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The Thermodynamics of it All: Do GSHPs Work in Cold Climates?

The Thermodynamics of it All: Do GSHPs Work in Cold Climates?. Coefficient of Performance (COP) = Heat Energy Output Electric Energy Input Industry claims COP ranging from 3 to 6 From the Second Law of Thermodynamics and the Carnot cycle: COP theoretical limit =

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The Thermodynamics of it All: Do GSHPs Work in Cold Climates?

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  1. The Thermodynamics of it All:Do GSHPs Work in Cold Climates? Coefficient of Performance (COP) = Heat Energy Output Electric Energy Input Industry claims COP ranging from 3 to 6 From the Second Law of Thermodynamics and the Carnot cycle: COP theoretical limit = Indoor Temperature (Indoor Temperature - Temperature of Heat Source/Sink) 1

  2. Thermodynamics (continued) Degrees Farenheit to degrees Kelvin conversion: °K = 5/9 (°F - 32) + 273 Example: • Heat a dwelling to 70 °F ~ 294 °K • Ambient groundwater temperature in Massachusetts is typically about 54 °F ~ 285 °K From the Carnot cycle: COP theoretical limit = 294 °K = 33 294 °K – 285 °K

  3. Thermodynamics (continued) What happens to the theoretical efficiency toward the end of the heating season if the entering water temperature has dropped to 35 °F? 35 °F ~ 275 °K COP theoretical limit = 294 °K = 15 294 °K – 275 °K

  4. Thermodynamics (continued) Why is the theoretical heat pump efficiency many times greater than the actual COP? • No heat pump is 100% efficient - Not all of the energy put into the heat pump is converted into the work of pumping heat – some energy “lost” as waste heat • It takes energy to pump the heat transfer fluid through the ground coupled part of the system and the air or water through the building’s heating ducts

  5. Ground Exchange vs. Air Exchange Heating season Cooling season Heating season °F

  6. Entering Heat Pump Temperature vs. Theoretical Maximum COP Entering Temperature °F Theoretical Coefficient of Performance

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