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Heat pumps and shape-memory alloys motor

  1. Jan 7, 2006 #1
    Heat pumps and shape-memory alloys motor....


    Would it be possible to get some electricity from a heat pump and shape-memory alloys motor/engins with a generator?

    A heat pump use 1 kW and give you 3-5 kW heat.

    Would it be possible to use shape-memory alloys motor and a generator to generate some electricity.

    Or is the efficiency too low?


    Or what about a solar parabol.

    The efficiency of electric solar panels is at the most 20%.

    Can you get more efficiency when using a shape-memory alloys motor?

    Regards Magi
    Last edited: Jan 7, 2006
  2. jcsd
  3. Jan 7, 2006 #2


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    The proposed motor seems to be a variation on a Sterling engine, or a K-cycle with NiTiNOL replacing the pistons. It's certainly workable, but I don't know what the efficiency would be compared to a regular Sterling.
  4. Jan 7, 2006 #3
    Are you talking about a perpetual motion device? Getting electricity out with no electricity input? Are you saying you would have a heat pump driven by a shape memory engine in a closed loop, but in which case the shape memory engine produces more mechanical energy than is required by the heat pump?
  5. Jan 7, 2006 #4


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    :surprised I hope that's not what he meant! It's possible, I guess, but I assumed that he meant that the thing produced 3-5kW from an input of 4-6kW (1kW wastage).
  6. Jan 7, 2006 #5
    Not a perpetual motion device.

    I were just asked if shape-memory alloys could be used in any way.
  7. Jan 7, 2006 #6
    I'm just wondering why you are considering using the thermal energy from a heat pump? What's the point of using electricity to generate some thermal energy? That's the part that had me confused. Some shape memory engines could run off the small temerpature differential of a heat pump, but what's the point of doing so?
  8. Jan 7, 2006 #7


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    Agreed. There is no point and it is not engineerably sensible to convert electricity into heat and then electricity again. The conversion of heat into any form of mechanical energy (and electricity is an energy with an extraordinary internal available amount of mechanical energy) is very inefficient.
  9. Jan 7, 2006 #8
    A guy just asked me a simpel question once.

    -I take 1 kW and feed it in to a heat engine and get heat out of it around 3-4 kW. Why cant I transform the heat to energy to get a perpetual motion device.

    I know about the carnot cycle.

    Then a guy asked what is possible to do with shape-memory alloys.

    A sterling engine has a efficiency around 30 % and a electric solar panel have 20- 30 % so what efficiency have a shape-memory alloys engine.

    And I just wanted to see what people said about this..... :)
  10. Jan 7, 2006 #9
    It sounds like you're still fuzzy about the issue. A Stirling or any other engine does not have a definite efficiency such as 30%. It totally depends on the temperature differential between thermal energy source and thermal energy sink. So if the Stirling engine is receiving thermal energy from a heat pump with a coefficient of performance of 5, then the heat engine will have an efficiency of no greater than 20% or 1/5th. If the Stirling engine is receiving thermal energy from a heat pump with a coefficient of performance of 10, then the heat engine will have an efficiency of no greater than 10% or 1/10th.
  11. Mar 3, 2006 #10
    Heat Pump COP and perpetual motion machine

    I'm a little fuzzy on the efficiency calculations. Wouldn't a heat pump with a COP of 5 put out five times the energy it requires to run? In that case it could be coupled to a Sterling engine with, say 25% efficiency for a net efficiency of 125% (enough for "perpetual motion"). The heat pump could be run by the shaft output of the Sterling, avoiding electircal conversion losses.
  12. Mar 4, 2006 #11


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    Ben is correct.

    Real heat pumps don't have efficiencies anywhere near that high, but the reall killer is that while the Stirling engine's efficiency is directly proportional to temperature difference, the heat pump's efficiency is inversely proportional to temperature difference. So the higher you get the heat pump in efficiency, the lower the Stirling engine gets. And vice versa.

    Just look at the equations for Carnot Efficiency (Equation 3 - same as Stirling Engine
    Heat Pump Efficiency

    Notice the terms are the same (Tc, Th).

    You can always get your heat from another source (ie, the sun), but then that isn't perpetual motion either.
    Last edited: Mar 4, 2006
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