread about these devices].

 

Having used a high power Stirling engine in reverse, power in = cold out for cooling a detector, I am not such a fan of these devices. Tell  me why I should be, politely please.

[KWILParticipant @kwil]

[Gordon WParticipant @gordonw]

No reason at all, except they are interesting, relativley easy to build, lots of stuff still to find out about them, loads of variety in design, interesting history, dont need expensive boilers, and expensive insurance etc. etc. Why build oily old steam engines?

[Ian S CParticipant @iansc]

Hi Kwil,your welcome on the other site,even if you don’t like them you may have some ideas that we don’t(we sort of wander about in the dark a bit,with a few glimmers here and there)Here in Christchurch New Zealand the local university engineering school has turned out a number of engineers in hot air engineering.A local firm is Whispertec founded by some of these chaps.The whispergen is their product,a small pressurised Stirling engine producing about 1Kw of electrical power plus hot water,use-home power supply,or on a boat.This background causes one to be a little curious about this seemingly simple machine.I want wheels going round,and I don’t intend to build boilers(have done little pot boilers).I do have an IC motor slowly evolving.There is one of the local sterling engine men who has rebuilt a cryo type engine,its used for creating frozen gasses,think it came from the university.Ian S C

[KWILParticipant @kwil]

Ian SC,  thanks for the welcoming thoughts, I could be interested in the wispertec, but only 1Kw!! That will not run the house I am afraid, but would probably keep the lights on and the heating running!

Edited By KWIL on 16/12/2009 16:18:45

[Tony MartyrParticipant @tonymartyr14488]

My wife is a great fan of small low delta-t stirling engines because she says they are the only things that I have ever build that don’t make a horrible noise.

[CirclipParticipant @circlip]

 

  

Why build a Stirling Engine?

 

  Cos you can, but can you get it running? THATS the trick, no brute force of high ( or medium or low) powered steam.

 

  Regards  Ian.

[Martin WParticipant @martinw]

Hi All

 

Guess what you soon will all be able to have your own Stirling engine to heat your water and put some power back into the grid. Whisper Tech have an off shoot called Whisper Gen and they have signed a NS$300 million deal  to supply Stirling engines/heat exchangers/generators to the UK. Evidently E.ON UK have stumped up NZ$10 million as part of an initial order for these things!!!!!

 

One thing you won’t see is any wheels going round cause there aren’t any. They drive the generator with a ‘Wobble Yoke’ which they make a great fuss about but I seem to remember the three cylinder engine at Sandown Exhibition was a wobble plate system to the final drive.

 

Stirling engines to combat global warming they claim!!!!!!

 

Cheers

 

Martin W

[Mark Smith 3Participant @marksmith3]

Hi Kwil, Good question, but we would welcome you on the stirling engine thread. I don’t know if you have noticed how many people have been reading it; there is obviously more interest than even M.E. realise.

There are probably more reasons not to build than to build such as:

Very low power compared to steam per cc. Measured in mere watts or fractions of watts.

Every part must be totally friction free.

No leaks can be tolerated.

People (women usually) ask “what does it do?”.

They sound more like a sewing machine than an engine.

Frequently they are made from junk or scrap.

Model engines only enjoy 1 or 2 percent efficiency compared to its theoretical efficiency of nearer 50 percent.

Better than even chance that after all the work the bl$%&dy thing wont go!

 

However

 

If you do build one and get it to run, well, the chest swells to match your head, you tell your sceptical friends (if you have any left) and you sit in the garage with a quite beer and just marvel at the magic of it as it clatters away.

Then… you start planning the next one having learned so much from the first… then, another and another…….

Mark, the lonely nutter.

[KWILParticipant @kwil]

Wobble plate,  real name is a swash plate. Nothing new, I have had one in my high pressure water jet for years, drives  3 or 4 small piston pumps at 10 bar. My stirling engine cooler went down to near absolute zero, compressed helium, inside a very low vacuum. When you switched off the electric motor, it could run on for ………as it warmed up, just alike a stirling engine but rather LOUDER!!

[Ian S CParticipant @iansc]

The Wobble yoke refers to the special patented linkage to the swash plate Martin W says most of the story,the unit is a battery charger,and a battery bank and inverter supply the power for house hold use.Its normal to use gas for cooking,and the cooling water is used to heat the house hot water,and can be used for heating the house.Surplus can be fed into the main grid,its a good idea if you are already hooked up to stay that way,if you need extra you can use that too.The unit is silent,you can feel a faint vibration if you put your hand on top of the cabnet.Multi choice fuel,you can selectwhat you want to use;diesle,kerosene/parafin,LPG,natural gas.The size is(wxdxh)450x500x650,weight 90kg.Ian S C

[Jonathan HowesParticipant @jonathanhowes93303]

I think I may have similar views to KWIL on this:

 

Reasons for building one:

 

Entertaining to design and build

 

Reasons for not building one from a paper I wrote a while ago:

 

The Stirling and Ericsson second cycle are often presented as textbook examples of real cycles having the potential to deliver the Carnot efficiency. This is, unfortunately, very misleading. Taking the Stirling cycle as an example, a Stirling engine combines a cold space, a hot space and an interconnecting regenerator. The principle is generally described as:

 

(a)        The working fluid (normally a gas although vapour and liquid cycles have been proposed and sometimes used) is compressed in the cold space, as it is compressed any heat of compression is removed such that the compression (and therefore heat rejection) takes place at a constant temperature.

(b)        The working fluid is then transferred to the hot space at constant volume via a finely divided thermal matrix which has a linear thermal gradient between the cold side and the hot side. The working fluid takes heat from this matrix and is delivered to the hot space at the same temperature as the hot space.>>

(c)        The fluid is then expanded within the hot space. During the expansion heat is supplied to maintain the temperature at that of the hot space such that the expansion (and therefore heat supply) takes place at a constant temperature.>>

(d)        The fluid is then transferred back to the cold space at constant volume via the thermal matrix leaving its heat in the matrix and being delivered to the cold space  at the same temperature as the cold space.

 

Edited By Jonathan Howes on 12/04/2010 17:54:55

[Jonathan HowesParticipant @jonathanhowes93303]

And what happens in a reduced form of reality:

 

What happens in practice is somewhat different and it is a truism that no Stirling engine has ever operated on a Stirling cycle as described above. The real situation is closer to the following:

(a)        The working fluid is compressed in the cold space. All the spaces, including the thermal matrix (regenerator) are interconnected in a Stirling engine and hence fluid in both regenerator and any clearance volume in the hot space is also compressed. The cold space is cooled as much as possible during this process but, since heat transfer requires a temperature gradient, the cooling is not even and the fluid cannot achieve the same temperature as the cold sink, it is also not kept at a constant temperature. The fluid within the regenerator and hot space clearance volume becomes warmer as the result of the compression and the regenerator temperature profile is affected by this warming.

(b)        Fluid transfers to the hot space via the regenerator and is heated by the regenerator. As this happens the pressure rises, due to the interconnection of the spaces the fluid within the cold space is compressed before entering the regenerator and the temperature of this fluid rises due to the compression. On passage through the regenerator the regenerator temperature profile is further corrupted by this temperature rise. Fluid that has already passed to the hot space is further compressed by the increase in pressure and also becomes hotter, since the regenerator has already warmed the fluid to the hot space temperature the fluid within the hot space is now hotter than the hot space walls.

(c)        The fluid is expanded within the hot space. Since the fluid in this space is initially overheated, heat initially leaves the fluid by conduction to the walls, as the temperature falls some heat is conducted into the gas, this heating is not evenly distributed or at a constant temperature.

(d)        The fluid is now transferred back to the cold space via the regenerator. As the temperature drops on passage through the regenerator the pressure drops, this pressure drop causes further expansion of the remaining fluid in the hot space lowering its temperature, passage of this cooled fluid further corrupts the temperature profile within the regenerator. The pressure drop also affects fluid in the cold space which expands and becomes over-cooled resulting in a lower rate of heat rejection through the cold space walls.

Edited By Jonathan Howes on 12/04/2010 17:57:36

Edited By Jonathan Howes on 12/04/2010 18:03:29

[Jonathan HowesParticipant @jonathanhowes93303]

If, for other reasons, a closed gas or vapour cycle machine is required then a Brayton or Ericsson style of machine is more practical. If reciprocating then a valve somewhere in the circuit must work at peak cycle temperature, if using turbomachinery this can be avoided and the main benefit is that heat exchange can take place at constant pressure in a very large space, ie, over several cycles (or compression/expansion segment transit times for turbo machinery).

 

Stirling engines are fun but will not be the saviour of the human race.

 

Jon

[Mark Smith 3Participant @marksmith3]

All very well, but the shortcomings of the Stirling engine will not stop people building them. I’m sure some very knowledgeable person could tell us all the shortcomings of the steam engine and its boiler, most engineers have some idea of what they are but many thousands have built them just the same. I think the point is being missed that model engineering is supposed to be an enjoyable hobby that anyone can enjoy. Of course there are problems with hot-air engines but that knowledge does nothing to diminish the joy of seeing something one has built spring into life, even if it has no practical application. It should be remembered, too, that some of the best hobby engineers have been fascinated by this engine and built some beautiful examples.There might be many reasons for not building a hot-air engine but, unless you are a believer, you might just miss the many reasons to build one.

Mark

[Jonathan HowesParticipant @jonathanhowes93303]

I agree with you, I have built several…. for fun.

[Ian S CParticipant @iansc]

Here here, Why build as steam loco, you should be scratch building a modern diesel motor and all, I mean to say steam’s obsolete, then again we should be open to all. There was a bloke in the States in the mid ninteenth century who said man will never fly, his name was Wright and he had two sons named Orvil and Wilber, so although hot air engines might be as they are, who’s to say what the future will bring for hot air or steam. Ian S C

[John Wood1Participant @johnwood1]

Whenever I start looking into Stirling engine building I am usually swamped by a load of theoretical stuff which, quite frankly is often way above my head, such stuff is, to me, rather boring so I like to leave all the design work to those of superior intelect who are interested in such things, I know… call me lazy. Me…? I just like to make stuff which works so, as long as I have a basic grasp of the theory of operation, so I can work out what’s important and what isn’t during construction, I value full detailed plans of tried and tested machines together with PRACTICAL hints and tips re construction to enable me to successfully build the engine and GET IT RUNNING. It’s most important to end a practical article with as much information as possible to help the contructor actually get his new project going, so many just finish with “so it was nice to give it a flick and off it went first time”  Arggg…. it virtually never happens to us mortals, we need nursing along so please spare us a thought!

 

So to answer the original post, my reasons for building Stirling engines are to gain pleasure from their construction and satisfaction in seeing the finished product working and looking good, even if the Wife does say “but what’s it for?”

 

John

[NJHParticipant @njh]

Well John

 

 ….” but what’s it for?”    – a question often asked of me by my friends when I tell them of  some engine or other project I’m doing. My reply is usually:- ” but you spend hours watching football and then more hours discussing it – what’s THAT for?  I sometimes wish I could see in it what they do but I guess, at least in the last couple of months, I’ve had the last laugh!

 

Cheers

 

Norman

 

 

 

 

[Sub MandrelParticipant @submandrel]

The practical application of a stirling or similar engine is pretty obvious; it is the simplest, most easily maintained solution for performing useful work from a low grade heat source*.

 

In the third world this might be dried cow pats, wood, charcoal or rubbish. In the ‘developed’ world this could be flue gases or some other heat that would otherwise go to waste. Some of these legendary ‘server farms’ that run many kilowatts of computer power could harvest the waste heat and put a worthwhile amount of electricity back into the input

 

If you want to burn a low-grade fuel, then at the price of (arguably) reduced efficiency a stirling engine is cheaper and has less to go wrong (and goes wrong in a less lethal fashion) than a steam engine, without requiring continual vigilance of the boiler water level.

 

So if one reason to be a fan is that it is “apprpriate technology”, the other has to be “you can see how it works”.

 

Neil

 

*You could argue that peltier elements or even multiple thermocouple elements are an alternative solution. Can anyone think of a better way to get useful work from a low-grade heat source?

[John OlsenParticipant @johnolsen79199]

Just a point on the server farm thing. Anything that tries to extract work from the waste heat will raise the temperature of the thing being cooled. This is not desirable, so you can’t actually extract useful work from the waste heat.

 

To put it more simply, they are already using fans to push the air past the heatsinks etc. If you could safely raise the temperature of the devices being cooled, you could just  turn the fans down a bit, or even off, and thus save some work going in. That would be even better than adding apparatus to try and recover heat on the way out.

 

But otherwise, it is a bit  like trying to extract energy from a stream without obstructing the  flow of the water.

 

What you can do is use the coolers (refrigerators) in the system to pump heat into parts of the building that you want warmed.  (in winter I guess!) That is better than trying to pump heat from outside to warm the building, since the heat pump will use less energy when pumping through a smaller heat difference.

 

I wonder if the honourable and learned editor would like a non mathematical article on thermodynamics?

[Ian S CParticipant @iansc]

John you could of course use the Stirling engine in reverse in the case of the server farm situation, its being done with miniature systems in computers now, this is quire an effective cooling method, and not as messy as water cooling. Ian S C

[Sub MandrelParticipant @submandrel]

Not so John,

 

According to an article in New Scientist there are already server farms doing this.

Basically these have huge racks of computers all cblowing out hot air which has to be pumped out of the building.  Passing that hot air over a heat engine on the way out dioesn’t affect the effectiveness of the air pumped in at the other end of the building for cooling the computers. – its not like the engines are fitted in the computer cases.

 

With a river the energy is all derived from the head of water. The flow of air is only the mechanism for transferring heat not the source of energy. Slowing the flow will cause the air to get hotter RAISING the temperature difference and hence the efficiency of the heat engines (and probably causing your computers to go pop).

 

It’s not unlike a condensing tumble dryer, which has a simple heat exchanger that pre-heats the air before it goes into the heater while extracting heat and water from the air on the way out.  A lot more effcient and it allows the dry air to be used to warm the building in winter. By your argument the extra load on the motor from circulating the air would exceed the efficeincy gains from teh heat exchanger, but it doesn’t.

 

No breaking of the rules of thermodynamics, you can improve efficiency but never work magic.

 

Neil

[Author]

Posts