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NAT'L INST, or STAND & TECH R. .C

PUBLICATIONS

NBSIR 80-2147

liratENCE

A111D5 3SMSSS uiti!

Daylighting, Window Management

Systems, and Lighting Controls

S. Treado and T. Kusuda

December 1980

Center for Building Technology

National Engineering Laboratory

National Bureau of Standards

U S. Department of Commerce

Washington, DC 20234

Prepared for: Naval Facilities Engineering Command

U.S. Navy

Washington, DC 20390

Directorate of Civil Engineering

U.S. Air Force

Washington, DC 20330 and

Office of Chief of Engineers

U.S. Army

Washington, DC 20314

U.S. Department of Energy

Iffice of Buildings and Community Systems

A/ashington, DC 20585

c. 2

NBSIR 80-2147

1

1

* DAYLIGHTING, WINDOW MANAGEMENT

SYSTEMS, AND LIGHTING CONTROLS

National Bureau of Standards

Library. l-Ol Admin. Bldg.

FEB 2 7 1981

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S. Treado and T. Kusuda

December 1980

Center for Building Technology

National Engineering Laboratory

National Bureau of Standards

U S. Department of Commerce

Washington, DC 20234

U.S. DEPARTMENT OF COMMERCE, Philip M. Klutznick, Secretary

Jordan J. Baruch, Assistant Secretary for Productivity. Technology, and Innovation

NATIONAL BUREAU OF STANDARDS. Ernest Ambler, Director
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ACKNOWLEDGMENTS

This project was sponsored jointly by the Tri-Services Committee, the

Department of Energy and the National Bureau of Standards. The authors

are grateful for the data collection and reduction assistance provided

by C. Kilb.

iii
ABSTRACT

This report investigates major factors concerning windows in buildings and

their effect on visual conditions, thermal conditions, and energy requirements. Empirically obtained data are presented for daylight illumination

as a function of solar radiation, sky condition, window size and orientation,

and interior reflectance. The thermal and visual effects of several window

management strategies are examined, along with an analysis of automatic

lighting controls. Daylight utilization is seen to offer great potential

for minimizing lighting load in perimeter building areas, and careful

determination of lighting needs and window management strategies can provide

additional benefit.

Key words: Daylight; energy conservation; fenestration design; illumination;

lighting control; solar heat gain; window management.

iv
NOMENCLATURE

COND = Heat conduction through window

CONV = Heat convection from window

LW = Long-wave IR radiation emitted by window

RAD = IR heat exchange between window and room

SW = Short-wave solar radiation transmitted through window

= Indoor air temperature

Tg = Window surface temperature

e = Surface emittance of window

a = Stefan-Boltzman constant

V
TABLE OF CONTENTS

Page

ACKNOWLEDGMENTS iii

ABSTRACT iv

NOMENCLATURE v LIST OF TABLES vii

LIST OF FIGURES viii

1, INTRODUCTION 1 2, DAYLIGHTING FACILITY AND EXPERIMENTS ^ 3, AVAILABLE DAYLIGHT 6

3.1 Effect of Sky Condition, Window Area and Wall Reflectance 6

3.2 Indoor Illumination Percentage Charts 8

3.3 Daylight Distribution 8

4, WINDOW MANAGEMENT SYSTEMS 10

4.1 Effect of Window Management Systems on Daylighting 11

4.2 Performance of Solar Control Devices 13

5, WINDOW HEAT GAIN AND LIGHTING ENERGY ANALYSIS 17

5.1 Lighting Energy Analysis 17

5.2 Cooling Load Due to Lighting and Window Heat Gain 18

5.3 Energy Consumption Due to Lighting and Window Heat Gain 18

6, PERFORMANCE OF LIGHTING CONTROLS 20

6.1 Continuous Dimming System 20

6.2 Sequential System 21

6.3 Comparison of Actual Systems with Perfect Dimmer 22

6.4 Dimming System Performance 22

6.5 Sequential System Performance 23

7, APPLICATION OF LIGHTING CONTROL SYSTEMS 26

8, LIGHTING CONTROL/WINDOW MANAGEMENT TEST CASE 27

9, SUMMARY AND CONCLUSIONS 30

REFERENCES 32

vl
LIST OF TABLES

Table 1. Surface Reflectances 5

Table 2, Window Management Systems 12

Table 3. Thermal Effects of Window Management Systems for South Exposure

Under Clear Sky 15

Table 4. Hourly Power Consumption of Dimming System 23

Table 5. Hourly Power Consumption of Sequential System 24

Table 6. Lighting, Window Management Test Case 28

vii
LIST OF FIGURES

Figure 1. Photographs of the NBS daylight research facility 33

Figure 2. Floor plan of Daylighting Lab 34

Figure 3. Effect of sky condition on indoor illumination - north-facing

. window 35

Figure 4. Effect of sky condition on indoor illumination - south-facing

window 37

Figure 5. Effect of window area and wall reflectance - north-facing

window 39

Figure 6, Percentage hours of solar radiation 41

Figure 7, Percentage hours of indoor illumination — north-facing

window, 1.5 m (5 ft) and 2.7 m (9 ft) wall reflectance

0.28, window area 22.9, 15.3 or 7.6 percent 42

Figure 8. Percentage hours of indoor illumination — north-facing

window, 1.5 m (5 ft) and 2.7 m (9 ft) wall reflectance

0.50, window area 22.9, 15.3 or 7.6 percent 43

Figure 9. Indoor horizontal illumination profiles for north-facing

window, wall reflectance 0.28 and varying window areas. Percentages indicate ratio of illumination at each grid

location to illumination immediately inside window 44

Figure 10. Indoor horizontal Illumination profiles for north-facing

window, wall reflectance 0.50, and varying window areas. Percentages indicate ratio of illumination at the center

of each grid location to illumination immediately Inside

window 45

Figure 11. Daily solar radiation and illumination cycle - north and

south-facing window 46

Figure 12. Thermal and illumination effects of windows 47

Figure 13. Indoor illumination with window management - south-facing

window 48

Figure 14. Indoor illumination with roll shades - south-facing window 50

Figure 15. Vertical solar radiation at inside surface of south-facing

window, with and without management 51

viii
LIST OF FIGURES (Cont’d)

Figure 16, Daily window radiation profiles ~ south-facing window,

with and without window management 52

Figure 17, Short-wave solar heat gain for test case, various window

systems — south-facing 53

Figure 18, Lighting load for test case 54

Figure 19, Cooling load for test case 56

Figure 20, Cooling and lighting energy for test case 58

Figure 21, Schematic of lighting control systems.. 60

Figure 22, Light output versus power consumption — two systems 61

Figure 23, Perfect dimmer performance comparison 63

Figure 24, Average indoor illumination for test room — south-facing

window 64

Figure 25, Lighting power consumption and energy savings — dimming

system 65

Figure 26, Lighting power consumption and energy savings — sequential

system 67

Figure 27, Lighting power consumption and window solar heat gain

trade-off — clear window versus silver film 69

ix

1 . INTRODUCTION

The role of windows in buildings is a complex one due to the variety of

ways windows affect interior illumination, artificial lighting, heating and

cooling loads, and occupant comfort (both physical and psychological), as well as their integration into the overall structural design.

Windows have been estimated to account for as much as five percent of the

total energy consumption in the United States, with electrical lighting

accounting for an additional 5.5 percent of total energy use [1]. Thus,

effective use of windows and lighting could have a significant impact on over 10 percent of the energy used in the United States. In addition, up

to" 50 percent of the energy used in commercial buildings has been shown to

be due to electric lighting, with an additional energy expenditure of 15

to 20 percent due to Increased cooling requirement to remove heat produced

by lighting [2, 3]. This indicates that commercial buildings in particular

have great potential for energy savings through optimum design and

utilization of windows and lighting systems.

Daylight utilization has been recognized as an effective way of reducing

the energy required for illuminating buildings. However, attendant impact

on building heat and cooling systems must also be considered before net

annual benefits can be determined. Comprehensive computer programs are available for calculating natural daylighting for a specific room and

window configuration. These procedures incorporate complex mathematical

representations of light-ray reflections from surface to surface within a room, requiring time-consuming (and costly) calculations for a building

energy analysis.

A simplified daylight calculation procedure would enable Incorporation

of daylight utilization, and related heating, cooling and lighting effects,

to be included when undertaking building energy analyses. This in turn

would promote energy-efficient building design, as well as enabling cost

effective retrofit procedures to be identified. Additional information is

needed concerning the performance characteristics of the various window

management strategies (such as shades, films or screens) and automatic

lighting control systems presently available. The use of these devices

can dramatically alter energy use in buildings.

Determining the optimum design and utilization of windows in buildings is

difficult due to the tradeoffs between the various window associated factors.

Windows may be designed to provide natural lighting, ventilation, and visual

contact with the outdoor environment for the building occupants [4] (favoring larger windows). At the same time, windows must be designed to minimize

unwanted solar heat gain or cold weather heat loss [5], as well as maintaining sufficient privacy (favoring smaller windows). Of course, windows must

be structurally sound and weathertight , with a long service life and low

maintenance. A movable sash is needed if ventilation is desired.

Various window management systems have been proposed to selectively alter

the thermal conductance and/or the solar transmittance of windows through

1
the use of screens, films or shutters [6], Special emphasis has been placed

on the Improvement of the thermal resistance of window systems to make them more compatible with the other portions of the building envelope. Efforts

in this area include the use of multiple-pane systems, low-conductivity fill

gases, internal partitions, partial evacuation of the air space, transparent

heat mirror coatings and movable insulating devices [7], These studies,

along with several computer model simulations [8, 9, 10], indicate that

properly designed and operated window systems can reduce overall operating

costs to the extent of making the window a net energy benefit as compared

to a solid wall.

The use of daylight to replace or supplement artifical lighting through the use of automatic controls has been shown to have significant potential for

energy savings, due to reduction of both lighting and cooling requirements

[10, 12]. Computer procedures have been developed to predict the amount of

natural lighting available in buildings as a function of window size, type

and orientation [12]. Potential energy savings through the use of automatic

lighting controls have been estimated assuming a unit-for-unlt replacement

of interior illumination from artificial lighting, but actual savings may

vary due to the light control strategy employed. This factor may be offset

[13] if visual performance is better with daylight than with an equivalent

level of electric lighting due to better contrast rendition.

Unlike most of the previous efforts, which are based on mathematical modeling,

this report will investigate several of the major window associated building

energy factors, based upon experimentally measured data. The mathematical

modeling studies examined the Interaction between lighting and cooling energy

savings through the utilization of daylight, and undesirable window-associated

thermal gains or losses, along with the resulting energy requirements.

Building energy analysis computer routines, such as NBSLD and DOE-2 use solar

radiation data as an input parameter to calculate building energy performance

data. In order to Include the effects of daylight utilization on building

energy performance, natural Illumination from daylight must be calculated

along with building thermal loads. Most daylight calculation routines require

outdoor illumination or sky luminance data as input parameters. Illumination

and sky luminance data are not available on a large scale basis nor for a wide range of geographical areas. However, solar radiation data is regularly

collected at a large number of locations. An accurate correlation between

indoor illumination and solar radiation would enable the effects of daylight

utilization to be included in building energy performance analyses. The

first topic of the report is daylight availability. The amount of available

interior illumination due to daylight is measured as a function of window

area and orientation, internal room reflectance, outdoor solar radiation,

and sky condition. Indoor illumination percentages, indicating the percent

of working hours during which indoor illumination meets or exceeds a set

level, are presented for a variety of room and outdoor conditions. The

distribution of daylight within a room is also examined.

The second topic of the report is the effect of various window management

strategies on the thermal and illumination characteristics of window systems.

2
Indoor illumination with window management is examined along with measurement

and analysis of the corresponding reduction in solar heat gain.

The final topic of the report concerns experimental observation of the

performance of automatic lighting control systems, and their interrelation

with windows and building energy requirements.

Results of several test cases are presented, with a comparison of the

performance of the actual lighting controls to the performance of a 'perfect'

lighting control system. Application notes for potential lighting control

users are also included.

The experimental data reported herein were obtained during spring and summer

of 1979, at the NBS Daylight research facility constructed on the solar test

site on NBS grounds in Gaithersburg, MD. (latitude 39°8', longitude 77°13').

This is an initial report which does not attempt to address the general question of the annual cost/benef its of daylight utilization, lighting controls

and window management, since a full year of test data is not yet available.

Additional fall and winter measurements are being performed with particular

emphasis on the annual window management systems and lighting controls,

results of which will be contained in a subsequent report.
2. DAYLIGHTING FACILITY AND EXPERIMENTS

The NBS Daylight Research Facility consists of a specially altered mobile

home, 3.7 m x 16.3 m (12 ft. x 54 ft.). The home was constructed of wood

framing and paneling, 7.6 cm (3 in.) glass fiber insulation and aluminum

siding. Photographs of the facility are shown in figure 1. The home was partitioned into four testing modules with various combinations of window

orientations and sizes. A floor plan of the lab is shown in figure 2. Three

of the modules were outfitted with fluorescent electric luminaires suspended

directly below the ceiling. Rooms 1 and 3 had four luminaires, while room 2 had eight. Each luminaire consisted of two-40W lamps with one ballast, either

standard or dimming, with a wrap-around prismatic diffuser. The power con- sumption of each lighting circuit was monitored individually. The lab is

cooled by a single air conditioner located in room 4 also with individually

monitored power consumption. Rooms 1 and 4 can be isolated from the rest

of the lab by closing their entry doors. The doorway between rooms 2 and 3

is blocked by a curtain with negligible light transmittance.

Instrumentation was installed to measure and record several parameters on an average hourly basis. These parameters included total hemispherical solar

radiation and diffuse solar radiation over a horizontal surface, horizontal

indoor illumination at various interior locations, and temperatures outdoor

and throughout the lab (TYPE T thermocouples). Additional sensors were

used in conjunction with the window management segment of the testing,

consisting of a vertically mounted pyranometer and a similarly mounted

long-wave infrared radiometer positioned at the center of the south window

in room 4 (see figure 2).

Sensor signals were logged with millivolt integrators and a multichannel

data logger. Additional measurements of illumination were performed by hand,

using illuminoraeters. A frequent visual examination of sky condition was

also performed.

The various electrical power consumptions were measured using pulsegenerating watt-hour meters connected to resetting counter/printers which

could be selected to print at 0.25, 0.5 or 1.0 hours.

The reflectances of the interior surfaces and the surrounding ground surfaces

are listed in table 1. These were determined with a ref lactometer . The

ground surfaces on the south, west and east sides of the lab were completely

asphalt, while on the north side asphalt extended approximately 10 m (33 ft.)

followed by a grassy downward slope. A small section of concrete was located

adjacent to the northwest corner of the facility, increasing the amount of

ground-reflected radiation impinging upon the north window of room 1 when

the sun was in the west.

4
Table 1. Surface Reflectances

Surface Type Reflectance

Interior Floor brown carpet 0.38

Ceiling white drjrwall 0.84

Wall a) wood paneling 0.28

Wall b) gray matte surface 0.50

Wall c) white matte surface 0.85

Exterior:

Ground asphalt 0.07

Ground concrete 0.25

The views out of the windows utilized for this testing schedule were

essentially unobstructed (<2 percent) with the exception of the large north

window in room 1. The bottom of the north window was partially blocked (12

percent) by the tops of large trees located approximately 122 m (400 ft.)

north of the facility on a downward slope. Window obstruction profiles were determined by viewing out the windows at a spot 0.9 m high (3.0 ft.) located

2.7 or 3.1 m (9 or 10 ft.) from the center of each window. Rooms 1 and 4 were only deep enough to allow measurements at the shorter depth.

All the windows were comprised of single-pane clear glass 1.6 mm (1/16 in.)

thick, except the large north window in room 1, which was double-pane clear

glass, each pane 3.2 mm (1/8 in.) thick. The measured visible transmittance

of the single pane windows was 0,91 and the double pane windows 0.82. Visible

transmittances were measured with an illuminometer by consecutively positioning

the sensor immediately adjacent to the inner and outer surfaces of the window.

Transmittance was computed from ratio of the measured illuminance transmitted

by the window to that incident upon the outer surface of the window.

5
3. AVAILABLE DAYLIGHT

One of the major objectives of this study was to evaluate the contribution of

daylight to interior illumination as a function of window size and orientation,

internal reflectances, and sky condition. North and south exposures were examined during this phase, through the use of rooms 1 and 3, respectively.

Indoor illuminance was measured and correlated as a function of total horizontal solar radiation and sky condition. This is a different approach from

a previous daylight factor approach in which the indoor daylighting level was correlated with the outdoor illuminance. A key point of such an approach is

the abundance of total solar radiation data available in the literature, since

this data is routinely collected at many locations by the Weather Service,

which enables application of this type of correlation to predict interior

daylight illuminance at a wide range of locations.

Throughout this report, references to solar radiation are meant to indicate

radiation power including the full solar spectrum (0.2-3 jm). Diffuse solar

radiation is simply total solar radiation less the direct component. Outdoor

illumination is considered to be radiation power in the visible light band

(0,4-0. 8 pm) as measured while shaded from direct sunlight. Indoor illumination is measured in a similar manner. All illumination measurements are filtered to respond according to the CIE standard eye curve.

3.1 EFFECT OF SKY CONDITION, WINDOW AEEA AND WALL REFLECTANCE

Solar radiation is the source of daylight illumination, but there is no direct relation between the level of solar radiation and the illuminance

levels, either indoors or out. This is because of the random effect of sky

condition. The relative amounts of the Infrared, visible, and ultra-violet

portions of the radiant power spectrum vary as a function of sky condition

(atmospheric moisture, cloud density and type). Under clear sky conditions,

the level of solar radiation is high, but the illuminance level is not since

a clear blue sky provides low levels of illuminance. On an overcast day, the

opposite situation occurs. The level of outdoor illuminance is more dependent upon the level of diffuse solar radiation than total solar radiation,

since greater levels of diffuse solar radiation indicate more overcast sky

conditions. The bright white clouds which occur on partially overcast days,

during which the sun is not directly obscured, provide the greatest daylight

illuminance levels.

For the purposes of this study, sky conditions were defined as follows:

clear sky - essentially very few or no clouds, bright sun; partially overcast - sun visible, partially cloudy;

overcast - sun not visible, completely cloudy.

Considerable variation in sky condition can occur, as combinations of overcast,

partially overcast, or clear conditions may be present at the same time in

different areas of the sky. This variation is greatest under partial

overcast conditions since by definition the clear and overcast conditions

result in nearly uniform sky conditions. However, measurements indicate a

6
good relation between level of solar radiation, sky condition and indoor

illumination from daylight.

Assuming equivalent levels of total solar radiation, an overcast sky will be

more effective than a clear sky in providing interior illumination, due to

reflected light from clouds. This is especially true for northern orientations since the sun is in the south sky causing a large amount of reflected

light from any clouds in the northern sky. Figures 3a and 3b examine the

effect of sky condition and total solar radiation on the amount of available

Indoor illumination for a northern exposure at depths of 1.5 m (5 ft.) and

2.7 m (9 ft.), respectively, from window at sill level. These figures were determined from many measurements performed in room 1, with a wall reflectance

of .28, and a window area of 1.6 m^ (17.4 ft^), or 22.9 percent of external

wall area. The celling and floor reflectances were measured and found to be

0.84 and 0.38 respectively.

A large difference in the amounts of available daylight is seen as a function

of sky condition. Each of the lines represents a least-squares fit of handmeasured and recorded simultaneous, instantaneous values of total solar radiation and indoor illumination. An overcast sky is seen to provide approximately 2.0 times the indoor illumination as a partially overcast sky, and

approximately 4.0 times as much as a clear sky, for a 1.5 m (5 ft.) depth at an equivalent level of total solar radiation. Similar calculations for the

2.7 m (9 ft.) depth indicate an overcast sky as providing 1.5 times the indoor

illumination as a partially overcast sky and over 2.0 times as much as a clear

sky. These figures indicate a range of expected indoor illumination values.

Since low values of total solar radiation usually mean overcast days, indoor

illumination levels under those conditions would probably correspond best with

the overcast sky. Similarly, since high values of total solar radiation

usually correlate with clear sky conditions, indoor illumination levels would

probably correspond best with the clear sky. Indoor illumination at medium

values of total solar radiation would probably fall at intermediate locations

within the clear/overcast band, but would experience the most variation and

be the most difficult to predict, without knowing sky condition or cloud

cover.

Figures 4a and 6 provide a similar examination for a southern exposure for two

depths. The window size was 1,1 m^ (12 ft^) or 16 percent of wall area. An

overcast sky is seen to provide 1.3 times the Indoor Illumination as a partially overcast sky and over twice the illumination for a clear sky for the

1.5 m (5 ft.) depth, with smaller differences for the 3.1 ra (10 ft.) depth,

at an equivalent level of total solar radiation.

Figures 5a and b examine the effect of window area and wall reflectance on

the amount of available daylight from a north window for depths of 1.5m

(5 ft.) and 2.7 m (9 ft.) respectively. The window areas included are 22.9 percent, 15.3 percent and 7.6 percent, as well as wall reflectances of

0,28 and 0.50. Window areas were altered by symmetrically reducing the width

of the window. Wall reflectances were altered by changing the wall covering.

The illuminances are based on an average of measurements under all sky

conditions for comparison purposes. Changes in window area are seen to

7
produce larger changes in illumination level than a change of wall reflectance

for the shallower depth location, while the opposite effect is seen for the

deeper location. This effect indicates that daylight illumination at the

deeper location is more dependent upon inter-reflected light from the walls

than light directly transmitted from the window.

3.2 INDOOR ILLUMINATION PERCENTAGE CHARTS

In analyzing the effect of daylight on illumination, it is important to know

the percentage of working hours during which indoor illumination from daylighting reached various levels. Such an indoor illumination percentage

would indicate the ratio of time during which the desired indoor illumination

level could be achieved through daylight alone, as well as percentage contributions of daylight and artificial lighting needed to maintain the illumination set point. Since Indoor illumination as a function of total solar

radiation, window area, and wall reflectance has been previously presented,

it is necessary to examine first the percentage of working hours for which

solar radiation met or exceeded various levels. These solar radiation

percentages for spring and summer are presented in figure 6.

The work day was assumed to be from 0600 to 1800 hours. Hourly averaged

total solar radiation values were examined and compiled according to their

level and the percentage hours they occurred. The spring percentage was slightly lower than that for summer.

Combining the indoor illumination versus total horizontal solar radiation

correlations previously presented with the percentage hours of solar radiation levels permits the determination of percentage hours of indoor illumination. These percentage hours of indoor illumination for a north window are shown in figures 7 and 8 as a function of window area, wall reflectance, depth

from window, and season. These figures indicate that daylight can provide a significant portion of the desired indoor illumination level. The relative

contribution of daylighting would be dependent upon the desired indoor

illumination level, window size and wall reflectance.

For example, if a 700 lux (70 F.C.) indoor illumination level was desired at

1.5 m (5 ft.) from the window, this could be achieved through daylight alone

for over 50 percent of working hours with the largest window area., as shown

in figure 7 . The figures indicate that more modest contributions would be

expected for the 2.7 m (9 ft.) deep location, but still significant ones,

especially for the 0.50 wall reflectance, as shown in figure 8. Window area

is seen to have a marked effect on the relative contribution of daylight. At

the smallest window area, approximatley 300 lux (30 F.C.) indoor illumination

occurs for 50 percent of the working hours at the shallow depth and less than

half that at the deep location. As noted earlier, wall reflectance is seen

to have a greater effect on the illumination at the deeper location.

3.3 DAYLIGHT DISTRIBUTION

Figures 9 and 10 show the effect of window area and the distribution of

indoor illumination due to daylight for a north window. The two figures

8
represent data taken with two different wall reflectances. These profiles

were determined from consecutive measurements with different window areas,

consisting of 22,9 percent, 15.3 percent and 7,6 percent of the exterior

wall area, for each of the wall reflectances. The percentage illuminances

were determined by comparing the illuminances in the various sub-areas of the room to the illuminance directly in front of the window. All measurements

were made at a height of 0.76 m (30 in.) above floor level on a horizontal

plane.

The significance of these profiles is in the distribution of daylight into

the deeper parts of the room. As would be expected, the higher wall reflectance produced a better distribution of light into the rear of the room. The

average illumination of the four deepest areas with the largest window area

(22,9 percent) and wall reflectance of 0.50 was 22,1 percent. Reducing window area to 7.6 percent resulted in a drop in average illumination to 8.7 percent in the same areas. A similar illumination level, 7.8 percent, was observed at the same areas for a wall reflectance of 0.28 and a window area

of 22.9 percent. In this case, increasing wall reflectance from 0.28 to 0.50

produced an increase in indoor illumination at the deepest areas comparable

to that which would have been obtained by increasing window size by a factor

of three. This indicates the importance of wall reflectance in influencing

illumination level. This conclusion is limited to the room sizes investigated

in this study.

It should be understood, however, that the distribution of daylight

throughout a room will vary with time of day, season and sky condition.

These illumination profiles are presented as mere examples, in an attempt to

evaluate the effect of window area and wall reflectance on indoor illumination.

Figure 11 presents typical diurnal profiles of total solar radiation, outdoor

illumination, and indoor illumination for a north and south exposure. Indoor

illumination was measured 1.5 m (5 ft.) from the center of the windows, at a height of 0,76 m (30 in.). The north-facing window area was 22.9 percent of

the wall area and the south-facing window area was 16 percent. Sky condition

was partially overcast. Indoor illumination is seen to be higher in the

morning, corresponding to a higher level of outdoor illumination at that

time due to sky conditions.

9
4. WINDOW MANAGEMENT SYSTEMS

The results in the previous section were obtained using clear glass, unaltered

or shaded. Many varieties of window management devices are available to alter

the thermal and/or visual performance of window systems. Some of these take

the form of sun control screens, films or shades, and are designed to reduce

unwanted solar heat gain during the cooling season by reflecting Incident

solar radiation. Additional benefits of glare control, cosmetic appearance

and reduced solar-induced material fading are attributed to these types of

window management systems.

In this section the effect of several window management systems on the

interior illumination level due to daylight will be examined. Some of the

thermal aspects of the sun control devices are also presented.

As compared to an opaque wall, a window in a room will affect the thermal and

visual characteristics of that room in several significant ways. First the

window will provide natural illumination through transmission of daylight.

Heat may be transferred into the room due to the admittance of solar radiation through the window. This can be termed short-wave solar radiation.

Radiation exchange in the long-wave infrared region will occur between the

window glass and the room, the net magnitude dependent upon the temperature

difference between the glass and the room, and their respective surface emittances. Heat transfer through the window will also occur due to temperature

differences between the interior and exterior air tempertures, by way of con- duction and convection. The amount of total solar radiation, sun angle, time

of day, sky condition, and wind conditions will all Influence the thermal or visual performance of windows. These factors are summarized in figure 12.

Heat gain through a window can be expressed by the following relation;

Window Heat Gain = SW + LW + CONV + COND (1)

where SW = short-wave solar radiation transmitted through window

(0.2-3 ym)

,

LW = long-wave radiation emitted by window (4-50 ym)

,

CONV = convection from window,

COND = conduction through window.

For the sake of simplicity, only those factors pertaining to summer conditions

will be examined. The short wave component is composed of direct and diffuse

solar radiation as well as ground-reflected solar radiation. The magnitude

of the short-wave component is dependent upon the level of solar radiation,

sky condition and ground reflectance. Most of the incident short-wave radiation will be absorbed by the floors, walls and furniture of a room, raising

their temperatures. Since glass is opaque to long-wave radiation emitted by

surface less than 121 °C (250°F), this energy cannot be transmitted back out

of the room through the window. The temperature of the glass will determine

the net transfer of heat due to radiation between the room and window. The

glass temperature is in turn dependent upon the Indoor and outdoor air

temperature, level of solar radiation, wind conditions and window management

10
system. Heat exchange due to eonduQtiye and convective processes is

essentially dependent upon the glass and air temperatures and air flow

conditions at the window surface.

Window management systems can reduce unwanted summer window heat gain by

rejecting short-wave radiation. directly, and by reducing window temperature,

thereby reducing the long-wave radiation component, as well as the convection

component. Since window temperature is primarily dependent upon air temperature, the magnitude of the last. effect may be .small for a non^heat-absorbing

glass. This is because the amount .of radiation emitted by a surface is

dependent upon its absolute temperature, so the temperature variations

normally encountered with standard glass, have a relatively small effect.

An increase in window temperature may occur if a film with a significant

solar absorptance or an interiqr shade is .utilized for solar control. This

phenomenon may be beneficial under winter’ conditions, increasing the heat

gain as well as reducing heat loss to the window by radiation, although

window conduction heat losses may Increase.

It is difficult to selectively suppress solar heat gain through windows

without also reducing daylight .transmission. Because of this, a trade-off

must be made between reducing cooling load through rejection of solar radiation with window management systems and increasing lighting load because of

lower levels of daylight lumlnatton available with window managment systems

installed. Window manageinent techniques which allow sun control only when

needed, enabling full utilization of daylight at other times, would partially

reduce the negative aspects of this tradeoff. However, many of the sun films

and screens presently available are designed for relatively permanent installation, at least on a seasonal basis, and would be difficult to remove during

the portion of the day when they are not needed for solar control purposes.

Several types of adjustable shades and louvers are available that can be

quickly and easily positioned in response to sun control or daylight transmission requirements determined by the user. Adjustment may be designed to occur manually or automatically, depending on the system, so the window will

be completely clear of obstructions.

Table 2 lists the various window management systems and devices examined

during this study. All the equipment listed is available commercially.

While many more types of window, management systems are available, the ones presented here were chosen to demonstrate the performance which might be

expected through utilization of typical systems.

A.l EFFECT OF WINDOW MANAGEMENT SYSTEMS ON DAYLIGHTING

Figure 13 presents the indoor illumination levels observed in a room with a south-facing window (area 13 percent) as a function of total outdoor hemispherical radiation for different window management systems. Illuminance was measured 1.5 m (5 ft.) and 3.0 m (l6,ft.) from the center of the window at a height of .76 m (30 in.). Illuminatibn levels for the roll shades are presented

in figure 14. In these figures, the illuminance plots are given for overcast

11
and partially overcast sky conditions for all the window management systems,

with additional clear data for the roll shades.

The screens and shade with film are seen to reduce natural illumination

significantly as compared with clear glass. Use of either the bronze screen

or the shade with film would provide very little natural illumination, even

at high levels of solar radiation. The reflective films and shade without

film produce a more moderate reduction in illumination levels. Indoor illumination on overcast days is seen to be higher than on partially overcast days,

for an equivalent level of solar radiation. Illumination provided by the

roll shade without film is higher on clear days than on partially overcast

days, due to the direct solar radiation falling on the shade being dispersed

into the room.

Table 2. Window Management Systems

Item Description

Bronze Screen Small horizontal louvers of bronze

bands; flat black; attached to exterior

window frame.

Aluminum Screen Small horizontal louvers stamped from

aluminum sheet; natural unpainted;

attached to exterior window frame.

Glass Fiber Screen Open weave fabric; dark brown; attached

to exterior window frame.

Silver Reflective Film 60 percent reflection, 17 percent visible

transmission; polyester base; adhered to

glass.

Grey Reflective Film 30 percent reflection, 20 percent visible

transmission; polyester base; adhered to

glass.

Bronze Reflective Film 17 percent reflection, 28 percent visible

transmission; polyester; adhered to glass.

Roll Shades, Single Various colors (white, grey) and weaves;

vinyl; interior use.

Roll Shades, Dual Same as single but with one layer of

fabric and one layer of reflective film

seprated by small air space (1.3 cm,

0.5 in.); installed with film facing

window side.

12
4.2 PERFORMANCE OF SOLAR CONTROL DEVICES ’

To evaluate the effectiveness of the different window management strategies

in reducing solar heat gain through the window, measurements were made of the

level of solar radiation incident upon a vertical plane inside the window

surface. These measurements are summarized in figure 15 as a function of

exterior horizontal total solar radiation and window management system. These

data were obtained by a linear least-squares fit of hourly averaged data for

each window management system. All measurements were made during a one-month

period, and no correction was made for the small differences in sun angle due

to seasonal changes in solar altitude.

The window utilized had an area of 0.66 (7.1 ft^) or 9.7 percent of wall

area. The bronze screen and the roll shade with film are seen to be the most

effective in reducing the transmission of solar radiation into the room, with

the other screens and silver film transmitting slightly higher amounts. All

the systems except the grey film, which did not have a high reflectance, provided at least a 50 percent reduction in short*^wave solar transmittance into

the room.

Typical daily profiles of outdoor horizontal Solar radiation, indoor

vertical solar radiation, indoor vertical long-wave IR radiation and outdoor

temperature for a sunny summer day are given in figure 16 for a clear window

and for a window with a sun control screen, for a south orientation. The

indoor radiation levels were measured immediately adjacent to the window

surface. The level of long-wave radiation emitted from the window is seen

to remain fairly constant, with only a slight increase in outdoor temperature

during the day. The level of indoor vertical solar radiation is significntly

reduced through the utilization of the sun control screen. This indicates

that the most significant benefit of the solar control devices is reduction

in the amount of solar radiation .transmitted through the window, rather than

a reduction of the long-wave radiation emitted from the window surface, since

window surface temperature does not change much as a function of solar

radiation.

It is difficult to analyze the thermal effects of the different window

management systems because of the variety of factors which influence their

performance. It was not possible to perform side-by-side comparison testing

of all the window management systems under identical conditions. Each of the

systems was installed for a set time Interval, and' its performance continuously monitored. For comparison purposes^ only a limited set of data was shown which had been obtained under relatively equivalent time-of-day, solar

and temperature conditions. Some variations in conditions did occur,

especially temperature during the later stages of testing.

Table 3 lists the measured levels of window heat gain along with a breakdown

of the individual components. Heat transfer due to conduction is not included,

since it is essentially a function of the indoor-to-outdoor temperature difference and the thermal transmittance (U-factor) of the window. Use of window

management systems may alter the U-value of a window, but this phenomenon was

not investigated. Under summer conditions, the U-value of a window management

13
system does not have as great a thermal influence on a room as the reduction

in solar heat gain due to reduced solar transmission. Air infiltration was

also neglected, since it was assumed to be approximately the same regardless

of the window management system.

The significance of table 3 is in the breakdown of percentage contributions

of the short-wave, long-wave and convection components. The convective heat

transfer is dependent upon the temperature difference between the window

surface and the indoor air. This parameter was calculated from the following

relation [14]

Indoor air temperature was measured directly and the window surface

temperature was determined from the long-wave radiometer readings and the

Stef an-Boltzman relation [14], The total radiation incident upon the radiometer (LW) is equal to the sum of the energy emitted and energy reflected by

the window, or:

e = surface emittance of window

0 = Stefan-Boltzman constant, 5,67 (10”^) ^ (0,1713 <' 10~8 '> Btu ) m2'°K^ h*ft2*°R'^

Rearranging the terms of this relation yields:

CONV =0,19 (Tg-T^)^/^ (Tg-T^) ( 2 )

where

Tg = window surface temperature

T^ = indoor air temperature.

LW = eoTg + (1 - e)dT^ (3)

where

(4)

The surface emittances of the glass and films were determined through

measurement, also utilizing the radiometer.

(5)

<,(T^ - T^)

14
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A surface thermocouple was attached to the window surface with a thin piece

of tape, and measurements were performed at night to eliminate solar absorption effects which might induce inaccuracy. Simultaneous measurements were made of Tg, and LW, allowing e to be computed. These measurements indicated surface emittance values of 0.94 for the glass and 0.82 for the films.

When the roll shades were installed, the radiometer viewed the shade surface,

which was composed of an open weave material. It was difficult to determine

an exact surface emittance value for this system, since the shade and glass

or film surface both contributed to the radiation intensity sensed by the

radiometer. Therefore, the surface emittances were assumed to be 0.94 for

the roll shade without film, and 0.82 for the roll shade with film, since

these were the measured values for the surfaces which were opaque to long wave IR radiation.

IR radiation heat gain to the room (RAD) was estimated from the relation:

RAD = eo(Tg - T^) (6)

based on the assumption that the room air temperature represents the mean radiant room temperature (since walls were highly insulated) and assuming

that the room is performing as a black body (since the window views the room

as a cavity).

In all cases, convective heat transfer is seen to make a small contribution

to the overall heat gain. The window surface temperature with the grey film

is the highest, due to the solar absorptance of the tinted film. The surfaces

of the roll shades also reached a higher temperature, due to the long-wave

radiation trapped between the shade and glass.

As would be expected, the main effect of the window management systems is seen to be the reduction of transmission of short-wave solar radiation.

Significant reductions in window heat gain are seen, especially with the

bronze screen and roll shade with film. The grey film, which does not have

a high reflectance, produced a small reduction in short-wave solar radiation

which was offset by an increase in long-wave radiation, producing no net

reduction in window heat gain. The long-wave components from the other

systems were not significantly affected.

16
5. WINDOW HEAT GAIN AND LIGHTING ENERGY ANALYSIS

To examine the tradeoff between natural and artificial illumination and solar

heat gain through the window system, several examples were considered. A

south facing window was chosen, since this orientation is of most interest

during the summer cooling season. Two window areas were examined, 26 percent

and 13 percent, along with three window systems; clear, fiber glass screen,

and bronze screen. These three window systems were chosen because their

solar rejection performance covers a large range, from high (bronze screen),

to medium (fiber glass screen), to low (clear).

Using the data from figure 15, which indicated short-wave solar radiation

levels at the inside window surface per unit area, short wave heat gain into

the room was determined for the two window areas with each window system.

This data is presented in figure 17, again as a function of outdoor horizontal

solar radiation. These relations were utilized in subsequent analysis of

cooling load due to solar heat gain and lighting.

5.1 LIGHTING ENERGY ANALYSIS

To integrate the effect of natural and artificial illumination on energy

consumption, as a function of window area and window system, several factors

must be considered. The required indoor Illumination level must be determined, as well as the location within the room of that illumination level.

The contribution of daylight is much greater nearer the window than deep

in a room. The illumination set point is determined mainly by occupant needs.

Some users require high illumination levels throughout the room, others are satisfied with high levels only in the center of the room or at another

specific location.

For the purpose of example, the locations of the illumination set points were chosen to be 1.5 m (5 ft.) and 3,1 m (10 ft.) in from the window center, and

the illumination levels chosen to be 500 lux and 1000 lux. These illumination

levels were considered to be moderate and high, respectively. Illumination

levels will usually be higher for locations nearer to the window than at the

set point locations, due to daylight entering through the window. Size and

type of artificial lighting must be chcsen also. Fluorescent lighting systems

were chosen since they were used throughout the testing and are commonly found

in buildings. Lab measurements indicated that four 80 W double- lamp fluorescent luminaires provided approximately 1000 lux illumination at the center of

the test modules. Considering additional power consumption by lamp ballasts

and control systems, total lighting power consumption was assumed to be 400 W

for the 1000 lux condition and half that, 200 W, for the 500 lux condition.

Based on the foregoing assumptions, lighting loads were calculated for the

various conditions assuming the operation of a perfect dimming system. A

perfect dimming system is defined here as one which varies lighting level in

direct proportion to the lighting power consumption while maintaining constant

indoor illumination level. This is done by reducing artificial lighting in

response. to the available Interior daylight. A perfect system maintains

this linear relationship down to zero light output. Perfect dimming systems

17
do not exist in reality, so reductions in lighting load could be less for

actual system. However, the assumption of a perfect; dimming system is used

in this analysis. System performance of several lighting control systems is

examined in a later section.

Figure 18 examines lighting load as a function of outdoor horizontal solar

radiation for the various window areas, window systems, and illumination set^

point levels and locations. The contributions of daylight were determined

based on the illumination levels for partially overcast conditions presented

in figure 13. Partially overcast conditions were used since they reflect

average data. Significant reductions in lighting load are seen for most con- ditions, especially with 500 lux Illumination set point and the window sytems

with higher visible transmission. Window area does not influence lighting

load as strongly as does the type of window system. For the clear window,

26 percent of the wall area, 500 lux setpoint condition, daylight is seen to

fully satisfy total illumination requirements at the 1.5 m (5 ft.) location

whenever horizontal solar radiation exceeds 200 W/m^. From figure 6, this

is seen to occur 76 percent of the working hours in spring and 79 percent in summer. However, under the same conditions at the 3.1 m (10 ft.) location,

some artificial lighting is always needed to maintain the illumination set

point, although significant reductions in lighting load are seen (35 percent

at moderate solar radiation level of 400 W/m^) for the clear windows. As a worst case, lighting load is only reduced approximately 6 percent for the

3.1 m (10 ft.) 1000 lux condition with the window screens, at a 400 W/m^

solar radiation level.

5.2 COOLING LOAD DUE TO LIGHTING AND WINDOW HEAT GAIN

Lighting operation affects energy consumption in two ways. First, energy is

used to provide Illumination. Second, heat dissipated must then be removed

from the conditioned space, increasing the cooling load by a factor dependent

upon the heat absorption and conduction capacity of the structure, as well as the COP (coefficient of performance) of the cooling system. A COP of 2.5 was chosen as a representative value for typical systems for use in this analysis.

Neglecting the conduction and absorption effect of the structure, cooling

load due to solar heat gain and lighting is given in figure 19 drawing from

figures 17 and 18, as a function of horizontal solar radiation. At low

levels of solar radiation, the majority of the cooling load is due to lighting, but with increasing solar radiation levels, short-wave heat gain begins

to dominate. Cooling load remains fairly constant at normally encountered

levels of solar radiation for the 1.5 m (5 ft.) set-point location, except

for the clear windows. For the 3.1 m (10 ft.) set-point location, cooling

load is seen to increase with increasing solar radiation, as solar heat gain

increases faster than the cooling load due to lighting decreases.

5.3 ENERGY CONSUMPTION DUE TO LIGHTING AND WINDOW HEAT GAIN

Energy consumption due to lighting and cooling to offset solar heat gain is

presented in figure 20. These plots are a combination of the previous two

sets of figures, and indicate the net effect of the window areas and window

18
systems on energy consumption. Results are very different for the 1.5 m

(5 ft.) and 3.1 m (10 ft.) set-point locations. For the 1000 lux condition

at the shallow depth, figure 20a, the contribution of daylight causes the

clear window to provide the least energy costs at normally encountered levels

of solar radiation. For the 500 lux condition, the medium screen (fiber

glass) is seen to be most effective when solar radiation exceeds low moderate

levels (400-600 W/m^), depending on window area.

At the deeper location, figure 20b, energy consumption is least for the screen

with highest solar rejection, and either screen is preferable to clear glass

for the larger window area. This is due to the fact that the contribution

from daylight is low at this room depth as compared with the shallower depth,

while solar heat gain remains the same. Therefore, increases in artificial

lighting load are more than offset by reduction in cooling load for this con- figuration. Energy consumption for cooling and lighting is seen to remain

almost constant regardless of solar radiation level for all systems except

the clear glass 26 percent window rea, indicating that for these situations

the various trade-offs virtually balance out.

19
6. PERFORMANCE OF LIGHTING CONTROLS

In this section, the performance of actual lighting controls will be examined,

to investigate their energy savings potential. In addition, information is

presented with respect to installation and operation of two typical lighting

control systems. Finally, the interaction between lighting power consumption

and window management system is examined with presentation of results from a test case using a dimming system in combination with a clear window or a solar screen.

In the previous section, the energy savings potential of automatic lighting

controls was examined assuming a "perfect" dimming system. No actual lighting control system can perform identically to a perfect system due to several

factors. These factors include power consumed to operate the control system,

inability of the control system to maintain a constant level of Illumination

due to sensor characteristics or room/window configuration, and power con- sumption of lighting not varying linearly with light output. Thus, the

lighting power consumption of actual systems could be different, probably

higher than for a perfect system. The magnitude of this difference would

be dependent upon lighting control system operation, amount of available

daylight and illumination set point.

Two commercially available lighting control systems were examined. One

system was a continuous dimming control and the other a sequential control

which switched individual fixtures on or off. Both control systems utilized

a photo-sensitive detector to monitor indoor illumination, and an internal

comparison circuit which determined if illumination level was not at the

required level. The control systems automatically adjusted the light output

of the electric lighting to maintain the Illumination set point. A schematic

diagram of the two systems is presented in figure 21.

6.1 CONTINUOUS DIMMING SYSTEM

With the dimming system, each luminaire was a complete system, including

photo sensor, controller and level adjustment. The photo sensor aperture was mounted on the fixture facing downwards, and was covered with a lens approximately 0,8 cm (5/16 in.) in diameter. A short piece of fiber optic cable

transmitted incident light from the lens to the actual photo detector located

within the controller. The controller was located adjacent to the standard

ballast. A comparator circuit within the controller determined if the illumination level sensed by the photo detector was at the required level. If

not, light output was varied until the illumination level was met. Light

output was varied through the use of a current limiting technique. Due to

the operating characteristics of fluorescent lighting, a minimum power level

of approximately 32 percent is required for proper operation. Reducing power

below that amount causes flickering, reduced lamp-life or no light output.

This minimum power level is maintained automatically by the controller. The

level adjustment control was a variable resistor which enabled any lighting

level to be chosen between full and the level at the minimum power point.

Figure 22a presents light output of a typical dimmer control unit as a function of power consumption for a single luminaire. Illumination is roughly

20
linear as a function of power consumption above the minimum power point. At

50 percent light output, power consumption is seen to be 68 percent.

To install and operate this particular system, it is simply necessary to

locate the luminaires and adjust the light level control to the desired

illumination level. The number of luminaires should be chosen such that the

; illumination level desired in the absence of daylight is achieved with the

1 level adjustment fully or nearly at its maximum. In this manner, the full

dynamic range of the control system can be utilized.

6.2 SEQUENTIAL SYSTEM

I

!i The sequential system is essentially an automatic switching system. Rather

j than continuous dimming, light output is controlled by switching luminaires

; on or off in response to the photo sensor. A single controller, level adjustment and photo sensor can accomodate multiple fixtures. The photo sensor and

level adjustment are wall mounted. The sensor is covered with a hemispherical

translucent white dome, approximately 3.5 cm (1,25 in.) in diameter, which is

oriented upward. Sensor positioning is critical for optimum performance..

I The output of the photo sensor passes through the level adjustment to the

controller. The controller activates the triac switches wired in series with

: each luminaire. An internal comparator in the controller determines if lumiI nalres should be switched to maintain a constant illumination level within a band. Since switching is used rather than continuous dimming, light output

! can be varied only in discrete steps, so illumination is not constant. The

j illumination level is maintained at or above the set point. Since all the

i fixtures can be switched off, the minimum power level is simply the controller

! power, approximately 12 watts.

Figure 22b presents light output of a typical sequential unit as a function

of power consumption for a sequential system controlling four luminaires.

The increments of illumination corresponding to the activation of each individual luminaire are dependent upon fixture placement and room configuration,

: This case represents a typical example. Each luminaire activated provides

I an additional 210 to 320 lux increase in illumination at the center of the

I room. Illumination levels for the discrete points fall very near the

' line for a perfect dimmer, as would be expected. There is no continuous

I relation between light output and power consumption, since the controller

operates in discrete steps.

To install and operate this particular system, the luminaires must be placed

I and the triac switches connected in series with the ballast. A low voltage

line connects the controller to the triacs and provides the control signal.

The photo sensor must be mounted at a suitable location, dependent upon window and lighting configurations. Several operations must be performed with

the level adjustment. The illumination set point must be adjusted to the

desired level. A second adjustment controls the magnitude of the illumination

difference (set point minus observed) that will trigger a switching function.

A third adjustment controls response time to variations in illumination.

Since each luminaire adds or subtracts a step of illumination, the controller
should be adjusted so a change in illutnination of the same magnitude as the

lighting step will trigger a switching action. In this case, since activation of an individual luminaire changes illumination level by as much as 320

lux, the controller should be adjusted to initiate switching when a change

in illumination of approximately 320 lux is observed. If the illumination

difference sensitivity is any less, the increment of illumination from the

activated fixture will cause the illumination level to overshoot the setpoint,

and illumination will be sensed by the controller as being greater than the

set point, causing the luminaire to be switched off. At that point, the

illumination level will be sensed as being too low and the fixture will be

reactivated. This cycle will continue repeatedly until illumination

conditions change substantially.

As with the dimming control system, the number of fixtures should be chosen

such that the desired illumination level in the absence of daylight is

achieved when all fixtures are activated.

6.3

COMPARISON OF ACTUAL SYSTEMS WITH PERFECT DIMMER

Figure 23 compares the performance of the dimming system and sequential system

with that of a perfect dimmer. Daylight illumination percentage on the

abscissa refers to the contribution of daylight to the illumination set point.

Whenever this percentage meets or exceeds 100 percent, daylight is supplying

all the required illumination and lighting power consumption should be zero. Both systems continue to consume lighting energy above the 100 percent point,

32 percent for the dimmer and 3 percent for the sequencer. Power consumption

for both systems below 100 percent daylight illumination also exceeds that

for a perfect dimmer.

6.4 DIMMING SYSTEM PERFORMANCE

The dimming system was installed in a room with a south window (area 16

percent, 1.2 m^, 13 ft^). Indoor illumination as a function of solar radiation for the room is presented in figure 24, for average solar conditions.

The level adjustment on each of the four luminaires was adjusted to be at

maximum light output in the absence of daylight, approximately 1300 lux at

1.5 m (5 ft.) from the window.

Lighting power consumption was monitored on an average hourly basis for a one week period. Figure 25a presents hourly lighting power consumption versus average hourly solar radiation for the same hour. Figure 24 indicates that

the 1300 lux set point will be achieved when solar radiation exceeds 500 W/m^,

for the 5 ft. location. Thus, a perfect dimmer would shut off the lights com- pletely at that point. Based on that, a perfect dimmer power consumption line was plotted. In the case of the actual dimmer, the minimum power level is

120 W, due to factors described earlier.

Actual lighting power consumption is seen to follow the perfect dimmer line

until the minimum power level is reached. In general, actual power

consumption exceeds that of a perfect dimmer for most hours, but not by much.

22
A daily breakdown of hourly average power consumption and solar radiation

is given in table 4, for the hours 0700-1700,

Table 4. Hourly Power Consumption of Dimming System

Day

Solar Radiation

W/m^ per hour*

Lighting Power

Consumption W/h

Lighting Power

Savings %

123

4

56

7

199 279 27

107 322 15

282 210 45

181 262 31

260 222 42

280 199 47

262 237 38

Average 35%

* average of hourly readings between 0700-1700 hours

The average solar radiation levels were low during this test period, the

hourly average for the week being 224 W/m^. There is a strong correlation

between lighting energy savings and level of solar radiation, as demonstrated

in figure 25b. If the level of solar radiation had been higher, more savings

would have occurred. The maximum possible savings with this system would be

68 percent, due to the minimum power level required for operation. The minimum power level would have been reached at a solar radiation level of

approximately 390 W/m^ or greater.

The amount of lighting energy savings is essentially dependent upon daylight

contribution and lighting control operation. If the operation of a lighting

control approximates the performance of a perfect dimming system, the maximum

lighting energy savings will be achieved. In this case, the actual dimming

system did perform similarly to a perfect system, until the minimum power

level was reached. If solar radiation levels (and consequently daylight

illumination levels) had been higher, the performance of the dimming system

would have deviated more from that of a perfect system, although lighting

energy savings would have increased due to the larger contribution, of

daylight to the indoor illumination level.

6.5 SEQUENTIAL SYSTEM PERFORMANCE

A similar test procedure was performed using the sequential system in a room with a north window (area 22.9 percent 1.6 m^) (17,3 ft^). Indoor illumination as a function of solar radiation was previously presented in figure 5.

Wall reflectance was 0,50. Illumination level, in the absence of daylight

with all four fixtures energized, was approximately 1000 lux at the center

of the room. The configuration of the lighting fixtures was identical to

that presented in figure 22b. The level adjustment was set to maintain this

illumination level.

23
Lighting power consumption was averaged on an hourly basis for a one-week

period. Figure 26a presents average hourly lighting power consumption versus average hourly solar radiation for the same hour. Figure 5 indicates that

the 1000 lux set point will be achieved when solar radiation exceeds 500

W/m^, for the 1.5 m (5 ft.) location.

A perfect dimmer would shut off the lights completely at that point. The

lighting power consumption for a perfect dimmer was plotted, similar to

figure 25.

Performance of the sequential system is seen to approximate that of a perfect

dimmer. The minimum power level is approximately 12 W, the power needed to

operate the controller. Lighting energy consumption is seen to be much lower

than for a perfect dimmer for some levels of solar radiation near 200 W/ra^.

Most of these points represent the late afternoon hours, when indoor illumination levels received a larger contribution of ground reflected daylight

from the concrete surface located to the west of the lab. A plateau is seen

at approximately 100 W lighting power consumption. This is due to the operation of a single lighting fixture which was sometimes needed to maintain

indoor illumination level even at high solar radiation levels. These points

occurred on a very clear day. A daily breakdown of hourly average power

consumption and solar radiation is given in table 5.

Table 5. Hourly Power Consumption of Sequential System

Day

Solar Radiation

W/m^ per hour

Lighting Power

Consumption W/hr

Lighting Power

Savings %

12

34

56

7

415

452

412

569

522

432

385

108 72

58 85

54 86

114 71

57 86

66 83

91 77

Average 80%

The average solar radiation levels were high for this test period (445 W/m^),

as was the contribution of daylight. During many hours of most days, daylight

completely satisfied indoor illumination requirements and all fixtures were switched off by the controller. Figure 26b presents daily lighting energy

savings as a function of solar radiation level. This relationship is very

similar to figure 25b except for slight difference due to window size and

orientation, and the minimum power level for the dimming system. System

performance for both the sequencer and the dimmer are nearly identical, until

the minimum power level is reached, in terms of lighting energy savings. If

solar radiation levels during the sequencer test period had been similar to

the levels during the dimmer test period, energy savings for the sequencer

probably would have been similar to the savings measured for the dimmer. On

24
the other hand. If the higher solar radiation levels had occurred during the

dimmer test period, energy savings would probably have approached 68 percent,

the maximum, possible for that system.

25
7. APPLICATION OF LIGHTING CONTROL SYSTEMS

Lighting control system requirements for a building are determined by several

factors, the main one being daylight contribution. If large daylight contributions are expected, more energy savings could be achieved through the utilization of a system which can completely shut off the lighting fixtures, as opposed to a dimming system which must remain at or above a minimum power

level. Of course, if high contributions of daylight are available, a dimming

system, or any lighting system, can simply be switched off manually. If more modest daylight contributions are expected, either type of system would

achieve reductions in lighting energy.

Another factor is uniformity of illumination. A dimming system will provide

fairly uniform distribution of illumination throughout the room, while a sequential system will provide more light directly beneath the luminaires

energized. Proper luminaire location, in respect to windows and work areas, can minimize this effect. Also, a dimming system provides fairly constant

indoor illumination over time, due to its ability to vary light output over

a continuous range. Operation of a' sequential system will cause illumination

levels to change in steps as luminaires are energized or de-energized. In some cases, that factor may be a problem or distraction, although it was not

felt to be one in this case.

With any type of lighting control system, proper sensor placement and level

adjustment is essential for maximum energy savings and proper system operation. The lighting controller should not respond to changes caused by people

walking through a room, or reflected glare or a cloud quickly passing in

front of the sun. Frequent and drastic changes in light output may be

unacceptable to building occupants.

Many existing buildings are targets for lighting control retrofit. As

compared with new buildings where illumintion levels can be specified and met

by choosing the proper number of lighting luminaires as well as their location,

existing buildings may be currently over or under-illuminated, most probably

the former. Potential lighting energy savings would be greater due to the

possible elimination of overlighting in addition to daylight utilization.

The economics of lighting control retrofit procedures are not examined here,

as each potential building would be an individual case requiring detailed

analysis of energy costs due to lighting, availability of daylight, and cost

of controls. It is noted, however, that the control systems examined herein

are not costly devices compared to the cost of the total lighting system and

the cost of lighting energy. If lighting fixtures are old and ready for

replacement anyway, lighting controls may be viable retrofit procedure.

Maintenance costs should not be significantly different for lighting systems

with and without controls, although long-term durabllty tests have not yet

been performed. The two control systems utilized for this testing have been,

and still are, operating properly after approximately one year.

26
8. LIGHTING CONTROL /WINDOW MANAGEMENT TEST CASE

The dimming system was utilized in conjunciton with a solar control film to

present an example of the lighting and cooling energy tradeoffs examined

earlier. A room with a south window exposure (area 16 percent, 1.2 m^,

13 ft^) was outfitted with 8 luminaires. Indoor Illumination, solar radiation and lighting power consumption were monitored for two test periods,

first with clear glass and then with a silver reflective film (item 4 from

table 2) installed.

The levels of each of the parameters were averaged for each hour of the day

for a five-day test period. Figure 27a presents an average profile for the

test period with a clear window. Indicating the average level of solar radiation, lighting power consumption, and indoor illumination which was observed

for each hour of the day. Figure 27b presents similar data for the test

period with the silver reflective film. With the clear window, indoor illumination greatly exceeds the 760 lux set point during the middle of the day,

although lighting power consumption levels off. This occurs because the

luminaires in front of the window are at the minimum power level and the

luminaires to the side of the window do not see the daylight as much. In any

case, lighting power consumption must be at least 220 W if all light fixtures

are at the minimum power level.

Installation of a solar control film would be expected to produce two effects,

namely reduction in daylight contribution and reduction in short-wave solar

heat gain through the window. As is seen in figure 27b, indoor illumination

remains fairly constant. This is because although daylight contribution is

reduced, most of the reduction eliminates the over-illumination observed with

the clear window. Lighting power consumption remains nearly the same. A

comparison of lighting and cooling energy for these two cases plus a non-dimmer case are presented in table 6, for the hours 0600 to 1800.

Lighting power consumption for the non-dimmer case was calculated assuming

constant power consumption for the 12-hour test days at the measured 460 W

level in the absence of daylight. A 28 percent savings in lighting energy is seen for the dimmer with clear window case, and a 22 percent savings for the

silver film case. Solar heat gain was calculated for each condition using the

indoor vertical solar-outdoor horizontal solar data presented in figure 15 on

an hour-by-hour basis. The solar radiation levels observed during the silver

film test period were used for all three cases, for comparison purposes. A

75 percent reduction in solar heat gain is seen for the silver film test

period. Cooling energy required to remove lighting and solar heat gain indicates a 43 percent savings for the silver film and 17 percent for the clear

window, as compared with the non-dim situation. Energy for lighting and cooling (due to lighting and solar heat gain) is seen reduced by 24 percent for

the clear window and 30 percentfor the silver film, as compared to no-dim,

clear window.

A comparison was made between the observed lighting energy savings and

that which might be predicted based on the analysis presented in the window

management section of the report. Figure 18 Indicates lighting power

27
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28
consumption as a function of solar radiation. Using the solar radiation

levels observed during each portion of the test on an hour- by-hour basis,

predicted lighting power savings would be 37 percent for the clear window

and 20 percent for the silver film. Actual lighting energy savings compared

to predicted was less for the clear window by 11 percent and more for the

silver film by 2 percent.

This indicates that in the clear window phase of the test, the dimming system

did not take full advantage of the available daylight, due to minimum power

requirement for proper operation of fluorescent lighting.

29
9. SU^^MARY AND CONCLUSIONS

The proper utilization of window and lighting systems requires consideration

of many factors relating to building use, orientation and configuration.

Natural illumination from daylight can make an important contribution to

indoor illumination levels. Measurements indicate that significant levels

of daylight Illuminance occur a high percentage of working hours. An indoor

illumination level of over 700 lux (70 F.C.) was observed more than 50 percent of the hours between 0600-0800 during the spring and summer test period

for a location 1.5 m (5 ft.) from the window. Similar measurements at a 3.1 m (10 ft.) location showed daylight levels of 400 lux (40 F.C.) occurring

50 percent of the working hours.

Sky condition and solar radiation level were seen to be the main determinants

of available daylight illumination, with window area and internal reflectance

controlling the actual indoor illumination at a particular location within a room. In particular, higher wall reflectances provided signficantly higher

levels of Indoor illumination for the locations at greater depth from the

window than that provided by lower wall reflectances.

Photo-sensitive lighting controls can take advantage of daylight to reduce

lighting energy while maintaining required illumination levels. Measurements

indicate lighting energy savings from 35 percent to 80 percent or more are possible in perimeter offices, depending upon window size and orientation

and required illumination level. Although actual lighting control systems

do not operate exactly as a perfect dimming system, analysis of potential

energy savings through the utilization of actual lighting controls based on

the operation of a perfect dimming system can adequately depict actual savings

in many cases.

Window management devices are seen to be of value under summer conditions,

although their use requires trade-offs to be made between daylight utilization ind solar heat gain. Solar control devices such as screens, films and

shades can reduce solar heat gain dramatically. Even though lighting energy

may increase due to a reduction in daylight contribution after installation

of a solar control device, this increase can be offset by the reduction in

cooling energy associated with the reduction in solar heat gain under summer conditions. It should be noted however, that these results pertain only to summer conditions. To examine the annual effect of window management strategies, winter conditions must also be considered. Solar heat gain in winter

can be effective in reducing building heating load as long as overheating of

perimeter areas does not occur. Adjustable window management devices would

be more beneficial in this respect since their flexibility would allow window

system characteristics to be altered selectively. Analysis of daylight con- tribution and lighting needs for a building will enable proper selection of

lighting control system and window management system to maximize energy

savings. The correlation between level of solar radiation and level of

natural indoor illumination is seen to be sufficient to permit analysis of

the performance of lighting control systems, for the purpose of predicting

energy savings through their utilization. Although significant variation

30
exists for indoor illumination from daylight as a function of solar

radiation, due to variations in sky condition, utilization of average sky

conditions (or partially overcast conditions) can be useful in predicting

average illumination levels within a room.

Significant potential exists for energy savings in existing buildings through

the use of lighting controls and window management devices. In new construction, careful analysis of illumination needs, window design and lighting con- trols can result in optimum utilization of these factors to minimize energy

consumption.

31
References

1. K-ing, W.J., High Performance Solar Control Office Windows , Lawrence

Berkeley Laboratory Report 7825, 1977. '

2. Bitterice, M.G., McKinley, R.W., Use Solar Daylight and Heat from

Windows to Save Fossil Fuel , PPG Industries, Inc.; 1978.

3. Owens, P.G.T., Energy Conservation and Office Lighting , Pilkington

Bros. Ltd., Technical Advisory Service; 1976.

4. Collins, B.L., Windows and People; A Literature Survey , National Bureau

of Standards BSS 70; 1975.

5. Ruegg, R.T., Chapman, R.E., Economic Evaluation of Windows in Buildings:

Methodology , National Bureau of Standards BSS 119; 1979.

6. Yellott, J.I., The Phoenix Fenestration Tests of 1977 - Thermal Aspects,

ASHRAE TRANSACTIONS , Vol. 85, Part 2; 1980.

7. Selkowitz, S.E., thermal Performance of Insulating Window Systems,

ASHRAE TRANSACTIONS , Vol. Part 2; 1980.

8. Kusuda, T., Collins, B.L., Simplified Analysis of Thermal and Lighting

Characteristics of Windows; Two Case Studies , National Bureau of

Standards BSS 109; 1978.

9. Owens, P.G.T., Analysis of the Energy Requirement of Buildings with

Particular Reference to the Window , Glass in Buildings Conference,

International Council for Building Research, Studies and Documentation

(CIB); 1977.

10. Crisp, V.H.C., a Preliminary Study of the Use of Automatic Daylight

Control of Artificial Lighting, Lighting Research and Technology ; Vol. 9 , 1977.

11. Smith, M.N., Automatic Light Sensing and Control of Lighting Systems

for Energy Conservation , Civil Engineering Laboratory, Tech.. Note

N-1486, 1977.

12. Kusuda, T., Bean, J.W., Comparison Between a Simplified Daylighting

Calculation Procedure and a Comprehensive Interreflection Model Calculation Procedure , Proc. 3rd Inti, Symposium on Use of Computers

for Environmental Engineering Related to Buildings, Aug. 1978,

National Research Council of Canada, Report CNRC 17376.

13. Pierpoint, P.E., Intelligent Lighting Control Principles , Civil

Engineering Laboratory, Tech. Note N-1558; 1979.

14. ASHRAE Handbook of Fundamentals , published by ASHRAE New York, 1977.

32
Photographs of the NBS daylight research facility

33

Figure 1.

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34
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TOTAL SOUR RADIATION (w/m 2

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Effect of sky condition on indoor illumination ~ north-facing

window , depth 1.5m

35
ILLUMINANCE

TOTAL SOLAR RA0UT10N (w/m^ 1

Figure 3.b Effect of sky condition on indoor illumination - northfacing window, depth 2.7m

I

36
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window, depth 1.5m

37
ILLUMINANCE

Figure 4 .b Effect of sky condition on Indoor illumination - south-facing

window , depth 3.0m

38
ILLUMINANCE

Figure 5. a Effect of window area and wall reflectance - north-facing

window , depth 1.5tii

39
000

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800

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SOLAR RADIATION (W/m^ |

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40
SOUR

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PERCENTASES

41

Figure

6.

Percentage

hours

of

solar

radiation
100 200 300 400 500 600 700 800 900 1000

' “ ILLUMINANCE |lux|

DEPTH 2.7m

Figure 7. Percentage hours of indoor illumination — north-facing

window, 1.5 m (5 ft) and 2,7 m (9 ft) wall reflectance

0.28, window area 22.9, 15.3 or 7.6 percent

42
Figure 8. Percentage hours of indoor illumination — north-facing

window, 1.5 m (5 ft) and 2.7 m (9 ft) wall reflectance

0.50, window area 22.9, 15.3 or 7.6 percent

43
WALL REFLECTANCE 23%

c] WINDOW AREA 7.6%

Figure 9. Indoor horizoni.al Illumination profiles for north-facing

window, wall reflectance 0,28 and varying window areas. Percentages Indicate ratio of illumination at each grid ocatlon to illumination immediately inside window

44
WALL REFLECTANCE 50%

Figure 10. Indoor horizontal illumination profiles for nurth~ facing

window, wall reflectance 0.50, and varying window areas. Percentages indicate ratio of illumination at the (enter

of each grid location to illumination immediately inside

window

45
INDOOR

ILLUMINANCE

S 7 8 f 10 11 12 13 14 15 16 17 18 19 20

HOUR

Figure 11. Daily solar radiation and illumination cycle - north and

south-facing window

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Figure 13.a Indoor illumination with window management - south-facing

window, 1.5 m (5 ft)

48
550

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400

350

300

250

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150

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NORIZ. SOLAR RADIATION (W/m^)

Figure 13. b Indoor illumination with window management - south-facing

window, 3.0 m (10 ft)

49
1100

1000

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NO

700

NO

NO

400

300

200

100

200 4N 800 800 1N0 1200 1400

SOUR RADIATION (W/m^

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PARTWL

OVERCAST

CLEAR

SHADE ONLY

ar w/lilm

. Indoor illumination with roll shades - south-facing window
MIZ. MIAR RADIATION |W/m’|

Figure 15. Vertical solar radiation at inside surface of south-facing

window, with and without manageinent

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systems — south-facing

53
550

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54
200 400 too too 1000 1200

NORIZ. SOLAR RADIATION (W/m^j

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55
Figure 19a Cooling load for test case DEPTH 1.5m

56
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ling and lighting energy for test case DEPTH 1.5m

58
NSnZ. S8LAR RADIATION |w/m’|

Figure 20. b Cooling and lighting energy for test case , depth 3.0m

59
DIMMING SYSTEM

Level adjustment

Photo sensor

'-Contrtller ^ Fiber optics

- 110V AC

Fluorescent lighting

fixture w/2 40W tubes

Each fixture is a complete system.

Multiple fixtures are connected

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SEQUENTIAL SYSTEM

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SCHEMATC OF UGHTMG CONTROL SYSTEMS

Figure 21. Schematic of lighting control systems

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Figure 24. Average indoor illumination for test room — south-facing

window

64
Dimming system

south window

area 16 % (1.2m2 j

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65
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BIBLIOGRAPHIC DATA

•v SHEET

1. PUBLICATION OR REPORT Ni, NBSIR 80-2147

8.Gov'Vj JPA No. 3. No,/

4. TITLE AND SUBTITLE

DAYLIGHTING, WINDOW MANAGEMENT SYSTEMS, AND LIGHTING CONTROLlS

5. Publication Date December 1980

$» Perfonniftg Organization Code

742,01

7. AUTHOR(S)

S. Treado, T. Kusuda

8. Performing Organ. Report No.

9. PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, DC 20234

10. Unit No. 742 5109

11. Contract/Grant No.

12. SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS rsfreer. City, state. ziP)

Department of Energy, Architecture & Engineering Branch

Washington, DC 20585

and the Tri-Services (US Army, Navy and Air Forces

^ Englnaaring Coimnand)

13. Type of Report & Period Covered

Final

14. Sponaoring Agency Code

15. SUPPLEMENTARY NOTES

Q Document describes a computer program; SF-185, FIPS Software Summary, is attached.

16. ABSTRACT (A 200-word or teas factual summary of moat significant information. If document includes a significant bibliography or

literature survey, merttion it here,)

This report investigates major factors concerning windows in buildings and their

effect on visual conditions, thermal conditions, and energy requirements. Empirically obtained data are presented for daylight illumination as a function of

solar radiation, sky condition, window size and orientation, and interior re- flectance. The thermal and visual effects of several window management strategies

are examined along with an analysis of automatic lighting controls. Daylight utilization is seen to offer great potential for minimizing lighting load in perimeter building areas, and careful determination of lighting needs and window

management strategies can provide additional benefit.

17. KEY WORDS (six to twelve entries; alphabetical order; capitalize only the lirst letter o{ the first key word unless a proper name;

separated by semicolons)

Daylight; energy conservation; fenestration design; illumination; lighting control;

solar heat gain; window management.

18. AVAILABILITY [§ Unlimited

I I For Official Distribution. Do Not Release to NTIS

Order From Sup. of Doc., U.S. Government Printing Office, Washington, DC

20402, $D Stock No. SN003-003-

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