ASHRAE Journal - September 2013 - 52

technical feature

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Lightning SavingS (%)

energy saved for the controlled lighting.
Figure 3 Lighting savings for each space both before and after our commissioning effort.
Our data for Period 1 shows that the median
100
automatic daylighting control system in
90
a population represented by our sample
After Cx
80
would save approximately 809 kWh for
Before Cx
70
every kW (2912 MJ/kW) of lighting that is
60
controlled by the system, or about 20%† of
50
the controlled lighting energy. This number
increases to 915 kWh per kW (3294 MJ/kW)
40
or 23% if we include the savings in associ30
ated HVAC energy. If we look at the units of
20
kWh/kW, this essentially simplifies to the
10
number of equivalent hours in a year for
0
which the lights would be fully off. If a typi–10
cal commercial space operates for 3,500
hours per year, it seems reasonable that a
typical daylighting control system could
keep the lights at or near off for a quarter
of that time. On an area basis, for a typical commercial
performance of the control itself. Initially, we found the
space lit with 1 W/ft2 (11 W/m2), this savings equates to
average controls effectiveness in the as-found condition
2 (9.7 kWh/m2).
0.9 kWh/ft
to be 51% (with a median of 66%). However, the spaces in
Savings for this as-found condition ranged from as high
our study displayed a wide range of performance. Four
as 74% or 2,463 kWh/kW (8867 MJ/kW) for controlled light- did not work at all, showing an effectiveness of zero. On
ing on a fully glazed north façade, to as low as zero. In fact,
the other end of our spectrum, over half of the spaces
four of the 20 systems that we studied were essentially not
had effectivenesses of more than 60%.
saving energy as they were found. And, in at least one case,
After our commissioning effort, controls effectiveness
the owner assumed they were saving substantial energy.
increased (on average) from 51% to more than 75%, or a
But after the spaces underwent recommissioning, sav- 50% improvement. This additional savings results solely
ings changed substantially. Figure 3 shows the lighting
from improving controls operation, not from any change
savings on a percent basis both before and after comto glazing design, blinds operation, furniture, or ballast
missioning for each space in our study. The median
minimum power—all more difficult or expensive items
savings increased to approximately 1,762 kWh for every
to modify. Figure 4 shows, for each of the 20 spaces, the
kW (6343 MJ/kW) of lighting controlled by the system, or correlation between effectiveness both before and after
about 55% of the controlled lighting energy.‡ These savour commissioning effort. Note that in all but one of the
ings numbers can be as high as 4,220 kWh per kW (15 192 spaces effectiveness improved after commissioning, and
MJ/kW) and 92% savings including HVAC energy. More
some dramatically so.
importantly, the minimum savings has increased from
Table 2 summarizes key results for both before and after
0% prior to commissioning, to 14% after commissioning. conditions in our study.
And, if we include the savings in HVAC energy, median
Impacts on Heating and Cooling
savings increased to approximately 1,976 kWh per kW
Results were presented previously for either 1) light(7081 MJ/kW), or 63% savings. For an average space at 1
W/ft2 (11 W/m2), this equates to 2 kWh/ft2 (21.5 kWh/m2). ing only or 2) the combination of lighting and HVAC
Next, we can examine the difference in controls effecimpacts. Daylighting controls generally reduce cooling
tiveness. Again, controls effectiveness normalizes for
loads and increase heating loads in the space due to
light level in the space, and, therefore, measures just the reduced light energy. We found that most of the spaces
† With a range of performances selected, with some systems working and others not, the standard deviation of these values based on our sample size is 1250 kWh and 27%.
‡ Though these savings values have nearly doubled from the pre-commissioning case, the absolute standard deviation remains similar at 1,260 kWh (4536 MJ)and 25%; there is considerably less (relative)
variation in results when systems are all commissioned.
52

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ASHRAE Journal - September 2013

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