Thermodynamic Cycles

Carlos SilvaNovember 18th 2009

THERMODYNAMIC PROCESSES

Different Processes

Isobaric Isometric

Adiabatic ΔT ≠ 0 but Q = 0

IsothermalΔT = 0 but Q ≠ 0

CyclicIf clockwise – heat engine

If counterclockwise – heat pump

THERMODYNAMIC CYCLES

Ideal (Carnot) Cycle

Carnot Theorem

•No engine operating between two heat reservoirs can be more efficient than a Carnot engine operating between those same reservoirs

Pressure-Volume Temperature-Entropy

Real Cycles

There are no ideal cycles

• Irreversible systems, losses of heat

Types of Cycles

Heat Engine

•Rankine

Gas Power Systems

• Brayton

Internal Combustion Engines

• Otto, Diesel,Stirling, Atckison

Refrigeration

Heat Pump

Air Conditioning

HEAT ENGINE

Heat Engines

Converts thermal energy (heat) to mechanical output (work)

The working fluids are gases and liquids.

Phase change cycles

•The engine converts the working fluid from a gas to a liquid.

• Rankine

• Regenerative

Gas cycles

• The working fluid is always gas

• Carnot

• Stirling

Rankine (Classical steam engine)

Generation power plants

• practical Carnot Cycle

• heat addition and ejection are isobaric (and not isothermal)

Working fluid is alternatively vaporized and condensed

PV vs TS diagrams

Alternative Rankine cycles

Super Heat Reheat Regenerative

Reheat and Regenerative

Efficiencies

Cycle Efficiencies Unmodified Rankine

CycleRankine Cycle

with Regeneration

Carnot efficiency 52.6% 52.6%

Thermal efficiency 36.2% 38.4%

% Increase in Power Generation

0% 6.1%

mean temperature of heat addition

226.7 C 251.5 C

GAS POWER SYSTEMS

Brayton Cycle (Joule Cycle)

Usually used in gas turbines

•Basis of jet engines

Examples

480 MW GE unit

Jet Engine diagram

1968 Howmet TX

J85 GE unit

A-37 Dragonfly

Combined Cycle

Combining Rankine and Brayton cycles

INTERNAL COMBUSTION

Special type of heat engines

Combustion of fuel used to produce work directly

• in heat engines is used to heat the fluid

• the expansion of the high temperature and pressure gases, produced by the combustion, directly applies force to a movable component of the engine, such as the pistons or turbine blades and by moving it over a distance, generate useful mechanical energy

• combustion is usually intermittent

Otto engine

Gasoline vehicles

• External ignition

•Octane rating- measure of the resistance of gasoline and other fuels to detonate at constant volume.

•The higher the value, the slower the fuel burns

Diesel engine

Diesel cars

• no external ignition

• highest efficiency due to compression ration

• low speed engines can exceed 50%

• Diesel Cetanes

• combustion quality during compression ingnition

Stirling Cycle

Similar to Otto cycle

• replace adiabatic per isothermals

Used in Micro CHP

Atkison Cycle

four-stroke cycle to occur in a single turn of the crankshaft

designed to bypass patents covering the existing Otto cycle engines

REFRIGERATION AND HEAT PUMP

Vapor compression cycle

Refrigerator vapor

• Freon

Heat Pump

Can pump heat in two directions

• reversing valve

Absorption Heat Pump

Heated by gas, solar thermal, etc..

Air Conditioning

Refrigerator + resistance

Heat pump