ENGINES, REFRIGERATORS, AND HEAT PUMPS

This lecture highlights aspects in Chapters 9,10,11 of Cengel and Boles. Every thermodynamic device has moving parts. To understand these movements, it is important that you watch some videos on the Internet. I will go through these slides in two 90-minutes lectures.

Zhigang Suo, Harvard University

How humans tell each other something?• The thing itself• Pictures• Words• Equations

• Language• Books• Movies• The Internet 2

Thermodynamics = heat + motionToo many devices to classify neatly

• Fuel (input): biomass, fossil, solar thermal, geothermal, nuclear, electricity.

• Application (output): mobile power plant (transpiration in air, land, sea), stationary power plant (electricity generation), refrigerator, heat pump. Power cycle, refrigeration cycle.

• Working fluid: Gas cycle (air), vapor cycle (steam, phase change). • Fluid-solid coupling: piston engine (reciprocating, crankshaft),

turbine engine (jet, compressor). • Site of burning: external combustion, internal combustion.

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Plan

• Internal combustion engines• Gas turbines• Stirling and Ericsson engines• Vapor power cycle• Refrigeration cycle• Thermodynamics in a nutshell

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External combustion engine Internal combustion engine (ICE)

Fayette Internal Combustion Engiine I

US Navy Training Manual, Basic Machines

Combustion engineburns to move

• Otto (gasoline) engine• Diesel engine• Gas turbine• Jet propulsion

• Steam engine• Stirling engine• Ericsson engine

PISTON

PISTON

COMBUSTION CHAMBER

WATER

STEAM

BOILER

6US Navy Training Manual, Basic Machines

Reciprocating enginealso known as piston engine, converts linear motion to rotation

PISTON

CONNECTING ROD

CRANKSHAFT

CYLINDER

7US Navy Training Manual, Basic Machines

1 cycle4 strokes 2 revolutions

INTAKE STROKE COMPRESSION STROKE

POWER STROKE EXHAUST STROKE

fuel-air mixtureentering cylinder

exhaust valveclosed

pistonmoving down

cam lobe liftingvalve tappet

intakevalve open

valve tappetlifting valve

Fuel dischargingfrom nozzle

air entering fuel-air mixturebeing compressed

both valvesclosed

pistonmoving up

spark igniting mixture

both valvesclosed

exhaust valveopen

intake valve closed

pistonmoving up

pistonmoving down valve tappet

lifting valve

cam lobe liftingvalve tappet

Animated engineshttp://www.animatedengines.com/

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Spark-ignition engine (gasoline engine, petrol engine, Otto engine)

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1. Model the engine as a closed system, and the working fluid as air (an ideal gas).

2. The cycle is internally reversible.

3. Model combustion by adding heat from an external source

4. Model exhaust by rejecting heat to an external sink

Air-standard assumptions

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Cold air-standard assumptionModel air as an ideal gas of constant specific heat at room temperature (25°C).

2 independent variables to name all states of thermodynamic equilibrium6 functions of state: PTvush4 equations of state

Gibbs equation

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Thermal efficiency of Otto cycleCompression ratio:

Conservation of energy:

Isentropic processes:

Thermal efficiency:

wout

win

Otto cycle represented in planes of different variables

12v

s

12

3 4

qinqout

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Spark-ignition engine (Otto, 1876) Compression-ignition engine (Diesel, 1892)

https://ccrc.kaust.edu.sa/pages/HCCI.aspx

Reciprocating engines of two types

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Compression-ignition engine (Diesel engine)

compression ratio:

cut-off ratio:

Conservation of energy:

Isentropic processes

Thermal efficiency:

Plan

• Internal combustion engines• Gas turbines• Stirling and Ericsson engines• Vapor power cycle• Refrigeration• Thermodynamics in a nutshell

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Gas turbine (Brayton cycle)4 steady-flow components: isobaric and isentropic

s

P

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2 3

qout

qin

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Thermal efficiency of Brayton cycleDefinition of pressure ratio:

Conservation of energy:

Isentropic processes:

Thermal efficiency:

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Brayton cycle has large back work ratio

wout

win

Intercooling, reheating, regeneration

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Gas turbine for jet propulsionThousands of years of history

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Who invented this? Hero of Alexandria Frank Whittle (UK), Hans von Ohain (Germany) (first century AD) (during World War II)

http://www.techknow.org.uk/wiki/index.php?title=File:Hero_4.jpg

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Propulsive force:

Propulsive power:

Propulsive efficiency:

Gas turbine for jet propulsion6 steady-flow components

22http://www.ae.utexas.edu/~plv955/class/propulsion/Cp_air.GIF

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Air as an ideal gas of variable specific heat

See section 7.9 for the use of this table

Plan

• Internal combustion engines• Gas turbines• Stirling and Ericsson engines• Vapor power cycle• Refrigeration cycle• Thermodynamics in a nutshell

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Displacer-type Stirling engine

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https://www.stirlingengine.com/faq/

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Stirling engine and regenerator (1816)reversible cycle between two fixed temperatures, having the Carnot efficiency

https://people.ok.ubc.ca/jbobowsk/Stirling/how.html

Stirling vs. Carnotfor given limits of volume, pressure, and temperature

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• On PV plane, the black area represents the Carnot cycle, and shaded areas represent addition work done by the Stirling cycle.

• On TS plane, the black area represents the Carnot cycle, and the shaded areas represent additional heat taken in by the Stirling cycle.

• The Stirling cycle and the Carnot cycle have the same thermal efficiency.• The Stirling cycle take in more heat and give more work than the Carnot cycle.

Walker, Stirling Engine, 1980.

Work out by Stirling cycle

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Specific work

Specific gas constant

Gas Formula R (kJ/kgK)

Air 0.2870

Steam H2O 0.4615

Ammonia NH3 0.4882

Hydrogen H2 4.124

Helium He 2.077

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Ericsson engine with regenerator (1853) reversible cycle between two fixed temperatures, having the Carnot efficiency

Plan

• Internal combustion engines• Gas turbines• Stirling and Ericsson engines• Vapor power cycle• Refrigeration cycle• Thermodynamics in a nutshell

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Coal power stationcoverts coal to electricity

Brayton Point Power StationSommerset, Massachusetts

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Mount Hope Bay

http://www.clf.org/blog/clean-energy-climate-change/brayton-point-retirement-means-game-coal-new-england/

Nuclear power stationconverts uranium to electricity

33http://www.nuclear-power.net/nuclear-power-plant/

Animationhttps://www.awesomestories.com/images/user/be4285df4b.gif

Nine Mile Point Nuclear Power Plant, New York

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Lake Ontario

Cooling tower

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Why water? Why steam?• Water is cheap.• Water flows!• Water is a liquid at the temperature of heat sink (rivers, lakes,...). • Vaporization changes specific volume greatly: a lot of work at relatively low pressure.

https://www.ohio.edu/mechanical/thermo

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Rankine cycle4 steady-flow components: isobaric and isentropic

s

P

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2 3qboiler,in

qcondenser, out

wpump,in = h2 - h1

qboiler,in = h3 - h2

wturbine,out = h3 – h4

qcondenser,out = h4 – h1

wturbine,out wpumo,in

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Rankin cycle has small back work ratio

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Rankin cycleVapor cycleSteam turbineSmall back-work ratio

Brayton cycleGas cycleGas turbineLarge back-work ratio

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Issues with the in-dome Carnot cycle

Process 1-2 limits the maximum temperature below the critical point (374°C for water)

Process 2-3. The turbine cannot handle steam with a high moisture content because of the impingement of liquid droplets on the turbine blades causing erosion and wear.

Process 4-1. It is not practical to design a compressor that handles two phases.

Issues with supercritical Carnot cycle

Process 1-2 requires isothermal heat transfer at variable pressures.

Process 4-1 requires isentropic compression to extremely high pressures.

Carnot cycle is unsuitable as vapor power cycle

Cogeneration

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Plan

• Internal combustion engines• Gas turbines• Stirling and Ericsson engines• Vapor power cycle• Refrigeration cycle• Thermodynamics in a nutshell

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Refrigerator and heat pump4 steady-flow components

animation

Selecting Refrigerant

1. Large enthalpy of vaporization2. Sufficiently low freezing temperature3. Sufficiently high critical temperature4. Low condensing pressure5. Do no harm: non-toxic, non-corrosive, non-flammable,

environmentally-friendly6. Low cost

• R-717 (Ammonia, NH3) used in industrial and heavy-commercial sectors. Toxic.

• R-12 (Freon 12, CCl2F2). Damage ozone layer. Banned.• R-134a (HFC 134a, CH2FCF3) used in domestic

refrigerators, as well as automotive air conditioners.43

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Plan

• Internal combustion engines• Gas turbines• Stirling and Ericsson engines• Vapor power cycle• Refrigeration cycle• Thermodynamics in a nutshell

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Rankin cyclePower stationExternal CombustionVapor cycleSteam turbinePumpSmall back-work ratio

Brayton cycleJet propulsion, power stationInternal combustionGas cycleGas turbineCompressorLarge back-work ratio

Refrigeration cycleRefrigerator, heat pumpElectricityVapor cycleNo turbineVapor compressorNo back work

wout

win

48https://flowcharts.llnl.gov/

Pure substance

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2 independent variables to name all states of thermodynamic equilibrium6 functions of state: PTvush4 equations of state

Incompressible liquid liquid-gas mixture ideal gas

liquid

weights

fire

vapor

T

s

P = 0.1 MPa

gasliquid

Concepts and definitions

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Isolated systemQuantum states of an isolated system

Fundamental postulateStates of thermodynamic equilibrium

Functions of statePhases

Number of quantum states of an isolated system:Entropy of an isolated system:

Isolated system generates entropy. IrreversibilityIsolated system conserves energy and volume:

Model a closed system as a family of isolated systems:

Definition of temperature (Gibbs equation 1):

Definition of pressure (Gibbs equation 2):

Definition of enthalpy:Definition of Helmholtz function (free energy):

Definition of Gibbs function:

Definition of heat capacities:

Theory of everythingthe world according to entropy

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• Entropy• Equilibrium• Irreversibility• Temperature, energy• Pressure, volume• Phases• Ideal gas• Osmosis• Turbines, compressors, throttling valves, heat exchangers, diffusers, nozzles• Engines, refrigerators, heat pumps

Summary• Engine converts fuel to motion.• Refrigerator and heat pump use work to pump heat from a place of low

temperature to a place of high temperature. • Many ideal cycles are internally reversible, but externally irreversible.• Stirling and Ericsson cycles are internally and externally reversible, so they

have the same thermal efficiency as the Carnot cycle.• Use ideal-gas model to analyze gas as working fluid.• Use property table to analyze vapor as working fluid.• Model piston engine as a closed system (Otto, Diesel, Stirling, Ericsson).• Model turbine (or compressor) device as steady-flow components in

series (Brayton cycle, Rankine cycle, refrigeration cycle).

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