CEA Lighting & HVAC Energy Analysis 5 — Questions and Answers
Question 1: A building energy model shows an annual cooling load of 500,000 ton-hours. If the chiller plant average efficiency is 0.65 kW/ton, what is the annual chiller electricity consumption?
- 125,000 kWh
- 250,000 kWh
- 325,000 kWh (Correct answer)
- 500,000 kWh
Correct answer: 325,000 kWh
Annual energy = 500,000 ton-hr × 0.65 kW/ton = 325,000 kWh.
Question 2: What is the primary advantage of a thermal energy storage (TES) system in a commercial building?
- It eliminates the need for mechanical cooling equipment
- It shifts cooling energy use to off-peak hours to reduce demand charges (Correct answer)
- It improves indoor air quality by reducing refrigerant use
- It increases chiller COP during peak hours
Correct answer: It shifts cooling energy use to off-peak hours to reduce demand charges
TES systems generate cooling during off-peak hours and store it, reducing peak demand charges and taking advantage of lower off-peak electricity rates.
Question 3: According to ASHRAE 90.1, what automatic controls are required for exterior lighting in commercial buildings?
- Occupancy sensors on all exterior areas
- Both time-of-day scheduling and photosensor (daylighting) controls (Correct answer)
- Dimming to 50% after midnight via timer
- Manual override switches at each entry point
Correct answer: Both time-of-day scheduling and photosensor (daylighting) controls
ASHRAE 90.1 requires exterior lighting to be controlled by both a time-of-day schedule and a photosensor to ensure lights extinguish during daylight hours.
Question 4: Which of the following best describes 'infiltration' in the context of building HVAC energy loads?
- Intentional outdoor air brought in through the AHU
- Uncontrolled air leakage through envelope gaps and cracks (Correct answer)
- Exhaust air returned to the supply stream
- Moisture migrating through building insulation
Correct answer: Uncontrolled air leakage through envelope gaps and cracks
Infiltration is unintentional air leakage into or out of a building through gaps in the envelope, adding heating or cooling load depending on outdoor conditions.
Question 5: A photoelectric dimming system reduces lighting to 40% of full output when daylight is sufficient. The connected load is 10 kW and the system dims for 1,500 hours per year. How many kWh are saved annually compared to full-output operation?
- 4,500 kWh
- 6,000 kWh
- 9,000 kWh (Correct answer)
- 15,000 kWh
Correct answer: 9,000 kWh
At 40% output, 60% of energy is saved; 10 kW × 0.60 × 1,500 hr = 9,000 kWh saved annually.
Question 6: What is 'chiller sequencing' and why is it important for energy efficiency?
- Operating chillers in series to achieve lower supply water temperatures
- Staging multiple chillers on/off to match load and keep each unit near peak efficiency (Correct answer)
- Rotating chiller operation weekly for maintenance purposes
- Running all available chillers simultaneously to reduce compressor wear
Correct answer: Staging multiple chillers on/off to match load and keep each unit near peak efficiency
Chiller sequencing stages units so each chiller operates near full load where COP is highest, avoiding inefficient part-load operation of multiple chillers simultaneously.
Question 7: Which measure best reduces heating energy use in a VAV reheat system serving a large office building?
- Increasing supply air temperature setpoint year-round
- Implementing supply air temperature reset based on outdoor air temperature (Correct answer)
- Installing larger ductwork to reduce static pressure
- Adding more VAV terminal unit reheat coils throughout the building
Correct answer: Implementing supply air temperature reset based on outdoor air temperature
Supply air temperature reset raises the supply air temperature during mild weather, reducing reheat energy and delivering less over-cooled air to terminal units.
CEA Lighting & HVAC Energy Analysis 5
A building energy model shows an annual cooling load of 500,000 ton-hours.
If the chiller plant average efficiency is 0.65 kW/ton, what is the annual chiller electricity consumption?