Design of a high-efficiency collection structure for daylight illumination applications Meng-Che Tsai,* Allen Jong-Woei Whang, and Tsung-Xian Lee Graduate Institute of Color and Illumination Technology, National Taiwan University of Science and Technology, #43, Sec. 4, Keelung Rd., Taipei 106, Taiwan *Corresponding author: [email protected] Received 17 July 2013; revised 22 September 2013; accepted 10 November 2013; posted 19 November 2013 (Doc. ID 193554); published 16 December 2013
In developing a high-quality natural light illumination system (NLIS), the primary considerations include how to increase system efficiency and broaden its applications. This paper describes the conception, design, and analysis of a daylight collector that presents the combined advantages of excellent efficiency and a compact size. The collector structure consists of extendable two-channel collecting units, a planar light guide, and a central coupler to improve light collection efficiency and increase surface area. In this study, two types of daylight collectors are proposed for illumination applications with different light patterns. With these collectors, the NLIS can now provide sufficiently powerful light for indoor illumination. © 2013 Optical Society of America OCIS codes: (080.2740) Geometric optical design; (080.4298) Nonimaging optics; (350.6050) Solar energy. http://dx.doi.org/10.1364/AO.52.008789
1. Introduction
In response to the energy crisis, solar energy, wind energy, hydroelectric power, ocean wave energy, geothermal energy, and energy from biomass have all been gaining popularity as green energy options. Research regarding the production and use of energy must address how to maximize energy efficiency as one of its top priorities. Energy-saving and carbon dioxide reduction are some of the most critical issues currently facing the globe. Among renewable energies, solar energy has the potential for widespread application [1]. One possible technique is to illuminate a solar cell so as to transform light energy directly into electricity. However, because of its low efficiency and high cost, solar cells are not yet economically feasible. In order to minimize energy expenditure and create a healthy illumination, our team has designed a series of products, called the natural light 1559-128X/13/368789-06$15.00/0 © 2013 Optical Society of America
illumination system (NLIS) that can transform natural light into indoor illumination [2]. The concept of collecting daylight for natural light illumination has the benefit of using solar energy directly without the loss of energy required to transform solar energy into electrical energy [3,4]. In past research, we present the light collector module of the NLIS, known as the Optical Brick, which uses prismatic elements, set on the roof to collect daylight [5,6]. Although the NLIS guides daylight to interior spaces, there are problems to overcome. The Optical Brick collecting area is much smaller than the whole module area, so utilization of sunlight is relatively low. The biggest difference between artificial lighting and the NLIS is that artificial lighting has a greater number of light source selections such as light bulbs and light tubes. There is only collimated source in a traditional NLIS, which is restricted through the demand of the high-efficiency guiding process. This study develops a method for maintaining high efficiency and designs a variety of lighting luminaries for various applications. 20 December 2013 / Vol. 52, No. 36 / APPLIED OPTICS
8789
2. NLIS Structure
In the past, we placed our Optical Brick collecting module, which is composed of prism elements and a transmission device (Fig. 1), in an area where sunlight can be received, such as the top floor or the balcony of a building. As the Brick collects and transmits light through its elements, sunlight effectively illuminates the interior of the building. The system is divided into three main structures: a light collection system, an optical transmission system, and a light emission system. Sunlight is collected by the light collection system, transmitted to the house by the optical transmission system, and emitted uniformly by the light-emitting system to achieve natural environmental illumination, as shown in Fig. 2. On the basis of the traditional NLIS, we recently proposed revised system architecture, as shown in Fig. 3. To increase the efficiency of light collection, we replaced the Optical Brick module with static concentrators to obtain two shapes of exit surface for the light emission system. By connecting different lighting devices with the two shapes of exit surface, natural light is more efficiently utilized in a variety of lighting applications. 3. Structure of the Static Collecting Module
Concentrating photovoltaic (CPV) systems and designs are well documented in the literature [7–12]. They require a thin light collection module to achieve high efficiency, as does the NLIS. The greatest difference between the CPV and NLIS design concepts is that the NLIS collection module must also guide the light for later usage, i.e., by connecting with a light transmitter or lighting device. To address this issue, we designed a two-layer static collecting structure.
Fig. 2. Illustration of the NLIS.
The top collecting layer contains collecting units and the central unit, and the bottom transmitting layer contains the planar light guide and a central coupler, as shown in Fig. 4. To provide different types of light-collecting modules, the collecting structure can be rotated into a circular collecting module or extruded into a square collecting module, as shown in Fig. 5. Through the circular collecting module, the light can be gathered into a small round exit surface, as shown in Fig. 6(a). Cost is very low for long-distance transmission by this shape of exit surface when connecting with optical fibers or an optical pipe, as in the traditional NLIS. When using the square module, the light is collected into a rectangular exit surface, similar to the light tube used for artificial lighting, as shown in Fig. 6(b). The components of this new collecting module, its design concepts, and their effect on the optical performance are detailed below. A. Collecting Unit Design
Fig. 1. Optical Brick collecting module. 8790
APPLIED OPTICS / Vol. 52, No. 36 / 20 December 2013
A novel compact, high-efficiency collecting module for guiding natural light was designed, as shown in Fig. 7. Each collecting unit has a spherical surface to compress the light and two aspherical surfaces to defocus the light into smaller parallel light beams. A mirror reflects the vertically incident sunlight, which
Input
System
Output
Natural Light Guiding System
Natural Light
Light Collector
Light Transmitter
Light Emitter
Static Concentrator
Optical Fiber/ Optical Pipe
Lighting Device for Point Source Lighting Device for Line Source
Sun Tracking System
Lighting Applications
Active Lighting Module Optical Sensor
Electricity
LED
Fig. 3. Schematic of the newly revised NLIS.
Fig. 4. Two-dimensional profiles of the static collecting structure.
Fig. 5. Illustration of (a) the circular and (b) the square collecting module.
Fig. 6. Illustration of (a) the collimated light source produced by the exit surface of the circular collecting module and (b) the line light source produced by the exit surface of the square collecting module. 20 December 2013 / Vol. 52, No. 36 / APPLIED OPTICS
8791
Fig. 9. Illustration of the RI-to-IR light guide.
into a single path, making it possible to increase the number of collecting units that can be stacked. C.
Fig. 7. Illustration of the collecting unit.
can be the input for the next unit. The angle between the mirror and the horizontal axis is 45°. The collecting unit is designed to separate the light beam into two channels: the RIR channel and the X-to-RXIR channel. The RIR channel compresses the direct sunlight from above and changes the light transmission direction over the total internal reflection surface (refer to red and green rays in Fig. 8). The X-to-RXIR channel is composed of two units, and collects the light from the mirror at the back unit (refer to brown rays in Fig. 8), while also compressing the light into an exit surface by the RXIR channel in front of the unit. With this newly designed collecting unit, a larger area of light from the input is compressed into a smaller area of light and transmitted to the output at a high energy density, while also changing the direction of transmission. These units are stackable and in close proximity to each other in the collecting module, so that the ratio of the collecting area to the total module area can reach 100%. B.
Coupler and Central Unit Design
The coupler is used to guide the light beams from the light guide and to change the direction of the light beams, as shown in Fig. 10. The RIR design is also used in the coupler design in this study. When the coupler design is complete, a central connector is still needed to link with the collecting units and to maintain the high ratio of the collecting area to the total module area. Therefore, we designed a central unit to collect the light from the top of the coupler and to guide the light from the mirror of the back collecting unit, as shown in Fig. 11. 4. Performance Analysis
For optical simulation software, we used BK7 (n 1.51872 at 546.1 nm) to define the two types of materials in the static collecting modules and to set up the coating of the 45° mirror at 95% reflectance. To obtain the simulation results for the different stacked amounts, we assumed that the light source was perpendicularly incident. The two types of collecting modules were simulated and compared, as follows.
Light Guide Design
The light guide is used to transfer light to the end of the collecting units. We propose an RI-to-IR light guide, as shown in Fig. 9. Each light guide unit is composed of a microstructure RI to guide the parallel light beams from the collecting unit and a planar light guide IR to transmit the light beams to the central coupler. With the RI-to-IR light guide, the light beams from different collecting units can be guided
Fig. 8. Illustration of the optical structure formed of the two collecting units. 8792
APPLIED OPTICS / Vol. 52, No. 36 / 20 December 2013
Fig. 10. Illustration of the coupler connected with the light guide.
Fig. 11. unit.
Illustration of the coupler connected with the central
Fig. 12. Simulated collection efficiency for different stacked amounts.
Fig. 14. Total gathered light flux with different numbers of stacks.
Stack amount 0 indicates that the top layer of the collecting modules had no collecting units and only a central unit. The simulation results showed that both types of collecting modules exhibited very good performance in collection efficiency. Even when the collecting structure was stacked with up to nine units, efficiency still reached more than 30%, as shown in Fig. 12. In terms of efficiency performance, the square module is slightly better than the circular module. As the two types of collecting modules had the same number of stacks, the diameter of the circular collecting module was equal to the side length of the square collecting module. Therefore, we could easily calculate the collecting areas of the two modules from their geometrical relationship, as shown in Fig. 13. After determining the efficiency and collecting area for both types of collecting modules, we compared their total gathered light flux. We assumed that the light source provided 1 lm∕mm2 . The square module was calculated to have a higher total gathered light flux than the circular module, as shown in Fig. 14. The trajectory of the sun will affect the efficiency of the light collector. In practice, light collection systems typically use a two-dimensional tracking system to maintain optimum efficiency. Thus, the simulation of the square type, that is, the data for two different axes, is also calculated. To calculate the angle tolerance of the collecting modules, the light source incidence angle was simulated from 0° to 50°. The attainable efficiency for each incidence angle is shown in Fig. 15. The results show that efficiency was reduced to 50% of the vertical incidence when
the tilt angle of the circular collecting module was 3.5° and when the tilt angle of the square collecting module was 46°.
Fig. 13. Collecting area for different stacked amounts.
5. Applications Analysis A. Circular Type
Regarding the application of the NLIS, a circular collecting module is used for long-distance transmission. A collimated light source is required for achieving low-loss transmission. The simulation of the nine-stack circular collecting module resulted in a candela distribution on the exit surface as shown in Fig. 16. The design of the circular collecting module provided a high-directivity light source, as expected. To verify the transmission performance of this light source, we simulated and compared the transmission efficiency through a plastic optical fiber of a Lambertian light source and the circular collecting module. In the simulation software, we set the absorbance of the plastic optical fiber at 5%/m. The results showed that the circular collecting module exhibited better performance, as shown in Fig. 17. B. Square Type
The NLIS application using a square collecting module is like a light tube source for an illumination design. Generally, a high-uniformity light source is preferred for lighting design. The high uniformity reduces the difficulty of secondary optical element design. The irradiance distribution on the exit surface of the nine-stack square collecting module is
Fig. 15. Simulated efficiency for different incidence angles. 20 December 2013 / Vol. 52, No. 36 / APPLIED OPTICS
8793
Fig. 16. Rectangular candela distribution on the circular collecting module.
The collector combined expandable collecting units, a planar light guide, and a central coupler to improve the light collection efficiency while increasing the surface area of the module. Different solutions for illumination applications were also considered. Circular and square collecting modules showed very good efficiency performance. The efficiency of the nine-stack circular collecting module reached 33% and that of the nine-stack square module reached 48%. Regarding their applications, the nine-stack circular collecting module provided a high-directivity light source for use in long-distance transmission, with transmission efficiency in a 1 m plastic optical fiber of 95.6%. The square collecting module provided a uniform-light tube for secondary optical element design, and the uniformity on the exit surface of the square collecting module was 87%. Future work will investigate how to maintain maximum output all day by using a sun tracking system design, followed by studies to optimize a low-cost manufacturing process and outdoor experimentation. References
Fig. 17. Transmission efficiency for different transmission distances in the plastic optical fiber.
Fig. 18. Irradiance distribution on the exit surface of the square collecting module.
shown in Fig. 18. The design of the circular collecting module provided a high-uniformity line light source, as expected. The uniformity on the exit surface of the square collecting module was 87%. 6. Conclusions
A novel static daylight collector with a two-layer structure was proposed and tested in this study.
8794
APPLIED OPTICS / Vol. 52, No. 36 / 20 December 2013
1. A. J. W. Whang, Y. Y. Chen, and B. Y. Wu, “Innovative design of Cassegrain solar concentrator system for indoor illumination utilizing chromatic aberration to filter out ultraviolet and infrared in sunlight,” Sol. Energy 83, 1115–1122 (2009). 2. A. J. W. Whang, Y. Y. Chen, S. H. Yang, P. H. Pan, K. H. Chou, Y. C. Lee, Z. Y. Lee, C. A. Chen, and C. N. Chen, “Natural light illumination system,” Appl. Opt. 49, 6789–6801 (2010). 3. A. J.-W. Whang, Y.-Y. Chen, and Y.-T. Teng, “Designing uniform illumination systems by surface-tailored lens and configurations of LED arrays,” J. Display Technol. 5, 94–103 (2009). 4. A. J.-W. Whang, P.-C. Li, Y.-Y. Chen, and S.-L. Hsieh, “Guiding light from LED array via tapered light pipe for illumination systems design,” J. Disp. Technol. 5, 104–108 (2009). 5. S.-C. Yeh, A. J.-W. Whang, H.-C. Hsiao, X.-D. Hu, and Y.-Y. Chen, “Distribution of emerged energy for daylight illuminate on prismatic elements,” J. Sol. Energy Eng. 133, 021007 (2011). 6. S. H. Yang, Y. Y. Chen, and A. J. W. Whang, “Using prismatic structure and brightness enhancement film to design cascadable unit of static solar concentrator in natural light guiding system,” Proc. SPIE 7423, 74230J (2009). 7. J. C. Miñano, P. Benítez, J. Liu, J. Infante, J. Chaves, and A. Santamaría, “Novel ideal nonimaging designs by multichanneling the phase-space flow,” Proc. SPIE 7652, 765220 (2010). 8. J. H. Karp, E. J. Tremblay, and J. E. Ford, “Planar micro-optic solar concentrator,” Opt. Express 18, 1122–1133 (2010). 9. O. Selimoglu and R. Turan, “Exploration of the horizontally staggered light guides for high concentration CPV applications,” Opt. Express 20, 19137–19147 (2012). 10. K. A. Baker, J. H. Karp, E. J. Tremblay, J. M. Hallas, and J. E. Ford, “Reactive self-tracking solar concentrators: concept, design, and initial materials characterization,” Appl. Opt. 51, 1086–1094 (2012). 11. S. Bouchard and S. Thibault, “Planar waveguide concentrator used with a seasonal tracker,” Appl. Opt. 51, 6848–6854 (2012). 12. J. M. Hallas, K. A. Baker, J. H. Karp, E. J. Tremblay, and J. E. Ford, “Two-axis solar tracking accomplished through small lateral translations,” Appl. Opt. 51, 6117–6124 (2012).
In developing a high-quality natural light illumination system (NLIS), the primary considerations include how to increase system efficiency and broaden its applications. This paper describes the conception, design, and analysis of a daylight collector that presents the combined advantages of excellent efficiency and a compact size. The collector structure consists of extendable two-channel collecting units, a planar light guide, and a central coupler to improve light collection efficiency and increase surface area. In this study, two types of daylight collectors are proposed for illumination applications with different light patterns. With these collectors, the NLIS can now provide sufficiently powerful light for indoor illumination. © 2013 Optical Society of America OCIS codes: (080.2740) Geometric optical design; (080.4298) Nonimaging optics; (350.6050) Solar energy. http://dx.doi.org/10.1364/AO.52.008789
1. Introduction
In response to the energy crisis, solar energy, wind energy, hydroelectric power, ocean wave energy, geothermal energy, and energy from biomass have all been gaining popularity as green energy options. Research regarding the production and use of energy must address how to maximize energy efficiency as one of its top priorities. Energy-saving and carbon dioxide reduction are some of the most critical issues currently facing the globe. Among renewable energies, solar energy has the potential for widespread application [1]. One possible technique is to illuminate a solar cell so as to transform light energy directly into electricity. However, because of its low efficiency and high cost, solar cells are not yet economically feasible. In order to minimize energy expenditure and create a healthy illumination, our team has designed a series of products, called the natural light 1559-128X/13/368789-06$15.00/0 © 2013 Optical Society of America
illumination system (NLIS) that can transform natural light into indoor illumination [2]. The concept of collecting daylight for natural light illumination has the benefit of using solar energy directly without the loss of energy required to transform solar energy into electrical energy [3,4]. In past research, we present the light collector module of the NLIS, known as the Optical Brick, which uses prismatic elements, set on the roof to collect daylight [5,6]. Although the NLIS guides daylight to interior spaces, there are problems to overcome. The Optical Brick collecting area is much smaller than the whole module area, so utilization of sunlight is relatively low. The biggest difference between artificial lighting and the NLIS is that artificial lighting has a greater number of light source selections such as light bulbs and light tubes. There is only collimated source in a traditional NLIS, which is restricted through the demand of the high-efficiency guiding process. This study develops a method for maintaining high efficiency and designs a variety of lighting luminaries for various applications. 20 December 2013 / Vol. 52, No. 36 / APPLIED OPTICS
8789
2. NLIS Structure
In the past, we placed our Optical Brick collecting module, which is composed of prism elements and a transmission device (Fig. 1), in an area where sunlight can be received, such as the top floor or the balcony of a building. As the Brick collects and transmits light through its elements, sunlight effectively illuminates the interior of the building. The system is divided into three main structures: a light collection system, an optical transmission system, and a light emission system. Sunlight is collected by the light collection system, transmitted to the house by the optical transmission system, and emitted uniformly by the light-emitting system to achieve natural environmental illumination, as shown in Fig. 2. On the basis of the traditional NLIS, we recently proposed revised system architecture, as shown in Fig. 3. To increase the efficiency of light collection, we replaced the Optical Brick module with static concentrators to obtain two shapes of exit surface for the light emission system. By connecting different lighting devices with the two shapes of exit surface, natural light is more efficiently utilized in a variety of lighting applications. 3. Structure of the Static Collecting Module
Concentrating photovoltaic (CPV) systems and designs are well documented in the literature [7–12]. They require a thin light collection module to achieve high efficiency, as does the NLIS. The greatest difference between the CPV and NLIS design concepts is that the NLIS collection module must also guide the light for later usage, i.e., by connecting with a light transmitter or lighting device. To address this issue, we designed a two-layer static collecting structure.
Fig. 2. Illustration of the NLIS.
The top collecting layer contains collecting units and the central unit, and the bottom transmitting layer contains the planar light guide and a central coupler, as shown in Fig. 4. To provide different types of light-collecting modules, the collecting structure can be rotated into a circular collecting module or extruded into a square collecting module, as shown in Fig. 5. Through the circular collecting module, the light can be gathered into a small round exit surface, as shown in Fig. 6(a). Cost is very low for long-distance transmission by this shape of exit surface when connecting with optical fibers or an optical pipe, as in the traditional NLIS. When using the square module, the light is collected into a rectangular exit surface, similar to the light tube used for artificial lighting, as shown in Fig. 6(b). The components of this new collecting module, its design concepts, and their effect on the optical performance are detailed below. A. Collecting Unit Design
Fig. 1. Optical Brick collecting module. 8790
APPLIED OPTICS / Vol. 52, No. 36 / 20 December 2013
A novel compact, high-efficiency collecting module for guiding natural light was designed, as shown in Fig. 7. Each collecting unit has a spherical surface to compress the light and two aspherical surfaces to defocus the light into smaller parallel light beams. A mirror reflects the vertically incident sunlight, which
Input
System
Output
Natural Light Guiding System
Natural Light
Light Collector
Light Transmitter
Light Emitter
Static Concentrator
Optical Fiber/ Optical Pipe
Lighting Device for Point Source Lighting Device for Line Source
Sun Tracking System
Lighting Applications
Active Lighting Module Optical Sensor
Electricity
LED
Fig. 3. Schematic of the newly revised NLIS.
Fig. 4. Two-dimensional profiles of the static collecting structure.
Fig. 5. Illustration of (a) the circular and (b) the square collecting module.
Fig. 6. Illustration of (a) the collimated light source produced by the exit surface of the circular collecting module and (b) the line light source produced by the exit surface of the square collecting module. 20 December 2013 / Vol. 52, No. 36 / APPLIED OPTICS
8791
Fig. 9. Illustration of the RI-to-IR light guide.
into a single path, making it possible to increase the number of collecting units that can be stacked. C.
Fig. 7. Illustration of the collecting unit.
can be the input for the next unit. The angle between the mirror and the horizontal axis is 45°. The collecting unit is designed to separate the light beam into two channels: the RIR channel and the X-to-RXIR channel. The RIR channel compresses the direct sunlight from above and changes the light transmission direction over the total internal reflection surface (refer to red and green rays in Fig. 8). The X-to-RXIR channel is composed of two units, and collects the light from the mirror at the back unit (refer to brown rays in Fig. 8), while also compressing the light into an exit surface by the RXIR channel in front of the unit. With this newly designed collecting unit, a larger area of light from the input is compressed into a smaller area of light and transmitted to the output at a high energy density, while also changing the direction of transmission. These units are stackable and in close proximity to each other in the collecting module, so that the ratio of the collecting area to the total module area can reach 100%. B.
Coupler and Central Unit Design
The coupler is used to guide the light beams from the light guide and to change the direction of the light beams, as shown in Fig. 10. The RIR design is also used in the coupler design in this study. When the coupler design is complete, a central connector is still needed to link with the collecting units and to maintain the high ratio of the collecting area to the total module area. Therefore, we designed a central unit to collect the light from the top of the coupler and to guide the light from the mirror of the back collecting unit, as shown in Fig. 11. 4. Performance Analysis
For optical simulation software, we used BK7 (n 1.51872 at 546.1 nm) to define the two types of materials in the static collecting modules and to set up the coating of the 45° mirror at 95% reflectance. To obtain the simulation results for the different stacked amounts, we assumed that the light source was perpendicularly incident. The two types of collecting modules were simulated and compared, as follows.
Light Guide Design
The light guide is used to transfer light to the end of the collecting units. We propose an RI-to-IR light guide, as shown in Fig. 9. Each light guide unit is composed of a microstructure RI to guide the parallel light beams from the collecting unit and a planar light guide IR to transmit the light beams to the central coupler. With the RI-to-IR light guide, the light beams from different collecting units can be guided
Fig. 8. Illustration of the optical structure formed of the two collecting units. 8792
APPLIED OPTICS / Vol. 52, No. 36 / 20 December 2013
Fig. 10. Illustration of the coupler connected with the light guide.
Fig. 11. unit.
Illustration of the coupler connected with the central
Fig. 12. Simulated collection efficiency for different stacked amounts.
Fig. 14. Total gathered light flux with different numbers of stacks.
Stack amount 0 indicates that the top layer of the collecting modules had no collecting units and only a central unit. The simulation results showed that both types of collecting modules exhibited very good performance in collection efficiency. Even when the collecting structure was stacked with up to nine units, efficiency still reached more than 30%, as shown in Fig. 12. In terms of efficiency performance, the square module is slightly better than the circular module. As the two types of collecting modules had the same number of stacks, the diameter of the circular collecting module was equal to the side length of the square collecting module. Therefore, we could easily calculate the collecting areas of the two modules from their geometrical relationship, as shown in Fig. 13. After determining the efficiency and collecting area for both types of collecting modules, we compared their total gathered light flux. We assumed that the light source provided 1 lm∕mm2 . The square module was calculated to have a higher total gathered light flux than the circular module, as shown in Fig. 14. The trajectory of the sun will affect the efficiency of the light collector. In practice, light collection systems typically use a two-dimensional tracking system to maintain optimum efficiency. Thus, the simulation of the square type, that is, the data for two different axes, is also calculated. To calculate the angle tolerance of the collecting modules, the light source incidence angle was simulated from 0° to 50°. The attainable efficiency for each incidence angle is shown in Fig. 15. The results show that efficiency was reduced to 50% of the vertical incidence when
the tilt angle of the circular collecting module was 3.5° and when the tilt angle of the square collecting module was 46°.
Fig. 13. Collecting area for different stacked amounts.
5. Applications Analysis A. Circular Type
Regarding the application of the NLIS, a circular collecting module is used for long-distance transmission. A collimated light source is required for achieving low-loss transmission. The simulation of the nine-stack circular collecting module resulted in a candela distribution on the exit surface as shown in Fig. 16. The design of the circular collecting module provided a high-directivity light source, as expected. To verify the transmission performance of this light source, we simulated and compared the transmission efficiency through a plastic optical fiber of a Lambertian light source and the circular collecting module. In the simulation software, we set the absorbance of the plastic optical fiber at 5%/m. The results showed that the circular collecting module exhibited better performance, as shown in Fig. 17. B. Square Type
The NLIS application using a square collecting module is like a light tube source for an illumination design. Generally, a high-uniformity light source is preferred for lighting design. The high uniformity reduces the difficulty of secondary optical element design. The irradiance distribution on the exit surface of the nine-stack square collecting module is
Fig. 15. Simulated efficiency for different incidence angles. 20 December 2013 / Vol. 52, No. 36 / APPLIED OPTICS
8793
Fig. 16. Rectangular candela distribution on the circular collecting module.
The collector combined expandable collecting units, a planar light guide, and a central coupler to improve the light collection efficiency while increasing the surface area of the module. Different solutions for illumination applications were also considered. Circular and square collecting modules showed very good efficiency performance. The efficiency of the nine-stack circular collecting module reached 33% and that of the nine-stack square module reached 48%. Regarding their applications, the nine-stack circular collecting module provided a high-directivity light source for use in long-distance transmission, with transmission efficiency in a 1 m plastic optical fiber of 95.6%. The square collecting module provided a uniform-light tube for secondary optical element design, and the uniformity on the exit surface of the square collecting module was 87%. Future work will investigate how to maintain maximum output all day by using a sun tracking system design, followed by studies to optimize a low-cost manufacturing process and outdoor experimentation. References
Fig. 17. Transmission efficiency for different transmission distances in the plastic optical fiber.
Fig. 18. Irradiance distribution on the exit surface of the square collecting module.
shown in Fig. 18. The design of the circular collecting module provided a high-uniformity line light source, as expected. The uniformity on the exit surface of the square collecting module was 87%. 6. Conclusions
A novel static daylight collector with a two-layer structure was proposed and tested in this study.
8794
APPLIED OPTICS / Vol. 52, No. 36 / 20 December 2013
1. A. J. W. Whang, Y. Y. Chen, and B. Y. Wu, “Innovative design of Cassegrain solar concentrator system for indoor illumination utilizing chromatic aberration to filter out ultraviolet and infrared in sunlight,” Sol. Energy 83, 1115–1122 (2009). 2. A. J. W. Whang, Y. Y. Chen, S. H. Yang, P. H. Pan, K. H. Chou, Y. C. Lee, Z. Y. Lee, C. A. Chen, and C. N. Chen, “Natural light illumination system,” Appl. Opt. 49, 6789–6801 (2010). 3. A. J.-W. Whang, Y.-Y. Chen, and Y.-T. Teng, “Designing uniform illumination systems by surface-tailored lens and configurations of LED arrays,” J. Display Technol. 5, 94–103 (2009). 4. A. J.-W. Whang, P.-C. Li, Y.-Y. Chen, and S.-L. Hsieh, “Guiding light from LED array via tapered light pipe for illumination systems design,” J. Disp. Technol. 5, 104–108 (2009). 5. S.-C. Yeh, A. J.-W. Whang, H.-C. Hsiao, X.-D. Hu, and Y.-Y. Chen, “Distribution of emerged energy for daylight illuminate on prismatic elements,” J. Sol. Energy Eng. 133, 021007 (2011). 6. S. H. Yang, Y. Y. Chen, and A. J. W. Whang, “Using prismatic structure and brightness enhancement film to design cascadable unit of static solar concentrator in natural light guiding system,” Proc. SPIE 7423, 74230J (2009). 7. J. C. Miñano, P. Benítez, J. Liu, J. Infante, J. Chaves, and A. Santamaría, “Novel ideal nonimaging designs by multichanneling the phase-space flow,” Proc. SPIE 7652, 765220 (2010). 8. J. H. Karp, E. J. Tremblay, and J. E. Ford, “Planar micro-optic solar concentrator,” Opt. Express 18, 1122–1133 (2010). 9. O. Selimoglu and R. Turan, “Exploration of the horizontally staggered light guides for high concentration CPV applications,” Opt. Express 20, 19137–19147 (2012). 10. K. A. Baker, J. H. Karp, E. J. Tremblay, J. M. Hallas, and J. E. Ford, “Reactive self-tracking solar concentrators: concept, design, and initial materials characterization,” Appl. Opt. 51, 1086–1094 (2012). 11. S. Bouchard and S. Thibault, “Planar waveguide concentrator used with a seasonal tracker,” Appl. Opt. 51, 6848–6854 (2012). 12. J. M. Hallas, K. A. Baker, J. H. Karp, E. J. Tremblay, and J. E. Ford, “Two-axis solar tracking accomplished through small lateral translations,” Appl. Opt. 51, 6117–6124 (2012).
