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Geothermal power

  1. Jul 3, 2015 #1
    Hi,
    i looking for install in my home geothermal system for heat/cool my house with lower electricity.
    Here is the spec of my heat pump.
    If i understand my heat pump extract energy from the earth.
    so with this pump, the total energy is 7 kWh input and output 21 kWh heating or 17 kWh for cooling.

    I supose the pump extract more energy in heat mode because the earth temp is 10C, so this is my think! if the earth was under 0C the pump will extract better energy in cool mod that heat mode but only supposition.
    If some people know why the pump extract more heat, let me know please, i'm curious.

    Ok! so now what i wanna know is, this thermal energy get by the heat pump, can i change it to mecanical energy for creat electricity?
    Using steam turbine and pump like sea thermal energy but using geothermal ?

    297971futusup.png
     
  2. jcsd
  3. Jul 4, 2015 #2

    russ_watters

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    Welcome to PF! The reason a heat pump works better in heating mode than in cooling mode is that in heatinig mode you get to keep the input energy as heat. However, yes the efficiency depends on temperature difference.

    And yes, you can use the energy to run a turbine, but the efficiency will be pooor - you will get less out than you put in
     
  4. Jul 4, 2015 #3
    Ok thank you, so if i use the rankine cycle for make electricity with the 21 kWh of heat energy i will change it to electricity lower that 7 kWh? So the rankine cycle have 33% or lower rentability?
    If in winter i use cool temperature from outside to generate more electricity?
    Is not possible to make 4 or 5 kWh more that pump and compresor use?
    How we calculate rankine energy generation?
     
  5. Jul 5, 2015 #4

    billy_joule

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    You can calculate the upper limit efficiency from:

    516f2b4734695a8660107ea0f12ae7b1.png

    https://en.wikipedia.org/wiki/Carnot's_theorem_(thermodynamics)

    What you're proposing is an ORC:
    https://en.wikipedia.org/wiki/Organic_Rankine_cycle

    Note that:
    I did an internship on designing a pilot ORC plant (~50 kW shaft power output), the expected efficiency was ~10% which is around what most working ORC plants you'll find in literature get.
    There is quite a lot of literature on ORC systems, some can be found on google scholar if you don't have access to journal databases.
     
  6. Jul 5, 2015 #5
    So for the sink temperature, i have imagine 2 heat pump, one on the heat side give 21 kWh for heat and on the other side make one on cooling mode 17 kWh. So if we do this do we add 21 kWh + 17 kWh for make electricity power?
    And when the winter coming, use the outside for cooling when temp is lower that cooling pump product.
    Do you know if this process will give more power that need to work?
     
  7. Jul 5, 2015 #6

    russ_watters

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    First, kW and kWh aren't quite the same thing....kW is power, kWh is energy (1 kW for one hour).

    No, those are two different modes of operation and it is the same energy going in as going out. IE, in cooling mode, 17 kWh is pulled-in at the inside coil and 17+7=24 kW dissipated at the outside coil.
    Again, no. It isn't an issue of power (or energy), per se, it is an issue of temperature difference. The efficiency equation Billy posted is depentent on temperature difference between the inside and outside reservoirs. Moving heat is more efficient with a lower temperature difference and generating mechanical power is more efficient with a larger temeperature difference. As a result, a heat pump and heat engine combination can never be more than 100% efficient.
     
  8. Jul 6, 2015 #7
    Ok but something i dont undestand, why i read, that gas engine have heat % between 40% to 90% heat rentability and electricity is 100% rentability but heat pump is 100% to 200% rentability.
    Tell me, if my heat pump use 7 kWh and give 21 kWh this is more that 100% efficient no?
    Or i dont understand something
     
  9. Jul 6, 2015 #8

    russ_watters

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    Coefficient of performance and efficiency have different names because they are different things. Efficiency is a measure of how good a conversion of one form of energy to another is. Coefficient of performance is how good the device is at moving heat from one place to another. See, the air conditioner doesn't (primarily) convert electricity to cooling, it just moves heat out of the warm place and into the cold place.
     
  10. Jul 6, 2015 #9
    Ok thank, do you know if exist and other way to make electricity using low heat temp?
    We can make electricity using solar panel, wind turbine, rankine cycle when high temp, exist any other way to make electricity with heat?
     
  11. Jul 7, 2015 #10

    russ_watters

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    For low grade heat, a sterling engine is probably the most efficient way.
     
  12. Jul 7, 2015 #11

    billy_joule

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  13. Jul 7, 2015 #12
    do you think is possible to gernerate electricity using heat pump lost energy?
    before return the cool anti freeze to the earth, can i use this energy to make little bite electricity.
    anyway this is lost energy because this need to return to the ground before heat again.
    thank you for all your reply very appreciate
     
  14. Jul 7, 2015 #13

    russ_watters

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    There isn't any heat to recover for generating work with a heat pump. Anything you do to harness the rejected heat (save for using it as heat) will interfere with the heat rejection and decrease the cycle efficiency.
     
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