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  • expandable fluids

    Some time ago I entertained the idea of making a solar powered generator of sorts, using a liquid with high thermal expansion. It seems one of the best liquids is gasoline- another is kerosene. Gasoline seems to have a shelf life, ie it goes rotten after some storage time. Does kero do this as well? Kerosene also has a comparatively high ratio of expansion with heat, and might be suitable in such an 'engine'. On the minus side, these are fuels and highly combustible, but may be ok in a system void of any other materials such as oxygen.

    The idea as I envisioned it some time ago is to circulate the fluid in bursts, allowing the heated volume to expand and do work, then cycle through to cooling fins. Not sure of the best way to utilize the fluid- probably very close to the point of focus, where it's absorbing the heat of concentrated sunlight. Just a slight detail to be worked out

    At any rate, my main question is whether kerosene is stable over time, or does it also suffer degradation?

    I'm still having trouble envisioning the energy flow in such a system. The fluid held in a chamber at the focus point of a solar reflector will absorb heat, and the fluid will expand. I think the fluid would continue to absorb heat as it expands, until a temperature is reached at which it is no longer expanding because the rate of heat loss matches the heat input. But what happens when the fluid is restricted, as it would be where there's some means of using the expansion to advantage?

    You would be drawing off some energy in the 'expansion engine', but that energy has a source- focused sunlight. It is used as heat, which causes the working fluid to expand, absorbing more heat because it has larger volume now- but you could just as easily not use the pressure of expansion, simply letting the fluid cycle through the cooling system. How can the temperature of the 'exhausted' fluid be less if it's pushing on a piston, as opposed to simply being allowed to bypass through to the cooling fins? I'm just not getting it- where do you measure the loss of heat as it's being converted to force?
    Last edited by darryl; 04-15-2013, 10:05 PM.
    I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-

  • #2
    What about freon or ammonia? They're both used in AC&R units and heat pumps.

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    • #3
      I thought about using a gas of some kind, even propane, but it seems like a fluid would give more output with less loss- part of which is in operating the mechanism at a slow speed rather than high speed. Low speed/high pressure would seem to me at least to be more efficient.
      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
        I don't think you will get much energy out unless you have a phase change in your working fluid, in other words, add enough heat to boil the liquid into a gas. Do a Google search on "naptha engines"

        John

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        • #5
          What JSR said... Of course, naptha engines did occasionally blow up, which lessened their appeal.
          ----------
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          Everybody is ignorant, only on different subjects. -- Will Rogers
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          Law of Logical Argument - Anything is possible if you don't know what you are talking about.
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          • #6
            +1 It's the phase change that does the magic.
            For just a little more, you can do it yourself!

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            • #7
              Apparently a liquid expansion engine can have an efficiency of 27%. That's pretty good when you compare it to affordable solar cells.

              Of note is that liquid co2 has the capability of expanding by about 10% over a temperature range of 10C to 20C. That's a pretty narrow range of temperature change, along with a substantial displacement capability. Co2 also cannot remain a liquid regardless of pressure if it's above 31C or so. Under well-constrained conditions it could be the ideal heat to force medium, but it doesn't appear to be practical to me anyway under what might be typical hsm conditions.

              I read somewhere that gasoline and kerosene will expand about 30%, but it takes a temperature differential of up to about 100 F- could be wrong with these figures, it's just from memory. These liquids don't vaporize at such a low temperature, so they might be more appropriate as a working fluid.

              I understand the use of phase change in materials. We have steam, which is rated at something under 20% efficiency in an expansion engine. I don't recall the figures for many other gasses, but propane beats many of the refrigerant gasses in cooling ability. Hmm, I should check into liquid propane- keeping it liquid is possible up to fairly high temps with moderate pressures.
              I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-

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              • #8
                Ha. Looks like I've asked about this a couple times before. The only thing I uncovered this time in my searching is that turpentine also has a relatively high thermal expansion ratio.

                One article I read suggests that pressures as high as 15,000 psi are used, and one design used a piston diameter of 5/8 inch. This makes sense, as you are not using a phase change to extract power from the heated liquid. Theoretically, if the liquid is not compressible, there would be much more pressure than that available. The efficiency of the process would be lessened by the springiness inherent in the materials used to construct the engine, amongst other things. There would then seem to be an optimum pressure to design for in an engine operating from heat-expanded working liquids. A high proportion of otherwise extractable energy could be lost past seals, and in bearing friction, etc.

                In the above example, with a pressure of 15000 psi and a 5/8 diameter piston, the actual pressure on the piston would be just under 5000 lbs, which doesn't seem unrealistic to a great extent- I think it could be designed for. I'm just wondering now if that figure of 27% for efficiency is realistically attainable.

                And, in a report which I've just read, Sharp and another company have hit 43.5% efficiency with solar cells-
                Last edited by darryl; 04-16-2013, 01:53 AM.
                I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-

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                • #9
                  Why would you run the liquid through cooling fins after the expansion? Don't you have to replace that heat loss on the next cycle?

                  Phil

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                  • #10
                    The fluid would need to be cooled so it would contract. When fed into the 'reaction chamber', it would need to be able to expand to a large degree in order to do the work. The cooler it can start off, the better. When the concentrated sunlight hits it, you'd want to be able to absorb as much of that as possible so the constrained fluid would expand as much as you can get it to.

                    If you could upgrade the heat that you waste to a point where it can give back a sort of fast 'pre-heat' to the fluid in the chamber, then you wouldn't need as much sunlight to push the expansion all the way. It would be a balancing act- you'd get the fluid into the chamber as cool as possible, then it goes through the widest temperature swing and expands the most.

                    Still trying to wrap my head around some of what's happening- basically, a valve opens and a charge of cooler fluid pushes into the chamber, pushing out the hotter fluid. While this is happening, the piston is being returned to the start of its stroke. Then the valves close and the quantity of fluid in the reaction chamber begins to heat and expand. It can go nowhere except to push on the piston. When that stroke is complete, the valves open again and the cycle repeats. The hotter fluid goes through a heat exchanger and is ready to go back into the chamber.

                    What I'm really not getting (and it's probably fairly simple) is how the heat gets traded for the push. On the face of it, the heated fluid is a volume of a certain mass- expanded or otherwise- and it will get hotter the more energy it absorbs from the solar concentrator. As it heats and expands, it's still the same amount of fluid. But it has done work by pushing the piston. If there was little resistance behind the piston, then little work has been done, but if there was lots of resistance, more work will have been done. None of this changes the mass of the fluid, which to me will have gotten up to the same temperature either way. The temperature of the fluid can't drop as it expands because then it wouldn't expand. Where does the energy for the push come from?

                    You can do the experiment mentally. Enclose a mass of something that will expand with heat, then try to stop it expanding as you heat it. It will develop an enormous pressure unless it has an escape route. On one hand, it would seem that it should be just as hot escaping by force as it would be escaping easily. It's the same mass of fluid, reaching a temperature determined by how long it remains in the heating area, how strong is the solar influx, and how fast the structure draws the heat out of it.

                    I know there' an easy answer- I'm just not seeing it. I don't know why-
                    I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-

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                    • #11
                      When the liquid expands against little resistance it expands more than if it is pushing agains a resistance. Liquids are compressible. So for ther same expansion you need more heat input (higher temperature).

                      How much heat you waste by cooling the liquid depends on the specific heat of the liquid vesus how much energy actually goes into the expansion. I think you are on a hiding to nothing if you do not use the "waste" heat.

                      Phil

                      Originally posted by darryl View Post
                      What I'm really not getting (and it's probably fairly simple) is how the heat gets traded for the push. On the face of it, the heated fluid is a volume of a certain mass- expanded or otherwise- and it will get hotter the more energy it absorbs from the solar concentrator. As it heats and expands, it's still the same amount of fluid. But it has done work by pushing the piston. If there was little resistance behind the piston, then little work has been done, but if there was lots of resistance, more work will have been done. None of this changes the mass of the fluid, which to me will have gotten up to the same temperature either way. The temperature of the fluid can't drop as it expands because then it wouldn't expand. Where does the energy for the push come from?

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                      • #12
                        This whole thread has the "earmarks" of a bunch of "Mechanics" trying to do
                        "Doctorial" level Physics. :-)
                        ...lew...

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                        • #13
                          Many threads on this forum follow in a similar vein. Lack of knowledge was, and never is, a reason not to have a go. Have you ever read the story of the Wright Brothers, if you did your clearly weren't paying attention. Actually high school physics is more than enough, but you may have a higher need in order to keep up. If you think this thread is inappropriate you probably should seek your kicks elsewhere

                          Phil

                          Originally posted by Lew Hartswick View Post
                          This whole thread has the "earmarks" of a bunch of "Mechanics" trying to do
                          "Doctorial" level Physics. :-)
                          ...lew...

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                          • #14
                            Malone Engine

                            This is a provocative thread. My first reaction was that it is kind of nuts to use a liquid as the working fluid in a heat engine. If the idea has merit, why hasn't it been tried before?

                            But thinking about it, I could not dismiss the idea on the basis of efficiency, or lack thereof. Consider the Stirling Cycle, which achieves the maximum possible efficiency for any cycle operating between a high temperature heat source and a low temperature heat sink. The working fluid of Stirling engine is typically air or helium. But there is no reason why a Stirling engine cannot use a liquid as the working fluid.

                            In fact, in the 1930s there was a Stirling machine which used a liquid working fluid: the Malone Engine described here: http://en.wikipedia.org/wiki/Malone_engine. The efficiency was reported to be 27%.

                            The third reference in that Wiki article make interesting reading. I think it would be fun to make a Malone Engine, just to see it run!
                            Last edited by aostling; 04-17-2013, 04:44 PM.
                            Allan Ostling

                            Phoenix, Arizona

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                            • #15
                              To answer your question, kerosene is extremely stable. It will last for years even in hot conditions as long as the system is sealed.
                              Free software for calculating bolt circles and similar: Click Here

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