Evaluation of a large dish‐type concentrator solar lighting system for underground car park

To utilize solar energy more efficiently and reduce lighting power consumption in underground public spaces such as car park, a large dish‐type concentrator solar lighting system is put forward along with its evaluation, which is a unique design to apply a laminated layer of beam split thin‐film coating and thin‐film solar cells onto the dish reflector. The collected sunlight is split into 2 parts, one being reflected into a fiber optical bundle and transmitted for daylighting, while the rest being absorbed by solar cells for electricity generation as the other way to replenish daylighting. A set of 4 solar lighting systems using 3.28‐m diameter dish are designed to meet the lighting requirement in a 1771‐m2 underground car park. A mathematical model is adopted to calculate the output power and conversion efficiency of solar cells distributed on the parabolic dish surface. The indoor illuminance distribution is given by lighting simulation. The results indicate that the average daylight illuminance in the car park can vary between 62.7 and 284 lx on February 25, 2016 and between 62.7 and 353 lx on August 17, 2016 for 2 chosen days, respectively. For the presented design, the electricity produced by solar cells is just enough to power light‐emitting diodes for lighting meeting a criterion at night. Considering about 19% conversion efficiency of solar cells and the efficacy of 129.5 lm/W of light‐emitting diodes, the hybrid solar lighting system can have about 40% utilization ratio of solar energy, so it can be concluded that a sufficient lighting provision can be provided by the proposed large dish‐type concentrator solar lighting system for applications in underground car park.


| INTRODUCTION
With fast global development, the contradiction between the demand and the supply of energy has become sharper. To address the associated challenge of climate change, most countries in the world have come together to sign the Paris Climate Agreement, 1 which has seen that the solar energy is playing a more and more important role in energy structure. In developed countries, 20 to 40% of gross energy consumption is used for buildings, 2,3 12 to 22% of which is for lighting. 3 There are some underground public places in our daily life such as underground car parks, tunnels, and basements, however, being in badly need of lighting all the time as a result of being not sunlit directly, which contributes to a greater energy consumption. And public lighting is an important part of current energy consumption. 4 Direct daylighting from sunlight [5][6][7] is an important way to solve these problems. Sunlight optical fiber lighting was proposed in 1980, 8 which has become an important way of implementation. [9][10][11][12] However, on account of its instability due to the weather and sun illumination time, its development has been restricted. A breakthrough of solution to this problem is required to increase the acceptance and utilization of sunlight and maintain the stability of system. The system presented in this paper can greatly obtain and concentrate solar radiation by applying a large dish. 13 Solar dish systems are applied widely to collect solar radiation for photovoltaic 14,15 and thermal energy employing [16][17][18][19] as a result of their high concentrated radiation intensity 20,21 to make solar energy utilized efficiently. 14,22 By the surface of dish concentrator coated with beam split thin film 23 and coupled with thin film solar cells, 24 accounting for 38.9% of the total radiant energy of sunlight, the visible (VIS) light (400-760 nm) 25 is split into 2 equal parts by beam split thin film, of which the first 50% VIS light beam is reflected and concentrated onto a secondary reflector and then reflected into fiber-optic bundle (FOB), while the rest 50% VIS light and other spectral radiation (300-400 nm and 760-1800 nm) will be transmitted through the beam split thin film and absorbed and transformed into electricity by thin-film solar cells underneath the beam split thin film. The generated electricity is stored and supplied to the complementary light-emitting diode (LED) lighting, 26 so as to improve the overall utilization of sunlight. The FOB mentioned is a bundle of optical fiber made of polymethyl methacrylate (PMMA) material with 200-dB/km 27 attenuation coefficient.
In this article, the output power and conversion efficiency of solar cells will be calculated from the spectra of sunlight; meanwhile, the utilization ratio of solar energy, the optical efficiencies of dish concentrator, and FOB will be also defined and given. The values of luminous flux from both FOB daylighting and LED lighting will be used to simulate and analyze the application of this system for lighting of an underground car park in Wuhan, China, as illustrated in Figure 1, to prove that the system can light that place up to lighting standard efficiently and have a performance of stable illuminance. Furthermore, self-sufficiency of the complementary solar lighting for a long period in a whole year will be analyzed.

| DESCRIPTION
The main body of the dish-type concentrator is a primary parabolic dish reflector as illustrated in Figure 2A. A typical Cassegrain optics structure 28 can be presented with a confocal secondary hyperbolic dish reflector, which is used to reflect light into FOB. The optical schematic diagram is given in Figure 2B. The concentrator is supported by a sun tracking system with 4 sensors on the quadrants of a bar on the back of the secondary reflector to detect the direction of the sunlight, so it can have a real-time track of the sun in practice and guarantee the system to collect the maximum amount of solar radiation by controlling the rotation of motors. 8 The reflectance of primary reflector is dependent of beam split thin film, and the reflectance R s (%) of 85% is assumed for the secondary reflector. Based on our design analysis, the chosen geometries of the primary and secondary reflectors such as diameter, focal distance, half field angle, and eccentricity are given in Table 1.
The optical surface of the primary reflector is divided into an array of 204 elements, 29 and the area of each elemental reflector is about 432.64 cm 2 (20.8 × 20.8 cm) for a single solar cell. As for each element, its area and curvature are small, so an identical structure can be used, which can help to reduce the manufacturing difficulty  Figure 3A, where a beam split thin film is coated on a curved glass, which is then laminated with a thin-film solar cell and a circuit layer to form a module with its edge sealed with butyl glue. Each solar cell module element is fixed in an array parabolic dish panes to from a large dish reflector, which are jointed with the cross metal skeleton, and the cross metal skeleton supports 4 metal bars to fix secondary hyperbolic dish reflector. The whole large dish concentrator is supported with a metal structure as shown in Figure 2A.
The described beam split thin film 30,31 is a 13-layer film system made up of ZnS and MgF 2 , and it can split VIS light into 2 equal parts, of which one part is reflected and the other is transmitted and absorbed. The beam split film can meanwhile realize antireflection of ultraviolet (UV) light (300-400 nm) and infrared (IR) light (760-1800 nm) and eliminate or decrease the composition of these light in the concentrated light, which is transmitted through FOB into a room. By removing UV and IR light, the concentrated light will be much less harmful to human body, 25 have much less thermal effect in the coupling pot of FOB, and thus slow down the aging speed of fibers, which can help to maintain the basic form and performance of FOB. When a beam of sunlight strikes on a parabolic dish, the incidence angle is different at different positions of a dish. So it is necessary to analyze the influence of incidence angle on the spectral reflectance R(λ) (%) of beam split thin film by the TFCalc ™ software. From Figure 3B, we can find that, although the reflectance band window slightly shifts by about 40 to 50 nm toward the short-wave direction when the incidence angle  changes from 0°to 30°, the basic shape of reflectance curve remains almost unchanged and had little effect on the actual spectroscopic effect; therefore, we used the R(λ) values for 0°incidence angle in calculation in this paper.
The FOB in this system needs to be light, soft, and resistant to bending and must be also cheap. The plastic optical fiber becomes the best candidate for this design as a result of its low optical loss, suitability for coupling with the light sources, and good flexible characteristics. In fact, we chose PMMA as the material of FOB. While the core diameter of single optic fiber is 14 mm, its numerical aperture (NA) is 0.51. 27 The FOB is composed of 36 optic fibers, with a circular component of 92-mm diameter.
The terminal lamps consist of 3 optical fibers, 4 LEDs, and diffusers. With the advantages of high optical efficacy, energy conservation, environment protection, long service, and cheap price, LED lighting has become a development trend of future lighting. The choice of LEDs in this design is XLamp XP-G3 of Cree, with its max efficacy of 190 lm/ W under the binning condition, the rated power of 6 W, and the luminous flux output of 777 lm, and it will supply lighting in cloudy weather and at night. The emergent luminous flux of lamps reaches 2625.6 lm when only 4 LEDs work, which means it can have very good lighting effect while saving energy. The VIS band of sunlight is guided into the lamps by FOB in daytime. The emergent angle of divergence of optical fiber 2 α is determined by α = arcsin (NA), and it is 61.3°in this design, but it does not conform to the uniform requirements. A diffuser can solve such problem, and it may be based on polycarbonate material with 88% transmittance. The refraction, reflection, and scattering of light between chemical particles and resin form the uniform effect of light and not only increase the illuminated area but also decrease the occurrence of dazzling when applied in underground car parks.
As illustrated earlier in Figure 1, the complementary sunlight and electric lighting system is based on the combination of a solar optical fiber daylighting device and electric LED lamps. The former mainly includes a primary dish reflector laminated with thin-film solar cell and beam split thin film, a secondary reflector, FOB, and a sun-tracking device. The beam split thin film is coating on the curved glass, and the curved glass is laminated with thinfilm solar cells. When sunlight irradiates beam split thin film, UV, IR, and 50% VIS light are transmitted through the beam split thin film and are absorbed and transformed into electricity by thin-film solar cells underneath, while the other 50% VIS light is reflected and concentrated to the secondary reflector, and then reflected again and coupled into the FOB, being transmitted to the lamps eventually. The LEDs in lamps are supplied by battery whose power is from the thin-film solar cells. If there is power shortage, the battery will get extra power supply from the grid. The LEDs will be controlled with photoswitch to provide light on cloudy weather or at night for complementary lighting.

| RESULTS AND ANALYSIS
For the calculation of the output power and conversion efficiency of solar cells and the simulation of lighting in underground car park, we chose Wuhan, China, as a location to gather outdoor irradiance E e (W/m 2 ) and illuminance data. Because the data are affected markedly by the seasonal fluctuation, while 2 typical days on February 25, 2016 and August 17, 2016 were chosen, we measured and calculated the hourly averaged values between 300 and 1800-nm wavelength λ (nm) for the outdoor irradiance (see Figure 5A).
Because the measured and averaged outdoor irradiance is an integrated for a spectral range, it is necessary to use this hourly outdoor irradiance to give a predicted spectral irradiance E e (λ) (W/(m 2 • nm)) according to the normalized spectral irradiance E e ′ (λ) (W/(m 2 • nm)), as shown in Figure 5B under the condition of air mass = 1.5 (AM1.5). The spectral irradiance E e (λ) (W/(m 2 • nm)) is represented by where I is proportionality coefficient, which can be calculated from

FIGURE 4
The spectral external quantum efficiency of the copper indium gallium diselenide thin-film solar cells 32 When it comes to calculating the output power P out (W) and conversion efficiency η (%) of solar cells in this system, the spectral irradiance transmitted through the beam split thin film and absorbed by the solar cells is represented as E e (λ, θ) (W/(m 2 • nm)), and it can be given as where θ is the incidence angle of sunlight to solar cells (see Figure 6) and R(λ) is the spectral reflectance of beam split thin film given in Figure 3B.
The short-circuit current density J sc (θ) (A/m 2 ) of solar cells can be calculated from 36 where q = 1.602 × 10 −19 C, h = 6.626 × 10 −34 J ⋅ s, and c = 2.998 × 10 8 m/s are electron charge, Planck constant, and speed of light. So, the short-circuit current I sc (A) is where Σ represents integration over the curved surface of parabolic dish and dS c (m 2 ) is infinitesimal of curved surface area of parabolic dish. The relationship between infinitesimal of aperture area of parabolic dish dS p (m 2 ) and dS c is shown in Figure 6, and it can be given as Therefore, short-circuit current can be calculated from Equations 3 to 6 as As for open-circuit voltage V oc (V), it can be given as 37 where k = 1.381 × 10 −23 J/K is Boltzmann constant and T (K) is the temperature in Kelvin, which is 298 K under the standard condition. As for saturation current density I 0 (A/cm 2 ), it can be calculated from where E g (eV) is band gap, and it is 1.02 eV. 38 Normalized open-circuit voltage v oc (V) is defined as and fill factor FF is followed as The solar irradiance: A, the hourly averaged value of outdoor irradiance between 300 and 1800 nm for 2 days; B, the normalized spectral irradiance between 300 and 1800 nm under the condition of AM1.5 35

FIGURE 6
The model of incidence of sunlight and infinitesimal area projection relationship between dS p and dS c Therefore, output power can be calculated from The conversion efficiency can be given as As for incident optical power P in (W) of solar cells in this system, it can be expressed as The hourly averaged output power and conversion efficiency are shown in Figure 7, where were calculated for 1 dish concentrator system between 4 and 20 o'clock on 2 example days, respectively.
As for simulation of this system applied to an underground car park, which was built in software with the geometries of 52.4 m long, 33.8 m wide, and 3.8 m high, the reflectance of car park ceiling, wall, and floor are 70%, 90%, and 22.3%, respectively. There are 60 parking spaces, and each parking area is 12.72 m 2 (5.3 × 2.4 m), 39 which is denoted in Figure 8A. Each dish concentrator FOB daylighting system supports 12 lamps. As is illustrated in Figure 8B Table 2.
The luminous flux at lamps can be deduced for one system from outdoor irradiance through FOB to lamps. Transmission efficiency of FOB is determined by attenuation coefficient, which is shown in Figure 9 for the attenuation coefficient A(λ) (dB/km) of PMMA optical fiber. Attenuation coefficient can be calculated in decibels from 41 A where L (km), Φ ei (λ) (W/nm), and Φ eo (λ) (W/nm) are length of FOB, input spectral radiation flux, and output spectral radiation flux. Because the length of FOB is long relatively, it becomes the main factor of light attenuation.
Transmission efficiency of FOB η(λ) (%) is So the transmission efficiency of FOB can be given from Equations 15 and 16 as While 1 system supports 12 lamps, for every lamp, the input spectral radiation flux is related by So the output spectral radiation flux is calculated from Radiometry is mentioned above, but photometry is needed for the simulation of daylighting in software. The relationship between radiometry and photometry in this paper is represented by luminous efficacy K(λ) (lm/ W) and spectral luminous efficiency of photopic vision V(λ) (see Figure 10) as follows:

FIGURE 9
The attenuation coefficient of polymethyl methacrylate optical fiber 40

FIGURE 10
The spectral luminous efficiency of photopic vision 42 where Φ ν (λ) (lm/nm) and K m (lm/W) are spectral luminous flux and the maximum luminous efficacy and K m is 683 lm/W. The spectral luminous flux is given from Equations 20 and 21 as Synthesizing 2, 17, 19, and 22, luminous flux Φ ν (lm) is calculated from Calculated for 1 system is the average daylight luminous flux of every hour from outdoor irradiance through FOB to lamps for the 4 types of lamps between 4:00 and 20:00 for 2 example days on February 25, 2016 and August 17, 2016, respectively, which are given in Figure 11.
The optical efficiencies of FOB η f (%) and the optical efficiency of dish concentrator η d (%) can be defined as The numerator of Equations 24 and 25 means the output radiation should be integrated from 300 to 1800 nm theoretically, but in fact, on the one hand, the   VIS to near IR (about 350-1100 nm) is the wavelength range in which the attenuation coefficient of PMMA optical fiber is minimum. 40 Meanwhile, those of the rest UV, IR, and red wavelength range are high 40,43 ; on the other hand, the most UV and IR sunlight are transmitted through the beam split thin film, and light transmitted by PMMA optical fiber are rare in this wavelength range according to the spectral reflectance R(λ) of beam split thin film. Therefore, the wavelength range of integral for the numerator of Equations 24 and 25 is simplified to 350 to 1100 nm when it is calculated. The optical efficiencies of FOB are shown in Table 3, and the optical efficiency of dish concentrator is 12.16%. According to the hourly daylight luminous flux from FOB in each lamp at daytime and the luminous flux of LEDs lighting only in lamps at night, we set values of luminous flux in lighting simulation for every lamp. Lighting is simulated every hour for 2 example days in the underground car park in the software to give the illuminance at the level of floor.
The average values of indoor illuminance are shown in Figure 12, indicating a performance of stable illuminance under 353 lx and above 62.7 lx. According to Standard for lighting design of buildings (GB50034-2013), 44 lighting in underground car parks, taking the floor as reference plane (height is 0 m), the recommended illuminance is 50 lx. It can be found from the simulation of 48 lamps supported by 4 FOB daylighting systems in the 1771-m 2 underground car park that the sunlight lighting only are for 9 and 11 hours on these 2 days from Figure 12, and the maximum values of illuminance on the days are 284 and 353 lx around 13 and 12 o'clock, while the average value of illuminance for using LEDs only at night is set at 62.7 lx. Therefore, not only does the system supply sufficient lighting by using solar radiation at daytime but also meet the standard of lighting basically for the whole day.
The direct daylight delivered by FOB is sufficient during most of day time. As for the electricity output of 1 system on February 25, 2016, the battery was charged about 4.298 kWh and discharged about 4.320 kWh for LEDs, so the electricity produced by solar cells is just enough to power LEDs for lighting at that night, which demands more appropriate spectral reflectance of beam splitting to realize the self-sufficiency of the system.
As for the uniformity of illuminance in underground car park, it is illustrated in Figure 13 by the distribution of illuminance. Figure 13A to C includes the distribution of illuminance at 13 o'clock on February 25, 2016 and at 12 o'clock on August 17, 2016 and by using LEDs only at night. Although there are peak value regions around the lamp space, the whole distribution of illuminance is nearly uniform, and we can conclude that the distribution of illuminance of all the hours are nearly uniform by the distribution of illuminance of maximum and minimum values of illuminance.
The utilization ratio of solar energy σ (%) of this system should be considered in 2 ways of utilization, 50% VIS for daylighting and the remaining sunlight for electricity generation and then lighting at night. As for the latter, the output power of solar cells is measured by electric power; therefore, it is necessary to calculate the equivalent electric power for the direct lighting by the first 50% VIS light by using the supposed rated efficacy E (lm/W) of LED. For an luminous flux of an LED same as that of sunlight, the equivalent electric power P e (W) of LED can be given as where Φ νn , (n = 1, 2, 3, 4) (lm) are luminous flux from the FOB system at the lamps of 4 types, so the utilization ratio of solar energy can be defined as When the efficacy E = 129.5 lm/W (the rated efficacy of XLamp XP-G3 of Cree), the system utilization ratio of solar energy every hour can be shown in Figure 14.

| CONCLUSIONS
A large dish-type concentrator solar lighting system has been described and evaluated for lighting in an underground car park.
The beam split thin film has been selectively designed to reflect part of VIS light and meanwhile transmit the rest VIS and all invisible light. The proposed structure of large dish can concentrate solar radiation, split spectra and produce electricity as well, by using a laminated layer of beam split coating and thin-film solar cell. The conversion efficiency of solar cells is about 19%. As for the transmission of sunlight, the optical efficiency of dish concentrator is 12.16%. The simulation of lighting a 1771 m 2 underground car park by the large dish-type concentrator solar lighting systems achieves the average daylight illuminance varying between 62.7 lx which is the setting point for LED light and the maximum 284 lx on February 25, 2016 and between 62.7 and 353 lx on August 17, 2016, respectively, in Wuhan, China, with about 40% utilization ratio of solar energy that proves the system can light that place up to lighting standard efficiently and have a performance of stable illuminance.
Furthermore, for the current design to evaluate the overall lighting performance of system, the provision is just enough and meets the standard of lighting basically for the whole day. In the follow-up research, we will try to balance the beam split between daylighting and LED lighting for different days and seek to optimize the most appropriate spectral reflectance in beam splitting to realize the self-sufficiency of the complementary solar lighting for a long period in a whole year.