Toplighting for the Tropics

Published in the Proceedings of PLEA 2005 Conference on Passive and Low Energy Architecture, Beirut Lebanon, Nov 13-16, 2005

Liliana O. Beltran, Ph.D.

College of Architecture, Texas A&M University, College Station, TX, USA [email protected]

ABSTRACT: This paper presents the findings of a study that investigates the daylight performance of traditional and innovative toplighting systems to maximize the energy savings by offsetting the electric lighting and air conditioning requirements during peak hours. This study also includes the daylight distribution of different systems’ spacing and their effects in the energy consumption of the proposed toplighting systems. The toplighting systems were designed for Lima, Peru (latitude 12°S). The studied toplighting systems include skylights with diffusing glazing; clerestories and roof monitors with external shading devices; and skylights with splay wells, reflectors and diffusers beneath aperture. The RADIANCE ray-tracing lighting software and the EnergyPlus energy analysis computer program were used to model, simulate and analyse the daylight and thermal performance of the different toplighting systems. Results showed that roof monitors with North- and South-facing glazing, achieved the most uniform and highest illuminance levels. The skylights with splay wells and reflectors beneath aperture achieved the most efficient overall performance: even illumination (~500 lux), reduced glare, and low summer solar heat gains. Results confirmed that toplighting systems with good solar control and reflective surfaces can save energy by providing adequate daylight illumination in buildings so that electric lights can be turned off during peak hours.

Conference Topic: Low Energy Architecture Keywords: daylighting, energy-savings, toplighting, solar control, Peru

1. INTRODUCTION 2. CLIMATE

Historically, toplighting systems have been widely The toplighting systems were designed used. We can find examples worldwide that range specifically for a low latitude location in the southern from small openings to large glazed areas. Roof hemisphere: Lima, Peru (12° South). The climate of monitors were the traditional toplighting systems in Lima is characterized by a hot and humid summer Lima’s architecture during the past four centuries. season and a relatively mild winter. Day and night Examples of these systems can be found in convents, temperatures have small differences. The number of libraries and old colonial courtyard houses. maximum hours of sun occurs during the summer Optimum sizing and placement of toplighting season ranging from 55% to 65% of sunlight systems have been studied and recommended for availability. The two predominant sky conditions are locations in high latitudes based on local climatic the clear and overcast skies. conditions and solar geometry [1]. However, locations near the equator with year-round bright skies around the zenith, and high potential to introduce high illuminance levels to single-story buildings and top floors, have not been investigated. The potential to use toplighting systems in Lima is fairly high, since more than 95% of buildings are single- and two- stories. Currently, toplighting systems are not being used as often as in the past. Nowadays, sidelight windows are the most common source of illumination in interior spaces. Toplighting systems provide many advantages compare to sidelight windows in low latitudes: (a) high illuminance levels, (b) uniform illumination throughout large floor areas, (c) low luminance ratios, (d) controlled solar heat gains, (e) low ratios of window area per floor area for similar illuminance levels with sidelight windows, and (f) easy integration in existing buildings. Figure 1: Lima’s weather: (top) solar, heating and cooling degree hours, (bottom) monthly diurnal average temperature.

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3. TOPLIGHTING SYSTEMS specular film (97.5%). The glazing is a clear spectrally selective glazing to let sun rays over the reflectors at Four toplighting systems were designed most times of the year. Splay wells are included near specifically for Lima’s climatic conditions: diffusing the aperture to increase the entrance of sunlight. skylight, sawtooth clerestory, roof monitor, and skylight with interior reflectors/diffusers. In all the designs, the goal was to improve lighting quality, control excessive daylight illuminance levels, and maximize the energy savings, compared to conventional diffusing skylights. The glazing area of each toplighting system was sized to minimize heat gains in the space. The systems were designed using the local construction materials. All the designs included high performance glazing, such as spectrally Figure 5: Skylight with interior reflectors and diffuser. selective clear glass (visible transmittance, Tvis=0.77). The reflectances of interior surfaces were: walls 45.1%, floor 21.7%, and ceiling 80%. 4. METHODOLOGY

3.1 Diffusing Skylight The RADIANCE ray-tracing lighting software [2] The base case skylight has a small aperture of was used to model, simulate and analyse the daylight 0.15m horizontal diffusing glazing (Tvis=0.77). performance of the different toplighting systems. The EnergyPlus [3], building energy analysis computer program, was used to evaluate the thermal performance of each toplighting system. Comprehensive sets of computer simulation were used to simulate annual daylight and energy Figure 2: Detail of diffusing skylight performance. The modelled space is 15m wide, 25m long and 3.2 Sawtooth clerestory 4.5m high. The spacing of the toplighting systems The sawtooth clerestory is designed with a north- was 1.5 times the height of the space. The spacing of facing vertical glazing of 0.95m high and a sloped 1 time the height of the space resulted in much higher ceiling surface of 90% reflectance. The glazing is illuminance levels at the cost of higher cooling loads. protected from the direct sun by external vertical and The design objectives of the systems were to horizontal shading devices. maximize daylight efficacy, increase uniformity and to minimize solar heat gains during summer months.

5. EVALUATION

5.1 Daylight Performance The most even workplane illuminance distribution is achieved with the roof monitors and skylights with reflectors. Illuminance ratios achieved with roof Figure 3: Detail of sawtooth clerestory monitors and skylights with reflectors did not exceed more than 1:2 throughout the year. Maximum 3.3 Roof Monitor recommended illuminance ratios for uniform The roof monitor is designed with north- and illumination are 1:5 [4]. Annual average illuminance south-facing glazing of 0.55m high with external under clear sky conditions at 12:00 PM was 1,650 lux vertical and horizontal shading devices for sun with roof monitors and 450 lux with skylights with control. reflectors. The roof monitor has the highest average daylight factor (2.8%) while the skylight with reflectors has the lowest (0.4%), as shown in Table 1. Figures 6 to 9 show the illumination within a single-story space during the summer solstice (Dec. 21) at 12:00 Pm. It is noticeable the uniform light distribution over the floor with the roof monitors and skylights with reflectors compared to the two other

Figure 4: Detail of roof monitor systems. Figures 10 to 11 depict illuminance levels along the centre of the space with the four systems at 3.4 Skylight with Interior Reflectors and Diffuser the solstices and equinox, at 12:00 PM. The highest The skylight with reflectors is similar in illuminance ratios are achieved with the diffusing dimensions to the base case diffusing skylight. The skylight 1:20 on Dec. 21. The sawtooth clerestory main difference is the set of interior reflectors and achieved also a fairly uniform distribution, 1:3 on Dec. diffusers that hangs underneath the small aperture. 21 and 1:2 the rest of the year. The reflectors are coated with a highly reflective 3/4

Table I: Daylight factors of the four toplighting systems.

DF Diffuse Sawtooth Roof Skylgt. Skylgt. Clerestory Monitor Reflect. Average 0.5 2.7 2.8 0.4 Min. 0.1 1.5 2.1 0.3 Max. 1.5 3.3 3.1 0.5

Figure 9: Radiance images of skylights with interior reflectors and diffusers on Dec. 21, 12:00 PM, (bottom) false color image.

The CIBSE code [5] establishes that spaces with uniform light distribution should have the ratio of minimum illuminance to average illuminance more than 0.8. The two systems that achieved higher ratios Figure 6: Radiance images of diffusing skylights on throughout the year were the roof monitor (0.64) and Dec. 21, 12:00 PM, (bottom) false color image. skylight with reflectors (0.61), while the sawtooth clerestory (0.5) and diffusing skylight (0.3) achieved lower ratios.

2500 Skylight+diffuse glass Clerestory Monitor Skylight+reflectors

2000

1500

Lux

1000

500

0 1 2 3 4 5 6 7 8 9 101112131415161718192021222324 Sensors Figure 7: Radiance images of sawtooth clerestories Figure 10: Illuminance level on Dec. 21, 12:00 PM, on Dec. 21, 12:00 PM, (bottom) false color image. clear sky conditions.

2500 Skylight+diffuse glass Clerestory Monitor Skylight+reflectors

2000

1500

Lux 1000

500

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 Sensors Figure 11: Illuminance level on Sep. 21, 12:00 PM, Figure 8: Radiance images of roof monitors on Dec. clear sky conditions. 21, 12:00 PM, (bottom) false color image. 4/4

600 Skylight+diffuse glass Monitor Clerestory Skylight+reflectors 3500.0 Skylight Clerestory 500 3000.0 Monitor Skylgt_reflectors

2500.0 400

2000.0 300 Lux Watts 1500.0

200 1000.0

100 500.0

0.0 0 1 2 3 4 5 6 7 8 9 101112131415161718192021222324 123456789101112131415161718192021222324 Hour Sensors Figure 12: Illuminance level on Jun. 21, 12:00 PM, Figure 15: Solar heat gains on Sep. 13, the coldest overcast sky conditions. day of the year.

800.0 5.2 Thermal Performance Skylight Clerestory 700.0 Indoor average temperature with all toplighting Monitor systems remains fairly similar year-round, with slightly Skylgt_reflectors 600.0 hotter temperatures (<0.5ºC) during the summer season (Figure 13). Solar heat gains introduced by 500.0 roof monitors and sawtooth clerestories were twice 400.0 the amount introduced by the two skylights during the Watts 300.0 hottest months (Figure 14). During the coldest months the solar heat gains introduced by all the 200.0 toplighting systems were about the same (Figure 15). 100.0 Heat losses during the coldest months are four times 0.0 higher using the roof monitors and clerestories than 123456789101112131415161718192021222324 the two skylights systems (Figure 16). Hour

30.0 Figure 16: Heat losses on Sep. 13, the coldest day of the year. 28.0

26.0 24.0 6. CONCLUSION 22.0 20.0 ºC Results showed that the skylights with reflectors 18.0 have the best overall daylight and thermal

16.0 performance of all the systems. They introduce

14.0 uniform light throughout the space (~500 lux) with minimum solar heat gains and heat losses. Roof 12.0 Outdoor Skylight Clerestory Monitor Skylgt_reflectors monitors provide the most uniform and highest 10.0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 illuminance levels. However, they also introduce the Hour highest solar heat gains during the cooling season of Figure 13: Interior temperatures on Jan. 6, the all the toplighting systems evaluated. Toplighting hottest day of the year. systems with adequate solar control and appropriate use of reflective surfaces can provide both good 3500.0 Skylight daylight illumination and energy savings. Clerestory 3000.0 Monitor Skylgt_reflectors 2500.0 7. REFERENCES

2000.0 [1] E.S. Lee, L.O. Beltrán, and S.E. Selkowitz, Proc. Watts 1500.0 of ACEEE 1996 Summer Study on Energy Efficiency

1000.0 in Buildings, Pacific Grove, CA (1996). [2] radsite.lbl.gov 500.0 [3] www.eere.energy.gov/buildings/energyplus/

0.0 [4] IESNA, IESNA Recommended Practice of 1 2 3 4 5 6 7 8 9 101112131415161718192021222324 Daylighting, Illuminating Engineering Society of North Hour America, New York, 1999. Figure 14: Solar heat gains on Jan. 6, the hottest [5] CIBSE, CIBSE Code for interior lighting, Chartered day of the year. Institution of Building Service Engineers, London, 1994.