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Coefficient of performance (COP)

According to the law of conservation of energy

Q2 = Q1 + W

Q2 = Heat out of the condenser (the back of the refrigerator).

Q1 = Heat absorbed by the evaporator (inside the refrigerator).

W = Work or electricity consumption of the refrigerator.

COP = Q1/W   The higher the COP the better, because more heat can be transferred with less work (electricity).

The COP depends primarily on the temperature of the evaporator and the condenser, the closer the two temperatures are, the higher the COP.  COP also depends on the refrigerant gas and the model of compressor used.

The COP of a heatpump is restricted by the second law of thermodynamics.

$COP_{\mathrm{heating}} = \frac{\Delta Q_{\mathrm{hot}}}{\Delta A} \leq \frac{T_{\mathrm{hot}}}{T_{\mathrm{hot}}-T_{\mathrm{cool}}} = \frac{1}{\eta_{\mathrm{carnotcycle}}}$
$COP_{\mathrm{cooling}} = \frac{\Delta Q_{\mathrm{cool}}}{\Delta A} \leq \frac{T_{\mathrm{cool}}}{T_{\mathrm{hot}}-T_{\mathrm{cool}}}$

All temperatures T are measured in kelvins.

Tcool  is the temperature of the refrigerator/freezer's (R/F) evaporator - inside the R/F.

Thot - Tcool  is the temperature difference between the R/F's condenser (outdoor temperature) and evaporator (its inside temperature).

References:  http://en.wikipedia.org/wiki/Heat_pump

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