Page 61 - BASIC CONCEPTUAL OF THERMOFLUID

CHAPTER 5: SECOND LAW OF THERMODYNAMICS

5.4 Limitation Second Law of Thermodynamics

The second law of thermodynamics stated that the total heat absorbed could not be completely
converted to mechanical work. It means there is always some heat that is wasted (in practical sense)
or transferred to the surroundings (in Physical sense).

It imposes a limitation in the nature. Although there are different forms of the same statement,
ultimately conclude to the one limitation of nature that in any cyclic process, the conversion of work
to energy and the reverse cannot be done with 100% efficiency [7].

The engine statement of the second law of thermodynamics based on its limitation to convert heat
into mechanical work. The second law of thermodynamics limitation is that no heat engine can
convert all the absorbed heat from a hot reservoir completely into mechanical work. This statement
is also called Kelvin-Plank statement of the second law of thermodynamics.

Fig. 5.1: The energy flow diagram showing the energy flow in a heat engine [7]
The Fig. 5.1 shows an energy flow diagram where the engine takes heat Qh from the hot reservoir at
temperature Th and converts some amount of the absorbed heat into work WW and some amount of
the absorbed heat Qc is thrown to the cold reservoir. It means, the total heat Qh absorbed is the sum
of the work done by the system and the heat thrown to the cold reservoir that is

Qh = W − Qc = W + | Qc | … … … . . (1)

Note that the heat thrown to the cold reservoir flows out of the system of the engine and it is negative.
Therefore, Qc is already negative and negative sign with Qc is used to correct the above expression
or you can add the absolute value of Qc to work done. The ratio of work done by the engine W to the
heat absorbed Qh is called the efficiency of the engine denoted by Ƞ. The efficiency Ƞ is always less
than unity. From Equation (1), W=Qh+QcW=Qh+Qc and the efficiency is

W Qc + Qh Qc |Qc|
Ƞ = + = 1 + = 1 − … … … (2)
Qh Qh Qh Qh

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