Entropy

What is Entropy?

Entropy is a scientific concept that is most commonly associated with a state of disorder, randomness, or uncertainty.

First law of thermodynamics

The first law of thermodynamics states that energy can neither be created nor destroyed, only altered in form.

Second law of thermodynamics

Kelvin & Planck statement

"No (heat) engine whose working fluid undergoes a cycle can absorb heat from a single reservoir, deliver an equivalent amount of work, and deliver no other effect"

Clausius statement
"No machine whose working fluid undergoes a cycle can absorb heat from one system, reject heat to another system and produce no other effect"

Both statements of the Second Law constrains the First Law of Thermodynamics by identifying that energy goes downhill. The Second Law is concerned with Entropy (S) which is produced by all processes and associated with the loss of ability to do work. The Second Law states that the entropy of the universe increases.

For energy to be available there must be a region with high energy level and a region with low energy level. Useful work must be derived from the energy that flows from the high level to the low level.

• 100% energy cannot be transformed to work.
• Entropy can be produced but never destroyed.
• For the universe as a whole - the entropy is increasing.

We will see the impact of Entropy on a system with real time example.

Here I choose Steam Turbine because isentropic expansion happens on steam turbine as per Rankine cycle.  Why it is mentioned as isentropic expansion process, since no heat is added to or taken from the fluid, this process is isentropic.

Case 1

Temperature Entropy (T-S) diagram of Ideal isentropic steam expansion (Assuming no entropy – such condition does not exist – For case study referred here).

In the diagram stage 3 to 4 is the isentropic expansion, the straight line represents the ideal expansion it means the 100% efficient process happened but practically such process not possible.

 

Case 2

Temperature Entropy (T-S) diagram of Back pressure Turbine.

In the diagram stage 1 to 3’ is the isentropic expansion (theoretic), but actual is 1 to 3, the small deflection is due to entropy on the system.

Case 3

Temperature Entropy (T-S) diagram of Condensing steam pressure Turbine.

In the diagram stage 1 to 2s is the isentropic expansion (theoretic), but actual is 1 to 2act, the large deflection is due to entropy on the system.

 

Why entropy is high on condensing steam turbine than back pressure is turbine, because the outlet condition of condensing steam turbine will be fixed based on its saturation temperature with respective corresponding vacuum condition so that the condensation of saturated steam to condensate will be easier otherwise very huge heat exchanger is required. The dryness fraction of the steam indirectly contributes the rise in entropy which results in less efficiency.

There is no comparison between back pressure and condensing steam turbine efficiency, since both are used for different purpose....