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Closed cycle rankine cycle engine using alternative fluid.

  1. Jul 16, 2007 #1
    I have a solar heat source that will deliver between 80 and 120C temperatures. I want to power a steam engine with this heat but obviously, when the source is below 100C I won't get any power generated. I have considered instead of using water in the steam engine, ethanol. On the output side of the steam engine I plan to place a condenser, condense the ethanol and pump it back into the system.

    I believe the boiling point of ethanol is 81C. Other than the fact that ethanol is flammable, does anyone see any drawbacks to this approach?
  2. jcsd
  3. Jul 16, 2007 #2


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    The specific heat capacity of Ethanol (and Methanol) is significantly lower than that of water, which means you will have to flow more working fluid per minute to make the same amount of power as might be made with water. This might not really be a problem per say, but at least a design consideration.

    Water: 4.18 J/g*K
    Ethanol: 2.44 J/g*K (42% less than water)
    Methanol: 2.55 J/g*K (39% less than water)


    Ethanol is also easily contaminated by absorbing water. As such, to keep the boiling point as low as possible you will need to keep water sealed away from your system.
    Last edited: Jul 16, 2007
  4. Jul 16, 2007 #3
    As I intend this to be a closed system, the contamination problem shouldn't be an issue ... I stress the word shouldn't ... :rofl:
  5. Jul 17, 2007 #4
    Anyone have the formula to calculate the pressure of ethanol above the boiling point at a given temperature?
  6. Jul 18, 2007 #5


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    Sounds to me like you want a T-S diagram for Ethanol. Unfortunately, I don't have one, but I just found a REALLY cool NIST website that has thermophysical properties of many fluids. Unfortunately, it doesn't have Ethanol, but it does have Methanol and a few other useful fluids (including water).


    You might look into a program called RefProp. It's made by NIST and has a huge amount of thermophysical properties in it (yes, it has properties for both Ethanol and Methanol). You can also make your own T-S or P-V diagrams of any fluid it has. A definite must for anyone that does a lot of thermodynamics.

  7. Jul 19, 2007 #6
    Thanks, that link gave me the info I needed.

    Does anyone have any ideas of something that can easily be converted to be a small steam engine without much effort? I have been considering an automotive air conditioning compressor. I just need something small to build a prototype of this system.
  8. Dec 27, 2008 #7
    if you are using a closed cycle steam engine you can try operating the cold side of your system at 15" mercury vacuum, to get a lower boiling point. and or adding ethanol or methanol to the water, methanol boil's lower than ethanol
  9. Jan 1, 2009 #8
    I would recommend against ethanol. It's flammable and building a steam engine around it that doesn't leak like a sieve is a difficult prospect. A better option is to use refrigerant as your working medium. Not only is this option non flammable, but you can also use off the shelf AC parts for it. There have been some tinkerers who have tried it, but nothing I know about in the small scale has been successful.

    Remember the modern scroll compressor was originally designed as a steam engine.

    I will provide some useful links when I wake up. I originally had a similar idea, but figuring in R&D costs and maintenance I decided to stick to the grid.
  10. Feb 12, 2009 #9
    If you convert an automotive compressor the version that uses a scroll design is probably the best you just have to run in reverse and remove a valve. When you run a scroll compressor in reverse it is then a scroll expander. I have been researching doing this my self. I will either do the scroll expander or will do a rotary sterling engine(not a Wankel engine)I would like to try to use a scroll expander and then have a scroll compressor on the cold side to make it more of a sterling engine design and possibly use LPG (propane) as the gas. It expands allot more than water or air so it could do more work and is easier to more around in the engine, takes less effort. I would like to use helium but once it converts to a gas I don't know if I could ever get it back to a liquid to run the engine. Helium has a very low boiling point. I want to run my engine on solar probably with a Fresnel lens .
  11. Feb 12, 2009 #10
    People who have built similar things have said not to use water because water will destroy the bearings and since ethanol will be part water you will have the same problem.
  12. Feb 28, 2009 #11


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    Why not just use a stirling cycle engine instead? If you're skirting close to the boiling point, steam engines tend to have problems with condensation.

    Carnot himself basically said that there's not a whole lot of benefit to using fluids other than air and water.
  13. May 27, 2009 #12
    I have collected I think all the pieces to build a system in this style. I will be posting videos soon. I think assembly will begin tomorrow.
  14. May 28, 2009 #13
    I am looking for a pump (possibly hand driven) that will move R134a from one collector to another. The inlet will be 60 psi liquid and the second chamber will be 120 psi gas.

    For now I dont need a lot of volume I am just building a small model. Appx 16 ounces of R134a will be the working fluid.

    I think a simple pump that could carry the liquid to the second chamber and dump it out would work fine but I am not sure I know what the name of that pump would be.

    Any help would be greatly appreciated.

    One final note. I have been visiting this forum for quit some time and I am very inspired from all the knowledge that is bing shared.
  15. May 29, 2009 #14
    If you are going from 60psi liquid to 120 psi gas then I'm assuming there is a temperature differential between these two tanks. Is this for feeding refrigerant into your boiler (evaporator)?

    One solution I came up with is using multiple boilers and then alternating which one you supply heat to. When the boiler is cold the refrigerant drains right into it, then when it's full you apply heat and start supplying with it. While this is happening the other boiler cools off and begins to receive feed. When the first boiler is out you take the heat off it and the cycle repeats. No mechanical pump needed, only a few valves that can be automated with level control and a few check valves.

    Remember superheat is your friend, assuming that it's bellow the breakdown point of your refrigerant.
  16. May 31, 2009 #15
    I was thinking of something like this. I did not think of 2 (boilers) I was thinking of reversing the flow.

    I will put some thought into your suggestion. It definitely sounds like it has some potential.

    I only need to decide on the pump and I a boiler design and I am ready for test 1. I was hoping to have it ready today.

    One other design I may use for the test is. My current system is going to be a little easier. The cold low pressure liquid will be stored above the heated portion. So i will be able to open and close a couple valves and allow the liquid to gravity flow into the boiler.

    Also I have been able to make 14 volts with 60 psi and appx 40 cfm. I am not sure on the amps just yet. I am having problems with my voltage tester. Also this is with compressed air. I do not know the difference in my air motor output when it is powered with a refrigerant.

    I will keep everyone updated.
  17. May 31, 2009 #16
    A single boiler with reversing flow could work, but you would then have your generator stoping each time you needed to feed. Also what is your heat supply? If you are using an oil flow system to transfer the heat then you have to deal with it heating up while you are filling the boiler.

    Annother option is to use a piston type AC compressor. I believe (don't quote me on this!) that they use some sort of check valve system, so they can be used just as easily as a pump vs a compressor, with the added benifit that they are already designed for refrigerant.

    I'm starting to think about this as a project again, but my big thing is having the whole system automated for little operator action. I do not plan on geting up before dawn to start the system then monitoring it every hour untill sunset. I already do enough of that at work.

    I wonder how difficult it would be to modify a hermaticaly sealed scroll compressor from a home AC unit to use it as a generator. Having a hermaticaly sealed gen set would be a real plus.
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