Although according to the Carnot theorem, the reversible Carnot cycle boasts a high level of efficiency, its practicability is not possible due to Carnot cycle limitations.
One of its limitations is its low net work output for a given stroke volume. This means that to generate more work, the Carnot engine’s size would need to be quite very large.
Therefore, it is important to examine the limitations of the Carnot cycle under different conditions. In this discussion, we will explore the Carnot cycle limitations under two specific conditions
Carnot cycle limitations
1. Carnot cycle limitations when the vapour is used as a working substance
The reversible Carnot cycle that uses vapour as a working substance involves two isothermal processes: condensation and evaporation. These processes are internally reversible and can be easily accomplished in practice.
However, there may be some difficulties in achieving partial evaporation since it can be challenging to design an expander that can handle a mixture of mostly liquid and partly vapour during the isentropic expansion process.
Additionally, due to internal irreversibilities in both the compressor and the expander, the actual efficiency of the cycle is significantly lower than the ideal cycle efficiency.
2. when Gas is used as a working substance
The reversed Carnot cycle that employs gas as a refrigerant has two primary limitations.
Firstly, it is impractical to achieve isothermal processes of heat absorption and rejection, with gas as the working substance since these processes would take an infinite amount of time.
Secondly, the Carnot cycle’s p-v diagram is very narrow because the volume changes during both the reversible isothermal and adiabatic processes.
The Carnot p-v diagram, when drawn to scale, is much thinner than the diagram commonly illustrated. As a result, the stroke volume of the cylinder becomes quite large.
Hence the cycle’s actual efficiency is therefore poor due to irreversibilities in both the compressor and the expander.
References
Refrigeration and Air Conditioning by C P Arora
Internal combustion engines by M.L. Mathur and R.P. Sharma