CARNOT REFRIGERATION CYCLE

The reversed Carnot cycle is shown in Fig. 18.3 on p-v and T-s. diagrams. The sequence of operations is given below:

1. The process 1-2 represents the isentropic compression of the working fluid with the aid of external work. The temperature of the working fluid rises from T₁ to T₂.
2. The process 2-3 represents the isothermal compression of the working fluid during which heat Q_r is rejected at constant high temperature T₂.
3. The process 3-4 represents the isentropic expansion of the working fluid. The temperature of working fluid drops from T₂ to T₁.
4. The process 4-1 represents the absorption of heat Q_a by the working fluid from refrigerator at constant low temperature T₁ during isothermal expansion.

The Carnot cycle is practically not feasible since isothermal energy rejection requires extremely slow motion followed by isentropic process during which the piston should move at extremely faster rate, which is mechanically not obtainable.

The COP of reversed Carnot cycle is the highest of all refrigeration cycles. COP increases as T₁ is increased or T₂ is lowered. The increase in COP for increased T₁is more than for decreased T₂. Since T₂ in winter is less than T₂ in summer therefore COP in winter is more than that of in summer.

If the cycle is used as a heat pump, then energy performance ratio,

Figure 18.3 Reversed Carnot cycle: (a) p-v diagram, (b) T-s diagram