Sep 2, 1981 - the operating characteristics of a commesclally available daylight- linked dimming system. To avold any inferred sanctions or condemnations.
A CASE STUDY OF A DAYLIGHT-LINKED DIMMING SYSTEM FOR FLUORESCENT LAMPS
bY 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 available daylightlinked 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. Staff 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 floor 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, the 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 period* (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 (CISTI), Buildfng M-55, National Research Council of Canada, O t t a w a .
TESTING PROCEDURES
Retrofitting The daylight-linked dimrdng system and monitoring equipment were TnstalPed in a small room w e l l outside the occupants' a c t i v i t y areas. The dimming system conveniently allowed for remote control of three zones (Figure 1 ) :
Zone 1: Zone 2: Zone 3:
two rows o f luminaires adjacent t o the windows, middle four rows 0 5 luminatres, 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 orfginal, 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 ballasts 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 at night) s e t s of photosensors from the artificial 1Sghts alone (i.e., 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 that 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 lmer 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
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 quickly returned to the manual mode.
During nonworking hours over a one week period, 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 sunlight, 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 registered by the occupants. Rearranging the o f f i c e furnfture and partttions t o make better use of daylight might also 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 night, (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 variations. 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 relatively 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 As phatosensors, unlike human occupants, v i s u a l performance5 t6. disregard luminous inhomogeneities, the response of the sensor controlling the output of the a r t i f i c f a l lights can be i n a p p r o p r i a t e .
the reference area, however, went from about 700 lx t o about 900 lx, an Increase in excess of 20% aver both the pretest readtng and the t e s t
area illuminance. Three factors contributed to these results: (1) higher luminous outputs 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 both 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 effect in the test area, illuminance meaeurements 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 results obtafned when the luminaire 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 convent 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 relatfve energy savings gained by daylightl 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 daylight-linked 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 windaws. These savings were achieved by an undetermined cambxnation of
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 direct sunlight 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 indicate savings from windows and f rum the adj 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 measurements 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 with t h e i r rated powers ( 9 7 W and 83 W, respectively). A t 322 V, the 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 63.8 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 This value would compare to an expected savllngs of l f g h t i n g was 322 V. 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~nsumptionand 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 conventional b a l l a s t . This difference in relative efficiency for the two b a l l a s t s held almost independent of supply voltage.
B a l l a ~ tCharacterlstfcs 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 the 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 the conventional ballast.
DISCUSSION
It is d i f f i c u l t to generalize to other s i t e s and other daylightl f n k e d dimming systems, but 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 tor 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 efflctent 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 testing 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 levels were n o t mafatafned. Even at the more typical 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 the interrelated
factors that affect dimming system performance, Some of these factors are: natural and artificial lighting geometries, characrterfstics of internal partitfons, sensor desfgn, proximity of dfmming zone to windaws, l i n e voltage, and occupant use of window 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 inception of the project 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 the 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.,
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 Office 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 Application, 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.,
7.
Hunt, D.R.G., Simple expression f o r predfcting energy savings from photo-electric control of lighting. 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,
The Evaluation of Discomfort Glare, Illuminating Engineering, November, 1956, p.722-728.
The Measurement of Visual Acuity, H i s Majesty's Stationary Off ice, Medical Research Council, Report No. 173, 1932.
p.24-55. 9.
Anonymous, Automatic Lighting Output System, Energy Engineering, V o l . 79, No. 1, 1982, p.49-55.
PERIMETER WINDOW
HOUSlNG VENETIAN BLINDS
--@
SKYLIGHTS
a ZONE 3 L U M I N A I R E S
PHOTO SENSOR
CONTROL AREA LUMINAIRES
ZONE 1 L U M I N A I R E S
S E C U R I T Y LUMINAIRES
ZONE 2 LUMINAIRES
LUMINAIRES NOT INCLUDED I N STUDY o
I N C A N D E S C E N T LAMPS
FIGURE 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 OF I L L U M I N A T I O N C O N S I D E R E D I N C A S E S T U D Y
FIGURE 2
C E I L I N G M O U N T E D PHOTOSENSOR S U P P L I E D B Y MANUFACTURER
FIGURE 4 C E I L I N G MOUNTED PHOTOSENSOR BAFFLE
MODIFIED WITH A
FIGURE 5
HYPOTHETlCAL DIMMING SYSTEM O P E R A T I O N
A
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B
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C
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I
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FICTITIOUS C H A N G E I N A V A I L A B L E D A Y L I G H T I N TEST A R E A RESPONSE 1OF D I M M I N G SYSTEM TO D A Y L I G H T BEFORE M O D I F I C A T I O N S RESPONSE OF D I F M I N G S Y S T E M TO D A Y L I G H T A F T E R MODIFICATIONS EXTENDED P L A T E A U RESULTS FROM ALTERAT r O N S TO ZCNE CONTROL U N I T SENSOR C A L I B R A T I O N ADJUSTMENT A N D FROM
INSTALLATION O F B A F F L E A R O U N D PHOTO2
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SEFESOR PLATEAU R E S U L T S F R O M M O D I F I C A T I O N TO
C O N T R O L UNIT SYSTEM L O W LIMIT ADJUST
I
1
1
C O N V E ~ T TIONAL I BALlAST
A /
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/ /
#
/
'd
/
/
0
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DIMMING BALLAST
3
LINE VOLTAGE
R E L A T I V E ILLUMINANCE AT V A R I O U S L I N E V O L T A G E S W I T H DIMMING A N D CONVENTIONAL BALLASTS UNDER LABORATORY CONDITIONS
ZONE 1
ZONE 2 7
T
lmr
1
1
1
E
ZONE 3
. I
0
JAN
FEB MAR APR MAY JUNE JULY AUG
R E L A T I V E E N E R G Y S A V I N G 5 IN T H R E E ZONES DURING TEST P E R I O D
