Thermodynamics III: 2nd Law & Cycles

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

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

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

TSource Qin Working Substance W Engine Qout Pump TSink Carnot Cycle

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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Thermodynamics III: 2nd Law & Cycles