A new design of luminescent solar concentrator and its trial run

INTERNATIONAL JOURNAL OF ENERGY RESEARCHInt. J. Energy Res. 2010; 34:1372–1385Published online 19 February 2010 in Wiley Online Library(wileyonlinelibrary.com). DOI: 10.1002/er.1681

A new design of luminescent solar concentrator and its trial run

Chen Wang1,�,y, H. Abdul-Rahman1 and S. P. Rao2

1Centre for Construction and Innovation and Project Management (CIPM), University of Malaya, Kuala Lumpur, Malaysia2Centre for Equatorial Sustainable Design (ESD), University of Malaya, Kuala Lumpur, Malaysia

SUMMARY

Normal luminescent solar concentrators (LSC) have a great limitation in their light transportation because the lightproduced by the recently reported LSCs is not a point light source. In order to enable LSCs to use optical fibers as theeffective remote light transport media, a new design of LSC, which uses three color luminescent fibers for solarabsorption and uses clear optical fiber bundles to transport the absorbed sunlight into a remote place, is fabricated andtested. Radiation flux ratios with a mean value of 5.7%, the luminous flux up to 114.1 lumens, and the light efficiency of0.56% have been achieved during the trial run. The luminous efficacy as 0.643 lmW�1 is higher than that of combustingcandles (0.3 lmW�1) but lower than that of the incandescent light bulbs (16–40 lmW�1). Further, since the sun light isfree, different from electrical light sources, the luminous efficacy of the new LSC does not result in any electricityconsumption when it is under operation. A color analysis and spectrum test proves that the light produced by the newLSC performs a great match to the direct sun light in color. Copyright r 2010 John Wiley & Sons, Ltd.

KEY WORDS: LSC; optical fibers; solar energy

1. INTRODUCTION

Luminescent solar concentrators (LSCs) haveattracted the attention of a large number ofscientists and engineers since the first proposal byWeber and Lambe in [1]. LSCs were initiallydeveloped as an alternative approach to lower thecosts of PV. As both direct and diffuse light isconcentrated by a factor of 5–10, without the needfor expensive tracking, smaller silicon or other solarcells can be used. As the cost of the transparentplastic is expected to be much lower than the areacost of the solar cell, the cost per Watt-peak is lower

when compared with the cost of a planar siliconsolar cell [2,3]. LSCs are also developed for lightingpurposes. For instance, an LSC daylighting systemproduced by Earp et al. [4] transports sunlight toremote areas using a stack of pink, green, and violetLSCs and clear polymethyl methacrylate (PMMA)light guides. A difficulty in remote light transporta-tion has been found in this LSC system. As ageneral limitation for the normal LSCs, the lightproduced is not a point light source, which remainsas the bottleneck problem in LSCs’ light transpor-tation. According to Enedir and John [5], opticalfibers are the best media for the transportation of

*Correspondence to: Chen Wang, Centre for Construction and Innovation and Project Management (CIPM), University of Malaya,Kuala Lumpur, Malaysia.yE-mail: [email protected]

Received 14 October 2009

Revised 30 November 2009

Accepted 31 December 2009Copyright r 2010 John Wiley & Sons, Ltd.

concentrated light because of its advantages inreticulation and the high transport rate. In orderto enable LSCs to use optical fibers to transportthe concentrated light, a new design of LSC isfabricated and tested in this research.

2. SOLAR CONCENTRATORS AND LSCS

Solar concentrators were early brought intoconsideration as alternative ways to reduce thecost of photovoltaic electricity and solar heat dueto the relatively high material and productioncosts of solar cells and solar thermal absorbers[6,7]. Concentrating systems use lenses or reflectorsto focus sunlight onto the light collector. Highconcentration of solar radiation requires trackingof the sun around one axis or two axes, dependingon the geometry of the system [8–10]. The higherthe concentration, the more concentrator materialper unit area of solar cell or thermal absorber areais generally needed. The Monte Carlo ray tracetechnique is used by Tanejain et al. [11] in a two-stage linear solar concentrator employing aperfectly tracked parabolic trough as the primaryand a seasonally adjusted compound parabolicconcentrator as the secondary stage with a flathorizontal absorber. The geometrical profile of aseasonally adjusted solar concentrator composedof plane mirror elements and employing a tubularabsorber has been optimized by Mullick andNanda [12]. The procedure maximizes concentra-tion for any specified number of mirror elements,and for acceptance angle, with the height of theconcentrator or the reflector area as a constraint.The use of concentrated solar energy and itstransport in optical fibers was early studied byCariou et al. [13]. Transmission properties of fibersas well as geometrical conditions of the associationbetween fibers and concentrator were investigated:from which it was shown that modules where onefiber is associated with a small parabolic mirrormight supply 2W with efficiency greater than70%, whereas the concentration on the exit end ofa 10m long fiber may exceed 3000. Such a devicehas been achieved and the experimental results arein good agreement with the preliminary study [13].

A disadvantage of conventional solar con-centrators is that systems using mirrors or lens

chiefly rely on beam solar irradiation and requiretracking mechanisms to avoid astigmatism andother light losses experienced during the collectionof solar energy so that they lose their functionsin cloudy and diffuse conditions [14]. A solar con-centrator reported by Estrada et al. [15] usingporous silicon photonic mirrors as secondary reflec-tors in solar concentration systems were fabricatedwith nano-structured porous silicon to reflect lightfrom the visible range to the near-infrared region(500–2500 nm), although this range could be tunedfor specific wavelength applications. The concen-trator was exposed to high radiation in solar con-centrator equipment as shown in Figure 1. Poulekand Libra [16] developed a tracking ridge con-centrator using proven tracker hardware. This sys-tem combines a simple low-cost tracker with flatbooster mirrors but unlike V-trough concentratoras reported by Klotz [17] and Nann [18]. For thenew ridge concentrator, the mirror has been elimi-nated as shown in Figure 2.

On single axis trackers, both horizontal andpolar, the mirrors have to be extended beyond PVpanels to ensure uniform illumination of panels atseasonally variable elevation of the sun. On polaraxis trackers with seasonally adjustable slope of

Figure 1. Acronym of device for the study of highlyconcentrated radioactive fluxes ([15], p. 1308).

LUMINESCENT SOLAR CONCENTRATOR AND ITS TRIAL RUN 1373

Copyright r 2010 John Wiley & Sons, Ltd. Int. J. Energy Res. 2010; 34:1372–1385

DOI: 10.1002/er

the axle the extended mirror is not needed. UnlikeV-trough concentrators, no additional mirrorsupporting structures are needed. However, itcould only double solar energy gain of PV panelsin comparison with fixed ones [16].

The luminescent planar solar concentrator wasproposed in the late 1970s [1,19,20] consisting of atransparent plastic sheet doped with organic dyes.Sunlight is absorbed by the dye and then re-ra-diated isotropically, ideally with high quantumefficiency, and trapped in the sheet by internalreflection. A stack of sheets doped with differentdyes can separate the light. The operation of theLSC, which can be considered as a peculiar kind oflight guide, is based on the following principles:one or more high quantum yield species are dis-solved in a rigid highly transparent medium ofhigh refractive index. Solar photons entering theplate are absorbed by the luminescent species andreemitted in random directions. Following Snell’slaw, a large fraction of the emitted photons will betrapped within the plate and transported by totalinternal reflections to the edge of the plate, as il-lustrated in Figure 3, where they will be convertedby appropriate photovoltaic cells or be collectedfor lighting purposes [2,21,22].

Conversion of the incident solar spectrum tomonochromatic light would greatly increase theefficiency of solar cells. LSCs consist of a highlytransparent plastic, in which luminescent species,

originally organic dye molecules, are dispersed.These dyes absorb incident light and isotropicallyemit it at a red-shifted wavelength, with highquantum efficiency. Internal reflection ensures col-lection of part of the emitted light in the solar cellsat the sides of the plastic body. The energy of theemitted photons ideally is only somewhat largerthan the band gap of the attached solar cells, toensure near-unity conversion efficiency [19]. A largefraction of the emitted photon losses come from theescape cones. The size and form of the cross sectioncould impact the proportion of photons trapped bythe LSC plate and the reduction of the cross-sec-tional area of the luminescent plate could increasethe photon loss [2,3,21,22]. It has been reported thatthin luminescent concentrator films could be im-plemented in the form of integrated devices or assensitive elements in the traditional four-detectordifferential position sensors [23,24].

Although the luminescent fibers have not ex-perienced the dramatic commercial success forcommunications and lighting purposes, they havebeen continuously and enthusiastically studied[25–27]. Luminescent fibers are often used in ra-diation protection and measuring devices. Theirworking principle is similar to luminescent con-centrators. The luminescent material absorbs high-energy alpha, beta, and gamma radiation andconverts that energy into forms that can be mea-sured by conventional visible light detectors [28].

3. TDGS AND OPTICAL FIBER FOR LIGHTTRANSPORTATION

Daylight guidance redirects natural light intobuilding areas that cannot be lit by conventionalglazing. The most commercially successful type is

Figure 2. The polar axis tracking ridge concentratorwith 8�55 Wp PV panels ([16], p. 201).

Figure 3. Schematic representation of LSC ([3], p. 245).

C. WANG, H. ABDUL-RAHMAN AND S. P. RAO1374


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tubular daylight guidance systems (TDGS), whichcomprise a clear polycarbonate domed light collec-tor that accepts sunlight and skylight from thewhole sky, a light transport tube lined with highlyreflective silvered or prismatic material, and adiffuser commonly made of opal or prismaticmaterial light to distribute light in an interior [29].The majority of commercially available TDGS aresimply empty tubes along which light can travel intothe interior of a building or other dark spaces. Theyare available from a number of manufacturers andare versatile enough to be installed in straight orangled assemblies, enabling them to bring daylightinto otherwise inaccessible rooms. The coating onthe internal surface of the TDGS is composed ofhighly reflective materials such as anodized alumi-num or coated plastic films such as Alcoa Everbriteand Silverlux, which have reflectances greater than95% [30]. TDGS use the principle of high efficiencyreflection, and as a result straight TDGS performbetter than angled ones as light energy decreaseswith increased reflections [31]. Each TDGS bendmay reduce light output by approximately 8% [32].The TDGS also transmits less solar heat thanwindows, preventing internal heat gains in summerand heat loss in winter. Finally, the diffuserdistributes the light more evenly into the space theTDGS is illuminating.

According to Enedir and John [5], since the early1990s, fiber optic cables using an artificial lightsource have been used in remote-source lightingsystems. Using this technology, light travels from itssource to one or more remote points through fiberoptic cables. The technology has been used in manyapplications such as museums and retail displaysand in architectural applications to emphasize thefeatures of a building or to outline its exteriorcontours; other applications have involved lightingexit signs and aisles in theatres and air planes, etc.During the past 30 years a new type of optical fiberhas been researched, namely the plastic opticalfiber. The situation of this transmission mediumhad remained rather stagnated for years becauseof its high attenuation and the lack of demand ofspecific commercial applications. However, sincethe development of the graded index plastic opticalfibers in 1990 and the later attainment of the low-attenuation per fluorinated fibers in 1996, plastic

optical fibers have received a lot of interest, which isexpected to give rise to a great deal of applicationsin the next several years [33]. Because of its highfrequencies, the optical portion of the electro-magnetic spectrum is currently being used byemploying optical fibers as transmission media.Specifically, the well-known plastic optical fiberswith PMMA core were introduced in the 1960s,although the first optical fibers that were used weremade of glass. In the past several decades, con-current with the successive improvements in glassfibers, POFs have become increasingly popular,owing to their growing utility [34,35].

In building integration, one of the most impor-tant features of the remote light transportationis the reticulation method, and the reticulationmethod is expected to be as simple as possible[5,13,34,35]. The TDGS have their difficulties forreticulation so that optical fibers are more suitablefor this requirement than TDGS. A point lightsource is always required for the optical fiber totransport light [13,34,35].

4. THE NEED FOR A NEW TYPE OF LSC

Daylight has a disadvantage that it may not able toreach many areas such as storerooms, basements,and corridors. One also brings heat gain with thelight [30,36]. TDGS were designed to transport thedaylight to the deeper parts in buildings. However,the TDGS have their difficulties for reticulation sothat daylight transportation through optical fibers isconsidered as the best approach [5,13]. In buildingintegration, one of the most important features ofthe remote light transportation is the reticulationmethod, and the reticulation method is expected tobe as simple as that of electrical wires. Only opticalfibers are suitable for this requirement. However,the optical fiber needs a point light source for it totransport [13,34,35].

Solar concentrators have been designed usingoptical approaches such as using mirrors and/orlens because of the high price for PV cells. Sincethey are only sensitive for the beam irradiation,they function poorly in the cloudy weather and thediffuse light conditions, and a tracker is alwaysneeded [37,38]. LSC and some static solar con-centrators were then designed as the diffuse light



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solution and the static solution respectively[1,19,20]. Static concentrators always come with apoor concentration rate without a tracker [39], andthe light concentrated by normal LSCs could notbe transported by optical fibers to a remote placesince the light produced by an LSC is not a pointlight source [37,38,40,41]. Therefore, a new con-cept, which is to use luminescent fibers for solarconcentration and to use clear optical fiber bun-dles to transport the absorbed daylight into a re-mote place, is developed by the author.Accordingly, a new LSC is designed and fabricatedin this study based on the concept.

5. A NEW DESIGN OF LSC AND ITSWORKING PRINCIPLE

The new LSC consists of totally 150 pieces of 1mlong 2mm diameter luminescent fibers. The AutoCAD Schematic of the new LSC is provided inFigure 4. The material for these luminescent fibersis acrylic with quantum dots seeded in them.

Detailed composition and structure of the quan-tum dots are proprietary, but the characteristics ofthe luminescent fibers are provided by the supplier:the luminescent fibers have a refractive index (Nd/251C) of 1.49 and a light transmission rate of 93%.The quantum yields of all colors range from 0.8 to1.0. The half-lengths of the green, yellow, and redluminescent fibers are 1.13, 0.96, and 0.79m,respectively. Relation between length and lightoutput is complicated; however, assuming the fiberhas absolutely no bending and uniform lightacross the entire fiber, the maximum light outputwould be produced by the luminescent fiber with alength around 1m. If fiber is coiled or bent, themaximum light output length will be shortened. Asshown in Figure 5, a 1300m� 1300m silver-giltspecular reflector with a approximate reflectivityof 95% is installed under the new LSC plate toincrease the light absorption. A pyranometer and alux sensor connected with a remote data logger arefixed together with the plate to monitor the realtime solar irradiation received by the new LSC.

Figure 4. Auto CAD schematic of the new LSC.



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The 150 pieces of luminescent fibers consist ofthree colors (green, yellow, and red) as shown inFigure 5. One reason for using three color fibers istrying to cover a full spectrum band [4]. Anotherreason for using three color fibers is because thetransported light yielding out of the finishing endcould considerably mix into a white color lightby self-scattering for illumination purpose. Themixed white light was proved by CIE color ana-lysis and naked eye observation. There are 50pieces for each color. The totally 150 pieces ofluminescent fibers are symmetrically embedded ina 1200mm� 1200mm PMMA plate with a spaceof 2mm between each two pieces of fibers. Thelight transmission rate for the PMMA plate is93%. At both edges of the new LSC plate, eachluminescent fiber is polished and connected witha 10m long 2mm diameter PMMA clear opticalfiber by an aluminum bushing and fixed by a typeof UV glue. In the conjunction, both the lumi-nescent fiber and the clear fiber have their endspolished to give a smooth finish. The detaildrawing for the aluminum bushing is illustrated inFigure 6. The light transmission rate for the clearfibers is 94%. The light absorbed by the lumines-cent fibers is transported by these clear fibers to aremote dark room for testing purposes. The sec-tional view of LSC is provided in Figure 7.

The new LSC consisting of transparent PMMAfibers doped with appropriate quantum dyes. Inbrief, sunlight is absorbed by the fibers and thenre-radiated isotropically by the quantum dots,ideally with high quantum efficiency, and trapped

in the fibers by internal reflection. As illustrated inFigure 8, the micro operation of the new LSC isbased on the following principles: quantum yieldspecies are dissolved in rigid highly transparentPMMA fibers of high refractive index. Solarphotons entering the LSC plate are absorbed bythe luminescent species and reemitted in randomdirections. A large fraction of the emitted photons

Figure 5. The new LSC installed on the building roof.

Figure 7. Sectional view of LSC.

Figure 6. The aluminum bushing for luminescent fiberand clear fiber.



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are trapped within the fibers and transported bytotal internal reflections to the edge of the lumi-nescent fibers, where they enter the edge of theclear fibers and are transported to the dark roomthrough the clear fiber bundle. Figure 9 shows howthe new LSC mounted on the building roof light-ens a windowless dark store room on the fourthfloor of a building. The building is the tallestbuilding in the surrounding 500m2. There is notree or any other construction blocking the newLSC from receiving sun light during the entiredaytime.

6. INSTRUMENTATION ANDEXPERIMENTAL PROCEDURES

The light concentrated by the new LSC istransported by two 30mm diameter clear fiberbundles. Each aluminum finishing end of the fiberbundles has a diameter of 30mm. The ends of thetwo clear fiber bundles are polished to give asmooth finish. In order to monitor its workingperformance, the new LSC is mounted on abuilding roof and the concentrated light is trans-ported by two 10m long clear fiber bundles into a

Figure 8. Principle of work for luminescent fiber.

Figure 9. Principle of work for the new LSC mounted on the roof.



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remote windowless dark room. A pyranometer anda lux sensor are installed with the LSC plate todetect the solar radiation and illuminance. Thepyranometer installed with the LSC plateis denoted as Pyranometer 1. The radiation ofan LSC finishing end is measured by anotherpyranometer denoted as Pyranometer 2. Pyrano-meter 2 is installed pressing against the finishingend. The model for both pyranometers isSKS1110 with a responding range from 400 to1100nm. The calibration factors of Pyranometersare 0.05644mAW�1m�2 and 10.22mVW�1m�2.The values measured by the Pyranometer 1 andPyranometer 2 are denoted as R1 and R2 respec-tively for analysis, which are recorded by a remotedata logger with an interval of 10min for 24h perday. The unit for R1 and R2 is Wm�2. Anotherfinishing end is installed upon a lux sensor at adistance of 100mm. The model of the lux sensorsare SKL 310 with a responding range from 350 to750nm. The calibration factors for the lux sensorsare 0.1597mAklux�1 and 100.00mVklux�1. Thevalues read by the lux sensor are marked as Ev andthey are also recorded by the remote data loggerwith an interval of 10min for 24h a day. The unitfor Ev is lux. EPP2000&ISA2000 spectrometer wasused for the CIE color analysis and the wavelengthtest, the responding range for which is from 190 to2200nm. The 24-h data recorded for the new LSCfrom 6:00 to 20:00 every day were selected. Variousanalyses have been conducted for the new LSCsystem, namely: radiation flux ratio (Zr), luminousflux, luminous efficacy, the light efficiency (Zl), andCIE analysis, and so on. The statistic methods suchas correlation and linear test are employed.

7. RESULTS FROM TRIAL RUN

7.1. The solar radiation flux (R1) and the LSCradiation flux (R2)

The mean values for R1 and R2 in the 6 months are307.73Wm�2 and 17.02Wm�2, respectively. Themaximum values for R1 and R2 in these 6 monthsare both at 13:00 on 14th June 2008, which are1308.31Wm�2 and 64.77Wm�2, respectively. Thehourly values of R1 and R2 on a random day 4thJune 2008 are compared in Figure 10. Figure 11

presents the R2 values to give a better reading.Figures 10 and 11 show that the hourly R1 and R2

have very similar curves, from which it is assumedthat the hourly values of R1 and R2 are linear. Thelinearity assumption is proved by the linear testconducted in the program ‘Statistical Package forSocial Science (SPSS) program for Windows98/XP’.The significance value from a linear test between R1

and R2 is under a 0.05 level, which indicates that R2

is significantly linear with the R1. This findingconcludes that the radiation yielded from the LSCfinishing end is significantly proportional with thesolar radiation irradiated on the LSC plate.

7.2. LSC radiation flux ratio (Zr)

System radiation flux ratio is defined as the ratioof the LSC radiation flux (R2) to the solarradiation flux (R1) as illustrated in Equation (1).Comparing the light efficiency, which focus on the

Figure 10. Hourly R1 and R2 on a random day.

Figure 11. Hourly R2 on a random day.



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luminous flux (lumens), the radiation flux ratiotakes account that how much energy of theradiation could be transported by the system.Since the luminous flux is only sensitive to thespectral range from 400 to 700 nm, the radiationflux ratio covers much larger spectral range from300 to 1100 nm. In this regards, the radiation ratiois an indispensable ministrant parameter to thelight efficiency in the lighting application.

Zr ¼ R2=R1 ð1Þ

The 6-month mean value of the Zr from 24 May2008 to 23 November 2008 is 0.057. The standarddeviation for the 6-month Zr is 0.0015, which isunder a 0.01 level, so that the mean value of 0.057 isconcluded to be significantly stable and representa-tive for these 6 months. The curve of the radiationflux ratio (Zr) from 7:00 to 17:00 on a random day4th June 2008 is illustrated in Figure 12. On the day4th June 2008, the maximum value of Zr is 0.11 (at7:50) and the minimum value is 0.046 (at 17:00). Themean value of Zr on that day is 0.056. The standarddeviation for the values at a 10-min interval is 0.012,which is under a 0.05 level, so that the radiation fluxratio mean value of 0.056 is significantly represen-tative for that day. The radiation flux ratio from9:00 to 16:00 appears stable. The peak value appearsat around 7:45 when the sun light is very weak.

7.3. Luminous flux

As described by [42], the lux takes into account thearea over which the luminous flux is spread. If alight source emits one candela of luminousintensity uniformly across a solid angle of onesteradian, its total luminous flux emitted into thatangle is one lumen. Alternatively, an isotropic one-candela light source emits a total luminous flux of

exactly 4p lumens. If the source were partiallycovered by an ideal absorbing hemisphere, thatsystem would radiate half as much luminous flux,which is only 2p lumens. The luminous intensitywould still be one candela in those directions thatare not obscured [42]. Accordingly, the luminousflux (F) for one LSC finishing end in this researchis calculated in Equation (2):

F ¼ Ev� S ð2Þ

Where Ev is the values detected by the lux sensorand the S is the half sphere’s surface area radiatedby the light source. The distance between thefinishing end and the lux sensor is 0.1m. Theluminous flux (F) of one finishing end is calculatedas shown in Equation (3):

F ¼ Ev� S ¼ 0:063Ev ð3Þ

Equation (3) indicates that the luminous flux (F) ofone of the new LSC finishing end is proportionalto the illuminance values. The hourly luminousflux of one LSC finishing end is illustrated inFigure 13. As presented in Figure 13, one LSCfinishing end could provide a luminous fluxbetween 20 lumens to 110 lumens during the officehour from 9:00 to 17:00.

The monitored peak value of Ev provided byone LSC finishing end was 1811 lux, which ap-peared at 13:00 on 31 May 2008, when the solarradiation was 1156.13Wm�2 and the solar illu-minance was 127174.3 lux at that moment. Theluminous flux (F) of a finishing end at that timeequals 114.1 lumens as calculated by Equation (3).It is higher than that of a 15W incandescent lamp(110 lumens) according to [43,44]. Compared to atypical 25W bulb (200 lumens), totally two LSC

Figure 12. Hourly radiation flux ratio (Zr) on a random day.



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finishing ends or one LSC system is required todeliver an equivalent lighting effect.

7.4. Light efficiency (Zl)

Light efficiency is the ratio of output luminous fluxto the input luminous flux [4,45]. Evaluation isconducted to calculate the ratio of the luminousflux yielding from the two LSC finishing ends to theluminous flux dropping on the luminescent fibers,and it is defined here as the light efficiency (Zl). Theluminous flux yielding from the LSC finishingdenoted as F has been calculated in Equation (3).The luminous flux dropping on the luminescentfibers is denoted as F0, which is the luminous fluxprovided by the sun light. Accordingly, the lightefficiency (Zl) is defined by Equation (4). Thecalculated LSC light efficiency (Zl) is 0.56%. Thesize and the form of the cross section could impacton the proportion of photons trapped by the LSCplate and the reduction of the cross-sectional areaof the luminescent plate could increase the photonloss [2,3,21,22]. Accordingly, the light efficiency ofthe LSC could be raised by increasing the diameterof the fluorescent fibers embedded.

Zl ¼ 2F=F0 ð4Þ

7.5. Luminous efficacy of the new LSC

Luminous efficacy is a figure of merit for lightsources. It is the ratio of luminous flux (in lumens)to power (usually measured in watts). As mostcommonly used, it is the ratio of luminous fluxemitted from a light source to the power consumedby the source, and thus describes how well the

source provides visible light from a given amountof energy [45]. Accordingly, the luminous efficacyfor the new LSC is defined by Equation (5):

Luminous efficacy ¼ 2F=E ð5Þ

Where F is the luminous flux produced by onefinishing end, and E is the solar energy irradiatedon the luminescent fibers. Since there are twofinishing ends equally sharing the irradiation fromthe luminescent fibers, the coefficient ‘2’ is usedhere in Equation (5). The luminous efficacy of theLSC finishing end is calculated as 0.643 lmW�1,which is higher than that of a combusting candleat 0.3 lmW�1 according to [46]. The new LSC’sluminous efficacy is lower than that of theincandescent light bulbs (16–40 lmW�1) accordingto [47]. Since the sun light is free, different fromthe electrical light sources, the low luminousefficacy of the new LSC does not result in anyelectricity consumption when it is under operation.

7.6. CIE color analysis and spectrum test

The CIE (Commission Internationale de l’Eclai-rage, the International Commission on Color)color analysis and spectrum test for the lightproduced by the new LSC were conducted on arandom day 16th Oct. 2008. The device used forthe CIE color analysis and the wavelength test isthe EPP2000&ISA2000 spectrometer, the respond-ing range for which is from 190 to 2200 nm. Sincethe visible light waveband drops between 400 and700 nm and the average wavelength for the whitelight is around 555 nm [48], this device is suitablefor the test described in this section. A CIE coloranalysis was conducted under the irradiantmode at around 12:30 (sunny with little clouds).Figure 14 presents the screen of the CIE diagram.As indicated in Figure 14, the solid curve on theright side of the diagram is the black body curve.There are two diagonals forming a cross in thecentral area and the cross point is indicated as‘central white point’ in the diagram, wherex5 0.333 and y5 0.333. Any detected point dropson or near this central white point is considered aswhite color [45]. The LSC point (x5 0.479 andy5 0.460) in the diagram drops between thecentral white point and the yellow area indicatingthat the new LSC light output leans a bit towards

Figure 13. Hourly luminous flux on a random day.



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the yellow area comparing to the absolutely whitelight. Referring to the coordinate of the direct sunlight (x5 0.427 and y5 0.401), the LSC point

(x5 0.479 and y5 0.460) appears a great match.Therefore, it concludes that the new LSC lightoutput under a sunny sky is a kind of yellow-whitecolor light and its color has a good match to that ofthe direct sunlight. According to [42,45], a yellow-white color light is comforting to human eyes.

The spectrum test was conducted on the sameday on 16th Oct. 2008. Figure 15 presents a wa-velength screen in the scope mode at 12:23 on 16thOct. 2008, from which it indicates on the top of thescreen that the centroid is 607.31 nm. Comparingto the wavelength centroid of 598.57 nm capturedfor the natural daylight under the clear sky con-dition at 11:47, the wavelength results reveal agreat match between the LSC light output and thenatural daylight.

8. ERROR ANALYSIS OF THEEXPERIMENT

First, as illustrated in Figures 16 and 17, there is astaircase that may probably cause some shadingfor the diffuse irradiation and some slight lightreflection on the LSC. The staircase locates on theNorth–West to the LSC and is about 1m higherFigure 14. CIE color analysis 12:27, 16 October 2008.

Figure 15. Wavelength 12:23, 16 October 2008.



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than the LSC plate. Because of the field limitation,there is not any way either to climb up to thestaircase roof to install the LSC on it or to find amore suitable place higher than the staircase roof.However, the staircase is considered causing onlyvery little shading for beam solar irradiation sincethat Kuala Lumpur is located in the northernhemisphere and the beam solar irradiation comesonly from the South and Top according to Dilshanet al. [49] and Kamaruzzaman et al. [50]. Further,the light reflected from the staircase’s South–Eastsurface may considerably counteract the blockeddiffuse irradiation. Therefore, the impact causedby the staircase was not experimentally studiedand it was considered as insignificant and negli-gible as mentioned above.

Second, the LSC is not exactly horizontallymounted but with a low gradient of around 31 tothe horizontal for the purpose to lead the rainwater

out of the surface, which may cause a slight un-derestimation of the solar radiation. However,the effect caused by the small gradient was notconsidered in the data analysis because of itslittle impact.

9. CONCLUSIONS ANDRECOMMENDATIONS FOR FUTURE

RESEARCH

The new designed LSC enables it to use opticalfibers as both its light concentration and lighttransport media. The fabricated new LSC hasachieved a radiation flux ratio of 5.7% and aluminous flux up to 114.1 lumens. The LSC coulddeliver a kind of light matching to the natural lightin color. The light efficiency is about 0.56% and itis expected to be raised by increasing the diameterof the luminescent fibers embedded in the LSC.Comparing to the conventional artificial lightingpowered by PV cells, which converts light toelectricity and then back to light again which haveinherent losses in each step in multi-converting, theidea of the new LSC as a shortcut light–lightconversion is considered to be a good alternativeapproach. As a limitation for this research, onlyluminescent fibers with a diameter of 2mm areused. The size and the form of the cross-sectionalarea could impact on the proportion of photonstrapped by the luminescent plate and the reductionof the cross-sectional area of the luminescentplate could remarkably increase the photon loss.Accordingly, if luminescent fibers with a largerdiameter could be embedded in the new LSC, say10mm diameter, the performance parameters ofthe new LSC such as the luminous efficacy and the

Figure 16. Front view of the building where LSC installed.

Figure 17. Top view of the staircase that close to LSC.



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light efficiency could increase. Experimentation forluminescent fibers with larger diameters is recom-mended in future study.

NOMENCLATURE

E 5 solar energy irradiated on theluminescent fibers (Watt)

Ev 5 illuminance (LUX)F 5 luminous flux yielding from a the

new LSC finishing end (lumen)F0 5 luminous flux dropping on lumi-

nescent fibers (lumen)R1 5 light radiation monitored by Pyr-

anometer 1 (Wm�2)R2 5 light radiation monitored by Pyr-

anometer 2 (Wm�2)S 5 the half sphere’s surface area

radiated by a light source (m2)

Greek

Zr 5 radiation flux ratio of the new LSCZl 5 light efficiency of the new LSCl 5wave length of light (nm)

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DOI: 10.1002/er

A new design of luminescent solar concentrator and its trial run

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