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Cost-effective optical fiber daylighting system using modified compound parabolic concentrators
Solar Energy 136 (2016) 145–152
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Cost-effective optical fiber daylighting system using modified compound parabolic concentrators Ngoc-Hai Vu, Seoyong Shin ⇑ Department of Information and Communication Engineering, Myongji University, San 38-2 Nam-dong, Yongin 449-728, South Korea
a r t i c l e
i n f o
Article history: Received 27 May 2015 Received in revised form 10 May 2016 Accepted 24 June 2016
Keywords: Optical fiber daylighting system Modified compound parabolic concentrator Concentration ratio Plastic optical fiber
a b s t r a c t We present a cost-effective optical fiber daylighting system composed of modified compound parabolic concentrators (M-CPC) coupled with plastic optical fibers (POFs). An M-CPC is made by combining two conventional CPCs into one component. Our simulation results demonstrate an optical efficiency of up to 84% when the concentration ratio of the M-CPC is fixed at 100. We have also used a simulation to determine an optimal geometric structure of M-CPCs. Because of the simplicity of the M-CPC structure, a lower-cost mass production process is possible. Our quest for an optimal structure has also shown that M-CPC has high tolerance for input angle of sunlight. The high tolerance allows replacing a highly precise active sun-tracking system with a lower accuracy sun-tracking system as a cost-effective solution. A prototype of M-CPC was fabricated by laser cutting method and preliminary experiments of a sunlight concentrator using M-CPC were performed in outdoor. The good agreement between simulation results and experimental results confirm that M-CPC is designed properly. The overall system cost is also estimated. Some considerations on the economic expansion of the system in terms of efficiency are discussed. The results show that the presented optical fiber daylighting system is a strong candidate for low-price and highly efficient solution for solar energy application to building energy savings. Ó 2016 Elsevier Ltd. All rights reserved.
1. Introduction As challenges related to sources of energy are becoming more serious worldwide, the number of potential solar energy solutions is rapidly increasing (Ullah and Shin, 2014). While many aspects of modern society have negative impacts on the environment, buildings have a particularly significant impact due to the amount of energy that they use. Several studies have estimated that buildings are responsible for 20–40% of the total energy consumption in most developed nations (EIA, 2009; Pelegrini, 2010). To reduce the total energy consumption of buildings, among all current renewable energy options, the optical fiber-based daylighting system is the most easily integrated with LED lighting, which can replace conventional artificial lighting. Optical fiber daylighting systems are a unique and innovative means of bringing direct sunlight into a building by collecting natural light and channeling it through optical fibers to the luminaires within the space. This technology has the ability to bring sunlight much deeper into buildings. As products become commercially available and more economically viable, these systems have the potential to conserve significant amounts of energy and improve the quality of indoor ⇑ Corresponding author. E-mail address: [email protected] (S. Shin). http://dx.doi.org/10.1016/j.solener.2016.06.064 0038-092X/Ó 2016 Elsevier Ltd. All rights reserved.
environments. Advancements of this technology have, however, been delayed due to the fact that it is very high cost (Lingfors and Volotinen, 2013; Rosemann et al., 2008). Basically, optical fiber daylighting systems are comprised of three main components: the sunlight collector with a tracking mechanism, optical fibers and associated connections, and luminaires that distribute light within the space (Kribus et al., 2000). The heart of optical fiber daylighting system is the sunlight collector and sun tracking system. Through the research and development of many public and private groups, two basic collector designs have proven to be the most effective and reliable. The first uses Fresnel lenses to refract and concentrate sunlight into optical fibers, while the second captures incoming light by the reflection of parabolic mirrors. However, in order to collect light efficiently into the optical fiber, it is necessary to concentrate the sunlight with a high concentration ratio, which requires very high-quality optical components, alignment, and a precise electromechanical tracking system. Most of existing solar collectors for daylighting applications that satisfy these requirements are not cost-effective and are difficult to implement widely; thus, they have not experienced success on the market to date (André and Schade, 2002). Fig. 1(a) illustrates a schematic of a commercial solar fiber optic lighting system SP3 (the third generation of the fiber optic solar light systems from Parans company). The collector consists of 36 Fresnel
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Fig. 1. (a) Schematic of a commercial solar fiber optic lighting system; (b) detailed structure of one element inside the collector, which is the array of Fresnel lenses. Source: http://www.parans.co.
lenses (6.5 cm 6.5 cm) made of silicone imprinted onto the inside of the glass panel in front of the collector (Ae et al., 2014). Each lens focuses solar light into an optical fiber as shown in Fig. 1(b). The conventional compound parabolic concentrators (CPCs) are popular non-imaging concentrators that are relevant for solar energy collection that is used for absorbers such as photovoltaic or thermal energy applications (Rabl, 1976a). Basically, the CPC structure can be used to collect sunlight in optical fiber daylighting systems; however, its optical efficiency is not good because of the high-diverging, non-uniform output beam and low concentration ratio. In this study, we propose a novel optical fiber-based daylighting system using modified CPCs (M-CPCs). With this specialty design, the concentration ratio can reach high enough and the output light is collimated for optical fiber coupling purposes. This makes it possible for a very simple sunlight-collecting system including an array of M-CPCs connected to optical fiber bundles to replace conventional, more complicated collectors that consist of the confocal lenses or parabolic imaging systems. The proposed design has some advantages, such as lower costs and higher tolerance, thus eliminating the need for high-quality optics, alignment, and a highly precise tracking system. The aim of this designfocused research was to design, analyze, and optimize a novel compact non-imaging optical component, namely an M-CPC, that can lead to the development of high-efficiency and low-cost solar daylighting collectors and thus facilitate the viable commercialization of the cost-effective mass production of daylighting systems. The remainder of the paper is organized in the following manner: Sections 2 discusses the M-CPC concentrator geometry and optical fiber coupling approaches. In Section 3, the optical fiber daylighting system based on M-CPCs is modeled in LightToolsTM software (Synopsys Inc., California, USA) to evaluate the performance of such a system (Bouchard and Thibault, 2014; Pei et al., 2012; Vu and Shin, 2016). This section also details the fabrication and operation of an initial working prototype. Section 4 presents a proposed optical fiber daylighting system using M-CPCs and discusses the expansion of the system economically. Finally, brief concluding remarks and possibilities for future work are included in Section 4. To the best of our knowledge, the optical fiber daylighting system using M-CPCs that is described here is the first optical fiber daylighting system with cost-effective potential when manufactured at high volumes.
The basic idea for the M-CPC was inspired by the concept of integrated optics (Hunsperger, 2009). By reviewing the current literature on solar concentrators of existing optical fiber daylighting systems, we established some preliminary requirements for the design and development of the M-CPCs, including a single integrated optical element, a high concentration ratio, high tolerance, the possibility of being integrated with an optical fiber, and low costs including manufacturing and maintenance. The M-CPC concentrator that is presented here utilizes double CPCs as a means to capture, concentrate, and collimate the direct sunlight and couple it into an optical fiber. The idea is based on a three-dimensional (3D) CPC that has been used for many different applications, from high-energy physics to solar energy collection (Winston et al., 2005). To modify the CPCs for our purpose, we first reviewed the theory of conventional CPCs. A symmetrical CPC consists of two identical parabolic reflectors that funnel radiation from the aperture to the absorber (Rabl, 1976b). According to Foster and Ghassemi (2009), a 2D CPC is composed of two truncated parabolic reflectors; neither one keeps its vertex point, but both rims must be tilted toward the sun. Fig. 2 (a) shows the 3D-CPC geometric structure. Its surface is formed by rotating a 2D-CPC. Fig. 2(b) is a cross-section of a 3D-CPC. It describes the geometric parameters of a 3D-CPC. They are overall length L, input aperture diameter Din, acceptance input angle hin, output diameter Dout, and output angle hout. Fig. 2(b) also shows the geometric relationship between two parabola segments for the construction of a 3D-CPC. The points FA and FB are focuses of Parabolas A and B, respectively. The output aperture rim can be called by the focal rim of CPC. The relationship between these parameters and the concentration ratio are determined by the following equations (Winston et al., 2005):
L¼
ðDin þ Dout Þ 2 tan hin
ð1Þ
2 sinhout sin hin
ð2Þ
CR ¼
Here CR denotes the concentration ratio. From Eqs. (1) and (2), we see that the enhancement of the concentration factor leads to an increase of the object length. In addition, a higher concentration ratio corresponds with a lower angular acceptance input angle of
2. M-CPC concept This section introduces the conceptual design and working principles of a non-imaging solar collector, namely, the M-CPC.
Fig. 2. Schematic representation of a 3D-CPC: (a) 3D structure generated by LightTools software; (b) all geometric parameters related to the structure of the 3DCPC.
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the system. However, since CPCs are attached to the sun-tracking system, the input sunlight is always parallel to the main axes of the CPCs. The output angle hout should be 30° to suit the angular aperture of the optical fiber. Fig. 3 shows the structure of the 3DCPC with a high concentration ratio, CR = 100; the shape becomes very slim with a CPC length of L = 219.72 mm, and the input acceptance angle hin is decreased to 2.86°. From ray tracing in LightToolsTM to analyze the output beam of this high-concentration 3D-CPC, we found that the light was focused from a large area to a small area; however, the output beam become highly divergent and non-uniform. This is not suitable for coupling with the optical fiber. To overcome this problem, a novel M-CPC was proposed. M-CPC is an optical component that is comprised of two confocal CPCs, as shown in Fig. 4(a). The primary CPC is designed with a high input area and a high concentration ratio to capture direct sunlight to the maximum level. While the primary CPC uses its major aperture as an input to harvest solar energy, another CPC is placed inversely for the purpose of being used as a collimator and uses its minor aperture as an input and captures the total output light from the primary CPC. The collimated CPC should be designed to achieve a high degree of collimation to facilitate an effective fiber-coupling performance (Kong et al., 2011). Some main parameters of the M-CPC are depicted in Fig. 4(a). They are input diameter Din, output diameter Dout, length of primary CPC L1, and length of collimated CPC L2. The overall length of the MCPC is L = L1 + L2. The diameter of the conjunction area between the primary and collimated CPCs is DF. DF corresponds to the diameter of the focal region. Two CPCs still keep intrinsic parameters of CPCs; however, for the best coupling between two CPCs, the minor aperture sizes of two CPCs need to be the same and the minor aperture angles need to be fixed to the same 90°. The concentration ratio of M-CPC is defined by the ratio between the input and output areas,
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Fig. 4. (a) Design of M-CPC enclosure with geometric parameters and (b) the result of the ray tracing analysis performed on the M-CPC.
limation properties of an M-CPC can be explained by the characteristics of the confocal optical system. This M-CPC concept introduced here fulfills the requirements for a concentrator of an optical fiber daylighting system, such as a high concentration ratio and an enough collimated output light for optical fiber coupling. An optimized structure for the M-CPC is considered in depth in next part of this paper. 3. Optical simulation and experimental results
CR ¼
Din Dout
2 ;
ð3Þ
where CR denotes the concentration ratio. Since the output of MCPC is coupled with a POF, the output diameter is fixed with the same size of the diameter of the POF. To ensure a simple assembly in the fabrication process, the output of collimating CPC can be designed as a form of fiber optic ferrule for POF coupling. POF coupling can be made simpler by injecting the fiber into the output of M-CPC and fixing it. Optical alignment manipulation is therefore eliminated. The optical performance of the M-CPC design was analyzed with Monte Carlo ray tracing using LightToolsTM. The collimation of the output beam was realized by adding an inversely placed CPC with an appropriate design as shown in Fig. 4(b). Since the minor aperture rim of a 3D-CPC is a focal rim, the structure of M-CPC with two CPCs has a joint focal rim, which forms a confocal CPC system like a confocal lens system. The concentration and col-
Optical modeling plays a crucial role in the efficiency evaluation of an optical system. Commercial optical modeling software, LightToolsTM, was used to design the geometrical form of M-CPCs. We analyzed the efficiency of the system and found that there was some light loss due to the intrinsic geometrical structure of the M-CPC and the optical fiber coupling. To address this issue, the relationship between the structure and each geometric parameter needed to be modeled correctly. Optical fibers were utilized to deliver sunlight to the interior with small losses. Silica optical fibers (SOFs) are known to be good light-transmission media and have the best resistance to heating; however, SOFs are expensive. Plastic optical fibers (POFs) have substantially higher attenuation coefficients than SOFs, but POFs are preferred in daylighting systems due to their lower cost, tighter minimum bend radius, ease of installation and durability for complex wiring in buildings (Han et al., 2013; Ullah and Shin, 2014). In this research, we employed POFs from Anchor Optics (‘‘Anchor Optics,” n.d.). The POF parameters are listed in Table 1.
Fig. 3. High-concentration 3D-CPC and ray tracing analysis for the output beam when input direct sun light is parallel to the main axis of the 3D-CPC.
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Table 1 POF parameters for design, simulation and experiment. Parameters
Normal core POF
Core/cladding diameter Refractive index: core/cladding Minimum bend radius Spectral trans. range
1.960/2.0 mm 1.492/1.402 50 mm 380–750 nm
We placed a commercial POF with a diameter of 2 mm at the output of the M-CPC. In other words, the output diameter of the M-CPC was fixed at Dout = 2 mm. A concentration ratio of 100 was chosen. This was reasonable ratio because although a CPC can be designed to achieve a very high concentration ratio to facilitate good sunlight collection, it would require very large physical dimensions relative to a given input aperture size. With a fixed input size of Dout = 2 mm and a concentration ratio of CR = 100, the input diameter Din = 20 mm was determined using Eq. (3). As previously explained, an M-CPC is an integrated structure with two CPCs whose minor apertures are put together, and the joint part is named by the focal diameter, DF. DF is a very important parameter, as the other parameters like the length of the M-CPC, the input angle, the output angle, and the efficiency can be determined by DF. Based on some initial conditions such as Din = 20 mm, Dout = 2 mm, concentration ratio CR = 100, we generated the M-CPC structures with several different focal diameters using LightToolsTM, ranging from 0.5 to 1.5 mm in the increment of 0.1 mm. The dependence of the input angle hin, the primary CPC length L1, the output angle hout, and the collimated CPC length L2 on the focal diameter is shown in Fig. 6(a) and (b). The primary CPC length decreased significantly with an increased DF, and the collimated CPC length became much shorter as DF increased. Since the optical conduit was based on commercial POF whose cladding diameter (D) was 2 mm, core refractive index (n1) was 1.49, cladding refracqffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi tive index (n2) was 1.39, numerical aperture (sin hc = n21 n22 ) was
Fig. 5. Illustration of the structure for the simulation and ray-tracing analysis for design verification.
0.3, and half acceptance angle (hc) was 30°, the focal diameter can be calculated theoretically DF = 1 mm. Although, the parameters of M-CPCS can be calculated based on the theory, the optical fiber coupling is a complicated mechanism and cannot be predicted theoretically. The simulation was carried out to verify the theoretical predictions and calculate optical efficiency. The simulation scheme is described in Fig. 5. For the simulation, M-CPC was assumed to have a 100% inner surface reflection coefficient. Two optical receivers were placed at the input mouth and at the end of the POF to evaluate the optical losses. A parallel monochromatic light source worked as a direct sun light source. The wavelength of light did not affect the efficiency of the system because the principle of M-CPC is based on reflection. Since the influence of the wavelength on the numerical aperture (NA) of fiber was small, it was not necessary to perform a simulation with a total visible range of sunlight. We carried out a simulation with several different focal diameters, ranging from 0.5 to 1.5 mm in the increment of 0.1 mm. The coupling efficiency between M-CPC and POF is illustrated in Fig. 7. The efficiency geff was measured by the ratio between the total luminous flux at the input receiver and that at the output receiver, such as:
geff ¼
/out ; /in
ð4Þ
where / is denoted as the total luminous flux in the lumens. In the LightToolsTM software, the luminous flux was calculated by the integration of illuminance E over the total area of the receiver as below:
I /¼
E dS:
ð5Þ
The integration in Eq. (5) was performed by dividing the receiver area into grids, and the total lumens was the sum of the illuminance of all grid cells. This method produced some errors, which are shown as bars in Fig. 7. The loss of M-CPC and POF coupling due to the Fresnel reflection at the entrance surface of POF is essential. One interesting observation from Fig. 7 is that the efficiency was constant to 84%, when the focal diameter, DF, increased from 0.5 mm to 1.05 mm; it decreased particularly fast when DF was larger than 1.05 mm. This can be explained by referring back to the output angle hout and the NA of the POF. The critical value of DF = 1.05 mm corresponded to 30° of the output angle of M-CPC, which coincided with the half acceptance angle of the POF. Therefore, 1.05 mm is the optimal size for focal diameter DF to obtain the highest efficiency of 84% and short M-CPC length. The simulation
Fig. 6. Dependency of (a) input angle and primary CPC length and (b) output angle and collimated length on the focal diameter.
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Fig. 7. Variation of optical efficiency at different focal diameters DF.
Table 2 Optimal parameters of an M-CPC with a concentration ratio of CR = 100. M-CPC parameters
Value (Unit)
Primary CPC length Collimated CPC length Total M-CPC length Input angle Output angle Focal diameter
200 (mm) 2.47 (mm) 202.47 (mm) 3 (°) 31 (°) 1.05 (mm)
results, which were very close to the theoretical predictions, confirmed that the design of M-CPC adapts to daylighting purpose. All other parameters of an optimized M-CPC are listed in Table 2. For proper operation of the proposed optical fiber daylighting system, direct sunlight should always be normal to input aperture of M-CPC. This is a difficult task since the position of the sun is always changing, and this led us to use a sun tracking system. The required accuracy of the sun tracking system is determined by the solar concentrating collector’s angle of tolerance. The tolerance of the system is the acceptable angular deviation of the sunlight direction from the two main axes of the system, within the allowable efficiency loss. It is defined as the angle where the efficiency drops by 10%. The acceptance angle determines the required accuracy of the tracking system mounted upon the concentrator (Xie et al., 2014). We performed a simulation for three cases—MCPC, conventional CPC, and Fresnel lens—to evaluate the advantage of M-CPC over conventional CPC and existing systems. The concentration ratio of CPC and M-CPC was the same at 100. We examined the efficiency with a different angular deviation of sunlight direction from 6 to +6°. The optical efficiency of M-CPC and CPC when coupled with POF depend on angular deviation is plotted in Fig. 8. The red line and triangular denotes M-CPC, blue line and rectangular denotes conventional CPC. In the case of M-CPC and POF coupling, the efficiency was 84%, and this did not change within the range of angular deviation from 3 to +3°, but it decreased dramatically when the angle became larger than ±3°. In the case of coupling between CPC and POF, the maximum efficiency was 75.2%. The efficiency decreased significantly from 75.2% to 55% when the angular deviation increased from 0 to ±2.5°, increased slightly when the angular deviation was from ±2.5 to ±6°, and decreased dramatically when the angular deviation became larger than ±6°. Although the acceptance input angle of CPC was only 2.8°, the light still escaped from the output of CPC when the angular deviation was from ±2.5 to ±6° because the rays inside the CPC make two reflections on average. This does not appear in the M-CPC because the primary CPC’s output aperture is very small. The lower and unstable efficiency of CPC and POF coupling in comparison with
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Fig. 8. Variation of optical efficiency at different angular deviation in three cases: M-CPC, conventional CPC and Fresnel lens.
M-CPC and POF coupling highlights one of the important characteristics of 3D-CPC, namely the very high nonhomogeneity in light distribution at the output aperture. The collimated CPC of M-CPC also works as a light mixer to redistribute the light on the exit area. This is the reason for the higher and more stable optical efficiency of the M-CPC and POF coupling. To evaluate the performances of M-CPC in comparison with existing devices, we performed another simulation for an optical fiber daylighting system based on Fresnel lens. Some parameters of Fresnel lens and optical fiber were based on SP3 system – the third generation of fiber optic solar light systems from Swedish Parans company (http://www.parans.com). The Fresnel lens has size of 65 mm 65 mm). Plastic optical fibers are used with a core diameter of 1 mm. The simulation results are shown in Fig. 8 by green line and circle. For daylighting systems using Fresnel lenses, the dispersion of the visible range is a problem, as this leads to an essential decrease in optical efficiency. As seen in Fig. 8, Fresnel lens system can achieve an optical efficiency of 77%. Although, optical efficiency is lower than M-CPC but using Fresnel lens as a concentrator can lead to high concentration ratio (CR > 3500). The advantage and disadvantage of high or medium concentration ratio that affect on system cost will be discussed in more detail in discussion part. The angular tolerance of the concentrator is defined as the angular deviation, within which the optical efficiency drops to 90% of its maximum value. The tolerance of conventional CPC, M-CPC, and Fresnel lens can be determined based on Fig. 8 and is ±1.3°, ±2.9°, ±0.25°, respectively. The high tolerance of M-CPC suggests that low accuracy sun tracking system can be utilized and this reduces the system cost (Xie et al., 2014). According to above description, a prototype of M-CPC was made by laser cutting. Inside of M-CPC was mirror coated with evaporated aluminum. Fig. 9(a) shows a structure design generated by LightToolsTM and Fig. 9(b) is a fabricated sample. The experimental configuration is shown in Fig. 10. The M-CPC was mounted with a dual axis sun tracking system whose angular tolerance is less than 3°. A 5 m length of POF was attached in the M-CPC output aperture. The degradation due to ultraviolet (UV) radiation and temperature is a problem in plastic material such as POFs. We suggested using UV protection thin film at input aperture of M-CPC to eliminate the UV wavelength (Mayhoub, 2014; Sapia, 2013) as shown in Fig. 10. The illuminance from the sunlight was measured at different times of the day. The site of application was located at 127° longitude and 37.5° latitude. Here we look at the illuminance on a sunny day. The measured illuminances of the input flux at the surface of the M-CPC and of the luminous flux at POF endface are listed in Table 3. The prototype system reached an optical efficiency of
150
N.-H. Vu, S. Shin / Solar Energy 136 (2016) 145–152 Table 3 Average illuminance at different times of the day and luminous flux at POF output for the proposed system.
Fig. 9. (a) A M-CPC structure generated by LightToolsTM and its cross sectional view; (b) fabricated M-CPC and its cross sectional view.
Fig. 10. A prototype M-CPC was used to collect sunlight in an outdoor setting. Insert shows the light exits at the end of POF.
65% including: M-CPC intrinsic loss, non-ideal of mirror coating, optical fiber coupling and optical fiber transmission loss for 5 m. The last column in Table 3 shows the luminous flux at POF endface that was calculated based on simulation results (the loss of 5 m length of POF was also included in calculation). Our experimental measurements were in close agreement with our optical model and support the notion that M-CPC design is reliable and the system would perform the work with high efficiency. The temperature measured at M-CPC/POF coupling region is around 35 °C, which shows that the heat is not a problem in the proposed M-CPC.
Time
Sunlight illuminance (lux)
Luminous flux on the M-CPC input (lm)
Luminous flux at POF endface (lm) (by experiment)
Luminous flux at POF endface (lm) (by calculation)
6 AM 7 AM 8 AM 9 AM 10 AM 11 AM 12 PM 1 PM 2 PM 3 PM 4 PM 5 PM 6 PM
20,105 39,969 60,051 79,989 95,005 105,007 110,000 102,023 94,005 80,989 59,958 40,125 19,800
6.3 12. 7 18.9 25.1 29.8 33.0 34.5 32.0 29.5 25.4 18.8 12.5 6.2
4.13 6.2 10.3 16.3 19.3 21.4 22.5 20.8 19.2 16.5 10.25 9.2 1.7
4.2 8.4 12.7 16.9 20.0 22.2 23.2 21.5 19.8 17.1 12.7 8.5 4.2
nent (lenses or reflectors) to focus sunlight and a secondary optical element for flux collimation. The requirement for the alignment between the primary optics, secondary optics and optical fiber bundles is very tight. The primary purpose of the present work is to find a low cost and highly efficient way to utilize the solar energy for daylighting. In this research, we proposed a costeffective approach for an optical fiber daylighting system using non-imaging optic devices such as M-CPCs. The primary collector is consisted of a 10 20 M-CPCs array. 200 M-CPC are required and the size of concentrator is 540 mm 270 mm 212.34 mm as shown in Fig. 11(a). Two hundred pieces of POFs were utilized as the sun light conduits. At the highest elevation (zenith) angle of sun, each M-CPC can provide 22 lux for interior lighting (Table 3) so total system can achieve 4400 lm. For the typical office building, average illuminance is required about 500 lux. This proposed system can cover a surface area of 8 m2. To distribute daylight at the destination, 200 pieces of fibers were arrange into twenty bundles and then placed at different positions to cover surface area of 8 m2. Total required POF length is 1000 m. Optical fibers in each bundle were organized into a circular shape. A commercial end fitting for POF using ball lens is best choice for the light distribution in term of cost efficiency. For simulation purpose, we used some parameters of a commercial POF end fitting from Shenzhen Everzone Technology Co., Ltd. (China) as seen in Fig. 12(a). Fig. 12(b) shows the ray tracing performed by optical fiber bundle and the end fitting using LightToolsTM. The room interior was designed in DIALuxTM and the light distribution modules were imported into DIALuxTM to show the room interior under lighting simulation. Twenty fiber bundles with end fitting were arranged in 4 5 array. Interior view of the room under lighting simulation is shown in Fig. 13(a). We have achieved the required illuminance value and uniformity on the floor by using this approach (Fig. 13(b)).
4. Proposed optical fiber daylighting system and discussion Optical fiber daylighting systems are concerned more about delivering daylight into remote and windowless spaces in buildings. Many types of optical fiber daylighting systems have been developed with various technologies and solutions over a few decades, but few of them have been successfully commercialized. Providing a cost effective system is the major challenge for optical fiber daylighting system. The concentrator parts of typical optical fiber daylighting systems consist of a large primary optical compo-
Fig. 11. Proposed a sunlight concentrator for optical fiber lighting system based on M-CPCs array.
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As discussed in the introduction, to be competitive, the M-CPCs must be fabricated using a low-cost process. According to this goal, they must be manufactured using plastic material. The advantage of using plastic to fabricate the M-CPC is the capacity to mold the system, which is a low cost solution for mass production. Fig. 14 shows a proposed injection molding method to fabricate the M-CPCs. Each half of M-CPC is fabricated by molding method and then makes the mirror coating inside. Two half of M-CPC are glued together to become a compact one. Recently, a recognized effective design strategy led to the simplification of product/system design and reduced the number of stages in manufacturing process (Edwards, 2002; O’Driscoll, 2002). Reducing the use of materials will also contribute to reduced manufacturing costs, supplier management costs, and recycling costs. To achieve these goals, the one component, one material, one manufacturing process concept (Pelegrini, 2010) are necessary for the design. The optical fiber daylighting system using M-CPC concept proposed here meets all of these requirements. M-CPC and POF are made of single material – plastic. M-CPC fabrication can utilize a simple manufacturing process, such as injection molding, which is suitable for mass production. These features have resulted in a reduction of manufacturing costs. For an estimate of the cost of whole proposed optical fiber daylighting system, we make the following considerations: the cost of one M-CPC in mass production is estimated by $5 so 200 M-CPCs have cost of $1000. The cost of mechanical bracket is $100. The UV film is about $30. The cost of the POFs is about $0.5/m for the 2 mm core POFs so 5 m long POF bundles (1000 m of POF) cost is $500. Automatic dual axis sun tracking system with low accuracy
Fig. 12. (a) An optical fiber bundle with end fitting as sun light distributer in the interior and (b) ray traycing.
151
Fig. 14. Proposed a simple injection molding method to manufacture the M-CPC for mass production.
(<3°) has cost of $400, roughly. The cost of twenty end fittings is $20. Therefore, total expected cost of the proposed system is $2050. We can evaluate our proposed system in terms of daylight collection capability and cost effectiveness by comparing our system with two other commercialized systems: Himawari and Parans systems. Due to variety of operating conditions, system comparison is not easy, however we used the data from developers’ publication so that we can estimate our proposed system (Vu and Shin, 2016). The Himawari system collects and concentrates sunlight using multiple sun tracking Fresnel lenses. Light is transported by quarts optical fibers. The total luminous flux of a Himawari system is 4000 lm which was measured under direct sun illuminance of about 100,000 lux. The net price of a Himawari system package (including 12 lens collector, two 5 m long optical bundles) is $6240 (‘‘HIMAWARI Solar Lighting System,” n.d.). The Swedish Parans system consists of an array of Fresnel lenses which simultaneously tracks and concentrates daylight. Paran system’s total output flux is about 3540 lm, which was measured under direct sun illuminance of 100,000 lux, 10 m away from the collector. Parans systems package (six 5 m long optical fiber bundles) net price is about $5425 (‘‘PARANS Product Information,” n.d.). The comparisons associated with the daylight collection capability and cost have been summarized in Table 4. As shown in the table, our proposed system is about 2.5 times cheaper than the other previous systems. This is reasonable because M-CPC fabrication cost is much cheaper than Fresnel lens which has micro-structure on the surface. Our proposed system has lower concentration ratio in comparison with previous ones so that it required more optical fiber. Although the cost for POFs increases but nowadays optical fiber cost has become cheaper so it is acceptable. However, high concentration ratio in conventional system requires very high quality of optical devices, and sun tracking system. The alignment
Fig. 13. (a) Simulation of the daylight illuminance distribution at the test site and (b) light distribution on the floor.
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Table 4 Comparison in cost and daylight collection capability with some commercialized optical fiber daylighting systems. Comparison categories
Proposed system
Himawari system
Parans system
Output flux (lm) Cost ($)
4400 2050
4000 6240
3540 5425
requirement between optical components is also very tight and the alignment becomes degraded fast when they operate under outdoor condition. Heat is also a problem in high concentration system and requires more cost for treatment. High condensed light in optical fiber needs a complicate and lossy optical system to distribute the light in the interior. Due to the simplicity in material, shape, form, function, manner of operation, assembly, and usage, and cost efficiency of the proposed system using M-CPCs, apparent and obvious commercialization of fiber optic daylighting system can be realized. 5. Conclusion An optical fiber daylighting system using M-CPCs has been designed and discussed with the purpose of saving the energy consumed by electric lighting. To explore the practical performance of the proposed system, a sample optical system was modeled and simulated with LightToolsTM. The simulation results indicate that 84% of optical efficiency was achieved at Cgeo = 100 for the proposed M-CPC. In addition, the tolerance (acceptance angles) also was analyzed. By using M-CPCs as the primary concentrator, an acceptance angle of ±2.9° was achieved. This allows us to use a lower accuracy sun tracking system as a cost effective solution. A prototype of M-CPC was fabricated using laser cutting method. The experimentation under real conditions was implemented to verify the accuracy of the simulation and the commercial viability of the system. This study is the first to use a non-imaging device M-CPC for an optical fiber daylighting system. It shows great potential for the commercial and industrial scale daylighting field. In the future, we will try to implement a completed system and install it in the real building office to evaluate all aspects of the system. Conflict of interest The authors declared that there is no conflict of interest. Acknowledgements This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIP) (No. 2014R1A2A1A11051888).
References Ae, A.P., Kumar, M.R., E, A.P., 2014. A review of solar fiber optic lighting systems. In: International Conference and Workshop on Electronics and Telecommunication Engineering, pp. 519–522. Anchor Optics [WWW Document], n.d. (accessed 10.8.15). André, E., Schade, J., 2002. Master’s Thesis: Daylighting by Optical Fiber. Bouchard, S., Thibault, S., 2014. GRIN planar waveguide concentrator used with a single axis tracker. Opt. Express 22, A248–A258. http://dx.doi.org/10.1364/ AO.51.006848. Edwards, K., 2002. Towards more strategic product design for manufacture and assembly: priorities for concurrent engineering. Mater. Des. 23, 651–656. http://dx.doi.org/10.1016/S0261-3069(02)00050-X. EIA, 2009. Annual Energy Outlook 2009, Annual Energy Outlook. doi: DOE/EIA-0383 (2008). Han, H.J., Riffat, S.B., Lim, S.H., Oh, S.J., 2013. Fiber optic solar lighting: functional competitiveness and potential. Sol. Energy 94, 86–101. http://dx.doi.org/ 10.1016/j.solener.2013.04.010. HIMAWARI Solar Lighting System [WWW Document], n.d. (accessed 12.14.15). Hunsperger, R., 2009. Integrated Optics: Theory and Technology, sixth ed. Springer, New York. Kong, C.R., Barman, I., Dingari, N.C., Kang, J.W., Galindo, L., Dasari, R.R., Feld, M.S., 2011. A novel non-imaging optics based Raman spectroscopy device for transdermal blood analyte measurement. AIP Adv. 1, 1–13. http://dx.doi.org/ 10.1063/1.3646524. Kribus, a., Zik, O., Karni, J., 2000. Optical fibers and solar power generation. Sol. Energy 68, 405–416. http://dx.doi.org/10.1016/S0038-092X(00)00009-8. Lingfors, D., Volotinen, T., 2013. Illumination performance and energy saving of a solar fiber optic lighting system. Opt. Express 21, A642. http://dx.doi.org/ 10.1364/OE.21.00A642. Mayhoub, M.S., 2014. Innovative daylighting systems’ challenges: a critical study. Energy Build. 80, 394–405. http://dx.doi.org/10.1016/j.enbuild.2014.04.019. O’Driscoll, M., 2002. Design for manufacture. J. Mater. Process. Technol. 122, 318– 321. http://dx.doi.org/10.1016/S0924-0136(01)01132-3. PARANS Product Information [WWW Document], n.d.
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Cost-effective optical fiber daylighting system using modified compound parabolic concentrators Ngoc-Hai Vu, Seoyong Shin ⇑ Department of Information and Communication Engineering, Myongji University, San 38-2 Nam-dong, Yongin 449-728, South Korea
a r t i c l e
i n f o
Article history: Received 27 May 2015 Received in revised form 10 May 2016 Accepted 24 June 2016
Keywords: Optical fiber daylighting system Modified compound parabolic concentrator Concentration ratio Plastic optical fiber
a b s t r a c t We present a cost-effective optical fiber daylighting system composed of modified compound parabolic concentrators (M-CPC) coupled with plastic optical fibers (POFs). An M-CPC is made by combining two conventional CPCs into one component. Our simulation results demonstrate an optical efficiency of up to 84% when the concentration ratio of the M-CPC is fixed at 100. We have also used a simulation to determine an optimal geometric structure of M-CPCs. Because of the simplicity of the M-CPC structure, a lower-cost mass production process is possible. Our quest for an optimal structure has also shown that M-CPC has high tolerance for input angle of sunlight. The high tolerance allows replacing a highly precise active sun-tracking system with a lower accuracy sun-tracking system as a cost-effective solution. A prototype of M-CPC was fabricated by laser cutting method and preliminary experiments of a sunlight concentrator using M-CPC were performed in outdoor. The good agreement between simulation results and experimental results confirm that M-CPC is designed properly. The overall system cost is also estimated. Some considerations on the economic expansion of the system in terms of efficiency are discussed. The results show that the presented optical fiber daylighting system is a strong candidate for low-price and highly efficient solution for solar energy application to building energy savings. Ó 2016 Elsevier Ltd. All rights reserved.
1. Introduction As challenges related to sources of energy are becoming more serious worldwide, the number of potential solar energy solutions is rapidly increasing (Ullah and Shin, 2014). While many aspects of modern society have negative impacts on the environment, buildings have a particularly significant impact due to the amount of energy that they use. Several studies have estimated that buildings are responsible for 20–40% of the total energy consumption in most developed nations (EIA, 2009; Pelegrini, 2010). To reduce the total energy consumption of buildings, among all current renewable energy options, the optical fiber-based daylighting system is the most easily integrated with LED lighting, which can replace conventional artificial lighting. Optical fiber daylighting systems are a unique and innovative means of bringing direct sunlight into a building by collecting natural light and channeling it through optical fibers to the luminaires within the space. This technology has the ability to bring sunlight much deeper into buildings. As products become commercially available and more economically viable, these systems have the potential to conserve significant amounts of energy and improve the quality of indoor ⇑ Corresponding author. E-mail address: [email protected] (S. Shin). http://dx.doi.org/10.1016/j.solener.2016.06.064 0038-092X/Ó 2016 Elsevier Ltd. All rights reserved.
environments. Advancements of this technology have, however, been delayed due to the fact that it is very high cost (Lingfors and Volotinen, 2013; Rosemann et al., 2008). Basically, optical fiber daylighting systems are comprised of three main components: the sunlight collector with a tracking mechanism, optical fibers and associated connections, and luminaires that distribute light within the space (Kribus et al., 2000). The heart of optical fiber daylighting system is the sunlight collector and sun tracking system. Through the research and development of many public and private groups, two basic collector designs have proven to be the most effective and reliable. The first uses Fresnel lenses to refract and concentrate sunlight into optical fibers, while the second captures incoming light by the reflection of parabolic mirrors. However, in order to collect light efficiently into the optical fiber, it is necessary to concentrate the sunlight with a high concentration ratio, which requires very high-quality optical components, alignment, and a precise electromechanical tracking system. Most of existing solar collectors for daylighting applications that satisfy these requirements are not cost-effective and are difficult to implement widely; thus, they have not experienced success on the market to date (André and Schade, 2002). Fig. 1(a) illustrates a schematic of a commercial solar fiber optic lighting system SP3 (the third generation of the fiber optic solar light systems from Parans company). The collector consists of 36 Fresnel
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Fig. 1. (a) Schematic of a commercial solar fiber optic lighting system; (b) detailed structure of one element inside the collector, which is the array of Fresnel lenses. Source: http://www.parans.co.
lenses (6.5 cm 6.5 cm) made of silicone imprinted onto the inside of the glass panel in front of the collector (Ae et al., 2014). Each lens focuses solar light into an optical fiber as shown in Fig. 1(b). The conventional compound parabolic concentrators (CPCs) are popular non-imaging concentrators that are relevant for solar energy collection that is used for absorbers such as photovoltaic or thermal energy applications (Rabl, 1976a). Basically, the CPC structure can be used to collect sunlight in optical fiber daylighting systems; however, its optical efficiency is not good because of the high-diverging, non-uniform output beam and low concentration ratio. In this study, we propose a novel optical fiber-based daylighting system using modified CPCs (M-CPCs). With this specialty design, the concentration ratio can reach high enough and the output light is collimated for optical fiber coupling purposes. This makes it possible for a very simple sunlight-collecting system including an array of M-CPCs connected to optical fiber bundles to replace conventional, more complicated collectors that consist of the confocal lenses or parabolic imaging systems. The proposed design has some advantages, such as lower costs and higher tolerance, thus eliminating the need for high-quality optics, alignment, and a highly precise tracking system. The aim of this designfocused research was to design, analyze, and optimize a novel compact non-imaging optical component, namely an M-CPC, that can lead to the development of high-efficiency and low-cost solar daylighting collectors and thus facilitate the viable commercialization of the cost-effective mass production of daylighting systems. The remainder of the paper is organized in the following manner: Sections 2 discusses the M-CPC concentrator geometry and optical fiber coupling approaches. In Section 3, the optical fiber daylighting system based on M-CPCs is modeled in LightToolsTM software (Synopsys Inc., California, USA) to evaluate the performance of such a system (Bouchard and Thibault, 2014; Pei et al., 2012; Vu and Shin, 2016). This section also details the fabrication and operation of an initial working prototype. Section 4 presents a proposed optical fiber daylighting system using M-CPCs and discusses the expansion of the system economically. Finally, brief concluding remarks and possibilities for future work are included in Section 4. To the best of our knowledge, the optical fiber daylighting system using M-CPCs that is described here is the first optical fiber daylighting system with cost-effective potential when manufactured at high volumes.
The basic idea for the M-CPC was inspired by the concept of integrated optics (Hunsperger, 2009). By reviewing the current literature on solar concentrators of existing optical fiber daylighting systems, we established some preliminary requirements for the design and development of the M-CPCs, including a single integrated optical element, a high concentration ratio, high tolerance, the possibility of being integrated with an optical fiber, and low costs including manufacturing and maintenance. The M-CPC concentrator that is presented here utilizes double CPCs as a means to capture, concentrate, and collimate the direct sunlight and couple it into an optical fiber. The idea is based on a three-dimensional (3D) CPC that has been used for many different applications, from high-energy physics to solar energy collection (Winston et al., 2005). To modify the CPCs for our purpose, we first reviewed the theory of conventional CPCs. A symmetrical CPC consists of two identical parabolic reflectors that funnel radiation from the aperture to the absorber (Rabl, 1976b). According to Foster and Ghassemi (2009), a 2D CPC is composed of two truncated parabolic reflectors; neither one keeps its vertex point, but both rims must be tilted toward the sun. Fig. 2 (a) shows the 3D-CPC geometric structure. Its surface is formed by rotating a 2D-CPC. Fig. 2(b) is a cross-section of a 3D-CPC. It describes the geometric parameters of a 3D-CPC. They are overall length L, input aperture diameter Din, acceptance input angle hin, output diameter Dout, and output angle hout. Fig. 2(b) also shows the geometric relationship between two parabola segments for the construction of a 3D-CPC. The points FA and FB are focuses of Parabolas A and B, respectively. The output aperture rim can be called by the focal rim of CPC. The relationship between these parameters and the concentration ratio are determined by the following equations (Winston et al., 2005):
L¼
ðDin þ Dout Þ 2 tan hin
ð1Þ
2 sinhout sin hin
ð2Þ
CR ¼
Here CR denotes the concentration ratio. From Eqs. (1) and (2), we see that the enhancement of the concentration factor leads to an increase of the object length. In addition, a higher concentration ratio corresponds with a lower angular acceptance input angle of
2. M-CPC concept This section introduces the conceptual design and working principles of a non-imaging solar collector, namely, the M-CPC.
Fig. 2. Schematic representation of a 3D-CPC: (a) 3D structure generated by LightTools software; (b) all geometric parameters related to the structure of the 3DCPC.
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the system. However, since CPCs are attached to the sun-tracking system, the input sunlight is always parallel to the main axes of the CPCs. The output angle hout should be 30° to suit the angular aperture of the optical fiber. Fig. 3 shows the structure of the 3DCPC with a high concentration ratio, CR = 100; the shape becomes very slim with a CPC length of L = 219.72 mm, and the input acceptance angle hin is decreased to 2.86°. From ray tracing in LightToolsTM to analyze the output beam of this high-concentration 3D-CPC, we found that the light was focused from a large area to a small area; however, the output beam become highly divergent and non-uniform. This is not suitable for coupling with the optical fiber. To overcome this problem, a novel M-CPC was proposed. M-CPC is an optical component that is comprised of two confocal CPCs, as shown in Fig. 4(a). The primary CPC is designed with a high input area and a high concentration ratio to capture direct sunlight to the maximum level. While the primary CPC uses its major aperture as an input to harvest solar energy, another CPC is placed inversely for the purpose of being used as a collimator and uses its minor aperture as an input and captures the total output light from the primary CPC. The collimated CPC should be designed to achieve a high degree of collimation to facilitate an effective fiber-coupling performance (Kong et al., 2011). Some main parameters of the M-CPC are depicted in Fig. 4(a). They are input diameter Din, output diameter Dout, length of primary CPC L1, and length of collimated CPC L2. The overall length of the MCPC is L = L1 + L2. The diameter of the conjunction area between the primary and collimated CPCs is DF. DF corresponds to the diameter of the focal region. Two CPCs still keep intrinsic parameters of CPCs; however, for the best coupling between two CPCs, the minor aperture sizes of two CPCs need to be the same and the minor aperture angles need to be fixed to the same 90°. The concentration ratio of M-CPC is defined by the ratio between the input and output areas,
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Fig. 4. (a) Design of M-CPC enclosure with geometric parameters and (b) the result of the ray tracing analysis performed on the M-CPC.
limation properties of an M-CPC can be explained by the characteristics of the confocal optical system. This M-CPC concept introduced here fulfills the requirements for a concentrator of an optical fiber daylighting system, such as a high concentration ratio and an enough collimated output light for optical fiber coupling. An optimized structure for the M-CPC is considered in depth in next part of this paper. 3. Optical simulation and experimental results
CR ¼
Din Dout
2 ;
ð3Þ
where CR denotes the concentration ratio. Since the output of MCPC is coupled with a POF, the output diameter is fixed with the same size of the diameter of the POF. To ensure a simple assembly in the fabrication process, the output of collimating CPC can be designed as a form of fiber optic ferrule for POF coupling. POF coupling can be made simpler by injecting the fiber into the output of M-CPC and fixing it. Optical alignment manipulation is therefore eliminated. The optical performance of the M-CPC design was analyzed with Monte Carlo ray tracing using LightToolsTM. The collimation of the output beam was realized by adding an inversely placed CPC with an appropriate design as shown in Fig. 4(b). Since the minor aperture rim of a 3D-CPC is a focal rim, the structure of M-CPC with two CPCs has a joint focal rim, which forms a confocal CPC system like a confocal lens system. The concentration and col-
Optical modeling plays a crucial role in the efficiency evaluation of an optical system. Commercial optical modeling software, LightToolsTM, was used to design the geometrical form of M-CPCs. We analyzed the efficiency of the system and found that there was some light loss due to the intrinsic geometrical structure of the M-CPC and the optical fiber coupling. To address this issue, the relationship between the structure and each geometric parameter needed to be modeled correctly. Optical fibers were utilized to deliver sunlight to the interior with small losses. Silica optical fibers (SOFs) are known to be good light-transmission media and have the best resistance to heating; however, SOFs are expensive. Plastic optical fibers (POFs) have substantially higher attenuation coefficients than SOFs, but POFs are preferred in daylighting systems due to their lower cost, tighter minimum bend radius, ease of installation and durability for complex wiring in buildings (Han et al., 2013; Ullah and Shin, 2014). In this research, we employed POFs from Anchor Optics (‘‘Anchor Optics,” n.d.). The POF parameters are listed in Table 1.
Fig. 3. High-concentration 3D-CPC and ray tracing analysis for the output beam when input direct sun light is parallel to the main axis of the 3D-CPC.
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Table 1 POF parameters for design, simulation and experiment. Parameters
Normal core POF
Core/cladding diameter Refractive index: core/cladding Minimum bend radius Spectral trans. range
1.960/2.0 mm 1.492/1.402 50 mm 380–750 nm
We placed a commercial POF with a diameter of 2 mm at the output of the M-CPC. In other words, the output diameter of the M-CPC was fixed at Dout = 2 mm. A concentration ratio of 100 was chosen. This was reasonable ratio because although a CPC can be designed to achieve a very high concentration ratio to facilitate good sunlight collection, it would require very large physical dimensions relative to a given input aperture size. With a fixed input size of Dout = 2 mm and a concentration ratio of CR = 100, the input diameter Din = 20 mm was determined using Eq. (3). As previously explained, an M-CPC is an integrated structure with two CPCs whose minor apertures are put together, and the joint part is named by the focal diameter, DF. DF is a very important parameter, as the other parameters like the length of the M-CPC, the input angle, the output angle, and the efficiency can be determined by DF. Based on some initial conditions such as Din = 20 mm, Dout = 2 mm, concentration ratio CR = 100, we generated the M-CPC structures with several different focal diameters using LightToolsTM, ranging from 0.5 to 1.5 mm in the increment of 0.1 mm. The dependence of the input angle hin, the primary CPC length L1, the output angle hout, and the collimated CPC length L2 on the focal diameter is shown in Fig. 6(a) and (b). The primary CPC length decreased significantly with an increased DF, and the collimated CPC length became much shorter as DF increased. Since the optical conduit was based on commercial POF whose cladding diameter (D) was 2 mm, core refractive index (n1) was 1.49, cladding refracqffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi tive index (n2) was 1.39, numerical aperture (sin hc = n21 n22 ) was
Fig. 5. Illustration of the structure for the simulation and ray-tracing analysis for design verification.
0.3, and half acceptance angle (hc) was 30°, the focal diameter can be calculated theoretically DF = 1 mm. Although, the parameters of M-CPCS can be calculated based on the theory, the optical fiber coupling is a complicated mechanism and cannot be predicted theoretically. The simulation was carried out to verify the theoretical predictions and calculate optical efficiency. The simulation scheme is described in Fig. 5. For the simulation, M-CPC was assumed to have a 100% inner surface reflection coefficient. Two optical receivers were placed at the input mouth and at the end of the POF to evaluate the optical losses. A parallel monochromatic light source worked as a direct sun light source. The wavelength of light did not affect the efficiency of the system because the principle of M-CPC is based on reflection. Since the influence of the wavelength on the numerical aperture (NA) of fiber was small, it was not necessary to perform a simulation with a total visible range of sunlight. We carried out a simulation with several different focal diameters, ranging from 0.5 to 1.5 mm in the increment of 0.1 mm. The coupling efficiency between M-CPC and POF is illustrated in Fig. 7. The efficiency geff was measured by the ratio between the total luminous flux at the input receiver and that at the output receiver, such as:
geff ¼
/out ; /in
ð4Þ
where / is denoted as the total luminous flux in the lumens. In the LightToolsTM software, the luminous flux was calculated by the integration of illuminance E over the total area of the receiver as below:
I /¼
E dS:
ð5Þ
The integration in Eq. (5) was performed by dividing the receiver area into grids, and the total lumens was the sum of the illuminance of all grid cells. This method produced some errors, which are shown as bars in Fig. 7. The loss of M-CPC and POF coupling due to the Fresnel reflection at the entrance surface of POF is essential. One interesting observation from Fig. 7 is that the efficiency was constant to 84%, when the focal diameter, DF, increased from 0.5 mm to 1.05 mm; it decreased particularly fast when DF was larger than 1.05 mm. This can be explained by referring back to the output angle hout and the NA of the POF. The critical value of DF = 1.05 mm corresponded to 30° of the output angle of M-CPC, which coincided with the half acceptance angle of the POF. Therefore, 1.05 mm is the optimal size for focal diameter DF to obtain the highest efficiency of 84% and short M-CPC length. The simulation
Fig. 6. Dependency of (a) input angle and primary CPC length and (b) output angle and collimated length on the focal diameter.
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Fig. 7. Variation of optical efficiency at different focal diameters DF.
Table 2 Optimal parameters of an M-CPC with a concentration ratio of CR = 100. M-CPC parameters
Value (Unit)
Primary CPC length Collimated CPC length Total M-CPC length Input angle Output angle Focal diameter
200 (mm) 2.47 (mm) 202.47 (mm) 3 (°) 31 (°) 1.05 (mm)
results, which were very close to the theoretical predictions, confirmed that the design of M-CPC adapts to daylighting purpose. All other parameters of an optimized M-CPC are listed in Table 2. For proper operation of the proposed optical fiber daylighting system, direct sunlight should always be normal to input aperture of M-CPC. This is a difficult task since the position of the sun is always changing, and this led us to use a sun tracking system. The required accuracy of the sun tracking system is determined by the solar concentrating collector’s angle of tolerance. The tolerance of the system is the acceptable angular deviation of the sunlight direction from the two main axes of the system, within the allowable efficiency loss. It is defined as the angle where the efficiency drops by 10%. The acceptance angle determines the required accuracy of the tracking system mounted upon the concentrator (Xie et al., 2014). We performed a simulation for three cases—MCPC, conventional CPC, and Fresnel lens—to evaluate the advantage of M-CPC over conventional CPC and existing systems. The concentration ratio of CPC and M-CPC was the same at 100. We examined the efficiency with a different angular deviation of sunlight direction from 6 to +6°. The optical efficiency of M-CPC and CPC when coupled with POF depend on angular deviation is plotted in Fig. 8. The red line and triangular denotes M-CPC, blue line and rectangular denotes conventional CPC. In the case of M-CPC and POF coupling, the efficiency was 84%, and this did not change within the range of angular deviation from 3 to +3°, but it decreased dramatically when the angle became larger than ±3°. In the case of coupling between CPC and POF, the maximum efficiency was 75.2%. The efficiency decreased significantly from 75.2% to 55% when the angular deviation increased from 0 to ±2.5°, increased slightly when the angular deviation was from ±2.5 to ±6°, and decreased dramatically when the angular deviation became larger than ±6°. Although the acceptance input angle of CPC was only 2.8°, the light still escaped from the output of CPC when the angular deviation was from ±2.5 to ±6° because the rays inside the CPC make two reflections on average. This does not appear in the M-CPC because the primary CPC’s output aperture is very small. The lower and unstable efficiency of CPC and POF coupling in comparison with
149
Fig. 8. Variation of optical efficiency at different angular deviation in three cases: M-CPC, conventional CPC and Fresnel lens.
M-CPC and POF coupling highlights one of the important characteristics of 3D-CPC, namely the very high nonhomogeneity in light distribution at the output aperture. The collimated CPC of M-CPC also works as a light mixer to redistribute the light on the exit area. This is the reason for the higher and more stable optical efficiency of the M-CPC and POF coupling. To evaluate the performances of M-CPC in comparison with existing devices, we performed another simulation for an optical fiber daylighting system based on Fresnel lens. Some parameters of Fresnel lens and optical fiber were based on SP3 system – the third generation of fiber optic solar light systems from Swedish Parans company (http://www.parans.com). The Fresnel lens has size of 65 mm 65 mm). Plastic optical fibers are used with a core diameter of 1 mm. The simulation results are shown in Fig. 8 by green line and circle. For daylighting systems using Fresnel lenses, the dispersion of the visible range is a problem, as this leads to an essential decrease in optical efficiency. As seen in Fig. 8, Fresnel lens system can achieve an optical efficiency of 77%. Although, optical efficiency is lower than M-CPC but using Fresnel lens as a concentrator can lead to high concentration ratio (CR > 3500). The advantage and disadvantage of high or medium concentration ratio that affect on system cost will be discussed in more detail in discussion part. The angular tolerance of the concentrator is defined as the angular deviation, within which the optical efficiency drops to 90% of its maximum value. The tolerance of conventional CPC, M-CPC, and Fresnel lens can be determined based on Fig. 8 and is ±1.3°, ±2.9°, ±0.25°, respectively. The high tolerance of M-CPC suggests that low accuracy sun tracking system can be utilized and this reduces the system cost (Xie et al., 2014). According to above description, a prototype of M-CPC was made by laser cutting. Inside of M-CPC was mirror coated with evaporated aluminum. Fig. 9(a) shows a structure design generated by LightToolsTM and Fig. 9(b) is a fabricated sample. The experimental configuration is shown in Fig. 10. The M-CPC was mounted with a dual axis sun tracking system whose angular tolerance is less than 3°. A 5 m length of POF was attached in the M-CPC output aperture. The degradation due to ultraviolet (UV) radiation and temperature is a problem in plastic material such as POFs. We suggested using UV protection thin film at input aperture of M-CPC to eliminate the UV wavelength (Mayhoub, 2014; Sapia, 2013) as shown in Fig. 10. The illuminance from the sunlight was measured at different times of the day. The site of application was located at 127° longitude and 37.5° latitude. Here we look at the illuminance on a sunny day. The measured illuminances of the input flux at the surface of the M-CPC and of the luminous flux at POF endface are listed in Table 3. The prototype system reached an optical efficiency of
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N.-H. Vu, S. Shin / Solar Energy 136 (2016) 145–152 Table 3 Average illuminance at different times of the day and luminous flux at POF output for the proposed system.
Fig. 9. (a) A M-CPC structure generated by LightToolsTM and its cross sectional view; (b) fabricated M-CPC and its cross sectional view.
Fig. 10. A prototype M-CPC was used to collect sunlight in an outdoor setting. Insert shows the light exits at the end of POF.
65% including: M-CPC intrinsic loss, non-ideal of mirror coating, optical fiber coupling and optical fiber transmission loss for 5 m. The last column in Table 3 shows the luminous flux at POF endface that was calculated based on simulation results (the loss of 5 m length of POF was also included in calculation). Our experimental measurements were in close agreement with our optical model and support the notion that M-CPC design is reliable and the system would perform the work with high efficiency. The temperature measured at M-CPC/POF coupling region is around 35 °C, which shows that the heat is not a problem in the proposed M-CPC.
Time
Sunlight illuminance (lux)
Luminous flux on the M-CPC input (lm)
Luminous flux at POF endface (lm) (by experiment)
Luminous flux at POF endface (lm) (by calculation)
6 AM 7 AM 8 AM 9 AM 10 AM 11 AM 12 PM 1 PM 2 PM 3 PM 4 PM 5 PM 6 PM
20,105 39,969 60,051 79,989 95,005 105,007 110,000 102,023 94,005 80,989 59,958 40,125 19,800
6.3 12. 7 18.9 25.1 29.8 33.0 34.5 32.0 29.5 25.4 18.8 12.5 6.2
4.13 6.2 10.3 16.3 19.3 21.4 22.5 20.8 19.2 16.5 10.25 9.2 1.7
4.2 8.4 12.7 16.9 20.0 22.2 23.2 21.5 19.8 17.1 12.7 8.5 4.2
nent (lenses or reflectors) to focus sunlight and a secondary optical element for flux collimation. The requirement for the alignment between the primary optics, secondary optics and optical fiber bundles is very tight. The primary purpose of the present work is to find a low cost and highly efficient way to utilize the solar energy for daylighting. In this research, we proposed a costeffective approach for an optical fiber daylighting system using non-imaging optic devices such as M-CPCs. The primary collector is consisted of a 10 20 M-CPCs array. 200 M-CPC are required and the size of concentrator is 540 mm 270 mm 212.34 mm as shown in Fig. 11(a). Two hundred pieces of POFs were utilized as the sun light conduits. At the highest elevation (zenith) angle of sun, each M-CPC can provide 22 lux for interior lighting (Table 3) so total system can achieve 4400 lm. For the typical office building, average illuminance is required about 500 lux. This proposed system can cover a surface area of 8 m2. To distribute daylight at the destination, 200 pieces of fibers were arrange into twenty bundles and then placed at different positions to cover surface area of 8 m2. Total required POF length is 1000 m. Optical fibers in each bundle were organized into a circular shape. A commercial end fitting for POF using ball lens is best choice for the light distribution in term of cost efficiency. For simulation purpose, we used some parameters of a commercial POF end fitting from Shenzhen Everzone Technology Co., Ltd. (China) as seen in Fig. 12(a). Fig. 12(b) shows the ray tracing performed by optical fiber bundle and the end fitting using LightToolsTM. The room interior was designed in DIALuxTM and the light distribution modules were imported into DIALuxTM to show the room interior under lighting simulation. Twenty fiber bundles with end fitting were arranged in 4 5 array. Interior view of the room under lighting simulation is shown in Fig. 13(a). We have achieved the required illuminance value and uniformity on the floor by using this approach (Fig. 13(b)).
4. Proposed optical fiber daylighting system and discussion Optical fiber daylighting systems are concerned more about delivering daylight into remote and windowless spaces in buildings. Many types of optical fiber daylighting systems have been developed with various technologies and solutions over a few decades, but few of them have been successfully commercialized. Providing a cost effective system is the major challenge for optical fiber daylighting system. The concentrator parts of typical optical fiber daylighting systems consist of a large primary optical compo-
Fig. 11. Proposed a sunlight concentrator for optical fiber lighting system based on M-CPCs array.
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As discussed in the introduction, to be competitive, the M-CPCs must be fabricated using a low-cost process. According to this goal, they must be manufactured using plastic material. The advantage of using plastic to fabricate the M-CPC is the capacity to mold the system, which is a low cost solution for mass production. Fig. 14 shows a proposed injection molding method to fabricate the M-CPCs. Each half of M-CPC is fabricated by molding method and then makes the mirror coating inside. Two half of M-CPC are glued together to become a compact one. Recently, a recognized effective design strategy led to the simplification of product/system design and reduced the number of stages in manufacturing process (Edwards, 2002; O’Driscoll, 2002). Reducing the use of materials will also contribute to reduced manufacturing costs, supplier management costs, and recycling costs. To achieve these goals, the one component, one material, one manufacturing process concept (Pelegrini, 2010) are necessary for the design. The optical fiber daylighting system using M-CPC concept proposed here meets all of these requirements. M-CPC and POF are made of single material – plastic. M-CPC fabrication can utilize a simple manufacturing process, such as injection molding, which is suitable for mass production. These features have resulted in a reduction of manufacturing costs. For an estimate of the cost of whole proposed optical fiber daylighting system, we make the following considerations: the cost of one M-CPC in mass production is estimated by $5 so 200 M-CPCs have cost of $1000. The cost of mechanical bracket is $100. The UV film is about $30. The cost of the POFs is about $0.5/m for the 2 mm core POFs so 5 m long POF bundles (1000 m of POF) cost is $500. Automatic dual axis sun tracking system with low accuracy
Fig. 12. (a) An optical fiber bundle with end fitting as sun light distributer in the interior and (b) ray traycing.
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Fig. 14. Proposed a simple injection molding method to manufacture the M-CPC for mass production.
(<3°) has cost of $400, roughly. The cost of twenty end fittings is $20. Therefore, total expected cost of the proposed system is $2050. We can evaluate our proposed system in terms of daylight collection capability and cost effectiveness by comparing our system with two other commercialized systems: Himawari and Parans systems. Due to variety of operating conditions, system comparison is not easy, however we used the data from developers’ publication so that we can estimate our proposed system (Vu and Shin, 2016). The Himawari system collects and concentrates sunlight using multiple sun tracking Fresnel lenses. Light is transported by quarts optical fibers. The total luminous flux of a Himawari system is 4000 lm which was measured under direct sun illuminance of about 100,000 lux. The net price of a Himawari system package (including 12 lens collector, two 5 m long optical bundles) is $6240 (‘‘HIMAWARI Solar Lighting System,” n.d.). The Swedish Parans system consists of an array of Fresnel lenses which simultaneously tracks and concentrates daylight. Paran system’s total output flux is about 3540 lm, which was measured under direct sun illuminance of 100,000 lux, 10 m away from the collector. Parans systems package (six 5 m long optical fiber bundles) net price is about $5425 (‘‘PARANS Product Information,” n.d.). The comparisons associated with the daylight collection capability and cost have been summarized in Table 4. As shown in the table, our proposed system is about 2.5 times cheaper than the other previous systems. This is reasonable because M-CPC fabrication cost is much cheaper than Fresnel lens which has micro-structure on the surface. Our proposed system has lower concentration ratio in comparison with previous ones so that it required more optical fiber. Although the cost for POFs increases but nowadays optical fiber cost has become cheaper so it is acceptable. However, high concentration ratio in conventional system requires very high quality of optical devices, and sun tracking system. The alignment
Fig. 13. (a) Simulation of the daylight illuminance distribution at the test site and (b) light distribution on the floor.
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Table 4 Comparison in cost and daylight collection capability with some commercialized optical fiber daylighting systems. Comparison categories
Proposed system
Himawari system
Parans system
Output flux (lm) Cost ($)
4400 2050
4000 6240
3540 5425
requirement between optical components is also very tight and the alignment becomes degraded fast when they operate under outdoor condition. Heat is also a problem in high concentration system and requires more cost for treatment. High condensed light in optical fiber needs a complicate and lossy optical system to distribute the light in the interior. Due to the simplicity in material, shape, form, function, manner of operation, assembly, and usage, and cost efficiency of the proposed system using M-CPCs, apparent and obvious commercialization of fiber optic daylighting system can be realized. 5. Conclusion An optical fiber daylighting system using M-CPCs has been designed and discussed with the purpose of saving the energy consumed by electric lighting. To explore the practical performance of the proposed system, a sample optical system was modeled and simulated with LightToolsTM. The simulation results indicate that 84% of optical efficiency was achieved at Cgeo = 100 for the proposed M-CPC. In addition, the tolerance (acceptance angles) also was analyzed. By using M-CPCs as the primary concentrator, an acceptance angle of ±2.9° was achieved. This allows us to use a lower accuracy sun tracking system as a cost effective solution. A prototype of M-CPC was fabricated using laser cutting method. The experimentation under real conditions was implemented to verify the accuracy of the simulation and the commercial viability of the system. This study is the first to use a non-imaging device M-CPC for an optical fiber daylighting system. It shows great potential for the commercial and industrial scale daylighting field. In the future, we will try to implement a completed system and install it in the real building office to evaluate all aspects of the system. Conflict of interest The authors declared that there is no conflict of interest. Acknowledgements This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIP) (No. 2014R1A2A1A11051888).
References Ae, A.P., Kumar, M.R., E, A.P., 2014. A review of solar fiber optic lighting systems. In: International Conference and Workshop on Electronics and Telecommunication Engineering, pp. 519–522. Anchor Optics [WWW Document], n.d.