Building a Low Power Stirling Generator

[cypher]

Hi,
I am a student from the UK and I'm doing a DT project to power one of our school 5w led light bulbs for 4 hours (5 watt is the minimum I need if the engine produces more power that would be great as well) but to get the higher marks the energy source needs to be constant and on demand. This rules out solar and wind power, the former not because of the day night cycle but because of cloudy days (and here in the UK there are quite a lot of those) and the latter because wind is not the most reliable thing.

I have already ruled out gravity as an energy source because I would need an improbably high mass or I would need to rent an aircraft to get to the height of the correct potential energy. I've also tried thermoelectrics but it's proving to be difficult to get the correct power as well as keeping a uniform temperature across the module. Funnily I actually did look into stirling engines for their efficiency but I thought that thermoelectrics would be easier.

I've come back to the idea of having an lpg powered stirling engine as my project and I'm here to ask for help in deciding the type of engine that would suit my project and more importantly, if someone can help me build one. I would like it to have a decent efficiency so to generate the required 5 watts every hour I would need it to use less than 300Btu/hr of lpg (88W).

To start off, I came across the following magazine article Push-Pull Generator but even though the design seems interesting the efficiency seems low (though even at 10% efficiency I would only need 170Btu/hr or 50W to generate the 5W).

This is just an idea, if you have a better idea for my project that would be amazing as well.

I really hope you can help plus any help and advice will be very much appreciated.

[fullofhotair]

This is just me .but I think you need to focus. What are you really trying to achieve? Low energy in means low energy out. You could use a capacitor to build up a charge then do some real work .bright light ,etc.

[vamoose]

Hey cypher,
So people on the forum can help with your project, and get an idea of where your at and what research you've undertaken so far, I will ask a few questions..

Will it be a pressurised or atmospheric engine?

Do you have access to a lathe and workshop?

If not, are you considering a 'tin can engine' design approach?
(this is potentially a valid method for low power engines)

Are you considering an LTD (Low Temp Differential) design or a HTD (High TD) engine?

From your research so far, Are you thinking of an Alpha Beta or Gamma configuration?

What is your budget?

How much time do you have for your project?

Do you have any prior exposure to Stirling engines?

If so, what might that be?

Are there any educators from you learning institution that have any knowledge of these fickled engines, and can they help guide you with your project?

Hopefully the answer's to these questions will help forum members figure out how they can assist..
vamoose

[Ian S C]

To supply 5W electric output, you will need a minimum of 10W at the crankshaft owing to the low efficiency of very small generators.
Vamoose has some very worthwhile questions.
Two books worth looking at are vols 1 and 2 " The Stirling Engine Manual" by James G. Rizzo, theyare quite expensive, but with any luck you'll find them, and other Hot Air Engine books in a library, or you might find stuff second hand. Stirling Engine builders tend toward being a bit stingy, but not mean. Ian S C

[vamoose]

Just so you know cypher,
If I was posed with these same questions a couple of years ago, these would be my responses..

Will it be a pressurised or atmospheric engine?
WTF..

Do you have access to a lathe and workshop? 
Lathe – no. Workshop- yes

If not, are you considering a 'tin can engine' design approach?
(this is potentially a valid method for low power engines)
Tin can maybe, I think there is good Information on the net about these..

Are you considering an LTD (Low Temp Differential) design or a HTD (High TD) engine?
Hmmm, (I will have to think (and research) about that) maybe LTD is easier, although then again maybe not..

From your research so far, Are you thinking of an Alpha Beta or Gamma configuration?
Gamma, (although still confused)

What is your budget?
F All - f^&$* #!!

How much time do you have for your project?
Forever (hopefully), but maybe about, F All + f^&$* #!!

Do you have any prior exposure to Stirling engines?
Yes!!

If so, what might that be?
I think I sat on one once!!

Are there any educators from you learning institution that have any real knowledge of these fickled engines, and can they help guide you with your project?
Are you drunk?

So its a learning curve, for all of us at our own pace, and if your willing to take the time, it can be good fun.

[cypher]

Thank you, I appreciate the replies.

This is just me .but I think you need to focus. What are you really trying to achieve?

To put it simply I want to generate a minimum of 5 watts an hour from a 300Btu/hr lpg source. Using a sterling engine is what I've settled on but if anyone here has a better idea about how I could do the project I would be really interested in that as well.

Will it be a pressurised or atmospheric engine?
If not, are you considering a 'tin can engine' design approach?
(this is potentially a valid method for low power engines)
Are you considering an LTD (Low Temp Differential) design or a HTD (High TD) engine?
From your research so far, Are you thinking of an Alpha Beta or Gamma configuration?

I think the answers to these questions would be 'whatever can help me reach my aim easily and efficiently'. Though I do seem to be leaning towards a thermomechanical generator (like the one youtube user barumman has built) due to it looking seemingly simple to make.

Do you have access to a lathe and workshop?
What is your budget?
How much time do you have for your project?

We do have access to both but I don't think that I have the skills to actually build one, one of the reasons that I posted on here is to see if someone would be willing to help me which brings me onto the second question. Because this is an important project I thought that I would put in some of the money that I've saved up so I have a budget $200 but if someone can build a working engine for me I can give them $100 more to make it $300. I know that this might not be a lot but it's most of the money that I have saved. As for the project time frame, preferably I'd like to have it done by July but I don't think this will happen so I would say that I have time till December.

Do you have any prior exposure to Stirling engines?
Are there any educators from you learning institution that have any knowledge of these fickled engines, and can they help guide you with your project?

The simple answer to these two questions is 'not really'. I can't seem to find much information about stirling engines in the power range that I need for my project, usually they make a couple hundred miliwatts or their power is measured in the kilowatt range.

[Junkie]

I managed around 2 watts from this machine without a lathe or machine tools.
I think with better construction (lighter parts (esp displacer), better balancing and
a better generator 5 watts might be possible. It won't be easy though.

[youtube]http://www.youtube.com/watch?v=U9vPdcW_qTg[/youtube]

[Ian S C]

In my gallery there are 3 motors that can produce power, the first is my second motor, based on Dyna by James G. Rizzo in his book "The Stirling Engine Manual" vol 1. The next one is the vertical one with the ornate legs, it's basicly Dyna scaled up. And the 3rd one is the Ross Yoke motor with the power hacksaw, it uses a Junior hacksaw blade, and will cut through a 1/2" steel bar in about 20min if the blade is sharp. The 2.5cc motor will run a 3V radio, the generator is the motor from a cassette tape recorder. Even the little beam engine could light an LED. Ian S C