Available energy referred to a cycle

Consider a cyclic heat engine as shown in Fig. 1.23. The maximum work output obtainable from this engine is the available energy (AE). The minimum energy that has to be rejected to the sink (as per 2nd law of thermodynamics) is called the unavailable energy (UE).

Fig. 1.23 Available and unavailable energy in a cycle

From energy balance

Q1 = AE + UE

1.198

Again,

Wmax = AE = Q1 - UE

1.199

For the given source and sink temperatures T₁ & T₂ respectively

1.200

If T₁ is given, η_rev will increase with decrease of T₂. The lowest practical temperature of heat rejection is the temperature of the surroundings, T₀.

1.201

And,

1.202