Study on Improving the Energy Efficiency of a Building: Utilization of Daylight through Solar Film Sheets

Abstract

Daylight can contribute to substantial reductions in the energy consumed by artificial lighting applications. However, issues such as visual comfort, illumination intensity, and availability represent major issues when daylight is relied upon to illuminate buildings. There are many technologies that are used to control received sunlight and minimize its side effects. The placement of solar film sheets on window glass is a common and popular method utilized in many buildings to minimize electric lighting energy consumption without causing undue visual discomfort to occupants. To examine the practicality of this application and its effect on room lighting, a modern office was selected in which to conduct this field study. Two measures were used to evaluate this technique: firstly, field measurements and their comparison to the specified standard illumination levels; and secondly, a short-form questionnaire survey conducted to obtain occupants’ opinions of the office lighting. Actual measurements were conducted in the selected office spaces, with and without applying solar control film coating on the window glass. Indoor luminance levels and lighting comfort were systematically recorded and analyzed. The findings of this study show that using a solar film with a visible light transmittance of 50% can achieve savings in energy consumption of up to 33% if utilized as part of an integrated lighting system.

1. Introduction

The need to reduce the use of fossil fuels and shift to renewable energy sources is critical for facing the major issues of climate change and attaining global sustainability [1,2,3]. Carbon dioxide emissions from fossil fuels like coal, oil, and natural gas play a significant role in the buildup of heat-trapping gases in the atmosphere and the consequent global warming. By shifting to renewable energy, such as solar [4], wind [5], hydro [6], and geothermal [7], carbon emissions can be significantly reduced, and the adverse effects of climate change can be mitigated. Another method that leads to the reduction in fossil fuel use is the adoption of waste-heat recovery techniques [8,9], which can improve the efficiency of existing systems and decrease energy consumption. Thus, there is a critical need for energy consumption reduction, since promoting energy efficiency measures and adopting sustainable practices in sectors such as transportation, buildings, and industry can help minimize energy waste and lessen the overall demand for fossil fuels. Additionally, this approach can considerably contribute to the achievement of the Sustainable Development Goals, since reducing energy consumption is considered one of the major contributing factors [10].

Large fractions of the electricity needed in buildings nowadays are used for electric lighting. Many studies confirm that it accounts for almost 40% of offices’ electricity consumption [11]. On the other hand, daylight generates outdoor illuminances that often exceed the required illuminances for building lighting by some orders of magnitude. Making daylight more available in buildings can therefore contribute to significant energy savings [12,13,14]. Additionally, it can enhance the occupants’ performance and wellbeing. Thus, highly efficient daylighting technologies were developed to take advantage of the available sunlight, such as a highly energy-efficient window system and a daylight-responsive lighting control system [15,16]. Natural illumination in buildings requires correctly designed apertures and floor plans to provide sufficient daylighting for a reasonable distance into the building. However, problems such as daylight’s availability, glare, and high illumination intensity cause occupants to close blinds. These drawbacks are considered real obstacles to relying largely on daylighting in offices [17]. To overcome some of these problems, a solar control technology is required in addition to the use of artificial illumination [18]. This can help optimize the value of daylight in offices over the course of a day or year. Two types of daylighting techniques are widely available: indirect and direct. The first delivers sunlight indirectly into windowless and remote spaces in buildings, for example, the Heliobus lighting system, where a roof-mounted static mirror heliostat is designed and located to collect and redirect the largest possible amount of daylight into prismatic vertical light guided through some floors [19]. Himawari [20] and Parans [21] are examples of indirect solar lighting systems where modular solar panels with Fresnel lenses are used to concentrate daylight into an optical fiber cable and then to distribute it through passive light fixtures and tubular daylight guidance systems [22]. These examples of indirect daylighting bring great benefits to buildings in remote areas since they can reduce lighting energy consumption. However, as custom-built solutions for individual applications, they have high capital costs, which is one of the reasons they are not widely used [23]. There may also be aesthetic concerns about their appearance, and they may additionally pose problems in the space planning of floor plates where it is not easy to redesign or modify the building to accommodate these technologies. The second type, which is direct daylighting control technology, is designed to modify sunlight as it passes through the windows of buildings, providing filtered light into the perimeter spaces. This type includes anidolic daylighting systems, which are a facade-integrated technology that collect sunlight outside the building and redeploy it inside [24,25]. Direct systems also comprise lighting control detectors that may include lux level sensors, photodetector sensors, and solar coating films.

The desire to create low-energy buildings with good daylight penetration means that daylight should be brought inside the building at acceptable levels and also with acceptable comfort characteristics that minimize glare and heat gain. Since the sufficiency and acceptability of daylight are subject to the external illuminance levels and direction, artificial light must be part of the complete lighting design. The method of lighting design that combines natural lighting with artificial lighting is called an integrated lighting system (ILS) [26,27]. Among the technologies that are used to control sunlight and minimize its side effects, solar film sheets placed on the windows represent a simple application that is claimed to be an efficient and effective solution that is able to reduce the cost of air conditioning by 30–40% [28]. Moreover, it is claimed that it minimizes the electric lighting consumption without causing undue visual discomfort to the occupants [29].

This study explores the potential efficiency and energy savings in lighting design in offices using solar film sheets applied to windows in conjunction with an artificial lighting regime. While the broader topic of daylight utilization is well established, this research aims to verify the effectiveness of employing solar film, with a specific focus on its impact on energy savings resulting from the reduction in electrical lighting usage within an ILS, as most previous studies primarily concentrated on the effects of solar films on cooling and heating loads. Additionally, this paper combines three methods to comprehensively verify the positive effects of solar films: field measurements, simulations, and surveys. Quantitative field measurements of the proposed whole integrated daylighting and artificial lighting system are combined with a qualitative analysis of the occupants’ responses. To eliminate direct sunlight and to avoid the problems of glare and excessive brightness, solar film coating is proposed as a simple technology to control the solar lighting entering the building. This study represents a continuation of a previous study from the literature to investigate energy-efficient lighting design in buildings [29]. In this study, we looked at the impact of utilizing daylight as a tool to enhance building lighting performance. The performance of the ILS was studied experimentally in an office room that was modified for these lighting installations. The lighting performance and real measurement was recorded for two types of installations over two months: first, with clear glass windows, and second, with solar-film-tinted glass that reduces the lighting transmittance by 50%. The actual lighting level in the office was recorded, and the occupant satisfaction was collected via surveys. DIAlux 4.10 simulation software was used to study and evaluate the efficient lighting schemes for the tested office through the use of LED light fixtures that were tested in [30] and found to be the best energy-efficient type of lamp for the building.

2. Experimental Configuration

The performance of the proposed ILS was studied through test building-room lighting installations located at a commercial building in Abu Dhabi, the capital of the UAE. The building is located on the eastern coast of the Arabian Peninsula, at a latitude of 25°15′ N and longitude 51°36′ E, north of the Tropic of Cancer [31]. An experimental cellular office room was selected for this study. The room is located in a commercial building in Masdar City on the 4th floor. On each floor, there are twenty-five identical offices. The office room is a rectangle shape 6 m wide and 4 m long, and the height of the room is 3 m. Northward-oriented windows are installed as the facade; their dimensions are 6 m in width and 3 m in height, and they are transversally placed as the fourth wall. The reflection coefficients (supposed diffuse) of the walls are 0.57, 0.2, and 0.7 for the wall, floor, and ceiling, respectively. The required lux level as per the CIBSE standard is 500 Lux [16]. The room is illuminated through the use of ERCO product reference ERCO 46815000 Quintessence Down light 9 × LED 28 W daylight white, equipped with an ERCO LED lamp of 28 W for use at night or as a complementary lighting solution. The price of energy consumption of a commercial building in the UAE is $0.12/kWh. The office working hours are 8 h a day, five days a week. Figure 1 shows the actual room configuration, taken with a camera during the daytime and with the window blinds closed to avoid any daylight effect on the photo. Four weather conditions were identified during the recording of the readings (overcast, cloudy, dusty, and sunny day). The following weather conditions were considered: Overcast weather refers to the condition where all of the sky is obscured with clouds and the sky is totally dull. Cloudy weather is defined as a sky condition in which clouds cover at least 70% of the sky. Dusty weather refers to a weather condition in which dust and sand particles spread in the air, obscuring the sun’s light, and sunny weather represents the weather in which the sky is cloudless and clear of dust.

3. Methods

3.1. Field Measurements

In the first scenario, nine white LED lights were installed, which were proven to be the most energy-efficient type of lamp for this kind of application. Daylight enters the room through dual clear windows with no special coatings applied to the glass. The dual panes of glass are spaced 15 cm apart; air fills the space and acts as an insulation barrier. The space between the glass panes allows the external venetian blind roller to open and close automatically and manually, reducing glare from the sun and providing solar control. The visible light transmittance of these windows is 0.81. The room dimensions were taken from the room design plan and verified on-site.

In the second scenario, a solar film sheet was placed on the inner side of the window glass. The solar film specification was as follows: double-glazed, visible light transmission of 50% (percentage of visible light allowed to pass through the film/glazing), and a shading coefficient of 0.33 (this represents the ratio of the film/glazing performance over the entire solar spectrum). The detailed specifications of the solar film sheets used are presented in

Table 1. Solar film specifications (https://www.guardianglass.com/me/en/our-glass/sunguard-double-silver/ds-50-t, accessed on 10 October 2023).

Visible light	Transmittance (%)		50
	Reflectance (%)	Out	12
		In	14
	General color rendering index (Ra)		86.9
Ultraviolet	Transmittance (%)		24
Solar energy	Transmittance (%)		24
	Reflectance	Out	27
		In	36
	Absorptance (%)		49
	Solar heat gain coefficient		0.29
	Shading coefficient		0.33
Thermal properties	U-value (W/m2.K)	Winter night	1.667
		Summer day	1.617
Light to solar gain			1.71

. As can be noticed, this type was chosen for several reasons: it provides good visible light transmission, UV protection, effective solar energy control, and thermal insulation. These factors collectively contribute to a more energy-efficient living area while maintaining visual comfort for a good working environment.

Daily measurements were made of the room illumination from April 2022 to June 2022 three times each day (9 a.m., 12 p.m. and 3 p.m.). These times were selected based on the typical working regime for offices in the UAE: (1) at 9 a.m., the workday starts in most offices, (2) at 12 p.m., most employees take their lunch breaks, and (3) 3 p.m. corresponds to the midafternoon period. The measurement of required illumination was taken on a work plane that has a height of 0.85 m (reading level), and a grid of 128 × 128 points was selected for complete room coverage. Three areas were selected for measurements, (A), (B), and (C), to check the lighting level in different places in the room: near the window, in the middle of the room, and on the opposite side of the room. The illumination level was examined and tested through the use of a HIOKI LUX HiTSTER 3423 Lux meter; the accuracy of this meter is ±4% recorded digit, ±1 digit at room temperature (23 ± 5 °C) at zero adjustment [32]. Calibration was performed at the beginning of each test, and records were taken and recorded after 5 s of reading per the meter manufacturer’s user manual and manufacturer recommendations.

3.2. Simulation

The illuminance incident angle (for direct illuminance from the sun in a clear sky) on the vertical windows was estimated for these three months based on the experiment location latitude, the vertical window angle, the time of measurement, and specific days of each month chosen to calculate these angles [33]. The average day in April is 15 April, in May is 15 May, and in June is 11 June, which correspond to days 105, 135, and 162 in the year, respectively. The incident angles (the angle between the sun’s rays and the perpendicularity to the surface) are calculated using Equation (1) [34], and the results are shown in Table 2.

Cos θ = sin ϕ sin δ cos β- sin δ cos ϕ sin β cos γ + cos ϕ cos δ cos ω cos β + cos δ sin ϕ sin β cos γ cos ω + cos δ sin β sin γ sin ω …

(1)

where:

θ is the incidence angle of the sun on the surface.

δ is the solar declination.

ϕ is the location latitude.

γ is the surface azimuth angle.

ω is the hour angle.

Table 2. Solar incident angle.

Month	Day	Incident Angle (Degree) at 9 a.m.	Incident Angle (Degree) at 12 p.m.	Incident Angle (Degree) at 3 p.m.
April	15	98	106	98
May	15	89	96	89
June	11	85	91	85

Table 2. Solar incident angle.

Month	Day	Incident Angle (Degree) at 9 a.m.	Incident Angle (Degree) at 12 p.m.	Incident Angle (Degree) at 3 p.m.
April	15	98	106	98
May	15	89	96	89
June	11	85	91	85

In the first step, we determined the room elements and actual dimensions in order to use them as inputs to calculate and modulate the design through computer software. Figure 2 shows the actual room elements as simulated in the DIAlux software. The detailed input parameters for the software are as follows:

• Office dimensions: office room is a rectangle shape 6 m wide and 4 m long, and the height of the room is 3 m.
• Windows: northward-oriented windows are installed as the facade; their dimensions are 6 m in width and 3 m in height, and they are transversally placed as the fourth wall.
• Interior wall, floor, and ceiling finishes: the reflection coefficients (supposed diffuse) of the walls are 0.57, 0.2, and 0.7 for the wall, floor, and ceiling, respectively.
• Light selection: the room is illuminated through the use of ERCO product reference ERCO 46815000 Quintessence Down light 9 LED 28 W daylight white, equipped with an ERCO LED lamp of 28 W for use at night or as a complementary lighting solution.
• Working hours: the office working hours are 8 h a day, five days a week.
• Furniture layout: This refers to the dimensions of each piece of furniture such as tables, desks, chairs, drawers, and shelves.
• Window blinds: venetian blind rollers.
• Weather inputs: four weather conditions were identified during the recording of the readings (overcast, cloudy, dusty, and sunny day).
• Office orientation: direction of the sun, east-rising.

Figure 2. Simulated test room configuration.

Figure 2. Simulated test room configuration.

The Lux meter was placed horizontally facing the ceiling at a height of 0.85 m from the floor with the three points selected as follows: point A was within one meter of the window, point B was within two meters of the window, and point C was within one meter of the opposite wall. The illumination measurements were taken with the electrical lighting switched off, and four weather conditions were identified during the recording of the readings (overcast, cloudy, dusty, and clear sky weather).

3.3. Survey

A face-to-face (walk-in) questionnaire survey was conducted to explore the occupants’ (employees’) satisfaction with the different lighting arrangements. Fifteen participants were selected and questioned to explore their opinions. The participants working in those offices were aged between 35 and 45 years old and included four managers, eight engineers, and three office administrators. The survey was administered in two stages: firstly, for the reference scenario, and secondly, for the improved model scenario [35]. In each stage, the questionnaire was conducted four times based on the weather conditions (overcast, cloudy, dusty, and sunny). The questionnaire focused on the three lighting elements that include lighting comfort, daylight level, and use of curtains/blinds and electric light. The statements were as follows:

• How do you rate the lighting level in this office?

□ Not at all satisfied	□ Not satisfied
□ Somewhat satisfied	□ Very satisfied
□ Extremely satisfied

2.

How well can you see to do your work in this office?

□ Very bad	□ Bad	□ Neutral	□ Well
□ Very well

3.

Do you need to control the amount of sunlight in your workplace by using curtain or electrical lighting?

□ No	□ If yes, what type
□ Window blinds
□ Switch on lighting

4. Results and Discussion

4.1. Simulation Results

4.1.1. Without Daylight

The test room was modeled with electrical lighting without the contribution of daylighting in order to assess the adequacy of the room lighting installation design for night and the amount of light that would contribute to the room lighting when integrated with daylighting (see Figure 3).

From the simulation results shown in

Table 3. Room with electrical lighting only; specific connected load: 12 W/m²; ground area: 24 m².

Surface	ρ (%)	Eav (lx)	Emin (lx)	Emax (lx)	u0
Work plane	/	372	47	477	0.126
Floor	20	233	13	424	0.055
Ceiling	70	71	52	85	0.734
Walls (4)	52	113	16	237	/

, it is clear that the average lighting level at workplace level is 372 Lux, and it is about 450 Lux at desk level, which is considered an acceptable lighting level per the CIBSE standard [16]. The power density is 12 W/m², which was confirmed through a previous study on the adequacy of using LED lighting for indoor lighting design due to its low energy consumption and delivery of a sufficient lighting level [29]. A false-color rendering scheme is illustrated in Figure 4, where it confirms the expected lighting level in the form of a color index and the accepted uniformity distribution of the LED installation.

Actual illumination levels were taken after dark to avoid any false readings due to daylight influence, and then, all the lamps were switched on. We also did not consider any contribution from street lights because of the height of the floor (eighth floor) and the closure of the window blinds during this measurement. The findings of the field measurements shown in

Table 4. Lighting level with electrical lighting only.

Location	Readings (Lux)	Tolerance (Lux)
A	330	±13
B	425	±17
C	300	±12

confirm the result obtained by simulation software (DIAlux software) with minor lighting level deviation in some factors such as luminaire actual performance and room elements performance including reflectance factor; despite these differences in illumination level, the light measurement was still at an acceptable level compared to software results.

4.1.2. With Daylight through Clear Glass

The room simulation results using daylight are shown in Figure 5. The simulation assumption was for a clear sky day on June 11 at noon; the sun incident angle is shown in Table 2, and the windows’ direction was considered in the simulation (330° north). The result indicates that the lighting level near the windows can reach more than 3700 Lux. This level is high and can cause high levels of reflection, glare, and visual discomfort to the room user. In the middle of the room, at the working desk, and at a distance of 2.5 m from the window, the light level was 1500 Lux, which is also high and can cause glare and discomfort.

The false-color rendering scheme, which is provided by the software (see Figure 6), shows the different areas in the room and the possible false-color rendering configuration of the lighting. Figure 6 shows the effect of lighting on different areas in the tested office room.

Light level measurements shown in

Table 5. Lighting level in overcast weather (clear glass).

Weather	Location	Reading (Lux)
		9:30 a.m.	12:00 p.m.	3:00 p.m.
Overcast	A	900	1000	900
	B	700	950	680
	C	650	820	630

represent average readings for two overcast days. In this overcast weather, in area (A), the measurements showed that the light intensity reached up to 900 Lux with a margin of ±50 Lux from morning till afternoon, while in area (B), the light intensities varied from 650 Lux in the morning up to 950 Lux at midday and again down to 680 Lux in the afternoon. In area (C), the light intensity was less, and its recorded values were 630 Lux in the morning, 800 Lux at noon, and 640 Lux at 3 p.m.

The values in

Table 6. Lighting level in cloudy weather (clear glass).

Weather	Location	Reading (Lux)
		9:30 a.m.	12:00 p.m.	3:00 p.m.
Cloudy	A	1490	1550	1410
	B	1050	1260	1100
	C	1000	1180	1050

represent the average readings for four cloudy days during the observation period. On these cloudy days, the lighting level at point (A) showed light intensity reached up to 1500 Lux with a margin of ±100 Lux from morning to afternoon, while at point (B), light intensities varied from 1050 Lux in the morning up to 1260 Lux at midday and again down to 1100 Lux in the afternoon; at point (C), readings showed even less lighting, which reached 1000 Lux in the morning, 1180 Lux at noon, and 1050 in the afternoon.

Table 7. Lighting levels in dusty weather (clear glass).

Weather	Location	Reading (Lux)
		9:30 a.m.	12:00 p.m.	3:00 p.m.
Dusty	A	1500	1600	1450
	B	1100	1480	1200
	C	1000	1300	1100

shows the light intensity readings in dusty weather that occurred on seven days during the two months of the experiment. The lighting level in area (A) showed light intensity reached up to 1500 Lux with a margin of ±100 Lux from morning to afternoon, while the readings in area (B) showed that light intensities varied from 1100 Lux in the morning to 1480 Lux at midday and 1200 Lux in the afternoon.

Sunny days with a clear sky occurred on 47 days during the 60 days of the experiment.

Table 8. Lighting levels in clear sky weather (clear glass).

Weather	Location	Reading (Lux)
		9:30 a.m.	12:00 p.m.	3:00 p.m.
Sunny day	A	1750	1920	1780
	B	1500	1600	1510
	C	1240	1340	1300

shows a sample measurement of the light intensity readings taken for a clear day on June 11, where the lighting level in area (A) reached an intensity of up to 1850 Lux with a margin of ±70 Lux from morning to afternoon, while the readings in area (B) showed light intensities that varied from 1500 Lux in the morning to 1600 Lux at midday to 1510 Lux in the afternoon.

An examination of scenario 1 shows that when allowing daylight to enter the office through clear glass, the illuminance levels in the office were high in all weather conditions (overcast, cloudy, dusty, and clear sky) and all office areas near the window, in the middle of the room, and on the opposite side; these levels were higher than the recommended lighting lux level for acceptable light level inside offices [16,36].

Comparing the results of the daylighting simulation to the field measurement showed a large difference between them (Figure 5 and Table 8). The values obtained from the simulation software were twice those of the field measurement. This might be due to reasons such as nonaccurate data fed to the software, the limitation of the software to simulate daylight contribution, and the reliance on specific reflected material type specifications that were changed. For this reason, we believe that DIAlux software may not be a good choice for daylight simulation.

4.1.3. With Daylight through Solar Film Sheet

The light level measurement, shown in

Table 9. Lighting levels in overcast weather (with solar film).

Weather	Location	Reading (Lux)
		9:30 a.m.	12:00 p.m.	3:00 p.m.
Overcast	A	260	290	250
	B	80	80	70
	C	50	55	50

, represents the average readings for two overcast days. In overcast weather in areas (A), (B), and (C), measurements showed low-intensity light coming through the window, where the highest light level was 290 Lux, which was near the window (A), while in the rest of the room, the light level was less than 100 Lux throughout the day.

During the observation period, the same issue was noticed in the four days of cloudy weather (see

Table 10. Lighting levels in cloudy weather (with solar film).

Weather	Location	Reading (Lux)
		9:30 a.m.	12:00 p.m.	3:00 p.m.
Cloudy	A	380	420	370
	B	120	160	100
	C	80	70	70

). On cloudy days, the lighting level in area (A) reached an intensity of 420 Lux at midday, while in the rest of the room (areas B and C), the light level remained low and was not adequate for reading or performing other office duties (160 Lux).

Table 11. Lighting levels in dusty weather (with solar film).

Weather	Location	Reading (Lux)
		9:30 a.m.	12:00 p.m.	3:00 p.m.
Dusty	A	500	580	540
	B	275	290	255
	C	120	150	140

shows the lighting level during the dusty weather (seven days during the two months of the test). The lighting level in area (A) was up to 580 Lux from morning to afternoon, and the readings in area (B) were 290 Lux, which can be considered a marginally acceptable light level for specific office work such as computer-based work. In area (C), a low level of light (150 Lux) was recorded, making this area unsuitable for general office work.

On days with a clear sky (sunny day), the average readings for 47 days during the 60 days of the measurement were recorded. The readings of the light intensity are shown in

Table 12. Lighting levels in sunny weather (with solar film).

Weather	Location	Reading (Lux)
		9:30 a.m.	12:00 p.m.	3:00 p.m.
Sunny day	A	550	800	560
	B	275	296	250
	C	180	190	160

. The lighting level in area (A) was up to 800 Lux during the day, while the readings in areas (B) and (C) varied from 296 to 190 Lux, which is considered a tolerable light level for area (B) but a low lighting level for area (C). As a result, area (C) appears dark, affecting employees’ ability to work there.

4.2. Survey Results

This section presents the results of the survey questionnaire considering the four different weather conditions. Concerning the lighting level in the case of clear glass, all of the participants were not satisfied with the lighting level in the office during the sunny days (see

Table 13. Participants’ replies to lighting level (clear glass).

1. How Do You Rate the Lighting Level in This Office?
Reply	Overcast	Cloudy	Dusty	Sunny
Not at all satisfied	0%	7%	80%	93%
Not satisfied	13%	53%	20%	7%
Somewhat satisfied	27%	40%	0%	0%
Very satisfied	60%	0%	0%	0%
Extremely satisfied	0%	0%	0%	0%

). This was also the case on days with dusty weather when those participants were not satisfied with the light level; these days represented 90% of the measurement period (54 days out of 60 days). The majority of respondents were “not at all satisfied” in dusty and sunny conditions with percentages of 80% and 93%, respectively, which shows that these lighting conditions are not preferable for the office environment. On cloudy days, a significant proportion (53%) of participants were not satisfied, while 40% were somewhat satisfied. On the other hand, acceptable satisfaction was observed in the case of overcast conditions with a “very satisfied” percentage of 60%.

Table 14. Participants’ replies to visual comfort (clear glass).

2. How Well Can You See to Do Your Work in This Office?
Reply	Overcast	Cloudy	Dusty	Sunny
Very bad	0%	7%	67%	93%
Bad	0%	53%	20%	7%
Neutral	7%	40%	13%	0%
Well	80%	0%	0%	0%
Very well	13%	0%	0%	0%

shows the participants’ replies to the second question, concerning their visual comfort and their ability to do office work under the four climate conditions. This question is linked with question number one, as many studies confirm that visual comfort is associated with lighting level [37]. Most of the participants indicated they were not able to do their work on sunny and dusty days. Regarding overcast days, their answers confirm that they were able to see well and do most of their work well and comfortably, while no respondents reported “well” or “very well” in other conditions. It is also worth noting that under these circumstances, visual comfort was only achieved on a few days, as such conditions refer to less than 10% of the test period.

Table 15. Participants’ replies to question 3 (clear glass).

3. Do You Need to Control the Amount of Sunlight in Your Workplace by Using Curtains or Electrical Lighting?
Reply	Overcast	Cloudy	Dusty	Sunny
No	80%	7%	0%	0%
Yes	20%	93%	100%	100%
Use curtain	7%	93%	100%	100%
Electrical light	13%	93%	100%	100%

shows the participants’ replies to question three regarding their tendency to control the amount of light in the office, due to either insufficient light or the participants closing the window blinds as a result of high light illumination. Most participants (80%) confirmed that during the overcast days, they did not need to close window blinds or switch on electrical lighting. On the other hand, during the cloudy days, most of the participants (93%) confirmed the need to close the curtain and use the window blinds at some time during most of those days. Moreover, all participants reported a preference for using electrical lighting instead of daylight on dusty and sunny days.

Table 16. Participants’ replies to the questionnaire (with solar film).

1. How Do You Rate the Lighting Level in This Office?
Reply	Overcast	Cloudy	Dusty	Sunny
Not at all satisfied	100%	0%	0%	0%
Not satisfied	0%	100%	7%	7%
Somewhat satisfied	0%	0%	27%	7%
Very satisfied	0%	0%	60%	80%
Extremely satisfied	100%	0%	0%	0%

,

Table 17. Participants’ replies to visual comfort (with solar film).

2. How Well Can You See to Do Your Work in This Office?
Reply	Overcast	Cloudy	Dusty	Sunny
Very bad	100%	93%	0%	0%
Bad	0%	7%	7%	7%
Neutral	0%	0%	0%	0%
Well	0%	0%	0%	13%
Very well	0%	0%	93%	80%

and

Table 18. Participants’ replies to question 3.

3. Do You Need to Control the Amount of Sunlight in Your Workplace by Using Curtains or Electrical Lighting?
	Overcast weather	Cloudy	Dusty	Sunny
No	0%	0%	87%	93%
Yes	100%	100%	13%	7%
Use curtain	0%	0%	7%	7%
Electrical light	100%	100%	13%	0%

illustrate the results of the questionnaire survey for office employees with scenario two, which uses solar film on glass to reduce daylight. Table 16 shows that employees were not satisfied with the lighting level during overcast and cloudy days. The low-level lighting caused them to be unable to do their work properly under daylight only. They confirmed that they needed additional lighting sources, such as electrical lighting, to achieve visual comfort and be able to perform various office tasks at that light level. During the dusty and sunny weather, more than 93% of the survey participants were either very satisfied or somewhat satisfied with the daylighting level and considered it suitable for doing their work. It was noticed that some of the participants were unhappy with the light setup even with the presence of solar film; the reason was that the light reflected on their computer screens. Thus, sitting against a window (with the window directly behind the employee’s working position) is not a suitable arrangement for an employee whose job requires the use of a visual display unit (VDU).

Table 17 presents the results of the surveyed employees’ responses to question two, where their visual comfort under the four climate conditions was presented. In overcast weather, all the participants were unhappy and did not feel comfortable with the lighting arrangement, as they saw the office as being dark and they needed an additional source of light (i.e., electrical lighting) so they could do their work comfortably. This result is understandable, as the light level was very low (Table 9 and Table 10). On the sunny and dusty days, which represented 90% of the test period (54 days out of 60 days of experimentation), most of the participants (93%) confirmed their satisfaction with the lighting arrangement and stated that they were able to do most of their work well and comfortably.

When the surveyed participants were asked about their need to control the lighting in the tinted-window-glass setup, their reply was as follows: On overcast and cloudy days, they confirmed that they needed to switch on the electrical lighting on most of these days, as the lighting level was insufficient for them to perform their usual daily tasks; they did not need to close the curtains (they stated that they enjoyed the view outside the building as an added benefit from tinted windows). During the dusty days, more than 87% of the respondents confirmed they did not need any additional electrical light or to close the curtain. However, 13% of participants who sat against the window confirmed the need to partially or fully close the curtain and use the electrical lighting during most of these days. In sunny weather, more than 93% of employees stated that there was no need to use electrical lighting or close the curtains, which means this is an acceptable and comfortable arrangement for them. Only a small portion of participants (7%) were not able to benefit from this daylight due to their desk location (against the windows), as their computer screens were affected by the light reflection from incoming sunlight (see Table 18).

Field measurement results showed that the only energy savings that can be achieved are through integrating and controlling the lighting in area (A), i.e., near the window, as long as it falls within an acceptable lux level. The daylight level in this area, as shown in Table 8, Table 9, Table 10 and Table 11, had lighting levels ranging from 300 to 800 Lux, which represents a high potential to be used as a reliable source of light in this area, especially when sunlight use was linked with electrical light through a lux level detector. Hence, there is a potential to reduce the number of operating lights from nine (Figure 4) to six during most weather conditions. This reduced lighting energy consumption by up to 33% during working hours. The lighting power density would be maximally reduced from 12 W/m² to 7 W/m² as a result of switching off lights in area (A). This power density represents 40% less than the stipulated lighting power density in the CIBSE standard, which was developed in order to reduce electric lighting consumption in buildings. Equation (2) represents the energy cost per month per floor when using LED only and when using LED integrated with daylight. Table 19 shows that the cost savings can reach thirty-three percent. On the other hand, it is essential to mention that the calculation of energy reduction in this study is only based on the lighting, while in fact, the total amount of energy reduction may be affected by the change in cooling and heating loads.

E = W × K × M

(2)

where E is the lighting energy cost in USD.

W is the lighting consumption per floor per month.

K is the consumption charges in Kwh/USD.

M is the total number of office floors (30).

Table 19. Lighting power consumption with and without daylight.

Type of Lamp	No. of Lamps Operating per Floor	Power Density (W/m2)	Power Consumed per Floor (kWh)	Energy Cost per Month/Building (USD)	Energy Cost Reduction %
LED	225	12	1512	5443	-
LED + DAYLIGHT	150	7	1008	3628	33%

Table 19. Lighting power consumption with and without daylight.

Type of Lamp	No. of Lamps Operating per Floor	Power Density (W/m2)	Power Consumed per Floor (kWh)	Energy Cost per Month/Building (USD)	Energy Cost Reduction %
LED	225	12	1512	5443	-
LED + DAYLIGHT	150	7	1008	3628	33%

5. Recommendations

While this study demonstrated the potential benefits of incorporating solar film sheets to enhance visual comfort and reduce electrical lighting power consumption, several avenues for future research and analysis require attention. In this context, this section aims to expand our understanding of the impact of solar film applications on energy consumption, glare assessment, durability, and office arrangement. The corresponding recommendations can be summarized as follows:

• Comprehensive energy evaluations are crucial, considering both heating and cooling energy influences. Annual energy performance using software tools should be conducted to assess the net energy savings achieved by solar film installations. This analysis can provide valuable insights into the broader energy-efficiency implications and help quantify the environmental and economic benefits associated with solar films.
• Future studies should include a thorough assessment of glare, since it is a critical aspect of visual comfort. Researchers should consider utilizing glare assessment metrics such as the daylight glare index (DGI) and the daylight glare probability (DGP) to quantitatively evaluate the potential glare reduction achieved by solar film. This evaluation will help ensure that improvements related to visual comfort do not cause glare-related issues.
• Investigating from the point of view of the long-term durability and performance of solar film under various weather conditions is also essential. Future research should focus on monitoring the effectiveness of solar film over extended periods, considering the effect of different factors such as weather, aging, and maintenance. This will provide valuable insights into sustainability and cost-effectiveness over time.
• The arrangement of an office has a significant impact on the efficient use of daylight and the wellbeing of employees. Therefore, prioritizing the layout and design of the office space is essential, ensuring that workstations and common areas are strategically positioned near windows to maximize access to natural light. Additionally, light-reflective surfaces could be useful, as they can help distribute daylight more effectively. It is also recommended to employ adjustable window coverings, which allow for precise control of light levels and heat gain during peak daylight hours.
• Optimizing an ILS to achieve greater energy savings is a crucial step toward creating more energy-efficient and cost-effective lighting solutions. To further enhance the ILS, advanced sensors and controls can be integrated. Occupancy sensors can be strategically placed throughout the office space to ensure that lights are only active when the room is occupied, reducing unnecessary energy consumption. Daylight harvesting sensors are also useful to adjust artificial lighting levels in response to the availability of natural light, ensuring that electrical lighting is only used when needed. Additionally, exploring the potential for smart lighting systems that can be remotely controlled and programmed can offer better flexibility and energy efficiency.

6. Conclusions

As discussed in this study, there are many technologies used to control sunlight and minimize high illumination levels, and in order to reduce lighting energy consumption, a solar film sheet placed on window glass is a simple and common application that is widely utilized in many buildings to reduce the reliance on electrical lighting to minimize electric lighting consumption and reduce high daylight illumination levels without causing undue visual discomfort to the occupants.

To examine the practicality of this application and its effect on room lighting for illuminance reduction, a modern office setting was selected to conduct this field study. Two measures were used to assess this application: first, through field measurement and comparing it to the specified standard illumination level; and second, through a short-form questionnaire survey conducted to obtain occupants’ opinions on the office lighting. This study revealed the following:

• Using a clear glass facade is not a preferred solution to bring daylight into office buildings, as field measurements confirmed that the daylighting level was very high and above the recommended levels when using clear glass windows. Accordingly, it caused visual discomfort.
• During the daytime, the windows’ blinds were closed, and all electrical lighting was turned on; hence, no energy savings were achieved when using clear glass. This is confirmed by the surveyed employees and previous studies [38,39].
• Using solar sheet film on glass resulted in a great reduction in daylight entering the offices (up to 70%). However, this reduction was not uniformly distributed around the office, as a higher light level was measured near the window area, average lighting levels were recorded in the middle, and a low light level was measured at the opposite end of the office.
• Office arrangement has a great impact on the use of daylighting and electrical lighting. This study shows that even when using solar sheets to reduce daylight levels in offices, employees who sit against the windows will suffer from light reflection on their computer screens.
• There is a potential to achieve savings in lighting energy consumption when utilizing solar sheet film on windows through the development of an integrated lighting system (ILS), i.e., connecting a lux level detector with certain lights. Under the investigated conditions, the proposed technique helped reduce energy consumption by up to 33%. This was also accompanied by the users’ satisfaction, as confirmed by other studies [40].
• This study confirmed previous studies that linked user satisfaction with lighting level [36,37,41]. The questionnaire survey along with field measurement showed that users appreciated the light levels that fell within the recommended standard.