Applied Energy 90 (2012) 225–232

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Applied Energy

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Introducing natural lighting into the enclosed lift lobbies of highrise buildingsby remote source lighting system

Irene Wong ⇑, H.X. YangRenewable Energy Research Group (REG), Department of Building Services Engineering, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong, China

a r t i c l e i n f o a b s t r a c t

Article history:Received 14 July 2010Received in revised form 1 March 2011Accepted 17 March 2011Available online 16 April 2011

Keywords:Remote light source systemPrismatic light pipeFiber OpticCentral core design

0306-2619/$ - see front matter � 2011 Elsevier Ltd. Adoi:10.1016/j.apenergy.2011.03.018

⇑ Corresponding author. Tel.: +852 2 27664559; faxE-mail address: 06901650r@polyu.edu.hk (I. Wong

In metropolitan cities like Hong Kong where land value is high, highrise commercial and residentialbuildings with ‘‘central core’’ design are developed. The lift lobbies in the buildings are usually locatedin the centre of the building to free up peripheral areas to provide valuable exterior view. The lift lobbieshave no window provisions and electric lighting is switched on for 24 h continuously consuming non-renewable energy. Different types of light guides/light pipes have been proven to be capable to transferdaylight into deep floor plan during both clear sky and overcast periods. However, the thickness of lightguide requires 3 m headroom clearance for installation whilst the headroom of the highrise buildings isusually limited to 2.8 m in order to build more storeys. In this research, a remote source lighting system(RSL) is introduced to illuminate the enclosed lift lobbies. The system composed of prismatic light pipeand optic fiber to address the problem of limited headroom. Simulation of the system was carried outand reported in the paper, indicating that this lighting system can solve the energy consumption problemin the lift lobby in terms of renewable energy use and natural lighting application. This research concen-trates on highrise residential buildings.

� 2011 Elsevier Ltd. All rights reserved.

1. Introduction

The burning of petroleum generates electric energy and posesdamaging impacts on the environment. Building energy use isresponsible for about half of the carbon dioxide emission and athird of sulphur dioxide in US [1] which causes global warming.As a metropolitan city, the building energy demand in Hong Kongis increasing. The harmful emissions and exploitation of fossil fuelresources have deeply affected the building industry to the extentthat methods of low-energy construction and passive solar energyuse have come to be dictated not only by public awareness, butalso by global necessity [2].

Due to high land cost, the buildings in Hong Kong are developedinto highrise with central core design (Fig. 1) to free up morespaces for the peripheral facades to provide more valuable externalviews for each unit. Common areas and lift lobbies are groupedcentrally without windows and views (Fig. 1). Electricity is re-quired 24 h in a day to provide artificial lighting in these lift lob-bies. The indoor environment is heated by the heat generatedfrom artificial lighting that increases the cooling load. Researchcarried out by McGill University revealed that each unit of electriclight contributed to an additional half unit of electricity for spaceconditioning. There is a growing interest to design energy con-

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scious buildings to minimize the energy required by lighting andcooling, whilst relying less on energy from polluting fuels. A newbuilding philosophy that optimizes solar energy so as to minimizethe dependency on fossil fuel should be explored.

2. Application of remote source lighting system in Hong Kong

The potential of utilizing daylight for saving energy use is highin Hong Kong [3]. Buildings should be designed to optimize day-light for interior space illumination. At present, the active designutilizes solar panels to capture sunlight and transfer it into electri-cal energy with an efficiency of 10%, which is converted back intolight energy with an efficiency of 20% for building use, whichmeans that only 2% of the solar energy is delivered as useful illumi-nation energy [4]. Passive solar design that utilizes daylight di-rectly for illumination is worth further study.

Remote source lighting system (RSL) is a technique in whichlight travel from its source through a medium to one or more re-mote points [5]. RSL usually consists of a sunlight collector, a lighttransfer medium and a light distribution device. The medium isusually a light guide that brings daylight further inside buildingsthan the peripheral areas currently illuminated by artificial light[6]. There are two main types of light guide, which are hollow mir-ror light pipe (MLP) and prismatic light pipe (Fig. 2) (PLP). MLP ismade from highly reflective metal and transfers light by multiplespecula reflections [7]. Efficiency depends on the reflectivity of

Fig. 1. Floor plan of a central core design.

Fig. 2. Prismatic light guide (Opdal 2001).

Fig. 3. Total internal reflection.

226 I. Wong, H.X. Yang / Applied Energy 90 (2012) 225–232

the material. Metallic reflection is inefficient, in that 5–20% of thelight is absorbed on each reflection [6]. PLP is a hollow tube linedwith optical light film with a smooth surface at one side and longi-tudinal micro-right angled prisms of height 0.18 mm at the other[8]. PLP transfers light by total internal reflection (Fig. 3) (TIR).The refractive index is lower on the other side of the boundary thatno light can pass through, so effectively all of the light is reflected.Very little light is absorbed by the pipe because light travels pri-marily in the air space within and the efficiency has been calcu-lated as approaching 99% [9]. PLP is preferable to MLP as a lightguide. In order to transmit sufficient light, the common size ofPLP is 400 mm in diameter or more.

The performance of a long light pipe can be enhanced by addinga Laser Cut Panel light deflector (Fig. 4) (LCP), which can work inboth clear and overcast sky condition, at the input aperture to di-rect sunlight into a more collimated beam that travels along thelight pipe. An array of transparent rectangular blocks is formedby laser cutting (Fig. 4) inside a thin layer of acrylic sheet, whichdeflects or transmits light. Reflected fraction fr, deflected fractionfd and undeflected fraction fu are governed by the light incidenceangle on the LCP (Fig. 5) [10]. High elevated sunlight is deflected

Fig. 4. LCP [10].

Fig. 5. LCP deflecting light axially [10].

Fig. 6. Schematic layout of RSL (plan).

I. Wong, H.X. Yang / Applied Energy 90 (2012) 225–232 227

more axially down the pipe and makes fewer reflections in travers-ing the pipe [11]. RSL can successfully remove most of the infra-redand ultra-violet light from the visible spectrum, thus reducing theheating effect and lower the cooling load [1]. It has the potential ofintegrating artificial and natural light into one system [7] and pro-vides a centralized lighting system in the building replacing manyelectrical fixtures and cabling [6], thus allowing flexibility in designand ease of maintenance.

There are two modes of light transport by horizontal and verti-cal light pipes [12]. Light pipe is widely used in N. America &Australia [13] which is a vertical light pipe and mainly applied inlow-rise buildings [14]. Due to its limiting length of 20–24 m forefficient performance, its application is not common in Hong Kongas most of the buildings are over 30 storeys. Horizontal light pipe ismore applicable in Hong Kong. Horizontal PLP has been applied tobring in daylight to illuminate working areas of deep plan offices

Fig. 7. Schematic layou

5–10 m deep with illuminance level up to 300 lx without turningon electric lighting for 4–6 h [15]. The authors have carried out astudy on different layouts of lift lobby in highrise residential build-ings of central core design in Hong Kong and concluded that thepotential of applying RSL in the enclosed lift lobby is high. TheRSL system should be supplemented by artificial lighting system[16].

Due to high land cost and restriction in Plot Ratio limiting thetotal height of the building, majority of the highrise buildings inHong Kong are built with floor heights not exceeding 2.8 m. Thislimitation in floor height poses particular difficulty in designingRSL with PLPs due to their sizes. Increasing the headroom meansfewer floors can be built thus reduces the revenue return of thedevelopers. To solve this problem, a combination of PLP and FiberOptic (FO) of small diameter is proposed to be used in the RSL(Fig. 6). PLP is installed in the external or service areas withoutheadroom problem or parts of the headroom can be reduced below2.8 m. FO will be installed in common areas and lift lobbies whereheadroom clearance is crucial. A hybrid system consisting of RSLand dimmable electric lighting operated by light sensors is pro-posed. During cloudy days when solar intensity is too low to pro-vide adequate illumination, electric lighting will be switched onautomatically. The design should be able to integrate into thearchitectural design of the lift lobby.

3. Proposed design of RSL

A schematic RSL (Fig. 7) is designed for the enclosed lift lobbyand consists of 3 main parts: (a) Laser Cut Panel, (b) prismatic lightpipe of length 1.2 m and 450 mm in diameter; and (c) side-emit-ting Fiber Optic of length 6 m and 25 mm in diameter. The RSL

t of RSL (section).

Fig. 8. Perspective of lift lobby showing RSL.

Fig. 9. Simulated RSL.

228 I. Wong, H.X. Yang / Applied Energy 90 (2012) 225–232

combines with a supplementary electrical lighting system. Thesupplementary lighting system consists of fluorescent lighting fix-tures, which will be switched on/off automatically by a control sys-tem when the light intensity falls below a designated level.

LCP is fixed at the external aperture of the PLP at an inclined an-gle of 55�, which is the optimal inclined angle for Hong Kong [3] tocollect and redirect sunlight more directly along the axis of the PLPcreating a collimated light beam traveling along the PLP [7]. As thefiber acceptance aperture is very small, light needs to be highlyconcentrated before entering the FO [17]. Converging lens is ap-plied to concentrate the incident beam into the FO. UV and IR fil-ters are installed before the converging lens to remove ultra-violet (UV) and infra-red (IR) light to protect the FO from overheat-ing and deterioration that caused by these rays. FO with the flexi-bility to bend around corners is installed along the lift lobby andcorridors without reducing the headroom. The length of the FOshould be minimized to reduce light loss along the length. Thelength is 6 m in this case study. The FO installation in the lift lobbyintegrates into the false ceiling system (Fig. 8). The FO, which runsalong the lift lobby and corridors, is concealed inside the reflectingceiling panels to imitate fluorescent lightings. Anidolic Ceiling Sys-tem is installed to conceal the fluorescent lightings and FO. Theceiling panels are made from aluminum of high reflectivity andact as parabolic reflectors. The lower panels reflect the light com-ing from either FO or fluorescent lightings to the upper panelswhich reflect the light in a downward direction to illuminate thelift lobby and corridors in a soft lighting effect.

Fig. 10. PLP with detector at end.

4. Computer simulation and findings

Theoretically LCP can redirect sunlight into a collimated beamaxially down the PLP. The light being redirected by the LCP is as-sumed to form a parallel light beam to the axis of PLP. A computersimulation study on the performance of the proposed RSL is carriedout by using ZEMAX-EE optical design software. The orientation ofthe RSL is assumed due south with LCP being inclined at the opti-mal angle of 55�; and the lens system is formed by two identicalplano-convex lenses being placed in opposite positions. The sizes

and configurations of LCP, PLP, converging lenses and FO are en-tered into the non-sequential ray-tracing program to simulatethe RSL as shown in Fig. 9. Detectors are placed at the followinglocations to record the amount of received light:

1. At the end of PLP.2. After lens 1.3. After lens 2.4. Detector 1: before entering FO.5. Detector 2: at FO length 1.66 m.6. Detector 3: at 1st 90�turn of FO; at FO length 2.16 m.7. Detector 4: at 2nd 90�turn; at FO length 2.67 m.8. Detector 5: at FO length 3.87 m.9. Detector 6: at 3rd 90 turn; at FO length 4.37 m.

10. Detector 7: at FO length 4.97 m.11. Detector 8: at 4th 45�turn; at FO length 5.52 m.12. Detector 9: at end of FO of length 6.02 m.

For the sake of analysis, the following data is entered forsimulation:

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

Simulation of PLP is carried out first to study the performance(Fig. 10). The Total Power in the Detector Viewer diagram of thedetector at end of PLP record the output power is 100 W which

Fig. 11. Efficiency of PLP.

I. Wong, H.X. Yang / Applied Energy 90 (2012) 225–232 229

means the efficiency of the PLP is 100% as shown in Fig. 11. Onehundred watt of light intensity is then delivered into the lens sys-tem and will be converged by lenses 1 and 2 into FO.

Fig. 12. Efficiency

From simulation, detectors after lenses 1 and 2 record efficien-cies of 96.8% and 83.2% (Fig. 12) respectively. Similarly, detectors1–9 records efficiencies as shown in Table 1. Efficiency at end of

after lens 2.

Table 1Transmission efficiencies of FO.

Detector @ Length of FO: L (m) Transmission efficiency: EE (%) Transmission efficiency: ES (%) Resulted transmission efficiency: ET (%)

After PLP – 100 –After lens 1 – 96.8 – –After lens 2 – 83.2 – –Detector 1 – 76.0 – –Detector 2 1.66 76.0 90 68Detector 3 2.16 74.1 87 64Detector 4 2.67 72.6 85 62Detector 5 3.87 72.6 79 57Detector 6 4.37 72.2 76 55Detector 7 4.97 72.2 73 53Detector 8 5.52 70.3 71 50Detector 9 6.02 63.3 68 43

Fig. 13. Efficiency of RSL.

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FO (Fig. 13) is 63.3%, which is the overall efficiency of the RSLsystem.

5. Results and discussion

ZEMAX likes most optical design software that are available inthe market, calculates the efficiency of end-emitting FO only. Inside-emitting FO, part of the light is emitted along the wholelength while the rest is transmitted towards the end. The linearemittance of studied FO is 6% per meter run i.e. the transmissionefficiency (ES) is 94(L) %; where L is the length of the FO. Combiningthe side-emitting and end-emitting properties of the FO, the re-sulted transmission efficiency (ET) is calculated to be EE � ES (%)where EE is the transmission efficiency of end-emitting FO. Table1 listed EE, ES and ET at different locations of FO.

The light intensity being emitted at the end of the FO is 43% ofthe incident light. The light intensity at this point is the lowest. Theside-emitted light intensity at this point is:

PðlxÞ � 43%� 6% ð1Þ

where P is the light intensity of the light beam entering the PLP. Therecommended light intensity for illumination in lift lobby is 150 lxthat means the minimum required incident light intensity P is 150/(0.43 � 0.06) which is 5814 lx.

Theoretically, the RSL can operate in both clear and overcastsky. T.M. Chung has carried out a study on the luminous efficacyof daylight in Hong Kong and calculated the mean direct solarluminous efficacy (Ks) and the mean overcast sky luminous effi-cacy (Koc) to be 94 ± 16 lm/W and 121 ± 3 lm/W respectively[18]. The luminous efficacy of clear sky with direct sunlight islower than overcast sky. To test the worst case scenario, the per-

Fig. 14. Ks vs. solar altitude (b) (Chung, 1992).

Fig. 15. fd vs. incidence angle [19].

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formance of the RSL in clear sky condition of lower Ks wasinvestigated.

The direct solar luminous efficacy is the ratio of horizontal di-rect illuminance to irradiance. In T.M. Chung’s research, Ks can berepresented by the best-fit regressive curve as shown in Fig. 14. Di-rect luminous efficacy (Ks) increases with solar altitude (b) as illus-trated by the following equation [18].

Ks ¼ 48:5þ 1:67b� 0:0098b2 ð2Þ

Edmond carried out a research on the performance of LCP of dif-ferent spacing-to-depth ratio (D/W) at various incident angles [19],which is also b. The results were plotted in Fig. 15. Based on Ed-mond’s research, Kwok studied the performance of a LCP (Fig. 5)of 0.7 D/W that is inclined at the optimal angle of 55� [10]. D isthe distance between the parallel laser cuts and W is the width ofthe LCP. Kwok’s LCP is chosen as case study in this paper. In the pro-posed RSL, the LCP of 0.7 D/W is inclined at 55� facing west to cap-ture the brightest sunlight at noon and onwards. With reference toChung’s research as illustrated in Fig. 14, the LCP collects sunlight atb from 10� to 70�. Assuming the thickness of the PLP in Fig. 7 is10 mm, the internal diameter of the PLP is 430 mm. The effectivearea (A) of the LCP for collecting sunlight is in oval shape of widthand height being 430 mm and 750 mm respectively:

A ¼ 0:43� 0:75� 0:8 ¼ 0:258 m2 ð3Þ

In Fig. 5, b is 70� and Ks is calculated to be 50.5 lm/W.From the record of Hong Kong Observatory, the daily average of

total solar radiation on a horizontal surface is 3.8889 kW h/m2. Onclear sky, the total daily average outdoor horizontal illuminance(lmout(S70)) when b is 70� can be estimated as follows:

lmoutðS70Þ ¼ 3:8889� 50:5� 1000 ¼ 196;389 lm=m2 ð4Þ

The incident light intensity (P(70)) received by the LCP when b is70� will be:

Pð70Þ ¼ 196;389� A ¼ 50;668 lx ð5Þ

In Figs. 5 and 15, the fraction of light deflected (fd) by the LCP is0.83. The intensity of the deflected light P0ð70Þ

� �is:

P0ð70Þ ¼ 50;668� 0:83 ¼ 42;054 lx ð6Þ

which is greater than the minimum required incident light intensity(P) of 5814 lx.

Ks is lowest when b is 10� and calculated to be 48.78 lm/W.Hence lmout(S10) is 189,705 lm/m2, which means that P(10) is48,944 lx. In Fig. 4, fd at b = 10� is 0.2. The intensity of the colli-

mated light beam P0ð10Þ

� �entering the lens system is 9789 lx,

which is still greater than the minimum required P.Solar altitudes (b) at different times and dates in the winter sea-

son from November 2010 to February 2011 in the period fromnoon to 16:00 h were measured. The average b decreases from70� to 40�. The proposed RSL can provide adequate lighting tothe enclosed lift lobby in the afternoon, and can be a potentiallighting system to reduce the energy consumption in illuminatingthe enclosed lift lobby in highrise residential buildings of centralcore design.

6. Conclusions

Enclosed lift lobbies of the central core design consume non-renewable energy continuously 24 h in a day for illumination pur-pose in commercial and residential buildings. This paper investi-gated the application of solar lighting in the enclosed lift lobbiesof highrise residential buildings in Hong Kong. The following con-clusions can be drawn from the study.

The RSL consisting of LCP as a sunlight collector and a combina-tion of the PLP with FO that utilizes daylight directly is a potentialalternative to illuminate the enclosed lift lobbies during daytime,and at the same time to solve the problem in applying light pipesto the lift lobbies of limited headroom. Application of a converginglens to focus collimated light beams into the side-emitting FO canincrease the transmission efficiency. The RSL system can also beeasily integrated into the architectural design of the lift lobbies.

Theoretically, the LCP can work in both clear and overcast skyconditions. However, there are limiting factors that affect the per-formance of the RSL such as orientation, solar azimuth angle, angleof incident light, and inclination angle of the LCP. The crucial factoris how effectively the LCP can deflect incident light into a highlycollimated beam along the axis of the lenses as light beam needsto be highly concentrated when entering the FO for efficient trans-mission. Furthermore, the side emission property of the FO has tobe designed and controlled precisely for appropriate application.The performance of the RSL can be improved if the percentage oflinear emission of the FO can be increased. Experiment will be car-ried out in next stage to study these effects on the performance ofthe proposed RSL.

Installation of the PLP together with other building services pro-visions will complicate the compliance of fire service require-ments. A fire damper should be installed at the junction of the

232 I. Wong, H.X. Yang / Applied Energy 90 (2012) 225–232

service room and lift lobby to prevent the spread of fire and smokeinto the lobby in case a fire occurs in the service room. A space pro-vision that is directly accessible to external daylight is recom-mended to be reserved in future designs of the central corehighrise buildings to accommodate the PLP and for routine mainte-nance purpose. The design of the RSL should also be able to inte-grate into the architectural design of the external facade and liftlobby.

Acknowledgement

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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