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Comparative study of window glass influence on daylighting in an open-plan office.

1 INTRODUCTION

Special glazings which reduce solar radiation transmittance may be used in glazed facades for elimination of energy demands for cooling and ventilation in buildings. On the other hand it is necessary to consider carefully application of these glazings. Solar radiation is the main life-determining factor. Building environment influences our health. Long term habitation in a building with improper daylighting could cause eye defects and disorders [Boubelri 2008]. It is know that the syndrome of seasonal affective disorder [Rea 2000] affects people living in cities during winter seasons because of deficiency of natural light. This syndrome decreases on springs and fades during summer seasons. The syndrome affects circadian biological rhythms [Ehrenstein 1995]. It was proven that the natural light activates the central nervous system [Illnerova 1986]. Research studies [Libermann 1991], [Ott 1974] aimed at the evaluation of influence of the artificial lighting on health brought indisputable accomplishments. No artificial lighting can permanently substitute daylight and solar radiation.

[FIGURE 1 OMITTED]

Indoor visual comfort should be considered in early stages of architectural design or in a project of building renovations. Building renovations include for example:

--replacement of common windows by new ones with thermally insulated bigger frames which reduce glazed area in a building facade,

--installation of movable blinds by permanently fixed shading elements,

--substitution of window single glass panes by double or triple glass units,

--exchange of common window glass by another one as low-emissivity or solar-control glazing.

All these renovation procedures could bring energy saving effects but in some cases also unwanted deterioration of daylight level in buildings.

2 VISUAL COMFORT DESIGN REQUIREMENTS

Daylight influences vision and also biological functions. Modern society is separated out of the natural conditions because many people spend vital parts of their life in buildings. Contemporary buildings have artificial constant indoor climate conditions without dynamic changes in the response to the outdoor climate. In many cases indoor environment does not serve habitants comfort. Improper daylighting is only one of the negative aspects of the unsuitable indoor climate. Special window and facade glazings can serve solar control for buildings. They have special reflective coatings or they can be tinted. Many of these glazings deform natural spectrum of solar radiation because they transmit only a limited spectral part and the rest of light is reflected or absorbed within the glass pane. Daylight filtrated through some solar control glazings looses its energy essential for health.

[FIGURE 2 OMITTED]

[FIGURE 3 OMITTED]

The light quality depends on the colors of natural light spectrum. White light is a composition of red, orange, yellow, green, blue and green colors.

Traditional artificial sources emit light spectrum shifted to yellow, red and infrared wavelengths; blue and near ultraviolet spectrum is mostly blocked, Fig. 1 [Ott 1974]. Predominance of red light activates stress. The long-lasting light deficiency causes disharmony.

[FIGURE 4 OMITTED]

Natural spectrum of light of convenient intensity is required for indoor illumination. Illuminance 300 lx is required in many cases for illumination of office desks. But this light intensity is only sufficient for thirty-year old or younger people. Older people need illumination of higher intensity.

It is commonly known that vision of older people is impaired and the fact could also be taken into consideration in the building design process. Figure 2 compares illuminance levels which are required for the some visual activities in dependence on habitants' age [Habel 1998].

In summary the indoor visual comfort depends on:

--appropriate illuminance level in accordance with required visual activities,

--natural solar spectrum,

--daylight uniformity in rooms,

--convenient direction of the affected light,

--proper luminance distribution in the field of sight,

--glare protection.

3 DAYLIGHT SIMULATIONS

The design of proper daylighting in buildings is coupled mainly with a window design and its glass transmittance [McMullan 2007; Szokolay 2008; Baker & Steemers 2002; Evans 1981] in the response with external daylight conditions. A comparative daylight simulation study focused on an evaluation of influence of different window glazing on daylighting in an office was carried out. The study was completed for the project of renovation of an administrative building [Creo 2008].

The influence of the window glazing on interior daylighting was compared in the open-plan office. The office floor dimensions are 33.30 m to 16.15 m and the room clearance height is 2.7 m. A continual window zone in aluminum frames is placed in the peripheral facade system of the open-plan office, Fig. 3.

The continual window in located the front eastern facade and partly in southern and northern facades. The window height is 2.0 m and it is positioned 0.9 m over the office floor level.

[FIGURE 5 OMITTED]

Calculation of daylight factor DF [%] was carried out for the determination of an effect of three different types of window glasses on the indoor daylighting level in the investigated office. There was a double glazed unit used for the facade glazing. The glazed unit (4 mm glass pane--12 mm air cavity--4mm glass pane) was considered in the following three variations for the daylight study:

--variation 1: two clear glass panes: light transmittance [tau] = 0.84,

--variation 2: clear glass pane and pane of clear solar control glass: light transmittance [tau] = 0.58,

--variation 3: clear glass pane and pane of tinted solar control glass: light transmittance [tau] =0.36.

[FIGURE 6 OMITTED]

The graph of spectral light transmittance of three glass samples of the clear float glass and solar control (SC) clear and tinted glass is presented in Fig. 4. Values of light transmittance [tau] determined for the above mentioned variations of the selected double glazed units were calculated in accordance with the DIN standard formula [DIN 1998]

[tau] = [[tau].sub.1]([lambda])[[tau].sub.2]([lambda]/1 - [[rho]'.sub.1]([lambda])[[rho].sub.2]([lambda])[-]

where

[[tau].sub.1]([lambda]) spectral transmittance of the outer glass pane

[t.sub.2]([lambda]) spectral transmittance of the second glass pane

[[rho]'.sub.1]([lambda]) spectral reflectance of the outer glass pane, measured against the direction of propagated light

[[rho].sub.2]([lambda]) spectral reflectance of the inner glass, measured in the direction of propagated light

[FIGURE 7 OMITTED]

Daylight evaluations for the all above mentioned window glass variations were completed by the computer program WDLS [ Stanek 2002]. The computer daylight simulations are based on the following design assumptions:

--overcast sky with sky, sky luminance gradation from horizon to zenith 1:3, time 1st March,

--position of the building is adequate to latitude 49[degrees]12', longitude 16[degrees]34', altitude 223.5 m,

--dark terrain (average light reflectance of the surrounding ground is [rho] = 0.1),

--interior surfaces which influence internally reflected component of the daylight factor calculations are: walls with white plaster [rho] = 0.5, white ceiling soffit [rho] = 0.7, light gray floor carpet [rho] = 0.3, wooden furniture (natural lime) [rho] = 0.4.

Results of the computer simulations give graphical distribution of daylight factor on the working plane 0.85 m over the floor level in the open-plan office, Fig. 5. Daylight factors were calculated for a mesh of nodal points on the working plane. Horizontal distances among the nodal points are 1.0 m. Graphical results from the daylight computer simulations are compared for the three glazing variations in Fig. 6, 7 and 8. There are two graphical solutions compared in every figure--the first graph represents the daylight factor distribution without influence of furniture and the second one shows daylight factor distribution with consideration of the furniture.

[FIGURE 8 OMITTED]

4 DISCUSSION

Daylight computer simulations were completed for three types of window glass units variations with light transmittance [tau] = 0.84, [tau] = 0.58 and [tau] = 0.36 and for alternatives with and without furniture. All calculation results are summarized in Table 1. There are values of calculated daylight factors compared in maximal ([DF.sub.max]), minimal ([DF.sub.min]) and average ([DF.sub.av]) values and finally as a ratio of minimal and maximal values r = [DF.sub.min]/[DF.sub.max] for all variations. Higher maximal values of [DF.sub.max] in the calculations with furniture are caused because of higher internally reflected component from inter-reflections on the furniture surfaces.

The influence of a window glass type on the interior illuminance is obvious from the simulation results. Values of average daylight factors are reduced about one third (33 percent) in case of double glazed units with clear solar control glass and even more than half (57.5 percent) for variation with tinted solar control glass compared to the calculation results for clear double glass unit.

Minimal daylight factor 1.57 percent as the calculation result for the evaluation with clear glass and without furniture is acceptable but value of [DF.sub.min] = 0.67 percent calculated for the variation of tinted double glass unit is very low to be accepted as convenient daylight conditions in the investigated open-plan office.

The floor area of the open-plan office is large and for this reason it is necessary to consider functional areas close to work stations. These areas would satisfy for the average daylight factor between 2.0 and 3 percent which is adequate to average internal illuminance between 100 and 150 lx, determined for average 5000 lx of the external illuminance for overcast sky conditions.

5 ENERGY EVALUATION

The daylight study completed in the above mentioned variations represents three daylight x artificial light regimes as follows:

Variation 1--overheating and glare problem. The facade must have the shading system activated between 10:00 and 14:00 during summer seasons, which requires fully artificial lighting service (10 ceiling lamps, power 100 W),

Variation 2--without shadings and substitutive artificial lighting during the day services,

Variation 3--the last work positions (11 working places) must be artificially illuminated by desk lamps (60 W) about 4 hours during winter seasons and transition periods (from October to March).

The electric energy consumption for artificial lighting is compared in Table 2.

6 CONCLUSION

Window glazing could influence indoor daylighting. It is possible to summarize design recommendations on basis of the above mentioned results:

--window double glass units with clear glass panes on the eastern and northern facades,

--solar control clear glass on the southern or western glazed facades,

--tinted glass should be used very carefully in the window design, maybe for only certain parts of the large glazed facades exposed to intensive solar radiation,

--design of higher windows and higher window sills allows daylighting of the distant places in large rooms like the open-plan offices,

--working desks at the last row position from the window is recommended to have additional artificial lighting with full white light spectrum.

Window design has to consider climatic conditions of the building site, orientation of windows and facade glazing towards the cardinal points, area of glazed parts and a floor area of rooms, visual activity of habitants, requirements for the indoor climate comfort (daylighting and natural ventilation, thermal and acoustics comfort, etc), protection of rooms against the glare effect (from the exterior and interior side), maintenance and durability of glazing systems, requirements for the low investment price and effective construction. These requirements determine the window design in building envelopes. In general a design principle can be recommended: in regions with moderate climatic conditions the design of ordinary window glasses with automatic shading devices is more convenient than special tinted glasses that permanently limit transmittance of daylight into buildings.

doi: 10.1582/LEUKOS.2010.07.01003

ACKNOWLEDGMENT

The presented daylight simulations were carried out under the support of project GAC R 101/09/H050 "Research of Energy-Efficient Systems and Installations for Indoor Climate Comfort". The daylight evaluation was completed in collaboration with the Creo, Ltd. company.

REFERENCES

Baker N, Steemers K. 2002. Daylight design of buildings. James&James Science Publishers Ltd. London.

Boubelri M. 2008. Daylighting, Architecture and Health. Building Design Strategies. Architectural Press, Elsevier, Oxford.

Creo, Ltd. 2008. Project of renovation of open plan offices of the administrative building. Brno 2008.

[DIN] 1998. EN 410:1998. Glass in Building - Determination of luminous and solar characteristics of glazing.

Ehrenstein V. 1995. Circadian lighting systems. International Lighting Review. 1995(2).

Evans BH. 1981. Daylight in Architecture. McGraw-Hill. New York.

Habel, J. 1998. Osvetlovani, CVUT. Prague.

Illnerova H. 1986. Circadian rhythms in the mammalian pineal gland. Academia, Prague.

Libermann J. 1991. Light-Medicine of Future. Bear&Company, Inc. Santa Fe.

McMullan R. 2007. Environmental Science in Buildings. 6th edition. Hampshire.

Ott, J.N. 1974. Health and Light. The Devin-Adair Company, Old Greenwich.

Rea MS, editor. 2000. Lighting Handbook-Reference and Application, 9th Edition, New York, IESNA, 2000.

Stanek P. 2002. Computer program: WDLS. Astra Zlin.

Szokolay S. 2008. Introduction to Architectural Science. The basis of Sustainable Design. Architectural Press. Elsevier. Oxford.

Faculty of Civil Engineering, Brno University of Technology, Czech Republic

Correspondence: mohelnikova.j@fce.vutbr.cz
TABLE 1.
Summarised Results of
Daylight Factor (DF)
Calculations

Calculation Variation                 Daylight Factor DF [%]

                           [DF.sub.min]   [DF.sub.max]   [DF.sub.av]
                               [%]            [%]            [%]

Variation 1 glass unit         1.57          14.34          5.09
  [tau] = 0.84, without
  furniture
Variation 1 glass unit         0.0            26.0          4.62
  [tau] = 0.84, with
  furniture
Variation 2 glass unit         1.05           9.53          3.38
  [tau] = 0.58, without
  furniture
Variation 2 glass unit         0.0           17.28          3.07
  [tau] = 0.58, with
  furniture
Variation 3 glass unit         0.67           6.10          2.16
  [tau] = 0.36, without
  furniture
Variation 3 glass unit         0.0           11.60          1.96
  [tau] = 0.36, with
  furniture

Calculation Variation            Daylight Uniformity

                           r = [D.sub.min]/[D.sub.max] [-]

Variation 1 glass unit                   0.11
  [tau] = 0.84, without
  furniture
Variation 1 glass unit                   0.0
  [tau] = 0.84, with
  furniture
Variation 2 glass unit                   0.11
  [tau] = 0.58, without
  furniture
Variation 2 glass unit                   0.0
  [tau] = 0.58, with
  furniture
Variation 3 glass unit                   0.11
  [tau] = 0.36, without
  furniture
Variation 3 glass unit                   0.0
  [tau] = 0.36, with
  furniture

TABLE 2.
Electric Energy Consumption
for Additional Artificial
Lighting

                   Energy Consumption for
Design Variation   Additional Artificial
                     lighting Per Year              Note

Variation 1               312 kWh           Less energy efficient
Variation 2                0 kWh             Convenient solution
Variation 3              412.8 kWh           Energy inefficient
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Author:Mohelnikova, Jitka
Publication:Leukos
Article Type:Report
Geographic Code:4EXCZ
Date:Jul 1, 2010
Words:2351
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