Effects of Building Color, Material, and Angle on Bifacial and Transparent Solar Panels

Abstract

Numerous studies have explored the placement of solar panels on the facades or roofs of buildings. This study investigates a new approach to estimating energy generation from transparent, double-sided solar panels integrated into the facade of an existing building, focusing on how the façade’s color influences panel performance. The most significant advantages of integrating double-sided and transparent solar panels on the sides of a building are the natural lighting provided by the sunlight entering the building and the additional energy generated when the radiation returns to the back of the panel. The light beam strikes the front panel, allowing some radiation to pass through the transparent panel to the back side, where it hits the surface. Part of the beam is then reflected toward the rear panel. The fraction of light reflected (albedo) depends on the surface’s color. We first constructed a double-sided, transparent solar panel and integrated it with MATLAB software 2024 code. The model was verified by comparing the simulation results, specifically the I–V and P–V graphs, with data from the manufacturer’s specifications. We conducted an extensive investigation into panels installed on surfaces made of different materials during each installation. This investigation aimed to understand the behavior and performance of the panels when installed on the surfaces of various materials.

1. Introduction

Bifacial solar cells, a technology dating back to 1960, are designed to capture solar irradiance from both sides of the cell, effectively utilizing reflected sunlight from surfaces such as the ground. By leveraging the albedo, these cells can enhance solar energy absorption by up to 50%. We anticipate that 40% to 50% of solar panels sold by 2028 will utilize bifacial technology [1]. Transparent solar cells have emerged as a solution to the challenges posed by traditional photovoltaic (PV) technologies, which were initially limited to rooftops and vacant lots. The constrained space in buildings made implementing these technologies effectively and increasingly difficult. In response, researchers and scientists have turned their focus to transparent PV technologies. One promising aspect of these advancements is their potential to integrate seamlessly into windows, offering dual benefits of energy generation and innovative architectural design enhancements throughout daylight hours [2,3]. PV cells can be integrated into buildings using three primary placements: rooftops, windows, and building facades [2]. In this way, two goals can be achieved: sunlight illuminates the interior of the building, and energy is generated. Thus, researchers have explored installing photocells and double-sided transparent cells on the facade of a building, significantly impacting the building’s color reflection and directly influencing energy production. Certain colors exhibit a high albedo, enhancing energy production, as demonstrated in [4]. Accordingly, this study selected colors to test their impact on the energy production of transparent bifacial panels installed on an existing building’s facade. This research used the tables of albedo values appearing in [5,6,7]. Furthermore, placing the panels near the building’s facades significantly impacts their transparency. The more transparent the panels are, the more irradiance passes through to the panel’s rear side. Thus, a direct correlation exists between panel transparency, the albedo value of the building’s facade color, and panel energy production. Choosing a cell with higher density necessitates the consideration of transparency because of their inverse relationship, as illustrated in [3] and Equation (1).

$${\mathrm{T}}_{\mathrm{M}}=\left(1-{\mathrm{D}}_{\mathrm{c}}\right)\times {\mathrm{T}}_{\mathrm{v}}$$

(1)

where $T_v$ is the transparency of the glass, $D_c$ is the cell density, and $T_v$ is the transparency of the glass. When installing panels on a sloped facade, according to [8], the building color affects the albedo (irradiance reflection) between surfaces, as does the angle between them. Therefore, energy production from the back side of the panels is impacted. This investigation aims to understand how specific angles affect energy production. This research presents a program in MATLAB that simulates bifacial and transparent solar panels and checks the background color effect according to the albedo tables in the literature [5,6,7]. We simulated the installation of a panel array on the front of a building without a gap to reduce the effects of scattered irradiance and reflections from other objects and tested the effect of building color mainly on the energy production that was added from the back side of the panel installed on the front of the building. Next, we tested the vertical installation of panels on a sloped facade and evaluated the effect of the reflection of the sloped facade on the energy production from the panels. The research was carried out in the following steps: First, we devised a software code to simulate a single double-sided solar cell, which could be separated based on the contribution of each side of the cell according to the electrical circuit shown in Figure 1. This approach was different from that taken in [9], which analyzed a bifacial panel using monofacial electrical models. Next, we expanded the single-cell software code to simulate the double-sided, transparent solar panel from [10]. In addition, we modified the code to simulate an array of solar panels installed on the facade of a building. The main goal of this research was to simulate panels on different color backgrounds. Therefore, we needed to project light at different intensities on the front side to examine the effect of the color of the base on the energy production, particularly on the back side of the panel. This goal was achieved by combining several parameters in the software code: the intensity of the radiation on the front side of the panel, the transparency of the panel, and the reflection of the color (i.e., albedo) from the surface on which the panel was installed. The albedo is critical because the intensity of the reflected radiation affects the energy production from the back side of the panel. This paper ends with a discussion of the results, the innovations of this study, its limitations, and improvements that could be made in the future.

Table 1. Nomenclature.

Abbreviation	Definition
$T_M$	Percentage transparency of module
$D_c$	Cell density
$T_v$	Transparency of glass
$I_0$	Reverse saturation current of bypass diode
$q$	Electron charge constant (1.60217646 × 10−19 C)
$N_C$	Number of cells
$k$	Boltzmann constant (1.3806503 × 10−23 J/K)
$T$	Temperature of the p-n junction (K)
a	Diode ideality constant
$I_{SC}$	Short circuit current
$V_{OC}$	Open circuit voltage
$G_f$	Irradiance received by front side of panel
$G_r$	Irradiance received by rear side of panel
$V_D$	Voltage drop across the circuit
$R_S$	Series resistance
$I_{0,1}$	First diode’s reverse saturation current
$I_{0,2}$	Second diode’s reverse saturation current
$I_{sc,f}$	Short circuit current on front side
$I_{sc,r}$	Short circuit current on rear side
$\alpha$	Measurement error rate
$I_{ph,f}$	Generation current of the front side of the panel.
$I_{ph,r}$	Generation current of the rear side of the panel
$\Delta T$	Temperature difference between the experimental temperature T and standard temperature (25 °C)
Albedo	Fraction of light that a surface reflects
GHI	Global horizontal irradiance
DNI	Direct normal irradiance
DHI	Diffuse horizontal irradiance
β	Angle between two surfaces
$G_{1,2}$	Intensity of the radiation transferred from surface 1 to surface 2
$F_{ij}$	View factor of radiation from area Ai to area Aj

presents the nomenclature used in this paper.

2. The Essence of the Problem and the Goal of This Study

2.1. Incorporating PV Cells into Buildings

The concept of nearly zero-energy buildings (NZEBs) involves enhancing the energy efficiency of the building envelope, followed by integrating renewable energy sources (RESs), such as PV systems, to meet electricity demands reliably in both existing and new buildings.

The best available method for NZEBs is the integration of transparent solar PV cells, allowing sunlight to penetrate the interior parts of the building. As reported in Section 1, the transparency of the panels directly affects the density of the cells; greater transparency means a lower cell density, which results in less energy. Therefore, for energy efficiency, double-sided transparent solar cells are a good choice, utilizing the light energy penetrating the building’s interior and returning it via the albedo to the back side of the panel.

Integrating bifacial PV modules into a building facade is a significant step in applying this relatively new technology. In addition to functioning as a renewable electricity source in its active role, such a facade decreases the building’s cooling requirements through its passive capabilities. In one study [11], the authors designed multilayer, one-dimensional dynamic thermal models of monofacial glass–back sheets and bifacial glass–glass PV modules integrated into a building facade. Given the geometry and PV technologies of the facades, as well as the weather conditions in Catania, Italy, the bifacial glass–glass PV modules yielded about 5% more energy than monofacial PV modules.

In 2013, the IEA illustrated that more than one-third of total energy consumption comes from buildings. Additionally, Xin Liang et al. [12] showed that buildings consume 40% of the world’s energy. These studies focused on how to select the type of glass or thickness of glass without considering the integration of solar panels on the glazing surface, which can hinder energy production. Therefore, to fill this gap, one study in [11] proposed an approach to determine the type of glass covering on the building and optimize the solar cell installation area on the facade to reduce the energy demand during the operation phase of buildings. The authors presented the results of a simulation performed on an existing 30-story building in Vietnam; the predicted energy generated from panels installed on four facades of the building were as follows: south side, 107.36 $\left[{\displaystyle \frac{\mathrm{KWh}}{{\mathrm{m}}^2\mathrm{year}}}\right]$; north side, 70.71 $\left[{\displaystyle \frac{\mathrm{KWh}}{{\mathrm{m}}^2\mathrm{year}}}\right]$; west side, 132.24 $\left[{\displaystyle \frac{\mathrm{KWh}}{{\mathrm{m}}^2\mathrm{year}}}\right]$; and east side, 131.03 $\left[{\displaystyle \frac{\mathrm{KWh}}{{\mathrm{m}}^2\mathrm{year}}}\right]$.

An integrated PV building offers the advantage of harnessing solar radiation from all sides, particularly during sunrise and sunset. The research detailed in [12], which examined the radiation intensity in Sharjah, UAE, revealed peaks between 5:00 am and 9:00 am in the east and between 3:00 pm and 7:00 pm in the west, reaching 800 W/m². This highlights the potential for energy generation from east- and west-facing facades.

The previous section emphasized the demand for integrating solar panels into the facades of newly designed buildings. This study aims to explore energy production via the installation of solar panels on existing commercial or residential buildings, some of which were constructed many years ago with facades made of concrete, stone, or marble. We identify the most efficient methods for installing solar panels on these buildings to generate green energy, and the optimal approach involves using transparent and bifacial solar panels on the facades, allowing energy generation from both sides. The challenge at hand is to develop a software module to assess the potential energy output from these panels. Maximizing energy efficiency entails generating the maximum energy from the rear side of the panel, influenced by the panel’s transparency and the building façade’s albedo, which varies with the building’s color and material. This study examines how the building’s color impacts energy production from these panels.

2.2. PV and Transparent Cell Electrical Module

Based on previous studies, we developed an electrical model for bifacial PV cells featuring two current generators and two diodes. This modeling approach simplifies the differentiation of energy production between the panel’s front and back sides [13,14,15,16]. In this study, separating the contributions of the front and back sides of the double-sided, transparent solar panel was critical. We focused on evaluating the change in the energy contribution of the panel compared to the change in the light intensity that penetrates the double-sided panel, hits the background, and returns to the back side of the panel. According to [17], manufacturers provide guidelines for constructing such models based on their specifications.

The following equations define the behavior of the solar panels:

$$I_{pv}=I_{ph,f}+I_{ph,r}-I_{01}\left[\exp \left({\displaystyle \frac{qv}{\left(a1 Nc K T\right)}}\right)-1\right]-I_{02}\left[\exp \left({\displaystyle \frac{qv}{a2 Nc K T}}\right)-1\right]-{\displaystyle \frac{V_D}{R_p}}.$$

(2)

$$I_{01}={\displaystyle \frac{\left(I_{sc}+K_i\Delta T\right)}{\exp \left(\frac{\left(v_{oc}+K_V\Delta T\right)q}{a1N_{c}kT}\right)-1}}$$

(3)

$$I_{02}={\displaystyle \frac{\left(I_{sc}+K_i\Delta T\right)}{\exp \left(\frac{\left(v_{oc}+K_V\Delta T\right)q}{a2N_{c}kT}\right)-1}}$$

(4)

$$I_{ph,f}=\alpha \times \left(I_{sc,f}+Ki\times \Delta T\right)\times {\displaystyle \frac{G_f}{G_s}}$$

(5)

$$I_{ph,r}=\alpha \times \left(I_{sc,r}+Ki\times \Delta T\right)\times {\displaystyle \frac{G_r}{G_s}}$$

(6)

2.3. Installing a Bifacial Transparent Panel and Studying the Effect of Color

This section examines the installation of a solar panel on a colored surface, see Figure 2 and Figure 3, and analyzes the impact of color on energy production from the panel. Our simulation utilized parameters from a RAYTECH (Hangzhou Bay New Zone, Ningbo, China) panel with the following specifications: a double-sided, transparent panel with model code BPDMJ36H(S)-215, as referenced in [10]. The transparency of these panels is 45%, and the efficiency is 12.5%. The array consisted of 115 panels, 11 in series and 5 in parallel. The software simulated their performance on surfaces mimicking building facades and systematically altered the facade color to assess its impact on energy production from these panels.

For this purpose, we adjusted Equation (6) to account for the albedo of the facade color and the effect of the transparency of the panel, yielding Equation (7) for the current generation produced by the back side of the panel.

$$I_{ph,r}=\alpha \left(I_{sc,r}+Ki\times \Delta T\right){\displaystyle \frac{Gf}{Gs}} \left(Transparent\right)\left(Albedo factor\right)$$

(7)

Equation (7) is valid, provided that the panels are installed directly on the surface (β = 0) with no diffuse irradiance. According to [8], the irradiance is transferred from surface 1 to surface 2, accounting for the albedo factor, global horizontal irradiance (GHI; the amount of irradiance incident on the horizontal surface), direct normal irradiance (DNI), and diffuse horizontal irradiance (DHI), and is calculated as follows:

$$G_{1,2}=\left(\mathrm{Albedo} \mathrm{factor}\right)\mathrm{DNI}{\displaystyle \frac{1+\cos \mathsf{\beta }}{2}}+\left(\mathrm{Albedo} \mathrm{factor}\right)\left(\mathrm{GHI}-\mathrm{DNI}\right)\left({\displaystyle \frac{1+\cos \mathsf{\beta }}{2}}-{\mathrm{F}}_{ij}\right)$$

(8)

$$F_{ij}={\displaystyle \frac{1-\cos \left(180°-\beta \right)}{2}}$$

(9)

where $\beta$ is the angle between the surfaces and $F_{ij}$ is the view factor of radiation from area A_i to area A_j. In terms of the irradiation from surface i directly to surface j, in this article, A_i is the surface area of the building, and A_j is the area of the back side of the panel, as illustrated in Figure 4. The view factor is a purely geometric quantity that is independent of heat and surface properties.

In this study, we neglected DHI because we assumed that the panel was installed directly (without a gap) on the base or front of the building; therefore, GHI = DNI. Otherwise, additional measurements or calculations would be necessary. Therefore, only DNI was considered, following Equation (7), which corresponds to (10), which is the equation for generating irradiance on the back side of a vertically installed inclined panel. Equation (10) is the current generation for the back side of the panel, combining the transparency value of the cell, the albedo factor of the base on which the panel was installed, the DNI, and the angle between two surfaces β:

$${\mathrm{I}}_{ph,r}=\alpha \left(I_{sc,r}+Ki\times \Delta T\right){\displaystyle \frac{G_f}{G_r}} \left(Transparent\right)\left(Albido factor\right)\left(\mathrm{DNI}\right)\times 0.5\times \left(1+\cos \mathsf{\beta }\right)$$

(10)

3. Simulation Results

3.1. Single-Panel Simulation

This research created a software code to simulate a solar panel, as described in [10]. Below are the simulation results for the module in [10]. Note that the radiation on the front side is always equal to the radiation on the back side. In addition, the current vs. voltage (I–V) and power vs. voltage (P–V) curves of RAYTECH were obtained under standard test conditions (STCs) when the projections were equal on both sides (Figure 5).

Figure 6 and Figure 7 show the I–V and P–V curves, respectively, from the simulation based on the STCs in [10].

3.2. Array Simulation

This section shows the simulation results for a panel array with 11 panels in series and 5 in parallel with an irradiance of 1000 W/m² on both sides. We aimed to obtain the array’s maximum power. In addition, the results are used for comparison in later sections.

Figure 8 and Figure 9 show the resulting I–V and P–V curves, respectively, where the maximum obtainable power is 11,637 W.

3.3. Array Performance on White-Coated Stainless Steel

We calculated the contribution of the back side of the 5 × 11 panel array on a building facade with white-coated stainless steel. The albedo of the white-coated stainless steel was 78.46 (i.e., albedo = 0.7846), and the transparency of the panel was 45%. The front side of the panel was exposed to different projection intensities.

According to the P–V curve in Figure 10, when the background color was a stainless-steel white coating, the maximum receivable power was 7947 W.

3.4. Array Performance on Aluminum Façade

We calculated the contribution of the back side to a 5 × 11 panel array on an aluminum facade without a coating. The albedo of the aluminum was set to 72.53%, and the transparency of the panel was 45%; the front side of the panel was exposed to different projection intensities.

Figure 11 shows that the maximum receivable power with this setup was 7759 W.

3.5. Array Performance on Stainless Steel

We calculated the contribution of the back side of a 5 × 11 panel array on a facade of stainless steel. The albedo of the stainless-steel color was set to 69.67%, and the transparency of the panel was 45%; the front side of the panel was exposed to different projection intensities.

According to Figure 12, when the background color was stainless steel without a coating, the maximum receivable power was 7671 W.

3.6. An Array of Panels on a Sloping White-Coated Stainless Steel

Installing an array of panels on a sloping stainless-steel white coating background requires the consideration of the background’s inclination. When the panels are installed at an angle to the building’s surface, the reflection of light from the background is affected. In this case, the simulation exposed the front side of the panel to different projection intensities with the panels and surfaces at an angle of 30°. Figure 13 and Figure 14 show the resulting I–V and P–V curves, where the maximum receivable power was 7780 W.

3.7. Summary of Simulation Results

Table 2. Simulation results efficiency of the color and the overall efficiency.

Material and Color	White Ceramics	Red Ceramics	Aluminum Without Coating	Stainless-Steel Without Coating	Concrete	Coral Concrete	Stainless-Steel White Coating
Albedo	0.531	0.331	0.7253	0.6967	0.3–0.4	0.34	0.7846
Power [W]	7153.19	6529.67	7759	7671.09	6433.63–6743.77	6557.89	7947.53
Percentage of maximum power	61.14%	56.111%	66.67%	65.91%	55–58%	56.35%	68.29%
General efficiency of the panel in [16]	7.64%	7.03%	8.333%	8.23%	6.8–7.24%	7.04%	8.5369%

shows the simulation results related to the efficiency of the color and the overall efficiency of the panel; as mentioned in [16], the efficiency of the panel was 12.5%.

The third row in Table 2 shows the maximum power when the radiation power on the front side was 1000 W/m², and the background was generated from a material that appears in the first row. The fourth row is the percentage ratio between the maximum power obtained when the radiation on the front side had an intensity of 1000 W/m² compared to the maximum power when 1000 W/m² was irradiated from both sides.

3.8. Color Effect

Figure 15 shows the I–V curve simulation results when the irradiance on the front sides of the panel was 1000 W/m², and the following background colors were used:

• 1—Stainless-steel white coating.
• 2—Aluminum without coating.
• 3—Stainless-steel without coating.
• 4—White ceramics.
• 5—Red ceramics.
• 6—Coral concrete.
• 7—Concrete.

Figure 16 shows the P–V curve simulation results when the irradiance on the front sides of the panel was 1000 W/m² with the following background colors used:

• 1—Stainless-steel white coating.
• 2—Aluminum without coating.
• 3—Stainless-steel without coating.
• 4—White ceramics.
• 5—Red ceramics.
• 6—Coral concrete.
• 7—Concrete.

3.9. Summary Figure and Analysis of Results

The power produced was measured under an irradiance of 1000 W/m² on the panel array from both sides under STCs, and the maximum obtainable power from this panel was 11,637 W.

As summarized in Table 2 and Figure 17, we simulated a solar panel mounted on bases of different colors when the radiation intensity was 1000 W/m² on the front side of the panel. The color of the most effective background was the white-coated stainless steel; we obtained a maximum power of 7947.53 W, which is 68.29% of the maximum power of the panel. For the background made of aluminum without a coating, we obtained a maximum power of 7759 W, which is 66.67% of the maximum power of the panel. For a background made of stainless steel without a coating, we obtained a maximum power of 7671 W, which is 65.91% of the maximum power of the panel. For a background made of white ceramic material, we obtained a maximum power of 7153.19 W, which is 61.14% of the maximum power of the panel. For a background made of coral concrete material, we obtained a maximum power of 6557.89 W, which is 56.35% of the maximum power of the panel. For a background made of red ceramics, we obtained a maximum power of 6529.67 W, which is 56.111% of the maximum power of the panel. For a background made of concrete material, we obtained a maximum power of 6743.77 W, which is 58% of the maximum power of the panel.

In addition to the above results, we carried out a test on a panel installed on a white-coated stainless-steel background at an inclination of 30° when the radiation on the front side was 1000 W/m². The maximum power was 7779.6 W, which means that it decreased by 2.4% compared to the power obtained without an angle. Therefore, an angle between the panels and the surface yields less energy.

4. Summary and Conclusions

Many studies have investigated the issue of transparent solar panels; for example, the authors in [18] investigated the installation of transparent panels in agricultural sites and the effects of various parameters on agriculture, and another study [19] used the solar-harvesting method to improve energy performance.

This work investigated and tested the installation of double-sided, transparent solar panels on backgrounds of different colors without reference to the diffuse irradiance. Additionally, we investigated and tested the effect of the background color on the production of energy from these panels, which enabled us to estimate the amount of energy that could be obtained from these panels. This study took into account two important points: light rays need to penetrate the building for lighting purposes and efficient energy production, and for this purpose, double-sided and transparent panels were chosen. Therefore, we needed to consider several important factors, including the transparency of the panels (which were also affected by the cell density) and the effect of albedo on energy production. As a result, we designed software that could simulate the installation of an array of solar panels on a surface, changing the surface color and considering the common materials and colors of buildings. The power produced by the panels was evaluated, taking into account the albedo, transparency of the panel, and the tilt angle. The simulations were accurate in terms of installing a panel directly without a gap on the facade of an existing building, and we were able to estimate the maximum energy that could be received.