Stirling power generator

Discussion on Stirling or "hot air" engines (all types)
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isty66
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Location: Hungary

Stirling power generator

Post by isty66 » Mon Apr 16, 2018 1:17 pm

Hello there.
I am new to this site, but thank god not for this topic. I have wanted to make a powerful stirling engine driven generator since years, but now that I have my part time job as a teacher amongst with my studies in the university, I finally have the time, funds and pretension. I find the old Philips machine incredibly charming, and I want to make something like that in fact. I have found it's manual, but many things are still unknown for me I fear. The working principle of the preheater and the main burner is still a mistery for example. And how did the regulation of the engine work? I mean the generator was a permanent magnet synchronous generator with direct drive from the engine. How did they managed to keep the frequency at about 50 Herz? Oh, and the pressurisation of the working side of the cylinders is also a problem. How would I know that at which pressure would mine need to work well? My plans are a bit more grandiose than a replica. I would attempt to reach 3 kilowatts of peak output, and about 1.5 kilowatts of continous. It would be lovely to use during a camping event with me mates, and that would require a music source, a fridge for beer, and lights to see where are everyone after three hours. And I would be able to put coffe on as well.

Thank you for your kindness and time!
Best Regards: Steven Blakeney

Blaf
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Joined: Fri Mar 02, 2018 2:50 am

Re: Stirling power generator

Post by Blaf » Wed Apr 18, 2018 1:23 pm

Im afraid 3 kW Stirling engine is nothing you would go camping with.

Ian S C
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Location: New Zealand

Re: Stirling power generator

Post by Ian S C » Thu Apr 19, 2018 2:22 am

Even a fairly small burner you could put a pot on to make coffee.
With the Philips genny it's a case of keeping it at 3000rpm, today the best way I see is to produce a DC current, then use an inverter if you require 50hz AC, and it is quite possible to synchronize the output to the grid.
You'v seen the Philips units performance. For a high power gen set aim for a few hundred Watts at the most, but you'll need quite a big motor, 1 Watt per cc is good going.
Ian S C

isty66
Posts: 8
Joined: Mon Apr 16, 2018 12:56 pm
Location: Hungary

Re: Stirling power generator

Post by isty66 » Fri Apr 20, 2018 1:07 am

The size and weight is not a big problem. I have found a stirling designer program, I suspect it is fairly good. It gave a relatively precise output for the Philips, using it's datas. It calculated that if I would use a 75 milimeter bore and stroke, with fairly big dead volumes, I would be able to get 3 kW of power at a charge pressure of 10 atm. That would require with an estimated 25% efficiency 12kJ of work each second, which is 43 MJ of work in a hour, given the diesel's calorific value of 4.2 MJ/kg, this would require 12 litres of it each hour. But I suspect I will never ever going to use 3 kW continously. 75 milis would not be an unfamiliar sight to me, as I am a car mechanic, so I have spares, and whistles and bells andbirds andthebees... I really quite like the idea of DC and inverter. The regulator would keep the voltage at around 14, so the revs are not concerning me anymore. In this case the unused power would spin the engine up, so if I'm right, a centrifugal regulator would be enough to open or close the atomiser, to keep the revs roughly at about 1000 or 1500, wouldn't it? The quick regulation is therefore done by the generator, but to keep the voltage regulator in it's comfort zone the engine speed regulation would be inevitable I suspect. Or should I regulate the engine with changing the pressure inside it?

Blaf
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Joined: Fri Mar 02, 2018 2:50 am

Re: Stirling power generator

Post by Blaf » Fri Apr 20, 2018 1:39 am

For power regulation you may either put a valve on a channel between cylinders (limiting airflow causes engine to lose power, or better say - to waste power on pushing the air through), or you could add container with a lot of dead volume, again connected through valve to the rest of the system (additional dead volume lowers compression and therefore also power output).
Ofc, there are many other ways how to regulate engine power, but i like the simplicity of these i mentioned :)

isty66
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Joined: Mon Apr 16, 2018 12:56 pm
Location: Hungary

Re: Stirling power generator

Post by isty66 » Fri Apr 20, 2018 4:01 am

The blocking valve sounds quite satisfying. I thought on a design exactly like the Philips machine, which if I am correct, channels the working fluid out of the piston cylinder, and pushes it down amongst the former, and the outer cylinder. Between the two are the hot heat fins, the regenerator coils and the cold heat fins. Then it returns to the inner cylinder via portholes under the displacer, and over the working piston. Is this correct? In this case I would be able to put a covering ring around those portholes, which then can be rotated to cover all the holes, or leave it open. How will this affect the efficiency? Because I am now thinking on that there is a huge gap between 3 kW and 500 Watts, so if the burner continues to supply the heat flow required to the output of 3kW, but the engine now can only take one sixth of it, this may result a melt hot-end.

isty66
Posts: 8
Joined: Mon Apr 16, 2018 12:56 pm
Location: Hungary

Re: Stirling power generator

Post by isty66 » Fri Apr 20, 2018 4:35 am

Here's my problem. On this diagram the two cylinders can be seen. But the displacer on this picture is likely in it's topdead, still it has curved line in the middle of it, similar to the top of the cylinder.
Image
On this picture however, there is no such thing, and it surely is in it's TDC, indicating that the displacer must cover the whole volume of the hot end.
Image
Question: Which one of these pictures is correct? I suspect it is the latter, but in that case, what is that curved line on the middle of the displacer on the former picture? Does anyone have some pictures about that engine where the piston and the displacer is visible? And maybe the inside of the outer cylinder?

Blaf
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Re: Stirling power generator

Post by Blaf » Fri Apr 20, 2018 4:44 am

Well, imho in an engine complex like this, connecting it to an expansion tank (with additional dead volume) might be more feasible.

Blaf
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Joined: Fri Mar 02, 2018 2:50 am

Re: Stirling power generator

Post by Blaf » Fri Apr 20, 2018 4:49 am

Basic rule of thumb - less dead space means more power, therefore i suppose in upper picture the displacer isn't in top end.

isty66
Posts: 8
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Location: Hungary

Re: Stirling power generator

Post by isty66 » Fri Apr 20, 2018 4:58 am

I thought so, but the wo drawings are from the same engine, and if that is the displacer's top, the length of the two displacers are not even close to each other. In other news.. I have just discovered that using the 1 Watt/ccm multiplied by the internal average pressure in atm gives a roughly correct result on output power in watts. At least in case of the Philips machnie and my engine's computed output.

Here is the stirling engine simulator:
http://www.solarheatengines.com/stirlin ... simulator/

Ian S C
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Location: New Zealand

Re: Stirling power generator

Post by Ian S C » Fri Apr 20, 2018 5:02 am

If you plan to burn 12L / hr I would expect a lot more than 3W(maybe 10s of Kw's ), the Philips motor ran for 8 hrs on a 3.3L tank, or about .4L per hr. Power regulation can be done by varying the pressure in the motor, but the best way is to have a constant load, the motor and load designed to match, so charge a battery bank, and take the load off the batteries. When power is not required on site either shut down or feed to the grid.
In the top diagram the curved bit in the middle of the displacer (I think)indicates that there is a heat dam/ the space is divided in two to reduce heat transfer. Bottom l/h diagram, on the l/h side is the air pump, this would be the point of pressure regulation. The motor on the R is one of Andy Ross's ALPHA motors.
Ian S C

isty66
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Joined: Mon Apr 16, 2018 12:56 pm
Location: Hungary

Re: Stirling power generator

Post by isty66 » Fri Apr 20, 2018 5:28 am

Oh, thanks :D So the Displacer is actually this long. That simplified it in my mind. I thought so that this amount of consumption would mean a nuclear grade output, but my calculations seem to be correct.

In case of the Philips machine:
-100W shaft output at low load, which is 100Joules a second, times 60 to minutes, times again 60 to one hour is 360'000 Joules/hr
-360'000 divided with 1'000'000 is 0,36 Megajoules, divided by 4.2, which is the calorific value of diesel or gasoline is 0,08 kg of fuel
-Because the engine is about 25% efficient divide the 0,08 with 0,25 which gives 0,32 kilograms, and it is equal to about 0,4 liters of fuel an hour.

In case of mine:
- 3000W * 60 * 60 = 10,8 MJ/hr
- 10,8 / 4,2 = 2,52 kg/hr
- 2.52 / 0,25(eff) = 10,2 kg/hr

Should you be able to notice my error, if there's any. I hope there will be though.
Now I want to determine the heat exchanging area. If I am correct the heat convection is in linear relationship with the sink area, so if the energy is two times bigger under the same amount of time, either the Area has to be two times bigger, or the temperature... I don't know how many times bigger. Am I being correct?

isty66
Posts: 8
Joined: Mon Apr 16, 2018 12:56 pm
Location: Hungary

Re: Stirling power generator

Post by isty66 » Sat Apr 21, 2018 3:09 am

Another question have just occured to me. I would like the whole engine to be constructed from as few parts as possible. I therefore thought that the inner cylinder or sleeve would be one piece, from the top of the hot side, to the top of the base block of the working piston. The outer sleeve would be of two parts because of the enormus heat difference. The housing of the crankcase would be cast iron, and would be made of two parts, because the crankshaft would have counterweights on both sides, wich would include the weight of the flywheel. The question would be, is the machining the whole inner sleeve from one part, a good idea, or it will crack? And the other, what type of metal would be adequate for the 3 or 4 cylinder sleeves, and for the working piston and working cilynder?

Ian S C
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Location: New Zealand

Re: Stirling power generator

Post by Ian S C » Sun Apr 22, 2018 4:36 am

The bore of the power cylinder should be cast iron, the hot cap should be stainless steel.
Ian S C

isty66
Posts: 8
Joined: Mon Apr 16, 2018 12:56 pm
Location: Hungary

Re: Stirling power generator

Post by isty66 » Mon Apr 23, 2018 12:57 am

Thanks for the advice. I am preparing the model of the engine from my sketches. Now I suspect that it won't be that compact little thing like the Philips machnie..

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