Applied Energy 92 (2012) 705–713

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Applied Energy

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Simulation and experimental studies on natural lighting in enclosed lift lobbiesof highrise residential buildings by remote source solar lighting

Irene Wong ⇑, H.L. Choi, H. YangRenewable Energy Research Group (RERG), Building Services Engineering Department, The Hong Kong Polytechnic University, Hong Kong

a r t i c l e i n f o

Article history:Received 12 April 2011Received in revised form 30 July 2011Accepted 7 August 2011Available online 17 September 2011

Keywords:Remote source lightingHeliostatSide-emitting fiber opticConverging lensCentral core design

0306-2619/$ - see front matter � 2011 Elsevier Ltd. Adoi:10.1016/j.apenergy.2011.08.013

⇑ Corresponding author. Tel.: +852 9028 6972; fax:E-mail address: changwongirene@yahoo.com.hk (I

a b s t r a c t

The residential buildings in Hong Kong are often developed into highrise central core blocks due to lim-ited buildable land and dense population. The lift lobbies in these buildings are enclosed without win-dows and their floor heights are restricted to not more than 2.8 m. Electric lighting is switched oncontinuously for 24 h a day. This paper investigated an alternative way to provide lighting for theenclosed space by solar energy via a remote source solar lighting system. The detailed analysis on differ-ent layouts of typical lift lobbies are reported first and then the remote source solar lighting system isproposed, which is composed of a simple heliostat and side-emitting fiber optic. Simulation on the lighttransmission performance in the system was carried out by the ZEMAX-EE. An experiment was carriedout to validate the simulation results. The validated simulation results can reveal the performance ofthe side-emitting fiber optic lighting system. The results show that the proposed remote source solarlighting system can be applied as an alternative lighting system to illuminate the enclosed lift lobby atdaytime in clear sky condition.

� 2011 Elsevier Ltd. All rights reserved.

1. Introduction

Hong Kong is characterized as a city of high-rise buildings.Forty-nine of the tallest one hundred residential buildings in theworld are in Hong Kong [1]. Approximately 80% of the 1902 squarekilometers of land is mountainous [2]. Population density was6500 persons per square kilometers in 2010 according to the statis-tic provided by the Census and Statistic Department [3]. High pop-ulation density and limited habitable land result in high land value.The average property price for a common size premise between 40and 70 m2 was HK$8627 per square meter in 1997 [2]. Residentialpremises are developed into highrise buildings to compensate thehigh land cost. The total height of a building is governed by the PlotRatio requirement in the Building (Planning) Regulations [4]. In or-der to build more floors within a building height, the floor heightseldom exceeds 2.8 m. Residential buildings usually adopt the cen-tral core design in which the lift lobbies and service areas aregrouped in the center to create more peripheral areas of valuableexterior views that can be sold or leased at a higher price. These liftlobbies are enclosed without natural lighting and depend on elec-tricity that is generated from fossil fuels to provide lighting. Theinterior of a typical lift lobby is shown in Fig. 1. Energy use in build-ings accounts for nearly half of the total primary energy use in theterritory [5]. Reduction in the use of electric lighting in these lift

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+852 2336 9994.. Wong).

lobbies can conserve our environment. This project investigatedthe potential of applying remote source solar lighting technologyto introduce natural lighting into the enclosed lift lobbies.

2. Survey of architectural layouts of enclosed lift lobbies

There are four main types of residential buildings in Hong Kong,which are public housings, villas and bungalows, home ownershipscheme (HOS) and private housings. Natural lighting is provided tothe lift lobbies of public housings while villas and bungalows donot have lift lobbies. These two types of residential buildings wereexcluded from the study. A detail study was carried out on the liftlobby layouts of 60% of the HOS and 50% of private buildings in2010 [6]. Under the Fire Services Regulations, lift lobbies musthave direct access to a staircase or link to a staircase through acommon passage [7] and natural light should be provided tostaircases at each floor level as required by the Building (Planning)Regulations [8]. However majority of the lift lobbies do not receivenatural lighting because staircases must be protected by a smokelobby under the Buildings (Planning) Regulations and Codes ofPractice [7,8].

The central core layouts of the studied residential buildings canbe categorized into four main type (Figs. 2–5). Windows are pro-vided at landings of staircase in type (a); windows are providedat flights of staircase in type (b); the staircase is totally enclosedin type (c) and lift lobby adjoins a light well in type (d). The lift lob-bies in 83% of the studied HOS and 98% of the private housings are

Fig. 1. Interior of a typical lift lobby.

Fig. 2. Type (a) lift lobby.

Fig. 3. Type (b) lift lobby.

Fig. 4. Type (c) lift lobby.

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enclosed without windows and use electrical lighting for the wholeday. Although type (d) lift lobby is abutting a light well, no windowis usually provided to avoid the undesirable view of the light well.The potential of applying remote source lighting technology (RSL)to illuminate these four types of lift lobby was analyzed. Naturallighting could be introduced into these four types of lift lobbiesby appropriately designed RSL. Layouts of the four proposed RSLwere diagrammatically represented in lines with arrows indicatingthe transfer of daylight in Figs. 2–5. To maximize ’’diamond

pattern’’. The domestic units are located along the diagonals ofthe central core to provide more windows to each domestic unit

Fig. 5. Type (d) lift lobby.

Fig. 6. Type (e) lift lobby.

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as shown in Fig. 6. This type of lift lobby layout is derived from type(b) and classified as type (e). The common corridors are longer andmore winding than other types. The RSL layout in type (e) is com-paratively more complicated than others. The worst case acenarioof type (d) lift lobby was selected as the case study for the applica-tion of RSL.

3. Remote source solar lighting system: case study

The RSL is a technique in which light is transmitted by a guidingdevice before being emitted in a remote location [9]. The mediumis usually a light guide that brings daylight further inside buildingsthat is currently illuminated by artificial lighting [10]. The commonguiding devices are metal light pipe (MLP) and prismatic light pipe(PLP) with diameter usually more than 400 mm and require 3 mheadroom clearance for installation [11]. K. Yeang had designed aRSL that directly utilized sunlight to illuminate an office buildingcalled Waterfront House in Kuala Lumpur in Malaysia (Fig. 7).The system composed of a laser cut panel light deflector (LCP)and MLP of 2 m wide, 0.8 m high and 20 m long as light transmis-sion medium. The LCP collected and directed sunlight in a colli-mated beam before entering the MLP, which transferred anddistributed sunlight to illuminate the interior space [12]. The LCPwas fixed at the external aperture of the PLP at an inclined angleof 55�, which is the optimal inclined angle for both Malaysia [12]and Hong Kong [13] to collect and redirect sunlight more directlyalong the axis of the horizontal light pipe. The horizontal MLP wereable to bring in daylight to illuminate working areas of 5–10 mdeep plan offices with illuminance level up to 300 lx without turn-ing on electric lighting for 4–6 h [12]. RSL that directly utilizes sun-light as light source is called remote source solar lighting system(RSSL). It is possible to integrate artificial light and natural lightinto one system [9]. RSSL can provide a centralized lighting systemand replace many electrical fixtures and cabling in a building [10],and allows flexibility in design and ease of maintenance.

4. The proposed solar lighting system and modifications

Hong Kong has a subtropical climate with abundant sunlight. Thepotential for utilizing daylight for saving energy is high [16] imply-ing that RSSL has the potential for development. However, the 3 mheadroom requirement for installing RSSL is not feasible in HongKong [13]. In type (e) lift lobby the RSSL was modified to suit the sit-uation in Hong Kong. The RSSL in Fig. 8 composed of a LCP as sunlightcollector; a 450 mm diameter MLP as light transfer medium, andside-emitting fiber optic (FO) as the second light transfer medium.As the acceptance aperture of fiber optics is very small, light needsto be highly concentrated before entering the FO [14]. Two plano-convex lenses of 450 mm in diameter were installed at the end ofthe MLP to converge light into the FO of small cross sectional areaof 10.4 mm diameter. Both the MLP and the converging lenses wouldbe installed in a service room where part of the headroom could bereduced below 2.8 m. The FO of smaller size with the flexibility tobend around corners was installed in the lift lobby and common cor-ridors without reducing the headroom. The FO layout could be inte-grated into the metal false ceiling as shown in Fig. 9. The anidolicceiling panels reflected the emitted light from the FO in a downwarddirection. The lighting effect would be similar to Fig. 1. Supplemen-tary electric lighting should be provided. A control system wouldswitch on the electric lighting automatically to maintain the interiorlight intensity at a minimum of 150 lx when the light intensity of thelift lobby fell below this level.

A preliminary experiment was set up to test the performance ofthe RSSL. The LCP was installed at the aperture of the 1.5 m longMLP and inclined at 55�. The plano-convex lenses were installedat the end of the MLP. The axis of the MLP, lens system and FO werein same alignment. The light beam that was collected by the LCPformed a blurred light spot after passing through the lenses andcould not be focused into the FO at all positions of the sun. A shar-per light spot began to form when the distance between the LCPand the lens was increased beyond 10 m. This distance is imprac-tical in application. The phenomenon can be explained by the fact

Fig. 7. A RSSL.

Fig. 8. The modified RSSL system.

Fig. 9. Perspective of a lift lobby with the RSSL.

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that light being redirected by the LCP was not in a perfect colli-mated beam. The light beam was slightly diverging. Multiplereflections occurred inside the MLP and formed a blurred image.A collimated light beam started to form when the distance be-tween the LCP and MLP was approaching infinity. The performanceof the LCP was also affected by the angle of the incident light. A re-search carried out by C.M. Kwok concluded that high elevated sun-light was deflected more axially down the pipe and made fewerreflections from traversing the pipe [15]. The RSSL system wasfurther modified. The LCP was replaced by a 760 � 760 mm plane

mirror which acted as a heliostat (Fig. 10). The position of the mir-ror was able to be adjusted both horizontally and vertically to re-flect sunlight parallel to the axis of the RSSL at all times.

5. Simulation analysis

ZEMAX-EE is an optical software for various aspects of opticalsystems with more advanced features like non-sequential ray trac-ing for lenses, light pipes, etc. in various medium. The accuracy ofsimulation is commented by professionals to be over 90%. In thefirst step, the orientation of the mirror was assumed at the optimaldue south position at noon when the sun was directly overhead.The mirror was adjusted to incline at an angle of 45� reflectingthe sun beam parallel to the axis of the lens system. The sizes, con-figurations and co-ordinations of each element of the RSSL wereinputted into the non-sequential ray-tracing program which simu-lated the RSSL. Thirteen light detectors were inputted to record theefficiencies of the RSSL at different points as shown in Fig. 11.

The following data were entered for simulation:

(a) Power input: 100 W.(b) Nos. of analysis rays: 10,000.

The power input into the mirror was automatically set to be111.25 W by the simulation program. The light intensity at eachdetector was calculated by the ZEMAX-EE and presented in theDetector Viewer Diagrams. The Total Power (W) that was shownin the Detector Viewer Diagram recorded the transmitted powerat respective detectors. With reference to Fig. 12, the Total Powerrecorded by D2 was 110.24 W. The transmission efficiency of themirror was calculated to be 99.1%. Similarly the efficiency of thelens system and the overall efficiency of the RSSL were calculatedto be 74.7% and 63.2% respectively.

In the next step, simulations of the mirror performance atvarious inclinations (h) from 5� to 175� were carried out. Theefficiencies ranged from 99 to 100%. The efficiency of a reflecting

Fig. 10. The proposed RSSL.

Fig. 11. The simulated RSSL.

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mirror orientated due south was averaged to 99.5% irrespective ofthe solar altitude (b), which is twice of h.

A preliminary experiment was carried out on 8 February 2011between 10:00 and 16:00 h to study the effect of solar altitudeand azimuth angle upon the efficiency of the RSSL. The mirrorwas installed with center line originally pointed to N32�W. A lightdetector was installed 1.5 m from the mirror and faced S32�W. Themirror was able to be adjusted both vertically and horizontallyresponding to the changes in b and azimuth angle so as to reflectthe sun beam perpendicular to the light detector at all times. Theaverage b and bearings of the mirror were recorded. As the trans-mitted indoor light intensity started to fall below 150 lx from14:40 h, records after this period were discarded. Simulations ofthe mirror behavior were carried out at 10:30 h, 12:10 h and14:38 h. The results were recorded in Table 1.

Deviation of the mirror orientation from south position loweredthe efficiency drastically from the ranges between 90% and 100% to30% and 40%; and consequently lowered the efficiency of the RSSL.The efficiency of the mirror increased from morning to afternoon inthe period between 10:00 and 16:00 h. Experiments were subse-quently carried out to verify the simulation results.

Note: D1: before mirror; D2: after mirror; D3: after lens 1; D4:after lens 2; D5: before entering FO; D6: at 1.66 m of FO; D7: at

2.16 m of FO; D8: at 2.67 m of FO; D9: at 3.87 m of FO; D10: at4.37 m of FO; D11: at 4.97 m of FO; D12: at 5.52 m of FO; D13:at 6.02 of FO.

6. Experiment and findings

Four numbers of RSSL were proposed to be installed in type (e) liftlobby as shown in Fig. 13. As the four RSSL were almost identical,only one quarter of the lift lobby was erected in a 1:2 wooden modelto test the performance of the system. The wooden structureorientated S32�E. A mirror of size 760 � 760 mm was mounted ona wooden stand opposite to the wooden structure and facedN32�W (see Fig. 14). Two plano-convex lenses of diameter450 mm were installed inside the wooden structure. 10.4 mm diam-eter large core LEF710M fiber optic, which was supplied by Fiberop-tics Technology Incorporation, was selected and installed in the ‘‘liftlobby’’. The FO was notched in factory at regular spacing to emit lightat 6% per meter run. The upper of the FO was covered by a reflectivejacketing that maximized the amount of light to be emitted in adivergent angle of 60� in downward direction. The length of the FOwas 6.02 m. The center lines of the mirror, the lenses and the aper-ture of the FO were in same alignment. The position of the mirrorcould be adjusted at both horizontal and vertical axes (Fig. 15)

Fig. 12. Detector viewer diagram of D2.

Table 1Changes in mirror efficiency at different times on 2 February 2011.

Time (h) Mirror bearings b (�) Efficiency of mirror (%) Overall efficiency of RSSL (%)

9:40 S62W 74 30.7 12.011:35 S76W 90 36.1 14.014:40 S84W 54 38.0 14.8

Fig. 13. Installation of four RSSL in enclosed lift lobby (Wong [6]).

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responding to the constant changes in azimuth angle and b in differ-ent times and dates so that the sun beam was always reflected

parallel to the axis of the lens system. Light transmission mediumwas not required because the mirror reflected the sun beam directly

Fig. 14. Mirror collector.

Fig. 15. Light emitted along FO.

Fig. 16. The trend of mirror bearing and b against time on 8 February 2011.

Fig. 17. Changes in indoor and outdoor illuminance vs. time on 8 February 2011.

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into the converging lens which converged the reflected light beaminto the FO. Light was transmitted and emitted uniformly alongthe whole length of the FO (Fig. 16). The modified RSSL system couldonly operate in clear sky condition.

From Hong Kong Observatory record, the mean daily solarradiation was lowest in February and March. The experimentwas carried on 8 February 2011 in the period from 9:30 to16:00 h to study the performance of the RSSL at different times.

The changes in mirror bearing and inclination were recorded. Theindoor and outdoor luminous intensities (lm/m2) were recordedby two sensors. Sensor 1 was located near the end of FO to recordthe indoor light intensity at the end of the system; and sensor 2was installed on the roof of the structure to record the changesin daylight intensity. The averages of the readings at differenttimes were plotted in Figs. 16 and 17. The value of b increased withtwice the increase in h from 9:30 to 11:35 around noon when h was45�. From noon b started to decrease in spite of an increase in hafter the sun changed from east to west position. b was lowest at14:30 h. The sunlight intensity was highest around noon between12:30 to 13:30 and the recorded indoor light intensity was 300 lm/m2 (Fig. 17). The RSSL could perform and emitted an average ofminimum 150 lm/m2 in the period from 10:30 to 14:30 whenthe external light intensity was 35,000 lm/m2 or above. The perfor-mance of the RSSL was affected by the sky condition and could notwork in cloudy condition.

7. Results and discussions

There is no available comprehensive optical design software inthe market to analyze the performance of side-emitting FO. Likemost optical design software, ZEMAX-EE calculates the efficiencyof end-emitting FO (EE) only [12]. In side-emitting FO, part of thetransmitted light is emitted along the length while the remainingis transmitted towards the end. The linear emittance of LEF710Mis 6% per meter, implying that 0.94(L) of the incident light intensityat specified length (L) is transmitted along the length of the FO. Thetransmission efficiencies (ES) of a straight side-emitting FO atvarious points were calculated to be 94(L)%. The efficiencies of anend-emitting FO at various points of the FO were calculated byZEMAX-EE and tabulated in Table 2.

Combining the side-emitting and end-emitting properties of theFO, the effective transmission efficiencies (ET) of the FO at variouspoints were calculated to be:

Table 2Transmission efficiencies at different locations of the RSSL.

Detection location @ Length of FO: L (m) Transmission efficiency: EE (%) Transmission efficiency: ES (%) Effective transmission efficiency: ET (%)

D1 – 100 –D2 – 99.1 – –D3 – 87.0 – –D4 – 74.7 – –D5 – 68.3 – –D6 1.66 68.3 90 62D7 2.16 66.6 87 58D8 2.67 65.3 85 55D9 3.87 65.8 79 52D10 4.37 65.0 76 50D11 4.97 65.0 73 48D12 5.52 63.2 71 45D13 6.02 56.9 68 39

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ETð%Þ ¼ EE � ES ð1Þ

Table 2 shows the EE, ES and ET at different locations of a RSSLlocated due south. The overall efficiency of the RSSL was calculatedto be 39%.

Comparing the performances of the RSSL in two different orien-tations with reference to Table 1 and 2, the RSSL is recommendedto be designed with minimum FO length and mirror facing south.

The transmitted solar energy is lowest at end of the FO. Theamount of emitted solar energy at the end of the RSSL is 6% ofthe remaining energy and should be 150 lx or lm/m2 minimum,which is the recommended light intensity in a lift lobby.

From the Hong Kong Observatory records, the daily average totalsolar radiation on a horizontal surface is 3.9 kW h/m2 with annualaverage daily sunshine hours of 5.28. Hence the average solar radi-ation that is received by a horizontal surface in 1 h is 740 W/m2.

The direct solar luminous efficacy (Ks) is defined as the ratio ofhorizontal direct illuminance to irradiance on the same surface inlm/W. C.M. Chung had carried out a study on the luminous efficacyof daylight in Hong Kong. The local Ks increased with b and couldbe calculated from the following equation [16]

Ks ¼ 48:5þ 1:67b� 0:0098b2 ð2Þ

The experiment demonstrated that the RSSL was able to deliver aminimum of 150 lm/m2 from 10:00 to 14:40 at most of the time on 8February 2011. h changed from 39� to 63� corresponding to bchanging from 78� to 54� in the period between 10:00 and 14:40 h(Fig. 16). b was lowest at 16:40 and Ks was calculated to be110.1 lm/W. The daylight intensity at this time was:

740 W=m2 � 110:1 lm=W ¼ 81;474 lm=m2

In Section 5 the efficiency of the RSSL with mirror facing S32�Wwas simulated to be 15%. The remaining light intensity at the endof the FO was estimated to be 12,221 lm/m2 and the emitted lightintensity at this point was estimated to be 733 lm/m2 maximum.Theoretically the RSSL could provide a maximum of 733 lm/m2 tothe lift lobby for about 4 h. However, the experiment showed thatthe RSSL could only deliver a maximum of 300 lm/m2 during thisperiod. The discrepancy in the two readings can be explained bythe difference in theoretical and measured outdoor intensity andthe actual efficiency of the RSSL being lower than the stimulatedvalue due to imperfection of materials.

8. Conclusions

Majority of lift lobbies in Hong Kong are enclosed without nat-ural lighting. A study was carried out on different layouts of liftlobby in residential buildings and concluded that installation of

appropriately designed RSSL could introduce natural lighting tothe enclosed lift lobbies.

The proposed RSSL that composed of mirror, converging lensesand side-emitting FO could solve the limited headroom problem inapplying RSSL in Hong Kong. A typical layout plan of the enclosed liftlobbies was selected as a case study. From the simulation analysiscarried out by ZEMAX-EE, the mirror should face south and thelength of the FO to be kept at minimum in the optimal design of aRSSL. The overall transmittance efficiency of the system was simu-lated to be 39%. A preliminary experiment was carried out in the per-iod between 9:30 and 15:00 h on 8 February 2011 in the winterseason of the lowest daily average total solar radiation in a year.The orientation of the RSSL was altered to S32�W and the positionof the mirror was adjusted responding to different positions of thesun in a day. ZEMAX-EE calculated the efficiencies of the RSSL at dif-ferent times. The efficiency of the RSSL decreased drastically whenthe system deviated from the due south location and was affectedby the changes in b and c. Theoretically the maximum emitted lightintensity was estimated to be 733 lm/m2, which is higher than therecommended light intensity of 150 lm/m2 in lift lobbies. Althoughin actual case could provide interior lighting in the range from 150to 300 lm/m2 in the period between 10:30 and 14:30 h as demon-strated by the experiment the RSSL and a minimum outdoor lightintensity of 35,000 lm/m2 was required to maintain the indoor lightintensity at 150 lm/m2, the proposed RSSL with supplementary elec-tric lighting can still be a potential alternative lighting system to pro-vide illumination to the enclosed lift lobbies in highrise residentialbuildings in daytime for about 4 h in clear sky condition. The poten-tial of RSSL is worth to be developed. The performance of the RSSL indifferent seasons should be further analyzed.

A sun-tracking system is proposed to be installed on the roofand connected to a central computer that can control the move-ment of all the mirrors installed on the facade of the building.The design of the RSSL should be compatible with the external fa-cade of the building for aesthetical purpose and able to integrateinto the architectural design of the lift lobby.

9. Future work

Experiment will be continued in spring season from March toMay. The lowest total solar radiation in a month occurs in March.The performance of the RSSL in this month will be studied in detail.The relationship between outdoor and indoor light intensities willbe analyzed. Other factors that may affect the collection of daylightsuch as orientation, shadowing effect by adjacent buildings will beinvestigated. Integration of the RSSL design into the architecturaldesign of different central layouts in highrise residential buildingswill also be explored.

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Acknowledgment

The work described in this paper was fully supported by a grantfrom the Sun Hung Kai Properties Limited (Project No. ZZ1T).

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