Roof lighting and sun protection

Energy and Buildings, 11 (1988) 283 - 287

Roof

283

Ughtingand Sun Protection

H. W. BODMANN, K. EBERBACH and P. R E U T E R Lichttechnisches Institut der Universitb't Karlsruhe, Kaiserstrasse 12, D-7500 Karlsruhe 1 (F.R.G.)

SUMMARY

E t o n inclined surfaces as a function of the

following parameters:

On the basis o f a radiation model o f the sky and direct sunlight, glare and heat loads can be calculated for different roof light systems. To avoid overheating, a m a x i m u m daily energy input through roof lights o f 0.65 k w h per square metre o f floor area is suggested, whereas the glare limit is set by a maximum luminance o f 6000 cd/m ~. With these criteria the efficiency o f various roof light systems is evaluated in terms o f annual hours o f daylight use with a set mimim u m for the interior iUuminance. Actual energy savings with roof lighting depend on the required lighting level and on the switching scheme for the electric lighting.

1. INTRODUCTION

The use of daylight as the cheapest light source in a building requires control of glare and overheating. Considering, for example, central Europe with a m a x i m u m irradiance up to 900 W/m 2 and a daily total energy of 8 kWh/m 2 incident on horizontal surfaces, it is obvious that r o o f lights need sun protection systems or their size must be limited. The objective o f our investigation [1] was to quantify t h e heat and glare loads with various possible r o o f lights and to evaluate the efficiency of r o o f light systems with regard to sun control.

ms TL ~z %

Pb Hs

Eg't

relative air mass (for direct sunlight) turbidity factor (after Linke) angle between the zenith and the normal of the surface angle between the sun and the normal of the surface reflectance of the ground altitude o f the sun total illuminance on horizontal surfaces

Et Eg,t

0.5(1 + cos ~z) + 1.75pb(~z/rad) + 8.8 cos %

=

1 + 8.8 sin(Hs) 1

Eg, t = 7.1 x 104 exp(--0.051 ms) + - ms

X e x p ( - - 0 . 0 0 8 8 m s TL) Adopting an average luminous efficacy of 111 lm/W [3] for light from the sun and sky, a simple conversion of illuminance into irradiance levels is possible. A typical result of a calculation is shown in Fig. 1. Corresponding calculations for overcast skies were combined with cloudless skies to obtain annual daylight hours with a given iUuminance or irradiance level using the sunshine probability as weighting factor.

3. G LA RE AND HEAT LOAD LIMITS 2. MODEL OF THE CLOUDLESS SKY

In addition to the well~stablished model for overcast skies, the irradiance on inclined surfaces with cloudless skies must be known to calculate glare and heat loads. A model developed at the University of Karlsruhe [2] describes the total illuminance 0378-7788/88/$3.50

To quantify the benefits and shortcomings of various possible r o o f light systems, many criteria have to be considered, whereas in this study only the glare and heat loads are considered. Owing to the lack of general specifications for the maximum luminance of r o o f lights © Elsevier Sequoia/Printed in The Netherlands

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---- ~

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.:

latitude:

~

turbidity

factor:

-

~1 ° N 5

ground r e f l e c t a n c e : azimuth:

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0,2

EAST

tilt angle:

~

= 6o o

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WEST

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Fig. 1. Incident irradiance (W/m 2 ) of 60 ° east sheds during daily and annual time.

~(X}O

example: factory hall

incident

floor area: heat

load

daylight

3000

162o m 2

factor:

5 %

azimuth:

SOUTH

2000.

X

SOUTH-EAST

O

EAST

+

NORTH-EAST

I

hEaTH

15oo.

1ooo sky

overcast

I

0"

I

30"

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I

60•

!

!

90"

tilt

angle

Fig. 2. M a x i m u m daily heat loads (kWh) incident on various roof light systems at latitude: 4o = 51°N.

and daily energy transfer t h r o u g h the r o o f light we a d o p t e d t h e f o l l o w i n g criteria: - - t h e luminance o f r o o f lights should n o t e x c e e d 6 0 0 0 cd/m2; - - the daily energy transfer t h r o u g h r o o f lights should be limited t o a p p r o x i m a t e l y 0 . 6 5 kWh per square metre o f floor area during the h o t season. These limits are m e t w i t h a 9 0 ° n o r t h shed o f 70% diffuse transmittance and 5% daylight factor.

The actual m a x i m u m glare and heat loads for various r o o f lights were calculated for an industrial building o n t h e f o l l o w i n g basis: - - floor area 1 6 2 0 m 2 - - daylight factor 5% - - turbidity factor TL 5 - - diffuse transmission r 0.7. The l u m i n a n c e o f a diffuse transmitting r o o f light can be calculated from its °°luminance E t by multiplication with a factor rDr. The results s h o w n in Fig. 2 and Fig. 3 prove the advantage o f n o r t h ~ r i e n t a t e d r o o f lights

285

luminance

example: diffuse

transmittance

of t h e w i n d o w s :

20

0.7

adO

azimuth: £3 SOUTH K 15

O

ooo

SOUTH-EAST

O EAST

\

"~- NC~TH-EAST •

NORTH

OOO

6 ooo

~

o v e r c a s t sky

I

O"

~

e

~

I

II

I

30 °

60 °

90 °

a . g l e ~-

tilt

Fig. 3. Maximum luminance (¢d/m 2) of various roof light systems at latitude: ~0= 51°N.

for both heat load and glare limitation. In cases of other orientations, glare and heat problems are not necessarily linked together. For instance, the 90 ° south shed is the worst case regarding heat loads whereas the 30 ° south shed presents the most critical condition with regard to glare.

between 60 ° and 90 °. Whenever sun control is necessary it should be realized by temporary rather than by permanent means for efficient use of daylight. To illustrate possible energy savings with combined use of roof lights and electric lighting, an example was calculated for a large f a c t o r y hall with the following presumptions: daylight factor up to 5%; -- sunshine probability 40%; -- minimum interior lighting level 300 Ix; - - c o n t r o l of electric lighting, see switching scheme in Fig. 5; -- r o o f lighting: 60 ° north shed and horizontal roof lights; - - t i m e period taken to be between sunset and sunrise. The calculated energy savings are obtained from Figs. 6 and 7. With a 5% daylight factor and a three-step lighting control we may save as much as 80% of electric energy. However, with the horizontal roof light system, overheating m a y occur if the daylight factor exceeds 2%. But even with a daylight -

4. EFFICIENCY OF R O O F LIGHTING

To characterize the efficiency of a r o o f light system, the annual hours providing an interior lighting level above a set m i n i m u m were chosen. With an average sunshine probability o f 40% for West Germany [4] and a m i n i m u m illuminance of 500 Ix, a result is shown in Fig. 4 for an industrial building as assumed in Fig. 2 and Fig. 3. Figure 4 clearly shows the impact of a perm a n e n t reduction of daylight factor (area or transmittance o f r o o f lights) to avoid overheating. Almost half of the annual hours of daylight use are lost except with n o r t h sheds

-

286

example: hours

indoor l i g h t i n g :

per

500 I x

sunshine p r o b i l i t y :

~o %

year

3000 without any sun protection daylight

factor:

5 %

2000

full sun protection by permanent

1000

of daylight

,

I

0*

i

i

30"

i

60"

reduction factor

I

90"

tilt

angle

Fig. 4. Maximumannua] hours of daylight use by various roof]ightsystems atlatitude: ~ = 51°N. percent

100%

electric

pow*r

|

67%

1 step control

I

I

~ 33% !

step control

I |

i

1001-x

. . / 3

300Ix

2 0 0 Ix

inaoor illuminonce from daylight

Fig. 5. Switching scheme for an illuminance of 300 lx maintained by electric lighting. percent electric energy

horizontal roof light

I00%"

~,~

~

.~.

I

increaeino

I

h e a t problems

50% I I !

~O/O

I

20//0

I

30/0

l

/.0/0

i

50/0

dOyl i g h t

factor

Fig. 6. Electric energy consumption during annual daylight hours by horizontal roof lights which maintained an illuminance of 300 Ix.

factor of 2% the energy savings would still amount to 60%. In reality the savings of electric energy much depend on the required illuminance as

well as on the working period. Furthermore the investment and running costs for sun protection and lighting control systems must be considered with various roof lighting systems.

287

percent electric

100 %

60 °

~ ~

energy

~

50 % ~

~

"

shea

north

without ~lk~

.

h e a t problems

"~''.,,. I

I

I

I

m

10/0

20//0

30/0

~e/O

5%

daylight

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Fig. 7. Electric e n e r g y c o n s u m p t i o n d u r i n g a n n u a l d a y l i g h t h o u r s b y 60 ° n o r t h s h e d s w h i c h m a i n t a i n e d a n illumin a n c e o f 3 0 0 Ix.

REFERENCES

1 H. W. Bodmann, K. Eberbach and P. Reuter, Oberlicht und Sonnenschutz, Forschungsbericht FB Nr. 415, Schriftenreihe der Bundesanstalt fiir Arbeitsschutz, Dortmund, 1985. 2 H. W. Bodmann, W. Burger and Ch. J. Liebelt, Photometrisches und radiometrisches M o d e U des triiben Himmels, CIE Compte Rendu, Kyoto, 1979, Publication CIE No. 50, 1980, 5451.

3 F. Kasten, K. Dehne, H. D. Behr and U. Berholter, Die r~umliche und zeitliche Verteilung der diffusen und direkten Sonnenstrahlung in der Bundesrepublik Deutschland, F o r s c h u n g s b e r i c h t 03 E4 1 6 7 - A , B u n d e s m i n i s t e r i u m fiir F o r s c h u n g u n d Technologie, 1983. 4 H. K r o c h m a n n a n d O. S c h m i d t , Uber die Sonnen-

scheinwahrscheinlichkeit in der Bundesrepublik Deutschland, L i c h t t e c h n i k 26 Nr. 1 0 / 1 9 7 4 , pp. 428 - 429, 4 6 6 - 468.

Lichttechnik

26 Nr. 1 1 / 1 9 7 4 , pp.