Daylight saving simulation in eQUEST

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Hi, all,

I simulated an office building with 49% window on each orientation, and
compared the results with daylighting and without daylight. The lighting
energy consumption is 48.5% less with daylighting, and the total energy
consumption is 18.5% less. I set the only sensor in each exterior zone
around 15ft from the exterior window and used default values for other
setup. I wonder if anybody with experience of daylighting think these
big(?) saving numbers are realistic?

Any input will be appreciated.

Lan Hu

Lan Hu's picture
Joined: 2011-09-30
Reputation: 0

It depends on what your original schedule is for lighting - if it is full on
in the exterior zones for working hours (say 8 to 5), then daylighting will
pretty much reduce all of this to a very minimum value. You can do a rough
estimate of savings by figuring out how much of the lighting load is due to
external zones during the daylight hours.
- Rohini

R B's picture
Joined: 2011-09-30
Reputation: 4

The numbers seem reasonable but there are a few ways you can check:

1. If you're using DOE-2/ Equest, check the depth of your zone &
photosensor location against head height of window. Rule of thumb is
daylit depth is ~1.5 times head height of window so typical zone depth
for 9-ft ceiling is about 15 ft with sensor at 2/3rds or 10 ft distance
from window. DOE-2 has limitations in calc with split-flux method:
internally reflected component is overestimated for distances greater
than approx 2 times head height of window which can lead to
overestimation of lighting energy use savings. Also, pay attention to
interior shading schedule/ modeling assumptions. If you have a bare
window, you're going to get great savings but uncomfortable occupants.

2. Check results against some other software packages:
DAYSIM or Daylight123 via Christoph Reinhardt, which uses Radiance as
core engine but friendly user interface
Run your facade design through
in about 5 minutes (database based on DOE-2)
Run SPOT tool (Radiance with GUI)
Compare switching strategies vs dimming strategies (ballast energy
consumption differs between strategies), include occupancy-sensors in
model for base & test cases

3. Field data:
New York Times Headquarters monitored data for open plan office with
actively managed shades:
30-48% avg annual lighitng energy savings for west facing windows and
75% for bilaterally daylit SW facing windows
with dimming, setpoint tuning, nighttime occupancy (no occ sensors
during daytime since open plan).
Owner informally reports an operating avg LPD of 0.35 W/ft2 for May/June
08 with installed LPD of 1.0 W/ft2.

HMG, NREL, Weidt Group reports savings that are significantly less than
predicted savings depending on attention paid to proper execution
(reports published on 0-10V systems) & in particular for sidelighting
applications. DALI-enabled digital lighting controls (used at NYT) is
making it easier to achieve more reliable results but still not a slam
dunk yet.

Hope this helps,

Eleanor Lee's picture
Joined: 2011-10-02
Reputation: 0


49% is large, but not unreasonable depending on a number of factors:

1. Percentage of Core v/s skin zones
2. Light level settings. Set the lights to dim at appropriate footcandle
levels (eQUEST defaults to 50fc)
3. Zone depth - If your zone is deeper than 20 feet, I would recommend
setting 2 lighting control sensors at different depths in the zone
controlling 50% (Or appropriate percentages). This is more realistic to
the controls that your actual building will (or should) have.
4. Glare Indices - I'm not sure how well this factor works in eQUEST,
but the default is set to 100. However if you go into the item help, it
suggests max glare of 22 for offices. If your window shades are set to
movable (either interior or exterior) this triggers the shades. This
would probably be more realistic - especially on east & west facades.

Hope this helps.

Vikram Sami, LEED AP

Vikram Sami's picture
Joined: 2011-09-30
Reputation: 0