Single Zone vs Multi Zone Energy Model for Early Stage Design

May 6, 2019 - 9 minutes read

Introduction:

The purpose of this study is to reduce the time and effort put into remodeling thermal zones whenever there is a small or big change in the building design made by the project team while maintaining the accuracy level. The main focus is on the sensitivity of building geometry and thermal zoning while keeping other parameters constant. The study is conducted using the small office building prototype provided by Pacific National Northwest Laboratory (PNNL).

Key Observations:

The Energy Usage Intensity (EUI) is compared for the small office model which has 4 perimeter zones and 1 core zone to the model which has only 1 zone. The models were simulated for all of the ASHRAE climate zones to obtain the results. The average overall EUI difference of 2.82% was observed between five zone model vs single zone model. The maximum and minimum percentage difference of 3.92 and 1.59 for overall EUI was recorded for Denver, CO (ASHRAE Climate Zone 5B) and San Francisco, CA (ASHRAE Climate Zone 3C) respectively.

Furthermore,
to study the sensitivity of glazing percentage on zone lumping the glazing
percentage for every orientation was increased by 20%. After running the
simulations, the average overall EUI difference of 4.23% was observed between
five zone model vs single zone model for all climate zones. The maximum and
minimum percentage difference of 5.7 and 2.88 for overall EUI was recorded for
Denver, CO (ASHRAE Climate Zone 5B) and San Francisco, CA (ASHRAE Climate Zone
3C) respectively.

Methodology:

Building Parameters:

The selected building is a small office prototype described
by Pacific Northwest National Laboratory (PNNL). Some of the building
parameters are mentioned below:

Total Area of the Building (Sqft) 5055
Number of Floors 1
Window Fraction 24.4% for South, 19.8% for other three
orientations
Thermal Zoning Perimeter Zone Depth 16.4 ft
Four Perimeter Zones and One Core Zone
Floor to Floor Height (Ft) 10
HVAC System Type Air-source heat pump with gas furnace as back up
Thermostat Setpoint 75°F Cooling/70°F Heating
Thermostat Setback 85°F Cooling/60°F Heating
Lighting Power Density (LPD) (W/Sqft) 0.98
Equipment Power Density (W/Sqft) 1.0935
Domestic Hot Water Consumption Rate (gal/ft2-day) 0.004908
Heating System COP 1.8
Cooling System COP 1.8

Thermal Zoning:

The thermal zoning for the prototype building model can be seen in the attached image.

Figure 1 Thermal Zoning for Small
Office according to PNNL

There are 4
perimeter zones and 1 core zone. The perimeter depth modeled is 16.4 ft on each
side. The thermal zoning can be a complex process depending upon the nature of
building design. The conceptual design decisions usually involve changes in the
geometry of the building, window to wall ratio (WWR) and envelope properties.

The proposed thermal zoning modifies the building to consist only on single zone instead of five different zones. This will significantly reduce the time spent in remodeling as there are lesser number of zones to be changed.

Figure 2 Proposed Thermal Zoning for
Small Office

Comparison
of Energy Usage Intensity:

The
different cities used to perform the comparison between single zone and five
zone small office models are listed below:

ASHRAE Climate Zone City ASHRAE Climate Zone City
1A Miami,
Florida
4C Seattle,
Washington
2A Houston,
Texas
5A Chicago,
Illinois
2B Phoenix,
Arizona
5B Denver,
Colorado
3A Atlanta,
Georgia
6A Minneapolis,
Minnesota
3B Las Vegas,
Nevada
6B Helena,
Montana
3C San
Francisco, California
7 Duluth, Minnesota
4A Baltimore,
Maryland
8 Fairbanks,
Alaska
4B Albuquerque,
New Mexico
   
  1. Zone 1A
    Miami, Florida:

The EUI difference between 5 zone model and single zone model was observed to be 2.84% for Miami, Florida. When the glazing was increased by 20% for all orientations the EUI difference increased to 3.58%.

  • Zone 2A
    Houston, Texas:

The EUI difference between 5 zone model and single zone model was observed to be 2.48% for Houston, Texas. When the glazing was increased by 20% for all orientations the EUI difference increased to 3.32%.

  • Zone 2B
    Phoenix, Arizona:

The EUI difference between 5 zone model and single zone model was observed to be 3.35% for Phoenix, Arizona. When the glazing was increased by 20% for all orientations the EUI difference increased to 4.25%.

  • Zone 3A Atlanta, Georgia:

The EUI difference between 5 zone model and single zone model was observed to be 2.78% for Atlanta, Georgia. When the glazing was increased by 20% for all orientations the EUI difference increased to 3.84%.

  • Zone 3B Las
    Vegas, Nevada:

The EUI difference between 5 zone model and single zone model was observed to be 3.45% for Atlanta, Georgia. When the glazing was increased by 20% for all orientations the EUI difference increased to 4.48%.

  • Zone 3C San
    Francisco, California:

The EUI difference between 5 zone model and single zone model was observed to be 1.59% for San Francisco, California. When the glazing was increased by 20% for all orientations the EUI difference increased to 2.88%.

  • Zone 4A
    Baltimore, Maryland:

The EUI difference between 5 zone model and single zone model was observed to be 2.92% for Baltimore, Maryland. When the glazing was increased by 20% for all orientations the EUI difference increased to 4.41%.

  • Zone 4B
    Albuquerque, New Mexico:

The EUI difference between 5 zone model and single zone model was observed to be 3.57% for Albuquerque, New Mexico. When the glazing was increased by 20% for all orientations the EUI difference increased to 4.88%.

  1. Zone 4C
    Seattle, Washington:

The EUI difference between 5 zone model and single zone model was observed to be 2.12% for Seattle, Washington. When the glazing was increased by 20% for all orientations the EUI difference increased to 3.35%.

  • Zone 5A
    Chicago, Illinois:

The EUI difference between 5 zone model and single zone model was observed to be 2.55% for Chicago, Illinois. When the glazing was increased by 20% for all orientations the EUI difference increased to 4.35%.

  • Zone 5B
    Denver, Colorado:

The EUI difference between 5 zone model and single zone model was observed to be 3.92% for Denver, Colorado. When the glazing was increased by 20% for all orientations the EUI difference increased to 5.70%.

  • Zone 6A
    Minneapolis, Minnesota:

The EUI difference between 5 zone model and single zone model was observed to be 2.77% for Minneapolis, Minnesota. When the glazing was increased by 20% for all orientations the EUI difference increased to 4.88%.

  • Zone 6B
    Helena, Montana:

The EUI difference between 5 zone model and single zone model was observed to be 3.15% for Helena, Montana. When the glazing was increased by 20% for all orientations the EUI difference increased to 5.19%.

  • Zone 7
    Duluth, Minnesota:

The EUI difference between 5 zone model and single zone model was observed to be 2.42% for Duluth, Minnesota. When the glazing was increased by 20% for all orientations the EUI difference increased to 4.63%.

  • Zone 8
    Fairbanks, Alaska:

The EUI difference between 5 zone model and single zone model was observed to be 2.40% for Fairbanks, Alaska. When the glazing was increased by 20% for all orientations the EUI difference increased to 3.66%.

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