**1. Introduction**

A high level of solar radiation is experienced throughout most of Thailand throughout the year. The average daily value of solar radiation is around 18.2 MJ/m2-day with the highest levels of solar radiation experienced in the months of April and May, between 20 and 24 MJ/m2-day [1–3]. These conditions cause significant thermal accumulation in 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. Nowadays, buildings are designed and constructed to provide their occupants with a better-quality environment and to improve energy conservation with optimal designs and functional practices [4,5].

Daylight is one solution to save energy consumption in buildings because it is free and a valuable light source for internal building areas throughout the day [6–8]. Effective daylight utilization can result in energy savings. The use of natural daylight in buildings also significantly improves the visual and physical comfort of the building. Individuals

**Citation:** Mahawan, J.; Thongtha, A. Experimental Investigation of Illumination Performance of Hollow Light Pipe for Energy Consumption Reduction in Buildings . *Energies* **2021**, *14*, 260. https://doi.org/ 10.3390/en14020260

Received: 19 November 2020 Accepted: 4 January 2021 Published: 6 January 2021

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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

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 were built using wood. The model room's walls each had an area of 1 m<sup>2</sup> and a volume of 1 m3, as shown in Figure 1a. A 20-W artificial LED lamp was used as the light source. The illumination changes, as the elevation angle was varied between 0◦ and 80◦ with a step size of 5◦, are exhibited in Figure 1. The illumination at nine positions—at the top and bottom ends—of each light tube was measured by using an illuminance lux meter (DIGICON LX-70) which is also based on the International Commission on Illumination standard (CIE standard) as shown in Figure 2. All values at the top and bottom side ends were calculated to determine the average illuminance of vertical aluminum and zinc alloy tubes with the diameters of 0.20 m, 0.25 m, and 0.30 m. The average luminous intensity at the top and bottom side ends was evaluated to identify the improved light transmission efficiency. The illuminance distribution in the model area was also tested and measured at 25 locations using an illuminance lux meter according to the CIE standard, as exhibited in Figure 3. The daylight factor was defined as the proportion of average internal illumination on the floor plane and the related luminance at ambient areas on horizontal and unshaded areas.

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

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

**Figure 3.** Fixed positions of the 25 illumination measurements on the floor plane.
