Thermodynamics III: 2nd Law & Cycles “It just don’t get no better than this…”

Objectives Understand types of state changes Comprehend thermodynamic cycles Comprehend the 2nd Law of Thermodynamics to include entropy, reversibility, & the Carnot cycle Determine levels of output and efficiency in theoretical situations

State Changes In addition to using flow/no-flow classifications for thermo processes, it is helpful to look at what happens to a medium also Isobaric: pressure remains constant throughout process (some pistons) q12 = h2 - h1

State Changes Isometric: volume remains constant during entire process q12 = u2 - u1 Adiabatic: no transfer of heat to or from medium during process -> usually in a rapid process w = u2 - u1

Thermodynamic Cycles Def’n: a recurring series of thermodynamic processes through which an effect is produced by transformation or redistribution of energy One classification: Open: working fluid taken in, used, & discarded Closed: working medium never leaves cycle, except through leakage; medium undergoes state changes & returns to original state

Five Basic Elements of all Cycles Working substance: transports energy within system Heat source: supplies heat to the working medium Engine: device that converts the thermal energy of the medium into work Heated: heat added in engine itself Unheated: heat received in some device separate from engine

Five Basic Elements of all Cycles Heat sink/receiver: absorbs heat from the working medium Pump: moves the working medium from the low-pressure side to the high-pressure side of the cycle Examples: Closed, unheated engine: steam cycle Open, heated engine: gasoline engine

Basic Thermodynamic Cycle HEAT SOURCE HEAT SINK Pump Engine W Qin Qout Working Substance

Second Law of Thermodynamics Reversibility: the characteristic of a process which would allow a process to occur in the precise reverse order, so that the system would be returned from its final condition to its initial condition, AND all energy that was transformed or redistributed during the process would be returned from its final to original form

Second Law of Thermodynamics Def’n 1: (Clausius statement) no process is possible where the sole result is the removal of heat from a low-temp reservoir and the absorption of an equal amount of heat by a high temp reservoir Def’n 2: (Kelvin-Planck) no process is possible in which heat is removed from a single reservoir w/ equiv amount of work produced

Second Law of Thermodynamics Overall: NO thermodynamic cycle can have a thermal efficiency of 100% (i.e., cannot convert all heat into work) Quick review: 1st Law: Conservation/transformation of energy 2nd Law: Limits the direction of processes & extent of heat-to-work conversions

Entropy Def’n: theoretical measure of thermal energy that cannot be transformed into mech. Work in a thermodynamic system It is an index of the unavailability of energy or the reversibility of a process In all real processes, entropy never decreases -> entropy of universe is always rising

Carnot Cycle Second Law states that no thermo system can be 100% efficient, and no real thermal process is completely reversible A French engineer, Carnot, set out to determine what the max efficiency of a cycle would be if that cycle were ideal and completely reversible

Carnot Cycle All heat is supplied at a single high temp and all heat is rejected at a single low temp Carnot used a simple cycle

Carnot Cycle TSource TSink Pump Engine W Qin Qout Working Substance

Carnot Cycle Carnot Principle: the max thermal efficiency depends only on the difference between the source and sink temps Does not depend on property of fluid, type of engine, friction, or fuel Example:

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