CEA Energy Modeling & Simulation Tools 5 — Questions and Answers
Question 1: In EnergyPlus, which object defines the performance curve for a centrifugal chiller's coefficient of performance (COP) as a function of part-load ratio?
- Chiller:Electric:EIR with CurveFit objects (Correct answer)
- Coil:Cooling:WaterToAirHeatPump:EquationFit
- PlantLoop:Sizing object
- HVACTemplate:Chiller:Electric
Correct answer: Chiller:Electric:EIR with CurveFit objects
The Chiller:Electric:EIR object uses bi-quadratic and quadratic performance curves (CurveFit objects) to model COP variation with part-load ratio, entering temperatures, and capacity.
Question 2: What is the key difference between 'occupied' and 'unoccupied' setback schedules in a building energy model?
- Occupied schedules use tighter temperature bands to reduce equipment cycling
- Setback schedules allow space temperatures to drift farther from comfort range during unoccupied hours to save energy (Correct answer)
- Occupied schedules disable economizer operation during business hours
- Setback schedules increase lighting power density to compensate for reduced occupancy
Correct answer: Setback schedules allow space temperatures to drift farther from comfort range during unoccupied hours to save energy
Temperature setback allows HVAC systems to operate less during unoccupied periods by widening the acceptable temperature range, reducing heating and cooling energy.
Question 3: In energy modeling for LEED compliance, what baseline system type is typically assigned to a building with a single-zone per floor and floor area under 75,000 ft²?
- System 5 - Packaged VAV with Reheat
- System 3 - PSZ-AC (Packaged Single Zone with Air Conditioning) (Correct answer)
- System 7 - VAV with PFP Boxes
- System 1 - PTAC (Packaged Terminal AC)
Correct answer: System 3 - PSZ-AC (Packaged Single Zone with Air Conditioning)
ASHRAE 90.1 Appendix G assigns System 3 (PSZ-AC) as the baseline for non-residential buildings with floor area under 75,000 ft² and fewer than 5 stories in climate zones 1-3.
Question 4: What is the purpose of 'shadowing' calculations in building energy simulation software?
- To determine noise levels from adjacent structures
- To compute which surfaces block solar radiation from reaching windows and roof surfaces (Correct answer)
- To calculate natural daylighting levels at workplane height
- To model interzone air transfer through interior partitions
Correct answer: To compute which surfaces block solar radiation from reaching windows and roof surfaces
Shadowing calculations determine which building surfaces, overhangs, or neighboring structures obstruct direct solar radiation from reaching specific exterior surfaces at each hour.
Question 5: In eQUEST or EnergyPlus, what does 'part-load ratio' (PLR) of HVAC equipment represent?
- The ratio of equipment runtime to total simulation period
- The ratio of actual load served to the equipment's full rated capacity (Correct answer)
- The ratio of outdoor air flow to total supply air flow
- The ratio of cooling energy to heating energy over a billing period
Correct answer: The ratio of actual load served to the equipment's full rated capacity
PLR is the fraction of rated capacity at which equipment operates at a given instant; PLR = 1.0 means full load while lower values indicate partial-load operation with different efficiency characteristics.
Question 6: Which EnergyPlus object is used to model daylighting controls that dim electric lighting in response to available natural light?
- Daylighting:Controls (Correct answer)
- Lights:Schedule
- Zone:InternalHeatGain
- Output:Variable:Illuminance
Correct answer: Daylighting:Controls
The Daylighting:Controls object in EnergyPlus links sensor reference points to lighting zones, specifying setpoint illuminance levels and dimming fraction to reduce electric lighting energy.
Question 7: What does 'normalization by floor area' accomplish when comparing Energy Use Intensity (EUI) across different buildings?
- It adjusts for climate differences between building locations
- It removes the effect of building size so energy efficiency can be compared on a per-square-foot basis (Correct answer)
- It accounts for occupancy density differences between building types
- It converts source energy to site energy for utility billing comparison
Correct answer: It removes the effect of building size so energy efficiency can be compared on a per-square-foot basis
Dividing annual energy consumption by gross floor area yields EUI (kBtu/ft²/yr), enabling fair comparison of energy efficiency regardless of absolute building size.
CEA Energy Modeling & Simulation Tools 5
In EnergyPlus, which object defines the performance curve for a centrifugal chiller's coefficient of performance (COP) as a function of part-load ratio?