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Design method of a planar solar concentrator for natural illumination
Solar Energy 194 (2019) 554–562
Contents lists available at ScienceDirect
Solar Energy journal homepage: www.elsevier.com/locate/solener
Design method of a planar solar concentrator for natural illumination a
a
Jiaqi Lv , Xiping Xu , Peng Yin a b
a,b,⁎
T
College of Optoelectronic Engineering, Changchun University of Science and Technology, Changchun, Jilin 130022, China School of Engineering, Monash University Malaysia, Jalan Lagoon Selatan, 47500 Bandar Sunway, Selangor, Malaysia
A R T I C LE I N FO
A B S T R A C T
Keywords: Nonimaging optics Natural illumination Concentrators Solar energy
This paper presents a design method of a natural light indoor illumination system using a planar solar concentrator as a daylighting module which can obtain high optical concentration ratios. In this design, the concentrator is composed of lens arrays, hemisphere-coupling structures, a lightguide, plate of beam splitters (PBS) and photovoltaic cells. The concentrated rays are divided into two parts by PBS, the visible rays for indoor illumination and the invisible rays for energy store. The converted electrical energy from invisible rays can drive the light emitting diodes (LEDs) at the entrance of the lightguide for illumination compensation in the condition of overcast and raining. Simulation results show that the proposed natural illumination system can achieve the illuminance more than 500 lx, which meets the commercial illumination requirement. In addition, the effects of collector width and parabolic coefficient on optical concentration ratio and concentration-acceptance product are discussed in detail. Finally, a prototype concentrator is constructed to demonstrate the viability of the proposed natural illumination system. The experimental measurements are in close agreement with simulation results. To the best of our knowledge, this is the first time that planar solar concentrators have been applied in the indoor illumination system, and our results can provide new insight into possible designs of novel natural illumination.
1. Introduction
Sangani and Solanki (2007) designed a single-v groove concentrator with a theoretical geometrical concentration ratio of 2 and carried out a concentrator experiment on a conventional monocrystalline silicon battery module. The results showed that the power was about 40% higher than that of a flat plate battery module. Although this design avoided tracking the sun, the trough concentrator was easily affected by the wind load to produce displacement deformation and reduce the performance (Zang et al., 2014). Lovegrove et al. (2011) proposed parabolic dish solar concentrators which contain metal laminated mirrors on glass. The solar concentrators have a 13.4 m focal length and 380 identical spherical mirror panels of 1.17 m × 1.17 m, which leads to a uniform spot. Hornung et al. (2011) tested the relationship between the optical efficiency and module performance of Fresnel lenses. The Fresnel lenses of PMMA showed considerable energy acquisition efficiency. The concentrator required low tracking accuracy of the sun, but it was easy to produce machining errors in manufacturing process. Araki et al. (2008) reported a photovoltaic device for concentration. It consisted of cylindrical Fresnel lenses for primary focusing, cylindrical optical elements for secondary focusing, transparent resin materials and solar cells. When the concentration ratio was high enough, additional heat dissipation devices need to be added behind the photovoltaic cells. The hybrid concentrator combines reflection, refraction and total
Along with economic development, the improvement of mankind energy demand year by year. Renewable energy plays a vital role in human society because of its low environmental impact and unfailing generation (Whang et al., 2009). With the introduction of low-carbon concepts and green lighting ideas, solar fiber illumination system has been widely researched and becomes a research hotspot owing to the simple structure, easy installation, low cost and favorable lighting effect (Chiang et al., 2009). In the past few decades, many scholars have researched a lot on optical fiber guiding system (Ullah and Whang, 2015; Kandilli and Ulgen, 2009). For example, Sapia (2013) developed a hybrid concentrating photovoltaic system including a major parabolic concentrator and a secondary planar optical reflector. Ullah and Shin (2014) proposed highly concentrated optical fiber-based daylighting systems for multi-floor office with two methods. The first method included a parabolic trough and the second method contained a linear Fresnel lens. It is not difficult to find that, as a light collecting device for natural illumination, the design of solar concentrators directly determines the illumination performance (Tsai et al., 2013). Solar concentrators can be divided into three types: reflectors, transmission lens and hybrid concentrators (Chong et al., 2013). ⁎
Corresponding author at: College of Optoelectronic Engineering, Changchun University of Science and Technology, Changchun, Jilin 130022, China. E-mail address: [email protected] (P. Yin).
https://doi.org/10.1016/j.solener.2019.10.056 Received 24 April 2019; Received in revised form 19 October 2019; Accepted 23 October 2019 0038-092X/ © 2019 International Solar Energy Society. Published by Elsevier Ltd. All rights reserved.
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J. Lv, et al.
internal reflection to focus light. The large array of collectors enables this type of concentrator to have a high geometrical concentrator ratio (Wu and Chu, 2013). Hybrid concentrators usually include three parts: primary optical elements, secondary optical elements and a lightguide. Moore et al. (2010) put forward a design method of the planar solar concentrator, and the dynamic concentration ratios can be achieved by changing the number of the primary optical elements. Unger et al. (2010) improved the structure of the lightguide and proposed a planar solar concentrator. However, the incident rays were reflected by air wedges repeatedly, which led to energy attenuation, low optical efficiency and short work distance. Karp (2010) optimized a flat-type concentrator, where the sunlights were focused by the spherical lens array and transmitted to the photovoltaic cells at the end of the lightguide. Bouchard and Thibault (2012) replaced the spherical lens array with a cylindrical lens array, which had the advantage of simplifying the tracking strategy, but the optical efficiency was unsatisfactory. Therefore, it has become an urgent problem to propose a kind of solar concentrator as the concentration module of natural illumination with high concentration ratios, simple manufacturing and high optical efficiency. This paper presents a design method of an indoor illumination system based on a planar solar concentrator and establishes the mathematical model of sunlights from concentration to propagation. The designed indoor illumination system can realize large area illumination with long runtime and low power consumption.
through the light emitter. The other part of system part is active lighting module which includes optical sensors and LED arrays. When solar energy is insufficient, the optical sensors can detect and drive the LED arrays for compensation. Under the cooperation of the natural light guiding system and active lighting module, the output illumination can be maintained on a stable level. The proposed natural illumination system can be used in basements with an efficient transmission distance of 20 m as shown in Fig. 2. On cloudy or rainy days, the illumination system still brings enough rays into the room by LED compensation system from dawn till dusk. Although the solar energy is inexhaustible, the effective utilization of solar energy is still very meaningful, and the proposed idea of natural illumination system can reduce the leakage of rays and increase the utilization of solar energy.
2. Design concept and model principles
Cg =
2.2. Design of concentrator As the most important part of the natural illumination system, the planar solar concentrator consists of three parts: collector arrays, coupling structures and a lightguide. The incident sunlights can be concentrated by collector arrays, get into the lightguide by coupling structures and propagate by multiple reflection times in lightguide. During the process, two important parameters of geometrical concentration ratio and the optical efficiency are introduced (Karp, 2010), which can be expressed as follows:
2.1. System structure
Sunlight vertically incident area Lightguide length = Lightguide exit port area Lightguide depth
(1)
and
The natural illumination system includes input part, system part and output part as shown in Fig. 1, and the system part is the core of the illumination system. In system part, the natural light guiding system works by capturing sunlights through a concentrator, propagating sunlights through the light transmitters and outputting sunlights
η=
Energy at lightguide exit port Vertically incident sunlight energy
(2)
For collector design, the profile of the paraboloid surface of the lens can be regarded as a parabola expression, as shown in Fig. 3(a):
Fig. 1. Structure diagram of the natural illumination system. 555
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Fig. 2. Illustration of natural illumination system.
(a)
(b)
(c)
(d)
Fig. 3. (a) The double parabolic curves after removal of bottom, (b) the double paraboloid structure, (c) the remaining part after outer edge cropped and (d) the collector structure.
y = az2
Fig. 4. Schematic diagram of height of concentrator after removing the sphere part.
(3)
where a is a coefficient. The incident rays can be concentrated at (y = 1 , z = 0), and the part under the focus surface needs to be re4a moved. After moving a copy of the parabola at a distance of 1/2a towards minus z axis, a double paraboloid structure can be established as shown in Fig. 3(b), where the height of the collector is h. To avoid some leakage from the outer edge of the double paraboloid structure, the double paraboloid structure is tailored with the remaining part width of D, as shown in Fig. 3(c). To prevent the rays from leaking out of the lower surface of the double paraboloid structure and the lightguide, the collector height should be limited as follows:
3/4a ≤ h ≤
2 (n1 +
n12 − 1 )/(n1 − 4a
n12 − 1 ) − 1 (4)
where n1 can be the refractive index of the lightguide. To make the collector lighter and separate it from the lightguide, the lower surface of the collector can be tailored as shown in Fig. 3(d). The radius of the tailored part can be expressed as:
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Fig. 5. Hemisphere-coupling structure on the lightguide.
Fig. 6. PBS transmits visible light and reflects invisible light onto photovoltaic cells.
Fig. 7. Schematic diagram of the coupling of the optical fibers and the lightguide outlet. Fig. 8. The rays from the LED is converged by CPC.
R=
h2 −
h 1 h 1 17 − · + 2 + 2a a a 4a 16a2
of the concentrator will be reduced accordingly. Suppose that the lowest point coordinate of the external paraboloid can be 1 1 + Δz, 4a + Δh , where Δz and Δh can be the distances that the 2a
(5)
(
As shown in Fig. 4, the blue dotted line indicates that the radius R of the cut sphere is exactly 1/2a. When R is greater than 1/2a, the height
)
paraboloid moves along the z axis and y axis after being cut, 557
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Fig. 9. Schematic of the planar solar concentrator for natural illumination system.
Table 1 Average illuminance (Whang et al., 2009) and luminous flux at different times of the day. Time
Sunlight illuminance (lx)
Luminous flux on the surface concentrator (lm)
6 AM 7 AM 8 AM 9 AM 10 AM 11 AM 12 PM 13 PM 14 PM 15 PM 16 PM 17 PM 18 PM
30,000 60,000 90,000 120,000 150,000 157,000 165,000 170,000 168,000 130,000 95,000 45,000 35,000
67,500 135,000 202,500 270,000 337,500 353,250 371,250 382,500 378,000 292,500 213,750 101,250 78,750
yR −
1 = Δh 4a
(7)
According to the triangle Pythagorean theorem, the following formula can be obtained: 2 1 Δh2 + ⎛ + Δz⎞ = R2 ⎝ 2a ⎠
(8)
From Eqs. (6)–(8), we can derive Δz and Δh :
2 Ra − Δz =
Δh =
1 4
−1
2a
(9)
2Ra − 1 2a
(10)
In this paper, a = 0.01 and R = 50.5 are selected. To eliminate rays leakage from the lightguide, the rays should be coupled into the lightguide. In this paper, a hemisphere-coupling structure can be added to the surface of the lightguide to couple the rays into the lightguide, as shown in Fig. 5. The center of the hemisphere can be selected at the focal point of the outer paraboloid of the collector, and the radius of the hemisphere can be negligible. 2.3. Design of beam splitter, solar cell and optical fiber Solar spectrum is a continuous spectrum of different wavelengths which can be divided into visible and invisible light. Visible light has a wavelength of 400–760 nm, and invisible light is further divided into the infrared band of 760–5300 nm and the ultraviolet band of 290–400 nm. Usually, active or passive cooler is applied to solve the solar energy LED lighting system radiation problem, which means that complex structures and more spending are needed to introduce. For the illumination system proposed in this paper, the plate of beam splitter (PBS) produced by DiYPRO Co., Ltd. was selected to eliminate heat (Vu and Shin, 2016). PBS is a multilayer dielectric coated spectroscopically selective thermal mirror, which can refract visible light and reflect both ultraviolet and near-infrared light (NIR) at the same time. The PBS is placed between the lightguide and the collectors to reflect the invisible light to the solar cells which are placed at the focus of the reflected light, as shown in Fig. 6. In this paper, the solar cells can be selected by CuInSe owing to its sensitivity to ultraviolet and nearinfrared light. In the daylighting system, we choose the traditional plastic optical fiber which has the advantages of low cost, light weight and flexibility. It is worth noting that the effect of heat on the optical fiber should be considered in the case of high concentration ratio, but the use of PBS
Fig. 10. Lumious flux versus different times from 6 AM to 18 PM.
respectively. Substituting the coordinate into Eq. (3), another equation can be obtained: 2 1 y = a·⎛ + Δz⎞ 2a ⎝ ⎠
(6)
At this point, the y-coordinate yR of the paraboloid after cutting is equal to the sum of the y-coordinate of the focus and the distance Δh along the y axis. So it follows that: 558
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Fig. 11. The diagram of influence of collection width and parabolic coefficient on optical concentration ratio.
Fig. 12. The influence of the collector height on the optical concentration ratio and CAP value in which case the radii of hemisphere-coupling structures are: (a) r = 0.1 mm, (b) r = 0.2 mm and (c) r = 0.5 mm.
exactly solves this problem. The propagation of light in optical fiber is due to the difference of refractive indexes between core and cladding. According to the fiber transmission theory, the angle between the central axis and the incident light can be θC = arcsin(NA) , where NA can be the numerical aperture of the fiber (Zhang, 2016). Optical fiber bundles compose of multiple optical fibers which have the advantages
of wide transmission spectrum, image transmission, uniform spot distribution, easy shaping and free bending. Thus, we can directly couple the optical fibers into the exit area of the lightguide, as shown in Fig. 7.
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3. Daylight simulation Optical model plays an important role in optical system. Commercial optical modeling software LightTools is used to design and simulate the geometric structure of the planar solar concentrator used in the natural illumination system. In this paper, the material of collectors, hemisphere-coupling structures and lightguide is polymethyl methacrylate (PMMA) which is one of the most common optical plastics. The concentrator of the natural illumination system consists of a 30 × 30 array of collectors, hemisphere-coupling structures, a lightguide, LED array, PBS array and photovoltaic cells, as shown in Fig. 9. The solar tracking system is not shown. 3.1. Natural light of large scale system The concentrator is located at 127.0° longitude and 37.5° latitude at 12:30 p.m., when sunlight intensity reaches maximum. In order to obtain the vertical incident sunlight, the concentrator has a sun tracking device which can rotate the collectors towards the sun. The area of the concentrator upper surface is 2.25 m2 and the calculation formula of luminous flux in lumens can be (Vu and Shin, 2016): (11)
F=E×S
where E can be the solar illuminance, S can be the area of the concentrator upper surface, and F can be the input luminous flux. The average solar illuminance at different time of the day are listed in Table 1, where the data are from China Meteorological Administration (Benítez et al., 2010). Based on average illuminance and solar altitude; luminous flux on the surface of the concentrator can be calculated as input for the simulation. Typically, a commercial building requires 500 lx. Fig. 10 shows the luminous flux provided by sunlight and LEDs and average lighting requirements of commercial buildings at different time of the day. When the light is strongest at 13:00 p.m. and the lightguide thickness is 2 mm, the output luminous flux can reach about 50,000 lm. That is to say, the natural illumination system can light up 100 m2. Other than 13:00 p.m., sunlights provide less than 50,000 lm, in which case the LED light sources can be tuned on for compensation. The equation for calculating the total luminous flux of indoor illumination is as follows:
Fig. 13. The prototype of (a) a collector and (b) a lightguide. (c) The experimental instrument including a three-dimensional mobile platform, a LED source, a collimator and a test platform. (d) When properly aligned to the parallel rays, rays incident on the collectors surface couple into the lightguide and exit at the lightguide edge, appearing as a bright line. (e) The optical power meter with the type of JW3216.
T=
∫ Fdt
(12)
where t is the time of day; T is the total received flux in the day (Vu and Shin, 2016). According to Eq. (12), it can be calculated that the sunlight is capable to afford about 416,172 lm and the LED needs to offset about 233,828 lm every day. Apparently, sunlight supplies 64% in total luminous flux during a day, and a part of the energy driving the LEDs comes from the PBS. That is to say, external electric power contributes a minuscule proportion of the total natural illumination system, which saves energy tremendously.
2.4. Light emitting diode (LED) integration In this section, a hybrid LED sunlight lighting system is selected to achieve uniform indoor lighting in the condition of changing luminous flux at different times of the day. In this paper, we introduce chip-onboard (COB) light source which is directly attached to the mirror metal substrate with high reflectivity (Zhu et al., 2014). When the LED is connected to the lightguide, only about 5% of the coupling efficiency can be lost (Vu and Shin, 2016). This design uses optical epoxy resin to bond the substrate to the lightguide inlet. Five arranged substrates are equipped with LEDs. If the substrate is coupled directly at the entrance of lightguide, the incident light will leak from the edge of the LED. Therefore, a compound parabolic concentrator (CPC) structure can be designed and attached at the entrance of the lightguide, which reduces the emission angle of the LEDs, as shown in Fig. 8. When the luminous flux is insufficient, the LED light source can be turned on to compensate. LED light sources can be powered by invisible energy stored in solar cells or external power sources.
3.2. The effect of collector width and parabolic coefficient on optical concentration ratio Optical concentration ratio is the product of geometrical concentration ratio and optical efficiency, given in the following relationship (Karp, 2010):
Co = Cg × η =
S1 ×η S2
(13)
where Co is the optical concentration ratio, S1 is the receiving area of the concentrator upper surface and S2 is the area at the exit of the lightguide. Since the variation of the collector width and the parabolic coefficient can change the surface profiles of the collectors, different optical efficiencies can be acquired in condition of the same geometric concentration ratio. Fig. 11 depicts the optical concentration ratio versus different collector widths and parabolic coeffcients. For 560
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Fig. 14. The optical efficiency versus the position errors of the LED source along (a) X-axis, (b) Y-axis and (c) Z-axis.
with the increase of the collector widths. When the collector widths are small, the downward trend of contour lines is rapid, and when the collector widths are large, the rising trend of contour lines is gentle. 3.3. Concentration-acceptance product For natural illumination system, concentration-acceptance product (CAP) is also an important parameter to evaluate the performance of concentrators. Meanwhile, the value of CAP depends on the geometrical concentration ratios and the acceptance angles. The CAP can be calculated as follows (Benítez et al., 2010):
CAP =
Cg sinα
(14)
where α is the acceptance angle where the optical efficiency decreases to 90% of the peak value. In this paper, the effects of different concentrator heights on the optical concentration ratios and CAP under three kinds of hemisphere-coupling structure radius were tested as shown in Fig. 12. It's easy to see that the optical concentration ratios and CAPs decrease with the increasing collector height. Similarly, the optical concentration ratios decrease with the increasing radius of the hemisphere-coupling structures, which attributes to the increasing rayleakage for reduced efficiencies. However, the CAPs increase with the increasing radius of the hemisphere-coupling structures, which means the increase of the acceptance angles.
Fig. 15. The optical efficiency versus the geometric concentration ratio with different LED source type and in simulation.
convenience, the length of the lightguide and the thickness are set as 500 mm and 1 mm, respectively, for the geometrical concentration ratio of 500. It can be clearly seen from that the optical concentration ratios distribute under different collector widths and parabolic coefficients. The optical concentration ratios decrease monotonically with the increase of parabolic coefficients. When the parabolic coefficients are small, the contour lines are thick, in which case the numerical change has a great influence on the optical concentration ratio. On the other hand, the optical concentration ratios increase first and then decrease
4. Experimental analysis of concentrator prototype Subject to experimental conditions, this part mainly focuses on the test of the concentration of rays. PMMA material is used to fabricate the collector through high-precision mold injection molding (Fig. 13(a)), 561
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concentration-acceptance product are discussed in detail. Finally, experimental analysis of the concentrator prototype is implemented and the optical efficiency versus the position errors of the LED source is discussed. The experimental results are in close agreement with simulation results, which demonstrates the feasibility and validity of the proposed design method.
and the lightguide and the coupling structures are processed with the help of five-axis computerized numerical control machine (Nanotech 350FG from Moore Nanotechnology Systems, LLC) (Fig. 13(b)). An experimental instrument is set up including a three-dimensional mobile platform, a LED source, a collimator and a test platform, as shown in Fig. 13(c). When the experimental instrument is operating, the diffusion rays from LED source are collimated by the collimator and shine on the upper surface of the concentrator. After concentrated by collectors, the rays are focus into the coupling structures and get into the lightguide. With the collectors focus aligned to the coupling structures, the output edge of the concentrator appears bright and very uniform in intensity, as shown in Fig. 13(d). The optical power of the emergent rays can be detected by the optical power meter with the type of JW3216, as shown in Fig. 13(e). Since the collimator cannot parallel the rays absolutely, some tilted rays cannot couple into the lightguide, which gives rise to decline of optical efficiency. Therefore, it is necessary to research the effect of different position errors of LED source on optical efficiency. The effect of different position errors of LED source on optical efficiency are shown in Fig. 14, and x, y and z direction are depicted in Fig. 13(c). On the whole, optical efficiency decreases with the increase of geometric concentration ratio. The effect of LED source along X-axis and Y-axis on optical efficiency is more obvious than that along Z-axis. This is blamed on the generation of less tilted rays after collimator when LED source changed along Z-axis. In addition, the curves in Fig. 14(b) are not symmetrical about point 0, which attributes to the symmetrical collector structure in Y direction. Finally, the optical efficiencies versus the geometric concentration ratio with different LED source types are depicted in Fig. 15. The divergence angle of all different LED sources are 120° to meet the same condition of simulated incident rays for comparison. The test results show that LED source with type of LUW HWQP from OSRAM can achieve higher efficiency in the same condition of geometric concentration ratio, but such a high efficiency is far from the simulation data owing to the loss from LED location errors, alignment error between collector array and lightguide, parallelism errors of incident rays and so on. The main goals of the prototype concentrator are to demonstrate proposed design method and rays coupling from multiple coupling structures into a common lightguide. The optical efficiency of the prototype system is lower than the simulation results. Despite its relative inefficiency, our experimental measurements are in close agreement with our simulation model and how to enhance the efficiency of the prototype system is placed at the heart of our plans for existing and future projects.
Funding National Nature Science Foundation of China (NSFC) (61605016). Declaration of Competing Interest The authors declare no conflict of interest. References Araki, K., Yano, T., Uozumi, H., 2008. Concentrator solar photovoltaic power generating apparatus. US. Benítez, P., Miñano, J.C., Zamora, P., Mohedano, R., Cvetkovic, A., Buljan, M., Chaves, J., Hernández, M., 2010. High performance Fresnel-based photovoltaic concentrator. Opt. Express 18 (S1), A25–A40. Bouchard, Sébastien, Thibault, S., 2012. Planar waveguide concentrator used with a seasonal tracker. Appl. Opt. 51 (28), 6848. Chiang, S.Y., Wei, C.C., Chiang, T.H., et al., 2009. Fiber optic lighting development trends in Asia. 2009 IITA International Conference on Services Science, Management and Engineering. IEEE Computer Society. Chong, KokKeong, et al., 2013. Design and development in optics of concentrator photovoltaic system. Renew. Sustain. Energy Rev. 19 (1), 598–612. Hornung, T., Steiner, M., Nitz, P., 2011. Estimation of the influence of Fresnel lens temperature on energy generation of a concentrator photovoltaic system. AIP Conf. Proc. 99 (1), 333–338. Kandilli, C., Ulgen, K., 2009. Review and modelling the systems of transmission concentrated solar energy via optical fibres. Renew. Sustain. Energy Rev. 13, 67–84. Karp, J.H., 2010. Planar micro-optic solar concentrator. Opt. Express 18 (2), 1122–1133. Lovegrove, K., Burgess, G., Pye, J., 2011. A new 500 m2 paraboloidal dish solar concentrator. Sol. Energy 85 (4), 620–626. Moore, D., Schmidt, G., Unger, B., 2010. Concentrated photovoltaics stepped planar light guide. International Optical Design Conference. International Society for Optics and Photonics. Sangani, C.S., Solanki, C.S., 2007. Experimental evaluation of V-trough (2 suns) PV concentrator system using commercial PV modules. Sol. Energy Mater. Sol. Cells 91 (6), 453–459. Sapia, C., 2013. Daylighting in buildings: developments of sunlight addressing by optical fiber. Sol. Energy 89 (Complete), 113–121. Tsai, M.C., Whang, J.W., Lee, T.X., et al., 2013. Adjustable planar lightguide solar concentrators with liquid-prism structure. Proc. SPIE – Int. Soc. Opt. Eng. 8620, 12. Ullah, I., Shin, S., 2014. Highly concentrated optical fiber-based daylighting systems for multi-floor office buildings. Energy Build. 72, 246–261. Ullah, I., Whang, A., 2015. Development of optical fiber-based daylighting system and its comparison. Energies 8, 7185–7201. Unger, B.L., Schmidt, G.R., Moore, D.T., 2010. Dimpled planar lightguide solar concentrators. International Optical Design Conference. Optical Society of America. Vu, N.H., Shin, S., 2016. Optical fiber daylighting system combined with LED lighting and CPV, based on stepped thickness waveguide for indoor lighting. J. Opt. Soc. Korea 20 (4), 488–499. Vu, N.H., Shin, S., 2016. Optical fiber daylighting system combined with LED lighting and CPV, based on stepped thickness waveguide for indoor lighting. J. Opt. Soc. Korea 20 (4), 488–499. Whang, J.W., Wang, C.C., Chen, Y.Y., 2009. Design of cascadable optical unit to compress light for light transmission used for indoor illumination. Renew. Energy 34 (10), 2280–2295. Wu, H.Y., Chu, S.C., 2013. Ray-leakage-free sawtooth-shaped planar lightguide solar concentrators. Opt. Express 21 (17), 20073. Zang, C., Gong, B., Wang, Z., 2014. Experimental and theoretical study of wind loads and mechanical performance analysis of heliostats. Sol. Energy 105, 48–57. Zhang, Q., 2016. Design of Conical Solar Fiber Concentrator. Jiangsu University. Zhu, C., Qu, Y.J., An, B., et al., 2014. Self-cooling chip LED-COB light source. Electron Technol. 5.
5. Conclusion This paper presents and discusses a natural light indoor lighting system based on a planar solar concentrator to output larger flux with a high optical concentration ratio. The concentrated solar energy can be divided into illumination part and stored energy part by PBS. The LED arrays can be drived by electric power from stored energy part to supply illumination in the condition of overcast and raining. LightTools software is introduced to design and simulate the proposed illumination system. The simulation results show that for a 100 m2 commercial building, the sunlight can supply 64% of the light flux during a day, and the rest can be compensated by LED. In addition, the effect of collector width and parabolic coefficient on optical concentration ratio and the
562
Contents lists available at ScienceDirect
Solar Energy journal homepage: www.elsevier.com/locate/solener
Design method of a planar solar concentrator for natural illumination a
a
Jiaqi Lv , Xiping Xu , Peng Yin a b
a,b,⁎
T
College of Optoelectronic Engineering, Changchun University of Science and Technology, Changchun, Jilin 130022, China School of Engineering, Monash University Malaysia, Jalan Lagoon Selatan, 47500 Bandar Sunway, Selangor, Malaysia
A R T I C LE I N FO
A B S T R A C T
Keywords: Nonimaging optics Natural illumination Concentrators Solar energy
This paper presents a design method of a natural light indoor illumination system using a planar solar concentrator as a daylighting module which can obtain high optical concentration ratios. In this design, the concentrator is composed of lens arrays, hemisphere-coupling structures, a lightguide, plate of beam splitters (PBS) and photovoltaic cells. The concentrated rays are divided into two parts by PBS, the visible rays for indoor illumination and the invisible rays for energy store. The converted electrical energy from invisible rays can drive the light emitting diodes (LEDs) at the entrance of the lightguide for illumination compensation in the condition of overcast and raining. Simulation results show that the proposed natural illumination system can achieve the illuminance more than 500 lx, which meets the commercial illumination requirement. In addition, the effects of collector width and parabolic coefficient on optical concentration ratio and concentration-acceptance product are discussed in detail. Finally, a prototype concentrator is constructed to demonstrate the viability of the proposed natural illumination system. The experimental measurements are in close agreement with simulation results. To the best of our knowledge, this is the first time that planar solar concentrators have been applied in the indoor illumination system, and our results can provide new insight into possible designs of novel natural illumination.
1. Introduction
Sangani and Solanki (2007) designed a single-v groove concentrator with a theoretical geometrical concentration ratio of 2 and carried out a concentrator experiment on a conventional monocrystalline silicon battery module. The results showed that the power was about 40% higher than that of a flat plate battery module. Although this design avoided tracking the sun, the trough concentrator was easily affected by the wind load to produce displacement deformation and reduce the performance (Zang et al., 2014). Lovegrove et al. (2011) proposed parabolic dish solar concentrators which contain metal laminated mirrors on glass. The solar concentrators have a 13.4 m focal length and 380 identical spherical mirror panels of 1.17 m × 1.17 m, which leads to a uniform spot. Hornung et al. (2011) tested the relationship between the optical efficiency and module performance of Fresnel lenses. The Fresnel lenses of PMMA showed considerable energy acquisition efficiency. The concentrator required low tracking accuracy of the sun, but it was easy to produce machining errors in manufacturing process. Araki et al. (2008) reported a photovoltaic device for concentration. It consisted of cylindrical Fresnel lenses for primary focusing, cylindrical optical elements for secondary focusing, transparent resin materials and solar cells. When the concentration ratio was high enough, additional heat dissipation devices need to be added behind the photovoltaic cells. The hybrid concentrator combines reflection, refraction and total
Along with economic development, the improvement of mankind energy demand year by year. Renewable energy plays a vital role in human society because of its low environmental impact and unfailing generation (Whang et al., 2009). With the introduction of low-carbon concepts and green lighting ideas, solar fiber illumination system has been widely researched and becomes a research hotspot owing to the simple structure, easy installation, low cost and favorable lighting effect (Chiang et al., 2009). In the past few decades, many scholars have researched a lot on optical fiber guiding system (Ullah and Whang, 2015; Kandilli and Ulgen, 2009). For example, Sapia (2013) developed a hybrid concentrating photovoltaic system including a major parabolic concentrator and a secondary planar optical reflector. Ullah and Shin (2014) proposed highly concentrated optical fiber-based daylighting systems for multi-floor office with two methods. The first method included a parabolic trough and the second method contained a linear Fresnel lens. It is not difficult to find that, as a light collecting device for natural illumination, the design of solar concentrators directly determines the illumination performance (Tsai et al., 2013). Solar concentrators can be divided into three types: reflectors, transmission lens and hybrid concentrators (Chong et al., 2013). ⁎
Corresponding author at: College of Optoelectronic Engineering, Changchun University of Science and Technology, Changchun, Jilin 130022, China. E-mail address: [email protected] (P. Yin).
https://doi.org/10.1016/j.solener.2019.10.056 Received 24 April 2019; Received in revised form 19 October 2019; Accepted 23 October 2019 0038-092X/ © 2019 International Solar Energy Society. Published by Elsevier Ltd. All rights reserved.
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internal reflection to focus light. The large array of collectors enables this type of concentrator to have a high geometrical concentrator ratio (Wu and Chu, 2013). Hybrid concentrators usually include three parts: primary optical elements, secondary optical elements and a lightguide. Moore et al. (2010) put forward a design method of the planar solar concentrator, and the dynamic concentration ratios can be achieved by changing the number of the primary optical elements. Unger et al. (2010) improved the structure of the lightguide and proposed a planar solar concentrator. However, the incident rays were reflected by air wedges repeatedly, which led to energy attenuation, low optical efficiency and short work distance. Karp (2010) optimized a flat-type concentrator, where the sunlights were focused by the spherical lens array and transmitted to the photovoltaic cells at the end of the lightguide. Bouchard and Thibault (2012) replaced the spherical lens array with a cylindrical lens array, which had the advantage of simplifying the tracking strategy, but the optical efficiency was unsatisfactory. Therefore, it has become an urgent problem to propose a kind of solar concentrator as the concentration module of natural illumination with high concentration ratios, simple manufacturing and high optical efficiency. This paper presents a design method of an indoor illumination system based on a planar solar concentrator and establishes the mathematical model of sunlights from concentration to propagation. The designed indoor illumination system can realize large area illumination with long runtime and low power consumption.
through the light emitter. The other part of system part is active lighting module which includes optical sensors and LED arrays. When solar energy is insufficient, the optical sensors can detect and drive the LED arrays for compensation. Under the cooperation of the natural light guiding system and active lighting module, the output illumination can be maintained on a stable level. The proposed natural illumination system can be used in basements with an efficient transmission distance of 20 m as shown in Fig. 2. On cloudy or rainy days, the illumination system still brings enough rays into the room by LED compensation system from dawn till dusk. Although the solar energy is inexhaustible, the effective utilization of solar energy is still very meaningful, and the proposed idea of natural illumination system can reduce the leakage of rays and increase the utilization of solar energy.
2. Design concept and model principles
Cg =
2.2. Design of concentrator As the most important part of the natural illumination system, the planar solar concentrator consists of three parts: collector arrays, coupling structures and a lightguide. The incident sunlights can be concentrated by collector arrays, get into the lightguide by coupling structures and propagate by multiple reflection times in lightguide. During the process, two important parameters of geometrical concentration ratio and the optical efficiency are introduced (Karp, 2010), which can be expressed as follows:
2.1. System structure
Sunlight vertically incident area Lightguide length = Lightguide exit port area Lightguide depth
(1)
and
The natural illumination system includes input part, system part and output part as shown in Fig. 1, and the system part is the core of the illumination system. In system part, the natural light guiding system works by capturing sunlights through a concentrator, propagating sunlights through the light transmitters and outputting sunlights
η=
Energy at lightguide exit port Vertically incident sunlight energy
(2)
For collector design, the profile of the paraboloid surface of the lens can be regarded as a parabola expression, as shown in Fig. 3(a):
Fig. 1. Structure diagram of the natural illumination system. 555
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Fig. 2. Illustration of natural illumination system.
(a)
(b)
(c)
(d)
Fig. 3. (a) The double parabolic curves after removal of bottom, (b) the double paraboloid structure, (c) the remaining part after outer edge cropped and (d) the collector structure.
y = az2
Fig. 4. Schematic diagram of height of concentrator after removing the sphere part.
(3)
where a is a coefficient. The incident rays can be concentrated at (y = 1 , z = 0), and the part under the focus surface needs to be re4a moved. After moving a copy of the parabola at a distance of 1/2a towards minus z axis, a double paraboloid structure can be established as shown in Fig. 3(b), where the height of the collector is h. To avoid some leakage from the outer edge of the double paraboloid structure, the double paraboloid structure is tailored with the remaining part width of D, as shown in Fig. 3(c). To prevent the rays from leaking out of the lower surface of the double paraboloid structure and the lightguide, the collector height should be limited as follows:
3/4a ≤ h ≤
2 (n1 +
n12 − 1 )/(n1 − 4a
n12 − 1 ) − 1 (4)
where n1 can be the refractive index of the lightguide. To make the collector lighter and separate it from the lightguide, the lower surface of the collector can be tailored as shown in Fig. 3(d). The radius of the tailored part can be expressed as:
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Fig. 5. Hemisphere-coupling structure on the lightguide.
Fig. 6. PBS transmits visible light and reflects invisible light onto photovoltaic cells.
Fig. 7. Schematic diagram of the coupling of the optical fibers and the lightguide outlet. Fig. 8. The rays from the LED is converged by CPC.
R=
h2 −
h 1 h 1 17 − · + 2 + 2a a a 4a 16a2
of the concentrator will be reduced accordingly. Suppose that the lowest point coordinate of the external paraboloid can be 1 1 + Δz, 4a + Δh , where Δz and Δh can be the distances that the 2a
(5)
(
As shown in Fig. 4, the blue dotted line indicates that the radius R of the cut sphere is exactly 1/2a. When R is greater than 1/2a, the height
)
paraboloid moves along the z axis and y axis after being cut, 557
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Fig. 9. Schematic of the planar solar concentrator for natural illumination system.
Table 1 Average illuminance (Whang et al., 2009) and luminous flux at different times of the day. Time
Sunlight illuminance (lx)
Luminous flux on the surface concentrator (lm)
6 AM 7 AM 8 AM 9 AM 10 AM 11 AM 12 PM 13 PM 14 PM 15 PM 16 PM 17 PM 18 PM
30,000 60,000 90,000 120,000 150,000 157,000 165,000 170,000 168,000 130,000 95,000 45,000 35,000
67,500 135,000 202,500 270,000 337,500 353,250 371,250 382,500 378,000 292,500 213,750 101,250 78,750
yR −
1 = Δh 4a
(7)
According to the triangle Pythagorean theorem, the following formula can be obtained: 2 1 Δh2 + ⎛ + Δz⎞ = R2 ⎝ 2a ⎠
(8)
From Eqs. (6)–(8), we can derive Δz and Δh :
2 Ra − Δz =
Δh =
1 4
−1
2a
(9)
2Ra − 1 2a
(10)
In this paper, a = 0.01 and R = 50.5 are selected. To eliminate rays leakage from the lightguide, the rays should be coupled into the lightguide. In this paper, a hemisphere-coupling structure can be added to the surface of the lightguide to couple the rays into the lightguide, as shown in Fig. 5. The center of the hemisphere can be selected at the focal point of the outer paraboloid of the collector, and the radius of the hemisphere can be negligible. 2.3. Design of beam splitter, solar cell and optical fiber Solar spectrum is a continuous spectrum of different wavelengths which can be divided into visible and invisible light. Visible light has a wavelength of 400–760 nm, and invisible light is further divided into the infrared band of 760–5300 nm and the ultraviolet band of 290–400 nm. Usually, active or passive cooler is applied to solve the solar energy LED lighting system radiation problem, which means that complex structures and more spending are needed to introduce. For the illumination system proposed in this paper, the plate of beam splitter (PBS) produced by DiYPRO Co., Ltd. was selected to eliminate heat (Vu and Shin, 2016). PBS is a multilayer dielectric coated spectroscopically selective thermal mirror, which can refract visible light and reflect both ultraviolet and near-infrared light (NIR) at the same time. The PBS is placed between the lightguide and the collectors to reflect the invisible light to the solar cells which are placed at the focus of the reflected light, as shown in Fig. 6. In this paper, the solar cells can be selected by CuInSe owing to its sensitivity to ultraviolet and nearinfrared light. In the daylighting system, we choose the traditional plastic optical fiber which has the advantages of low cost, light weight and flexibility. It is worth noting that the effect of heat on the optical fiber should be considered in the case of high concentration ratio, but the use of PBS
Fig. 10. Lumious flux versus different times from 6 AM to 18 PM.
respectively. Substituting the coordinate into Eq. (3), another equation can be obtained: 2 1 y = a·⎛ + Δz⎞ 2a ⎝ ⎠
(6)
At this point, the y-coordinate yR of the paraboloid after cutting is equal to the sum of the y-coordinate of the focus and the distance Δh along the y axis. So it follows that: 558
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Fig. 11. The diagram of influence of collection width and parabolic coefficient on optical concentration ratio.
Fig. 12. The influence of the collector height on the optical concentration ratio and CAP value in which case the radii of hemisphere-coupling structures are: (a) r = 0.1 mm, (b) r = 0.2 mm and (c) r = 0.5 mm.
exactly solves this problem. The propagation of light in optical fiber is due to the difference of refractive indexes between core and cladding. According to the fiber transmission theory, the angle between the central axis and the incident light can be θC = arcsin(NA) , where NA can be the numerical aperture of the fiber (Zhang, 2016). Optical fiber bundles compose of multiple optical fibers which have the advantages
of wide transmission spectrum, image transmission, uniform spot distribution, easy shaping and free bending. Thus, we can directly couple the optical fibers into the exit area of the lightguide, as shown in Fig. 7.
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3. Daylight simulation Optical model plays an important role in optical system. Commercial optical modeling software LightTools is used to design and simulate the geometric structure of the planar solar concentrator used in the natural illumination system. In this paper, the material of collectors, hemisphere-coupling structures and lightguide is polymethyl methacrylate (PMMA) which is one of the most common optical plastics. The concentrator of the natural illumination system consists of a 30 × 30 array of collectors, hemisphere-coupling structures, a lightguide, LED array, PBS array and photovoltaic cells, as shown in Fig. 9. The solar tracking system is not shown. 3.1. Natural light of large scale system The concentrator is located at 127.0° longitude and 37.5° latitude at 12:30 p.m., when sunlight intensity reaches maximum. In order to obtain the vertical incident sunlight, the concentrator has a sun tracking device which can rotate the collectors towards the sun. The area of the concentrator upper surface is 2.25 m2 and the calculation formula of luminous flux in lumens can be (Vu and Shin, 2016): (11)
F=E×S
where E can be the solar illuminance, S can be the area of the concentrator upper surface, and F can be the input luminous flux. The average solar illuminance at different time of the day are listed in Table 1, where the data are from China Meteorological Administration (Benítez et al., 2010). Based on average illuminance and solar altitude; luminous flux on the surface of the concentrator can be calculated as input for the simulation. Typically, a commercial building requires 500 lx. Fig. 10 shows the luminous flux provided by sunlight and LEDs and average lighting requirements of commercial buildings at different time of the day. When the light is strongest at 13:00 p.m. and the lightguide thickness is 2 mm, the output luminous flux can reach about 50,000 lm. That is to say, the natural illumination system can light up 100 m2. Other than 13:00 p.m., sunlights provide less than 50,000 lm, in which case the LED light sources can be tuned on for compensation. The equation for calculating the total luminous flux of indoor illumination is as follows:
Fig. 13. The prototype of (a) a collector and (b) a lightguide. (c) The experimental instrument including a three-dimensional mobile platform, a LED source, a collimator and a test platform. (d) When properly aligned to the parallel rays, rays incident on the collectors surface couple into the lightguide and exit at the lightguide edge, appearing as a bright line. (e) The optical power meter with the type of JW3216.
T=
∫ Fdt
(12)
where t is the time of day; T is the total received flux in the day (Vu and Shin, 2016). According to Eq. (12), it can be calculated that the sunlight is capable to afford about 416,172 lm and the LED needs to offset about 233,828 lm every day. Apparently, sunlight supplies 64% in total luminous flux during a day, and a part of the energy driving the LEDs comes from the PBS. That is to say, external electric power contributes a minuscule proportion of the total natural illumination system, which saves energy tremendously.
2.4. Light emitting diode (LED) integration In this section, a hybrid LED sunlight lighting system is selected to achieve uniform indoor lighting in the condition of changing luminous flux at different times of the day. In this paper, we introduce chip-onboard (COB) light source which is directly attached to the mirror metal substrate with high reflectivity (Zhu et al., 2014). When the LED is connected to the lightguide, only about 5% of the coupling efficiency can be lost (Vu and Shin, 2016). This design uses optical epoxy resin to bond the substrate to the lightguide inlet. Five arranged substrates are equipped with LEDs. If the substrate is coupled directly at the entrance of lightguide, the incident light will leak from the edge of the LED. Therefore, a compound parabolic concentrator (CPC) structure can be designed and attached at the entrance of the lightguide, which reduces the emission angle of the LEDs, as shown in Fig. 8. When the luminous flux is insufficient, the LED light source can be turned on to compensate. LED light sources can be powered by invisible energy stored in solar cells or external power sources.
3.2. The effect of collector width and parabolic coefficient on optical concentration ratio Optical concentration ratio is the product of geometrical concentration ratio and optical efficiency, given in the following relationship (Karp, 2010):
Co = Cg × η =
S1 ×η S2
(13)
where Co is the optical concentration ratio, S1 is the receiving area of the concentrator upper surface and S2 is the area at the exit of the lightguide. Since the variation of the collector width and the parabolic coefficient can change the surface profiles of the collectors, different optical efficiencies can be acquired in condition of the same geometric concentration ratio. Fig. 11 depicts the optical concentration ratio versus different collector widths and parabolic coeffcients. For 560
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Fig. 14. The optical efficiency versus the position errors of the LED source along (a) X-axis, (b) Y-axis and (c) Z-axis.
with the increase of the collector widths. When the collector widths are small, the downward trend of contour lines is rapid, and when the collector widths are large, the rising trend of contour lines is gentle. 3.3. Concentration-acceptance product For natural illumination system, concentration-acceptance product (CAP) is also an important parameter to evaluate the performance of concentrators. Meanwhile, the value of CAP depends on the geometrical concentration ratios and the acceptance angles. The CAP can be calculated as follows (Benítez et al., 2010):
CAP =
Cg sinα
(14)
where α is the acceptance angle where the optical efficiency decreases to 90% of the peak value. In this paper, the effects of different concentrator heights on the optical concentration ratios and CAP under three kinds of hemisphere-coupling structure radius were tested as shown in Fig. 12. It's easy to see that the optical concentration ratios and CAPs decrease with the increasing collector height. Similarly, the optical concentration ratios decrease with the increasing radius of the hemisphere-coupling structures, which attributes to the increasing rayleakage for reduced efficiencies. However, the CAPs increase with the increasing radius of the hemisphere-coupling structures, which means the increase of the acceptance angles.
Fig. 15. The optical efficiency versus the geometric concentration ratio with different LED source type and in simulation.
convenience, the length of the lightguide and the thickness are set as 500 mm and 1 mm, respectively, for the geometrical concentration ratio of 500. It can be clearly seen from that the optical concentration ratios distribute under different collector widths and parabolic coefficients. The optical concentration ratios decrease monotonically with the increase of parabolic coefficients. When the parabolic coefficients are small, the contour lines are thick, in which case the numerical change has a great influence on the optical concentration ratio. On the other hand, the optical concentration ratios increase first and then decrease
4. Experimental analysis of concentrator prototype Subject to experimental conditions, this part mainly focuses on the test of the concentration of rays. PMMA material is used to fabricate the collector through high-precision mold injection molding (Fig. 13(a)), 561
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concentration-acceptance product are discussed in detail. Finally, experimental analysis of the concentrator prototype is implemented and the optical efficiency versus the position errors of the LED source is discussed. The experimental results are in close agreement with simulation results, which demonstrates the feasibility and validity of the proposed design method.
and the lightguide and the coupling structures are processed with the help of five-axis computerized numerical control machine (Nanotech 350FG from Moore Nanotechnology Systems, LLC) (Fig. 13(b)). An experimental instrument is set up including a three-dimensional mobile platform, a LED source, a collimator and a test platform, as shown in Fig. 13(c). When the experimental instrument is operating, the diffusion rays from LED source are collimated by the collimator and shine on the upper surface of the concentrator. After concentrated by collectors, the rays are focus into the coupling structures and get into the lightguide. With the collectors focus aligned to the coupling structures, the output edge of the concentrator appears bright and very uniform in intensity, as shown in Fig. 13(d). The optical power of the emergent rays can be detected by the optical power meter with the type of JW3216, as shown in Fig. 13(e). Since the collimator cannot parallel the rays absolutely, some tilted rays cannot couple into the lightguide, which gives rise to decline of optical efficiency. Therefore, it is necessary to research the effect of different position errors of LED source on optical efficiency. The effect of different position errors of LED source on optical efficiency are shown in Fig. 14, and x, y and z direction are depicted in Fig. 13(c). On the whole, optical efficiency decreases with the increase of geometric concentration ratio. The effect of LED source along X-axis and Y-axis on optical efficiency is more obvious than that along Z-axis. This is blamed on the generation of less tilted rays after collimator when LED source changed along Z-axis. In addition, the curves in Fig. 14(b) are not symmetrical about point 0, which attributes to the symmetrical collector structure in Y direction. Finally, the optical efficiencies versus the geometric concentration ratio with different LED source types are depicted in Fig. 15. The divergence angle of all different LED sources are 120° to meet the same condition of simulated incident rays for comparison. The test results show that LED source with type of LUW HWQP from OSRAM can achieve higher efficiency in the same condition of geometric concentration ratio, but such a high efficiency is far from the simulation data owing to the loss from LED location errors, alignment error between collector array and lightguide, parallelism errors of incident rays and so on. The main goals of the prototype concentrator are to demonstrate proposed design method and rays coupling from multiple coupling structures into a common lightguide. The optical efficiency of the prototype system is lower than the simulation results. Despite its relative inefficiency, our experimental measurements are in close agreement with our simulation model and how to enhance the efficiency of the prototype system is placed at the heart of our plans for existing and future projects.
Funding National Nature Science Foundation of China (NSFC) (61605016). Declaration of Competing Interest The authors declare no conflict of interest. References Araki, K., Yano, T., Uozumi, H., 2008. Concentrator solar photovoltaic power generating apparatus. US. Benítez, P., Miñano, J.C., Zamora, P., Mohedano, R., Cvetkovic, A., Buljan, M., Chaves, J., Hernández, M., 2010. High performance Fresnel-based photovoltaic concentrator. Opt. Express 18 (S1), A25–A40. Bouchard, Sébastien, Thibault, S., 2012. Planar waveguide concentrator used with a seasonal tracker. Appl. Opt. 51 (28), 6848. Chiang, S.Y., Wei, C.C., Chiang, T.H., et al., 2009. Fiber optic lighting development trends in Asia. 2009 IITA International Conference on Services Science, Management and Engineering. IEEE Computer Society. Chong, KokKeong, et al., 2013. Design and development in optics of concentrator photovoltaic system. Renew. Sustain. Energy Rev. 19 (1), 598–612. Hornung, T., Steiner, M., Nitz, P., 2011. Estimation of the influence of Fresnel lens temperature on energy generation of a concentrator photovoltaic system. AIP Conf. Proc. 99 (1), 333–338. Kandilli, C., Ulgen, K., 2009. Review and modelling the systems of transmission concentrated solar energy via optical fibres. Renew. Sustain. Energy Rev. 13, 67–84. Karp, J.H., 2010. Planar micro-optic solar concentrator. Opt. Express 18 (2), 1122–1133. Lovegrove, K., Burgess, G., Pye, J., 2011. A new 500 m2 paraboloidal dish solar concentrator. Sol. Energy 85 (4), 620–626. Moore, D., Schmidt, G., Unger, B., 2010. Concentrated photovoltaics stepped planar light guide. International Optical Design Conference. International Society for Optics and Photonics. Sangani, C.S., Solanki, C.S., 2007. Experimental evaluation of V-trough (2 suns) PV concentrator system using commercial PV modules. Sol. Energy Mater. Sol. Cells 91 (6), 453–459. Sapia, C., 2013. Daylighting in buildings: developments of sunlight addressing by optical fiber. Sol. Energy 89 (Complete), 113–121. Tsai, M.C., Whang, J.W., Lee, T.X., et al., 2013. Adjustable planar lightguide solar concentrators with liquid-prism structure. Proc. SPIE – Int. Soc. Opt. Eng. 8620, 12. Ullah, I., Shin, S., 2014. Highly concentrated optical fiber-based daylighting systems for multi-floor office buildings. Energy Build. 72, 246–261. Ullah, I., Whang, A., 2015. Development of optical fiber-based daylighting system and its comparison. Energies 8, 7185–7201. Unger, B.L., Schmidt, G.R., Moore, D.T., 2010. Dimpled planar lightguide solar concentrators. International Optical Design Conference. Optical Society of America. Vu, N.H., Shin, S., 2016. Optical fiber daylighting system combined with LED lighting and CPV, based on stepped thickness waveguide for indoor lighting. J. Opt. Soc. Korea 20 (4), 488–499. Vu, N.H., Shin, S., 2016. Optical fiber daylighting system combined with LED lighting and CPV, based on stepped thickness waveguide for indoor lighting. J. Opt. Soc. Korea 20 (4), 488–499. Whang, J.W., Wang, C.C., Chen, Y.Y., 2009. Design of cascadable optical unit to compress light for light transmission used for indoor illumination. Renew. Energy 34 (10), 2280–2295. Wu, H.Y., Chu, S.C., 2013. Ray-leakage-free sawtooth-shaped planar lightguide solar concentrators. Opt. Express 21 (17), 20073. Zang, C., Gong, B., Wang, Z., 2014. Experimental and theoretical study of wind loads and mechanical performance analysis of heliostats. Sol. Energy 105, 48–57. Zhang, Q., 2016. Design of Conical Solar Fiber Concentrator. Jiangsu University. Zhu, C., Qu, Y.J., An, B., et al., 2014. Self-cooling chip LED-COB light source. Electron Technol. 5.
5. Conclusion This paper presents and discusses a natural light indoor lighting system based on a planar solar concentrator to output larger flux with a high optical concentration ratio. The concentrated solar energy can be divided into illumination part and stored energy part by PBS. The LED arrays can be drived by electric power from stored energy part to supply illumination in the condition of overcast and raining. LightTools software is introduced to design and simulate the proposed illumination system. The simulation results show that for a 100 m2 commercial building, the sunlight can supply 64% of the light flux during a day, and the rest can be compensated by LED. In addition, the effect of collector width and parabolic coefficient on optical concentration ratio and the
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