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Wood boiler to Stirling engine, generate electricity with wood. Ideas?

Post in 'The Boiler Room - Wood Boilers and Furnaces' started by jrlogan1, Mar 31, 2008.

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  1. jrlogan1

    jrlogan1 New Member

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    There are a few companies that are close to making a small stirling engine for consumer use, around 6kw. In case you don't know what a stirling is, it's a motor that runs off of heat. It doesn't require fuel otherwise, and they have been around for centuries. See http://travel.howstuffworks.com/stirling-engine.htm. Currently they are used in some submarines to generate electricity off of the nuclear reactor's heat, and also as backup generators at power plants that run off of natural gas.

    It looks like the engines will need a temperature around 600 degrees fahrenheit to get any real power. My idea is to run a high temperature coil into the firebox of my boiler and run a stirling which will turn a big truck alternator and generate 12V power. I will invert this and run it into the grid just as folks who generate solar do. Coil will have to be filled with oil or other non-volatile liquid. If I can generate 5kw, that's 3600 kw-h per month, enough even for a large home.

    I just wish there was a way to do this with a gasifier, but I don't see how. Since they only burn for a couple hours, you're then limited to 200 degree in the thermal storage, which is too low. I'm going to be buying my boiler soon, and I think because of this I'm going to be forced to get a standard type as opposed to a gasification unit.

    Ideas?

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  2. Kemer

    Kemer Member

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    I'm very interested in this .Once my boiler is up and running ,this will be my next project of interest
  3. MarcM

    MarcM New Member

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    Off the top of my head-

    Be sure you know what you're doing when modifying the fire box of any wood fired unit...
    Stirling engines were invented in the early 1800's... just coming up on 200 years old...
    Remember they operate off a temperature differential- i.e. the cold side temperature is as important as the hot side...
    Size your generator appropriately knowing the torque out put on Stirlings is relatively low... I'd seriously consider a gear reduction...

    Have you considered a Seton style gasifier with the hot side as part of the fire box (make a hole in the refractory for it)? Just thinking out loud there, I don't own one nor do I know anything about these engines and their configuration. Might consider cooling the cold side with a groundwater loop or outside air.
  4. Eric Johnson

    Eric Johnson Mod Emeritus

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    Welcome to the Boiler Room, jr.

    For this theoretical application, I don't see any difference between a gasifier and a conventional wood boiler, say an OWB. How about a Garn?

    There's really no difference between firing intervals and system water temps in either conventional wood-fired boilers or gasifiers that I'm aware of.

    What is there about gasifiers that you think would make them impractical to drive a Sterling engine? I think that if you bought a big gasifier like an EKO 60 and rerouted some of the tubes into the Sterling before sending it up the stack, you'd get temps in excess of 1,000 degrees F. That way you'd be generating home heat and electricity at the same time.

    Gasifiers like nothing better than a consistent load. If you could divert excess electrical production back into the grid or into a battery bank, all you would need to do would be keep the boiler full of wood and clean out the ashes periodically. You'd need to load it probably every 4 to 6 hours, I'm guessing for, in the case of the EKO 60, about 200 K btu/hour.

    Of course, maybe I'm missing something.
  5. Sting

    Sting Feeling the Heat

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  6. MarcM

    MarcM New Member

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  7. Sting

    Sting Feeling the Heat

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    To qoute a friend smarter than I am regarding off grid generation:

    I suspect that it would cost more to generate electricity with this device than the ten cents per kwh I currently pay to the power company, but I couldn't determine that from the website.

    I like it, and I wish I owned one. It seems to be a better option than a gas or diesel generator.
  8. Eric Johnson

    Eric Johnson Mod Emeritus

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    Civilization as we know it comes to a grinding halt when the grid goes down for any length of time. It would be a big load off my mind to know that I could keep the house warm and maybe even some lights and communications equipment going with nothing but wood, if necessary. Then all I'd have to do is modify my chain saw to run on wood alcohol.
  9. Sting

    Sting Feeling the Heat

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    I think that modification changes the design of the machine enough that we can call it and axe!
  10. Bob Rohr

    Bob Rohr Minister of Fire

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    6 kw would be 20,400 BTU/hr I'm not sure you could "find' that much excess energy in your boiler?

    There is no free energy, you would need to fuel that output with wood burned at a less than 80% efficiency and you would be taking that energy from the heating side of the boiler. Possibly pulling your boiler out of the gasification mode?

    I saw a Baxi mod con boiler a while back in Germany that had about a 1KW Sterling.

    http://thatsgreen.blogspot.com/2007/12/baxi-unveils-micro-chp-heating-homes.html


    hr
  11. jrlogan1

    jrlogan1 New Member

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    The stirling will need something around 600 degrees to run with any real power. This means that in either unit, a fire will have to be actively burning to generate the electricity. You won't be able to run the stirling from thermal storage (200 degrees). Sure, a gasification could get me 1000 degree temps if I kept it burning, but they burn so quickly that this is impractical. I'd have to load it constantly. The only feasible way I can see to use a gasifier is to buy a much larger stirling that can generate 24 hours of electricity in a 2 hour period and run the two full tilt for a couple hours a day (maybe 2 wouldn't be enough, but you get the point). This would mean much more expense in the stirling, controls, etc.

    With an OWB, you can load it once a day and keep a constant fire going, making constant 600 degrees much more convenient and practical, and keep a smaller stirling running 24/7.

    At least this is the conclusion I've come to. I don't want a system that will need constant babysitting.

    For the cold side, I will initially use outside air, but I am considering do a groundwater loop in the future for my air conditioning in the summer, so I could use that as well.

    Thanks for the links to the CHP units. I've seen some of these that are currently being sold for boats. Notice they use either natural gas or pellets, something that can be fed automatically to maintain constant temperature for the stirling.
  12. renewablejohn

    renewablejohn Member

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    If your gasifier can handle thermal oil instead of water as the heat transfer fluid then you have the facility to use a traditional steam engine at 150PSI to generate your electricity. You would then need a steam evaporator which converts thermal oil at 260C into steam at 150 PSI. We are doing this on a larger scale with two 300kw steam engines but could be scaled down to possibly 6kw.
  13. webbie

    webbie Seasoned Moderator Staff Member

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    The first person to figure out a low cost wood burning design with an electrical generator is going to have a lot of sales! As easy as this sounds, there are a lot of barriers. Steam engines are inefficient. I'm not sure of the conversion potential of a Stirling. IC engines wear out quickly. It would seem, at first glance, that true gasifications and an IC engine would work out best, but there are many problems involved.....not to make it work, but to get longevity, etc.

    There are actually some solid state modules which can soak up radiant heat and produce a small amount of electric, but these are expensive also. The idea of such a device would eliminate a lot of complexity.

    This whole issue sounds deceptively simply - but is actually very complex. A successful unit would have to have a decent efficiency OR be able to use the other heat for the house.
  14. Eric Johnson

    Eric Johnson Mod Emeritus

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    This is a general area which I obviously know very little about, but I do think that you could pretty easily build a gasifier that would put out a consistent level of hot gas and burn for an extended period. If I block off one of the nozzles in my EKO 60, for example, I get longer burns and lower heat output, but good gasification for the duration of the (longer) burn. If you mated an EKO 80 firebox to an EKO 18, I bet with a little modification you could get a gasification boiler that would run in excess of 12 hours, and probably closer to 24. Or, go with the Seton design and heat a large refractory mass that would hold higher temps for longer. Or some combination of the two.

    Bottom line, I think it would be a mistake to go with a primary heat source that is so much less efficient than a gasifier. There are much greater longterm gains with gasification.

    And, as I said earlier, I don't see why you couldn't divert some of the heat for space heating or DHW. Diverting any excess into water storage would allow a more consistent delivery of energy to the Stirling. Of course, as Craig points out, if this was as easy as it sounds, somebody would already be getting rich doing it. I have heard that Fred Seton has some sort of electrical generation option for his boilers, so who knows?
  15. MarcM

    MarcM New Member

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    http://www.johnsonems.com/jhtec.html

    If these have the potential to reach the effeciencies he claims, they'd be great for this application... and assuming they're easily scalable, may have much further reaching impacts than just wood heat to electricity conversion.
  16. renewablejohn

    renewablejohn Member

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    We use the excess heat for district heating therefore overall efficiency is very high.

    On a smaller scale 6kw electric would produce approx 24kw of heating so could be used for home heating with the right accumulator.

    As for longevity our engines were built in 1937
  17. Willman

    Willman Minister of Fire

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  18. renewablejohn

    renewablejohn Member

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    Willman

    I think a woodgun gasifier can produce 15 PSI so you would be able to connect to this engine and produce electric.

    Please be careful steam pressure can kill and humans burn very easily with steam. Do not attempt this without proper safety valves
  19. Eric Johnson

    Eric Johnson Mod Emeritus

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    How do you get steam out of a Wood Gun?
  20. Bob Rohr

    Bob Rohr Minister of Fire

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    I visited Sensible Steam a few years back in nearby Branson missouri. They build small 1.5KW steam powered engin/gen-sets and boilers. They also had a cool steam powered boat on the lake, maybe a 15- 20 footer

    I took an old homemade steam boiler and engine my Dad built many years ago. It was a copper tube boiler, held about a gallon of water. Skip warned me that amount of steam was about the same as a stick on dynamite.

    I've never fired it off since then :) It was my high school science project 30 some years ago.

    Many of the steam shows run old stuff on compressed air instead of steam these days.

    www.mikebrownsolutions.com/stmpwr.htm

    Here is a pic of the boiler and engine my dad built, I don't even know when :)

    hr

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  21. renewablejohn

    renewablejohn Member

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    Eric

    They apparently have a steam coil as a standard extra but when asked they said is was only good for 15PSI and I was wanting 150PSI
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