Low temp difference beta help

Discussion on Stirling or "hot air" engines (all types)
joedog44
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Joined: Wed Nov 19, 2014 3:33 pm

Low temp difference beta help

Post by joedog44 »

hello people.
First off like to say this forum is great. i'm a student like a lot of people on here asking questions, and the information here has been really helpful. For a pretty big project of mine i have to design a stirling engine to power a nightlight with a temperature difference of only 65 degrees. I've read a few of the standard suggested books, like James G Rizzos, Roy Darlingtons and a couple more technical ones. I Have a pretty good idea (theoretically only) of the standard design parameters etc but i'm struggling to get solid information on the diameter cylinder i should use. Thus far i've decided to make a beta type engine, and i've got a reasonable budget for materials and access to a fully kitted workshop with skilled technicians to lend a hand. If anyone could offer any advice, it would be much appreciated.
Cheers, joe
I
Aviator168
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Joined: Tue Feb 08, 2011 2:29 pm
Location: Brokeville, NY. USA

Re: Low temp difference beta help

Post by Aviator168 »

You should find out what the power requirement first. Most common LED diodes 20mA @ 2v, and that's 0.04w. Depending on the definition of nightlight. For some people, one is good enough. After you have the power requirement, you work your way backwards to get rpm, and torque, then on to how much heat you need and reject, and those should give you a good idea of the engine size.
joedog44
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Joined: Wed Nov 19, 2014 3:33 pm

Re: Low temp difference beta help

Post by joedog44 »

hi, thanks for your reply.
i've not got a set power requirement. The brief i've been giving just says a nightlight, speaking to my supervisor as long as it can light an led, that classes as a nightlight. I'm going to power it with a linear alternator running off the crank because i doubt i can make the torque to turn a generator with the limited temperature difference. What steps would you suggest to give me an indication of engine dimensions?
cheers, joe
Aviator168
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Joined: Tue Feb 08, 2011 2:29 pm
Location: Brokeville, NY. USA

Re: Low temp difference beta help

Post by Aviator168 »

It is difficult to design if you don't know the power requirement. A typical night light is 0.25~0.5w and I think you should start from that. For the generator, you might want to build that yourself as well. I don't know what the thermal efficiency is for LTDs in general. The heater and cooler is a challenge since you have to design in all the loses in it. For example. If your engine is 10% efficient and you want 0.5w output. Your heater has to be able to pump 5 watts into the engine. If your power estimate is too high, you might be designing something that is not possible for that temperature range.

Edit. Just found this paper. LTDs can have efficiency as high as 24% and that's high. This is a very good paper and it should help you in designing your engine.
http://www.gistconpro.com/230-Pobitra.pdf
Ian S C
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Location: New Zealand

Re: Low temp difference beta help

Post by Ian S C »

I'm not a strong advocate of LTD motors for power generation, a motor designed for higher temperatures is generally easier to get running with some power. On page two of my gallery you will find a 2.5 CC motor with a generator that will run a 3volt radio, and would run a number of LEDs. I have not measured it's maximum power output, but it would be between .5 and 1 Watt, the generator is the electric motor from a cassette tape recorder.
Ian S C
Ian S C
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Location: New Zealand

Re: Low temp difference beta help

Post by Ian S C »

Aviator168, I had a look at the link, the LTD shown there is almost identical to my LTD motor, mine has ball bearing races at the big ends of both con rods, and the main bearings of the crankshaft, and when placed on a bowl of boiling water 100*C it runs at about 120 rpm, I have not tried putting magnets on the flywheel, and coils nearby, it would probably light up a red LED (red lowest voltage), must try one day.
Ian S C
Aviator168
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Joined: Tue Feb 08, 2011 2:29 pm
Location: Brokeville, NY. USA

Re: Low temp difference beta help

Post by Aviator168 »

With due respect Ian, 120rpm is low for a LTD. Check this one out. The displacer with regenerators.
https://www.youtube.com/watch?v=CdX7hwjhgA4
joedog44
Posts: 5
Joined: Wed Nov 19, 2014 3:33 pm

Re: Low temp difference beta help

Post by joedog44 »

hi gents,
thanks for your replies. i have done some calculations, all based on ideal cycle, heat transfer, zero friction etc, dead space etc. i need to produce about 0.7 watts, my design is based on an estimated indicated engine power of 1.45 watts with the following dimensions:

cylinder diameter : 41mm
compression piston diameter : 41mm
compression stroke : 41mm
expansion piston diameter : 39mm
expansion piston stroke: 60mm
displacer length : 82mm
full cylinder length: 205mm
Th : 80
Tl: 14
mean system pressure : 0.5 bar (gauge)
max presssure : 1.24 Bar (gauge)
expected engine speed: 180rpm

if anyone has any comments or can see i've done something pretty dumb, please feel free to comment.
cheers, joe
Aviator168
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Joined: Tue Feb 08, 2011 2:29 pm
Location: Brokeville, NY. USA

Re: Low temp difference beta help

Post by Aviator168 »

This does not look like a beta to me. How do you plan to raise the temperature
Bumpkin
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Joined: Fri Mar 02, 2012 1:42 pm

Re: Low temp difference beta help

Post by Bumpkin »

Joe, for a Beta, I'm assuming where you say "expansion piston" you mean "displacer?" Anyway, for a low temperature difference engine I think you'll need a lot more displacer than your approximate 1.5/1 ratio. I don't know if I've ever seen an L.T.D. Beta.
Bumpkin
joedog44
Posts: 5
Joined: Wed Nov 19, 2014 3:33 pm

Re: Low temp difference beta help

Post by joedog44 »

yeah, expansion piston would be my displacer. the heat source is the waste heat from a domestic hot water cylinder, so i'm taking the high temperature as 80 deg C. i've not seen any LTD beta engines which is why the design phase is a real struggle. Theres very little to take examples from.
Aviator168
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Joined: Tue Feb 08, 2011 2:29 pm
Location: Brokeville, NY. USA

Re: Low temp difference beta help

Post by Aviator168 »

joe, my question was how are you planning to heat the working fluid from 15c to 80c and cool it down to 15c. In order words, you have not showed the heater and cooler design. Getting to maximum temperature differential is always a challenges for stirling engines.
joedog44
Posts: 5
Joined: Wed Nov 19, 2014 3:33 pm

Re: Low temp difference beta help

Post by joedog44 »

ah ok, well i was planning on running the setup vertical, having the hot side of the cylinder fixed to the top of the hot water tank. the mid section of the cylinder to be taken down on the lathe to cut down on heat transfer. The cooling side of things, i am going to run a combination of cooling fins and a water-cooled conduction pipe fed by the cold water supply of the water tank.
Aviator168
Posts: 308
Joined: Tue Feb 08, 2011 2:29 pm
Location: Brokeville, NY. USA

Re: Low temp difference beta help

Post by Aviator168 »

A picture will help
Ian S C
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Joined: Thu Dec 02, 2010 5:15 am
Location: New Zealand

Re: Low temp difference beta help

Post by Ian S C »

The usual pancake shaped LTD motor would have a diametrical ratio of perhaps 6:1, set up as a Gamma motor, and both cranks (going by my own motor) the same stroke[fairly short] for the displacer, and the power piston.
Ian S C
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