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4 Problem on Building Energy Audits - Assignment V - Fall 2007 | CVEN 5020, Assignments of Civil Engineering

University of Colorado Boulder (CU Boulder)Civil Engineering
Prof. Moncef Krarti

Material Type: Assignment; Professor: Krarti; Class: BUILDING ENERGY AUDITS; Subject: Civil Engineering; University: University of Colorado - Boulder; Term: Unknown 1989;

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CVEN 5020 BUILDING ENERGY AUDITS FALL 2007
University of Colorado at Boulder 1
Homework No. 5
Due October 18, 2007
Problem No. 1:
A daylighting system using dimming controls is being considered for a 4-story office building
located in Denver, CO. A typical floor is 200 ft by 50 ft with perimeter offices extending 15 ft from
the vertical windows. The electrical lighting has a density of 1.1 W/ft2 and is operated (without
daylighting controls) 5,000 hours/year. The cost of the daylighting system is estimated at $40,000.
The energy cost is $0.08/kWh.
(a) Assess the cost-effectiveness of the daylighting system using 20-yr cycle and 5% discount
rate if double clear glazing is used. Consider three WWR values 10%, 20%, and 30%.
(b) Repeat (a) for double reflective tinted glazing.
Problem No. 2:
The nameplate of a motor provides the following information:
Full Load HP 100
Volts 440/220
Amperes 123/246
Full Load RPM 1,775
During an audit, measurements on the motor indicated that:
Average Volts 440
Amperes 116
Full Load RPM 1,779
(a) Calculate the following parameters: the slip RPM, the efficiency, and the power factor of
the motor
(b) Determine if it is cost-effective to replace this motor with a more efficient motor (You need
to document the reference for the motor cost). Perform the calculations with and without
adjusting the speed of the energy efficient motor.
Problem No. 3:
Two motors operate 5,500 h/yr at full load. One is 60 HP with an efficiency of 0.78 and a power
factor of 0.75 while the other motor is 100 HP with an efficiency of 0.94 and a power factor of
0.80. Determine:
1. The overall power factor.
2. The capacitor in kVAR to be added to ensure that the overall power factor is about 0.90.
Instead of using a capacitor, provide an alternative to improve the overall power of both
motors.
3. The simple payback period of replacing the standard motors by energy-efficient motors.
Assume that the costs of standard and efficient motors are respectively, $1,500 and $1,950
(for 60 HP) and $3,100 and $3,700 (for 100 HP).
Assume that the cost of electricity is $0.09/kWh.
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CVEN 5020 BUILDING ENERGY AUDITS FALL 2007

University of Colorado at Boulder 1

Homework No. 5 Due October 18, 2007

Problem No. 1: A daylighting system using dimming controls is being considered for a 4-story office building located in Denver, CO. A typical floor is 200 ft by 50 ft with perimeter offices extending 15 ft from the vertical windows. The electrical lighting has a density of 1.1 W/ft 2 and is operated (without daylighting controls) 5,000 hours/year. The cost of the daylighting system is estimated at $40,000. The energy cost is $0.08/kWh.

(a) Assess the cost-effectiveness of the daylighting system using 20-yr cycle and 5% discount rate if double clear glazing is used. Consider three WWR values 10%, 20%, and 30%. (b) Repeat (a) for double reflective tinted glazing.

Problem No. 2: The nameplate of a motor provides the following information: Full Load HP 100 Volts 440/ Amperes 123/ Full Load RPM 1,

During an audit, measurements on the motor indicated that: Average Volts 440 Amperes 116 Full Load RPM 1,

(a) Calculate the following parameters: the slip RPM, the efficiency, and the power factor of the motor (b) Determine if it is cost-effective to replace this motor with a more efficient motor (You need to document the reference for the motor cost). Perform the calculations with and without adjusting the speed of the energy efficient motor.

Problem No. 3: Two motors operate 5,500 h/yr at full load. One is 60 HP with an efficiency of 0.78 and a power factor of 0.75 while the other motor is 100 HP with an efficiency of 0.94 and a power factor of 0.80. Determine:

  1. The overall power factor.
  2. The capacitor in kVAR to be added to ensure that the overall power factor is about 0.90. Instead of using a capacitor, provide an alternative to improve the overall power of both motors.
  3. The simple payback period of replacing the standard motors by energy-efficient motors. Assume that the costs of standard and efficient motors are respectively, $1,500 and $1, (for 60 HP) and $3,100 and $3,700 (for 100 HP).

Assume that the cost of electricity is $0.09/kWh.

CVEN 5020 BUILDING ENERGY AUDITS FALL 2007

University of Colorado at Boulder 2

Problem No. 4: Consider a three-phase 1,500-kVA transformer to step-down the volt from 13.8 kV to 480Y/ volt. The transformer is old and needs to be replaced. Two options are available:

i) replace it by the same transformer with an efficiency of 98.6% and an installed cost of $45,000 and ii) replace it by more energy efficient transformer with an efficiency of 99.0% and an installed cost of $60,000.

(a) Determine the best option for the transformer replacement considering a lifetime of 25 years, and electricity price of $0.06, a discount rate of 4.1%, and an average load factor of 50%. (b) Determine the electricity price for which the energy efficient motor is not cost-effective using the same assumptions of question (a). (c) Determine the size of the main secondary feeders for both the standard and the energy efficient transformers. Calculate the energy savings if the next higher feeder size is used.