Practical Reasons due to which a Carnot Cycle is not considered as Ideal Cycle for Vapour Power Plants

Though the Carnot cycle is the most efficient cycle between the specified temperature limits of T, and T, however, the following inherent practical difficulties make the Carnot cycle uneconomical for producing power.

1. The condensation process (4-1) has to be controlled accurately to end up with the desired quality of steam at state 1 which is difficult to achieve in practice.
2. The work of compression of two phase mixture is appreciable and it can be compared in magnitude with the work of expansion. Consequently the net work done per cycle will be small. i.e. it has a very low work ratio.
3. Isothermal heating at constant pressure in process (2-3) can be achieved in practice. However, the extension of isothermal process into the superheated region (3-5) will require a continuous reduction in pressure. Though the decrease in pressure can be accomplished through performance of work but such type of process is extremely difficult to perform in practice in a boiler.
4. Since the heat transferred to the cycle is normally obtained by combustion process in a furnace (source), the products of combustion can be cooled, theoretically upto the highest temperature of the cycle, Tj. But, in practice, the products of combustion for rapid heat transfer can only be cooled upto several hundred degrees above the temperature T, hence, they carry lot of heat energy. It follows that the overall thermal efficiency of the source and that of the cycle will be very low.
5. If the two phase mixture is to be compressed isentropically, during the process (1-2), it needs to be carried out rapidly. In such a case there will be negligible heat transfer from vapour to liquid because the compression of liquid phase is accompanied by slight increase in temperature while the compression of vapour results in superheating, hence, the equilibrium between the two phases (i.e. liquid and vapour) cannot be maintained. However, the phase equilibrium can be maintained if the process is carried out slowly but in this case the heat transfer to the surroundings would prevent the approximation to an adiabatic process.

Conclusions of Vapour Power Plants

From the above discussions we conclude that even though the Carnot cycle is theoretically most efficient but it is impracticable to build an engine on this cycle. Therefore, we have to look for another cycle which should correspond and to be free from inherent practical limitations of the Carnot cycle. Such an ideal cycle for steam power plants is Rankine cycle which will be discussed in the next section.