EGR 1301

Systems, Energy, & Efficiency

EGR 1301: Introduction to Engineering

EGR 1301

• System A particular subset of the universe specified in

time and space by a boundary (Ch 17, p. 484)

Systems

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Source: Professor Thomas

System boundarytinitial = start time

tfinal = stop time

EGR 1301

System Definition

• Rules the engineer must follow: Once a system is specified, it cannot be

changed midway through a calculation. The system boundary can be any shape, but

it must be a closed surface. It must also be closed (or bounded) in time.

The system boundary can be rigid (defining a volume of space) or it can be flexible (defining an object).

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EGR 1301

Importance of System Definition

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Source: Foundations of Engineering, Holtzapple & Reece, 2003

EGR 1301

Intensive vs. Extensive

• Extensive quantities Change with size of the system

• Intensive quantities Remain constant, regardless of size

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Quantity Intensive Extensive

Volume

Mass

Density

Temperature

X

X

X

X

EGR 1301

What is Energy?

• “The capacity for doing work”

OR• Unit of exchange (Ch 22, p. 572)

• Examples: Electricity light or heat Chemical energy in gasoline torque in car

or heat Natural gas electricity or hot water

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Source: Webster’s New Collegiate Dictionary

EGR 1301

Units of Energy

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Source: Foundations of Engineering, Holtzapple & Reece, 2003

EGR 1301

1st Law of Thermodynamics

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• “Law of Conservation of Energy”

• Energy can neither be created nor destroyed Therefore, energy must be conserved Energy can only be transformed

• Work can be converted into another form of work• Work can be converted into heat

• Need to keep track of, or “account” for, these changes

EGR 1301

Money Accounting• Can “account” for the money in your bank:

• Ex: Start with $1000 Pay you $500 for coming to class Spend $800 on new laptop How much do you have (i.e. final balance)?

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Final balance – Initial balance = Deposits - Withdrawals

Final balance = Initial balance + Deposits - Withdrawals

Accumulation = Net input

EGR 1301

Energy Accounting

• For any system, the same relationship is true:

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System Boundary

Accumulated Energy

(State Quantities)

Energy in/out(Path Quantities)

Final energy – Initial energy = Input - Output

Accumulation = Net input

State quantities = Path quantities

EGR 1301

State Quantities

• Kinetic Energy Energy associated with motion

• Potential Energy Energy associated with position, either

against a field (e.g. gravity or electric field), compressed spring, or stretched rubber band

• Internal Energy Energy associated with atoms, such as

temperature, phase changes, or chemical reactions

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2

2

1mvEk

EGR 1301

Path Quantities

• Work Energy flow due to a driving force other than

temperature: mechanical (shaft, hydraulic), electrical, photonic (laser, solar PV)

• Heat Energy flow due to temperature: conduction,

blackbody radiation

• Mass Energy flow due to mass crossing the

boundary: fuel12

2mcE

EGR 1301

Universal Accounting Equation

• Mathematical version of the accounting equation:

• All have the form:

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outinoutinoutinpk MMQQWWUEE

kikfk EEE pipfp EEE if UUU

Change in kinetic energy Change in potential energy Change in internal energy

Change = Energy at tfinal - Energy at tinitial

EGR 1301

Universal Accounting Equation

• Mathematical version of the accounting equation:

• Heat and Mass have the form:

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outinoutinoutinpk MMQQWWUEE

onWWork input = work done on the system from its surroundings

Work output = work done by the system to its surroundings

Energy added to system – Energy removed from system

byW

EGR 1301

Joule’s Experiment

Source: Foundations of Engineering, Holtzapple & Reece, 2003

xFW maF

outinoutinoutinpk MMQQWWUEE

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System boundary

tinitial = mass is raised

tfinal = after mass falls and

propeller and water stop movingAssume perfect insulation.

How are variables related?

EGR 1301

2nd Law of Thermodynamics

• Naturally occurring processes are directional Closely tied to idea of reversibility Reversible processes have no directionality

• Entropy

• Ex: balloon, car, office

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EGR 1301

Energy Conversion

• A system converts energy from one form to another

• The process is not always perfect

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Energy Conversion Device (System)

Energy OutEnergy In

Wasted Energy (often heat)

EGR 1301

Efficiency

• Measure of how well a system can convert energy

• Greek letter eta, η

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input

output 10

%100%0

EGR 1301

Example

• If a system outputs 70,000 J and η = 0.7, what is the input energy?

• How much was wasted?

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input

output

output

input 7.0

000,70 000,100 J

30,000 J

EGR 1301

• Can connect multiple systems together and do several conversions

Cascaded Conversion

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η1

Gas turbine

Natural gas

E1

Waste 1(heat)

η2

Generator

η3

Light bulb

Waste 2(heat)

Waste 3(heat)

E2 E3 E4

Rotating shaft Electricity Light

1

21 E

E

2

32 E

E

3

43 E

E

EGR 1301

• Treat multiple conversions as a single process

Overall Efficiency

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η1

ηoverall

E1 η2 η3

Total waste(heat)

E4

1

4

E

Eoverall 321

3

4

2

3

1

2

E

E

E

E

E

E

35.01 98.02 03.03

%03.10103.0

EGR 1301

Recap

• Systems – boundary (time & space)

• Energy – unit of exchange

• Intensive vs. Extensive Quantities

• State vs. Path Quantities

• Universal Accounting Equation

• Efficiency

• Cascaded systems

• Next: examples22