energies

Article Experimental Investigation of Illumination Performance of Hollow Light Pipe for Energy Consumption Reduction in Buildings †

Jiraphorn Mahawan and Atthakorn Thongtha *

Department of Physics, Faculty of Science, Naresuan University, Phitsanulok 65000, Thailand; [email protected] * Correspondence: [email protected]; Tel.: +665-596-3550 † This paper is an extended and revised article presented at the International Conference on Sustainable Energy and Green Technology 2019 (SEGT 2019) on 11–14 December 2019 in Bangkok, Thailand.

Abstract: This work investigates the light illumination intensity, light transmission performance, light distribution on the floor, and daylight factor of vertical light hollow tubes at various incident elevation angles of a light source. The light tubes were made from commercial aluminum alloy sheets and commercial zinc alloy sheets to investigate internal illuminance for buildings and reducing the demand of electrical energy from artificial lighting. The vertical light tubes with a constant length of 0.5 m and diameters of 0.20, 0.25, and 0.30 m were designed in a testing room model, with dimensions of 1 m × 1 m × 1 m. A 20-W light-emitting diode (LED) lamp was used as the light source for the lighting simulations, which was placed away from the top of the light tube. The incident elevation angle of the light source was changed between 0◦ and 80◦ with 5◦ increments. It was found that the elevation angle of the incidence light had an influence on the light intensity distribution on both ends of light tube. The average illuminance performance of both material types increased with an


increase of the incidence angle from 0◦ to 80◦ and an increase of the tube diameter from 0.20 m to
 0.30 m. The commercial aluminum alloy tube promotes greater light transmission and daylight Citation: Mahawan, J.; Thongtha, A. factor when compared with the commercial zinc alloy tube in each condition. This illuminance Experimental Investigation of measurement demonstrates that the light tube could be included in the lighting systems of some Illumination Performance of Hollow deeper or windowless areas of buildings to decrease the demand of energy consumption in the Light Pipe for Energy Consumption lighting of buildings. Reduction in Buildings . Energies 2021, 14, 260. https://doi.org/ Keywords: daylight; light pipe; light transmission; daylight factor; illumination 10.3390/en14020260

Received: 19 November 2020 Accepted: 4 January 2021 1. Introduction Published: 6 January 2021 A high level of solar radiation is experienced throughout most of Thailand throughout 2 Publisher’s Note: MDPI stays neu- the year. The average daily value of solar radiation is around 18.2 MJ/m -day with the tral with regard to jurisdictional clai- highest levels of solar radiation experienced in the months of April and May, between ms in published maps and institutio- 20 and 24 MJ/m2-day [1–3]. These conditions cause significant thermal accumulation in nal affiliations. buildings. Therefore, energy consumption in buildings is due mainly to the air conditioning system and lighting, which has been increasing due to the energy demands of modern society to improve the comfort of the occupants. Approximately 30% of total energy consumption in residential and commercial buildings is demand for artificial lighting. Copyright: © 2021 by the authors. Li- Nowadays, buildings are designed and constructed to provide their occupants with a censee MDPI, Basel, Switzerland. better-quality environment and to improve energy conservation with optimal designs and This article is an open access article distributed under the terms and con- functional practices [4,5]. ditions of the Creative Commons At- Daylight is one solution to save energy consumption in buildings because it is free tribution (CC BY) license (https:// and a valuable light source for internal building areas throughout the day [6–8]. Effective creativecommons.org/licenses/by/ daylight utilization can result in energy savings. The use of natural daylight in buildings 4.0/). also significantly improves the visual and physical comfort of the building. Individuals

Energies 2021, 14, 260. https://doi.org/10.3390/en14020260 https://www.mdpi.com/journal/energies Energies 2021, 14, 260 2 of 17

spend most of their time inside [9–13]. Windows permit daylight to illuminate interior building spaces, but parts of the deeper internal building areas do not obtain daylight, and heat transfers through windows as well [14]. Effects of insufficient daylight within deeper building areas and thermal accumulation in buildings due to daylight illumination mean that the artificial lighting system can be an option for illumination and contributes to reducing approximately 30% of total building energy consumption [15]. Reducing the artificial lighting energy consumption during the daytime is an issue for energy savings [13,16,17]. The design of architectural structures that can carry adequate daylight into the internal building areas results in energy conservation for both illumination and the air conditioning system [14,15,18–23]. Light pipes are an alternative way to provide daylight into indoor spaces of buildings, which is useful for spaces with or without glazing opening [24,25]. Light pipe systems may be straight or have bends. Commercial light pipes can be defined as a hollow tube to allow the illumination into deeper parts of buildings that do not receive sufficient daylight. Light pipes can be responsible for reducing the electricity consumption of artificial lighting systems [5]. Many studies have investigated various components of light pipe systems to improve the performance, such as integrating other functions such as ventilation. The integration of a light pipe system into a natural stack ventilation and solar heating was investigated by Shao and Riffat [26]. A horizontal light pipe with a trapezoidal shape was designed by Canziani et al. who used an active reflector to track the solar rays and further improved illumination into deeper areas by increasing the uniformity [27]. Light tubes with apertures attached have also been used to transmit daylight [28]. Uniform lighting levels on different levels of buildings can be provided by vertical light pipes, lighting multiple areas [29]. An illuminance proportion of 14% under cloudy sky and 7% for sunny conditions in winter and cloudy conditions was investigated using the light pipes [30]. Mohelnikova evaluated the efficiency of light pipes to be between 0.2 and 0.5, when different diameters of light pipes were studied [31]. Apart from these, the term “daylight penetration factor” (DPF) of light pipes was used to determine the performance of a daylight system, which describes the relation of the internal illuminance due to a light pipe against the total external illuminance [32–34]. A good lighting system requires 1–2% daylight factor (DF) for activity in residence and 2–4% DF for activities in office buildings under International Commission on Illumination (CIE) standard for overcast conditions [35]. The relationship of the average inside illuminance and different diameters of light pipes was investigated by Vasilakopoulou et al. [36]. The performance of light pipes was experimentally investigated under subtropical climates in Instanbul [25], Hong-Kong [37], Korea [38], Beijing [39], and Jordan [40] which demonstrated that good results and a uniform light distribution can be provided to buildings using light pipes. In Thailand, aluminum and zinc alloy sheet metal were generally used as roofing and siding material of buildings because of its lightweight, superior corrosion resistance and higher chemical durability than steel sheets [41–43]. Our previous presented work [44] designed and constructed the light tubes with a fixed length of 0.5 m and the different diameters of 0.20, 0.25, and 0.30 m, which were made from commercial aluminum and zinc alloy sheets. That work demonstrated only the reflection performance of hollow light tubes at each condition. To appropriately consider and utilize the illumination of the vertical light hollow tubes for transmitting light into buildings, this current investigation was focused on examining the improved illumination distribution at the top and bottom ends, light transmission performance, the internal illuminance distribution on the floor plane, and the daylight factor at various incident angles of the light source. Furthermore, the correlation to these obtained experimental values of the vertical light hollow tubes in each material type at different diameters and incident elevation angle was also investigated and compared.

2. Materials and Methods The light tubes were produced with either a commercial aluminum alloy sheet or a zinc alloy sheet. Both were designed as tubes with a length of 0.5 m and different diameters Energies 2021, 14, x FOR PEER REVIEW 3 of 17 Energies 2021, 14, x FOR PEER REVIEW 3 of 17

2. Materials and Methods 2. Materials and Methods Energies 2021, 14, 260 The light tubes were produced with either a commercial aluminum alloy sheet or a 3 of 17 The light tubes were produced with either a commercial aluminum alloy sheet or a zinc alloy sheet. Both were designed as tubes with a length of 0.5 m and different diame- zinc alloy sheet. Both were designed as tubes with a length of 0.5 m and different diame- ters of 0.20 m, 0.25 m, and 0.30 m. Each light tube was installed on the top of a testing ters of 0.20 m, 0.25 m, and 0.30 m. Each light tube was installed on the top of a testing room to allow light to transmit into the interior space. The six sides of the testing room ofroom 0.20 to m, allow 0.25 m,light and to 0.30transmit m. Each into light the inte tuberior was space. installed The onsix thesides top of of the a testingtesting roomroom were built using wood. The model room’s walls each had an area of 1 m2 and a volume of towere allow built light using to transmitwood. The into model the interiorroom’s walls space. each The had six an sides area of of the 1 m testing2 and a room volume were of 1 m3, as shown in Figure 1a. A 20-W artificial LED lamp was used as the light source.2 The built1 m3, usingas shown wood. in Figure The model 1a. A 20-W room’s artificial walls eachLED hadlamp an was area used of 1as m theand light a source. volume The of illumination changes,3 as the elevation angle was varied between 0° and 80° with a step 1illumination m , as shown changes, in Figure as1 a.the A elevation 20-W artificial angle LEDwas lampvaried was between used as 0° the and light 80° source. with a Thestep size of 5°, are exhibited in Figure 1. The illumination at nine positions—at ◦the top ◦and illuminationsize of 5°, are changes, exhibited as thein elevationFigure 1. angleThe il waslumination varied between at nine 0positions—atand 80 with the a steptop sizeand bottom ends—of◦ each light tube was measured by using an illuminance lux meter (DIGI- ofbottom 5 , are ends—of exhibited each in Figure light tube1. The was illumination measured atby nineusing positions—at an illuminance the lux top meter and bottom (DIGI- CON LX-70) which is also based on the International Commission on Illumination stand- ends—ofCON LX-70) each which light is tube also was based measured on the Inte byrnational using an Commission illuminance on lux Illumination meter (DIGICON stand- ard (CIE standard) as shown in Figure 2. All values at the top and bottom side ends were LX-70)ard (CIE which standard) is also as based shown on in the Figure International 2. All values Commission at the top on and Illumination bottom side standard ends were (CIE calculated to determine the average illuminance of vertical aluminum and zinc alloy tubes standard)calculated as to shown determine in Figure the average2. All values illuminance at the top of vertical and bottom aluminum side ends and were zinc calculatedalloy tubes with the diameters of 0.20 m, 0.25 m, and 0.30 m. The average luminous intensity at the towith determine the diameters the average of 0.20 illuminance m, 0.25 m, ofand vertical 0.30 m. aluminum The average and luminous zinc alloy intensity tubes with at thethe top and bottom side ends was evaluated to identify the improved light transmission effi- diameterstop and bottom of 0.20 side m, ends 0.25 m,was and evaluated 0.30 m. to The iden averagetify the luminous improved intensity light transmission at the top andeffi- ciency. The illuminance distribution in the model area was also tested and measured at 25 bottomciency. sideThe illuminance ends was evaluated distribu totion identify in the themodel improved area was light also transmission tested and measured efficiency. at The 25 locations using an illuminance lux meter according to the CIE standard, as exhibited in illuminancelocations using distribution an illuminance in the modellux meter area acco wasrding also testedto the andCIE measuredstandard, atas 25exhibited locations in Figure 3. The daylight factor was defined as the proportion of average internal illumina- usingFigure an 3. illuminance The daylight lux factor meter was according defined to as the the CIE proportion standard, of as average exhibited internal in Figure illumina-3. The tion on the floor plane and the related luminance at ambient areas on horizontal and un- daylighttion on the factor floor was plane defined and asthe the related proportion luminance of average at ambient internal areas illumination on horizontal on the and floor un- shaded areas.plane and the related luminance at ambient areas on horizontal and unshaded areas. shaded areas.

(a) (b) (a) (b) Figure 1. (a) View of the testing room. (b) Incident angle. FigureFigure 1. (a 1.) View(a) View of the of testing the testing room. room. (b) Incident (b) Incident angle. angle.

Figure 2. Fixed location of the nine illuminance measurements at the top and bottom end positions Figure 2. Fixed location of the nine illuminance measurements at the top and bottom end positions of light tube. Figure 2. Fixed location of the nine illuminance measurements at the top and bottom end positions ofof lightlight tube.tube.

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FigureFigureFigure 3.3. 3. FixedFixed Fixed positionspositions positions ofof of thethe the 2525 25 illuminationillumination illumination measurementsmeasurements measurementson on onthe the thefloor floor floor plane. plane. plane.

3.3.3. ResultsResults Results andand and DiscussionDiscussion Discussion FigureFigureFigure4 4 4presents presents presents thethe the internalinternal internal illumination illumination contourscontours of ofof the the top top positions positions positions of of of the the the alumi- alumi- alu- minumnumnum alloy alloy alloy light light light tube, tube, tube, with with with a a adiameter diameter diameter of of 0.25 0.25 m, m, at atat various variousvarious incident incident light light angles.angles. angles. AA A colorcolor color mapmapmap onon on thethe the two-dimensional two-dimensional two-dimensional horizontal horizontal horizontal plane plan plan demonstratesee demonstrates demonstrates the the the illuminance illuminance illuminance distribution distribution distribution at theat the top top position position of the of hollowthe hollow light light pipe. pipe. The illuminationThe illumination of the of top the position top position was between was be- at the top position of the hollow light pipe. The illumination◦ of the top position was be- 248tween lux 248 and lux 282 and lux 282 at thelux incidentat the incident light anglelight angle of 0 , of which 0°, which was uniformlywas uniformly distributed. distrib- tween 248 lux and 282 lux at the incident light◦ angle◦ of 0°, which was uniformly distrib- Whileuted.uted. While increasingWhile increasing increasing the incident the the incident incident light angle light light fromangle angle 0 fr frtoomom 30 0° 0°, to theto 30°, 30°, illuminance the the illuminance illuminance distribution distribution distribution at the topat the position top position was nearly was nearly uniform uniform in value. in va Whenlue. When the incident the incident light angle light increasedangle increased from at◦ the top◦ position was nearly uniform in value. When the incident light angle increased 30fromfromto 30° 8030° ,to to the 80°, 80°, internal the the internal internal illuminance illuminance illuminance distribution distribution distribution became became became nonuniform nonuniform nonuniform in some in positions,in some some posi- posi- as exhibited in Figure4d–f. As the incident light angle increased from 0 ◦ to 80◦, the internal il- tions,tions, as as exhibited exhibited in in Figure Figure 4d–f. 4d–f. As As the the incident incident light light angle angle increased increased from from 0° 0° to to 80°, 80°, the the lumination distribution at the top end changed in each condition. The illuminance value at internalinternal illumination illumination distribution distribution at at the the top top end end changed changed in in each each condition. condition. The The illumi- illumi- each position on the top increased in value when the incident angle increased, as displayed nancenance valuevalue atat eacheach positionposition onon thethe toptop increasedincreased inin valuevalue whenwhen thethe incidentincident angleangle in-in- in Figure4. creased,creased, as as displa displayedyed in in Figure Figure 4. 4.

(a(a) ) (b(b) ) (c(c) )

(d(d) ) (e(e) ) (f(f) )

FigureFigureFigure 4.4. 4.Internal Internal Internal illuminationillumination illumination contourscontours contours ofof of thethe the aluminumaluminum aluminum alloyalloy alloy tubetu tubebe onon on thethe the toptop top endend end forfor for thethe the incidentincident incident lightlight light anglesangles angles ofof of ((a a()a) 0)0°, ◦0°,, (b(b()b) 15 )15°, 15°,◦,( (c c()c) 30)30°, 30°,◦,( (d d()d) 45)45°, 45°,◦,( ( ee()e) ) 6060°, 60°,◦, andand and (( ff()f) ) 7575°. 75°.◦.

FigureFigureFigure5 5 illustrates5 illustrates illustrates illumination illumination illumination distribution distribution distribution of theof of topthethe endtop top ofendend the of of zinc the the alloyzinc zinc hollowalloy alloy hollow hollow light ◦ pipelightlight with pipe pipea with with diameter a a diameter diameter of 0.25 of of m 0.25 0.25 with m m various with with vari vari anglesousous angles angles of incidents. of of incidents. incidents. At the At incidentAt the the incident incident angle ofangle angle 0 , theofof 0°, illumination0°, the the illumination illumination of the endof of the the was end end between was was between between 165 lux 165 and165 lux 199lux and and lux, 199 which199 lux, lux, uniformly which which uniformly uniformly illuminated illu- illu- minatedminated the the area, area, as as shown shown in in the the color color map. map. In Increasingcreasing the the incident incident angle angle from from 0° 0° to to 30° 30°

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theled area,to nearly as shown uniform in theilluminance color map. for Increasing all angles. theWhen incident the incident angle fromlight 0angle◦ to 30 increased◦ led to nearlyfrom 30° uniform to 80°, illuminancethe illumination for all became angles. nonu Whenniformly the incident distributed, light angleas exhibited increased in Figure from 305d–f.◦ to When 80◦, the increasing illumination the becameincident nonuniformly angle from 0° distributed, to 80°, the as illumination exhibited in distinctly Figure5d–f. in- ◦ ◦ Whencreased. increasing the incident angle from 0 to 80 , the illumination distinctly increased.

(a) (b) (c)

(d) (e) (f)

FigureFigure 5.5. IlluminationIllumination contours from zinc zinc alloy alloy light light tube tube on on the the top top end end at at the the incident incident angles angles of of (a) ( a0°,) 0 (◦b,() b15°,) 15 (◦c,() 30°,c) 30 (d◦,) (d45°,) 45 (◦e,() 60°,e) 60 and◦, and (f) (75°.f) 75 ◦.

FigureFigure6 6 demonstrates demonstrates the the illumination illumination levels levels on on the the bottom bottom of of both both aluminum aluminum alloy alloy andand zinczinc alloy hollow light light pipes, pipes, with with the the same same diameter diameter of of 0.25 0.25 m mand and tube tube length length of 0.5 of 0.5m at m different at different incident incident angles. angles. When When using using the thealuminum aluminum alloy alloy hollow hollow light light pipe, pipe, the illu- the illuminationmination was was between between 12 lux 12 luxand and 52 lux 52 luxat the at theincident incident light light angle angle of 0°, of which 0◦, which was wasuni- uniformlyformly distributed. distributed. The The zinc zinc alloy alloy tube tube had had illumination illumination values values between between 2 lux and 3434 luxlux withwith aa nonuniformnonuniform illuminanceilluminance distributiondistribution at at the the incident incident angle angle of of 0 0°.◦. WhenWhen thethe incidentincident angleangle waswas increased increased from from 0◦ 0°to 30to ◦30°,, the the illuminance illuminance distribution distribution at the at bottom the bottom of both of types both wastypes nearly was uniform.nearly uniform. When the When incident the lightincident angle light increased angle fromincreased 30◦ to from 80◦, the 30° illuminance to 80°, the distributionilluminance became distribution nonuniformly became distributed,nonuniformly as exhibiteddistributed, in Figure as exhibited6d–f. The in illuminationFigure 6d–f. distributionThe illumination for both distribution aluminum for alloy both and aluminum zinc alloy alloy tubes and at zinc the alloy bottom tubes varied at the when bottom the ◦ ◦ incidentvaried when light the angle incident was changed light angle from was 0 to chang 80 .ed The from illuminance 0° to 80°. at The the illuminance bottom increased at the ◦ ◦ withbottom the increased increase of with the incidentthe increase angle of from the 0incidentto 80 , angle as illustrated from 0° in to Figure 80°, as6. illustrated in FigureThe 6. nine measurements of illumination at the top and bottom were used to calculate the averageThe illuminance nine measurements for both aluminum of illumination alloy and at the zinc top alloy and hollow bottom light were pipes used with to calculate a length ofthe 0.5 average m and illuminance different diameters for both of aluminum 0.20 m, 0.25 alloy m, and zinc 0.30 m.alloy When hollow using light the pipes aluminum with a alloylength hollow of 0.5 light m and pipe different with diameters diameters of 0.20 of m,0.20 0.25 m, m, 0.25 and m, 0.30 and m, 0.30 the averagem. When illuminance using the ataluminum different alloy incident hollow angles light is shownpipe with in Figurediameters7. When of 0.20 the m, incident 0.25 m, angle and was0.30 m, varied, the av- a changeerage illuminance of luminous at intensity different at incident the top andangles bottom is shown of the in tube Figure was 7 achieved.. When the At incident the top positionangle was of thevaried, tube a with change the diameterof luminous of 0.20 inte m,nsity the at average the top luminous and bottom intensity of the increased tube was fromachieved. 243 lux At to the 3022 top lux; position at the of bottom the tube end with of the the tube diameter with a of diameter 0.20 m, ofthe 0.20 average m, the luminous average luminous intensity increased from 22 lux to 1702 lux, when the incident light angle to the intensity increased from◦ 243 lux◦ to 3022 lux; at the bottom end of the tube with a diameter tubeof 0.20 increased m, the average from 0 luminousand 80 , as intensity displayed increased in Figure from7a. For 22 lux the diametersto 1702 lux, of when 0.25 m the and in- 0.30cident m, light the trendangle ofto thethe averagetube increased illuminance from at0° theandtop 80°, and as displayed bottom positions in Figure was 7a. similarFor the todiameters that of the of 0.25 tube m with and a 0.30 0.20 m, m the diameter, trend of as the exhibited average in illuminance Figure7b,c. at For the the top tube and withbottom a diameter of 0.25 m, the average luminous intensity increased from 261 lux to 3142 lux at the positions was similar to that of the tube with a 0.20 m diameter, as exhibited in Figure top end and from 31 lux to 2304 lux at the bottom end position of the tube with an increase in 7b,c. For the tube with a diameter of 0.25 m, the average luminous intensity increased the incident light angle to the light tube between 0◦ and 80◦, as illustrated in Figure7b. For from 261 lux to 3142 lux at the top end and from 31 lux to 2304 lux at the bottom end

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Energies 2021, 14, 260 6 of 17 position of the tube with an increase in the incident light angle to the light tube between position0° and 80°, of theas illustratedtube with anin Figureincrease 7b. in For the theincident diameter light of angle 0.30 tom, the the light average tube luminous between 0° and 80°, as illustrated in Figure 7b. For the diameter of 0.30 m, the average luminous theintensity diameter increased of 0.30 m,from the 220 average lux to luminous 3549 lux intensityat the top increased end and fromfrom 22040 lux lux to 35492591 luxlux atat intensity increased from 220 lux to 3549 lux at the top end and from 40 lux to 2591 lux at thethe topbottom end andend fromwhen 40 there lux towas 2591 an lux increase at the of bottom the incident end when angle there to wasthe light an increase tube from of the 0° theincidentto 80°, bottom as angle illustrated end to when the lightin there Figure tube was 7c. from an increase 0◦ to 80◦ of, as the illustrated incident inangle Figure to 7thec. light tube from 0° to 80°, as illustrated in Figure 7c.

FigureFigure 6.6. IlluminationIllumination contourscontours ofof aluminumaluminum alloyalloy andand zinczinc alloyalloy hollowhollow lightlight pipepipe onon thethe bottombottom endend positionspositions atat thethe incidentFigureincident 6. light lightIllumination anglesangles ofof contours ((aa)) 00°,◦,( (bb ))of 1515°, aluminum◦,( (cc)) 3030°,◦,( ( dalloy) 4545°,◦ ,(and (e) 60°, 60zinc◦, and andalloy ( (f ))hollow 75°. 75◦. light pipe on the bottom end positions at the incident light angles of (a) 0°, (b) 15°, (c) 30°, (d) 45°, (e) 60°, and (f) 75°.

(a) (b) (a) (b)

Figure 7. Cont.

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((c))

FigureFigure 7. Average 7. Average illuminance illuminance at the at top the and top bottom and bottom end positions end positi ofons aluminum of aluminum alloy alloy tube withtube awith diameter a diameter of (a) of 0.20 ((a)) m, 0.200.20 (b )m,m, 0.25 m, (b) 0.25 m, and (c) 0.30 m. and (c()b 0.30) 0.25 m. m, and (c) 0.30 m.

The illuminance of zinc alloy tubes with di differentfferentfferent diameters diameters of of 0.20 0.20 m, m, 0.25 m,m, andand 0.30 m, and height of 0.5 m, at various incident angles areare shownshown inin FigureFigure8 8.. A A variation variation inin the the incident incident light light angle angle to to the the tube tube led led to to thethe changeschanges inin luminousluminous intensityintensity at at the the toptop and bottom of the zinc alloy tube. The The trend trend of the illuminance of zinc alloy tubes with the diameters ofof 0.200.20 m,m, 0.250.25 m, m, and and 0.30 0.30 m m the the at at the the top top and and bottom bottom was was similar similar to that to that of the of thealuminumthe aluminumaluminum alloy alloyalloy tube, tube,tube, as illustrated asas illustratedillustrated in Figure inin FigureFigure8. At 8.8. the AtAt top thethe endtoptop end ofend zinc ofof zinczinc alloy alloyalloy light lightlight tubes, tubes,tubes, the theilluminancethe illuminanceilluminance intensity intensityintensity increased increasedincreased from fromfrom 200 lux 200200 to luxlux 2889 toto lux 28892889 for luxlux 0.20 forfor m, 0.200.20 from m, 180 from lux to180 3052 luxlux lux toto 3052for 0.25 luxlux m, forfor and 0.250.25 from m, and 190 from lux to 190 3469 lux lux to for 3469 the lux 0.30 for m the diameter 0.30 m pipes, diameter respectively, pipes, respec- with ◦ ◦ tively,antively, increase withwith inanan the increaseincrease incident inin thethe light incidentincident angle to lightlight the anan lightgle tubeto the between light tube 0 andbetween 80 . At0° and the bottom80°. At theofthe the bottombottom zinc of alloyof thethelight zinczinc alloy tubes,alloy lightlight the illuminancetubes,tubes, thethe illuminanceilluminance intensity increased intensityintensity increasedincreased from 12 lux fromfrom to 12 150212 luxlux lux toto 1502for 0.20 lux m,for from0.20 m, 16 from lux to16 2133lux to lux 2133 for lux 0.25 for m, 0.25 and m, fromand from 17 lux 17 tolux 2448 to 2448 lux lux for for 0.30 0.30 m, ◦ m,respectively, respectively, when when there there was anwas increase an increase of the of incident the incident light angle light to angle the light to the tube light from tube 0 ◦ fromtofrom 80 0°0°. It toto was 80°.80°. observed ItIt waswas observedobserved that the thatthat luminous thethe luminousluminous intensity intensityintensity increased increasedincreased at the top atat andthethe toptop bottom andand bottombottom of both ofaluminum both aluminum alloy and alloy zinc and alloy zinc vertical alloy vertical light tube light with tube an increasewith an increase of the incident of the incident angle to anglethe light to the source. light When source. considering When considering the low incident the low angles incident into angles the light into tube, the light a major tube, part a majorof the lightpart of beam the waslight reflected beam was and reflected little was an directlyd little was transmitted directly throughtransmitted the vertical through tube, the verticalwhich could tube, lead which to thecould lower lead illuminance to the lowe intensityr illuminance at the topintensity and bottom at the top end and positions bottom at endthe lowpositions incident at the angles low of incident the light angles source. of Whilethe light the source. higher incidentWhile the angles higher of incident light source an- glesinto of the light light source tube wasinto examined,the light tube an was increase examined, of the illuminancean increase of intensity the illuminance at the top inten- and sitybottom at the of thetop light and tubesbottom was of achieved, the light whichtubes was aachieved, result of morewhich direct was lighta result penetration of more directand less light incidental penetration light and reflection. less incidental light reflection.

((a)) ((b)) Figure 8. Cont.

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((cc))

FigureFigureFigure 8. Average 8.8. AverageAverage illuminance illuminanceilluminance at the atat the topthe toptop and andand bottom bottombottom tube tubetube position positionposition of ofof zinc zinczinc alloy alloyalloy tube tubetube withwithwith diametersdiameters of of (( (aaa)) ) 0.200.20 0.20 m,m, m, ((bb ()b) 0.25)0.25 0.25 m, and (cm,m,) 0.30 andand m. ((cc)) 0.300.30 m.m.

WhenWhen considering considering thethe the differentdifferent different typestypes types ofof of tubetube tube materials,materials, materials, itit waswas it was observedobserved observed thatthat that thethe theav-av- erageaverageerage luminousluminous luminous intensityintensity intensity atat thethe at bottombottom the bottom endend ofof end thethe aluminumofaluminum the aluminum alloyalloy tubestubes alloy havinghaving tubes aa having diameterdiameter a 0.20diameter0.20 mm waswas 0.20 higherhigher m was thanthan higher thatthat than ofof thethe that zinczinc of alloyalloy the zinc tubetube alloy forfor each tubeeach incident forincident each incidentlightlight angle,angle, light asas angle, demon-demon- as strateddemonstratedstrated inin FigureFigure inFigure 9a.9a. ForFor9a. thethe For diametersdiameters the diameters ofof 0.250.25 of 0.25mm andand m and 0.300.30 0.30 m,m, m,thethe the averageaverage average illuminanceilluminance illuminance ofof theofthe the aluminumaluminum aluminum alloyalloy alloy tubetube tube andand and zinczinc zinc alloyalloy alloy tubetube tube wawa wasss similarsimilar similar toto to thatthat that ofof thethe light light tube tube with with aa diameterdiameter ofof 0.200.20 m,m, asas shownshown inin FigureFigure9 9b,c.b,c.9b,c. This ThisThis indicates indicatesindicates that thatthat the thethe internal internalinternal surface surfacesurface of ofof the thethe aluminumaluminum alloyalloyalloy tube tubetube light lightlight showed showedshowed better betterbetter reflection, reflection,reflection, leading leadingleading to an toto improvement anan improvementimprovement in the inin light thethe lighttransmissionlight transmissiontransmission performance performanceperformance within withinwithin the tube. thethe tube.tube.

((aa)) ((bb))

((cc))

FigureFigure 9.9. AverageAverage illuminanceilluminance atat thethe bottombottom endendend positions positionspositions ofofof aluminumaluminumaluminum alloyalloyalloy tubetube andanandd zinczinc alloyalloy tubetube withwith thethe diametersdiameters ofof ((aa))) 0.200.200.20 m,m,m, (((bb))) 0.250.250.25 m,m,m, and andand ( (c(cc))) 0.30 0.300.30 m. m.m.

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AllAll of of the the luminous luminous intensity intensity values values at at the the top top and and bottom bottom ends ends of of both both aluminum aluminum alloyalloy and and zinc zinc alloy alloy tubes tubes were were considered considered to to calculate calculate the the light light transmission transmission performance performance ofof the the tubes, tubes, as as exhibited exhibited in in Figure Figure 10. 10 .Light Light transmission transmission of of the the vertical vertical aluminum aluminum alloy alloy tubetube with with a diameter a diameter of of0.20 0.20 m increased m increased from from 9.1% 9.1% to 56.3% to 56.3% when when the incident the incident light angle light toangle the tube to the increased tube increased from 0° from to 80°, 0◦ asto 80sh◦own, as shownin Figure in Figure10a. When 10a. considering When considering the diam- the etersdiameters of 0.25 of m 0.25 and m 0.30 and m, 0.30 the m, light the transmis light transmissionsion performance performance was similar was similar to those to those of the of 0.20the m 0.20 diameter m diameter tube, tube, as illustrated as illustrated in Figure in Figure 10a. 10Lighta. Light transmission transmission performance performance of the of aluminumthe aluminum alloy alloy tubes tubes with witha diameter a diameter of 0.25 of m 0.25 increased m increased between between 11.8% 11.8% and 73.3%, and 73.3%, and thatand of that the of 0.30 the m 0.30 tube m tubeincreased increased fromfrom 18.3% 18.3% to 73.1% to 73.1% when when the incident the incident light light angle angle in- creasedincreased from from 0° to 0◦ 80°.to 80 ◦.

(a)

(b)

FigureFigure 10. 10. LightLight transmission transmission performance performance of of (a (a) )aluminum aluminum alloy alloy tube tube and and (b (b) )zinc zinc alloy alloy tube. tube.

WhenWhen using using the the zinc zinc alloy alloy tube tube with with a a diamet diameterer 0.20 0.20 m m and and a a height height of of 0.50 0.50 m, m, light light transmissiontransmission increased increased from from 4.7% toto 52.5%52.5% whenwhen thethe incident incident light light angle angle increased increased from from 0◦ 0°to to 80 80°.◦. For For the the diameters diameters ofof 0.250.25 mm andand 0.300.30 m, the light transmission performance performance was was similarsimilar to to those those of of the the 0.20 0.20 m m diameter diameter tube, tube, as as illustrated illustrated in in Figure Figure 10b. 10b. Light Light transmission transmission performanceperformance of of the the zinc zinc alloy alloy tube tube with with a diameter a diameter of 0.25 of 0.25m increased m increased between between 8.7% 8.7%and 69.9%,and 69.9%, and that and of that 0.30 of m 0.30 tube m increased tube increased between between 9.1% and 9.1% 70.6% and with 70.6% an with increase an increase of the ◦ ◦ incidentof the incident light angle light from angle 0° fromto 80°. 0 Thisto 80 increase. This increaseof light transmission of light transmission performance perfor- is achievedmance is from achieved an increase from an of increase the incident of the light incident angle, light demonstrating angle, demonstrating the reduced the number reduced ofnumber reflections of reflections within the within light tube the lightsurfaces tube, which surfaces, leads which to minimal leads to losses minimal of luminous losses of intensity.luminous intensity.

Energies 2021, 14, 260 10 of 17

Energies 2021, 14, x FOR PEER REVIEW 10 of 17 Additionally, when using the aluminum alloy tube with a height of 0.5 m and diam- eters of 0.20 m, 0.25 m, and 0.30 m, the light transmission increased from 9.1% to 18.3% ◦ with anAdditionally, increase of thewhen tube using diameter the aluminum from 0.20 alloy m tube to 0.30 with m a at height the incident of 0.5 m lightand diam- angle of 0 . ◦ ◦ ◦ ◦ ◦ ◦ Theeters light of transmission0.20 m, 0.25 m, performance and 0.30 m, atthe the light incident transmission light angles increased of 5from, 10 9.1%, 15 to, 20 18.3%, 25 , 30 , 35◦with, 40◦ an, 45 increase◦, 50◦, 55of ◦the, 60 tube◦, 65 diameter◦, 70◦, and from 80 0.20◦ were m to similar 0.30 m toat the those incident of the light incident angle angleof of 0◦,0°. as The illustrated light transmission in Figure performance 10a. For the at case the inci of thedent zinc light alloy angles tube, of 5°, the 10°, light 15°, 20°, transmission 25°, performance30°, 35°, 40°, also 45°, increased50°, 55°, 60°, from 65°, 4.7% 70°, an tod 9.1% 80° were when similar the tube to those diameter of the increasedincident angle from 0.20 mof to 0°, 0.30 as millustrated at the incidentin Figure angle10a. For of the 0◦ case. At of the the incident zinc alloy light tube, angles the light of transmission 5◦, 10◦, 15 ◦, 20◦, 25◦performance, 30◦, 35◦, 40 also◦, 45 increased◦, 50◦, 55 from◦, 60 4.7%◦, 65 to◦ ,9.1% 70◦ ,when and 80the◦ ,tube the diameter light transmission increased from performance 0.20 alsom increasedto 0.30 m at from the incident 7.3% to angle 14.2%, of 0°. 6.7% At the to 16.8%,incident 7.1% light to angles 17.7%, of 5°, 8.8% 10°, to 15°, 23.1%, 20°, 25°, 18.0% to 30°, 35°, 40°, 45°, 50°, 55°, 60°, 65°, 70°, and 80°, the light transmission performance also 21.5%, 18.2% to 24.7%, 20.0% to 29.2%, 21.7% to 31.8%, 24.2% to 37.4%, 31.5% to 40.6%, increased from 7.3% to 14.2%, 6.7% to 16.8%, 7.1% to 17.7%, 8.8% to 23.1%, 18.0% to 21.5%, 36.5%18.2% to to 45.6%, 24.7%, 36.8% 20.0% to to50.7%, 29.2%, 44.2%21.7% to to 31.8%, 57.2%, 24.2% 48.3% toto 37.4%, 65.1%, 31.5% 51.1% to 40.6%, to 69.9%, 36.5% and to 52.5% to 70.6%,45.6%, 36.8% respectively, to 50.7%, as 44.2% exhibited to 57.2%, in 48.3% Figure to 10 65.1%,b. This 51.1% demonstrates to 69.9%, and that 52.5% higher to 70.6%, diameters ofrespectively, aluminum alloyas exhibited and zinc in Figure alloy 10b. light This tubes demonstrates lead to an that increase higher diameters in light transmission of alu- performance,minum alloy relating and zinc to alloy less light losses tubes of luminous lead to an intensityincrease in that light occur transmission from the perfor- decrease in themance, number relating of reflections to less losses in the of innerluminous tube intensity surfaces. that occur from the decrease in the numberWith comparingof reflections thein the aluminum inner tube alloy surfaces. tube and zinc alloy tube of the same diameter, averageWith light comparing transmission the aluminum performance alloy tube of theand aluminumzinc alloy tube alloy of the tube same was diameter, higher than thataverage of zinc light alloy transmission tube in each performance incident lightof the angle aluminum and diameteralloy tube ofwas light higher tube, than as that shown in Figureof zinc 11 .alloy It was tube observed in each incident that the light average angle light and transmissiondiameter of light performance tube, as shown of the in aluminum Fig- alloyure tube11. It canwas transmitobserved that the luminousthe average intensity light transmission more than performance 4% when of compared the aluminum with that alloy tube can transmit the luminous intensity more than 4% when compared with that of of zinc alloy tube in all conditions. This demonstrates that the aluminum alloy tube zinc alloy tube in all conditions. This demonstrates that the aluminum alloy tube shows shows more reflectivity than zinc alloy tubes. This indicates that this technology could be more reflectivity than zinc alloy tubes. This indicates that this technology could be con- consideredsidered as as an an alternative alternative daylight daylight system system in deeper in deeper rooms rooms or could or could substitute substitute artificial artificial lightinglighting in in windowless windowless spaces,spaces, leading leading to to greater greater energy energy conservation conservation in buildings. in buildings.

(a) (b)

(c)

Figure 11. Light transmission performance of light tubes with different materials with diameters of (a) 0.20 m, (b) 0.25 m, and (c) 0.30 m.

Energies 2021, 14, 260 11 of 17

The illumination distribution on the floor of the model room from the aluminum alloy and zinc alloy light tubes, with a diameter of 0.25 m and a tube height of 0.5 m, are presented in a 2-D color map as exhibited in Figures 12 and 13. The illuminance distribution from the aluminum alloy was between 12 lux and 31 lux, and the zinc al- loy light tube was between 12 lux and 19 lux, at the incident light angle of 0◦. When Energies 2021, 14, x FOR PEER REVIEW ◦ ◦ 11 of 17 the incident angle increased from 0 to 45 , the illuminance distribution on the floor from both types was uniformly distributed at the same value throughout. When the in- cident light angle increased from 45◦ to 80◦, the internal illuminance distribution was Figure 11. Light transmissionnonuniformly performance distributed,of light tubes aswith shown different in Figures materials 12 withd–f anddiameters 13d–f. of When (a) 0.20 the m, incident(b) 0.25 m, angle ◦ ◦ and (c) 0.30 m. increased from 0 to 80 , a higher illuminance in each position was achieved, as shown in Figures 12 and 13. TheThe 25illumination positions ofdistribution illumination on measurementsthe floor of the onmodel the floorroom were from averaged the aluminum for both al- aluminumloy and zinc and alloy zinc light alloy tubes, lighttube with with a diameter a length of of 0.25 0.5 mm andanddifferent a tube height diameters of 0.5 (0.20 m, are m, 0.25presented m, and in 0.30 a 2-D m) color as exhibited map as exhibited in Figures in 14 Figures and 15 12. When and 13. considering The illuminance the aluminum distribu- alloytion from tube, the with aluminum a diameter alloy of 0.20was m,between the average 12 lux illuminance and 31 lux, and on the the floor zinc increasedalloy light from tube ◦ ◦ 11was lux between to 236 lux12 lux when and the 19 incidentlux, at the angle incident increased light angle from 0of to0°. 80When. For the the incident cases of angle the diametersincreased offrom 0.25 0° m to and 45°, 0.30 the m, illuminance the average distribution illuminance valueson the onfloor the from floor both were types similar was to thoseuniformly of the distributed 0.20 m diameter at the tube, same as value illustrated throughout. in Figure When 14a. Thethe averageincident illuminancelight angle onin- thecreased floor from with the45° aluminumto 80°, the alloyinternal tube illumi withnance a diameter distribution of 0.25 m was increased nonuniformly between distrib- 16 lux anduted, 311 as lux,shown and in that Figures of 0.30 12d–f m increased and 13d–f. between When the 9 lux incident and 418 angle lux increased when there from was 0° an to ◦ ◦ increase80°, a higher in the illuminance incident light in each angle position from 0 wasto 80 achieved,. as shown in Figures 12 and 13.

(a) (b) (c)

(d) (e) (f)

FigureFigure 12.12.Illuminance Illuminance distributiondistribution onon thethe floorfloor from from the the aluminum aluminum alloy alloy light light tube tube at at thethe incidentincident angles angles of of ( a(a)) 0 0°,◦,( (bb)) 1515°,◦, (c(c)) 30 30°,◦,( (dd)) 4545°,◦,( (ee)) 6060°,◦, andand ((ff)) 7575°.◦.

The 25 positions of illumination measurements on the floor were averaged for both aluminum and zinc alloy light tube with a length of 0.5 m and different diameters (0.20 m, 0.25 m, and 0.30 m) as exhibited in Figures 14 and 15. When considering the aluminum alloy tube, with a diameter of 0.20 m, the average illuminance on the floor increased from 11 lux to 236 lux when the incident angle increased from 0° to 80°. For the cases of the diameters of 0.25 m and 0.30 m, the average illuminance values on the floor were similar to those of the 0.20 m diameter tube, as illustrated in Figure 14a. The average illuminance on the floor with the aluminum alloy tube with a diameter of 0.25 m in- creased between 16 lux and 311 lux, and that of 0.30 m increased between 9 lux and 418 lux when there was an increase in the incident light angle from 0° to 80°. When considering the zinc alloy tube with a diameter of 0.20 m, the average illumi- nance on the floor increased from 9 lux to 20 lux when the incident light angle increased from 0° to 80°. For the cases of the diameters of 0.25 m and 0.30 m, the average illuminance values on the floor were similar to those of the 0.20 m diameter tube, as illustrated in Figure 14b. The average illuminance on the floor with the zinc alloy tube with a diameter of 0.25 m increased between 16 lux and 302 lux, and that of the 0.30 m diameter tube in- creased between 6 lux and 400 lux when there was an increase in the incident angle from 0° to 80°.

Energies 2021, 14, x FOR PEER REVIEW 12 of 17

Energies 2021, 14, x FOR PEER REVIEW 12 of 17

Comparing the diameter of both aluminum alloy tubes and zinc alloy tubes, the av- erageComparing illuminance the on diameter the floor of wasboth between aluminum 9 lux alloy to tubes28 lux and when zinc the alloy tube tubes, diameter the av- in- eragecreased illuminance from 0.20 onm tothe 0.30 floor m wasat the between incident 9 lightlux to angles 28 lux of when 0°, 5°, the 10°, tube 15°, diameter20°, 25°, andin- creased from 0.20 m to 0.30 m at the incident light angles of 0°, 5°, 10°, 15°, 20°, 25°, and Energies 2021, 14, 260 30°. For the incident light angles of 35°, 40°, 45°, 50°, 55°, 60°, 65°, 70°, and 80°, the average12 of 17 30°.illuminance For the incident on the floorlight anglesof both of aluminum 35°, 40°, 45 al°,loy 50°, tube 55°, and 60°, zinc 65°, alloy70°, and tube 80°, increased the average with illuminancean increase onin diameterthe floor fromof both 0.20 aluminum m to 0.30 alm,loy as tube illustrated and zinc in Figurealloy tube 14. increased with an increase in diameter from 0.20 m to 0.30 m, as illustrated in Figure 14.

(a) (b) (c) (a) (b) (c)

(d) (e) (f)

(d) (e) (f) FigureFigure 13. 13.Illuminance Illuminance distribution distribution on on the the floor floor from from the the zinc zinc alloy alloy light light tube tube at the at incidentthe incident angles angles of (a )of 0 ◦(,(a)b 0°,) 15 (b◦,() 15°,c) 30 (◦c,) (Figured30°,) 45 (◦d ,()13. e45°,) Illuminance 60 (◦e,) and60°, (andf) 75distribution (◦f.) 75°. on the floor from the zinc alloy light tube at the incident angles of (a) 0°, (b) 15°, (c) 30°, (d) 45°, (e) 60°, and (f) 75°.

(a) (b) (a) (b) Figure 14. Average internal illuminance contours on the floor for the (a) aluminum alloy and (b) zinc alloy light tubes. Figure 14. AverageAverage internal illuminance contourscontours on the floorfloor for the (a)) aluminumaluminum alloyalloy andand ((b)) zinczinc alloyalloy lightlight tubes.tubes.

Energies 2021,, 14,, 260x FOR PEER REVIEW 13 of 17

(a) (b)

(c)

Figure 15. AverageAverage internal internal illuminance illuminance of of light light tube tube with with different different materials materials and and diameters diameters (a) ( a0.20) 0.20 m, m,(b) ( b0.25) 0.25 m, m,and and (c) (0.30c) 0.30 m. m.

ComparingWhen considering the aluminum the zinc and alloy zinc tube alloy with tubes a diameter at the same of 0.20 diameter m, the average of 0.20 illumi-m, the averagenance on illuminance the floor increased on the floor from from 9 lux the to 20alum luxinum when alloy theincident tube was light higher angle than increased that of zincfrom alloy 0◦ to tube 80◦. Forat all the incident cases of light the diameters angles, as of shown 0.25 m in and Figure 0.30 m, 15a. the The average aluminum illuminance alloy tubevalues was on more the floor than were7% brighter similar when to those compared of the 0.20 with m zinc diameter alloy tubes tube, at as incident illustrated light in anglesFigure 14betweenb. The average0° and 80°. illuminance For the ondiameters the floor of with 0.25 the m zinc and alloy 0.30 tube m, an with increase a diameter of the of average0.25 m increased illuminance between on the 16 floor lux and from 302 the lux, aluminum and that of alloy the 0.30tube m was diameter more than tube increased3% when comparedbetween 6 with lux and that 400 of zinc lux when alloy theretube at was all anincident increase light in angles, the incident as shown angle in from Figure 0◦ to15b,c. 80◦. This Comparingindicates that the the diameter aluminum of bothalloy aluminum tube has more alloy illuminance tubes and zinc efficiency alloy tubes,than zinc the alloyaverage tubes illuminance at each diameter on the size floor and was incident between light 9 lux angle. to 28 lux when the tube diameter ◦ ◦ ◦ ◦ ◦ ◦ increasedThe daylight from 0.20 factor m to is 0.30defined m at as the the incident average lightinternal angles illumination of 0 , 5 , on 10 the, 15 floor, 20 plane, 25 , ◦ ◦ ◦ ◦ ◦ ◦ ◦ ◦ ◦ ◦ and illuminance 30 . For the incidenton the external light angles testing of model 35 , 40 in, 45an unshaded, 50 , 55 , 60area., 65 The, 70 exterior, and 80 illumi-, the nanceaverage was illuminance measured on to thebe floorbetween of both 890 aluminum lux and 1140 alloy lux tube and and obtained zinc alloy an tubeaverage increased lumi- nouswith anintensity increase of in950 diameter lux. The from daylight 0.20 factor m to 0.30 for both m, as aluminum illustrated alloy in Figure and zinc14. alloy light tubesComparing in all patterns the is aluminum demonstrated and zincin Figu alloyre 16. tubes When at the considering same diameter the aluminum of 0.20 m, alloy the tubeaverage with illuminance a diameter onof 0.20 the floorm, the from average the aluminum daylight factor alloy increased tube was from higher 1.2% than tothat 22.7% of whenzinc alloy the incident tube at alllight incident angle lightincreased angles, from as shown0° to 80°. in For Figure the 15casesa. The of the aluminum diameters alloy of 0.25tube m was and more 0.30 thanm, the 7% average brighter daylight when comparedfactor was withsimilar zinc to those alloy tubesof the atdiameter incident of light 0.20 ◦ ◦ mangles as displayed between in 0 Figureand 80 16a.. For The the average diameters daylight of 0.25 factor m andfrom 0.30 the m,aluminum an increase alloy of tube the withaverage a diameter illuminance of 0.25 on m the increased floor from between the aluminum 1.6% and alloy32.7%, tube and was that more of 0.30 than m increased 3% when betweencompared 0.9% with and that 43.9% of zinc when alloy there tube was at all an incident increase light of incident angles, light as shown angle infrom Figure 0° to 15 80°.b,c. This indicates that the aluminum alloy tube has more illuminance efficiency than zinc alloy tubes at each diameter size and incident light angle. The daylight factor is defined as the average internal illumination on the floor plane and illuminance on the external testing model in an unshaded area. The exterior illumi- nance was measured to be between 890 lux and 1140 lux and obtained an average luminous

Energies 2021, 14, 260 14 of 17

intensity of 950 lux. The daylight factor for both aluminum alloy and zinc alloy light tubes in all patterns is demonstrated in Figure 16. When considering the aluminum alloy tube with a diameter of 0.20 m, the average daylight factor increased from 1.2% to 22.7% when the incident light angle increased from 0◦ to 80◦. For the cases of the diameters of 0.25 m and 0.30 m, the average daylight factor was similar to those of the diameter of 0.20 m as displayed in Figure 16a. The average daylight factor from the aluminum alloy tube Energies 2021, 14, x FOR PEER REVIEW 14 of 17 with a diameter of 0.25 m increased between 1.6% and 32.7%, and that of 0.30 m increased between 0.9% and 43.9% when there was an increase of incident light angle from 0◦ to 80◦.

(a) (b)

Figure 16. Daylight factor of the vertical light tube using (a) aluminum alloy andand ((b)) zinczinc alloy.alloy.

Considering thethe casecase of of the the zinc zinc alloy alloy tube tube with with adiameter a diameter 0.20 0.20 m, them, the average average daylight day- factorlight factor increased increased from from 0.9% to0.9% 23.2% to 23.2% with anwith increase an increase in the in incident the incident light anglelight angle from 0from◦ to 800° ◦to. For80°. the For cases the cases of the of diameters the diameters of 0.25 of m 0. and25 m 0.30 and m, 0.30 the m, trend the trend of average of average daylight daylight factor wasfactor similar was similar to those to th ofose the of diameter the diameter of 0.20 of m,0.20 as m, exhibited as exhibited in Figure in Figure 16b. 16b. The The average aver- daylightage daylight factor factor from from zinc zinc alloy alloy tube tube with with a diameter a diameter of 0.25 of 0.25 m increased m increased between between 1.7% 1.7% and 34.5%,and 34.5%, and thatand ofthat 0.30 of m 0.30 increased m increased between between 0.7% and 0.7% 42.2% and when42.2% there when was there an increasewas an in- of ◦ ◦ incidentcrease of lightincident angle light from angle 0 to from 80 . 0° When to 80 comparing°. When comparing the incident the lightincident angle light to theangle tube to ofthe both tube aluminum of both aluminum alloy tube alloy and tube zinc and alloy zinc tube, alloy the tube, average the average maximum maximum daylight daylight factor ◦ wasfactor found was found at the at incident the incident light anglelight angle of 80 offrom 80° bothfrom aluminumboth aluminum alloy alloy tubes tubes and zincand alloyzinc alloy tubes. tubes. Comparing the diameter variation of both aluminum alloy tubes and zinc alloy tubes, the daylight daylight factor factor was was similar similar and and between between 0.7% 0.7% and and 2.9% 2.9% when when the the tube tube diameter diameter in- ◦ ◦ ◦ ◦ ◦ ◦ increasedcreased from from 0.20 0.20 m m to to 0.30 0.30 m m at at the the incident incident light light angles of 00°,, 55°,, 1010°,, 1515°,, 2020°,, 2525°,, and ◦ ◦ ◦ ◦ ◦ ◦ ◦ ◦ ◦ 3030°.. For For the the incident incident light light angles angles to to the the tube tube of of 35 35°, 40°,, 40 45°,, 45 50°,, 50 55°,, 55 60,°, 60 65°,, 65 70°,, 70and, and80°, ◦ 80the ,daylight the daylight factor factor of both of both aluminum aluminum alloy alloy tube tube and and zinc zinc alloy alloy tube tube increased increased with anan increase in diameter from 0.200.20 mm toto 0.300.30 m,m, asas illustratedillustrated inin FigureFigure 1616.. ThisThis observationobservation demonstrated thatthat thethe aluminum aluminum alloy alloy tubes tubes can can provide provide a better a better daylight daylight factor factor compared com- withpared zinc with alloy zinc tubes alloy fortubes each for condition. each condition. This daylight factor factor obtained obtained from from the the aluminum aluminum alloy alloy tubes tubes and and zinc zinc alloy alloy tubes tubes in in this work corresponded to those of previous works [29,31], which demonstrated the this work corresponded to those of previous works [29,31], which demonstrated the day- daylight contribution in areas with 1–2% of daylight factor for activities in residence and light contribution in areas with 1–2% of daylight factor for activities in residence and 2– 2–4% of daylight factor for activities in office buildings [29,31]. For the case of window, the 4% of daylight factor for activities in office buildings [29,31]. For the case of window, the occupants visualize the outdoor environment and use natural ventilation, while the shading occupants visualize the outdoor environment and use natural ventilation, while the shad- from adjacent obstacles and barriers appeared in many cases and insufficient daylight ing from adjacent obstacles and barriers appeared in many cases and insufficient daylight within deeper building areas was achieved. Skylights can provide high light intensity, within deeper building areas was achieved. Skylights can provide high light intensity, natural ventilation, and uniform daylighting, but immoderate solar gains and overheating natural ventilation, and uniform daylighting, but immoderate solar gains and overheating in the internal spaces of rooms was obtained. The distant areas in buildings achieved in the internal spaces of rooms was obtained. The distant areas in buildings achieved in- insufficient light transmission. Hollow light tubes in this work can transport daylight into sufficient light transmission. Hollow light tubes in this work can transport daylight into some deeper or windowless areas of buildings and uniformly distribute light. Importantly, thesome investment deeper or price windowless of this system areas was of cheap build forings inventing and uniformly the aluminum distribute alloy light. tubes andIm- portantly, the investment price of this system was cheap for inventing the aluminum alloy tubes and zinc alloy tubes. Although an investigation of thermal behavior and inside illu- minance of the models under natural weather conditions, and an analysis of the reduction of energy consumption using systems on a full scale could be led to more sound scientific knowledge for integrating the vertical light tubes into buildings; a more widespread con- text will be studied and analyzed under natural weather conditions in the future work. In view of the benefits, the invented light tubes can use commercial aluminum alloy sheets and commercial zinc alloy sheets to provide illumination for activities in residences and offices for some incident light angles for each diameter of light tube, which leads to an

Energies 2021, 14, 260 15 of 17

zinc alloy tubes. Although an investigation of thermal behavior and inside illuminance of the models under natural weather conditions, and an analysis of the reduction of energy consumption using systems on a full scale could be led to more sound scientific knowledge for integrating the vertical light tubes into buildings; a more widespread context will be studied and analyzed under natural weather conditions in the future work. In view of the benefits, the invented light tubes can use commercial aluminum alloy sheets and commercial zinc alloy sheets to provide illumination for activities in residences and offices for some incident light angles for each diameter of light tube, which leads to an alternative inexpensive material for producing a lighting system. This investigation could be considered as an alternative daylight system in deeper parts of buildings or the windowless spaces to conserve the energy consumption for lighting buildings.

4. Conclusions This investigation evaluated the light transmission performance of aluminum alloy and zinc alloy hollow light tubes as a function of both incident angle and the diameter of the light tubes. The luminous intensity at the bottom of the aluminum alloy and zinc alloy light tubes with a diameter of 0.25 m increased from 31 lux to 2304 lux and 16 lux to 2133 lux, respectively, when the incident angle of light was increased from 0◦ to 80◦. This is a result of the proportion of reflected light and direct light being transmitted through the vertical tube. The light transmission performance of both light tube types increased when the incident light angle increased from 0◦ to 80◦ in each diameter and the diameter of tube increased from 0.20 m to 0.30 m for each incident angle. The type of light tube material has an effect on the reflective performance of the light tube. The aluminum alloy pipe can improve the light transmission performance by approximately 4% and achieved a better daylight factor when compared with the zinc alloy tube for each condition. This work could be the initial investigation to contribute to enhancing daylight in some deeper areas of buildings or replacing artificial lighting in windowless areas. Light tubes can conserve energy consumption for lighting in buildings.

Author Contributions: Conceptualization, A.T.; methodology, A.T. and J.M.; formal analysis, A.T. and J.M.; investigation, A.T. and J.M.; resources, A.T.; data curation, A.T.; writing—original draft preparation, A.T. and J.M.; writing—review and editing, A.T.; visualization, A.T.; supervision, A.T.; project administration, A.T.; funding acquisition, A.T. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by Faculty of Science, Naresuan University, grant number P2564C010. Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Acknowledgments: The authors would like to thank the Thailand Science Research and Innovation (TSRI), Faculty of Science, Naresuan University for providing financial support to this research work, and our research center. Thanks are given to Kyle V. Lopin for editing this document. Conflicts of Interest: The authors declare no conflict of interest.

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