Calculating the Coefficients of Performance of a Heat Pump
Consider a residential heat pump that uses lake water as a heat source in the winter and as a heat sink in the summer. The house is to be maintained at a winter temperature of 18°C and a summer temperature of 25°C. To do this efficiently, it is found that the indoor coil temperature should be at 50°C in the winter and 5°C in the summer. The outdoor coil temperature can be assumed to be 5°C during the winter months and 35°C during the summer.
a. Compute the coefficient of performance of a reverse Carnot cycle (Carnot cycle heat pump) in the winter and summer if it is operating between the temperatures listed above.
b. Instead of the reverse Carnot cycle, a vapor-compression cycle will be used for the heat pump with HFC-134a as the working fluid. Compute the winter and summer coefficients of performance for this heat pump. Assume that the only pressure changes in the cycle occur across the compressor and the expansion valve, and that the only heat transfer to and from the refrigerant occurs in the indoor and outdoor heat transfer coils.
\begin{array}{cccl}\hline \text { Point } & \begin{array}{c}\text { Heating } \\\text { Temperature }\end{array} & \begin{array}{c}\text { Cooling } \\\text { Temperature }\end{array} & \text { Fluid State } \\\hline 1 & 50^{\circ} C & 35^{\circ} C & \text { Saturated liquid } \\2 & & & \text { Vapor-liquid mixture } \\3 & 5^{\circ} C & 5^{\circ} C & \text { Saturated vapor } \\4 & & & \text { Superheated vapor } \\\hline\end{array}