- Email: [email protected]
Study on remote source solar lighting system application in high-rise residential buildings in Hong Kong
Energy and Buildings 60 (2013) 225–231
Contents lists available at SciVerse ScienceDirect
Energy and Buildings journal homepage: www.elsevier.com/locate/enbuild
Study on remote source solar lighting system application in high-rise residential buildings in Hong Kong Irene Wong ∗ , H.X. Yang Renewable Energy Research Group (RERG), Department of Building Services Engineering, The Hong Kong Polytechnic University, Hong Kong
a r t i c l e
i n f o
Article history: Received 9 May 2012 Received in revised form 8 October 2012 Accepted 5 January 2013 Keywords: Remote source solar lighting system Fiber optic Solar altitude Solar azimuth angle Solar irradiance Shadowing effect
a b s t r a c t This research investigates the design constraints and potential of applying the remote source solar lighting system to the enclosed lift lobbies of high-rise residential buildings in Hong Kong. No natural light is usually provided to these lift lobbies, which depend on electric lighting for illumination. The application of conventional light pipes requires minimum 3 m headroom and is not feasible to be applied in the high-rise residential buildings of 2.8 m floor height in Hong Kong. A remote source solar lighting system is specifically designed to solve the headroom problem. The daylighting system uses small diameter side-emitting fiber optic as light transmission medium. The factors of solar altitude, solar azimuth angle and solar irradiance are investigated. The shadowing effects caused by neighboring buildings and the supporting framework are analyzed in details. Design parameters of the natural daylight system are defined. Design guidelines and a model design are developed as a reference for building designers in designing a remote source solar lighting system. The design addresses both the functional application as well as the esthetic design. The remote source daylighting system can be integrated into the architectural design of the external facade and lift lobbies. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Greenhouse gases cause global warming [1] and yield a steep, relentless increase in global temperature throughout the twentyfirst century [2] with warming of several degrees Celsius by 2100 [3–5]. Carbon dioxide is the main source of greenhouse gases generated from fossil fuel combustion to produce electricity. Greater use of renewable energy sources that produce little or no CO2 is required to reduce the carbon dioxide emission [3]. Hong Kong is a metropolitan city characterized by high-rise buildings. The population density was 6500 persons per square kilometers in 2010 according to the statistic provided by the Census and Statistic Department [6]. Residential premises are developed into high-rise buildings to compensate the high land cost. In order to build more floors within the limited building height, the floor height seldom exceeds 2.8 m [7]. The lift lobbies are enclosed in the center to free up the peripheral spaces of valuable view for domestic units, which are enclosed without natural lighting. The layout of a typical floor with enclosed lift lobby is shown in Fig. 1. Energy use in buildings accounts for nearly half of the total primary energy use in Hong Kong [8]. Reduction in the use of electric lighting in the enclosed lift lobbies by introducing natural lighting can help to
∗ Corresponding author. Tel.: +852 2766 4559; fax: +852 2336 9994. E-mail address: [email protected] (I. Wong). 0378-7788/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.enbuild.2013.01.010
conserve our environment. This project is to investigate how to use solar lighting in lift lobbies for energy saving in Hong Kong.
2. The remote source solar lighting system As the floor height is limited to 2.8 m, the size of the light transmission medium must be small in order not to reduce the floor height. A remote source solar lighting system (RSSL) using sideemitting fiber optic as light transmission medium is designed. The RSSL composes of a 760 × 760 mm plane mirror, 2 numbers of 225 mm diameter converging lenses and a 10.4 mm diameter large core LEF710 fiber optic (FO). The plane mirror acts as a heliostat and reflects sunlight into the converging lenses, which is installed on the external wall. The mirror can rotate in both horizontal and vertical planes responding to the constant changes of the sun’s position in solar azimuth angle () and solar altitude (ˇ) as shown in Fig. 2 at different times so as to reflect the sun beam always parallel to the axis of the lenses. The lenses converge and concentrate the reflected light beam into the small sectional area of the FO, which emits light uniformly along the whole length at 2–6% per meter. Transmitted light is emitted in a divergent angle of 60 in downward direction. The RSSL system operates in direct sunlight under clear sky condition. An RGB LED lamp of 40 W is installed at the end of the FO to act as supplementary lighting device, which will be instantly turned on when the light detector at the far end of the FO detects the light intensity below the recommended 150 lx.
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Fig. 3. Relation between indoor and outdoor light intensities with time.
Fig. 1. A typical residential floor layout.
Fig. 4. Solar altitude vs. time.
Fig. 2. The RSSL.
Light emitted from the LED lamp will be transmitted in the reverse direction inside the FO to illuminate the lift lobby. The lens system can be installed in any service room which part of the headroom can be reduced below 2.8 m. 3. Factors affecting the RSSL’s performance An experiment was carried from February to August 2011 in the period from 10:00 to 16:30. The changes in indoor light intensity responding to the changes in outdoor light intensity, solar altitude (ˇ) and solar azimuth angle () at different times and dates were recorded. The outdoor light intensity was fluctuating due to the changes in sky condition even within a few minutes especially in partly cloudy sky condition. The collected data was used to derive a general trend for analysis. The collected data was plotted in graphs to analyze the results. The findings indicated that the indoor light intensity increases with the increase in outdoor light intensity. A minimum outdoor light intensity of 42,500 lx is required to maintain the indoor light intensity at 150 lx, which occurs mainly in the afternoon period (Fig. 3). As the RSSL can only operates under clear sky condition, the direct solar luminous efficacy is used to estimate the required solar irradiance for the operation of the RSSL. The direct solar luminous efficacy (Kbc ) can be calculated from Chung’s equation as below [9]: Kbc = 48.5 + 1.67ˇ − 0.0098ˇ2
(1)
Kbc increases with the increase in solar altitude. From Fig. 4, the lowest solar altitude of 36◦ in a day is used to calculate Kbc , which is 96 lm/W. The outdoor light intensity of 42,500 lx is converted to 443 W/m2 by the direct luminous efficacy (Kbc ). A minimum of 443 W/m2 in a clear day is required to maintain the indoor light intensity at 150 lx. The latest available comprehensive records of solar irradiance in different hours and months issued by the Hong Kong Observatory from 2004 to 2007 are used to estimate the functioning hours of the RSSL in a year. From these records, the total hours of average solar irradiance at or above 443 W/m2 for different months in a year in the period between 9:00 and 17:00 h are summarized in Table 1. The annual operating hour of the RSSL is 1235 h. The RSSL can contribute about 3.4 h of daylight in a day on an average basis. 4. Shadowing effects The optimal orientation of the mirror in the RSSL is facing south so that it can receive sunlight for most of the day. However the mirror in the south-orientated RSSL needs to be adjusted in both the horizontal and vertical planes responding to the changes in solar azimuth angle and solar altitude, respectively. The installation method of the mirrors to the external wall is structurally complicated to cater for the mirror rotations in 2 axes. Although a RSSL that is installed on the west facade can only operate in the post meridiem period, the mirror is required to rotate in the vertical plane only and the construction of the supporting frame is much simpler and less expensive. From Table 1, the west-orientated RSSL still has 1068 annual operating hour, which is 3 h per day. Furthermore, the operating period of the RSSL mainly falls within the post meridiem. Conclusively, a west-orientated RSSL is preferred.
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Table 1 Total No. of hours of average solar irradiance at 443 W/m2 or above. Month
Average No. of hours in a day
Total hour in a month
No. of hours from 12:00–17:00
Total No. of hours from 12:00–17:00 in a month
January February March April May June July August September October November December Total
1.75 2.25 0.75 2.75 3.5 3.5 5.75 4 4.75 5 3 3.5 1234.75
54.25 63 23.25 82.5 108.5 105 178.25 124 142.5 155 90 108.5 1068.25
1.75 2.25 0.75 2.75 2.75 3 4.75 3.5 3.75 4 2.75 3
54.25 63 23.25 82.5 85.25 90 147.25 105.5 112.5 124 82.5 93
Fig. 5. Relations of indoor light intensity and mirror inclination with time.
4.1. Shadowing effect from buildings Hong Kong is a densely populated city with buildings developed into high-rise of over 20 storeys. Such developments may result in a large degree of shading effect from nearby buildings and the effect can be significant [10]. Fig. 5 shows the relationship of indoor light intensity and the recorded mirror inclination () responding to the changes in the solar altitude with time. The RSSL can generally provide 150 lx from around 10:00 to 16:30. The solar altitude is twice the value of the mirror inclination, which changes from 60◦ at 10:00 h to 36◦ at 16:30, respectively. The angles of obstruction are 36◦ and 60◦ as shown in Fig. 6. Even there is no external obstruction, the RSSL that is installed on the western facade can only receive direct sunlight from half of the sky hemisphere. Therefore, 12:00 is used as the demarcation time for designing the RSSL. The period from 12:00 to 16:30 h occupies 64% of the total time in a day with the indoor light intensity generally above 150 lx. No obstruction shall fall within the shadowfree zone as shown in Fig. 6.
that it will not cast any image on the mirror below within the operating period of the RSSL (Fig. 7). Let ‘A’ be the inclination of the upper mirror that allow the incident ray just to pass without casting a shadow on the lower mirror in a west-orientated RSSL. ‘A’ can be calculated from the following equations: tan 2 = 2 tan
A 1
− tan A2
4.2. Shadowing effect within the RSSL As the mode of light transmission is horizontal, a RSSL is installed on every floor. The mirrors in the upper floors may cast a shadow to the mirrors below. Part of the sunlight will be blocked. Theoretically the RSSL can operate from 12:00 to 16:30 with no obstruction. The inclination of the mirror is adjusted responding to the change in solar altitude. The upper mirror should be designed
Fig. 6. Shadowing effect from buildings.
(2)
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a ; a` and b` are the 2 legs of the right-angled triangle b
tan A =
formed by the mirror
tan 2A =
2a + H ; 2b
H = 2800 mm
(3)
(4)
Hence 2ab2 = 2800b2 − 2a3 − 2800a2 2
2
4a + 4b = 760
2
(5) (6)
Therefore: b2 =
7602 − 4a2 4
(7)
Substitute Eq. (8) into (6): 5600a2 + 7602 ×
a − 7602 × 700 = 0 2
(8)
a = 269.4 sin A =
2a 760
‘A’ is calculated to be 45.16◦ ; which means the solar altitude is about 90◦ and occurs around 12:00. In this situation, the upper mirror will cast a shadow on part of the lower mirror and the efficiency of the RSSL will be reduced. ‘A’ must be smaller than 45◦ to avoid the shadowing effect. H should be adjusted to exceed 2.8 m. To solve this problem, 2 adjoining locations are reserved for the RSSL installation at each floor. The RSSL will be installed at each location on alternate floors. H is now changed to 5.6 m, and A becomes 44.96◦ , which is below 45◦ . From 12:18 onwards, decreases from 45◦ to 18◦ at 16:30 h. Doubling H to 5.6 m enables the RSSL to operate for a longer period. The minimum grid dimensions of the metal supporting frame for the mirrors are 0.8 m wide and 5.6 m high. 5. Detail design of a RSSL Fig. 7. Shadowing effect within the RSSL.
The design concept of “Forms follow function” [11] is adopted. The design of the RSSL should be primarily based on and derived from its functional use and can integrate into the architectural design of the facade and the lift lobby.
Fig. 8. Design concept of a RSSL.
I. Wong, H.X. Yang / Energy and Buildings 60 (2013) 225–231
229
Fig. 10. Modified configuration of the metal frame.
A fiberglass casing is designed to house the fiber optic, which also serves as a fixing device. A slit is formed along the metal casing to discharge the light being emitted from the notched fiber optic. The casings run along the lobby and corridors and act as an illuminator and decorative element (Fig. 12). A space of 1.2 × 1.2 m2 with an external wall preferably facing west is recommended to be reserved to house the lenses; or the lenses can also share the space of any service room that is accessible to the exterior. An aperture of 225 mm is formed on the external wall for light transmission. Fig. 9. Metal supporting framework.
5.3. Maintenance 5.1. Integration into the external facade The mirrors are supported by an external metal frame, which allows the mirror to rotate in the vertical plane. Combining the conceptual design of the mirror and supporting system (Fig. 8) evolves the detail design of the metal supporting framework as shown in Fig. 9. The framework can be designed as an architectural feature anodized in either striking colors to be an eye-catching element or in similar color tone with the building as a harmonized feature. The shape of the metal frame can be modified as shown in Fig. 10 to allow the mirrors always facing the east even though the external wall is not orientated to the west. The principle of shadow-free zone should be applied in the design.
The lift expectancy of high quality waterproof mirror is over 5 years; and over 10 years for lens and FO. Replacement of these components is not frequent. Routine maintenance such as cleaning the surfaces of the mirrors and lenses once a month is recommended as accumulation of dirt can reduce the efficiencies of these components.
5.2. Integration into the lift lobby design In the popular design of the conventional lighting system of an enclosed lift lobby, a series of florescent lamps joining together are installed along the 2 sides of the ceiling. False ceiling in usually installed to conceal the lighting fixtures (Fig. 11). In the RSSL, the notched fiber optic acts as an illuminator and replaces the florescent lamps. As fiber optic comes in a continuous bundle, the alignment problem of the florescent lamps is solved. False ceiling is not required to conceal the notched fiber optic. Anidolic reflecting ceiling panels are not required because notched fiber optic emits light downwards in a 60◦ cone. Light intensity is measured at eyelevel of 1.5 m high. The measured light intensity at 0.2 m from the margins of the emitted light zone drops to 50% of the light intensity at the center. One notched fiber optic can illuminate an area of 1 x L m where L is the length of the fiber optic. In a corridor of 2 m wide, 2 numbers of notched fiber optic are required.
Fig. 11. Conventional lighting system in lift lobby.
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6. Design guidelines
Fig. 12. Notched fiber optic as illuminator inside lift lobby.
Maintenance platforms and cat ladders are installed to the metal framework of the RSSL as shown in Fig. 9. The platforms are fixed to the inner side of the metal frame right below the mirrors to avoid blocking the reflected sun rays. Structural inspection is recommended to be carried annually.
In designing a RSSL, the abundance of solar energy is a crucial factor. The average hours of solar irradiance that is 443 W/m2 or above is recommended to be 3 h in a day. No neighboring buildings shall cast a shadow within the shadow-free zone. In the next step, a space of 1.2 × 1.2 m2 with the external wall orientating to the west is reserved for the installation of the lenses. The installation space should be as close to the lift lobby as possible. The metal frame can be installed to the north-west or south-west facade if the west facade is blocked by buildings or other obstructions. Modification to the design of the metal frame is required to allow the mirrors facing the east. The metal framework is divided into grid size of 0.8 m wide and 5.6 m high for the buildings of 2.8 m storey height. The floor layout should be studied in detail to determine the numbers of RSSL to be installed at each floor. The length of the fiber optic should not be greater than 20 m for efficient transmission [12]. A design flow chart is shown in Fig. 13.
Fig. 13. Design flowchart of a RSSL.
I. Wong, H.X. Yang / Energy and Buildings 60 (2013) 225–231
7. Conclusion The availability of daylight in Hong Kong is high [12,13]. An RSSL composing of a plane mirror as a simple heliostat, 2 converging lenses and side-emitting FO is developed to transfer daylight into the enclosed lift lobbies of high-rise residential buildings. The performance of the RSSL at different times and dates of a year has been studied and the required sky conditions for operation have been discussed. The solar irradiance is the dominant factor that affects the efficiency of the RSSL. A minimum of 443 W/m2 is required to maintain the indoor light intensity at 150 lx. As Hong Kong is crowded with high-rise buildings, the shadowing effect is critical to the operation of the RSSL. The RSSL must be designed to avoid the shadowing effects from nearby buildings and the supporting frame of the RSSL itself. A model RSSL is designed in detail to integrate into the architectural design of the building. The design of the RSSL should also facilitate routine maintenance. A design flowchart is formulated as a design reference for architects and building services engineers. 8. Recommendation for future works The development of the side-emitting technology in fiber optic is still in the preliminary stage and the light emission efficiency is about 2–6%. This research basically explores the potential of applying a west-orientated RSSL to the enclosed lift lobbies of high-rise residential buildings. When the side-emitting efficiency is improved to allow the RSSL to operate for longer hours, the design of the south-orientated RSSL should be investigated. Further research is required to improve the side-emitting efficiency of fiber optic in light transmission before the product can be commercialized. The research only concentrates on the application of the RSSL in enclosed lift lobbies in high-rise residential buildings. Based on
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the findings of the study, further research can be conducted to extend the application of the RSSL to provide daylight to the inner zone of office building, commercial centers, institutional and other buildings where natural light cannot reach. Acknowledgement The work described in this paper was fully supported by a grant from Sun Hung Kai Properties Limited (Project No. ZZ1T). References [1] J. Hansen, M. Sato, R. Ruedy, Forcing and chaos in interannual to decidal climate change, Geophysical Research 102 (22) (1997) 697–720. [2] J. Houghton, M. Filho, B.A. Callander, N. Harris, A. Kattenberg, K. Kaskell, Climate Change 1994, Cambridge University Press, Cambridge, 1995. [3] S. Manabe, R. Manabe, Numerical results from a nine-level general circulation mode of atmosphere, Monthly Weather Review 93 (12) (1965) 727–768. [4] J. Charney, A. Arakawa, D.J. Baker, B. Bolib, Carbon Dioxide and Climate, National Academic Press, Washington D.C., 1979. [5] J. Hansen, M. Sato, Trends of measured climate forcing agents, Proceedings of the National Academy of Science 19 (18) (2001) 14778–14783. [6] HKSAR, Census and Statistic Department Latest statistic. Website: http://www.censtatd.gov.hk kong statistics/statistics by subject/index.jsp dated 1 June 2011. [7] I. Wong, H.X. Yang, Introducing natural lighting into the enclosed lift lobbies of high-rise residential buildings by remote source lighting system, Applied Energy 90 (1) (2012) 225–232. [8] T.Y. Chen, J. Burnett, C.K. Chau, Analysis of embodied energy in the residential buildings in Hong Kong, Energy 26 (4) (2001) 323–340. [9] T.M. Chung, A study of luminous efficacy of daylight in Hong Kong, Energy and Buildings 19 (1) (1992) 45–50. [10] D.H.W. Li, S.L. Wong, Daylighting and energy implications due to shading effects from nearby buildings, Applied Energy 84 (2007) 1199–1209. [11] L.H. Sullivan, The tall office building considered artistically, in: Lippincott’s Magazine March, Fili-Quarian Classic, Philadelphia, 1896. [12] K. Yang, G. Hansen, I. Edwards, R. Hyde, Light pipes: an innovative design device for bringing natural daylight and illumination into buildings with deep floor plan, Far East Economic Review Innovative Awards (2003). [13] T.M. Chung, Daylighting in Hong Kong: potential and problems, Lighting Research and Technology 35 (2003) 39–41.
Contents lists available at SciVerse ScienceDirect
Energy and Buildings journal homepage: www.elsevier.com/locate/enbuild
Study on remote source solar lighting system application in high-rise residential buildings in Hong Kong Irene Wong ∗ , H.X. Yang Renewable Energy Research Group (RERG), Department of Building Services Engineering, The Hong Kong Polytechnic University, Hong Kong
a r t i c l e
i n f o
Article history: Received 9 May 2012 Received in revised form 8 October 2012 Accepted 5 January 2013 Keywords: Remote source solar lighting system Fiber optic Solar altitude Solar azimuth angle Solar irradiance Shadowing effect
a b s t r a c t This research investigates the design constraints and potential of applying the remote source solar lighting system to the enclosed lift lobbies of high-rise residential buildings in Hong Kong. No natural light is usually provided to these lift lobbies, which depend on electric lighting for illumination. The application of conventional light pipes requires minimum 3 m headroom and is not feasible to be applied in the high-rise residential buildings of 2.8 m floor height in Hong Kong. A remote source solar lighting system is specifically designed to solve the headroom problem. The daylighting system uses small diameter side-emitting fiber optic as light transmission medium. The factors of solar altitude, solar azimuth angle and solar irradiance are investigated. The shadowing effects caused by neighboring buildings and the supporting framework are analyzed in details. Design parameters of the natural daylight system are defined. Design guidelines and a model design are developed as a reference for building designers in designing a remote source solar lighting system. The design addresses both the functional application as well as the esthetic design. The remote source daylighting system can be integrated into the architectural design of the external facade and lift lobbies. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Greenhouse gases cause global warming [1] and yield a steep, relentless increase in global temperature throughout the twentyfirst century [2] with warming of several degrees Celsius by 2100 [3–5]. Carbon dioxide is the main source of greenhouse gases generated from fossil fuel combustion to produce electricity. Greater use of renewable energy sources that produce little or no CO2 is required to reduce the carbon dioxide emission [3]. Hong Kong is a metropolitan city characterized by high-rise buildings. The population density was 6500 persons per square kilometers in 2010 according to the statistic provided by the Census and Statistic Department [6]. Residential premises are developed into high-rise buildings to compensate the high land cost. In order to build more floors within the limited building height, the floor height seldom exceeds 2.8 m [7]. The lift lobbies are enclosed in the center to free up the peripheral spaces of valuable view for domestic units, which are enclosed without natural lighting. The layout of a typical floor with enclosed lift lobby is shown in Fig. 1. Energy use in buildings accounts for nearly half of the total primary energy use in Hong Kong [8]. Reduction in the use of electric lighting in the enclosed lift lobbies by introducing natural lighting can help to
∗ Corresponding author. Tel.: +852 2766 4559; fax: +852 2336 9994. E-mail address: [email protected] (I. Wong). 0378-7788/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.enbuild.2013.01.010
conserve our environment. This project is to investigate how to use solar lighting in lift lobbies for energy saving in Hong Kong.
2. The remote source solar lighting system As the floor height is limited to 2.8 m, the size of the light transmission medium must be small in order not to reduce the floor height. A remote source solar lighting system (RSSL) using sideemitting fiber optic as light transmission medium is designed. The RSSL composes of a 760 × 760 mm plane mirror, 2 numbers of 225 mm diameter converging lenses and a 10.4 mm diameter large core LEF710 fiber optic (FO). The plane mirror acts as a heliostat and reflects sunlight into the converging lenses, which is installed on the external wall. The mirror can rotate in both horizontal and vertical planes responding to the constant changes of the sun’s position in solar azimuth angle () and solar altitude (ˇ) as shown in Fig. 2 at different times so as to reflect the sun beam always parallel to the axis of the lenses. The lenses converge and concentrate the reflected light beam into the small sectional area of the FO, which emits light uniformly along the whole length at 2–6% per meter. Transmitted light is emitted in a divergent angle of 60 in downward direction. The RSSL system operates in direct sunlight under clear sky condition. An RGB LED lamp of 40 W is installed at the end of the FO to act as supplementary lighting device, which will be instantly turned on when the light detector at the far end of the FO detects the light intensity below the recommended 150 lx.
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I. Wong, H.X. Yang / Energy and Buildings 60 (2013) 225–231
Fig. 3. Relation between indoor and outdoor light intensities with time.
Fig. 1. A typical residential floor layout.
Fig. 4. Solar altitude vs. time.
Fig. 2. The RSSL.
Light emitted from the LED lamp will be transmitted in the reverse direction inside the FO to illuminate the lift lobby. The lens system can be installed in any service room which part of the headroom can be reduced below 2.8 m. 3. Factors affecting the RSSL’s performance An experiment was carried from February to August 2011 in the period from 10:00 to 16:30. The changes in indoor light intensity responding to the changes in outdoor light intensity, solar altitude (ˇ) and solar azimuth angle () at different times and dates were recorded. The outdoor light intensity was fluctuating due to the changes in sky condition even within a few minutes especially in partly cloudy sky condition. The collected data was used to derive a general trend for analysis. The collected data was plotted in graphs to analyze the results. The findings indicated that the indoor light intensity increases with the increase in outdoor light intensity. A minimum outdoor light intensity of 42,500 lx is required to maintain the indoor light intensity at 150 lx, which occurs mainly in the afternoon period (Fig. 3). As the RSSL can only operates under clear sky condition, the direct solar luminous efficacy is used to estimate the required solar irradiance for the operation of the RSSL. The direct solar luminous efficacy (Kbc ) can be calculated from Chung’s equation as below [9]: Kbc = 48.5 + 1.67ˇ − 0.0098ˇ2
(1)
Kbc increases with the increase in solar altitude. From Fig. 4, the lowest solar altitude of 36◦ in a day is used to calculate Kbc , which is 96 lm/W. The outdoor light intensity of 42,500 lx is converted to 443 W/m2 by the direct luminous efficacy (Kbc ). A minimum of 443 W/m2 in a clear day is required to maintain the indoor light intensity at 150 lx. The latest available comprehensive records of solar irradiance in different hours and months issued by the Hong Kong Observatory from 2004 to 2007 are used to estimate the functioning hours of the RSSL in a year. From these records, the total hours of average solar irradiance at or above 443 W/m2 for different months in a year in the period between 9:00 and 17:00 h are summarized in Table 1. The annual operating hour of the RSSL is 1235 h. The RSSL can contribute about 3.4 h of daylight in a day on an average basis. 4. Shadowing effects The optimal orientation of the mirror in the RSSL is facing south so that it can receive sunlight for most of the day. However the mirror in the south-orientated RSSL needs to be adjusted in both the horizontal and vertical planes responding to the changes in solar azimuth angle and solar altitude, respectively. The installation method of the mirrors to the external wall is structurally complicated to cater for the mirror rotations in 2 axes. Although a RSSL that is installed on the west facade can only operate in the post meridiem period, the mirror is required to rotate in the vertical plane only and the construction of the supporting frame is much simpler and less expensive. From Table 1, the west-orientated RSSL still has 1068 annual operating hour, which is 3 h per day. Furthermore, the operating period of the RSSL mainly falls within the post meridiem. Conclusively, a west-orientated RSSL is preferred.
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227
Table 1 Total No. of hours of average solar irradiance at 443 W/m2 or above. Month
Average No. of hours in a day
Total hour in a month
No. of hours from 12:00–17:00
Total No. of hours from 12:00–17:00 in a month
January February March April May June July August September October November December Total
1.75 2.25 0.75 2.75 3.5 3.5 5.75 4 4.75 5 3 3.5 1234.75
54.25 63 23.25 82.5 108.5 105 178.25 124 142.5 155 90 108.5 1068.25
1.75 2.25 0.75 2.75 2.75 3 4.75 3.5 3.75 4 2.75 3
54.25 63 23.25 82.5 85.25 90 147.25 105.5 112.5 124 82.5 93
Fig. 5. Relations of indoor light intensity and mirror inclination with time.
4.1. Shadowing effect from buildings Hong Kong is a densely populated city with buildings developed into high-rise of over 20 storeys. Such developments may result in a large degree of shading effect from nearby buildings and the effect can be significant [10]. Fig. 5 shows the relationship of indoor light intensity and the recorded mirror inclination () responding to the changes in the solar altitude with time. The RSSL can generally provide 150 lx from around 10:00 to 16:30. The solar altitude is twice the value of the mirror inclination, which changes from 60◦ at 10:00 h to 36◦ at 16:30, respectively. The angles of obstruction are 36◦ and 60◦ as shown in Fig. 6. Even there is no external obstruction, the RSSL that is installed on the western facade can only receive direct sunlight from half of the sky hemisphere. Therefore, 12:00 is used as the demarcation time for designing the RSSL. The period from 12:00 to 16:30 h occupies 64% of the total time in a day with the indoor light intensity generally above 150 lx. No obstruction shall fall within the shadowfree zone as shown in Fig. 6.
that it will not cast any image on the mirror below within the operating period of the RSSL (Fig. 7). Let ‘A’ be the inclination of the upper mirror that allow the incident ray just to pass without casting a shadow on the lower mirror in a west-orientated RSSL. ‘A’ can be calculated from the following equations: tan 2 = 2 tan
A 1
− tan A2
4.2. Shadowing effect within the RSSL As the mode of light transmission is horizontal, a RSSL is installed on every floor. The mirrors in the upper floors may cast a shadow to the mirrors below. Part of the sunlight will be blocked. Theoretically the RSSL can operate from 12:00 to 16:30 with no obstruction. The inclination of the mirror is adjusted responding to the change in solar altitude. The upper mirror should be designed
Fig. 6. Shadowing effect from buildings.
(2)
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I. Wong, H.X. Yang / Energy and Buildings 60 (2013) 225–231
a ; a` and b` are the 2 legs of the right-angled triangle b
tan A =
formed by the mirror
tan 2A =
2a + H ; 2b
H = 2800 mm
(3)
(4)
Hence 2ab2 = 2800b2 − 2a3 − 2800a2 2
2
4a + 4b = 760
2
(5) (6)
Therefore: b2 =
7602 − 4a2 4
(7)
Substitute Eq. (8) into (6): 5600a2 + 7602 ×
a − 7602 × 700 = 0 2
(8)
a = 269.4 sin A =
2a 760
‘A’ is calculated to be 45.16◦ ; which means the solar altitude is about 90◦ and occurs around 12:00. In this situation, the upper mirror will cast a shadow on part of the lower mirror and the efficiency of the RSSL will be reduced. ‘A’ must be smaller than 45◦ to avoid the shadowing effect. H should be adjusted to exceed 2.8 m. To solve this problem, 2 adjoining locations are reserved for the RSSL installation at each floor. The RSSL will be installed at each location on alternate floors. H is now changed to 5.6 m, and A becomes 44.96◦ , which is below 45◦ . From 12:18 onwards, decreases from 45◦ to 18◦ at 16:30 h. Doubling H to 5.6 m enables the RSSL to operate for a longer period. The minimum grid dimensions of the metal supporting frame for the mirrors are 0.8 m wide and 5.6 m high. 5. Detail design of a RSSL Fig. 7. Shadowing effect within the RSSL.
The design concept of “Forms follow function” [11] is adopted. The design of the RSSL should be primarily based on and derived from its functional use and can integrate into the architectural design of the facade and the lift lobby.
Fig. 8. Design concept of a RSSL.
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Fig. 10. Modified configuration of the metal frame.
A fiberglass casing is designed to house the fiber optic, which also serves as a fixing device. A slit is formed along the metal casing to discharge the light being emitted from the notched fiber optic. The casings run along the lobby and corridors and act as an illuminator and decorative element (Fig. 12). A space of 1.2 × 1.2 m2 with an external wall preferably facing west is recommended to be reserved to house the lenses; or the lenses can also share the space of any service room that is accessible to the exterior. An aperture of 225 mm is formed on the external wall for light transmission. Fig. 9. Metal supporting framework.
5.3. Maintenance 5.1. Integration into the external facade The mirrors are supported by an external metal frame, which allows the mirror to rotate in the vertical plane. Combining the conceptual design of the mirror and supporting system (Fig. 8) evolves the detail design of the metal supporting framework as shown in Fig. 9. The framework can be designed as an architectural feature anodized in either striking colors to be an eye-catching element or in similar color tone with the building as a harmonized feature. The shape of the metal frame can be modified as shown in Fig. 10 to allow the mirrors always facing the east even though the external wall is not orientated to the west. The principle of shadow-free zone should be applied in the design.
The lift expectancy of high quality waterproof mirror is over 5 years; and over 10 years for lens and FO. Replacement of these components is not frequent. Routine maintenance such as cleaning the surfaces of the mirrors and lenses once a month is recommended as accumulation of dirt can reduce the efficiencies of these components.
5.2. Integration into the lift lobby design In the popular design of the conventional lighting system of an enclosed lift lobby, a series of florescent lamps joining together are installed along the 2 sides of the ceiling. False ceiling in usually installed to conceal the lighting fixtures (Fig. 11). In the RSSL, the notched fiber optic acts as an illuminator and replaces the florescent lamps. As fiber optic comes in a continuous bundle, the alignment problem of the florescent lamps is solved. False ceiling is not required to conceal the notched fiber optic. Anidolic reflecting ceiling panels are not required because notched fiber optic emits light downwards in a 60◦ cone. Light intensity is measured at eyelevel of 1.5 m high. The measured light intensity at 0.2 m from the margins of the emitted light zone drops to 50% of the light intensity at the center. One notched fiber optic can illuminate an area of 1 x L m where L is the length of the fiber optic. In a corridor of 2 m wide, 2 numbers of notched fiber optic are required.
Fig. 11. Conventional lighting system in lift lobby.
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6. Design guidelines
Fig. 12. Notched fiber optic as illuminator inside lift lobby.
Maintenance platforms and cat ladders are installed to the metal framework of the RSSL as shown in Fig. 9. The platforms are fixed to the inner side of the metal frame right below the mirrors to avoid blocking the reflected sun rays. Structural inspection is recommended to be carried annually.
In designing a RSSL, the abundance of solar energy is a crucial factor. The average hours of solar irradiance that is 443 W/m2 or above is recommended to be 3 h in a day. No neighboring buildings shall cast a shadow within the shadow-free zone. In the next step, a space of 1.2 × 1.2 m2 with the external wall orientating to the west is reserved for the installation of the lenses. The installation space should be as close to the lift lobby as possible. The metal frame can be installed to the north-west or south-west facade if the west facade is blocked by buildings or other obstructions. Modification to the design of the metal frame is required to allow the mirrors facing the east. The metal framework is divided into grid size of 0.8 m wide and 5.6 m high for the buildings of 2.8 m storey height. The floor layout should be studied in detail to determine the numbers of RSSL to be installed at each floor. The length of the fiber optic should not be greater than 20 m for efficient transmission [12]. A design flow chart is shown in Fig. 13.
Fig. 13. Design flowchart of a RSSL.
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7. Conclusion The availability of daylight in Hong Kong is high [12,13]. An RSSL composing of a plane mirror as a simple heliostat, 2 converging lenses and side-emitting FO is developed to transfer daylight into the enclosed lift lobbies of high-rise residential buildings. The performance of the RSSL at different times and dates of a year has been studied and the required sky conditions for operation have been discussed. The solar irradiance is the dominant factor that affects the efficiency of the RSSL. A minimum of 443 W/m2 is required to maintain the indoor light intensity at 150 lx. As Hong Kong is crowded with high-rise buildings, the shadowing effect is critical to the operation of the RSSL. The RSSL must be designed to avoid the shadowing effects from nearby buildings and the supporting frame of the RSSL itself. A model RSSL is designed in detail to integrate into the architectural design of the building. The design of the RSSL should also facilitate routine maintenance. A design flowchart is formulated as a design reference for architects and building services engineers. 8. Recommendation for future works The development of the side-emitting technology in fiber optic is still in the preliminary stage and the light emission efficiency is about 2–6%. This research basically explores the potential of applying a west-orientated RSSL to the enclosed lift lobbies of high-rise residential buildings. When the side-emitting efficiency is improved to allow the RSSL to operate for longer hours, the design of the south-orientated RSSL should be investigated. Further research is required to improve the side-emitting efficiency of fiber optic in light transmission before the product can be commercialized. The research only concentrates on the application of the RSSL in enclosed lift lobbies in high-rise residential buildings. Based on
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the findings of the study, further research can be conducted to extend the application of the RSSL to provide daylight to the inner zone of office building, commercial centers, institutional and other buildings where natural light cannot reach. Acknowledgement The work described in this paper was fully supported by a grant from Sun Hung Kai Properties Limited (Project No. ZZ1T). References [1] J. Hansen, M. Sato, R. Ruedy, Forcing and chaos in interannual to decidal climate change, Geophysical Research 102 (22) (1997) 697–720. [2] J. Houghton, M. Filho, B.A. Callander, N. Harris, A. Kattenberg, K. Kaskell, Climate Change 1994, Cambridge University Press, Cambridge, 1995. [3] S. Manabe, R. Manabe, Numerical results from a nine-level general circulation mode of atmosphere, Monthly Weather Review 93 (12) (1965) 727–768. [4] J. Charney, A. Arakawa, D.J. Baker, B. Bolib, Carbon Dioxide and Climate, National Academic Press, Washington D.C., 1979. [5] J. Hansen, M. Sato, Trends of measured climate forcing agents, Proceedings of the National Academy of Science 19 (18) (2001) 14778–14783. [6] HKSAR, Census and Statistic Department Latest statistic. Website: http://www.censtatd.gov.hk kong statistics/statistics by subject/index.jsp dated 1 June 2011. [7] I. Wong, H.X. Yang, Introducing natural lighting into the enclosed lift lobbies of high-rise residential buildings by remote source lighting system, Applied Energy 90 (1) (2012) 225–232. [8] T.Y. Chen, J. Burnett, C.K. Chau, Analysis of embodied energy in the residential buildings in Hong Kong, Energy 26 (4) (2001) 323–340. [9] T.M. Chung, A study of luminous efficacy of daylight in Hong Kong, Energy and Buildings 19 (1) (1992) 45–50. [10] D.H.W. Li, S.L. Wong, Daylighting and energy implications due to shading effects from nearby buildings, Applied Energy 84 (2007) 1199–1209. [11] L.H. Sullivan, The tall office building considered artistically, in: Lippincott’s Magazine March, Fili-Quarian Classic, Philadelphia, 1896. [12] K. Yang, G. Hansen, I. Edwards, R. Hyde, Light pipes: an innovative design device for bringing natural daylight and illumination into buildings with deep floor plan, Far East Economic Review Innovative Awards (2003). [13] T.M. Chung, Daylighting in Hong Kong: potential and problems, Lighting Research and Technology 35 (2003) 39–41.