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Will this be more efficient than the Sterling engine?

  1. Jul 12, 2016 #1
    Hi everyone,I am hoping to discuss the value of a design.
    The original idea was intended to receive the thermal energy from the exhaust from an internal combustion (IC) engine and produce additional power. The working principle is much like a Stirling engine except that instead of a regenerator it relies on a phase change to create a substantial efficiency advantage over the Stirling design. Water as an example will expand about 1600 times its volume when boiled. Water vapor, nitrogen and all other gases change at a linear rate when the temperature changes. Therefore a chamber with a very small quantity of water at its phase change equilibrium will require a fraction of the energy required to produce the same pressure change as is common in a Stirling engine.

    The engine is a closed cylinder with many sliding vanes creating chambers that increase and decrease in volume. Other than a drive shaft and perhaps a valve stem to add a liquid and pressurize the unit there is no entrance or exit for the working fluid (it is not a water wheel). The external radius will have attached heat exchange surfaces or insulation as is required to handle the intended thermal exchange. The activity in the circular cycle of each chamber can be visualized when separated into quadrants (red outline), including two areas where the chamber volume changes very little, one large (3) and one small (1) and two where the chamber volume is either increasing (2) or decreasing (4) when the rotor is turned (clockwise).

    Before starting a cylinder allowing the conduction transfer of thermal energy though the circular wall of the outside circumference is closed. Confined in the formed chambers of the closed cylinder is a specified pressure of gas and a less than 3% by volume quantity of working phase change liquid. A gas such as helium is specified for its heat transfer coefficient and the liquid is specified for its boiling point at the pressure and temperature range anticipated.

    The cycle starts by transferring thermal energy into the chambers adjacent to the quadrant having the small average chamber volume (1). The transferred thermal energy both expands the gas contained and causes the liquid contained to boil (change phase), increasing the pressure. The increased pressure pushes the chamber through the next quadrant, where the volume is increasing, (2) turning the rotor. As each chamber moves though the quadrant having the large average volume (3), the thermal energy is transferred out, causing the gas to contract and the boiled vapor to condensate, reducing the pressure contained. The reduced pressure will now cause the described chambers to move through the quadrant having a declining volume, (4) then back to the start, carrying with it the quantity of condensate ready to be expanded to vapor again.
    Thank you in advance for contributing to this idea. Thermal exchange.png
     
  2. jcsd
  3. Jul 12, 2016 #2

    russ_watters

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    Well, I think if you include the heat of vaporization, you will find the efficiency is lower, not higher. However if having a liquid enables you to recover more energy due to better heat transfer you still might see an overall benefit.
     
  4. Jul 12, 2016 #3

    jack action

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    I'm not sure if you mean that all your fluid is liquid when in phase 1, but if that is the case, it cannot be in phase 3: Liquid is incompressible.

    Even if the fluid is only partially liquid, the biggest challenge of this type of design is proper lubrication/sealing of the sliders: Oil and water don't mix well and too much water tends to remove the needed oil film.
     
  5. Jul 12, 2016 #4
    Reminds me of a Wankel Engine.
     
  6. Jul 12, 2016 #5

    Mech_Engineer

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    It seems to me the O.P. is possibly describing a Rankine Cycle steam engine. Calculation of efficiency for a Rankine Cycle vs. Stirling Cycle can be done by anyone with some education/understanding of thermodynamics.

    https://en.wikipedia.org/wiki/Rankine_cycle
    The Carnot efficiency is a useful tool for comparing theoretical maximum efficiency between two differing processes, all you need is the high and low temps for the process. Take for example the Rankine cycle's max temperature of about 565 °C vs. its low temperature of 30 °C; this results in an estimated Carnot efficiency of 63%. Still, to compare two cycles apples-to-apples you have to look at their individual parameters and do some old-fashioned calculation.
     
  7. Jul 13, 2016 #6

    billy_joule

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    It's a rotary vane motor, they're found in many air tools. Also used as pumps.
    https://en.m.wikipedia.org/wiki/Rotary_vane_pump

    There are a few experimental (organic) Rankine cycle systems that use them as expanders (turbines). The papers can be found online.

    A die grinder with a rotary vane motor can be bought for less than 15USD so a rough prototype would be cheap. I can't see it lasting long, most expanders are damaged by condensation, let alone boiling.
     
  8. Jul 13, 2016 #7

    RonL

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    If you compare your illustration to the Rotary_vane_pump linked by billy_joule, you can see the rotation is produced by a flow through process. My mind sees your pump to need power applied to the shaft to get rotation, the sliding vanes need some stationary or high pressure to push against.
    I just can't quite see the rotation I think you are implying. :)

    RonL
     
  9. Jul 13, 2016 #8

    jack action

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    Isn't this true with any combustion engine?
     
  10. Jul 13, 2016 #9

    RonL

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    My air motors have flat plate surfaces that seal the sliding vanes into individual chambers, until each vane crosses a port of intake or exhaust, I can't grasp the red line portion of the OP drawing, when the vanes move past the dotted red line what happens in each chamber ?
    Did I misunderstand that this is a power provider design, driven by waste heat ? I'll go back and look again. :smile:
     
  11. Jul 13, 2016 #10
    Thank you for this, your response is great.
    Please let me try to describe the cycle with some probable numbers to answer some of your questions. Think of it as a heat exchanger with a counter flow middle man. The cycle could be viewed as a self-contained organic Rankine cycle, without most of the ancillary equipment. If it were to be installed on a thermal solar location with a lower temperature a liquid like ammonia or silicone oil will be installed in place of the water.

    http://www.greencarcongress.com/2014/06/20140604-orc.html

    https://en.wikipedia.org/wiki/Organic_Rankine_cycle

    In my drawing the red lines only indicate how the circular path can be understood during the cycle of the 12 chambers moving clockwise. The engine needs to be warmed up before it functions properly. The sliding vanes and rotor are lubricated with graphite. The chambers are more like a pressure cooker than a tea kettle and will be pre filled with 10 bar of helium and 1% of the volume is water. If too much water is present the absorbed thermal energy and be moved by the sliding vane much like a squeegee, reducing the efficiency. Most Stirling engines are pressurized with helium or hydrogen for the superior heat transfer coefficient.

    (1) As a sample chamber enters the first of four quadrants at the 4:30 point of the drawing and will start to acquire thermal energy with a conduction transfer. Heat from a source like the exhaust header of an IC engine is channeled through the adjacent “hot in” passage of area 1. With 10 bar of helium pressure present the water contained will start to change phase (boil) at about 180°C. A sample chamber enters the first of the four areas at 200°C and is able to acquire 50°C, bringing the temperature up to 250°C, with a few milliliters of water remaining. The additional energy will increase the pressure of the saturated steam from 14.5 bar to 38.7 bar. It is the 24.2 bar of additional pressure that will push the chamber through to (2) the second area where the volume is increasing, allowing the pressure to expand, similar to the Wankel engine after the combustion event. (3) When the chamber enters the third area starting at around the 10:30 point of the cycle and continuing to the 1:30 area of the drawing it will transfer the 50°C of energy out to the adjacent “cold in” passage, causing the pressure to drop. (4) Finally the chamber will move through the fourth area, bringing the condensate from the previously saturated steam with it.
     
  12. Jul 13, 2016 #11

    jack action

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    It is a closed system with an external combustion («Heat Exchanger»). There is also an external «Heat Sink» to cool down the enclosed fluid. It is some kind of rotating Stirling engine with that regard:

     
  13. Jul 13, 2016 #12

    RonL

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    Do you have a side view ? my mind can't comprehend what keeps the quadrants from equalizing when a black vane section moves past a red dotted line of the quadrant. I think I know exactly what your thoughts are (and think you have a good idea) the mechanics just seem to elude me at this point.
    Best wishes in your success as per this idea.

    RonL
     
  14. Jul 13, 2016 #13

    Nidum

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    This contraption would be very unlikely to work at all let alone provide any useful energy recovery from IC engine exhaust gas .

    I see a long list of difficulties but let us start with the problem which RonL asks about :

    Where are there any differences of area and / or differences of pressure in the vane system which would generate any torque on the rotor ?
     
  15. Jul 13, 2016 #14
    Here is an 3d cutaway image with red pencil lines indicating the 4 area of the cycle the thermal exchange passages removed.and one without the pencil lines.
    12.png
     
  16. Jul 13, 2016 #15
    The torque comes from the 24.2 bar increase in pressure created by the saturated steam produced by increasing the temperature in the chamber as it is moved past the heated sidewall. The pressure expands similar to the power event in a Wankel Engine moving the rotor forward. The sidewall is heated by the adjacent passage where the hot exhaust gas is being channeled, before being discarded.
     
  17. Jul 13, 2016 #16

    RonL

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    Sorry I can't see it. Unless there is a high pressure input that pushes each vane toward a low pressure outlet, there will be no movement of the rotor. This is the same as lifting oneself by pulling up on the bootstraps. The shape of the chamber being a wedge will not move the front vane forward if the same pressure is pushing back on the rear vane.
     
  18. Jul 13, 2016 #17
    From Post #6
    Since a prototype would be so cheap, how about just build one to avoid all this "I suppose", non-productive chatter. Hot side could be heated with a propane torch for proof of concept. Someone else can come up with a cheap-and-dirty way for the cold side. (afterthought: A water spray perhaps?)
    Actually, I see how the temp., therefore the pressure, could be higher at the 7-o'clock position than at the 5-o'clock position. Note that the image in the OP shows a counter-flow hot-side heat exchanger.
     
  19. Jul 13, 2016 #18

    jack action

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    Actually it will because the area are different and force is pressure times area, so the greatest area gives the greatest force. The vane motor is a proven concept in both pneumatics and hydraulics (http://enginemechanics.tpub.com/14105/css/Vane-Type-Motor-161.htm).
     
  20. Jul 14, 2016 #19

    Nidum

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    The action of a vane motor depends on a through flow of fluid acting on net differential areas .

    This engine has no through flow of fluid and there are no net differential areas .This engine is symmetrical about a vertical centre line . Forces on vanes causing clockwise motion are balanced by forces on vanes causing anti clockwise motion .
     
  21. Jul 14, 2016 #20
    Correct the two sides are the same on a vertical line. Steam is clear, therefore what you see coming from a boiling kettle is condensate. A graph of steam pressure shows a curved line because the vapor is changing the pressure at one rate, while the water becoming vapor effects the pressure at a different rate. When running, a chamber at 3 o clock will have more condensate in the form of water water droplets than a chamber at the 9 o clock position.
     
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