Systems, Energy,
& Efficiency
EGR 1301: Introduction to
Engineering
EGR 1301
Systems
• System
 A particular subset of the universe specified in
time and space by a boundary (Ch 17, p. 484)
System
boundary
tinitial = start time
tfinal = stop time
Source: Professor Thomas
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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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Importance of System Definition
Source: Foundations of Engineering, Holtzapple & Reece, 2003
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Intensive vs. Extensive
• Extensive quantities
 Change with size of the system
• Intensive quantities
 Remain constant, regardless of size
Quantity
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Intensive
Extensive
Volume
X
Mass
X
Density
X
Temperature
X
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What is Energy?
• “The capacity for doing work”
OR
• Unit of exchange (Ch 22, p. 572)
• Examples:
Source: Webster’s New Collegiate Dictionary
 Electricity  light or heat
 Chemical energy in gasoline  torque in car
or heat
 Natural gas  electricity or hot water
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Units of Energy
Source: Foundations of Engineering, Holtzapple & Reece, 2003
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1st Law of Thermodynamics
• “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
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Money Accounting
• Can “account” for the money in your bank:
Final balance – Initial balance = Deposits - Withdrawals
Accumulation = Net input
Final balance = Initial balance + Deposits - Withdrawals
• 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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Energy Accounting
• For any system, the same relationship is true:
Final energy – Initial energy = Input - Output
Accumulation = Net input
State quantities = Path quantities
System Boundary
Accumulated
Energy
(State Quantities)
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Energy in/out
(Path Quantities)
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State Quantities
• Kinetic Energy
 Energy associated with motion
1 2
Ek  mv
2
• 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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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: fuel
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E  mc2
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Universal Accounting
Equation
• Mathematical version of the accounting
equation:
Ek  E p  U  Win  Wout  Qin  Qout  M in  M out
Ek  Ekf  Eki
E p  E pf  E pi
Change in kinetic energy
Change in potential energy
U  U f  U i
Change in internal energy
• All have the form:
Change = Energy at tfinal - Energy at tinitial
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Universal Accounting
Equation
• Mathematical version of the accounting
equation:
Ek  E p  U  Win  Wout  Qin  Qout  M in  M out
Work input = work done
on the system from its
surroundings
Won Wby
Work output = work done
by the system to its
surroundings
• Heat and Mass have the form:
Energy added to system – Energy removed from system
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Joule’s Experiment
System
boundary
tinitial = mass is raised
tfinal = after mass falls and
propeller and water stop
moving
Assume perfect insulation.
How are variables related?
W  Fx
F  ma
Source: Foundations of Engineering, Holtzapple & Reece, 2003
Ek  E p  U  Win  Wout  Qin  Qout  M in  M out
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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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Energy Conversion
• A system converts energy from one form to
another
• The process is not always perfect
Energy In
Energy Conversion
Device (System)
Energy Out
Wasted Energy
(often heat)
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Efficiency
• Measure of how well a system can convert
energy
• Greek letter eta, η
output

input
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0  1
0%    100%
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Example
• If a system outputs 70,000 J and η = 0.7,
what is the input energy?
output

input
input 
output

70,000

 100,000 J
0 .7
• How much was wasted?
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30,000 J
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Cascaded Conversion
• Can connect multiple systems together and do
several conversions
Natural gas
Rotating shaft
Gas turbine
E1
η1
Waste 1
(heat)
E2
1 
E1
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Electricity
Generator
E2
η2
Light
Light bulb
E3
η3
Waste 2
(heat)
Waste 3
(heat)
E3
2 
E2
E4
3 
E3
E4
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Overall Efficiency
• Treat multiple conversions as a single process
ηoverall
E1
η1
η2
η3
E4
Total waste
(heat)
E4
E2 E3 E4






 overall 
 
1
2
3 
E1
E1 E2 E3
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1  0.35 2  0.98 3  0.03
  0.0103  1.03%
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Recap
•
•
•
•
•
•
•
•
Systems – boundary (time & space)
Energy – unit of exchange
Intensive vs. Extensive Quantities
State vs. Path Quantities
Universal Accounting Equation
Efficiency
Cascaded systems
Next: examples
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