Transcript 346N_083104_electrical_intro

Beginning Question
1. How much wood would a woodchuck chuck
if a woodchuck could chuck wood?
A. A lot
B. A little
C. None
D. Don’t know
Admin
• I sent a test message yesterday
• If you didn’t get it:
• Send me an e-mail message with ARE346N in the
subject line
• If you change your e-mail address over the course
of the semester you are responsible for forwarding
me your new address
Admin
• Textbook
• 3rd edition of Tao and Janis is on order at the co-op
• Only 5 copies were ordered, so let them know if
more or you want them
Objectives
• Critically analyze conservation claims
• Introduction to course themes
• List and define terms associated with electrical design
in buildings
• Calculate electric quantities with Ohm’s Law and
Kirchov’s Law (review)
• Calculate electricity costs for residential and
commercial buildings
• Major emphasis – how do you begin designing an
electrical system?
Critical Analysis
• Central objective of this course
• Myths website – myths aren’t always false
• http://ficp.engr.utexas.edu/conservationMyths/
• Goal is to evaluate them
• Project I
2) Myth 1: How long would the
average American have to leave the
bathroom tap running to use as much
water as one shower
A.
B.
C.
D.
60 minutes
30 minutes
15 minutes
5 minutes
3) How long is your average shower?
A.
B.
C.
D.
E.
2 minutes
5 minutes
10 minutes
20 minutes
30 minutes
Myth 1: Tight With the Tap
t sink
Qshower
 t shower
Qsink
t sink
4.5 gallon/min
 10 min
 30 min
1.5 gallon/min
• Claim is unlikely to be true because
very few people leave their taps
running for ½ hour.
4) Myth 2: Using drapes reduces heat loss
by ~1/3 for single glazed windows.
A.
B.
C.
D.
Untrue, heat loss is unaffected by drapes
Untrue, heat loss is only reduced by 10–15%
Approximately true
Untrue, a drape reduces heat loss by more
than 40%
Conclusions
• Claims are often made without verification
• We have the tools to evaluate these claims
• Critical analysis is necessary/important for
engineering
• Law of unintended consequences
Where are we going?
• Electrical systems
• Lighting technology and design
Applied Electrical Concepts
• Use terms to describe building electrical
system
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•
•
•
•
•
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AC vs DC
Ohm’s and Kirchov’s Laws
Power vs. Energy
Inductors and Capacitors
Power Factor
Single-phase vs. Three-phase
Grounding
ch. 9
ch. 8
ch. 9
ch. 9
ch. 9
ch. 9
Electrical Codes
• National Electrical Code (NEC aka NFPA 70)
• Library website
• Focus on safety
• Talk to the electrical inspector
AC DC
• Direct Current (DC)
• Sign matters
• Alternating Current (AC)
• Switching polarity
• Which are the following?
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•
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Photovoltaic solar cell
Automobile alternator
Household outlet
Outlet in ECJ
Battery
High voltage lines
AC terms
• Frequency, f [Hz]
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•
•
•
Wall outlet?
Fluorescent light?
Clock?
Electric resistance heater?
• Capacitor (plates separated by dielectric)
• Inductor (coil of wire)
• Transformers
Why use AC?
• Easier to generate
• Able to use inductors and capacitors in
meaningful way
• Voltage changes much easier
• Lower distribution losses
Voltage and Current
•Voltage (potential difference)
•Units?
•Symbol V or E
•Current (electron flow)
•Units?
•Symbol I
Across a transformer
I1E1 = I2E2
Ref: Tao and Janis (2001)
Ohm’s and Kirchov’s Laws
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•
•
•
V = IR, E = IR (E = energy [V])
P = EI = I2R
∑I = 0 for a node
∑E = 0 for a closed loop
• Are these for AC or DC or both?
Single-Phase vs. Three-Phase
For three phase:
P=√3 E I
Ref: Tao and Janis (2001)
5) Why do commercial buildings use
3 power?
a)
b)
c)
d)
Requires fewer wires
Higher motor efficiency
Requires bigger wires
Smaller space requirements
Why Three-Phase?
• Larger loads
• Smaller wire sizes (because higher voltages)
• P = I2R
• More efficient use of neutral wire
• 3 needed for efficient operation of equipment
Power Factor
• Difference between power supplied and power
that does useful work “working power”
• P=E I cos θ
• θ is the phase difference between current and
voltage
• For three phase
• P=√3 E I cos θ
• Commercial buildings pay for power factor.
6) Are kWh units of energy or power?
A. Energy
B. Power
Electricity Billing (ch. 13)
• Electrical Use (energy)
• Peak Demand (power)
• Power factor
• Which is largest portion of residential bill?
• What about for commercial buildings?
• http://www.austinenergy.com/About%20Us/Ra
tes/Commercial/index.htm
Example: September cost of Electricity
for ECJ
• Assume ECJ is 120,000 ft2 and that it needs,
on average, 8 W/ft2 for 8 hours a day, 6 W/
W/ft2 for 4 hours a day, and 4 W/ft2 for 12
hours a day
• Use Austin Energy Large Primary summer
service rate
• 1.5¢/kWh, 12.60 $/peak kW/month
• Assume no power factor charges
7) What is peak demand?
A.
B.
C.
D.
0.018 kW/ft2 × 120,000 ft2 = 2,160 kW
0.008 kW/ft2 × 120,000 ft2 = 960 kW
0.018 kW/ft2 × 120,000 ft2 × 8 hr = 17,280 kWh
0.008 kW/ft2 × 120,000 ft2 × 30 days = 28,800 kW
Solution
hours
8
4
12
Daily
kW
960
720
480
TOTAL
large primary service
$/kW
$/kWh
$ 12.60
$
0.015
kWh
7680
2880
5760
16320
demand
energy
$12,096.00
$ 7,344.00
Total
$19,440.00
Other Pricing Strategies
• Time of use pricing
• Becoming more common for residential and
commercial
• Electricity cost related to actual cost
• Requires meter
• Interruptible pricing
• Utility can shut off electricity for periods of time
Summary
• Know where to find NEC
• Compare DC and AC
• Calculate current/voltage across a transformer
• Describe 3-phase power and compare to 1phase
• List reasons for using 3-phase
• List and explain components of electrical
billing
• Calculate commercial electricity bill