Beam daylighting: an alternative illumination technique

Energy and Building, 1 (1977) 43 - 50 © Elsevier Sequoia S.A., Lausanne -- Printed in the Netherlands

43

Beam Daylighting: an Alternative Illumination Technique ARTHUR H. ROSENFELD Department of Physics and LBL, University of California, Berkeley, Calif. 94720 (U.S.A.) STEPHEN E. SELKOWITZ* California Institute of the Arts, Valencia, Calif. 91355 (U.S.A.)

This article is concerned with the energy savings and peak p o w e r reductions associated with the m a x i m u m utilization o f natural light. The general characteristics o f diffuse daylighting are discussed in terms o f a standard office plan. A n innovative technique o f daylighting using direct beam radiation from the sun is treated in some detail. Beam daylighting, at 100 lm/W, is found to be more efficient than fluorescent fixtures and is capable o f being directed up to 30 ft inward from the exterior facade by reflective louvers or blinds. The estimated cost o f the system is calculated to be equivalent to the savings in electrical energy costs within a three-year period. It is also noted that relatively small savings in electrical consumption for air-conditioning are a by-product o f the utilization o f such a system. INTRODUCTION The power of sunlight as a source of heat and light is rarely appreciated. Consider the thermal balance across a one-foot wide section of a typical office curtain wall consisting of four square feet of glass and four square feet of insulated wall. On a clear winter day, sunlight provides in excess of 800 Btu/h to the space. A temperature differential of almost 140 °F is required (equivalent to an outside air temperature of --70 °F!) before the conduction and ventilation losses exceed the solar gains. From a lighting perspective, sunlight acts as an intense collimated source with a color temperature pleasing to the h u m a n eye. If distributed uniformly, the lumens contained in a single square foot of sunlight could *Now at Lawrence Berkeley Laboratory, Berkeley, Calif. 94720 (U.S.A.}.

provide fifty footcandles (FC) of illumination over an area of 180 ft 2. It is precisely because of the powerful nature of these heat and light sources that the sun is generally treated as a force to be excluded from a modern office environment. The designer has traditionally lacked both the economic incentive and conservation ethic necessary to a t t e m p t to control these sources effectively. Thus neglected, their power, variability and unpredictability overwhelm and dominate their potential usefulness. This article outlines a particular strategy to realize substantial energy savings by harnessing sunlight for illumination based on the use of an innovative, yet simple, control mechanism. Any a t t e m p t to promote the widespread use of daylighting techniques to conserve energy must address the set of problems which has prevented its adoption and use. A glance at a modern building with its identical facades and permanent reflecting glass shows that it was not designed with the intent of using daylighting techniques for illumination. The remainder of this paper examines the limitations of existing daylighting techniques using diffuse solar radiation, and establishes the advantages, from a combined thermal/lighting energy perspective of using natural lighting. We then propose a novel daylighting system utilizing reflected solar beam radiation which appears to offer performance improvements over diffuse daylighting techniques.

LUMENS/WATT Light from the sun is pleasant to us owing to its color temperature, and it is a comparatively efficient source of illumination,

44 TABLE 1 Daylighting parameters: lumens/watt, solar heat grain, illuminance Direct beam Skylight (incident) sunlight on vertical window) 1. Lumens/watt* (a) Measured, clear day 106 +- 2** (b) Nominal values 100 t 2. Solar heat gain t t (Btu/ft 2 h) 3. Illuminance t (footcandles)

116 +- 7*** 120 t

300

35

9000

1200

*Ross has found some mistakes in the Ross and Baruzzini report on Daylighting in Commercial Buildings [1]; this explains why lm/W tabulated here from those of Ross and Baruzzini, page III-25, eqns. 3 and 4, but agree with an Erratum to be issued by Ross. **From I.E.S. Trans., (May) (1925), 493. ***From M. J. Blackwell, Meteorol. Res. Publ. No. 895, London, 1954. t A 5800 °K black body provides 92 lm/W. Approximately 50% of the energy lies in the visible spectrum, 50% in the infrared. The atmosphere preferentially filters out the infrared, thus raising the lm/W (see Threlkeld, Thermal Environmental Engineering, 2nd edn., 1970, Fig. 13.12, p. 295). Diffuse sky radiation is the result of Rayleigh scattering, favoring visible over infrared diffuse sky radiation thus has higher lm/W than direct beam radiation and may be characterized as a 10 000 °K source. t t F r o m ASHRAE Handbook of Fundamentals, 1974, Table 22.4, p. 390. Divide these values by 3.415 to obtain W/ft 2. r Can be obtained by multiplying the solar heat gain (in W/ft 2) by the lm/W. p r o d u c i n g 100 - 120 l m / W , as c o m p a r e d w i t h nearly 70 l m / W f o r i n c a n d e s c e n t b u l b s (see T a b l e 1). T h e r e is a s u b s t a n t i a l v a r i a t i o n in b o t h t h e i n t e n s i t y and s p e c t r a l c o m p o s i t i o n o f d a y l i g h t o w i n g to c h a n g i n g a t m o s p h e r i c and climatic c o n d i t i o n s , a n d t h e diurnal and seasonal a p p a r e n t m o v e m e n t o f t h e sun across t h e sky. Average availability can be r e c o r d e d or c a l c u l a t e d b u t t h e d a y l i g h t c o n d i t i o n s at a n y given t i m e are u n p r e d i c t a b l e .

b e e n a d o p t e d as t h e s t a n d a r d design condition. R e l a t i v e l y large w i n d o w s are t h e n n e c e s s a r y to p r o v i d e a d e q u a t e illumination. These guidelines change, h o w e v e r , in climates c h a r a c t e r i z e d b y clear s u n n y w e a t h e r w i t h b l u e skies o f low brightness. Here, diffuse d a y l i g h t i n g is based largely on sunlight reflected f r o m a d j a c e n t s t r u c t u r e s , and smaller w i n d o w s p r o v i d e sufficient i l l u m i n a t i o n . In n e i t h e r case is b e a m r a d i a t i o n f r o m t h e sun used d i r e c t l y to p r o v i d e illumination. E x p e r i e n c e tells us t h a t diffuse d a y l i g h t i n g can a l m o s t a l w a y s p r o v i d e a d e q u a t e illuminat i o n i m m e d i a t e l y a d j a c e n t t o a w i n d o w wall (see Fig. 1). It b e c o m e s c o n s i d e r a b l y m o r e difficult to p r o v i d e g o o d i l l u m i n a t i o n as o n e m o v e s t o w a r d s t h e i n t e r i o r o f an office space. T o t a l available d a y l i g h t declines e x p o n e n tially as o n e m o v e s a w a y f r o m t h e w i n d o w . ( A c t u a l values d e p e n d on w i n d o w size a n d l o c a t i o n , r o o m d i m e n s i o n s , etc.) T h e daylight r e a c h i n g an i n t e r i o r p o s i t i o n is the s u m o f (a) direct light f r o m t h e sky, (b) light r e f l e c t e d f r o m e x t e r n a l objects, and ( c ) i n t e r r e f l e c t e d light w h i c h has b e e n s c a t t e r e d f r o m at least o n e internal surface. Far f r o m the window, the internally reflected c o m p o n e n t replaces s k y l i g h t as t h e d o m i n a n t s o u r c e o f i l l u m i n a t i o n a n d t h e r e f l e c t a n c e s o f walls, ceilings and f l o o r b e c o m e i m p o r t a n t design considerations. I

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DIFFUSE DAYLIGHTING T h e d a y l i g h t i n g o f buildings has b e e n e x t e n s i v e l y studied and is r o u t i n e l y practised. In p r e d o m i n a n t l y o v e r c a s t climates, diffuse radiation (sunlight s c a t t e r e d a n d r e f l e c t e d b y t h e a t m o s p h e r e and clouds) has

Fig. 1. Daylight penetration in side-lit rooms with window height of 1.8 m. Daylight factor (DF) is converted to illumination using weather-averaged value of 800 fL on north elevation during winter. Figure from N. O. Milbank, Energy Consumption in "Other" Buildings, British Building Research Establishment, BRE PD 136/1975.

45

D i f f u s e daylighting - - n o r t h w i n d o w s

M o d e m office building design has produced a variety of glass boxes with identical building elevations. Not only is the design o f north and other elevations frequently identical, b u t the same solar control glazing is used, presumably for uniformity. Little summer sun is incident on a north wall, so the main effect of the reflective glazing is to reduce the daylighting potential. We suggest that there would be a market t o d a y for "facsimile" solar control glass for north windows. Its color would match that of the solar control glass on the other elevations, b u t it could have a much higher light transmission. In current energy conserving recommendations, the pendulum n o w swings the other way: codes to reduce heating and cooling loads now, under some conditions, suggest the elimination of all glazing on north-facing exposures, thus again removing the possibility of diffuse daylighting. D i f f u s e d a y l i g h t i n g -- o t h e r elevations

For orientations other than north, diffuse daylighting is possible during certain hours of the day, b u t sun control must be provided to exclude the direct rays of the sun. Reflective and/or tinted glazing is n o w used extensively in new construction to turn back the sun's heat. This approach, however, reduces the available diffuse radiation that might be used for daylighting. For an unshaded building, diffuse radiation is the dominant available m o d e on a north elevation, b u t represents only 50% of available lighting hours on an east or west elevation and contributes little on a south elevation (see Fig. 2). 5

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With the introduction o f overhangs and fins t o exclude direct radiation, an increased proportion of natural light is available in diffuse mode. Any a t t e m p t to control direct radiation b y the use of glazing with a fixed, low shading, coefficient will reduce the opportunities to utilize diffuse daylighting. Other factors, such as the trend to lower ceiling heights, have reduced the daylighting potential in buildings. Perhaps the greatest impediment to the widespread utilization of daylighting techniques, has been the existence of a cheap and versatile substitute -- an electric light. When used in a sensitive and intelligent manner, electric lighting produces comfortable and effective levels of task illumination. However, the power and versatility of electric lighting and an era of cheap and plentiful electricity have combined to p r o m o t e the careless use of electric lighting and disinterest in daylighting possibilities. The energy crunch and rapidly rising energy costs are rekindling an interest in the potential of daylighting to reduce electrical energy consumption. Traditional methods of diffuse daylighting can and should be utilized wherever practical. They are subject, however, to the limitations described on the preceding pages. We now examine a novel daylighting design utilizing reflected beam radiation from the sun, which will extend daylight utilization to other building types and climates not suited to diffuse daylighting techniques.

BEAM DAYLIGHTING

Direct solar radiation from the sun on a clear day provides approximately 9000 FC (see Table 1). We propose to reflect beam radiation from the sun onto the white ceilings typical o f most commercial buildings so as to penetrate up to 30 ft into the building, as shown in Fig. 3. A relatively small glazed area (approximately 2 ft high) near the ceiling of a standard window admits enough light throughout much of the year to provide deskt o p illumination levels in excess of 50 FC after reflection from the ceiling. Figure 4 shows the average lm/ft cast on a ceiling 30 ft deep from a south window at latitude 40 °N on an average clear day. With the exception of summer, illumination levels on the ceiling are consistently above 100 FC. Not all o f this

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light reaches the work plane. Consider the illumination at noon at the equinox. In the center of a large r o o m with a typical ceiling reflectance of 70% the work plane illumination will be approximately 210 FC when 300 lm/ft 2 strike the ceiling. Near the sides of the room the level would fall off to perhaps 175 FC owing to the proximity of walls of 50% reflectance. Figure 5 shows an LBL office, pleasantly daylighted by a single window of 4 ft width.

(c) Lighting level under identical c o n d i t i o n s with normal venetian blinds. Fig. 5. (a) The single w i n d o w with a blind whose t o p 15 slats are reflective, illuminates an inner office, (b). In (c) the reflecting films were r e m o v e d ; nothing else, including the exposure, was changed.

47 In a smaller office, these levels would be raised by a factor of two or three, providing illumination equivalent to that under an overcast sky, which would not be unpleasantly high. The height of the beam daylighting blinds could be reduced to one foot to drop the illumination level back to the level provided in the larger office.

Beam daylighting apparatus The apparatus used to provide beam daylighting is illustrated in Fig. 6 as part of an overall optimized window design. Two different sorts of Venetian blinds, both m o u n t e d behind a clear window, provide visual and thermal control: a silvered " b e a m " blind m o u n t e d behind the upper window and a solar control (partially reflective) blind located behind the lower (Vision) window. {This latter blind is mentioned in an article by Silverstein contained in this report.)

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Beam daylighting controls We divide the discussion of controls into two cases: (a) a small, one-person perimeter office, and (b) a large, open-landscaped office. 9

Fig. 6. Schematic section through southfacing wall. For the lower (view) part of the window, the solar control shade (e.g. Scotchtint) and the optional, light-colored, opaque Venetian blind, could be replaced by the dual mode shades discussed by Siiverstein or the dual mode venetian blind discussed by Rosenfeld. The beam daylighting blinds function independently of the solar control blinds. When there is no direct sun on a particular building elevation, the blinds would provide some diffuse daylighting. With direct sunlight, the slat tilt and spacing should be designed so as to prevent direct sunlight from traveling between slats and striking work areas. In addition, to provide o p t i m u m illumination at a constant depth in the room, the slat angle should be adjusted continuously to compensate for changing sun angles. In practice, only seasonal adjustment should suffice. Typical spacing and tilt conditions are shown in Fig. 7.

(a) Perimeter office, one occupant Here we see two options, the first of which is sketched in Fig. 6. In order to obtain the variation in both slat spacing and tilt angle, the conventional cloth straps have been replaced by elastic straps. The conventional tilt mechanism is unchanged. We then visualize that the occupant will occasionally adjust the blind tilt so that bands of fight penetrate to the back of the office. The slat spacing can then be adjusted by means of a cord which runs over a pulley, shown recessed into the ceiling in Fig. 6. We can envisage no circumstances under which the occupant could not maintain conditions of visual comfort. An alternative which we have not fully studied is to install " f i x e d " blinds (actually adjusted seasonally) which provide adequate, though not o p t i m u m , illumination. Preliminary studies indicate that illumination in excess of 45 FC can be provided for about 1000 clear-day hours per year from most window exposures. If some direct sunlight passes between the slats and enters the workspace in the one-man office with continously adjustable blinds, the occupant will readjust the blinds. In the large office with

48

fixed blinds, precautions must be taken that a direct beam never gets below eye level. We are studying several solutions: A second, fixed, Venetian blind, serving as a collimator which accepts only the shaded rays of light shown in Fig. 7. An eggcrate grill m o u n t e d almost horizontally -- actually sloping and grazing the b o t t o m of the daylight beam - - w h i c h stops a downward sloping direct beam but permits light scattered from the ceiling to travel downwards onto the task. The only serious control problem we foresee in a small office is turning off the lights once sunlight becomes available. The cheapest solution seems to be that the light switch should have a built-in timer which could b e controlled either centrally or b y the occupant who could dial light for up to four hours. This would turn lights o f f at lunchtime and after a forgetful occupant has g o n e home. In addition, on days with highly variable daylight, some conservationists might wish to dial artificial light for brief periods, knowing that daylighting conditions would probably return. Is it irritating to have the lights turn o f f automatically at 5:30 PM when one is on the phone working late? We suspect not, because of the trend towards task lighting or multi-level lighting within a single office. However, a thoughtful timer could always signal shortly before the lights were to be turned off. Independently of daylighting, timed light switches would amortize their additional cost quickly. A typical 150 ft 2 office, even with future lighting levels of 2 W/ft 2, consumes 0.3 kW, or about 5 kWh, for each night they are inadvertently left on (or 20 kWh per weekend). At 3C/kWh, forgetfulness costs 15~ per night or 60¢ per weekend. Our experience indicates that one light in five is often left on at the end of a work day. At that rate, savings amount to $12.00 per office per year, suggesting a payback period of about one year.*

(b ) Large open landscaped office In a large office, we anticipate using the beam daylighting apparatus to provide 50 FC illumination up to 30 ft from the windows. * $ 1 2 / o f f i c e - y e a r r e p r e s e n t s an a n n u a l savings o f 4 0 0 kWh. At a h e a t rate o f 13 0 0 0 B t u / k W h e , o r 0.1 gallon o f oil/kWhe, t h e savings are 40 gallons o f oil p e r office per year.

Figure 4 shows that on a clear day sufficient lumens are available from a south-facing clerestory window of height two feet to provide more than 100 FC on the ceiling from 9 AM to 3 PM throughout most of the year, averaging about 6 h per day. Figure 8 shows the beam day lighting availability in hours per clear day for window orientations other than south. The relatively small differences in the annual averages over a wide range of orientations hide the large hourly and seasonal variations which occur.

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Translating available solar energy to useful illumination is a more demanding task in the large office than in the small office previously considered. Differing illumination requirements and the larger area to be lit make it more difficult to provide continuous illumination at the levels desired at any given location in the office. The larger space to be lit and greater potential savings will justify an increased investment in either a more sophisticated blind control mechanism and/or a "smarter" electric light controller (continuous dimming or multi-level control). Our studies indicate that a reliable and cheap photoelectric controller could be produced if ultimate demand justified the initial research and preproduction costs. A detailed study has not yet been made of the tradeoffs inVolved in increasing the complexity and cost of the beam control mechanism, thu~ permitting a " f i x e d " beam blind and more sophisticated

49

electric lighting controls. In either case, a version of the timed light switch discussed in case (a) will still return excellent economic and energy savings. THERMAL CONSIDERATIONS

North windows lose heat in winter and admit heat in summer. For a 5000 degree-day climate, small north windows daylight enough area to easily pay for their heat loss, but above 2 0 % north glass area the increased cost of heating oil cancels electric savings. Natural gas is still cheaper, but in designing a new building one cannot plan on gas remaining cheaper than oil. For non-north windows we shall now show that thermal considerations are not very important.

of its exponential decay inwards it. overlights and overheats near the window and probably averages 50 - 70 lm/W (i.e., averaged over a whole office). A 1 ft height of clerestory window is adequate for beam daylighting a one-person 150 ft 2 office with a 12 ft wide south exposure. Thermal input for this window is shown in Fig. 9. On a clear January day at noon, the office is brighter than necessary, but the heat and light are pleasant. During the cooling season, typified by the August curve, we note that the window is about as " h o t " as the lights, even assuming future conservative lighting loads of only 2 W/ft 2. I

Winter Windows which see direct sunlight in winter roughly break even (solar heat gain tends to cancel conduction loss), so any electricity saved by daylighting is simply a net gain.

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We conclude that extra windows for diffuse daylight are about as hot as the luminaires t h e y displace. Small clerestory windows for beam daylight are slightly cooler than artificial lights, but in general the side effects of reduced air conditioning are negligible. There are significant thermal considerations for the "view" part of the window, but these are discussed elsewhere in this report by Silverstein and by Rosenfeld.

50 COST/BENEFIT

Small office In the Introduction, the potential national energy and peak power savings were discussed. We now consider the potential cost/savings balance for the single-occupant office evaluated in a previous section. A 150 ft 2 office with a 12 wide south exposure would contain 12 ft 2 of beam daylighting blinds, assuming a 1 ft high clerestory window. This would provide a light intensity on the ceiling in excess of 100 lm/ft 2 for more than 8 h during an average clear day. Assume then that 80% of the occupied hours could utilize beam daylighting. Typical figures for sunshine availability are 65%. Thus, during approximately 50% (80% X 65%) of the 2000 annual working hours, beam daylighting is both available and feasible. The power and energy savings are summarized below. Power: 2 W/ft 2 X 150 ft 2 = 300 W. Peak power charges are estimated (in a longer version of this paper, to appear in the FEA lighting symposium) to be about $4/year (see f o o t n o t e on p. 44). Energy: 50% X 2000 h X 300 W = 300 kWh/ year. At 3 C/kWh, energy saved is $9/year. Total savings (peak + energy charges): $13/ year. Costs: we assume timed light switches pay for themselves in a year or so by night-andweekend savings, so the only cost is for the reflective blinds. Venetian blinds in the Sears and Wards catalogs cost $1/ft 2 and upwards. We assume the reflective ones will sell for $2/ft 2. Cost: 12 ft 2 X $2/ft 2 = $24. Maintenance: little is foreseen. Payback period: $24 + $13/year = 2 year payout.

Large office Similar calculations were performed for one example o f the large open landscaped office {south exposure, 120 ft × 30 ft deep). Instead of Venetian blinds adjusted by the occupant and yielding 1000 h of daylight annually, we assume seasonal adjustment and only 800 h annually. For peak power we assume that only 1 W/ft 2 can be saved. Peak power savings, annually $75 Energy savings, annually: 800 h X 2 W/ft 2 X 3600 ft 2 X 3 t/kWh $173

Total annual savings -$250 Costs: blinds, $2/ft 2 X 240 ft 2 $480 Photocell control for lights $320 Total costs $800 Payback period: 3.2 years. We conclude that the small office is the best target for beam daylighting, but that the large office is worthy of further R and D.

CONCLUSIONS

Increased use of daylighting is proposed in virtually every shopping list of energy conservation strategies published in the last few years. Traditional methods of diffuse daylighting do work in perimeter offices, although they have n o t been extensively used for a number of reasons that are reviewed in this paper. We have proposed a new daylighting technique utilizing beam radiation from the sun which will substantially extend the applicability of daylighting. Reflecting Venetian blinds m o u n t e d behind a small clerestory window will reflect sufficient lumens o f f the ceiling to provide adequate illumination throughout much of the year. Further work is being performed to optimize the design of the blind mechanisms and their integration with the electric lighting controls. Substantial energy and peak power savings were shown to be possible in typical small and large offices. Energy savings should repay capital investm e n t in the mechanisms and controls in two to three years. On a national scale, we estimate that 1/8 quad (1/8 X 1015 Btu) of resource energy might be saved each year and peak power demand reduced by 6 gigawatts if beam daylighting were installed in 20% of existing commercial floor space. A longer version of this article, with details on peak power costs and a Table on optimal window size for diffuse daylighting appears in the Proceedings of the 1975 FEA Lighting Symposium published by the Federal Energy Agency, Washington, D.C. (April 1976).

REFERENCES 1 Ross

and Baruzzini, Inc., Energy conservation applied to office lighting, FEA Rep. (Contract No. 14-01-0001-1845), April 1975.