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Stirling Engine Application

  1. Dec 25, 2005 #1
    Hi. I know of the commercially available "battery-less" flashlights that use either squeezing or shaking mechanisms to generate electricity. I was wondering, however, whether I can create one that uses the difference of temperature between the hand and the room temperature. I propose to use a LTD stirling engine to drive a magnet inside a coil of wires to generate electricity. My question is, can a carefully constructed stirling engine with the size of this: (cylindrical, 10cm in diameter, and no longer than 30m in lenght), be powerful enough to generate electricity enough for 1 or 2 high output LEDs to light up? If it can, can someone guide me to building this stirling engine? I'm hoping to get an university professor to help me and I can access the school's metalshop's lathe and welding machines and grinders and others. So please respond. Thank you very much.

    I am also trying to build a Stirling-Stirling refrigerator. I propose to drive the first stirling using the heat from concentrated sunlight, which powers a second stirling to achieve cooling. I hope to get this machine to be able to make ice. I have a question about the stirling cycle cooling though. I want the stirling engine to be able to cool an area of 10cm x 10cm x 10cm. I know that a small gas compression chamber can yield better results, so how do I go about cooling that area? Should I just attach the cooling area outside the cooling gas chamber? Can I achieve a 0 degrees celsius cooling with a stirling built from scratch?
     
  2. jcsd
  3. Dec 26, 2005 #2
    You are pretty much asking about terra incognita. No one has ever engineered a flashlight shaped LTD stirling engine, and you'd really have to start from scratch on your own.

    You may already be familiar with it, but if not I'd reccomend An Introduction to Low Temperature Differential Stirling Engines by James R. Senft. This is probably available on the web somewhere.

    I haven't found any books that have any detailed info on using Stirlings as refridgeration devices, only mention that there is alot of promise for this.

    I have some experience as a machinist and, though I've never made a Stirling engine I did once engineer a little steam engine that ended up working pretty well. It is about the simplest possible design, but was still a lot of work and planning to make.
     
  4. Dec 27, 2005 #3
    Okay, thanks. I've basically ditched the flashlight idea because LTDs just barely overcome friction so I'm 99% sure they'll be unable to drive ANY external load.

    I'm fully concentrating on the stirlig-stirling cooler for my science fair. One thing good about it is that the hotter something get, the cooler the, well, cooler gets. Once the hot object's cooled down, the engine is no longer cool. Hmm... there can be an application of this somewhere, maybe to cool down an engine or a nuclear power plant...

    I'm working on the stirling-stirling cooler for my science fair project. However, I have a few questions about it:

    1) Is there a way to calculate the maximum temperature my cooler can reach, given its rpm, the space it needs to cool, the load the first stirling needs to drive etc. ? If so, how? I really want to know beforehand how cold my stirling-stirling cooler can get.

    2) Is there a way to make the heat reject side only slightly hotter while the heat accept side (the cold side) a great deal colder? I don't want my fridge to turn into an oven on the heat reject side and only slightly chillier on the cold side.

    3) This is a stupid question, but I can't find the answer to it. What exactly is a heater cap and why use it? I've seen a few designs on Koichi Hirata's site and I see a lot of his engines use the heater caps. Why? If I use an alcohol burner to heat it, then wouldn't it be in a very awkward position?

    Thanks so much for your responses!
     
  5. Dec 27, 2005 #4
    I wouldn't worry about the cold end not getting cold enough. From what I've read it seems they get plenty cold with no special adaptations.

    What people do when they want to use a Stirling as a heat pump is to power it with an electric motor. One advantage of this is that you can get it up to very high speeds, speeds much faster than it attains when operating as a heat powered engine.

    Another book by Senft I have, Introduction to the Stirling Engine says a company called Phillips started doing this with Stirlings a long time ago. They just let them sit humming away under electric power and liquified air simply drips off the cold end. That's pretty spectacular.

    How cold it gets depends on how fast you run it. If you want to power your Stirling heat pump with a Stirling engine I think the important thing is to use a larger engine for the "motor" and gear it such that the driven heat pump operates at much higher RPM's than the engine. Of course you also want your heat pump to be a high temperature differential king of Stirling: long and skinny rather than flat with a large diameter.

    I'm just a garage-tinkerer, not an engineer, so I can't steer you to the right set of formulas you'd need to figure out all the considerations you want to figure on paper ahead of time. In general, though, It's probably safe to say you'd have to settle on specific dimensions and calculate the outputs for those particular circumstances.

    I'm not sure what those "heat caps" are, either. Do you have a link to the site where I can look at them?
     
  6. Dec 27, 2005 #5
    Okay, thanks very much. I will probably gear the heat driven stirling to drive the second smaller heat pump. I'll create a long and skinny displacer type stirling for it, or are alpha stirlings better?
    For my cooler, I'm not so ambitious as to make it liquefy air, although I must admit, now that sounds tempting. How fast does it have to be running for it to liquefy air?
    For the heater caps, I got the term from the Stirling Engine enthusiast Koichi Hirata's website, http://www.bekkoame.ne.jp/~khirata/english/make.htm
    That's the URL.
     
  7. Dec 27, 2005 #6
    I have no idea. Senft doesn't mention which configuration they used.
    Here, too, I have no idea. All I can tell you is the obvious: the larger your heat pump the more it will cool because there'll be more surface area on the cold end.
    As far as I can tell the term seems just to refer to the hot side displacer cover. This is usually made separately from some material that can withstand the particular temperature you'll be subjecting it to.
     
  8. Dec 27, 2005 #7
    Cold-generating stirling engines are the heart of all liquid air and liquid nitrogen plants. See if you can visit a Linde factory.

    You probably know that you can buy model stirling engines from the Edmund Scientifics catalog, although I don't think they have regenerators.

    There's a Battelle-spinoff company, Stirling Technology, Kennewick, WA. that markets high-tech engines for space travel and for implanting withing living bodies. The engines have no bearings in them and they are expected to run forever. Some years ago they had one inside of a cow.

    Making cold from sun light would be a LOT easier if you used an absorption refregeration process. It has no moving parts (exept for flowing gas). You see lots of them in camper refrigerators. When I worked at Trane Co. I worked on a 1000-T unit for air conditioning office buildings.
     
  9. Dec 28, 2005 #8
    Thanks.
    The one thing that I think is good for this kind of stirling-stirling cooler is that the hotter the temperature to which you're subjecting the first stirling, the colder the second stirling gets, so this makes it valuable in cooling processes. Like I said in a previous post, say if I have a nuclear power plant that's at extreme temperatures. When using this kind of stirling-stirling cooler, the hotter the inside gets, the colder the cooler gets. That's the ideal property of a cooler isn't it? Can an absorption refrigerator do that?
     
  10. Dec 30, 2005 #9
    You got me there. An absorption refrigerator will get colder as you put in more heat, however, it will have a "coldest" temperature depending upon the gas used in it--like you say.
    A stirling cold generator could have the same limitation, if for example, it were charged with ammonia (gas commonly used in absorption refrigerators) or other common gas. Real stirling cold generators are charged with helium, so in principle, they could get down to almost absolute zero.
    The model stirling engines that you could buy from Edmond Scientific, are charged with air and, therefore, would be limited by liquid-air boiling point. I doubt that that temperature could be even approached, because the engines lack insulation. I could imagine you could get frost, however.
     
  11. Dec 30, 2005 #10
    Alright, thanks for the reply, Paulanddiw.
    I'm doing this cooler for demonstration purposes only, so it need not reach cryogenic temperatures. I'd be VERY happy if the cooler could make ice, which would be more than enough for my demonstration. I could show that a little flame from a low temperature candle makes the cooler not quite achieve 0 degrees, and a large high temperature flame making the stirling-stirling cooler achieve a lower temperature.
     
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