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A hybrid method for using sunlight and light-emitting diode (LED) illumination powered by renewable solar energy for indoor lighting is simulated and presented in this study. We can illuminate an indoor space and collect the solar energy using an optical switching system. When the system is turned off, the full spectrum of the sunlight is concentrated by a concentrator, to be absorbed by solar photovoltaic devices that provide the electricity to power the LEDs. When the system is turned on, the sunlight collected by the concentrator is split into visible and non-visible rays by a beam splitter. The visible rays pass through the light guide into a light box where it is mixed with LED light to ultimately provide uniform illumination by a diffuser. The non-visible rays are absorbed by the solar photovoltaic devices to provide electrical power for the LEDs. Simulation results show that the efficiency of the hybrid sunlight/LED illumination with the renewable solar energy saving design is better than that of LED and traditional lighting systems.
©2010 Optical Society of America
1. Introduction
The rapid development of efficient high power LEDs has led to the production of a variety of lighting applications, broadening our horizons and giving us a different view of lighting design. The advantages of LEDs mean they can now compete with, even surpass, traditional illumination [1]. Furthermore, powerful new legislation demands consideration of the environmental impact of a product over its life cycle, from production to disposal. All of this makes LEDs the ideal candidate for an environmentally-friendly light source [2]. Furthermore the worsening of the problems of global warming has made the development of renewable energy sources the focus of world-wide attention, one of which is solar energy.
The use of controlled beams of sunlight for lighting a building’s indoor spaces was first proposed in 1977 [3], followed in 1983 by the development of the concept of concentrating and piping sunlight for indoor illumination [4]. Such economical lighting designs can brighten interior spaces in the daytime. In 2008, Sun et al. took a step further to describe the illuminance and efficiency of LED and fluorescent light sources for interior illumination [5].
Photovoltaic systems use solar cells to directly convert the radiance of sunlight into electric power. They are very attractive energy sources, because they do not generate any pollution during operation, they can have a lifetime as long as 20 to 30 years and they need very little in the way of maintenance besides keeping the surface clean. This new technology has great potential encouraging solar cell production [6–9].
Designs for utilization of sunlight illumination, concentrators [10], light guides [11] and solar cells have been discussed, but their use in combination is rare. In this study we combine the most up-to-date in sunlight and LED illumination designs, especially with the goal of energy savings.
2. Theory
A hybrid sunlight and LED lighting system powered by renewable solar energy for indoor illumination is presented in this study. First we designed a sunlight concentrator to track and collect direct sunlight. Indoor illumination and collection of solar energy are controlled by an optical switching system. When the system is turned off, the full spectrum of the sunlight is concentrated by a concentrator to be absorbed by solar photovoltaic devices that provide the LEDs with electrical power. When the system is turned on the collected sunlight passes through a beam splitter to be separated into visible and non-visible rays. The visible rays pass through the light guide and are introduced into a light box while the non-visible rays are collected by the solar cells to provide electrical power when the illuminance is not enough. The light box can combine the visible sunlight and LED light to provide uniform lighting, and finally, more uniform illumination is provided by a diffuser. The LEDs will be illuminated when sunlight is insufficient. We simulated the illumination and calculated the indoor lighting efficiency of our hybrid sunlight and LED lighting system, based on the renewable solar energy saving concept, and compared this with that of LED and florescent lighting systems.
2.1 Sunlight concentrator
The sunlight concentrator, as shown in Fig. 1 , is composed of two reflectors, one beam splitter, an optical switching system for saving solar energy, and a light guide. The concentrator is supposed to be equipped with a tracking system to collect sunlight in the normal direction. The sunlight is collected by reflectors and focused on the light guide. There is a beam splitter between the reflectors and the light guide. The separation of the sunlight into visible and non-visible rays by the beam splitter is controlled by the optical switching system.
2.2 Reflectors for the sunlight concentrator
The con-focal system designed to concentrate the light is shown in Fig. 2 . In the sunlight concentrator, a parabolic reflector (S1) focuses the parallel rays of sunlight to the focal point F; the ellipsoidal reflector (S2) focuses sunlight from F to the other focal point F'. The area of the sunlight concentrator in this design is 0.1075 m2, and the concentration angle is 23.04°.
The distance between the focal point and the reflective surface (as shown in Fig. 3 ) can be calculated using Eq. (1) through Eq. (3) [12], and the designed parameters are listed in Table 1 .
Fig. 3 Distance between the light source point (focal point) and the reflective surface for: (a) a parabolic surface; and (b) an ellipsoidal surface.
Table 1. Design parameters of the reflectors
2.3 Beam splitter
The plate beam splitter, as shown in Fig. 4 , is designed to separate the sunlight into visible and non-visible rays. The visible rays pass through the beam splitter to be focused by the light guide, after which they are introduced into the light box. The non-visible rays are collected by the solar cells. In this study, the sunlight is simply separated by a plate beam splitter, where the concentration angle θ is 23.04°, θ1/2 is 11.52°. The variation of the transmittance of the plate beam splitter with the wavelength is shown in Fig. 5 .
2.4 Optical switching system for solar energy saving
We describe a conceptual design for an optical switching system for saving solar energy. The system is designed to control the path of the sunlight so as to save solar energy during the daytime. When sunlight is not needed for illuminating the indoor space, the system is turned off. The concentrated sunlight is directly reflected by tilted mirrors, and the full spectrum of sunlight is collected by solar cells, as shown in Fig. 6(a) .
Fig. 6 Optical switching system for saving solar energy saving when in the: (a) off state; and (b) on state.
The collimating lens is used to ensure uniformity of the concentrated sunlight to avoid damage to the solar cell [13,14]. When sunlight is needed for illumination, the system is turned on. The visible and non-visible rays are split by a beam splitter. The visible rays pass through the light guide and are guided for indoor lighting, while the non-visible rays, made uniform by the collimating lens, are collected by solar cells, as shown in Fig. 6(b).
The focal length of the collimating lens can be calculated by using Eq. (4) [12]
where NA is the numerical aperture of the sunlight concentrator; D is the diameter of the collimating lens, which is also the diagonal length of the illuminated surface of the solar cell, so the focal length of the collimating lens can be calculated. The design parameters of the collimating lens are listed in Table 2 .Table 2. Design parameters of the collimating lens
2.5 LED and LED reflector
We simulated the performance of the Nichia NS6W183T with 215 lm as shown in Fig. 7(a) , operated at a current of 700 mA and a voltage of 2.8V. Its efficiency value is equal to 109.69 lm/W. The candle power distribution curve is shown in Fig. 7(b) [15]. A parabolic reflector is included in the design to control the LED rays, where the LED light source is located at the focal point of the parabolic reflector, as shown in Fig. 7(c).
2.6 Sunlight
The illuminance from the sunlight in Taipei was measured at different times. Here let us look at the illuminance for the date July 29, 2010 as an example. Six sunlight concentrators are arranged to collect sunlight to illuminate an indoor space. The illuminance is simulated from 8 a.m. to 9 p.m. The measured illuminances are listed in Table 3 . The area of the sunlight concentrator is 0.1075 m2. The conversion into luminous flux in lumens can be calculated by Eq. (5) [16]
where E is the measured illuminance; dS is the area of the sunlight concentrator; and dF is the input luminous flux in lumens. With Eq. (5), we can calculate the simulated input flux from the measured illuminance, to simulate the indoor illumination.Table 3. Illuminance measured for each sunlight concentrator on 07/29/2010 in Taipei, Taiwan
2.7 Solar Cells
The desired solar photovoltaic efficiency range is from 19% to 28% [8], considering the spectrum that we want to be absorbed and to illuminate at the same time. CuInSe2(CIS) thin-film photovoltaic devices are the best candidate for this job.
Complete CIS thin-film photovoltaic devices are fabricated using absorbers produced that offers the potential to significantly lower the fabrication cost of CIS solar cells. CIS offers high efficiency, long-term stability, and potential for low cost processing. An efficiency of as high as 19.9% has been achieved by a National Renewable Energy Laboratory group [7]. Power conversion efficiencies of as high as 12.2% have also been achieved [6], where the received spectrum is from 280 nm to 1300 nm. The absorption spectrum of CIS thin-film is shown in Fig. 8 [9]. The visible rays (400 nm to 700 nm) are designed for indoor lighting; the non-visible rays (280 nm to 400 nm and 700 nm to 1300 nm) for solar energy power.
2.8 Glare and Diffusion
Glare occurs in two ways [17]. One is too much light (e.g., direct or reflected sunlight) which can cause the observer to squint, blink, or look away. The other is a large intensity of luminance in the visual environment. Glare that reduces visual performance is called disability glare and is due to light scattering in the eye reducing the luminance contrast of the retinal image. The effect of scattered light on the luminance contrast can be mimicked by adding a uniform veil of luminance to the target.
Both iluminance performance and glare discomfort are taken into consideration. We are not only able to improve the illuminance, but also decrease the glare. An LG VEGACHEM Gr1(2t) diffuser (as described in Table 4 ) is installed below the light box to reduce glare.
Table 4. Specifications for the LG VEGACHEM Gr1(2t) diffuser
2.9 Light box
Figure 9 shows the layout of the cubic light box. BaSO4 paint with 95% reflectivity is applied on the inside surface of the light box, to detect, collect and make uniform the visible sunlight and LED light. There are six conical reflectors placed below the light-guide exits at the surface of the ceiling and 140 LEDs with parabolic reflectors arranged around the inside surface of the light box. Four sensors are placed at each corner on the top surface of the box. For the LED illumination design, each LED has a reflector. The candle power distribution curve of each LED with the reflector is shown in Fig. 10 . The basic light box model hung from the ceiling of the laboratory is 8.8 m long, 2.94 m wide and 1.20 m high.
If the sunlight is not bright enough to illuminate the table plane, some or all of the 140 LEDs can be turned on. The illuminance on the table plane is maintained above the standard level. When nighttime comes, all 140 LEDs can be turned on to illuminate the indoor space. Figure 11 shows an indoor lighting simulation in our laboratory space. The simulation was carried out using the DIALux software.
3. Simulation Results
Here in this study, the simulated reflectivity of both mirrors is 0.85; the simulated transmissions of the filters are 0.9435 and 0.9191, respectively. Since the simulated system is to be set on the roof, it is assumed that the indoor space model is on the top floor. The simulated length of the light guide is 0.5 meters, in which case the loss from the light guide is 0.738%. We first set up six sunlight concentrators to collect sunlight and introduce it into the light box. The light box is designed to make the visible sunlight and LED light uniform. Finally, we calculate the illuminance efficiency at the table plane in the laboratory.
After building the elements in the model light box using the LightTools software, we plug in the illuminance values measured on a sunny day for calculation of the simulated illuminance on the table plane. The scale model is shown in Fig. 11. The table plane used as the basis for simulating the illuminance in our laboratory is 8.8 m long, 2.94 m wide, and 0.74 m high. The standard required illuminance at the table plane is above 500 lux, and the average difference below 10%. The average difference can be computed by [5]
where N is the amount of receiver mesh. The average difference indicates the degree of uniformity on the table plane. The lower the percentage is, the better the uniformity.We find the candle power distribution curve for the simulated light box with only sunlight illumination as shown in Fig. 12 . The simulated table plane illuminances at different hours are shown in Fig. 13 and listed in Table 5 . Because of the BaSO4 surface and diffuser design, the candle power distribution curves look alike and are lambertian in form. The simulation shows that no LEDs are needed from 8 a.m. to 4 p.m. For these eight hours, we were able to obtain enough illuminance from sunlight without using any LEDs (the illuninance exceeds the standard required of 500 lux). Figure 13 shows a summary of the illuminance distributions, average illuminances and average differences at the table plane for the period from 8 a.m. to 9 p.m. with and without LED lighting. We find that the average illuminances from 8 a.m. to 4 p.m. are higher than the standard required 500 lux, and the average difference is always 6.75%. The light box design ensures that the average differences are all the same at different hours on a sunny day. The light provides the same contrast for the human eye on sunny days and prevents uncomfortable glare.
Fig. 13 Sunlight illuminance distributions, average illuminances and average differences on the table plane from 8 a.m. to 9 p.m.
Table 5. Average illuminance on the table plane obtained using only sunlight and the hybrid sunlight-LED sources
The average illuminances are shown in Table 5. Some or all of the LEDs need to be turned on to enhance the illuminance on the table plane when it falls below 500 lux. On an ideal sunny day, 66 LEDs are needed at 5 p.m., 100 LEDs at 6 p.m., and 140 LEDs after 7 p.m. The simulated illuminance distributions from 5 p.m. to 9 p.m. are shown in Fig. 14 . The average illuminances from 5 p.m. to 9 p.m. are all above 500 lux, and the average differences are all better than 6.75%.
Fig. 14 Sunlight and LED Illuminance distributions, average illuminance and average differences on the table plane during the period from 5 p.m. to 9 p.m.
Calculation and comparison shows the electricity charge of 10 T8 fluorescent tubes, 10 T5 fluorescent tubes, 140 LEDs, and hybrid sunlight and LEDs to have the same standard illuminance on the table plane during the time from 8 a.m. to 9 p.m. 10 T8 or T5 fluorescent tubes are needed to reach the standard illuminance in the same indoor environment.
The power consumption for a single T8 fluorescent tube is 40W and the ballast consumes 20W of electricity. The power consumption for a T5 fluorescent tube is 28W and the ballast consumes 4W of electricity. The consumption a single LED light is 1.96W. The daily, weekly, monthly and yearly power consumption for hybrid sunlight/LED illumination with the renewable solar energy saving concept are calculated to be less than 1.15kW, 5.74kW, 22.97kW and 275.65kW, respectively. The power and electricity consumption for these four different types of illumination are listed in Table 6 . The advantages of hybrid sunlight/LED illumination with the renewable solar energy saving concept are very clear.
Table 6. Power and electricity consumption for the different types of illumination
The hybrid sunlight and LED illumination system can meet the requirement standards for average illuminance. Calculation of the savings in power for a year using this system shows its advantages, let alone the illumination with renewable solar energy saving equipment.
4. Conclusions
In this study we find that sunlight is not always bright enough for purposes of indoor lighting, but we can illuminate our space with a system combining sunlight and LEDs to achieve the level of illuminance we need. Sunlight is the most natural of light sources, causing no pollution, and it is available free of charge. LEDs are also environmentally-friendly and energy saving light sources. If we could use a combination of sunlight and LEDs for illumination in our daily life, we gain the advantages of both, and do better in terms of energy saving and CO2 reduction.
Acknowledgement
This study was sponsored by the National Science Council under contract number NSC 98-2221-E-008-021-MY3.
References and links
1. R. Devonshire, “The Competitive Technology Environment for LED Lighting,” J. Light Visual Environ. 32, 275–287 (2008). [CrossRef]
2. N. Zheludev, “The life and times of the LED- a 100-year history,” Nat. Photonics 1, 189–192 (2007). [CrossRef]
3. M. A. Duguay and R. M. Edgar, “Lighting with sunlight using sun tracking concentrators,” Appl. Opt. 16(5), 1444–1446 (1977). [CrossRef] [PubMed]
4. L. M. Fraas, W. R. Pyle, and P. R. Ryason, “Concentrated and piped sunlight for indoor illumination,” Appl. Opt. 22(4), 578–582 (1983). [CrossRef] [PubMed]
5. C. H. Tsuei, J. W. Pen, and W. S. Sun, “Simulating the illuminance and the efficiency of the LED and fluorescent lights used in indoor lighting design,” Opt. Express 16(23), 18692–18701 (2008). [CrossRef]
6. W. Liu, D. B. Mitzi, M. Yuan, A. J. Kellock, S. J. Chey, and O. Gunawan, “12% Efficiency CuIn(Se,S)2 Photovoltaic Device Prepared Using a Hydrazine Solution Process,” Chem. Mater. 22, 1010–1014 (2010). [CrossRef]
7. I. Repins, M. A. Contreras, B. Egaas, C. DeHart, J. Scharf, C. L. Perkins, B. To, and R. Noufi, “19.9%-efficient znO/CdS/CuInGaSe2 solar cell with 81.2% fill factor,” Prog. Photovoltaics 16, 235–239 (2008). [CrossRef]
8. G. Sun, F. Chang, and R. A. Soref, “High efficiency thin-film crystalline Si/Ge tandem solar cell,” Opt. Express 18(4), 3746–3753 (2010). [CrossRef] [PubMed]
9. H. Chen, S. M. Yu, D. W. Shin, and J. B. Yoo, “Solvothermal Synthesis and Characterization of Chalcopyrite CuInSe2 Nanoparticles,” Nanoscale Res. Lett. 5, 217–233 (2010). [CrossRef]
10. C. Domínguez, I. Antón, and G. Sala, “Solar simulator for concentrator photovoltaic systems,” Opt. Express 16(19), 14894–14901 (2008). [CrossRef] [PubMed]
11. J. Mohelnikova, “Evaluation of Indoor Illuminance from Light Guides,” J. Light Visual Environ. 32, 20–26 (2008). [CrossRef]
12. D. Malacara, Optical Shop Testing 2ndEdition, (Wiley, 1992).
13. H. Ries, J. Gordon, and M. Lasken, “High-flux photovoltaic solar concentrators with kaleidoscope-based optical designs,” Sol. Energy 60, 11–16 (1997). [CrossRef]
14. M. Hernández, A. Cvetkovic, P. Benítez, and J. C. Miñano, ““High-performance Köhler concentrators with uniform irradiance on solar cell”, Invited paper Nonimaging Optics and Efficient Illumination Systems V,” Proc. SPIE 7059, 705908 (2008).
15. Nichia, “NS6w183T Datasheet,” http://www.nichia.com/specification/led_09/NS6W183T-H3-E.pdf
16. V. N. Mahajan, Optical Imaging And Aberrations - Part 1 Ray Geometrical Optics, (SPIE press,1998).
17. Illumination Engineering Society of North America, “Glare,” in IESNA Lighting Handbook 9thEdition, (IESNA, 2000), pp. 128–131.
