Sadi Carnot introduced the concept of reversibility and laid the foundations of the second law. A reversible process, also called an ideal process, can reverse itself exactly by following the same path it took in the first place. Thus, it restores to the system or the surroundings the same heat and work previously exchanged.

In reality, there are no ideal (reversible) processes. Real processes are irreversible.

However, the degree of irreversibility varies between processes. There are many sources of irreversibility in nature.

The most important ones are friction, heat transfer, throttling, and mixing. Mechanical friction is one in which mechanical work is dissipated into a heating effect.

One example would be a shaft rotating in a bearing. It is not possible to add the same heat to the bearing to cause rotation of the shaft.

An example of fluid friction is when the fluid expands through the turbine, undergoing internal friction. This friction results in the dissipation of part of its energy into heating itself at the expense of useful work.

The fluid then does less work and exhausts at a higher temperature. The more irreversible the process, the more heating effect and the less the work.

Heat transfer in any form cannot reverse itself. Heat transfer causes a loss of availability because no work is done between the high- and low-temperature bodies.

External irreversibilities are those that occur across the boundaries of the system. The primary source of external irreversibility in power systems is heat transfer both at the highand low-temperature ends.

Internal irreversibilities are those that occur within the boundaries of the system. The primary source of internal irreversibilities in power systems is fluid friction in rotary machines, such as turbines, compressors, and pumps.

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