Second Law of Thermodynamics P M V Subbarao Professor Mechanical Engineering Department In search of Cycles with Better Cost-to-Benefit Ratio…….…

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Second Law of Thermodynamics P M V Subbarao Professor Mechanical Engineering Department In search of Cycles with Better Cost-to-Benefit Ratio…….…

Thermodynamic Model for a General Heat Engine First law: High Temperature Reservoir (Source) Low Temperature Reservoir (Sink) HE

Thermodynamic Model for a General Heat Pump First law: HTR (Sink) LTR (Source) HP

Kelvin Planks postulate “ It is impossible to construct a heat engine which produces no effect other than the extraction of heat from a single source and the production of an equivalent amount of work” On the other side It is possible to construct a heat engine which produces no effect other than the extraction of work from a single source and the production of an equivalent amount of heat

The Impossible Engine HTR (Source) LTR (Sink)

Clausius postulate “Heat cannot pass spontaneously (unaided) from a region of lower temperature to a region of higher temperature” However, Heat can pass spontaneously (unaided) from a region of higher temperature to a region of lower temperature

The Impossible Heat Pump HTR (Sink) LTR (Source) 

Discussion of Statements of Second Law Both are negative statements. They cannot be proved. They will remain correct till they are disproved. Violation of Kelvin Planks statement leads to violation of Clasius statement and vice versa.

Violation of any one Statement lead to violation of the other automatically!

Perpetual Heat Pump Model QHQH QLQL First law |Q H | = QL HTR (Source) LTR (Sink) Let us assume this is possible 

Perpetual Heat Pump & A General Heat Engine Q HP Q LP HTR LTR  Q HE Q LE E Wnet

Compound Heat Engine Q HP Q LP LTR (Sink) Q HE E  Wnet Q HE = Q HP = Q LP Q LE = Q LP -Wnet HTR Q LE HTR

Perpetual Motion Machine II : Compound Engine Q HP Q LP First law Q HP = Q LP Q HE E  Wnet Q HE = Q LP Q LE = Q LP -W net Q LP - Q LE =W net LTR (Source) HTR (Sink)

Consequences of Second Law What is the possible value of Maximum Efficiency? What is the model of Maximum Efficiency Engine? What is the possible value of Maximum CoP? What is the model of Maximum CoP Pump? Any machine (cycle) which can works as engine and also works as heat pump, without loss of performance can have maximum efficiency. These are called as reversible machines. All reversible machines working between same reservoirs should equally perform. It is impossible to construct a reversible machine better than another reversible machine working between same reservoirs.

A Blue Print for Construction of Most Efficient Machine!!!!

Famous Models for Maximum Efficiency Machines The Stirling Cycle: Reverend Robert Stirling patented a hot air engine in 1816 called “The Economiser”. The Carnot Cycle :1824 : Réflexions sur la puissance motrice du feu et sur les machines propres à développer cette puissance which includes his description of the "Carnot cycle". The Regenerative Cycle

The Reversible Cycles : Carnot Cycle The first model (1824) for reversible machine is the Carnot cycle. This consists of two isothermal processes and two adiabatic processes. This model can be used to construct either a heat engine or a heat pump. Hence Carnot Cycle is a Reversible Cycle.