A Case Study of a Daylight-Linked Dimming System for Fluorescent Lamps

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A Case Study of a Daylight-Linked Dimming System for Fluorescent

Lamps

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A CASE STUDY OF A

DAYLIGHT-LINKED

DIMMING SYSTEM

FOR

FLUORESCENT LAMPS b

Y

R.R. Jaekel and M,S. R e a

INTRODUCTION

A pilot case study was conducted to obtain first-hand knowledge of the operating characteristics of a commesclally a v a i l a b l e daylight-

linked dimming system.

To

avold any inferred sanctions or condemnations of particular products, trade names have not been used In this report. The particular aspects of the s i t e of thLs study limit the general use of these results for other installations. However, it is hoped that the

issues presented in t h i s repart would be considered when evaluating the

performance p o t e n t i a l af any phatoelactrtc dimming system.

TIIE TEST SLTe

An open-plan offfce, hausing library adnrLnistrative staff, was

chosen as a test area*. The test area is an the south side of the second f l o o r (Figure I ) , with an adjacent o f f i c e as a reference area.

S t a f f members in both areas were n o t informed of the testing.

'Phe south sFde of the bulldlng faces an open f i e l d . Windows extend

t h e full length of the t e s t area's south s i d e . Each wfndow is 1.7 m

high, extending from 81 cm above the f l o o r to 3 cm below the ceiling.

Each window cantaLns a venetian blind assembly between its glazing5 and although the pitch of the venetian b l i n d lalrvers can be manipulated by t h e occupants, t h e b l i n d s cannot be retracted. A covered atrim with

skylights and incandescent lamps bounds the north s i d e of the t e s t

area,

AttZficial illumination in the two areas 1s supplded by recessed

fluorescent luminaires using two warm white fluorescent lamps. These l u d n a i r e s are flush with the ceiling and covered with clear prismatic

l e n s e s . One switch c o n t r o l s the artificial lighting far both the t e a t

and reference areas. Although the lighting was ortginally deafgned to

operate on 347 V, i t 8 actual e l e c t r i c a l potential was approximately

322 V prior to

and

during the test p e r i o d * (The managing e l e c t r i c a l

eagfneer reduced the voltage to save energJr s u p p l i e d to incandescent

Lamps in the bu%1dfngts atrium.)

*

Canada Institote f o r Scientific and Techntcal Information ( C I S T I ) , B u i l d f n g

M-55,

National Research Council of Canada, O t t a w a .

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TESTING PROCEDURES

Retrofitting

The daylight-linked dimrdng system and monitoring equipment were

TnstalPed in a small room w e l l o u t s i d e the occupants' a c t i v i t y areas.

The dimming system conveniently allowed for remote control of three

zones ( F i g u r e 1 ) :

Zone 1: two rows o f luminaires adjacent t o the windows,

Zone 2: middle four rows 0 5 luminatres,

Zone 3: two rows of luminaires adjacent to the atrium.

The output of the lamps in each zone was modulated by two

photosensors, wtred in parallel, for that zone (Figures 1 , 2 ) . The

hours of usage and the energy consumed (kflowatt hours)

in

each zone and

fn the reference area were monitored separately. Only energy consumed

during working hours, 0800 t o 1700, was used for analysis.

New b a l l a s t s and lamps were installed in the test area. The

o r f g i n a l , conventtonal ballaets w e r e replaced with dfmmlng ballasts. New lamps were also substituted in the reference area, but the o r i g i n a l

b a l l a s t s were retafned.

The operation of the dimming system was established

In

accordance

with the manufacturer's recommendations, Figure 3 is a schematic of the control system. The d i d n g system was f i r s t adjusted t o supply maximum

current ta the lamps in the t e s t area (A, F i g u r e 3 ) . To save l i g h t i n g

energy, the managing electrical engineer had implemented a delamping program independent of t h i s case study

and

well before the t e s t period. Because the illuminance levels were already lower t h a n the original

d e s i g n l e v e l , maximum output of the lamps was d e s i r a b l e , even though the dimming system afforded the p o s s i b i l i t y of fureher reducing the luminous output. Second, the integrated luminous flux reaching each of the three s e t s of photosensors from the artificial 1Sghts alone (i.e., at night)

w a s taken as producing the criterion photocurrent for daylight-linked

d i d a g in a zone (B, F i g u r e 3). Therefore, light reaching the

photosensors from natural and a r t i f i c i a l sources t h a t produced a photocurrent larger than the "night t i m e " criterion resulted in a

reduction of the luminous output from the ceiling lamps. Increasing the amount of l i g h t reaching the photosensors would dim the output of the luminaires u n t i l a l m e r l i m i t , 20% of maximum output, was reached (C,

Figure 3).

Protocol

For three weeks prior to the actual testing period, the energy consumption rate ( k i l m a t t hours/hwr) was measured durkng working hours in bath the test and reference areas, whfLe the d l d n g system was in the "manual" mode. This rate (or average load) d i d n o t vary by more

than 2% in either area for every monitored p e r i o d during this pretesting

session.

O n the first day of operating the dimmfng system in t h e "automa~ic" mode, however, occupants complained about low task brightness. On

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checking the site it was agreed that the task brightnesses were very low despite "a l o t of l i g h t

in

the a f f l c e areab*. The dimmfng system was

q u i c k l y returned to the manual mode.

During nonworking hours over a one week p e r i o d , adjustments were

made to the dimming s y s t m

i n

an effort t o make the luminous environment

acceptable to the occupants. Because light could reach the sensors d i r e c t l y from the windrrws, circular b a f f l e s vere fitted on the sensors

(Figure 4) to l i m i t t h e i r phetosensitive f i e l d to the floor and task

areas. This appeared to be s t i l l inadequate f o r e l e v a t i n g task

brightness levelst The criterion photocurrent necessary far d i d n g was

increased so that more light from the w i n d w s was required b e f o r e

dinunkng took place. F i n a l l y , in Zone 1, which was p e r i o d i c a l l y bathed

with direct s u n l i g h t , the minimum output of the lamps was e l e v a t e d from

20% t o 50%. Therefore whatever the natural 1um;lnous conditions i n Zone 1 were, the output from the lamps would never be lesa than half of

maxi~mrm. Figure 5 i l l u s t r a t e s the changes designed to make the lumfnoue

environment acceptable to the occupants and more conducfve t o good

visual performance.

After these modifications, msniroring was c a r r i e d out between

January 10 and September 2, 1981. The dimming system performed r e l i a b l y throughout the t e s Ling period and no f a r t h e r complaints about the

l i g h t i n g conditions were r e g i s t e r e d by the occupants. Rearranging the o f f i c e furnfture and partttions t o

make

better use of d a y l i g h t might a l s o have eliminated workersf complaints about the l i g h t h g .

RE SUETS Illudnance Measurements

Rough illuminance measurements w e r e taken In bath the reference and t e s t areas under the o r i g i n a l operating conditions (before the dimming

system was installed) and after 100 hours in the pretesting session. Illuminance measurements were always taken (a) a t n i g h t , ( b ) in the center of the area away from partitions, (c) directly under one luminaire, and ( d ) with a "standard" position for the hand-held

instrument. After Installation, when the output of the lamps In the test area

was

maximum, the illuminance waa about: the same as i t had been o r i g i n a l l y (illuminances ranged from 680-720 1x1. The illuminance in

*

Sensors used with dimudng systems integrate luminous flux from all

areas within t h e i r photosensitive f i e l d ukthout regard for luminous v a r i a t i o n s . In an o f f i c e with regular, closely-spaced c e i l i n g luminaires, the distribution of flux reflected ta a ceiling-mounted

sensor

is

r e l a t i v e l y u n f f o m at night. Conversely the distribution of

flux reaching a ce~ling-mounted sensor from windows on one s f d e of an o f f i c e c a n be q u i t e inhomogeneous. Similar l u d n o u s variations seen

by occnpants can k both uncomfortablel D~ p 3 b 4 and deleterious to v i s u a l performance5 t6. As phatosensors, unlike human occupants, disregard luminous inhomogeneities, the response of the sensor

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the reference area, however, went from about 700 lx t o about 900 lx, an Increase in excess of 20% aver both t h e pretest readtng and the t e s t

area illuminance.

Three factors contributed to these results: (1) higher luminous

o u t p u t s from new lamps, ( 2 ) differences

in

ballast characteristics in

the two areas, ( 3 ) l o w operating voltage supplied to the lighting.

The higher illuminance in the reference area is e a s i l y explained by

the f f r s t factor: new lamps have higher l u m i n m s output, and, because

the other t w o factors

were

h e l d constant, higher illuminances w e r e obtained. The change in b o t h ballasts and laraps in the test area,

however, coincidently compensated f o r one another at t h e Pow operating voltage, resulting in the same illuminance.

To i l l u s t r a t e t h i a e f f e c t in the test area, illuminance meaeure-

ments from a single luminaire were taken i n the Laboratory, comparing one ballast of each type at d i f f e r e n t operating voltages. The

illuminance measurements were taken under constant conditions for both l i g h r i n g geometry and lumtnaire cavity temperature. Figure 6 shows the

r e s u l t s obtafned when the l u m i n a i r e lamps w e r e operated a t d i f f e r e n t

l i n e voltages with a d i d n g ballast (at maximum output) and a conven-

t i o n a l ballast, as was used in the reference area

(and

also like the original ballasts

in

the test area). At 322 V, t h e mean measured

potential during the testLng period, t h e illuminance was 24.5X less d t h

the dimming ballast relative t o the conventional ballast, This valve i s

close t o the 22% difference in illuminance measured between the test and

reference areas.

It

should be noted that at the standard operating

voltage, 347 V , the output of the lamps controlled by the dimming ballast was about 9% less than from the conventional b a l l a s t . Based

upon these laboratory measurements, then, one would not have expected comparable (maximum) illumFnances dn the test and reference areas even

if the l f g h t i n g had been operated at normal voltage. Energy Savings

Figure 7 presents the r e l a t f v e energy savings gained by daylight-

l i n k e d dimming of t h e a r t i f f c i a 1 l i g h t i n g during the testing period. (Some of the savings from so-called d a y l i g h t - l i n k e d dimming resulted from artificial lighting in the atrium,) In order to determine the

average consumption rate within a zone, the ktlowatt hours used p e r

month w e r e divided by the number of hours the Efghts were used. This corrected for any variations in light usage due t o inconsistent working hours or holidays. Consumption rate within the zones was also

derermined for the pretesting session, when the dimming system was operated in t h e "manual"' mode. The values presented in Figure 7 are energy savings based upon the average consumptLon rate in a zone with

dtmming capabilities (testlng p e r i o d ) relative to consumption r a t e in the same zone without dimming capabilities (pretesting p e r i o d ) . In

t o t a l , about 12% less energy was consumed i n the test area because of

daylight-linked dimming.

Energy savings were greatest in Zone 1, closest to the south

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diffuse and

direct natural fllumination. Further, there w a s ad unknown amount of window blind manipulation by the occupants. Modest and

relatively constant energy savings

were

also observed in Zone 3 , c l o s e s t t o the atrium, Skylights and artificial incandescent lamps in the

atrlum provided some indirect i l l u m i n a t i o n , and consequently, energy

constmption i n t h i s zone

was

reduced. Energy savings In Zone 2, the

middle zone, were apparent only durPng winter months; these s m a l l

savings resulted from deeper penetration o f d i r e c t s u n l i g h t into t h i s

test zone from the south windows, Because the energy savings in Zones 2

and 3 were not correlated, and because the savings in Zone 2 were zero

f o r mast months, it was assumed that Zones 1 and 2 were unaffected by

light from the atrium. Thus these results i n d i c a t e savings from windows and f rum the a d j acent atrium separately.

An addletonal, but not precisely determined, amount of energy was saved in the t e s t area relative to the reference area. From measure-

ments obtained from the one sampled dimming ballast and the one sampled conventional ballast, relative energy savings can be estimated far the test and reference areas. Using the same 40 W lamps, the measured power consmption for t h e conveatioual hallast: was 96.6 W; f o r the dZmming

ballast it was 81.2 W. These values agree f a l r l y closely w i t h t h e i r rated powers ( 9 7 W and 83 W, respectively). A t 322 V, t h e mean measured potential s u p p l i e d t o the lighting in the test and reference areas, the pcrwer consumption dropped to 99.2 W f o r the conventional ballast and 6 3 . 8 W for the dimming ballast. Extrapolating t o predict the relative

savings

in

the t e s t area, one

would

expect a savings o f about 30%

relative to the reference area because the electrical p o t e n t i a l to the l f g h t i n g was 322

V.

This value would compare to an expected savllngs of

about L6X if the p o t e n t i a l had been the standard 347 Y. These valvee

are

independent of the daylight-linked savings.

One further point should be made about the energy c~nsumption and the IllurrtinatFon levels. For l f n e voltages between 315 and 355 V, the

flluminance provided by the lamps controlled by the dimming ballast was l e a s than if contralPed by the conventional ballast. However, the lamps

controlled by the d i d n g ballast produced abaut 8 1 more illuminance p e r w a t t under laboratory condftions than the same lamps

with

the conven-

tional b a l l a s t . This difference in r e l a t i v e e f f i c i e n c y for the two

b a l l a s t s held almost independent of supply voltage. B a l l a ~ t Characterlstfcs

Power factors were also determined for t h e conventional and dimming ballasts. The l i n e voltage was varied from 315 to

355

VAC f o r these determinatfons. The conventional b a l l a s t provided power factors higher

than 0.9 fox these voltages, whereas the dlmming ballast p r o d d e d lagging power factors between 0.5 and 0 . 6 .

It should also

be

noted t h a t t h e l i n e current for the d3mming

ballast, resulting from bath l i g h t i n g load and reactive load, was rated 67% higher than with t h e conventional ballast.

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DISCUSSION

It

is d i f f i c u l t to generalize to other s i t e s and other daylight-

l f n k e d dimming systems, b u t these results provide some useful guidelines

t o potential users of such a system.

F i r s t , A t appears that the daylight-linked systems must be

carefully integrated into the envtronment, because the response of the

photosensors fs not lfke the response of Ehe occupants' eyes. Sensors

are "blind" to glare and excessive luminance ratios that can cause

d ~ s c o m f o r t or low visual performance for occupants. Making the

daylight-l-lnked dimming system amenable t o occupants can reduce the

expected energy savings.

Second, given a luminous environment that does not reeult i n occupant dissatlafaction, i t is difficult t o obtain any energy savings from south window d a y l i g h t in interior zones, V i r t u a l l y no savings were

measured in the Interior zone (Zone 2) except for those winter months when there was deep penetration of sunlight. Only in peripheral zones

close to windows (or other sources of illumination) were daylight-linked

savings r e a l l z e d .

Third, dimming ballasts clearly consumed less energy and w e r e more

e f f l c t e n t than the conventional ballasts tested, These advantages would

he offset, however, by higher l i n e currents and paor power factor, that

could result in additional c o s t s f o r larger gauge wiring and adjuvant power factor correcting equipment.

Fourth, the b a l l a s t a employed in the dimdug system substantially

reduced the amount of energy consumed i n the t e s t area. These savings were realized by a coebFnatfon of daylight-linked d i d n g , relatively mote efficient ballasts, and a lower than t y p i c a l voltage s u p p l i e d to

t h e lighting system. Relatfve t o t h e reference area, savings were

estimated to be in the neighbourhood of 42%

f o r

the entire floor. The

bulk of these savings, about 302, would come from the combination of

dimming ballasts and lower than t y p i c a l operating potential. The other

12% would come from daylight. The 30% savings would have been about 1 6 %

if _the_potential_had_been_the_more _{t y p i c a l}_{347 V.}

The combination of d i d n g ballasts and low operating p o t e n t i a l ,

hawever, a l s o produced lower illumination levels; without the relamping

procedure implemented prior t o the t e s t i n g sessfan, illumfnances would have been well below pretesting levels. Further, as the l a m p s aged I n

the test area, the early light l e v e l s were n o t mafatafned. Even at the

more t y p i c a l 347 V , t h e illuminance l e v e l s would have been a l i t t l e lower in the test area. The lower absolute illuminances produced by the

dimming sys tern should always be considered, especially f f reducing

voltage In buildfngs becomes more widespread with emphasis on energy

savings

.

F i f t h , a few estimates of energy savings from daylight-linked

dimming systems have been publ~shed ,7 '8 *9 P r e d i c t i n g savings from daylight 5s difficult because of the complexity in t h e interrelated

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factors that a f f e c t dimming system performance, Some of these factors

are: natural and artificial lighting geometries, characrterfstics of i n t e r n a l partitfons, sensor desfgn, proximity

of

dfmming zone to

windaws,

l i n e voltage, and occupant use of w i n d o w b l i n d s or manual

swiltcMng. U n t i l algorithms consider these factors, it is difficult to predict accurately the actual energy and financial savings p o s s f b l e with dimmlng systems.

ACKNOWLEDGMENTS

The authors would llb to thank Dr. A.W. L e v y and Mr, R. Boudreau for their contributions at the i n c e p t i o n of the p r o j e c t and for

commenting on the manuscri-pt. We would a l s o l i k e to thank MT. T. West f o r permission to use t h e areas i n buildfng M-55.

REFERENCES

1. Kaufmaa, J.E. (ed.), T.E.S. Lighting Handbook, Reference Volume, Illuminating Engkneering Society, New York, 1981.

2. F r y , G.A., The Evaluation of Discomfort Glare, Illuminating

Engineering, November, 1956, p.722-728.

3 . S a n d e r s , J . E , , The Role of the Level and Diversity of Rorizontal

Illumination in an Appraisal of a S l m p l e O f f i c e

Task,

LLghting

Research and Technology, V o l . 1, No. 1 , 1969, p.37-46.

4. Bennett, C . A , , The Demographic Variables of Discomfort Glare, Lighting Design and A p p l i c a t i o n , Janaury, 1977, p.22-24.

5. Kaufman, J.E, ( e d . ) , I.E.S. Lighting Handbook, A p p l i c a t i o n Volume, Illuminating Engineering S o c i e t y , New York, 1981(b).

6. Lythgoe, R . J . , The Measurement of Visual Acuity, H i s Majesty's

Stationary

Off

ice, Medical Research Council, Report No. 173, 1932.

7. Hunt, D.R.G., S i m p l e e x p r e s s i o n f o r predfcting energy savings from

photo-electric control of l i g h t i n g . Lighting Research 6 Technology,

Vol. 9 , No, 2, 1977, p.93-102.

8. Hunt, D.R.G., Field s t u d i e s of the use of a r t i f i c i a l lighting in o f f i c e s : 2. Possible energy savings from photoelectric control8. Bullding Research Establishment Current Paper

(CP)

47/47,

1978,

p.24-55.

9 . Anonymous, Automatic L i g h t i n g Output System, Energy Engineering,

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V E N E T I A N B L I N D S F I G U R E 1 T E S T S I T E A N D V A R I O U S S O U R C E S

A Case Study of a Daylight-Linked Dimming System for Fluorescent Lamps

Publisher’s version / Version de l'éditeur:

Building Research Note, 1983-01

NRC Publications Archive

Archives des publications du CNRC

A Case Study of a Daylight-Linked Dimming System for Fluorescent

Lamps

DAYLIGHT-LINKED

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REFERENCES

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