The Influence of Electrochromic Film on Indoor Environmental Quality

Abstract

This study was conducted at SPINLab. The full-scale experiments were performed using two experimental spaces of identical specifications to investigate the effects of electrochromic film (OG + EC_ON or OG + EC_OFF) on indoor environment and air conditioning electricity consumption in buildings with different orientations (East and West). The electricity-saving effects are more pronounced on the building’s west-facing side than on its east-facing side. For the east-facing side, the average electricity savings for OG + EC_ON and OG + EC_OFF were 4.5%, and 5.1%, respectively. For the west-facing side, the average electricity savings increased to 9.2% and 9.4% for OG + EC_ON and OG + EC_OFF. The research results on thermal comfort indicate (PMV) that applying electrochromic film (OG + EC_ON or OG + EC_OFF) significantly improved indoor thermal comfort compared to using clear glass (OG) alone. The visual comfort analysis results indicate that the opaque (OG + EC_OFF) and transparent (OG + EC_ON) states of electrochromic film could reduce daylight glare probability (DGP) values. However, due to the light-scattering properties of the liquid crystal droplets, the OG + EC_OFF and OG + EC_ON states of the electrochromic film increased DGP values in 26.5% and 41.5% of the cases, respectively, when sunlight directly entered the interior.

1. Introduction

The majority of building energy consumption is influenced by the heat entering through walls, windows, and ceilings, with windows being the most susceptible to heat ingress. Therefore, the thermal insulation efficiency of windows has a significant impact. Over the past few years, numerous thermal insulation facilities have been developed, including external shading, heat-insulating films, Low-E glass, electrochromic film, etc., with many scholars also comparing the thermal insulation effects of different types of glass. The optical properties of glass also play a crucial role in determining its insulation effectiveness. Thermal insulation films enhance the glass’s solar reflectance and absorption, thereby reducing its solar transmittance and subsequently lowering the heat entering indoors. Solar radiation consists of ultraviolet light, visible light, and infrared light. However, reducing visible light transmittance simultaneously reduces indoor brightness, and increasing solar absorption raises the film temperature, leading to secondary radiation and reducing the lifespan of the film. Many scholars have explored the effects of thermal insulation film application through practical experiments or by building energy simulations.

Smart photochromic glass is a material that can alter its optical properties based on external conditions, and includes piezochromic, thermochromic, and electrochromic glass. Among these, electrochromic glass can change its color and transparency level by applying an electric current, thereby regulating environmental temperature. There are three types of electrochromic technology: electrochromic, suspended particle device (SPD), and polymer-dispersed liquid crystal (PDLC), which we will introduce in detail in the following sections.

Electrochromic glass consists of two layers of tungsten oxide or nickel oxide as the electrochromic layer, with an electrolyte sandwiched in between [1]. The principle involves applying a direct current to the transparent conductive layer, which attracts ions from the electrolyte into the electrochromic layer, initiating a reduction reaction with electrons. Reversing the voltage causes the glass to transition from an opaque state back to a transparent state. Here are some simulated or experimental comparisons between electrochromic glass and other shading methods found in the literature.

Aste et al. [2] compared the effects of electrochromic glass, regular glass, and external blinds systems in an office building in Milan using EnergyPlus simulations. The results showed that electrochromic glass reduced energy consumption by 39.5% compared to regular glass and by 26.2% compared to external blinds.

Sbar et al. [3] evaluated the energy-saving effect of using electrochromic glass in buildings with a window to wall ratio of 60% in different climate zones of the United States. Simulation results using eQuest indicated that compared to static glass, electrochromic glass could reduce energy consumption by over 45% and decrease the demand on air conditioning systems, resulting in cost savings.

Piccolo et al. [4] conducted experiments and simulations on a building located in Messina, Italy. The results indicated that the temperature difference between the inner and outer surfaces of the glass was greater in the opaque state compared to the transparent state. This is attributed to the electrochromic (EC) effect, which increases absorption of thermal radiation, resulting in secondary solar heat generation. The author also analyzed the different heat components entering the interior through opaque glass, observing an 83% reduction in direct radiation, a 46% increase in thermal radiation, and a 14% increase in convective heat transfer. Simulation results showed that in climates with predominant cooling demands, EC glass effectively reduces building energy consumption, whereas in climates with predominant heating demands, the energy-saving effects are less evident.

Ardakan et al. [5] compared the glare control performance of two case sites using fritted glass (an opaque ceramic material deposited on glass during heat treatment) and electrochromic (EC) windows, one being a skylight and the other a vertical glass panel. They found that in both cases, transparent Low-E glass and fritted glass were insufficient for adequate sunlight glare control. In contrast, EC glass provided suitable glare control and daylight utilization, while maintaining clear external visibility at all times.

Wang et al. [6] studied the Meditation Hall at the New Materials Research Institute in Jinwan District, Zhuhai City, China, and simulated three glass configurations: electrochromic windows, Low-E windows, and regular glass windows. The results showed that the shading performance of the electrochromic windows surpassed that of the regular and Low-E windows, particularly during summer days, reducing approximately 40% of the total solar radiation. Compared to regular glass windows, electrochromic windows could save about 90% of energy consumption annually and effectively reduce the annual cooling energy consumption of buildings in subtropical regions.

Another type of electrochromic technology is the Suspended Particle Device (SPD). Its principle involves placing SPD material between two glass panes. When no voltage is applied, the SPD particles are randomly suspended, blocking light. When voltage is applied, the particles align vertically within the glass, allowing light to pass through.

Ghosh and Norton [7] explored the performance and advantages of SPD when powered by a photovoltaic (PV) device supplying alternating current (AC) and under electrically switchable direct current (DC) power. Ghosh et al. [8] also tested the optical properties of SPD switchable glazing using an outdoor test cell in Dublin, Ireland. The experimental results showed that the overall heat transfer coefficient (U-value) for both opaque and transparent states was very similar, approximately 5.9 W/m² K. The light transmittance was about 5% in the opaque state and 55% in the transparent state.

Another type of electrochromic material is polymer-dispersed liquid crystal (PDLC), which consists of a polymer base and tiny liquid crystal droplets [9]. By applying a small voltage, the orientation of the liquid crystals can be changed, resulting in two states: “transparent” and “translucent/opaque”. When the electrochromic film is in the translucent/opaque state, PDLC scatters light due to the refractive index mismatch between the droplets and the polymer base [10].

Through experiments and simulations, Mesloub et al. [11] analyzed the thermal and optical properties and building energy consumption of PDLC windows applied in hot climates. The results showed that both the transparent and opaque states of PDLC windows have significant advantages in terms of thermal and optical performance. Additionally, simulation results demonstrated that PDLC windows contribute to energy savings and carbon reduction.

Using a spectrometer (Perkin Elmer^® Lambda 1050 UV/VIS/NIR, Waltham, MA, USA) and experimental analysis, Hemaida et al. [12] investigated the optical and thermal properties of PDLC glass windows. Results revealed that the SHGC of the PDLC glass system was 0.68 in its transparent state and 0.63 in its opaque state, with corresponding U-values of 2.79 W/m² K and 2.44 W/m² K, respectively. This suggests its potential to effectively reduce heating loads in cold climates. However, caution is warranted due to the higher temperature of the glass inner surface, which may lead to secondary heat radiation.

Ghosh and Mallick [13] also investigated the optical properties of PDLC glass using a spectrophotometer. The experimental results showed that the total solar transmittance in transparent and opaque states were 41% and 23%, respectively. Additionally, due to the scattering phenomenon of liquid crystal particles, the opaque state exhibited high diffuse transmission, with a haze value of 82.6%, providing a high level of protection for human skin. The author also suggested that when applying PDLC in buildings, consideration should be given to the durability of the glass and its performance under switching cycles.

Oh et al. [14] proposed two retrofit methods using PDLC film to regulate solar radiation in old office buildings. Additionally, the energy-saving effects and daylight performance after the retrofit were analyzed. The first method involved applying PDLC film on the existing double-glazed curtain walls. The second method entailed sandwiching PDLC film between two glass substrates and installing a separate frame on the indoor side for new windows. Simulation results demonstrated the effectiveness of both retrofit methods using PDLC film in reducing heating and cooling energy consumption and improving daylight performance. Specifically, the retrofit with PDLC windows showed a greater performance improvement compared to simply applying PDLC film.

Field and Ghosh [15] investigated various glass configurations, including vacuum windows, aerogel windows, vacuum–aerogel windows, and switchable smart windows, including PDLC, PDLC–aerogel, and PDLC–vacuum. Taking into account heating, cooling, and lighting considerations, the overall findings suggest that PDLC–vacuum is the most effective strategy for attaining Nearly Zero Energy Buildings (NZEB), boasting a U-value ranging from 0.810 to 0.831 W/m² K and offering the greatest reduction in building energy consumption. However, its performance is influenced by factors such as window-to-wall ratio and building orientation. Furthermore, PDLC stands out for its affordability, having the shortest payback period among the options, with less than 25 years required for return on investment.

Chidubem Iluyemi et al. [16] examined the energy-saving benefits of electrochromic glass and PDLC glass compared to traditional clear glass in residential construction. The results indicate that smart windows offer improved optical characteristics over clear glass. Specifically, electrochromic glass with daylight control demonstrated the highest overall energy savings, reducing energy consumption by 23.56% when compared to single-pane glass windows. Moreover, in PDLC window setups, utilizing silver-coated glass for the inner pane of double-glazed windows further decreased energy usage.

Alghamdi and Almawgani [17] implemented a smart energy-saving system using PDLC film combined with programming and sensors, which can adjust the glass state according to weather changes. Compared to traditional shading systems, it saved approximately 39% of energy consumption, with lower installation and maintenance costs than curtains or blinds. The authors also suggest that PDLC film can reduce reflection by absorbing sunlight, thereby converting heat pollution factors into thermal energy that can be utilized for solar power generation.

According to the relevant literature, electrochromic glass consists of two layers of tungsten oxide or nickel oxide as the electrochromic layer, with an electrolyte sandwiched in between [1]. The principle of Suspended Particle Device (SPD) involves placing SPD material between two glass panes. Both of these technologies incorporate a color-changing layer or material between layers of glass. Another type of electrochromic material is polymer-dispersed liquid crystal (PDLC), which consists of a polymer base and tiny liquid crystal droplets [9]. PDLC can be made into a thin film that can be attached to other glass surfaces, similar to heat insulation films. Therefore, PDLC films have greater potential for application. However, when applied to glass windows, the effects of PDLC films on indoor temperature, air conditioning electricity consumption, and indoor environment have not yet been fully studied.

The objective of this study is to use SPINLab [18,19,20] as an experimental platform to investigate the effects of using clear glass or clear glass with electrochromic film on indoor environmental quality, electricity consumption of air conditioning, indoor thermal comfort, and visual comfort in various building façades (east-facing side or west-facing side). These research efforts aim to reduce building energy consumption, improve indoor environments, and enhance the comfort and livability of residential and office spaces.

The electrochromic film used in this study is based on PDLC technology. However, for convenience, we will refer to it as “electrochromic film” in the following discussion. The transparent state will be denoted as OG + EC_ON, while the opaque state will be referred to as OG + EC_OFF.

The following sections, respectively, detail the experimental equipment and the research parameters, and examine the effects of OG, OG + EC_ON, and OG + EC_OFF on indoor environment, electricity consumption of air conditioning, indoor thermal comfort, and indoor visual comfort.

2. Experimental Equipment and Method

The layout of the experimental test rooms is depicted in Figure 1. Both test rooms (room A and room B) were equipped with a variable-frequency air conditioner (RAS-110NJP, Hitachi, Japan), with the airflow direction fixed vertically and oscillating horizontally. The opening of a rolling shutter on the glass curtain wall was set to adjust the window-to-wall ratio to 40% on this side of the room. Only 40% (in height) of the glass was allowed for incident sunlight. A total of four clear glass (OG) panels were used in a single indoor space (room A or room B). Two of the panels measured 1136 mm × 12 mm × 1826 mm (L × W × H), while the other two measured 1176 mm × 12 mm × 1826 mm (L × W × H). The U-value of the clear glass (OG) was 5.02.

Each test room (room A and room B) was furnished with four desks and chairs, as well as two dummies to simulate the heat capacity of office occupants. Additionally, four temperature and humidity sensors were suspended at a height of 170 cm from the ground, similar to the height of a person’s head. Furthermore, flux meters were installed on the inner surface of the glass to measure the heat flux and temperature passing through the glass. Each test room (room A and room B) also contained four illuminance meters placed on the desktops. Two PMV analysis systems were positioned, one closer to the windows and the other closer to the air conditioner, to measure parameters such as dry bulb temperature, humidity, globe temperature, and wind speed at these two test points. These parameters were subsequently inputted into a computer to calculate the PMV values. The information of measurement sensors is based on reference [20] and

Table 1. Measurement sensors [20].

Sensors	Measuring Range and Accuracy
Temperature and Humidity	0–50 °C ± 0.2 °C and 0–100% ± 2%
Heat flux	Measurement range: −10 to +10 × 103 W/m2 Sensitivity: 5.5 × 10−6 V/(W/m2) Operating temperature: −40 to +150 °C
Illuminometer	0 to 167,731 lux ±10% typical for direct sunlight
Daylight Glare Probability (DGP)	Canon EOS 5D Mark IV (Canon, Tokyo, Japan) Canon EF8–15 mm f/4L Fisheye USM (Canon, Tokyo, Japan)
Predicted mean vote (PMV)	Delta OHM HD32.3 (Delta OHM, Caselle di Selvazzano, Italy)

.

Moreover, both test rooms (room A and room B) were equipped with a glare analysis system, consisting of a camera and an illuminance meter mounted above it. The camera took a photo every 30 min from indoors to outdoors, and the daylight glare probability (DGP) value was calculated later through computer software based on the current vertical illuminance value.

Table 2. The optical properties of the glass.

Test Items	OG (12 mm)	OG + ECON	OG + ECOFF
Visible light transmittance (380~780 nm)	85.82%	56.5%	10%
UV rejection (300~380 nm)	99%	100%	100%
Infrared rejection (780 ~2500 nm)	45.08%	99%	99%

and Figure 2 illustrate the properties of clear glass (OG) (room B) before and after the application of electrochromic film. The transparent state will be denoted as EC_ON, while the opaque state will be referred to as EC_OFF (room A). It can be observed that OG + EC_ON or OG + EC_OFF reduce the visible light transmittance of clear glass from 85.82% to 56.5% and 10%, respectively, while increasing the infrared rejection rate from 45.08% to 99%. This demonstrates that electrochromic film can effectively reduce the amount of heat entering the indoor environment. Figure 3 shows the experimental flowchart.

3. Results and Discussion

3.1. Analysis of Indoor Environment

Figure 4 shows that due to direct sunlight, the indoor temperatures and surface temperatures of the glass (OG, OG + EC_ON) in both rooms on the east-facing side in the morning and the west-facing side in the afternoon are higher than at other times. The period during which direct sunlight enters an east-facing side is from 7:00 to 10:00. For a west-facing side, direct sunlight enters the interior from 14:00 to 17:00 p.m. During these times, more heat is transmitted into the indoor space due to direct sunlight. On an east-facing side, the glass temperatures in rooms A and B increase significantly between 7:00 and 10:00. On a west-facing side, the glass temperatures in rooms A and B show a noticeable rise between 14:00 and 16:00. After 16:00, the impact of direct sunlight on the interior decreases, and the glass temperatures in rooms A and B gradually decline.

The outdoor temperature is denoted as T_out. The temperature of the air conditioner sensor is denoted as AC. The indoor temperature of room A and room B was controlled by the air conditioner to remain at 26 °C.

In room A (OG + EC_ON), the indoor temperatures (A12, A34, AC) reach maximums of 27.2 °C and 27.5 °C, respectively, while in room B (OG) (B12, B34, AC), they reach maximums of 28.2 °C and 29.55 °C. Additionally, during periods of direct sunlight, the temperatures (A12, B12) around the windows are notably higher than those around the air conditioner compared to other times.

Figure 5 illustrates that in the morning on the east-facing side and in the afternoon on the west-facing side, the glass temperatures (AG) in room A reach maximums of 39.3 °C and 43.55 °C, respectively, while in room B (BG), they reach maximums of 36.75 °C and 39.7 °C, all significantly higher than the outdoor temperatures. Moreover, the glass heat flux in room A (AH) reaches maximums of 121 W/m² and 130 W/m², respectively, while in room B (BH), it reaches maximums of 84 W/m² and 84.5 W/m². It can be observed that, compared to the OG alone, the application of OG + EC_ON results in a further increase in glass temperature and glass heat flux, especially during periods of direct sunlight.

Figure 6 shows that due to direct sunlight, the indoor temperatures in both rooms (room A and room B), with the east-facing side in the morning and the west-facing side in the afternoon, are higher compared to other times of the day. In room A (OG + EC_ON), the indoor temperatures (A12, A34, AC) reach maximum values of 28 °C and 27.7 °C, while in room B (OG) (B12, B34, AC), they reach maximum values of 29.05 °C and 30 °C respectively. During periods of direct sunlight, the temperatures (A12, B12) around the windows are notably higher than those around the air conditioner.

Figure 7 shows that during the morning on the east-facing side and the afternoon on the west-facing side, the glass temperatures (AG) in room A reach maximums of 42.95 °C and 44.1 °C, respectively. In room B (BG), they reach maximums of 38.5 °C and 39.85 °C, all significantly higher than the outdoor temperatures. The glass heat flux in room A (AH) peaks at 132.5 W/m² and 142.5 W/m², respectively, while in room B (BH), it peaks at 86.5 W/m² and 84.5 W/m². It is evident that, compared to the OG, OG + EC_OFF results in a further increase in both glass temperature and glass heat flux, especially during periods of direct sunlight irradiation.

Figure 8 shows that when the building’s façade faces East, before approximately 11:00 in the morning, and faces West in the afternoon after 14:00, the indoor temperature difference between room A and room B (green and blue lines) mostly ranges between 0 °C and 2 °C. There is a noticeable gap between the temperature differences near the windows (green line, B12-A12) and near the air conditioning (blue line, B34-A34) in both rooms. This indicates that compared to clear glass (OG), the electrochromic film (OG + EC_ON or OG + EC_OFF) can effectively lower the indoor temperature during periods of direct sunlight, with the cooling effect near the windows often being better than near the air conditioning. In contrast, during other times when sunlight is not directly entering the rooms, both the green and blue lines oscillate between −1 °C and 1 °C, with the temperature difference near the windows (green line, B12-A12) being only slightly higher than that near the air conditioning (blue line, B34-A34). This shows that the indoor environment is controlled by the air conditioning, and electrochromic film (OG + EC_ON and OG + EC_OFF) does not have a significant cooling effect indoors during these periods.

The electrochromic films (OG + EC_ON or OG + EC_OFF) can lower indoor temperatures due to their lower solar transmittance compared to clear glass (OG), which reduces the heat radiation entering the room. However, because their solar absorptance is higher than that of clear glass (OG), the inner surface temperature of the glass increases. Figure 6 shows that when the building’s façade faces East before approximately 11:00 in the morning and faces West in the afternoon after 14:00, the temperature difference of the glass in rooms A and B (red line) mostly ranges between −2 °C and −4 °C. This indicates that compared to clear glass (OG), electrochromic film (OG + EC_ON or OG + EC_OFF) significantly increases the glass temperature during periods of direct sunlight. The glass temperature of OG + EC_OFF is higher than OG + EC_ON.

During periods when sunlight is not directly entering the room, the glass temperature difference in rooms A and B (red line) mostly ranges between 0 °C and −2 °C. This shows that, regardless of direct sunlight, the electrochromic film increases the glass temperature compared to clear glass, with a more pronounced effect during periods of direct sunlight.

3.2. The Electricity Consumption of Air Conditioning

Figure 9 depicts the air conditioning electricity consumption of electrochromic film (OG + EC_ON or OG + EC_OFF) (represented by the red line for room A) and their corresponding clear glass (OG) (represented by the black line for room B) on the east- or west-facing side. The electrochromic films can reduce the amount of solar heat entering indoor spaces, thereby lowering the operation and electricity consumption of air conditioning systems. Compared to the OG + EC_ON configuration, the OG + EC_OFF setup is more effective at minimizing solar heat gain, resulting in greater electricity savings.

The electrochromic film (OG + EC_ON or OG + EC_OFF) showed significant electricity-saving effects. On the east-facing side, electricity-saving benefits are noticeable in the morning, with the slopes of the red and black lines nearly identical in the afternoon, indicating similar electricity consumption levels for rooms A and B. On the east-facing side, the air conditioning electricity consumption of room A (OG + EC_ON) and room B (OG) is almost the same from 08:00 to 10:00. After 10:00, the air conditioning electricity consumption of room A (OG + EC_ON) starts to be lower than that of room B (OG). At 16:00, the air conditioning electricity consumption of room A (OG + EC_ON) is about 6% lower than that of room B (OG), and this difference remains around 6% from 16:00 to 18:00.

For room A (OG + EC_OFF) and room B (OG), the air conditioning electricity consumption is almost the same from 08:00 to 08:30. After 08:30, the air conditioning electricity consumption of room A (OG + EC_OFF) starts to be lower than that of room B (OG). At 11:00, the air conditioning electricity consumption of room A (OG + EC_OFF) is about 9% lower than that of room B (OG), and this difference remains around 9% from 11:00 to 18:00.

On the west-facing side, the air conditioning electricity consumption of room A (OG + EC_ON) and room B (OG) is almost the same from 08:00 to 15:00. After 15:00, the air conditioning electricity consumption of room A (OG + EC_ON) starts to be lower than that of room B (OG). At 18:00, the air conditioning electricity consumption of room A (OG + EC_ON) is about 5% lower than that of room B (OG).

For room A (OG + EC_OFF) and room B (OG), the air conditioning electricity consumption is almost the same from 08:00 to 15:00. After 15:00, the air conditioning electricity consumption of room A (OG + EC_OFF) starts to be lower than that of room B (OG). At 16:00, the air conditioning electricity consumption of room A (OG + EC_OFF) is about 7% lower than that of room B (OG). At 18:00, the air conditioning electricity consumption of room A (OG + EC_OFF) is about 10% lower than that of room B (OG).

Conversely, on the west-facing side, the red and black lines overlap in the morning, indicating similar electricity consumption for both rooms until around 15:00, when an electricity consumption difference emerges. This result shows that the electrochromic film (OG + EC_ON or OG + EC_OFF) has significant power-saving benefits only during periods of direct sunlight.

To make a more comprehensive comparison of the electricity-saving benefits of electrochromic film versus clear glass, we conducted a 2- to 3-day average electricity savings analysis for each orientation to minimize potential errors, as shown in Figure 8. On the east-facing side, the average electricity savings for OG + EC_ON and OG + EC_OFF are 4.5% and 5.1%, respectively, while on the west-facing side, the average electricity savings for EC_ON and EC_OFF are 9.2% and 9.4%, respectively.

Figure 10 indicates that the electricity-saving ratio on the west-facing side (9.2–9.4%) is significantly higher than that on the east-facing side (4.5–5.1%). Moreover, in terms of electricity-saving effectiveness, EC_OFF outperforms EC_ON.

3.3. Analysis of Indoor Thermal Comfort and Indoor Visual Comfort

Figure 11 illustrates the predicted mean vote (PMV) values for electrochromic films (OG + EC_ON or OG + EC_OFF) along with their corresponding clear glass (OG) under air conditioning operation. The left column represents the east-facing side, while the right column represents the west-facing side. From top to bottom, they show EC_ON and EC_OFF, respectively. A1 and A2 denote the PMV values around the window and air conditioner in room A (OG + EC_ON or OG + EC_OFF), while B1 and B2 denote the PMV values around the window and air conditioner in room B (OG).

The parameters used for calculating the predicted mean vote (PMV) index include wet bulb temperature, black bulb temperature, air temperature, radiation temperature, wind speed, and indoor humidity. The results of this study indicate that the application of electrochromic window film leads to reductions in air temperature (as shown in Figure 7), black bulb temperature, and radiation temperature. Consequently, the PMV values associated with electrochromic films are lower than those observed with clear glass.

A comparison between the OG + EC_OFF and OG + EC_ON configurations reveals that OG + EC_OFF, with a lower visible light transmittance of 10%, compared to 56.5% in the “OG + EC_ON” state, is more effective in mitigating solar penetration into the indoor space. During periods of direct solar exposure, the PMV values under the OG + EC_OFF condition are slightly lower than those under the OG + EC_ON condition.

The PMV values near the window (A1 or B1) in rooms A and B are consistently higher than the PMV values near the air conditioning (A2 or B2). This is because positions closer to the window are more susceptible to outdoor temperature and direct sunlight, while positions closer to the air conditioning are more influenced by the air conditioning system, making them relatively more comfortable. Additionally, it is evident that the PMV values in room B (B1 or B2) are consistently higher than the PMV values in the corresponding positions in room A (A1 or A2). This indicates that, compared to clear glass, electrochromic film contributes to improving overall thermal comfort indoors throughout the day.

When the building’s façade faces east in the morning (before approximately 11:00) and west in the afternoon (after 14:00), the PMV values near the window (B1) in room B mostly range between 1 and 4, indicating a hot environment in room B.

Figure 12 and Figure 13 present the daylight glare probability (DGP) values and illuminance data for EC_ON and EC_OFF across the east and west building façades. An important parameter influencing visual comfort is indoor illuminance, particularly during periods of direct sunlight, which significantly increases both horizontal and vertical illuminance levels. This study found that variations in vertical illuminance have a greater impact on indoor glare indices.

Under the OG + EC_ON configuration, when vertical indoor illuminance exceeds 2300 lux, the daylight glare probability (DGP) surpasses 0.35, indicating the onset of glare discomfort. Similarly, in the OG + EC_OFF configuration, glare effects begin to occur when vertical illuminance exceeds 2000 lux, as the DGP also rises above 0.35.

The first and second columns, respectively, show the DGP values and vertical illuminance, recorded at 30 min intervals. In the graphs, the red line represents the electrochromic film, while the blue line represents the clear glass. The third column shows horizontal illuminance, recorded every minute. The red and black lines represent the average illuminance values around the window and the air conditioner for the electrochromic film, respectively, while the blue and green lines represent the average illuminance values around the window and the air conditioner for the clear glass. Despite the lower sensitivity of DGP and vertical illuminance data compared to horizontal illuminance, they still effectively illustrate the differences between the rooms.

Figure 12 illustrates that in the morning on the east-facing side and in the afternoon on the west-facing side, horizontal illuminance around the window increases due to direct sunlight entering the room. The impact of direct sunlight on the west-facing side is particularly pronounced, with both rooms A and B experiencing DGP values exceeding 0.4 starting from 15:30. Comparing the data for rooms A and B in both orientations (east-facing side and west-facing side), it is evident that EC_ON, compared to OG, reduces both vertical and horizontal illuminance, thereby also lowering the DGP values.

Figure 13 also demonstrates a similar phenomenon. In the morning on the east-facing side and in the afternoon on the west-facing side, both rooms A and B experience increases in DGP and illuminance due to direct sunlight. On the east-facing side, OG’s DGP values exceed 0.4 before 9:00, while for EC_OFF, this occurs before 10:00. On the west facing side in the afternoon, OG’s DGP exceeds 0.4 after 16:30, whereas for EC_OFF, it surpasses 0.4 as early as 14:00. This indicates that EC_OFF is affected by direct sunlight earlier and for a longer duration compared to OG. The reason is that the thermal insulation mechanism of the electrochromic film scatters incoming light. Instead of allowing sunlight to concentrate on the window-side floor or desk, EC_OFF diffuses it throughout the room, resulting in elevated DGP values persisting for a longer period.

Under the OG + ECON configuration, when vertical indoor illuminance exceeds 2300 lux, the daylight glare probability (DGP) surpasses 0.35, indicating the onset of glare discomfort. Similarly, in the OG + ECOFF configuration, glare effects begin to occur when vertical illuminance exceeds 2000 lux, as the DGP also rises above 0.35.

Next, we analyzed the DGP reduction effect of electrochromic film compared to clear glass. The DGP reduction at each time point was calculated by subtracting the DGP value of room A (with electrochromic film) from that of room B (with clear glass), and then standardizing it by dividing by the DGP value of room B. The results are presented in Figure 14, which shows the distribution of DGP reduction percentages for electrochromic film (OG + EC_ON or OG + EC_OFF). It can be observed that EC_OFF shows a reduction in DGP values in 58.6% of the data (10.7 + 16.4 + 19.5 + 10.1 + 1.9 = 58.6%), with 31.5% (19.5 + 10.1 + 1.9 = 31.5%) exhibiting a reduction greater than 20%. However, 41.4% of the data (1.8 + 3.8 + 10.7 + 9.7 + 15.7 = 41.4%) show an increase in DGP values.

On the other hand, EC_ON shows a reduction in DGP values in 73.4% of the data (32.7 + 26.5 + 11.7 + 1.9 + 0.6 = 73.4%), with 14.2% (11.7 + 1.9 + 0.6 = 14.2%) showing a reduction greater than 20%. However, 26.6% of the data (1.9 + 0.6 + 3.7 + 6.2 + 14.2 = 26.6%) show an increase in DGP values.

4. Conclusions

This study utilized the Subtropical Performance-Testbed for Innovative eNergy Research in Buildings Laboratory (SPINLab) to conduct full-scale experiments. It investigated the effects of electrochromic film on the indoor environment, air conditioning electricity consumption, and indoor comfort across different building façades. The main findings of this research are as follows:

• Direct sunlight has a significant impact on the indoor environment. During the summer, when a building’s façade faces east, sunlight directly enters the room before approximately 11:00; when the façade faces west, sunlight enters the room after approximately 14:00. Compared to other times of day, this significantly increases not only the indoor temperature but also the glass temperature and heat flux. Applying electrochromic film (OG + EC_ON and OG + EC_OFF) to the interior surface of clear glass effectively reduces indoor temperature. However, due to their insulation mechanisms, these films also increase the glass temperature and heat flux.
• Applying electrochromic film to the interior surface of clear glass on the east-facing side and the west-facing side effectively reduces air conditioning electricity consumption compared to using clear glass alone. For the east-facing side, the average electricity savings for OG + EC_ON and OG + EC_OFF were 4.5% and 5.1%, respectively. For the west-facing side, the average electricity savings increased to 9.2% and 9.4% for OG + EC_ON and OG + EC_OFF, respectively.
• Applying electrochromic film to the interior surface of clear glass significantly improves indoor thermal comfort compared to using clear glass alone. OG + EC_ON and OG + EC_OFF showed similar effects.
• Regarding indoor visual comfort, electrochromic film (OG + EC_ON and OG + EC_OFF) can reduce both vertical and horizontal illuminance and lower DGP values compared with OG. However, when OG + EC_OFF is exposed to direct sunlight, the light-scattering properties of electrochromic film increase vertical illuminance and DGP values, while decreasing horizontal illuminance. Although OG + EC_ON and OG + EC_OFF reduced DGP values by more than 20% in 14.2% and 31.5% of the data, respectively, they also increased DGP in 26.6% and 41.4% of the data, respectively, indicating unstable DGP value reduction performance.

In the future, studies could further explore the effect of seasonal changes (autumn, winter, or plum rain), direct sunlight, daylight glare probability, or night on indoor thermal comfort and air conditioning electricity consumption with different building orientations.