A household cooling and heating appliance can operate either as a refrigerator or as a heat pump while working between the same two temperature reservoirs.
a. (i) Define the coefficient of performance (COP) of (a) a refrigerator and (b) a heat pump.
b. (ii) Without performing any numerical calculation, state which of the two COPs is numerically larger for a given pair of reservoirs and give a clear physical reason for your answer.
c. (iii) The appliance works between an interior temperature of 270K and an exterior temperature of 300K. Explain qualitatively how the numerical difference in the two COPs would influence the choice of operating mode in summer and in winter.