Development of a Solar-Tracking Movable Louver with a PV Module for Building Energy Reduction

Abstract

In response to rising energy consumption in buildings, this study proposes a solar-tracking movable louver integrated with a photovoltaic (PV) module and evaluates its performance to verify its energy-saving potential. First, the louver system can be configured as either vertical or horizontal by modularizing and rotating its slats. A solar-tracking mechanism for single-axis louver control was also developed and proven effective. Second, for optimal energy-saving performance, the louver operation must respond to external environmental conditions. Its control should account for PV power generation and building energy demands for heating, cooling, and lighting to maintain comfortable indoor and outdoor environments. Third, the proposed louver system achieved a building energy reduction of 4.7–8.8% compared to conventional fixed technologies. However, in winter, the louver may obstruct solar gains, potentially diminishing its effectiveness in reducing energy consumption. While this study demonstrates the potential of the proposed louver technology for energy efficiency, it is limited by the scope of environmental and operational conditions considered in the performance evaluation. Further studies under diverse climatic scenarios are necessary to substantiate its broader applicability.

1. Introduction

According to the Global Energy Trends 2023 report published by Enerdata, global energy consumption in 2022 increased by 1.7% compared to pre-pandemic levels, with this upward trajectory projected to continue [1]. This ongoing increase is expected to double or even triple by the mid-21st century [2]. The sustained rise in energy consumption presents a critical challenge, as it contributes to elevated CO₂ emissions, exacerbating air pollution, and global warming [3]. A significant driver of this trend is the building sector, which has witnessed a marked surge in energy use [4,5,6]. The 2022 Global Status Report for Buildings and Construction, published by the Global Alliance for Buildings and Construction, reports that, in 2021, the buildings and construction sector accounted for 34% of total global energy consumption. Furthermore, energy use in buildings reached an all-time high in 2021, marking an approximate 4% increase from 2020. This underscores that the energy consumption of the building sector exceeds that of both the industrial and transportation sectors [7]. Consequently, reducing energy consumption in buildings has become a pressing issue, with research actively underway in areas such as building envelopes [8,9,10], green retrofitting [11,12,13], and environmentally friendly construction materials [14,15,16]. Among these strategies, louvers have emerged as a representative building envelope technology. Installed primarily on windows, louvers function as shading devices and are recognized for their simple structure and high efficiency in daylighting and shading. This has led to increasing application and research interest [17,18]. Recent studies have integrated photovoltaic (PV) systems into louver slats, adjusting their angles to enhance energy performance and reduce building energy demand [19,20,21]. However, these studies have primarily concentrated on maximizing power generation, without fully leveraging the respective advantages and disadvantages of vertical and horizontal louver configurations.

In response, this study proposes a movable louver system composed of vertical slats capable of reconfiguring between vertical and horizontal orientations. It aims to develop a louver-based technology that not only enhances PV power generation through solar tracking but also contributes to building energy reduction and improved indoor lighting environments.

1.1. Types of Louvers and Literature Review

Louvers are common external shading systems installed on windows to regulate the amount of natural light entering indoor spaces [22,23]. By adjusting the slat angles, louvers can provide partial or complete shading while also enabling daylighting, as illustrated in Figure 1 [24]. This capability allows them to reduce cooling energy consumption during summer by limiting the heat gain caused by solar radiation [25].

Louvers are generally classified as vertical, horizontal, or mixed types based on their installation orientation and shading approach, as summarized in

Table 1. Types and characteristics of louver.

Type		Characteristics	Best Orientation	Image
Horizontal	Overhang	-Prevents the rise of hot air from outside -Potential risk of damage due to wind pressure and snow accumulation	South
	Overhang with Multiple Blades	-Installed in the form of multiple horizontal slats -Potential risk of damage due to wind pressure and snow accumulation	South
	Overhang Horizontal Louvers	-Horizontal slats divided into uniform sizes -Allows free movement of air -Low snow and wind load	South
	Overhang Vertical Panel	-Allows free movement of air -Low risk of damage due to snowfall -Causes visual obstruction	South, East, West
Vertical	Vertical Fin	-Typically used only in hot climates for north-facing windows -May obstruct visibility	East, West
	Slanted Vertical Fin	-Installed at an angle (not perpendicular) to the window -May obstruct visibility	East, West
Mixed	Eggcrate	-Used in very hot climates -Tends to trap hot air -Causes high visual obstruction	East, West
	Eggcrate with Horizontal Louvers	-Applies horizontal slats in the form of overhang horizontal louvers -Partially mitigates the trapping of hot air -Suitable for use in very hot climates -Causes high visual obstruction	East, West

[26,27]. Vertical louvers are most effective for blocking sunlight when the sun is at lower angles in the east or west, making them ideal for installation on the east and west facades of buildings [28]. Horizontal louvers, in contrast, are effective in shielding against high-angle sunlight typical of summer and are best suited for south-facing windows [29]. In winter, adjusting the slat angles of horizontal louvers can allow low-angle solar radiation to enter indoor spaces, contributing to passive heating and energy savings [30,31]. Therefore, the choice between vertical and horizontal louvers should be made with careful consideration of solar orientation and environmental context. Mixed-type louvers, which combine elements of both vertical and horizontal configurations, offer the benefits of each. However, they tend to increase the shading surface area, potentially obstructing outdoor views and limiting solar heat gain. This makes them less effective in winter, when increased solar penetration is desirable for heating efficiency [30,31].

This study reviewed previous research on louver systems [19,20,21,32,33,34,35,36,37], as summarized in

Table 2. Review of previous studies on louver shading systems.

Reference	Objective	Louver			PV Application	Use of Solar-Tracking Technology
		Type (Shape)	Width (m)	Angle
[19]	Developed optimal control for BIPV to improve energy efficiency and visual comfort	Horizontal	-	−78° to 78°	Applied (PV attached to only half of the slats)	Not applied
[20]	Developed three prototype adaptive facade systems compatible with curvature by applying biomimicry and SMC	A shape that mimics the curvature of a fabric membrane	-	Changes by 20°	Not applied	Not applied
[21]	Presented a responsive envelope using hygroscopic material properties	A form that mimics the shape and function of cactus stomata	-	It closes at 50% relative humidity and opens at 85%	Not applied	Not applied
[32]	Evaluated fixed shading systems with integrated solar power generation in Mediterranean countries	Horizontal	-	-	Applied (Attached to the top surface)	Not applied
[33]	Developed a new ventilated BIPV double skin facade system consisting of transparent amorphous silicon PV modules and inward-opening windows	Horizontal	1.1	Fixed	Applied (Only to the external glass)	Not applied
[34]	Assessed translucent PV ventilated glazing for energy and cost savings	Horizontal	0.88	-	Applied (Attach translucent PV to the window)	Not applied
[35]	Tested prototype PV-integrated louver system for optimal shading, daylighting, and solar energy production	Horizontal	0.41	−90°, −60°, −30°, 0°, 30°, 60°, 90°	Applied (PV attached only to 2/3 of slats)	Not applied
[36]	Hybridized the physical characteristics of thermodynamic SMA springs and PFEA, which balance forces, to propose facade panel module designs	A form that mimics cactus surface	-	The length changes to 31.5, 50.7, and 68.1mm depending on the pressure	Not applied	Not applied
[37]	Designed building envelopes for improved thermal performance through a biomimetic optimization algorithm	A form that mimics the hygroscopic structure of a pine cone	-	Optimal rotation angle 43°, optimal enclosure angle 69°	Not applied	Not applied

. Prior studies primarily focus on technological development and performance evaluation based on slat specifications. Among these, several have examined the integration of photovoltaic (PV) modules into louver slats to reduce building energy consumption [19,32,33,34,35]. However, research incorporating actual solar-tracking mechanisms into louver systems remains limited. Recent advancements in louver technologies have introduced non-traditional configurations, enhancing performance through novel forms and adaptive behaviors [20,21,36,37]. Nevertheless, such complex forms often entail significant challenges in terms of maintenance and operability. In contrast, the louver system proposed in this study retains the simple and practical structure of conventional vertical and horizontal louvers. It differentiates itself by integrating both PV modules and a solar-tracking mechanism to evaluate the resulting performance improvements. This approach aims to balance technological enhancement with structural simplicity and ease of management.

1.2. PV Module Concept

PV modules are composed of multiple interconnected PV cells, which serve as the fundamental units of solar power generation [38,39]. As illustrated in Figure 2, a PV cell is typically formed by joining P-type and N-type semiconductors, each possessing distinct electrical properties. To assemble a PV module, multiple PV cells are connected using conductive ribbons that facilitate the flow of current between cells [40]. When photons strike the surface of a PV cell, they are absorbed by the semiconductor material, generating electron–hole pairs. The excited electrons migrate toward the N-type semiconductor, while the corresponding holes move toward the P-type region. These separated charge carriers are driven through an external low-resistance circuit, producing an electric current and thereby generating electrical power. The efficiency of a PV cell is significantly influenced by temperature; as the cell temperature increases, its efficiency tends to decrease [41,42,43]. In addition, PV cells achieve their highest efficiency when sunlight strikes them perpendicularly. Any deviation from this optimal angle reduces the intensity of light absorption and thereby lowers the energy conversion efficiency [44,45].

PV cells are generally classified into two major categories: crystalline and thin-film types. Crystalline cells are further subdivided into monocrystalline and polycrystalline types based on their fabrication processes. While monocrystalline PV cells offer higher efficiency than polycrystalline variants, they are also associated with higher production costs [46]. In this study, crystalline PV modules were integrated onto the planar slats of the louver system.

1.3. Concept of Solar-Tracking Technology

Solar-tracking technology in photovoltaic (PV) systems is designed to enhance the performance of fixed solar generation systems by rotating PV modules along the sun’s trajectory [47]. By adjusting the orientation of solar panels to follow the sun’s movement, this technology significantly improves power output and conversion efficiency, making it a critical component in various solar energy applications [48].

Solar-tracking methods are broadly categorized into program-based and sensor-based systems [49,50,51]. Program-based tracking utilizes astronomical algorithms that define the relative motion of the Earth and the sun. This method calculates the sun’s position in real time based on geographic coordinates and time data for a specific location. Since it operates according to predefined algorithms, it is unaffected by weather conditions, offering the advantage of eliminating unnecessary mechanical movements. However, it lacks adaptability to dynamic environmental factors such as cloud cover or shading [52]. In contrast, sensor-based tracking systems use multiple light sensors mounted on the tracking structure to detect the direction of maximum solar irradiance. By comparing the light intensity received by each sensor, the system adjusts the PV module’s orientation accordingly. This method enables responsiveness to changing weather and environmental conditions, thereby enhancing overall energy yield. However, it is generally less accurate in determining the exact solar position compared to program-based systems. Solar-tracking devices are also classified by the number of axes involved in the tracking mechanism, as shown in

Table 3. Operating method for solar tracking.

Method	Characteristics	Image
Single-axis tracking method	-Pivots along a single axis responding to the sun’s azimuth or altitude (divided into vertical and horizontal tracking) -Tracks the sun’s movement from east to west based on its position -Stability and power generation efficiency can be improved with external support -Does not account for external factors such as weather and shading
Dual-axis tracking method	-Rotates around two perpendicular axes, simultaneously tracking solar azimuth and altitude angles -Produces energy at a faster rate than single-axis tracking systems -Tracks both the sun’s movement from east to west and its altitude -Less affected by clouds and other environmental conditions

. These include single-axis and dual-axis systems. A single-axis solar tracker rotates around one axis—either aligned with the solar azimuth (east–west) or the solar altitude (elevation)—to follow the sun’s daily movement [53,54]. Typically, the system tilts the PV module toward the north while tracking the sun’s position from east to west, thereby improving solar concentration and power generation. Single-axis trackers offer enhanced mechanical stability and increased energy output with relatively simple structure. However, they are limited to tracking the sun’s daily movement and do not accommodate seasonal altitude variations, which reduces overall tracking precision and efficiency. Dual-axis trackers use two perpendicular axes to follow both the solar azimuth and altitude angles. This configuration enables comprehensive tracking throughout the day and across seasons, accounting for annual changes in the sun’s position [55,56]. Dual-axis systems can employ either passive or active tracking mechanisms. Although they provide superior energy gains and flexibility compared to single-axis systems, they suffer from increased structural complexity. The PV module is typically connected to the tracking frame only at the central rear, which weakens mechanical integrity and increases the risk of damage under adverse weather conditions [57].

2. Materials and Methods

2.1. Design Suggestion for a PV-Integrated Movable Louver System Utilizing Solar-Tracking Technology

The solar-tracking-based movable louver system proposed in this study is designed to improve photovoltaic (PV) generation efficiency by integrating solar-tracking technology with PV-attached slats. The system also allows for seamless transformation between vertical and horizontal louver configurations, thereby optimizing the shading and daylighting benefits of both types. The system is initially configured as a vertical louver, as shown in Figure 3a. Each slat is divided into modular segments, which are mechanically linked to rotate synchronously at a fixed angle. This design enables the structure to transform into a horizontal louver configuration, thereby combining the benefits of both orientations. The transformation capability allows the louver to be controlled dynamically in response to external environmental conditions, such as sun position and seasonal variations. In addition, as shown in Figure 3b, the slat support frames are designed to shift and overlap in one direction, enabling improved visibility and increased solar access in winter to reduce heating energy demand. Each slat module incorporates a PV panel on one side and a high-reflectivity aluminum surface on the opposite side, which reflects approximately 90% of incident light. By adjusting the slat angle, the system can either generate electricity or enhance indoor lighting, depending on orientation and seasonal need. As shown in Figure 3a, the system includes a solar-tracking device mounted at the top to adjust the slat angles in real time. This tracking mechanism uses a single-axis drive system and features a “┴”-shaped cross-section with a protruding center, as shown in Figure 4. Light sensors are positioned symmetrically on either side of the central protrusion. As the tracker rotates around a pivot axis, the irradiance values measured by the two sensors vary based on the angle of incidence from the sun. When the difference in sensor readings decreases, the sensor surface becomes more perpendicular to sunlight (Figure 4b). This principle allows the system to track the sun’s position accurately and apply the optimal slat angle for both shading and PV performance. Moreover, as shown in Figure 3d, the slat module support frames can move laterally to create an open-louver configuration, further enhancing view access and daylight penetration.

2.2. Performance Evaluation Environment Setup

To evaluate the performance of the solar-tracking-based movable louver with a PV module, this study constructed a testbed capable of simulating artificial climatic conditions. As illustrated in Figure 5, the testbed comprises four main components: an indoor space, a daylighting window, an artificial climate chamber, and an energy monitoring system.

The setup is detailed as follows. First, the indoor test space has internal dimensions of 6.6 m × 4.9 m × 2.5 m, with walls constructed from 0.1 m thick insulated panels. The surface reflectance values for the floor, walls, and ceiling are 25%, 46%, and 86%, respectively. Second, the indoor space includes four LED-type lighting fixtures and one floor-standing air conditioner, both of which were used to evaluate the energy-saving performance of the louver system. The lighting system supports eight levels of dimming, excluding the OFF state. The installation positions of the lighting fixtures follow the Illuminating Engineering Society (IES) four-point method [58]. Third, eight illuminance sensors were installed within the space, as shown in Figure 5. These were positioned at a height of 0.85 m above the floor to align with the typical working plane height. A temperature sensor was installed at the center of the interior space to monitor ambient thermal conditions. Fourth, the daylighting window into which the proposed louver was installed measures 1.9 m × 1.7 m. The window is composed of double-glazed panes with a light transmittance of 80%. Notably, the window is not centered on the wall but positioned off to one side, as per the testbed configuration. Fifth, the artificial climate chamber is equipped with a solar irradiation apparatus and a temperature control system. The solar simulator can replicate solar azimuth and altitude angles by adjusting both the intensity and elevation of artificial light. This Grade-A simulator, compliant with ASTM E927-85 international standards, ensures high measurement accuracy and reliability [59]. However, due to equipment constraints, it can simulate solar azimuth angles only within the range of 120° to 190°. Finally, the study implemented control of lighting and HVAC systems to maintain appropriate indoor illuminance and thermal comfort levels. Power consumption associated with these operations was continuously recorded using an integrated energy monitoring system.

2.3. Performance Evaluation Method

This study evaluates the shading and power generation performance of the solar-tracking-based movable louver system with an integrated PV module, as proposed in Section 2.1. The evaluation methodology is detailed below.

To verify the effectiveness of the proposed system (Case 4), three reference configurations were established for comparison: vertical louvers (Case 1), horizontal louvers (Case 2), and mixed-type louvers (Case 3). The specifications for each case are presented in

Table 4. Case setting for performance evaluation.

Case	Image	Louver Type	Movable or Not	Slat				Slat module		PV Module Area (Number of PV Cells Used)
				Count	Size	Spacing	Angle	Size	Angle
1		Vertical	Fixed	5	0.03 m (W) × 0.35 m (D) × 1.65 m (H)	0.375 m	Moves within a range of −90° to 90° in 15° increments	-	-	2.43 m2 (100)
2		Horizontal	Fixed	4	1.85 m (W) × 0.35 m (D) × 0.03 m (H)	0.41 m	Moves within a range of 0° to 180° in 15° increments	-	-	2.14 m2 (88)
3		Mixed	Fixed	9	Combination of Cases 1 and 2	Vertical 0.37 m, horizontal 0.41 m	0	-	-	5.84 m2 (240)
4		Vertical and horizontal	Movable	5	Each slat is divided into 4 modules	-	0°, 90°	0.03 m (W) × 0.31 m (D) × 0.38 m (H)	Moves within a range of 0° to 360° in 15° increments	1.95 m2 (80)

. Slat dimensions were determined based on the physical size of the PV cells used. In Cases 1 and 2, PV cells are affixed to one side of each slat. In Case 3, vertical slats are installed on both lateral sides of the window, while horizontal slats with PV cells are mounted on top. The proposed Case 4 adopts a configuration similar to Case 1 but incorporates modular slats, resulting in a slight reduction in total PV cell count. In addition, Case 4 offers dynamic transformation between vertical and horizontal orientations. As illustrated in Figure 6, the slat angles in Cases 1 and 2 are adjustable within a range of 180°, in 15° increments. For Case 4, the slat modules rotate within a full 360° range, also in 15° increments, allowing for enhanced operational flexibility. PV specifications are summarized in

Table 5. Specifications of the PV Cell.

Item	Specifications	Item	Specifications
Type	Monocrystalline Silicon	Open-Circuit Voltage	0.678 V
Max. Power (Pmax)	5.35 W	Fill Factor	80.75%
Voltage at Max. Power Point (Vmpp)	0.577 V	Size	0.156 m × 0.156 m
Current at Max. Power Point	9.329 A	Cell Efficiency	21.9%

. Slats or modules without PV cells were treated with aluminum surfaces having a reflectivity of 90% to support daylight reflection.

Second, a mockup was fabricated to evaluate the performance of the proposed louver system. This mockup was constructed using the profiles shown in Figure 7. A solar-tracking device, illustrated in Figure 7e, was mounted on the upper portion of the structure. The specifications of the pyranometer (illuminance sensor) used in the solar-tracking device are presented in

Table 6. Specifications of the illuminance sensor used in the solar-tracking device.

Specifications	Values	Specifications	Values
Model	ML-02	Non-linearity at 1000 W/m2	<0.2%
Wavelength range	400~1100 nm	Directional response at 1000 W/m2	<10 W/m2
Response time 95%	<1 ms	Sensitivity (μV/W/m2)	Approx. 50
Non-stability (change/year)	±2%	Operating temperature range	–30 to +70

. White-painted aluminum was used for the rear part of the louver system, with a reflectance of 0.6.

Third, the study calculated the electricity generated by the PV modules attached to the louvers. This was performed by recording the voltage at the maximum power point (Vmp) and the corresponding current (Imp) and then calculating the power output as the product of these two values. Since the total PV module area differed across the four test cases, the power generation values were normalized to a per-square-meter basis (W/m²) to enable fair comparison among cases.

Fourth, lighting energy consumption was evaluated based on the required dimming levels necessary to maintain appropriate indoor illuminance, with the goal of ensuring a visually comfortable environment. A target indoor illuminance of 500 lx was established in accordance with the general office lighting standards specified by the IES of North America [60]. Dimming control was implemented when the minimum illuminance value among the ten installed indoor sensors fell below 500 lx. The dimming level was increased incrementally, starting with the light closest to the sensor registering the lowest value. This process continued until all sensors measured illuminance levels of 500 lx or higher. Lighting energy consumption was derived based on the final dimming configuration for each scenario, providing a basis for evaluating the lighting energy reduction potential of each louver configuration. Lastly, the study determined the optimal slat and slat module angles for minimizing lighting energy consumption. These were defined as the angles at which energy use was lowest. In cases where multiple angles yielded the same minimum energy consumption, the optimal angle was selected based on the highest minimum illuminance value among the sensors—thereby prioritizing uniform lighting distribution while achieving energy efficiency.

Fifth, this study evaluated the operation of air conditioning systems required to maintain appropriate indoor temperatures across different louver configurations. Based on prior research [61], the target indoor temperatures were set to 26 °C for summer and 20 °C for winter. Instead of using the built-in temperature sensor of the air conditioner, this study used a centrally located sensor installed in the middle of the indoor space (as shown in Figure 5). This sensor provided real-time feedback for controlling the air conditioner, enhancing the accuracy and responsiveness of indoor temperature regulation. Heating and cooling energy consumption was monitored to assess the thermal energy-saving performance of each louver configuration. To ensure the validity of the temperature-based evaluation, lighting levels were kept constant at a designated dimming setting during the measurement of heating and cooling energy usage. This control ensured that lighting did not introduce variability in thermal load conditions.

Sixth, the performance evaluation was conducted under controlled seasonal conditions using the artificial climate chamber integrated into the testbed. This study was conducted in Seoul, South Korea, which has distinct seasonal climatic changes. Seoul sits at a latitude of 37.57° N and a longitude of 126.98°; the external environmental conditions of the artificial climate chamber were set based on the climate data of this region. The criteria reflected the climatic characteristics of a typical mid-latitude city and ensured the generalizability of performance evaluations. As presented in

Table 7. Simulated solar altitude, external illuminance, and solar radiation by time for summer and winter.

Season	Temperature of the Artificial Climate Chamber	External Illuminance, Azimuth Angle, Solar Altitude, and Solar Radiation by Time Zone
		10:00–11:00	11:00–12:00	12:00–13:00	13:00–14:00
Summer	27.1 °C	70 k lx, 120°, 60° 530 W/m2	75 k lx, 150°, 70° 574 W/m2	80 k lx, 180°, 76.5° 638 W/ m2	70 k lx, 203°, 70° 574 W/m2
Winter	–3.2 °C	20 k lx, 157°, 22.5° 289 W/m2	25 k lx, 172°, 28° 296 W/m2	30 k lx, 180°, 29.5° 332 W/m2	20 k lx, 187°, 26.5° 296 W/m2

, the environmental parameters were simulated in one-hour intervals from 10:00 a.m. to 2:00 p.m. Due to equipment constraints in the artificial solar radiation system, the 10:00 a.m. condition was partially adjusted to align with the testbed’s physical limitations. Each time slot simulated solar azimuth, altitude, external illuminance, and solar radiation levels consistent with seasonal variations. The seasonal temperature settings of the artificial climate chamber were based on the 30-year average climate data for Seoul provided by the Korea Meteorological Administration [61]. Additionally, the solar radiation of the artificial solar radiation system was measured based on illuminance control. The solar radiation information was calculated based on the solar radiation value that reached the vertical surface of the window where the louver was installed (Table 7).

3. Results and Discussion

3.1. Performance Evaluation Results

To validate the effectiveness of the solar-tracking-based movable louver system with a PV module (Case 4), comparative performance evaluations were conducted against existing louver technologies (Cases 1–3). The results are as follows.

First, the functionality of the solar-tracking system was verified by analyzing the ratio of illuminance sensor values recorded between 12:00 and 13:00 in both summer and winter conditions. As shown in Figure 8, the ratio was calculated by dividing the lower illuminance sensor value by the higher one. This ratio was used to determine the directional alignment of the solar-tracking device. In addition, the study evaluated power generation in Case 4 by varying the slat module angle. The resulting power generation curve exhibited a pattern similar to the sensor ratio trend observed in the solar-tracking system. When the maximum power output of the slat module and the maximum sensor ratio were normalized to the same reference, the average difference between the two indicators was found to be 9.9%. This close correlation demonstrates the functional validity and effectiveness of the solar-tracking device developed in this study.

Second, Figure 9 presents the power output of the PV modules based on the slat and module angles for each case. For the vertical louver configuration (Case 1), the optimal slat angles to maximize power generation during summer from 10:00 a.m. to 2:00 p.m. were identified as 60°, 30°, 0°, and −30°, respectively. In winter, the corresponding optimal angles were 30°, 15°, 0°, and 0°. The total power generation for Case 1, using these optimal angles, amounted to 1.326 kWh. For the horizontal louver configuration (Case 2), the optimal slat angles during summer were 60°, 75°, 75°, and 75°, while in winter they were 15°, 30°, 30°, and 30°, respectively. The resulting total power generation was 1.366 kWh. The mixed louver configuration (Case 3) was assumed to remain static (non-operational) during the performance evaluation. Despite its larger number of PV modules, the total power generation was 1.539 kWh. For the solar-tracking-based movable louver proposed in this study (Case 4), the optimal slat module angles between 10:00 a.m. and 2:00 p.m. were 60°, 75°, 90°, and 105° in summer and 45°, 75°, 90°, and 105° in winter. The total power generation for Case 4 was 1.387 kWh. These results demonstrate that adapting the slat angle in response to external environmental conditions improves PV generation efficiency. Although Case 4 has fewer PV cells than Cases 1 and 2, its total power generation was 4.4% higher than Case 1 and 1.5% higher than Case 2 when operated at optimal angles. More notably, when normalized by PV module area (1 m²), Case 4 achieved 23.3% and 10.3% higher power generation compared to Cases 1 and 2, respectively, as shown in

Table 8. Optimal angles and total power generation for improving power generation by case.

Case	Optimal Angle of the Slat Module for Improving Power Generation								Total Power Generation (kWh)	Power Generation per 1 m2 (kWh)
	Summer				Winter
	10:00–11:00	11:00–12:00	12:00–13:00	13:00–14:00	10:00–11:00	11:00–12:00	12:00–13:00	13:00–14:00
1	60°	30°	0°	–30°	30°	15°	0°	0°	1.326	0.546
2	60°	75°	75°	75°	15°	30°	30°	30°	1.366	0.638
3	-	-	-	-	-	-	-	-	1.539	0.264
4	60°	75°	90°	105°	45°	75°	90°	105°	1.387	0.711

. Although Case 3 incorporated 58.3%, 63.3%, and 66.7% more PV cells than Cases 1, 2, and 4, respectively, its power generation only increased by 13.1%, 11.2%, and 9.9% compared to each of those cases. This limited performance is attributed to the fixed nature of Case 3 and its structural design, which includes PV modules on both sides of vertical slats. Due to direct sunlight conditions, only one side receives effective irradiance, while the other remains shaded. Furthermore, the combination of vertical and horizontal slats causes mutual shading, further reducing overall PV efficiency.

Third, the lighting energy consumption required to maintain appropriate indoor brightness for each case is illustrated in Figure 10. In Case 1 (vertical louver), the optimal slat angles for reducing lighting energy between 10:00 a.m. and 2:00 p.m. were −90°, −90°, 90°, and 75° during summer and −90°, −90°, 90°, and 90° during winter. Based on these angles, the total lighting energy consumption was 0.831 kWh, as shown in

Table 9. Optimal angles and total lighting energy usage for reducing lighting energy consumption by case.

Case	Optimal Angle of the Slat Module for Improving Power Generation								Total Lighting Energy Consumption (kWh)
	Summer				Winter
	10:00–11:00	11:00–12:00	12:00–13:00	13:00–14:00	10:00–11:00	11:00–12:00	12:00–13:00	13:00–14:00
1	–90°	–90°	90°	75°	–90°	–90°	90°	90°	0.831
2	150°	165°	165°	165°	105°	120°	120°	120°	0.843
3	-	-	-	-	-	-	-	-	0.970
4	* 255°	195°	240°	270°	195°	195°	240°	240°	0.825
	** 60°	75°	90°	105°	45°	75°	90°	105°	0.950

* Slat module angle control to reduce lighting energy consumption in Case 4. ** Operation based on the optimal angle derived from the solar-tracking device of Case 4.

. For Case 2 (horizontal louver), the optimal slat angles were 150°, 165°, 165°, and 165° during summer and 105°, 120°, 120°, and 120° during winter. The total lighting energy consumption in this case was 0.843 kWh. Case 3 (mixed louver), which was non-operational during evaluation, consumed 0.970 kWh of lighting energy. In Case 4 (the proposed solar-tracking-based movable louver), the optimal slat module angles for reducing lighting energy were 255°, 195°, 240°, and 270° in summer and 195°, 195°, 240°, and 240° in winter. The total lighting energy consumption under these optimal angle conditions was the lowest among all cases at 0.825 kWh. This represents a reduction in lighting energy consumption by 0.7%, 2.1%, and 10.8% compared to Cases 1, 2, and 3, respectively. However, when Case 4 was operated using angles derived from the solar-tracking device—optimized for PV power generation rather than lighting—its total lighting energy consumption was 0.950 kWh. This outcome highlights the trade-off between maximizing power generation and minimizing lighting energy use, as the optimal slat angle for lighting reduction does not always align with that for PV efficiency. During the 10:00–11:00 and 11:00–12:00 time periods, Case 1 demonstrated effective lighting energy reduction by reflecting natural light into parts of the room distant from the daylighting window, as shown in Figure 11. This effect is attributed to the asymmetrical placement of the window in the testbed, which is offset to one side. Similarly, in Case 4, the optimal slat module angles for lighting reduction exceeded 180°, enabling effective reflection of natural light from the rear aluminum panel (opposite the PV side) into the interior space.

Fourth, the heating and cooling energy consumption required to maintain appropriate indoor temperatures for each case is shown in Figure 12. These calculations were performed in conjunction with dimming control for lighting to ensure consistent indoor illuminance levels. During summer, the optimal slat angles for reducing cooling energy from 10:00 a.m. to 2:00 p.m. were 0° for all time intervals in Case 1 and 0° (or 180° due to the horizontal orientation) for all intervals in Case 2. The corresponding cooling energy consumption was 1.574 kWh for Case 1 and 1.619 kWh for Case 2. In both cases, it was beneficial to block direct sunlight to minimize internal heat gain, making a slat angle of 0° preferable. During winter, the optimal slat angles for maximizing solar heat gain and thus reducing heating energy consumption were −45°, −90° (or 90°), −90° (90°), and −90° (90°) in Case 1 and 105°, 120°, 120°, and 120° in Case 2. The calculated heating energy usage was 2.643 kWh for Case 1 and 2.746 kWh for Case 2. These results suggest that maximizing indoor solar exposure through open slat angles is effective in reducing heating demand. In Case 3 (mixed louver), the total heating and cooling energy consumption was 4.864 kWh, which is 1.2% and 3.1% higher than in Cases 1 and 2, respectively. This increase is attributed to its fixed structure, which prevents the system from adapting to varying external environmental conditions. In Case 4 (proposed louver system), the heating and cooling energy consumption based on the optimal slat module angles was 5.0% higher than that of Case 1. This is primarily due to its structural configuration, which slightly reduces its ability to block solar radiation. However, when Case 4 was operated using the solar-tracking system—optimized for PV generation—the total heating and cooling energy dropped to 4.503 kWh. This variation again highlights the performance trade-offs between optimizing for energy savings and for solar energy production.

3.2. Discussion

This study compared and analyzed the energy reduction performance of the solar-tracking-based movable louver with a PV module (Case 4) against conventional louver systems.

Table 10. Total energy consumption based on the optimal slat/module angles for integrated energy reduction by case.

Case	Optimal Angle of the Slat Module for Energy Reduction								Total Summer Energy Usage (kWh)	Total Winter Energy Usage (kWh)	* Total Energy Usage (kWh)
	Summer				Winter
	10:00–11:00	11:00–12:00	12:00–13:00	13:00–14:00	10:00–11:00	11:00–12:00	12:00–13:00	13:00–14:00
1	60°	30°	0°	–30°	–30°	90°	–90°	–90°	1.424	2.844	4.268
2	60°	75°	60°	75°	105°	120°	60°	120°	1.493	2.965	4.458
3	-	-	-	-	-	-	-	-	1.431	2.889	4.320
4	60°	75°	90°	105°	45°	75°	90°	105°	1.425	2.642	4.067

* Energy consumption total (kWh) = PV module power generation−Lighting energy consumption−Heating and cooling energy consumption.

presents the optimal slat/module angles for each case when considering three criteria in combination: PV module power generation, lighting energy consumption required to maintain appropriate indoor illuminance, and heating/cooling energy required for thermal comfort. These integrated optimal angles differ from the individually optimized values previously shown in Table 8, Table 9 and

Table 11. Optimal angles and total heating and cooling energy usage for reducing energy consumption by case.

Case	Optimal Angle of the Slat Module to Improve the Total Heating and Cooling Energy Usage								Total Cooling Energy (kWh)	Total Heating Energy (kWh)	Total Heating and Cooling Energy (kWh)
	Summer				Winter
	10:00–11:00	11:00–12:00	12:00–13:00	13:00–14:00	10:00–11:00	11:00–12:00	12:00–13:00	13:00–14:00
1	0°	0°	0°	0°	–45°	–90° (90°)	–90° (90°)	–90° (90°)	1.574	2.643	4.217
2	0° (180°)	0° (180°)	0° (180°)	0° (180°)	105°	120°	120°	120°	1.619	2.746	4.365
3	-	-	-	-	-	-	-	-	1.787	3.077	4.864
4	* 90°	60°	90°	90°	0°	180°	270°	0°	1.715	2.628	4.343
	** 60°	75°	90°	105°	45°	75°	90°	105°	1.718	2.785	4.503

* Controlling the angle of the slat module to reduce heating and cooling energy consumption in Case 4. ** Operation based on the optimal angle derived from the solar-tracking device in Case 4.

, which were based on single-parameter evaluations. This indicates that controlling louver slats in PV-integrated systems should not rely solely on maximizing PV power generation but must also consider overall building energy usage, including lighting and HVAC loads. The proposed system (Case 4) incorporates solar tracking to autonomously respond to changing environmental conditions and combines the advantages of both vertical and horizontal louver orientations. As a result, Case 4 achieved overall energy reductions of 4.7%, 8.8%, and 5.9% compared to Cases 1 (vertical), 2 (horizontal), and 3 (mixed fixed), respectively. Although Case 4 does not optimize for lighting and heating/cooling energy directly—being driven by solar-tracking logic to enhance PV output—it still demonstrates superior overall performance. Importantly, Case 4 utilizes a smaller PV surface area than the other cases, yet it achieves comparable or improved results, underscoring its efficiency and practicality. In addition, this study examined the benefit of increasing solar access during winter to reduce heating energy consumption. As shown in Figure 3d and Figure 7d, an additional performance evaluation was conducted in which the slat modules were retracted to one side of the daylighting window to maximize solar radiation. Under this configuration, the total energy consumption was measured at 2.259 kWh (

Table 12. Total energy consumption of the open structure through slat movement in winter (Case 4).

PV Module Power Generation (kWh)	Total Lighting Energy (kWh)	Total Heating Energy (kWh)	* Total Energy Usage (kWh)
0.014	0.176	2.097	2.259

* Energy consumption total (kWh) = PV module power generation−Lighting energy consumption−Heating and cooling energy consumption.

). This represents an average energy reduction of 14.5% compared to the scenarios where the louver remained in standard operating positions, highlighting the effectiveness of passive solar gain strategies during colder seasons.

4. Conclusions

This study proposed a solar-tracking-based movable louver integrated with a photovoltaic (PV) module to enhance the energy performance of conventional louvers. The effectiveness of the system was verified through controlled testbed experiments, and the following conclusions were drawn:

First, the proposed louver system is fundamentally based on a vertical configuration. However, by modularizing the slats and allowing them to rotate, the system can be transformed into either vertical or horizontal configurations. This design enables the benefits of both louver types. A solar tracking device is mounted at the top of the structure, operating on a single-axis rotation mechanism. It features two illuminance sensors positioned on either side of a frame with a cross-sectional shape resembling “┴”. When the difference in sensor readings decreases, the angle of the sensor surface approaches perpendicularity to the sunlight, thus enabling accurate solar tracking based on this principle.

Second, to improve PV power generation, it is essential for sunlight to strike the PV modules perpendicularly. Therefore, the operation of the louver system should be adjusted according to the time of day. The proposed system achieved increases in power generation of 4.4% and 1.5% compared to vertical and horizontal louvers, respectively, when operated at optimal angles. Notably, this performance was achieved despite using fewer PV cells. In contrast, the mixed louver, despite having the largest number of PV cells, showed lower energy efficiency because it was fixed and unable to adapt to changing conditions.

Third, this study considered not only PV power generation but also lighting and heating/cooling energy requirements in evaluating louver performance. The analysis showed that the optimal slat angles for minimizing total energy consumption differ from those that only maximize PV output. This finding highlights the importance of integrated energy management. When accounting for all three energy components—PV generation, lighting, and thermal loads—the proposed louver system reduced total energy consumption by 4.7%, 8.8%, and 5.9% compared to vertical, horizontal, and mixed louver systems, respectively. This demonstrates the proposed system’s effectiveness in comprehensive energy savings.

Fourth, in winter, applying PV modules to the louver slats is not as effective for reducing total energy consumption. Instead, allowing natural sunlight to penetrate indoor spaces is more beneficial for minimizing lighting and heating energy. Therefore, the system is designed with an open–close mechanism to allow the slats to be retracted when solar radiation is desirable, as demonstrated in the winter energy-saving scenario. This highlights the importance of adaptable control based on seasonal and environmental conditions.

This study was validated through performance evaluations to verify the effectiveness of technology development aimed at improving the energy reduction performance of louvers. However, the performance evaluations were conducted only under limited conditions, such as specific regions and time periods. Furthermore, the performance evaluations were conducted with an artificial environment rather than a real one. The artificial sunlight used in this study was also limited in its simulation of the azimuth angle of the actual sun. As a result, additional constraints arose from conducting performance evaluations restricted to specific time periods. Therefore, it is necessary to conduct follow-up performance evaluations considering various temporal, climatic, and environmental conditions in the future.