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materials with high thermal expansion

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  • materials with high thermal expansion

    The thread on solar powered shop got me going on this. Thinking about what materials would be good solar energy converters, I wound up learning that gasoline has one of the highest levels of thermal expansion when heated. It would seem that ethyl alcohol, gasoline, and paraffin all have high expansion characteristics with temperature changes. Paraffin (wax) in particular is a good model because it's used in thermostats and endures countless cycles of heating and cooling without failure or change in operating parameters.

    Now I'm wondering how efficiently this process can convert absorbed heat into mechanical motion, and how well the process deals with the high pressures that could result from a closed system containing one of these materials. I wonder for instance how much pressure can be generated by a material before it is caused to deviate from its expansion characteristics as heat is applied. In other words, can you stop it from expanding by containing it within a sealed pressure vessel?

    Wax is one thing, as it goes through a phase change between a liquid and a solid, but ethyl alcohol for example, or gasoline, both remain liquids. They could be pumped into a chamber which is then exposed to concentrated sunlight, made to do work as the temperature rises, then flushed into a radiator to cool as replacement liquid is allowed to take its place. The circulation of the fluid could be passive, with the operating of the valves done electronically. The work done by the expansion of the liquid is stored in some way for later use.

    Thoughts on this anyone?
    I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-

  • #2
    Originally posted by darryl
    can you stop it from expanding by containing it within a sealed pressure vessel?
    Liquids and solids are incompressible under non-extreme circumstances.

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    • #3
      That may be true, but if an extreme torque is available from the expanding fluid, then it should mean that more energy can be extracted from the expansion before the high temperature design limit has been reached, and the fluid cycled. Because this would be a slow process where the exposed fluid is heated to it's limit, you'd have to work with extreme pressures to get the most out of it.

      One thing I'm having a hard time seeing is where the absorbed heat energy goes after the fluid has heated and expanded, done work, then been cycled through the cooling fins. Seems that the volume of fluid would exit at the same high temperature limit that it reached before the cycling valves opened. In that case, all of the heat energy absorbed must still be in the fluid- but it has done a lot of work as it has expanded and torqued over a distance-

      What goes on here- does the temperature of the fluid suddenly drop when the cycle valves open and the heated fluid moves out of the expansion chamber?
      I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-

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      • #4
        Originally posted by darryl
        That may be true, but if an extreme torque is available from the expanding fluid, then it should mean that more energy can be extracted from the expansion before the high temperature design limit has been reached, and the fluid cycled. Because this would be a slow process where the exposed fluid is heated to it's limit, you'd have to work with extreme pressures to get the most out of it.

        One thing I'm having a hard time seeing is where the absorbed heat energy goes after the fluid has heated and expanded, done work, then been cycled through the cooling fins. Seems that the volume of fluid would exit at the same high temperature limit that it reached before the cycling valves opened. In that case, all of the heat energy absorbed must still be in the fluid- but it has done a lot of work as it has expanded and torqued over a distance-

        What goes on here- does the temperature of the fluid suddenly drop when the cycle valves open and the heated fluid moves out of the expansion chamber?
        I don't know anything a out the device you're describing (an engine of some sort?). I'm from the boring bar thread where one fellow described filling boring bars with lead and then torquing them up, but freezing them before that to take advantage of the differences in thermal co-efficients in order to create additional internal pressure.

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        • #5
          Google Naptha Marine Engine, from the 1890's, a launch engine that operated by boiling petrol!

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          • #6
            You can build Sterling cycle engines that will run off the temperature difference between the palm of you hand and ambient air. Several designs of water pump have been based on the Sterling or Manson engine.
            Paul Compton
            www.morini-mania.co.uk
            http://www.youtube.com/user/EVguru

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            • #7
              Take a garbage bag and cut it into strips that are several inches wide. Take each strip and pull it so that it stretches to a point where it markedly resists further stretching. You will break a few.

              The strips so produced now have the long chain molecules of the polymer oriented in the stretching axis. They are very sensitive to temperature and will contract tremendously if heated slightly. The change is entirely reversible so if cooled the strip will lengthen again.

              Then take 10 strips and wind them together to make a thin cord. Make six of these cords and wind them together to make a thick cord. This cord will have a very strong response to change in temperature and will produce a significant amount of torque as well as shortening markedly when heated. The response time is very fast, fast enough to respond to a camera flash once per second.

              A solar powered motor may be made by using the cords as the spokes in a bicycle wheel. Only six spokes are required. The wheel is mounted free to turn and perpendicular to the ground so that one face of the wheel is presented to the sun. Half the wheel is shaded with the dividing point vertical. The sunlight will warm the exposed portion shortening the spokes and causing the rim to become unbalanced. This causes it to turn and as the spokes become shaded they cool and lengthen in a repeating continuous cycle. The amount of torque produced depends on the total weight of the rim as well as the weight lifting capacity of the spokes.
              Free software for calculating bolt circles and similar: Click Here

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              • #8
                You have to watch what type of fluids you use. Like the diesel engine, there's a temperature and pressure point where the fluid will self ignite.

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                • #9
                  The autoignition temperature of various combustible fluids is not related to the volatility or to the flash point of the fluid. Diesel, for example, is required to autoignite and is formulated to have a low autoignition temperature while having a high flash point. Gasoline however must vaporize without help but should not autoignite. As a result gasoline has a flash point of nearly 50 below zero but an autoignition temperature of around 500 degrees F. Diesel has a flash point of between 120 to 200F but an autognition temp of about 410F. Butane makes an excellent working fluid because it is easily liquified and has a high autoignition temperature of 760F.

                  All that said it won't ignite at all unless sufficient oxygen is present.
                  Free software for calculating bolt circles and similar: Click Here

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                  • #10
                    "Twenty years from now you will be more disappointed by the things that you didn't do than by the ones you did."

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                    • #11
                      To get some useable work out you need a reasonable tradeoff between force and distance, or pressure and volume. The problem with the OP is high pressures but little volume change, making it difficult to extract work. For example, a steam boiler works great because the water phase change turning to steam creates a large volume difference, therefore much easier to extract work.

                      Evan's suggestion of making cords from twisted film is an attempt to multiply distance to make it easier to extract work. Another method is bimetal where small expansion produces large deflection.

                      Regarding compressibility: Poisson's ratio is how much a material squirts out sideways when squeezed from the top. Poisson's ratio of 0.5 means no volume change, the material squirts out the side in equal volume to pressed down from above. Many materials have a ratio around 0.3. Rubber is one example of nearly incompressible material. It's ratio is very nearly 0.5. Often assumed to be 0.49 for the purpose of calculation because the numbers don't work at 0.5 because you'll get a "divide by zero" problem. This is one reason rubber is good for metal forming press work. On topic here! I do not know the equivalent for liquid hydrocarbons but I'd be surprised if they would be considered as incompressible as rubber.

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                      • #12
                        After seeing lazlo post I have one comment: Invar is an alloy used for it's low thermal expansion. However, that only applies near room temperature. If you expand the curve to temperature extremes it's overall coefficient is around 15 (don't have the curve in front of me now). The curve has a flat spot around room temperature and makes up for it at extremes.

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                        • #13
                          I would like to see that curve. Are you sure? The Hubble uses Invar struts to avoid length changes in some parts and it undergoes changes from -200 to +200 every orbit.
                          Free software for calculating bolt circles and similar: Click Here

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                          • #14
                            I'll try to find it.

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                            • #15
                              http://www.cartech.com/techarticles.aspx?id=1664

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