Energy Conversion

Systems

Efrain Teran, M. Sc.

(FIMP – 03897)

Course programi. WORLD AND NATIONAL ENERGY SITUATION.

ii. THERMODYNAMICS REVIEW: EXERGY ANALYSIS AND COMBUSTION.

iii. STEAM, GAS TURBINE, AND COMBINED CYCLE POWER PLANTS.

iv. INTERNAL COMBUSTION ENGINE POWER PLANTS

v. FEASIBILITY AND TECHNOLOGIES FOR USING BIOMASS AS AN ENERGY SOURCE

vi. HYDROPOWER PLANTS

vii. SOLAR THERMAL AND PHOTOVOLTAIC POWER PLANTS

viii. WIND POWER PLANTS

ix. ENVIRONMENTAL AND SOCIAL IMPACT ANALYSIS OF ENERGY SYSTEMS

x. ENERGY USE AND EFFICIENCY IN THE INDUSTRIAL AND BUSINESS SECTORS

Chapter 2: EXERGY ANALYSIS AND

COMBUSTION

Chapter 8: Exergy

• • Cengel, Yunus A., Boles, Michael A. Thermodynamics: AnEngineering Approach. McGraw Hill

6th Edition, 2007. 5th Edition, 2004.

Chapter 8: Exergy

• • Cengel, Yunus A., Boles, Michael A. Thermodynamics: AnEngineering Approach. McGraw Hill

7th Edition, 2011. 8th Edition, 2014.

Combustion

https://www.youtube.com/watch?v=zEjEqnMBdEM

Combustion

https://www.youtube.com/watch?v=xokHLFE96h8

First law of thermodynamics

• The law of conservation of energy states that the totalenergy of an isolated system is constant; energy can betransformed from one form to another, but cannot becreated or destroyed.

Second law of thermodynamics

• The second law of thermodynamics states that, in everynatural thermodynamic process, the sum of theentropies of all participating bodies is increased. ,

• The second law is an empirical finding that has beenaccepted as an axiom of thermodynamic theory.

Exergy• Consider de following experiment: an isolated system

consisting initially of a small container of fuel surroundedby air in abundance.

Exergy• Suppose the fuel burns so finally there is a slightly warm

mixture of air and the combustion products formed.

• Since air is abundantly present, the temperature of thefinal mixture is nearly the same as the initial airtemperature.

• The total quantity of energyassociated with the system isconstant because no energytransfers take place across theboundary of an isolated system and,by the first law of thermodynamics,energy is conserved.

Exergy• The initial fuel-air combination has a

much greater potential for use than thefinal warm mixture. For instance, thefuel might be used to generateelectricity, produce steam, or power a carwhereas the final warm mixture is clearlyunsuited for such applications.

• Actually, during the process shown in thefigures the initial potential for use ispredominately destroyed owing to theirreversible nature of that process.

ExergyThe fuel present initially also has

economic value, but economic valuediminishes as fuel is consumed. The finalwarm mixture has negligible economicvalue.

Exergy is the property that quantifiesthe potential for use and it is exergy thathas economic value.

Conceptualizing Exergy

• Consider a body at temperature Ti placed in contact withthe atmosphere at temperature T0.

• If Ti > T0, the body cools spontaneously until it is inthermal equilibrium with the atmosphere.

Conceptualizing Exergy

• However, by controlling the cooling, work can bedeveloped as shown.

• Instead of the body cooling spontaneously, heat transferQ passes to a power cycle that develops work Wc. Thework is fully available for lifting a weight, developingshaft work, or generating electricity.

Exergy

• Exergy is the work potential of a source or system, thatis, the amount of energy we can extract as useful work.

• The work potential of a system at a specified state is themaximum useful work that can be obtained from thesystem.

• Exergy is also called the availability or available energy.

Exergy

• Recall that the work done during a process depends onthe initial state, the final state, and the process path.

Exergy

• The Exergy of a system is the maximum useful workpossible during a process that brings the system intoequilibrium with a heat reservoir.

Exergy• A system is said to be in the dead state when it is in

thermodynamic equilibrium with the environment it is in .

• At the dead state, a system is:

• at the temperature and pressure of its environment (in thermaland mechanical equilibrium);

• it has no kinetic or potential energy relative to the environment(zero velocity and zero elevation above a reference level);

• and it does not react with the environment (chemically inert).

Exergy• It is important to realize that exergy does not represent

the amount of work that a work-producing device willactually deliver upon installation.

• Rather, it represents the upper limit on the amount ofwork a device can deliver without violating anythermodynamic laws.

• There will always be a difference,large or small, betweenexergy and the actual work delivered by a device.

Exergy

• Note that the exergy of a system at a specified statedepends on the conditions of the environment (the deadstate) as well as the properties of the system.

• Therefore, exergy is a property of the system–environment combination and not of the system alone.

• Altering the environment is another way of increasingexergy, but it is definitely not an easy alternative.

Exergy

• Using energy and entropy balances, the followingexpression is obtained for the exergy, E, of a system at aspecified state,

where U, V, S, KE, and PE denote, respectively, internalenergy, volume, entropy, kinetic energy, and potentialenergy of the system at the specified state. U0, V0, and S0denote internal energy, volume, and entropy, respectively,of the system when at the dead state. At the dead state,the kinetic and potential energy of the system are eachzero.

Exergy• Once the environment is specified, a value can be

assigned to exergy in terms of property values for thesystem only, so exergy can be regarded as a property ofthe system.

• Exergy is an extensive property.

• We can also express the change in exergy between twostates as

Exergy

• Expressing Exergy on a unit mass basis, the specificexergy is:

Exergy

• Exergy (Work Potential) Associated with Kinetic andPotential Energy.

• Kinetic energy is a form of mechanical energy, and thus it can be converted to work entirely.

• Potential energy is also a form of mechanical energy, and thus it can be converted to work entirely

Exergy• Potential energy is also a form of mechanical energy, and

thus it can be converted to work entirely.

• Therefore, the exergy of the potential energy of a system is equal to the potential energy itself regardless of the temperature and pressure of the environment.

ExampleMaximum Power Generation by a Wind Turbine

• A wind turbine with a 12-m-diameter rotor, as shown in the figure, is to be installed at a location where the wind is blowing steadily at an average velocity of 10 m/s. Determine the maximum power that can be generated by the wind turbine.

ExampleMaximum Power Generation by a Wind Turbine

ExampleMaximum Power Generation by a Wind Turbine

ExampleMaximum Power Generation by a Wind Turbine

ExampleMaximum Power Generation from a lake

A lake located 200 m above the closest riverlevel has a water volume of 1,000,000 m3.

What is the total exergy and specific exergyof this water reservoir?

If the turbine pipeline takes a volume of 10m3/s, what is the maximum possible powergeneration?

Exergy of potential energy:

pe (kJ/kg)x pe gz

ExampleMaximum Power Generation from a lake

Exergy of potential energy:

pe (kJ/kg)x pe gz

ExampleMaximum Power Generation by a Wind Turbine

ExampleMaximum Power Generation by a Wind Turbine

Exergy

• Expressing Exergy on a unit mass basis, the specificexergy is:

Exergy• If the system temperature at the final state is greater

than (or less than) the temperature of the environment itis in, we can always produce additional work by running aheat engine between these two temperature levels.

• If the final pressure is greater than (or less than) thepressure of the environment, we can still obtain work byletting the system expand to the pressure of theenvironment.

• If the final velocity of the system is not zero, we can catchthat extra kinetic energy by a turbine and convert it torotating shaft work.

Exergy

• The exergy of the thermal energy of thermal reservoirs isequivalent to the work output of a Carnot heat engineoperating between the reservoir and the environment.

Carnot engine• https://www.youtube.com/watch?v=kJlmRT4E6R0

Carnot engine• https://www.youtube.com/watch?v=s3N_QJVucF8

Carnot engine• An amount of heat QH flows from a high temperature TH

furnace through the fluid of the "working body" (workingsubstance) and the remaining heat QC flows into the cold sinkTC, thus forcing the working substance to do mechanical workW on the surroundings, via cycles of contractions andexpansions.

Carnot's theorem

• No engine operating between two heat reservoirs can be moreefficient than a Carnot engine operating between the samereservoirs.

Example:Exergy Transfer from a Furnace

Example:Exergy Transfer from a Furnace

Example:Exergy Transfer from a Furnace

Example:Exergy Transfer from a Furnace

REVERSIBLE WORK AND IRREVERSIBILITY

• The evaluation of exergy alone, however, is not sufficientfor studying engineering devices operating between twofixed states.

• This is because when evaluating exergy, the final state isalways assumed to be the dead state, which is hardly everthe case for actual engineering systems.

• In this section, we describe two quantities that are relatedto the actual initial and final states of processes and serveas valuable tools in the thermodynamic analysis ofcomponents or systems.

• These two quantities are the reversible work andirreversibility (or exergy destruction).

REVERSIBLE WORK AND IRREVERSIBILITY

• But first we examine the surroundings work, which is thework done by or against the surroundings during aprocess.

• The work done by work-producing devices is not alwaysentirely in a usable form. Some of it can go to theenvironment.

REVERSIBLE WORK AND IRREVERSIBILITY

• Reversible work, 𝑊𝑟𝑒𝑣 , is defined as the maximumamount of useful work that can be produced as a systemundergoes a process between the specified initial andfinal states.

REVERSIBLE WORK AND IRREVERSIBILITY

• Reversible work is the useful work output obtained whenthe process between the initial and final states isexecuted in a totally reversible manner.

• When the final state is the dead state, the reversible workequals Exergy.

REVERSIBLE WORK AND IRREVERSIBILITY

• Any difference between the reversible work, 𝑊𝑟𝑒𝑣, andthe useful work, 𝑊𝑢, is due to the irreversibilities presentduring the process, and this difference is calledirreversibility 𝐼.

Example: The Rate of Irreversibility of a Heat Engine

• A heat engine receives heat from a source at 1200 K at a rate of500 kJ/s and rejects the waste heat to a medium at 300 K. Thepower output of the heat engine is 180 kW. Determine thereversible power and the irreversibility rate for this process.

Example: The Rate of Irreversibility of a Heat Engine

• The reversible power for this process is the amount of powerthat a reversible heat engine, such as a Carnot heat engine,would produce when operating between the sametemperature limits, and is determined to be:

Example: The Rate of Irreversibility of a Heat Engine

• This is the maximum power that can be produced by a heat engineoperating between the specified temperature limits and receivingheat at the specified rate. This would also represent the EXERGY(available power) if 300 K were the lowest temperature available forheat rejection.

• The irreversibility rate is the difference between the reversible power(maximum power that could have been produced) and the usefulpower output:

Example: The Rate of Irreversibility of a Heat Engine

• Discussion Note that 195 kW of power potential is wastedduring this process as a result of irreversibilities.

• Also, the 500 - 375 = 125 kW of heat rejected to the sink isnot available for converting to work and thus is not partof the irreversibility.

Total Power500 kW Reversible Power

(available power)375 kW

Unavailable power125 kW

Actual Power180 kW

Irreversibility195 kW

FIRST LAW EFFICIENCY

• In general, energy conversion efficiency is the ratiobetween the useful output of a device and the input, inenergy terms.

η𝐼 = η𝑡ℎ =𝐸𝑜𝑢𝑡𝐸𝑖𝑛

SECOND LAW EFFICIENCY• The first law efficiency makes no reference to the best

possible performance, and thus it may be misleading.

• Consider two heat engines:

• With 30% efficiency

SECOND LAW EFFICIENCY

These engines, at best, can perform as reversible engines, in which case their efficiencies would be:

SECOND LAW EFFICIENCY