**1. Introduction**

Building energy consumption figures have risen from 26% in 1980 to 54% in 2010 and are predicted to rise to 84% in 2050. They will continuously increase day by day because of the rise in population, the growth of modern society, and quality of life improvements [1,2]. Lighting in offices contributes to approximately 30% of the total energy consumption in buildings [3–6]. In recent years, buildings have been appropriately developed, constructed, and maintained so as to supply their inhabitants with a better environment quality and electrical energy conservation through optimal design and functional practices [6,7].

Natural daylight is a very important source of illumination in buildings because it is free and helps reduce energy consumption [8–10]. Sufficient and effective daylight utilization results in energy saving. The use of natural daylight in buildings will also be significant for the visual and physical comfort and health of the people working within them [6,11–14]. The advance of daylight investigation contributes to energy conservation, improves human welfare, enhances physiological capacities, and avoids disease. One of the consequences of the adjustment of the human eyes to light over time is visual ability, and subsequently, the significant function of the psychophysical levels of daylight for numerous activities is affected. Undoubtedly, daylight conditions can impact the inhabitants of buildings both mentally and physically. Many studies have proven that the hormone melatonin is suppressed by daylighting, which enables us to improve regular light–dark rhythms that can help individuals gain satisfactory rest [15–18]. Daylighting will be used for various applications and it

has outstanding solutions to reduce energy consumption for internal building areas. The regular daylight system comprises a top skylight and a window. Windows allow natural light to illuminate the interior of a building while also allowing for the transfer of heat. Some areas of a building cannot be reached by natural light, so they do not heat up as much as the external parts of the structure [19]. The effects of inadequate daylight deeper in a building necessitate extra use of electricity from lighting, which contributes to approximately 30% of total energy consumption [20].

The conservation of artificial lighting during the daytime allows for great energy saving [21,22]. Architectural structures can be designed to provide adequate and efficient daylight for the internal areas of a building, which will result in energy conservation [19,23–26]. Daylight illuminates the inside of a building and transfers infrared radiation, which can be significantly absorbed by water vapor in the air, which is an important cause of thermal gain in buildings [27,28]. As a result of excessive heat gain in the building, ventilating fans and air conditioners are essential for providing a cool, controlled environment, removing the hot air from the building, or cooling the interior air. This is necessary for providing a comfortable environment for the inhabitants. This requires significant power consumption. To significantly reduce thermal accumulation in buildings and appropriately utilize natural lighting, the top of the test box was designed and constructed using wood and glass cylinders filled with distilled water. Our experiment concentrated on testing both thermal performance and the amount of natural light inside the box by using various glass cylinders that were constructed to meet the required heat transfer reduction and energy conservation levels within buildings.

#### **2. Materials and Methods**

#### *2.1. Testing Model Design, Temperature, and Illuminant Measurements under Controlled Conditions*

The thermal and illuminant performance of our model, which was constructed from wood on five sides and insulated using polyethylene sheets with different configurations of glass inserted into the top of the box, were investigated using nine different cylindrical glass designs. The dimensions of our model had an area of 1 m2, and a volume of 1 m3, as shown in Figure 1. The top of the testing unit was interchanged using two glass cylinders (2S), four glass cylinders (4S), and six glass cylinders (6S) to allow light to enter the box, creating illumination within it. Each glass design was constructed as either a single layer of glass (G), two layers of glass (2G) or two layers of glass filled with distilled water (2GW), as shown in Figure 2. Insolation was provided by nine 500 W halogen lamps placed 0.25 m away from the top of the unit. The nine lamps were calibrated to deliver a light intensity of 1000 W/m2, which was regulated using a basic voltage control device. K-type thermocouples with an accuracy of ±0.5 ◦C were employed to measure the temperature changes, and were attached to both the exterior and interior surfaces of the glass and the top of the unit using thermal paste to ensure good thermal contact, and were insulated using aluminum foil tape. The ambient temperature in the laboratory was set at approximately 25 ◦C using an air conditioner. The temperature inside the box was measured by suspending a thermocouple in the center of it. All data were recorded at 2 min intervals continuously for 3 h using a data logger, and the illuminance was measured using a lux meter (DIGICON LX-70).

**Figure 1.** (**a**) View of the test unit in controlled conditions. (**b**) Fixed locations of the thermocouples for temperature testing.

**Figure 2.** Views of the glass units.
