Post on 12-Jan-2016
description
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