Chapter 2 - Energy Concepts

8/10/2019 Chapter 2 - Energy Concepts

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Chapter 2

BASIC ENERGY CONCEPT,ENERGY TRANSFER,

AND GENERAL ENERGY ANALYSIS


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FORMS OF ENERGY Energy exists in numerous forms such as thermal, mechanical, kinetic,

potential, electric, magnetic, chemical, and nuclear, and their sumconstitutes the total energy, E of a system.

Macroscopic forms of energy : possesses by a system as a wholewith respect to some outside reference frame, eg. KE & PE.

Microscopic forms of energy : related to the molecular structure of asystem and the degree of the molecular activity.

Internal energy, U : The sum of all the microscopic forms of energy.

The macroscopic energy of anobject changes with velocity andelevation.

Kinetic energy, KE : The energythat a system possesses as a result

of its motion relative to somereference frame.

Potential energy, PE: The energythat a system possesses as a resultof its elevation in a gravitational

field.


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Some Physical Insight to InternalEnergy

The internal energy of asystem is the sum of all formsof the microscopic energies.

The various forms ofmicroscopicenergies that makeup sensible energy.

Sensible energy: IE associated with the KE of themolecules.Latent energy: IE associatedwith the phase of a system.Chemical energy: IEassociated with the atomic

bonds in a molecule.Nuclear energy: IE associatedwith the strong bonds within thenucleus of the atom itself.

Internal = Sensible + Latent + Chemical + Nuclear

Thermal = Sensible + Latent


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Total energyof a system

Energy of a systemper unit mass

Potential energyper unit mass

Kinetic energyper unit mass

Potential energy

Total energy

per unit mass

Kinetic energy

Mass flow rate

Energy flow rate


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Mechanical EnergyMechanical energy: The form of energy that can be converted tomechanical work completely and directly by an ideal mechanical device such

as an ideal turbine.KE & PE: The familiar forms of mechanical energy.

Mechanical energy of aflowing fluid per unit mass

Rate of mechanicalenergy of a flowing fluid

Mechanical energy change of a fluid during incompressible flow per unit mass

Rate of mechanical energy change of a fluid during incompressible flow


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ENERGY TRANSFER BY HEAT

Energy can cross theboundaries of a closed systemin the form of heat and work.

Temperature difference is the driving force for heat transfer. The larger thetemperature difference, the higher is the rate of heat transfer.

Heat : The form of energy that is transferred between two systems(or a system and its surroundings) by virtue of a temperaturedifference.

Heat transferper unit mass

Amount of heat transferwhen heat transfer ratechanges with time

Amount of heat transfer

when heat transfer rateis constant


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Historical Background on Heat

Heat transfer mechanisms: Conduction: The transfer of

energy from the moreenergetic particles of asubstance to the adjacent lessenergetic ones as a result ofinteraction between particles.

Convection: The transfer ofenergy between a solidsurface and the adjacent fluidthat is in motion, and it

involves the combined effectsof conduction and fluid motion. Radiation: The transfer of

energy due to the emission ofelectromagnetic waves (orphotons).

In the early nineteenth century, heat wasthought to be an invisible fluid called thecalor ic that flowed from warmer bodies tothe cooler ones.


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ENERGY TRANSFER BY WORK Work : The energy transfer associated with a force acting through a distance.

A rising piston, a rotating shaft, and an electric wire crossing thesystem boundaries are all associated with work interactions

Formal sign convention : Heat transfer to a system and work done by asystem are positive; heat transfer from a system and work done on a systemare negative .

Alternative to sign convention is to use the subscripts in and o u t to indicatedirection.

Specifying the directionsof heat and work.

Work doneper unit mass

Power is thework done perunit time (kW)


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THE FIRST LAW OF THERMODYNAMICS The f i r s t l aw o f t he rmodyn amics (the conserva t ion of energy

pr inc ip l e ) provides the relationships among the various forms of energy

and energy interactions. The first law states that energy can be ne i ther c reated no r des t roy ed

dur ing a p rocess ; i t c an on ly change fo rms . The Firs t Law : For all adiabatic processes between two specified states of

a closed system, the net work done is the same regardless of the nature ofthe closed system and the details of the process.

The increase in the energy of apotato in an oven is equal to theamount of heat transferred to it.


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Energy BalanceThe net change (increase or decrease) in the total energy of the system

during a process is equal to the difference between the total energy

entering and the total energy leaving the system during that process.

The energy changeof a system duringa process is equal

to the net work andheat transferbetween the

system and itssurroundings.


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Energy Change of a System, E system

Internal, kinetic, and

potential energy changes


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Mechanisms of Energy Transfer, E in and E out Heat transfer Work transfer

Mass flow

The energycontent of acontrol volumecan be changedby mass flow aswell as heat and

work interactions.

(kJ)

A closed massinvolves onlyheat transfer

and work.

For a cycle E = 0,thus Q = W .


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ENERGY CONVERSION EFFICIENCIES

Efficiency indicates how well an energy conversion or transfer process isaccomplished.

Efficiency of a waterheater: The ratio of theenergy delivered to the

house by hot water tothe energy supplied tothe water heater.


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Heating value of the fuel : The amount of heat released when a unitamount of fuel at room temperature is completely burned and thecombustion products are cooled to the room temperature.Lower heating value (LHV) : When the water leaves as a vapor.Higher heating value (HHV) : When the water in the combustion gases iscompletely condensed and thus the heat of vaporization is also recovered.

The definition of the heating value ofgasoline.


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Generator: A device that converts mechanical energy to electricalenergy.

Generator efficiency: The ratio of the electrical power output to themechanical power input.

Thermal efficiency of a power plant: The ratio of the net electricalpower output to the rate of fuel energy input.

A 15-W compact fluorescent lampprovides as much light as a 60-W

incandescent lamp.

Overall efficiencyof a power plant

Using energy-efficient appliances conserveenergy.

helps the environment by reducing the amountof pollutants emitted to the atmosphere duringthe combustion of fuel.

The combustion of fuel produces carbon dioxide, causes global warming nitrogen oxides and hydrocarbons, cause

smog carbon monoxide, toxic

sulfur dioxide, causes acid rain.


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Efficiencies of Mechanical and Electrical Devices

The mechanicalefficiency of a fan is theratio of the kineticenergy of air at the fanexit to the mechanical

power input.

The effectiveness of the conversion process betweenthe mechanical work supplied or extracted and themechanical energy of the fluid is expressed by the

pump efficiency and turbine efficiency ,

Mechanical efficiency


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Generator

efficiency

Pump-Motoroverall efficiency

Turbine-Generatoroverall efficiency

The overall efficiency of a turbine generatoris the product of the efficiency of the turbineand the efficiency of the generator, andrepresents the fraction of the mechanicalenergy of the fluid converted to electricenergy.

Pumpefficiency


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ENERGY AND ENVIRONMENT Energy conversion processes are often accompanied by environmental

pollution

Pollutants emitted during the combustion of fossil fuels are responsiblefor smog , acid rain , and global warming .

The environmental pollution became a serious threat to vegetation ,wild life , and human health .

Motor vehicles are the largest source of airpollution.


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Ozone and Smog Smog : Made up mostly of ground-level ozone (O 3), but it also contains numerous other

chemicals, including carbon monoxide (CO), particulate matter such as soot and dust,volatile organic compounds (VOCs) such as benzene, butane, and other hydrocarbons.

Hydrocarbons and nitrogen oxides react in the presence of sunlight on hot calm days toform ground-level ozone. Ozone irritates eyes and damages the air sacs in the lungs where oxygen and carbon

dioxide are exchanged, causing eventual hardening of this soft and spongy tissue. It also causes shortness of breath, wheezing, fatigue, headaches, and nausea, and

aggravates respiratory problems such as asthma.

Ground-level ozone, which is the primary componentof smog, forms when HC and NO x react in thepresence of sunlight in hot calm days.

The other serious pollutant in smog is carbonmonoxide , which is a colorless, odorless, poisonousgas.

It is mostly emitted by motor vehicles. It deprives the bodys organs from getting enough

oxygen by binding with the red blood cells that would

otherwise carry oxygen. It is fatal at high levels. Suspended particulate matter such as dust and soot

are emitted by vehicles and industrial facilities. Suchparticles irritate the eyes and the lungs.


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Acid Rain The sulfur in the fuel reacts with oxygen to form sulfur dioxide (SO 2), which is an

air pollutant. The main source of SO 2 is the electric power plants that burn high-sulfur coal.

Motor vehicles also contribute to SO 2 emissions since gasoline and diesel fuelalso contain small amounts of sulfur.

Sulfuric acid and nitric acid are formedwhen sulfur oxides and nitric oxides react withwater vapor and other chemicals high in the

atmosphere in the presence of sunlight.

The sulfur oxides and nitric oxides reactwith water vapor and other chemicals highin the atmosphere in the presence ofsunlight to form sulfuric and nitric acids.

The acids formed usually dissolve in thesuspended water droplets in clouds orfog.

These acid-laden droplets, which can beas acidic as lemon juice, are washed from

the air on to the soil by rain or snow. Thisis known as acid rain .


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The GreenhouseEffect: Global

Warming

Greenhouse effect : Glass allows the solarradiation to enter freely but blocks theinfrared radiation emitted by the interiorsurfaces. This causes a rise in the interior

temperature as a result of the thermalenergy buildup in a space (i.e., car). The surface of the earth, which warms up

during the day as a result of the absorptionof solar energy, cools down at night byradiating part of its energy into deep spaceas infrared radiation.

Carbon dioxide (CO 2), water vapor, andtrace amounts of some other gases suchas methane and nitrogen oxides act like ablanket and keep the earth warm at nightby blocking the heat radiated from theearth. The result is global warming .

These gases are called greenhousegases , with CO 2 being the primarycomponent.

CO 2 is produced by the burning of fossil

fuels such as coal, oil, and natural gas .The greenhouse effect on earth.


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A rise of this magnitude can cause severe changes in weather patterns with storms and heavy rains and flooding at some parts and drought inothers, major floods due to the melting of ice at the poles, loss of wetlandsand coastal areas due to rising sea levels, and other negative results.

Improved energy efficiency, energy conservation, and usingrenewable energy sources help minimize global warming.


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THE SECOND LAW OFTHERMODYNAMICS

MAJOR USES OF THE SECOND LAW

used to identify the direction of processes. second law asserts that energy has quality

as well as quantity. The second law provides the necessary

means to determine the quality as well asthe degree of degradation of energy duringa process.

also used in determining the theoretical

limits for the performance of commonlyused engineering systems, such as heatengines and refrigerators etc.

A cup of hot coffee

does not get hotter ina cooler room.


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HEAT ENGINES

Work can alwaysbe converted toheat directly andcompletely, but the

reverse is not true.

Part of the heatreceived by a heatengine isconverted to work,while the rest isrejected to a sink.

The devices that convert heat to work.

1. receive heat from a high-T source(solar energy, oil furnace, nuclearreactor, etc.).

2. convert part of this heat to work (inthe form of a rotating shaft.)

3. reject the remaining waste heat to alow-T sink (the atmosphere, rivers,etc.).

4. They operate on a cycle.

Heat engines and other cyclic devicesusually involve a fluid to and fromwhich heat is transferred whileundergoing a cycle. This fluid iscalled the working fluid .


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A steam power plant

A portion of the work outputof a heat engine is consumed

internally to maintaincontinuous operation.


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Thermal efficiency

Some heat engines perform betterthan others (convert more of theheat they receive to work).

Schematic ofa heat engine.


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Can we save Q out ?

A heat-engine cycle cannot be completed without rejecting some heat to alow-temperature sink.

In a steam power plant, the condenser is the device where large quantities

of waste heat is rejected to rivers, lakes, or the atmosphere.

Can we not just take the condenser out of the plant and save all that wasteenergy?

n o for the simple reason that without a heat rejection process in acondenser, the cycle cannot be completed.


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The Second Law ofThermodynamics:Kelvin Planck Statement

A heat engine that violates theKelvin Planck statement of thesecond law.

It is impossible for any devicethat operates on a cycle toreceive heat from a singlereservoir and produce a netamount of work.

No heat engine can have a thermalefficiency of 100 percent , or as for a

power plant to operate, the working fluidmust exchange heat with theenvironment as well as the furnace .

The impossibility of having a 100%efficient heat engine is not due tofriction or other dissipative effects. It is alimitation that applies to both theidealized and the actual heat engines.


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THECARNOTCYCLE

Reversible Isothermal Expansion (process 1-2, T H = constant)Reversible Adiabatic Expansion (process 2-3, temperature drops from T H to T L)Reversible Isothermal Compression (process 3-4, T L = constant)Reversible Adiabatic Compression (process 4-1, temperature rises from T L to T H )

Execution ofthe Carnotcycle in aclosedsystem.


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P-V diagram of the Carnot cycle. P-V diagram of the reversedCarnot cycle.

The Reversed Carnot CycleThe Carnot heat-engine cycle is a totally reversible cycle.

Therefore, all the processes that comprise it can be reversed ,in which case it becomes the Carnot refrigeration cycle .


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THE CARNOTPRINCIPLES

1. The efficiency of an irreversible heat engine is always less than theefficiency of a reversible one operating between the same tworeservoirs.

2. The efficiencies of all reversible heat engines operating between the

same two reservoirs are the same.

The Carnot principles.


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THE CARNOT HEAT ENGINE

The Carnotheat engineis the mostefficient ofall heatenginesoperatingbetween thesame high-and low-temperature

reservoirs.

No heat engine can have a higherefficiency than a reversible heat engine

operating between the same high- andlow-temperature reservoirs.

Any heatengine

Carnot heatengine


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The Quality of Energy

The fraction of heat thatcan be converted to workas a function of sourcetemperature.

The higher the temperatureof the thermal energy, thehigher its quality.

How do you increase thethermal efficiency of a Carnotheat engine? How about foractual heat engines?


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THE CARNOT VAPOR CYCLE

T-s diagram of two Carnot vapor cycles.

The Carnot cycle is the most efficient cycle operating between two specified temperaturelimits but it is not a suitable model for power cycles. Because:Process 1-2 Limiting the heat transfer processes to two-phase systems severely limits the

maximum temperature that can be used in the cycle (374C for water)Process 2-3 The turbine cannot handle steam with a high moisture content because of theimpingement 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.The cycle in (b) is not suitable since it requires isentropic compression toextremely high pressures and isothermal heat transfer at variable pressures.

1-2 isothermal heataddition in a boiler2-3 isentropic expansionin a turbine3-4 isothermal heat

rejection in a condenser4-1 isentropiccompression in acompressor

RANKINE CYCLE THE IDEAL CYCLE


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RANKINE CYCLE: THE IDEAL CYCLEFOR VAPOR POWER CYCLESSolutions : superheating the steam in the boiler and condensing it completely inthe condenser. Rankine cycle , which is the ideal cycle for vapor power plants.

The simple ideal Rankine cycle.


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Energy Analysis of the Ideal Rankine Cycle

The efficiency of power plants in

the U.S. is often expressed interms of heat rate , which is theamount of heat supplied, in Btus,to generate 1 kWh of electricity. The thermal efficiency can be interpreted

as the ratio of the area enclosed by thecycle on a T-s diagram to the area underthe heat-addition process.

Steady-flow energy equation


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HOW CAN WE INCREASE THE EFFICIENCY OF THERANKINE CYCLE?Idea : Increase the average temperature at which heat is transferred to the

working fluid in the boiler, or decrease the average temperature at which heat isrejected from the working fluid in the condenser.

Lowering the Condenser Pressure ( Low ers T low,avg )

condensers of steam power plants usuallyoperate well below the atmospheric pressure.

lower limit to this pressure depending on thetemperature of the cooling medium

Side effect: Lowering the condenserpressure increases the moisture content ofthe steam at the final stages of the turbine.


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Superheating the Steam to High Temperatures(Inc reases T high,avg )

The overall effect is an increase in

thermal efficiency since the averagetemperature at which heat is addedincreases.

Superheating to higher temperaturesdecreases the moisture content of thesteam at the turbine exit, which isdesirable.

The temperature is limited bymetallurgical considerations. Presentlythe highest steam temperature allowedat the turbine inlet is about 620C.


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THE IDEAL REHEAT RANKINE CYCLEHow can we take advantage of the increased efficiencies at higher boiler pressureswithout facing the problem of excessive moisture at the final stages of the turbine?1. Superheat the steam to very high temperatures. It is limited metallurgically.2. Expand the steam in the turbine in two stages, and reheat it in between ( reheat )

The ideal reheat Rankine cycle.


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The average temperature at

which heat is transferred duringreheating increases as thenumber of reheat stages isincreased.

The single reheat in a modern powerplant improves the cycle efficiency by 4 to5% by increasing the averagetemperature at which heat is transferredto the steam.

The average temperature during thereheat process can be increased byincreasing the number of expansion andreheat stages. As the number of stages isincreased, the expansion and reheat

processes approach an isothermalprocess at the maximum temperature.The use of more than two reheat stagesis not practical. The theoreticalimprovement in efficiency from thesecond reheat is about half of that which

results from a single reheat.The reheat temperatures are very closeor equal to the turbine inlet temperature.

The optimum reheat pressure is aboutone-fourth of the maximum cycle

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Chapter 2 - Energy Concepts