Thermodynamic
Cycles
Carlos Silva
November 18th 2009
THERMODYNAMIC
PROCESSES
Different Processes
Isobaric
Isometric
Isothermal
ΔT = 0 but Q ≠ 0
Adiabatic
Cyclic
ΔT ≠ 0 but Q = 0
If 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
Cycle
Rankine 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
1968 Howmet TX
480 MW GE unit
J85 GE unit
Jet Engine diagram
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