- Email: [email protected]
Luminous efficacy of daylighting in intertropical region: An analysis for toplighting systems
WREC 1996
LUMINOUS EFFICACY OF DAYLIGHTING IN INTERTROPICAL REGION: AN ANALYSIS FOR TOPLIGHTING SYSTEMS Ricardo C. Cabfis and Feruando O. IL Pereira CCT-CTEC-Universidade Federal de Alagoas-UFAL - 57.083-000 - Macei6-AL, Brazil [email protected] LABCON-ARQ-CTC-Universidade Federal de Santa Catarina-UFSC - 88.040-900-Flofian6polis-SC, Brazil arql [email protected]
ABSTRACT Toplighting in warm and humid climates have always been a problem for designers due to associated solar heat gain. This paper aims to analyse toplighting fenestration systems and the relationship between their heat gain and luminous performance of buildings in intertropieal regions. Using simplified methods to evaluate solar heat and daylight admission the paper analyses three types oftoplighting systems at the equator. It also examines the effect of shading devices and reflectance of external surfaces on the system luminous efficacy.
KEYWORDS Daylighting; toplighting; luminous efficacy; energy conservation; tropical region.
INTRODUCTION Since the mid 70's, after the oil crisis, and more recently with the environmental movement, energy issues have been the focus of many important world research centres. The need for saving energy has joined the need for developing alternative sources. Natural light exploitation has become a target in order to reduce the global electric energy use. During that period, in the USA, windows were responsible for a large fraction of the country's energy use or the equivalent of the energy produced by the Alaska pipeline (Arasteh, 1994). In Brazil, on the other hand, in 1989, artificial lighting represented 23% of electric energy use in residential building, and 44% in commercial buildings (Geller, 1994). As an alternative, daylight has been widely recommended and used in order to replace artificial electric lighting. Among several attributes, natural light has a better quality, mainly due to its colour rendering, it is free of charge, and helps saving non-renewable resources. So far, daylighting use means increase in work productivity and a general improvement on people's well-being. However, a whole set of problems caused by daylighting has to be solved: iUuminance internal distribution, associated heat gain, and variability of intensity across the day. Toplighting systems can be used to solve some of these problems, even though heat gain tends to grow.
210
WREC 1996 In view of this, the present paper aims to analyse the feasibility of toplighting systems in intertropical regions, where solar radiation is plentiful. Employing the Solar System Luminous Efficacy - SSLE - concept, proposed by Batunan et al. (1987), comparative studies between solar heat gain and natural light admitted by toplighting apertures are developed.
METHODOLOGY
The work is based on a comparative study of three toplighting fenestration systems. Sunlight and sky light, as well as shading devices are considered in order to produce different model systems. The following hypothesis are taken into account: (i) exterior and interior surfaces are ideally matte surfaces; (ii) the toplighting apertures are closed by an ideally diffuse translucent material; (iii) longwave radiation heat and reflected diffuse solar radiation (which increase heat gain) and heat loss by convection (which decreases heat gain) are considered; (iv) differences in solar and visible reflectance of opaque surfaces are assumed to be negligible. In order to consider the worst situation of solar radiation exposure, the study assumes to be at the equator (0° latitude), at noon of equinox(March/Sept. 21st) with clear sky conditions. Since the main objective of this work is to develop a comparative study of fenestration systems in the same conditions, simplified analysis methods are used to estimate mean internal illuminances and solar heat gain (ASHRAE, 1993; Incropera and Witt, 1992; Moore, 1991; Robbins, 1986). The room model is described in fig. 1 while toplighting fenestration systems are shown in section in fig. 2, 3 and4.
"~,oo,~ 5,00m i
iwl
Fig.
1 -
Perspective view of room model
i
Fig. 2 -Model 1: Horizontal roof aperture
l,OOm q
5.00m
I
5,00m
Fig. 4 - Model 3: Clerestory roof system.
Fig. 3 - Model 2: Monitor roof light. 211
WREC 1996 ANALYSIS The data set used in the calculations is presented below.
Climate data (ASHRAE, 1993; Rivero, 1986) Diffuse solar radiation Gsaif= 120 W/m2; direct solar radiation GSd~= 880 W/m2; sky light illuminance Ehdif= 20klux; sunlight illuminance Ehdir= 80klux (all data is for an external horizontal surface).
Room model data Area = 35m2 ; height = 3 m , workplane height = 0.75 n~
Material properties (ASHRAE, 1993; Burkhardt, 1975, 1976) Acrylic visible transmittance tv=0.45; acrylic solar transmittance ts=0.55; acrylic solar absorptanee as=0.20.
Performance Comparison Several calculation runs were developed to obtain average workplane illumimnce (Eu), solar heat gain (qs) and SSLE for each of the three toplighting systems. Aperture width (a) and external surfaces reflectance (p) were varied, assuming, respectively, the following values 0.09, 0.60, 0.83 and 0.70, 0.85 and 0.99. Table 1 and 2 summarise the main results.
Table 1. Average workplane illuminance, solar heat gain and SSLE for the three models, varying the aperture width (w) and keeping external surface reflectance p = 0.85.
w(m) 0.09 0.60 0.83
Modl 502 3348 4631
Eu (lux) Mod2 79 506 677
Mod3 67 390 503
Modl 410 2730 3777
q (W) Mod2 45 287 382
Mod3 40 230 295
Modl 43 43 43
SSLE (lm/W) Mod2 Mod3 61 58 62 59 62 60
Table 2. Average workplane iUuminance, solar heat gain and SSLE for the three models, varying the external surface reflectance (p) and keeping aperture width w = 0.60m.
0.70 0.85 0.99
Modl 3348 3348 3348
Eu (lux) Mod2 406 506 617
Mod3 297 390 491
Modl 2730 2730 2730
q (W) Mod2 225 287 355
212
Mod3 172 230 294
Modl 43 43 43
SSLE (lm/W) M o d 2 Mod3 63 60 62 59 61 58
WREC 1996 CONCLUSIONS
This paper has presented a simplified study developed for assessing and comparing the thermal and luminous performance of toplighting fenestration systems in intertropical region. From the results obtained it is poss~le to conclude that: (i) the optimisation of the aperture design is fundamental for getting more natural light with little solar heat gain into a toplit building; (ii) a well designed shading device improves the SSLE; once the cut-off angle has been optimised, the type and surface reflectance of toplighting system are not relevant for SSLE, although these aspects still have a great influence on the absolute values of workplam illuminaace; (iii) internal surfaces reflectances and interior design are significant to average workplane illmninance and, consequently, to SSLE; and (iv) because of the collimated attn'butes of direct solar radiation, fenestration systems which are capable of capturing and redistributing small portions of direct sunlight, can produce well lit spaces with little heat gain. REFERENCES Arasteh, D. K. (1994). Advances in window technology: 1973-1993. In: Advances in solar energy, an annual review of research and development. (Karl W. BOer, Ed.), Vol. 9, Boulder. ASHRAE (1993). Fenestration. In: Handbook Fundamentals. Chap. 27, ASHRAE, Atlanta. Bauman, F.S., J. W. Place, B. Anderson, J. Thornton and T.C. Howard (1987). The experimentally measured performance of a linear roof aperture daylighting system. In: ASHRAE Transactions, Vol. 93, Part 1,259-278. ASHRAE, Atlanta. Burkhardt, W.C. (1975). Solar optical properties of gray and brown solar control series transparent acrylic sheet. In: ASHRAE Transactions, Vol. 81, Part 1,384-397. ASHRAE, Atlanta. Burkhardt, W.C. (1976). Acryfic plastic glazing: properties, characteristics and engineering data. In: ASHRAE Transactions, VoL 82, Part 1,683. ASHRAE, Atlanta. Geller, H. (1994). O uso eficiente da eletricidade: uma estratdgia de desenvolvimento para o BrasiL INEE, Rio de Janeiro. Incropera, F. and D. Witt (1992). Fundamentos de transfer~ncia de calor e massa. Guanahara Koogan S.A, Rio de Janeiro. Moore, F.( 1991 ). Concepts and practice of architectural daylighting. Van Nostrand Reinhold, New York. Rivero, R. (1986). Arquitetura e clima: acondicionamento t~-mico natural. D.C. Luzzatto, Porto Alegre. Robbins, C. L. (1986). Daylighting: design and analysis. Van Nostrand Reinhold, New York.
213
LUMINOUS EFFICACY OF DAYLIGHTING IN INTERTROPICAL REGION: AN ANALYSIS FOR TOPLIGHTING SYSTEMS Ricardo C. Cabfis and Feruando O. IL Pereira CCT-CTEC-Universidade Federal de Alagoas-UFAL - 57.083-000 - Macei6-AL, Brazil [email protected] LABCON-ARQ-CTC-Universidade Federal de Santa Catarina-UFSC - 88.040-900-Flofian6polis-SC, Brazil arql [email protected]
ABSTRACT Toplighting in warm and humid climates have always been a problem for designers due to associated solar heat gain. This paper aims to analyse toplighting fenestration systems and the relationship between their heat gain and luminous performance of buildings in intertropieal regions. Using simplified methods to evaluate solar heat and daylight admission the paper analyses three types oftoplighting systems at the equator. It also examines the effect of shading devices and reflectance of external surfaces on the system luminous efficacy.
KEYWORDS Daylighting; toplighting; luminous efficacy; energy conservation; tropical region.
INTRODUCTION Since the mid 70's, after the oil crisis, and more recently with the environmental movement, energy issues have been the focus of many important world research centres. The need for saving energy has joined the need for developing alternative sources. Natural light exploitation has become a target in order to reduce the global electric energy use. During that period, in the USA, windows were responsible for a large fraction of the country's energy use or the equivalent of the energy produced by the Alaska pipeline (Arasteh, 1994). In Brazil, on the other hand, in 1989, artificial lighting represented 23% of electric energy use in residential building, and 44% in commercial buildings (Geller, 1994). As an alternative, daylight has been widely recommended and used in order to replace artificial electric lighting. Among several attributes, natural light has a better quality, mainly due to its colour rendering, it is free of charge, and helps saving non-renewable resources. So far, daylighting use means increase in work productivity and a general improvement on people's well-being. However, a whole set of problems caused by daylighting has to be solved: iUuminance internal distribution, associated heat gain, and variability of intensity across the day. Toplighting systems can be used to solve some of these problems, even though heat gain tends to grow.
210
WREC 1996 In view of this, the present paper aims to analyse the feasibility of toplighting systems in intertropical regions, where solar radiation is plentiful. Employing the Solar System Luminous Efficacy - SSLE - concept, proposed by Batunan et al. (1987), comparative studies between solar heat gain and natural light admitted by toplighting apertures are developed.
METHODOLOGY
The work is based on a comparative study of three toplighting fenestration systems. Sunlight and sky light, as well as shading devices are considered in order to produce different model systems. The following hypothesis are taken into account: (i) exterior and interior surfaces are ideally matte surfaces; (ii) the toplighting apertures are closed by an ideally diffuse translucent material; (iii) longwave radiation heat and reflected diffuse solar radiation (which increase heat gain) and heat loss by convection (which decreases heat gain) are considered; (iv) differences in solar and visible reflectance of opaque surfaces are assumed to be negligible. In order to consider the worst situation of solar radiation exposure, the study assumes to be at the equator (0° latitude), at noon of equinox(March/Sept. 21st) with clear sky conditions. Since the main objective of this work is to develop a comparative study of fenestration systems in the same conditions, simplified analysis methods are used to estimate mean internal illuminances and solar heat gain (ASHRAE, 1993; Incropera and Witt, 1992; Moore, 1991; Robbins, 1986). The room model is described in fig. 1 while toplighting fenestration systems are shown in section in fig. 2, 3 and4.
"~,oo,~ 5,00m i
iwl
Fig.
1 -
Perspective view of room model
i
Fig. 2 -Model 1: Horizontal roof aperture
l,OOm q
5.00m
I
5,00m
Fig. 4 - Model 3: Clerestory roof system.
Fig. 3 - Model 2: Monitor roof light. 211
WREC 1996 ANALYSIS The data set used in the calculations is presented below.
Climate data (ASHRAE, 1993; Rivero, 1986) Diffuse solar radiation Gsaif= 120 W/m2; direct solar radiation GSd~= 880 W/m2; sky light illuminance Ehdif= 20klux; sunlight illuminance Ehdir= 80klux (all data is for an external horizontal surface).
Room model data Area = 35m2 ; height = 3 m , workplane height = 0.75 n~
Material properties (ASHRAE, 1993; Burkhardt, 1975, 1976) Acrylic visible transmittance tv=0.45; acrylic solar transmittance ts=0.55; acrylic solar absorptanee as=0.20.
Performance Comparison Several calculation runs were developed to obtain average workplane illumimnce (Eu), solar heat gain (qs) and SSLE for each of the three toplighting systems. Aperture width (a) and external surfaces reflectance (p) were varied, assuming, respectively, the following values 0.09, 0.60, 0.83 and 0.70, 0.85 and 0.99. Table 1 and 2 summarise the main results.
Table 1. Average workplane illuminance, solar heat gain and SSLE for the three models, varying the aperture width (w) and keeping external surface reflectance p = 0.85.
w(m) 0.09 0.60 0.83
Modl 502 3348 4631
Eu (lux) Mod2 79 506 677
Mod3 67 390 503
Modl 410 2730 3777
q (W) Mod2 45 287 382
Mod3 40 230 295
Modl 43 43 43
SSLE (lm/W) Mod2 Mod3 61 58 62 59 62 60
Table 2. Average workplane iUuminance, solar heat gain and SSLE for the three models, varying the external surface reflectance (p) and keeping aperture width w = 0.60m.
0.70 0.85 0.99
Modl 3348 3348 3348
Eu (lux) Mod2 406 506 617
Mod3 297 390 491
Modl 2730 2730 2730
q (W) Mod2 225 287 355
212
Mod3 172 230 294
Modl 43 43 43
SSLE (lm/W) M o d 2 Mod3 63 60 62 59 61 58
WREC 1996 CONCLUSIONS
This paper has presented a simplified study developed for assessing and comparing the thermal and luminous performance of toplighting fenestration systems in intertropical region. From the results obtained it is poss~le to conclude that: (i) the optimisation of the aperture design is fundamental for getting more natural light with little solar heat gain into a toplit building; (ii) a well designed shading device improves the SSLE; once the cut-off angle has been optimised, the type and surface reflectance of toplighting system are not relevant for SSLE, although these aspects still have a great influence on the absolute values of workplam illuminaace; (iii) internal surfaces reflectances and interior design are significant to average workplane illmninance and, consequently, to SSLE; and (iv) because of the collimated attn'butes of direct solar radiation, fenestration systems which are capable of capturing and redistributing small portions of direct sunlight, can produce well lit spaces with little heat gain. REFERENCES Arasteh, D. K. (1994). Advances in window technology: 1973-1993. In: Advances in solar energy, an annual review of research and development. (Karl W. BOer, Ed.), Vol. 9, Boulder. ASHRAE (1993). Fenestration. In: Handbook Fundamentals. Chap. 27, ASHRAE, Atlanta. Bauman, F.S., J. W. Place, B. Anderson, J. Thornton and T.C. Howard (1987). The experimentally measured performance of a linear roof aperture daylighting system. In: ASHRAE Transactions, Vol. 93, Part 1,259-278. ASHRAE, Atlanta. Burkhardt, W.C. (1975). Solar optical properties of gray and brown solar control series transparent acrylic sheet. In: ASHRAE Transactions, Vol. 81, Part 1,384-397. ASHRAE, Atlanta. Burkhardt, W.C. (1976). Acryfic plastic glazing: properties, characteristics and engineering data. In: ASHRAE Transactions, VoL 82, Part 1,683. ASHRAE, Atlanta. Geller, H. (1994). O uso eficiente da eletricidade: uma estratdgia de desenvolvimento para o BrasiL INEE, Rio de Janeiro. Incropera, F. and D. Witt (1992). Fundamentos de transfer~ncia de calor e massa. Guanahara Koogan S.A, Rio de Janeiro. Moore, F.( 1991 ). Concepts and practice of architectural daylighting. Van Nostrand Reinhold, New York. Rivero, R. (1986). Arquitetura e clima: acondicionamento t~-mico natural. D.C. Luzzatto, Porto Alegre. Robbins, C. L. (1986). Daylighting: design and analysis. Van Nostrand Reinhold, New York.
213