Heat engines

Discussion in 'The Projects Forum' started by jpanhalt, Feb 1, 2010.

  1. jpanhalt

    Thread Starter AAC Fanatic!

    Jan 18, 2008
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    I posted this question last night and perhaps used the wrong forum: http://forum.allaboutcircuits.com/showthread.php?t=33661

    What ways are there to convert heat (say 60° to 80°C) into useful mechanical work?

    I need about 300W to run a circulation pump and have essentially an endless supply of the heat.

    My initial thought was a Sterling engine. Then I came across thermoeletric generators capable of 70 to 80 watts each. Are there any other ideas? Does anyone have experience with either (I just joined a Sterling engine site)?

    Electrical supply in the rural area is fairly reliable, and so this project would initially be just for backup.

    This may seem like a double post for which I apologize. A moderator may want to move that post here.

    Thanks. John
     
  2. Wendy

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    Mar 24, 2008
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    Sorry about that, I should have read your post more carefully before flying off at the mouth. I've tried to build a stirling engine with no luck, but ran across a dude using the same configuration with good results.

    The big difference was the pistons. They are critical.
     
  3. jpanhalt

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    I have made toy Stirling engines, but never anything that gave real power. The thought of a completely independent heating system, except for old logs, intrigues me. In the Winter, the temp differential will not be a problem. Even in the Summer, the air very rarely gets over 35°C, and I have a river and lots of dirt for my heat sink, if needed.
     
  4. Bernard

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    Many years ago, 1970s, I read about Mintos' [ sp?] BIG Wheel. Using low grade heat to heat tanks of refrigerant, butane or propane attached to large wheel, 10' or greater??, producing ?horse power at a few RPM but large torque. Tanks on opposite sides were cross connected so that tank on bottom passed thru trough of warm water would force liquid up to top tank, gravity then turned the wheel. Published in Popular Science I think.
     
  5. Bernard

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    I just Googled Minto's Big Wheel & found several hits & video of working model.
     
  6. boriz

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    Jul 16, 2009
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    “What ways are there to convert heat (say 60° to 80°C) into useful mechanical work?”

    The temperature is essentially irrelevant. What’s important is the temperature difference. You can only extract energy from the temperature gradient between two places of different temperature. The larger the difference, the more energy you can extract.

    Most small Stirling engines have fins on the outside of one of the cylinders to exchange heat with the ambient air, thereby cooling the cylinder. A small, efficient Stirling engine can work without a flame, just by applying ice to the cool cylinder. The heat for the hot cylinder being provided by the ambient room temperature.

    The Stirling engine is like a heat pump. It pumps heat from the hot side to the cool side, warming the cool side. As the cool side warms up, the engine goes slower and slower because the temperature difference between the two sides is getting smaller and smaller.

    If you are in the Sahara desert, then you might have trouble getting much power from your 60° source, but in the arctic, you’ll be laughing. One of the reasons Iceland is such a good place for geothermal energy.
     
  7. jpanhalt

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    @boriz My cold side will be room temperature or lower. In the Winter, I can depend on 0 to 10°, i.e., a temperature differential of 50 to 60 °C.

    However, these low temperature differential Stirling engines are massive -- about 4000 # of 1 KW version (http://www.bekkoame.ne.jp/~khirata/academic/kiriki/yama2/index.html).

    I did talk to the principal in an Ohio company. He used to market and plans to re-introduce a high temperature differential engine of 5HP. Current price is $9000.

    Maybe technology will improve that, but for now, I need to read more on thermoelectric conversion. Anyone have experience with that for generating 300 W or more or maybe a Cassini backup I could get real cheap?

    John
     
  8. beenthere

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  9. studiot

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    Unsure as to your ultimate objective.

    Are you trying to generate electricity with your 300W or do you actually want mechanical output?

    You could use some of that rural low boiling point moonshine to run a very ineffiecient expansion engine in closed cycle to turn a turbine.
     
  10. jpanhalt

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    @beenthere: It is actually a good sized river (Black River), but I think the state would object if I put a turbine on it. I don't actually own the river.

    @studiot: 300W is on the large end of motor size that I need to circulate the hot water from the wood burning furnace. Right now, I use electricity, and the cheapest alternative would be to supplement that with a small diesel generator for emergencies. My (day)dream in this case is to make it a totally self-sufficient heating system. In other words, just add wood and the house stays warm. It is my first experience with this type of heating system, and I am pretty impressed with it. Wood plus a little electricity gives me heat and hot water. It is very quiet and constant. Not at all like forced air heating. Of course, if it is going to require a 2-ton, $9000 Stirling engine, I will just go with a conventional back-up generator.

    John
     
  11. boriz

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    Jul 16, 2009
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    Get a few of these to experiment with: http://cgi.ebay.co.uk/100W-TEC-Thermoelectric-Cooler-Peltier-12V-TEC1-12709_W0QQitemZ220548919666QQ

    They work both ways. Electricity in, temperature differential out. Or. Temperature differential in, electricity out. I saw some military device using this principle. A flame powered generator.

    If you can arrange for good cooling on one side and good heating on the other, you’ll probably get some useful power. Suggest hot side pressed up against the stove, cool side with aluminium heatsink in flowing water.
     
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