Evaluating the real-world performance of vertically installed bifacial photovoltaic panels in residential settings: empirical findings and implications

Abstract

This research examines the extended performance of vertically positioned bifacial photovoltaic (BiPV) panels in actual environmental settings, considering various factors such as solar irradiance and the random surrounding structures. Two bifacial photovoltaic panel systems connected to the grid are set up on the roof of a residential structure. The first system consisted of seven panels installed at a tilt angle of 27^o, facing south. The second system comprises seven vertically installed panels facing west. A data acquisition system was employed to continuously monitor and record the electrical parameters of both systems. To quantify the performance of the systems, specific metric parameters, like the yearly energy output and the specific yield of the systems, are computed. The findings reveal that the vertically installed BiPV panels can achieve an energy yield as high as 100% compared with the tilted installation in certain months. Furthermore, the vertical installation demonstrated inherent anti-soiling properties akin to self-cleaning. Additionally, the vertical installation exhibited a multiple peak phenomenon, which could potentially alleviate the peak load issues on the electrical grid. The vertical installation also exhibited an exceptional ground coverage ratio, making it an attractive solution for space-constrained applications. The vertical installation exhibited a ~ 1678 kWh/kWp performance ratio, retaining ~82% of the tilted installation energy yield. The results underscore the feasibility and advantages of employing vertically installed bifacial photovoltaic panels in residential settings, particularly in limited areas. Moreover, the study provides insights into the viability and potential of this technology for small-scale residential applications.

1 Introduction

The rising need for eco-friendly and renewable energy solutions has amplified the focus on photovoltaic (PV) systems. Bifacial PV (BiPV) panels, among these technologies, have garnered considerable interest due to their capability to capture sunlight from both surfaces, enhance energy output, and lower the average cost of electricity [1].

Numerous nations have extensively implemented BiPV panels of late. Some instances include [2]: China [3], the United States [4], Japan [5], Spain [6], Qatar [7, 8], the United Arab Emirates, and other countries in the Middle East [9]. Other countries, including Australia, Brazil, India, and several European countries, are deploying BiPV panels on a smaller scale.

However, even though large-scale BiPV use is growing significantly, the deployment of BiPV systems in residential buildings or small-scale facilities faces resistance owing to a variety of factors, such as economic constraints, spatial limitations, maintenance challenges, technological complexity, regulatory obstacles, and the lack of mature knowledge in utilizing BiPV to benefit from their optimal potential. Despite this, bifacial implementation for small-scale residential house applications is becoming increasingly popular in Jordan.

Various studies have employed software tools such as Psys to gauge the performance of BiPV systems [10, 11]. In addition, some research efforts have turned to mathematical models to predict the energy output of BiPV systems [12]. Several studies have conducted practical experiments on the performance of BiPV systems in meticulously designed field experiments [13–15]. However, many factors, including the surrounding environment, albedo effect, and rear-side shading, influence the performance and energy yield of the BiPV panels. In light of this, our study adopts an experimental approach that evaluates BiPV systems under real-world conditions. This contrasts with studies set in controlled field experiments. We aim to assess the performance and suitability of BiPV systems for residential applications. It’s noteworthy that the majority of research on BiPV panels centers on horizontally installed configurations. This leaves the advantages of vertically installed panels and their potential benefits relatively unexplored [16].

Vertically installed BiPV (VI-BiPV) panels offer several advantages, such as reduced soiling, better self-cleaning, and lower wind loads [17, 18]. Moreover, they can be installed in areas with limited space, which makes them suitable for urban and industrial environments [19]. Despite these advantages, little research has been conducted on VI-BiPV panels for domestic houses and small-scale applications under realistic operating conditions. This may vary significantly from the idealized conditions often assumed in laboratory settings and mathematical and numerical models [20].

Despite increasing interest in VI-BiPV panels, a comprehensive understanding of the long-term factors that affect their performance under realistic conditions remains limited. Factors like fluctuating solar radiation, dirt accumulation, shadowing, and ground reflection can impact the energy output of bifacial PV panels [5].

Calculating the energy production gain is essential for evaluating bifacial PV systems’ profitability [21]. However, bifacial modules are more complex in simulations due to variations in the illumination of the rear surface [22]. In addition, compared to monofacial PV modules, the rear side of BiPV modules is more dependent on ground-reflected light [23, 24].

Numerous research efforts have delved into the performance of BiPV panels and systems [12, 25–31]. These studies have investigated different aspects of BiPV performance, such as energy yield, installation configuration, and shading loss.

In addition to our research on photovoltaic (PV) systems in Jordan [32], this article comprehensively analyzes the performance of VI-BiPV panels under authentic conditions. It takes into account factors such as varying solar radiation and shadowing effects. Through observations and findings, we aimed to elucidate the advantages of VI-BiPV panels and provide valuable insights for future applications and research. The findings from this research enhance our comprehension of the capabilities of VI-BiPV panels in actual situations, further promoting the embrace of more efficient energy alternatives.

Considering these factors, this work aims to present the long-term performance of VI-BiPV panels under realistic conditions. By analyzing our observations and results, we aim to offer meaningful perspectives on the benefits, constraints, and possible uses of VI-BiPV panels. This provides a better understanding of the practical implications of VI-BiPV panels in real-world scenarios, ultimately supporting the adoption of efficient and sustainable energy solutions.

Our research focuses on key findings from our current long-term experimental project. Instead, we mainly concentrated on the impact of soiling, energy yield, economic considerations, and the feasibility of BiPV systems. Consequently, our core findings and insights are disseminated across the following sections: Introduction, Theoretical Background and Methodology, System Installation Configurations, Results and Discussions, and Conclusions.

2 Materials and methods

BiPV panels are uniquely designed to capture solar power from both their front and rear sides, producing more energy than traditional monofacial panels. The installation orientation of the BiPV panels play a vital role in their performance. In this study, we investigate the performance of two installation configurations of BiPV panel systems by conducting practical experiments and measurements. Multiple factors, such as the tilt angle (β), elevation from the ground (H), and the azimuth angle (γ) of the panels, are taken into account to assess and compare the performance of the two PV systems, with emphasis on vertically installed VI-BiPVs. For this purpose, two solar PV configurations are established in real-world operational settings:

I. A bifacial PV system inclined towards the south.

II. A bifacial PV system vertically installed with east–west orientation.

This section outlines the experimental design, equipment, and methodologies employed to assess the performance of VI-BiPV panels. This exposition provides readers with an understanding of the practical aspects and considerations underpinning this study’s findings.

2.1 Theoretical background

Bifacial PV panels can capture light reflected or dispersed from the ground or adjacent areas on their back sides, enhancing the total energy output relative to monofacial units. A key attribute of BiPV panels is the bifaciality factor (BF). This factor represents the proportion of power output from the rear to the front of the module under standard testing conditions. Essentially, the BF is determined by the relationship between the efficiencies of the rear and front sides, as [12]:

$$\begin{equation} BF=\frac{\eta_{rear}}{\eta_{front}} \end{equation}$$

(1)

where (⁠|${\eta}_{front}$|⁠) represents the efficiency of the bifacial module’s front side, and (⁠|${\eta}_{rear}$|⁠) signifies the efficiency of its rear side under standard testing conditions. It is important to note that the actual energy output of a bifacial module is influenced not just by the BF but also by factors like the setup configuration, ground reflectivity (albedo), and the surrounding conditions [33].

Energy production is influenced by the total solar radiation absorbed by the front and back sides of the BiPV panels. While one side typically captures direct and diffused irradiance, the opposite captures reflected and diffused irradiance. As a result, to gauge the energy output of the BiPV panel, one must calculate the combined solar radiation striking both sides of the panels. Nonetheless, solar radiation can be divided into different elements. Figure 1 represents a simplified depiction of the different possible components of solar irradiance incidents on the BiPV sides.

Figure 1

Schematic depiction of the solar irradiance components incident on the sides of the BiPV panel.

Contrary to monofacial PV panels, albedo, and diffused irradiance significantly affect the overall energy production of BiPV panels. To analyze the energy output of the BiPV panels, it is essential to compute the cumulative solar radiation received by both faces of the BiPV panels. As depicted in Figure 1, various forms of solar radiation are crucial for examining the energy production of BiPV panels. These are outlined briefly below:

a. The global horizontal irradiance (GHI): this represents the total irradiance obtained from above by a surface that’s level with the ground. This measurement encompasses Direct Normal Irradiance (DNI) and Diffuse Horizontal Irradiance (DHI).

$$\begin{equation} GHI= DNI\times \mathit{\cos}\left(\theta \right)+ DHI \end{equation}$$

(2)

where (θ) symbolizes the angle of the solar zenith.

b. Direct normal irradiance (DNI): this is the solar radiation incident per unit of surface area consistently oriented to the sun’s rays at a right angle. This light originates directly from the sun’s present location in the sky along a linear trajectory.

c. Diffuse horizontal irradiance (DHI): the solar irradiance, per unit area, that incident on a consistently horizontal surface, accounting for the solar radiation dispersed by the atmosphere.

d. Plane of array irradiance (POA): this is the irradiance incident on the PV array plane. It includes direct, diffuse, and reflected irradiance components, typically essential for BiPV system performance analysis. The bifacial POA, on the front and rear sides can be approximated as [34]:

$$\begin{align} BPOA&= DNI\times \cos (AOI)+ DHI\times \frac{1+\cos \left(\beta \right)}{2}\nonumber\\&\quad+ GHI\times \rho \times \frac{1-\cos \left(\beta \right)}{2}+ GHI\times \rho \times \frac{1+\cos \left(\beta \right)}{2} \end{align}$$

(3)

where AOI is the angle of incidence, β is the tilt angle, and ρ is the ground albedo. These equations allow for calculating different types of solar irradiance depending on the sun’s position in the sky and the surface orientation. Equation (3) outlines the solar irradiance components. Given this, the Energy Yield (EY) for BiPV can be expressed as:

$$\begin{equation} EY=\left[{I}_{front}+\left({I}_{rear}\times BF\right)\right]\times \eta \times A \end{equation}$$

(4)

where (⁠|$\eta$|⁠) represents the nominal efficiency of the BiPV module, A denotes the area of the panel surface, BF is the panel bifaciality factor, |${I}_{front}$| is the cumulative irradiance on the front side and |${I}_{rear}$| is the cumulative irradiance on the rear side.

While these equations serve as a solid predictive model that predicts the performance and, hence, the energy yield of BiPV panels based on their installation angles, they fall short in complex settings where BiPV panels are placed in non-standard environments with multiple reflections and varying albedos. Therefore, an extended study in actual operational conditions, like ours, is essential.

2.2 Performance parameters

Key performance indicators are vital for assessing the efficiency of PV systems. Typically employed metrics for such evaluations include annual energy, performance ratio, specific yield, capacity utilization factor, and ground coverage ratio. A concise overview of each parameter is as follows:

a. The annual energy parameter represents the total output energy produced by the PV panels over a year. This provides a measure of the energy production of the system.

$$\begin{equation} {E}_a={I}_{dc}\times{V}_{dc}\times t \end{equation}$$

(5)

where |${E}_a$|denotes annual PV energy, |${I}_{dc}$| denotes DC current, |${V}_{dc}$| denotes DC voltage, and |$t$| denotes time [37].

b. The performance ratio (PR) is a metric that assesses the impact of temperature, inefficiencies, and malfunctions on a PV system’s output. It quantifies the ratio of the yearly energy yield to the product of the total annual plane of array (POA) radiation and the module’s efficiency percentage. The PR helps gauge the system’s performance relative to its rated capacity, accounting for shading and soil accumulation factors given by [38].

$$\begin{equation} PR=\frac{ Annual\ Enery\ yield\ (kWh)}{Annual\ POA\ to\mathrm{t} al\ radiation\ (kWh)\times module\ efficiency\ } \end{equation}$$

(6)

c. Specific yield: this is used to compare various sites, analyze various designs, and evaluate the viability of an array and measured in kWh/kWp as [35]:

$$\begin{equation} {Y}_s=\frac{Enery\ yield}{Module\ capacity} \end{equation}$$

(7)

d. Capacity utilization factor (CUF): is commonly employed to estimate the efficiency of a PV facility [36]. It is determined by comparing the actual output energy of a PV facility to its optimal output under perfect conditions over a year. CUF is usually presented as a percentage [37]:

$$\begin{equation} CUF=\frac{Actual\ energy\ from\ plant(kWh)\ }{Module\ \mathrm{Plant}\ \mathrm{capacity}\left(\mathrm{kWp}\right)\times 24\times 365} \end{equation}$$

(8)

e. Ground coverage ratio (GCR): in PV systems, the GCR represents the proportion of the photovoltaic array’s area relative to the entire land area [38]. Figure 2 provides a comparative illustration of ground coverage ratios for BiPV panels installed in two different orientations: (a) horizontal and (b) vertical.

Figure 2

Illustration comparing ground coverage ratios for (a) horizontally and (b) vertically installed BiPV panels, highlighting the spatial implications of each configuration.

The diagram delineates the spatial implications of each configuration, offering a visual understanding of how the orientation of the BiPV panels can significantly impact the area they cover on the ground. This comparison is crucial for optimizing space utilization and maximizing solar energy capture in architectural designs. Figure 2 demonstrates the importance of considering the installation orientation of BiPV panels in the early stages of building design to ensure the efficient use of space and energy resources.

2.3 PV panel selection

The types of BiPV panel selected for this experimental work are becoming popular and are among the most readily available in the local market. These panels can be considered representative of other types of BiPV modules. Table 1 provides the key specifications of the BiPV panels as follows:

Table 1

The specifications of the BiPV panels, Model: PS-M72(BF)-370 .

Cell type	Monocrystalline PERC
Dimensions	1968 x 990 x 35 mm
Glass type	High-transparency, low-iron, tempered glass (front and rear)
Weight	22.5 kg
Rear-side efficiency	Up to 75% of front-side efficiency
Cell arrangement	6 x 12 (72 cells)
Frame	Anodized aluminum alloy
Maximum power (Pmax)	370 W
Front-side efficiency	19.0%
Maximum power voltage (Vmp)	40.60 V
Maximum power current (Imp)	9.12 A
Open-circuit voltage (Voc)	48.36 V
Short-circuit current (Isc)	9.7 A
Temperature coefficient	−0.36%/°C

3 Experiment setup

Two different systems are installed on the rooftop. The first system consists of seven BiPV panels facing south with a tilt angle of 27°. The second system is composed of seven BiPV panels installed vertically, with a tilt angle of 90°, facing west. Figs 3 and 4 visually represent these systems, which are situated on a residential rooftop at latitudes of 32.5°N and 36.0°E in Jordan.

Figure 3

(a) A side perspective of the BiPV panels’ installation setup, oriented towards the south. (b) A three-dimensional representation of the south-facing BiPV panels arranged in a tilted configuration. (c) A photograph showcasing the actual installation of the panels on a rooftop.

Figure 4

(a) Illustration of the front-side view of the vertically installed BiPV panels. (b) Side perspective of the same installation. (c) A photograph capturing the actual system installed on the rooftop. (d) A detailed depiction of the installation process and setup.

To evaluate the performance of the vertical installations, we established an experimental system incorporating multiple BiPV panels in a vertical layout. This framework facilitates long-term monitoring of panel performance under various realistic conditions. Moreover, our system’s meticulous design and implementation offer an accurate assessment of the energy yield and the other performance indicators of the BiPV panels when installed vertically.

3.1 Horizontally installed configuration

Figure 3 depicts that the horizontally oriented system consists of seven BiPV panels installed with a southern-facing tilt angle of 27°. The base of these panels is elevated to a height of 1.2 meters, a design choice that significantly enhances the reflected irradiance on the rear side [12]. To maximize the benefits of bifaciality, the support framework is thoughtfully designed and constructed to avoid any hindrance to solar radiation on the backside of the panels. This strategic design ensures optimal exposure and maximizes the potential benefits of bifacial panel technology.

It is essential to highlight that the conventional approach of installing BiPV panels in a horizontal configuration using a standard support structure often fails to fully exploit the advantages of bifaciality. Such an installation method typically results in a marginal enhancement of the BiPV performance over its monofacial counterpart, with an increase of less than ~5%. This figure is derived from a comprehensive statistical analysis of systems within the local area, not included in this study. Consequently, the unique structure provided in this study for horizontally installed BiPV (HI-BiPV) panels ensures the complete utilization of bifacial benefits, thereby optimizing the performance of the BiPV system.

3.2 Vertical installation configurations

The system comprises seven BiPV panels installed vertically and facing —east–west, 90° tilt angle, and 270° azimuth angle, as demonstrated in Figure 4. The panels are installed on the rooftop at the height of 1.2 m above the roof surface, as shown in Figure 4(a). The seven panels are connected as a string to one of the inverters’ maximum power-point tracking ports. The holding structure is composed of stainless steel, designed to ensure minimum shading to the panels. It is worth mentioning that the holding structure, in terms of mass and cost, is less than that of the typical horizontally installed structure. Stainless steel balustrades were added to provide high stability under wind burst loads. A gap is created between the vertical PV panels to provide a lower wind load, as shown in Figure 4(c). In a concurrent study not presented within this manuscript, we have ascertained that the gaps mentioned above are superfluous when incorporating stainless steel balustrades.

In this arrangement, solar irradiance can be regarded as direct exposure impacting both the rear and front facets of the panels, contingent upon the time of day. Conversely, with HI-BiPV panels, the front side predominantly receives direct solar irradiance throughout the day and year, except during the early morning and late afternoon hours on summer days.

Acknowledging that the environmental conditions depicted in Figure 1 represent an idealized scenario is crucial. As demonstrated in Figure 4, the terrain and surroundings can be highly irregular and exhibit varying albedo values. Consequently, the approximations made in the mathematical models may not precisely capture the behavior of BiPV panels in such intricate environments. This underscores the necessity for empirical in-field studies, which is the focus of this study.

3.3 Data collection

In this study, data are collected using various measurement devices. This section briefly describes the data collection instruments used along with their specifications.

3.3.1 Irradiance measurements

The pyranometer measures global horizontal irradiance (GHI), which includes direct and diffuse sunlight falling on a surface. The pyranometer logs the data to National Instruments’ CompactRIO, a rugged, high-performance control and automation system for applications requiring high performance and reliability [39]. CompactRIO logs these data, stores them onboard, and sends them to another system for further processing, analysis, or storage. LabVIEW software analyzes the data, looks for patterns or trends, and controls other systems based on incoming solar irradiance data. The supplementary material in Appendix (C) shows the CompactRIO and a sample of the LabVIEW code used to monitor and analyze the logged data. The frequency of the irradiance measurements is flexible, thanks to the CompactRIO capabilities, and is selected as a sample every minute. The pyranometer specifications are presented in Appendix (B).

3.3.2 Electrical input/output measurements

A Fronius inverter with an integrated data logging system facilitates electrical output measurements. This mechanism is configured to secure readings at consistent five-minute intervals, contributing to forming a comprehensive dataset that promises regularity and detail.

The data logging feature of the inverter is engineered to document all measurements using a dual-modality data-storage approach. The merits of local data storage include ensuring data integrity and safeguarding against potential losses that may arise during Internet disruptions. Concurrently, online storage provides the benefit of remote data access and creates the potential for real-time monitoring, thereby enhancing the overall accessibility and versatility of data usage. The Fronius inverter can measure various parameters, including the DC voltage and current inputs, AC voltage and current, and generated power. These values are logged every five minutes and stored locally and via the Internet.

4 Results and discussion

Continuous biennial performance outcomes have been documented, encapsulating the energy yield and solar irradiance data points. The subsequent subsections represent and discuss these measurements, illustrating daily, monthly, and yearly performance under varying conditions and presenting samples of these measurements. The supplemental resources, Appendix (B), contain the daily, monthly, and yearly energy yield throughout the experimental timeframe.

4.1 Energy yield and performance

In the subsequent sections, we present the energy yield and the performance of Bi-PV panels, focusing on daily and monthly variations and examining trends over extended periods.

4.1.1 Daily energy yield and performance

The unique multi-peak characteristic of vertically installed bifacial photovoltaic (VI-BiPV) panels has been a focal point in numerous theoretical analyses, predicting a symmetrical power profile for such vertically oriented BiPV modules [24, 40]. Through the defined mathematical framework (Equations 1–3), we modeled the power output profile of BiPV panels, enabling the creation of a power profile that correlates power output with daily time. Figure 5 illustrates the power profile of BiPV systems under two distinct orientations: horizontally with a southern inclination and vertically oriented in an east–west direction, providing a normalized power profile that allows a comparative view of the performance dynamics of BiPV installations in these configurations.

Figure 5

Normalized power profile of BiPV panel installations. The power performance of BiPV systems in two orientations: horizontal (tilted south) and vertical (east–west). The analysis assumes a bifaciality factor of 100% under clear sky conditions on a specific day in the year and location on Earth.

However, despite our practical observations revealing a similar trend, we noticed an asymmetrical profile, as shown in Figure 6. The asymmetrical profile can be attributed to two key factors: the bifaciality factor (BF), which was 75% in our case, as defined in Equation (1), and the shading caused by the module’s frame. Multiple peaks in the power profile of VI-BiPV modules offer significant advantages in load distribution and peak power management. This can help to avoid the power concentration typically observed in HI-BiPV panels, which has important implications for grid stability.

Figure 6

Samples of the measured daily power profile of the Hi-BiPV and VI-BiPV systems under different weather conditions several times a year.

Figure 6 illustrates the measured daily performance of the BiPV panels installed in horizontal and vertical orientations across different seasons. This representation provides insight into how environmental conditions and seasonal variations significantly influence the energy yield of both configurations. The plot legend in Figure 6 shows each system’s daily energy yield, allowing a day-to-day performance comparison. The inset in the graphs shows the total daily energy yield for both systems, indicating the overall performance in terms of energy yield. The subfigures in Figure 6 represent the performance from sample days in various seasons under various conditions. The complete data gathered during this long-term experimental work are presented in the figures in Appendix (A).

The measured GHI is superimposed on the graph, highlighting the correlation between solar irradiance and energy yield. Notably, the power profile of the vertically installed BiPV modules exhibited a distinct pattern characterized by multiple peaks. This contrasts with the power profile of the horizontally installed modules, which typically concentrate power at peak times, usually at noon.

On sunny days, the power profile became more pronounced, revealing the impact of sunlight intensity on energy yield. The total energy yields of the VI-BiPV and HI-BiPV modules varied depending on the season. For instance, the energy yield from HI-BiPV modules on sunny winter days surpasses that of VI-BiPV modules despite the overall lower energy availability in winter than in summer. Conversely, during the summer months (May, June, and July), the VI-BiPV modules yielded more energy than the HI-BiPV modules.

Similar trends in the power generation performance of both systems are observed on partially cloudy days, with both systems producing comparable energy yields, as depicted in Figure 6(d). For the HI-BiPV, there is a notable utilization of solar irradiation during the mornings and afternoons on summer days, which could be highly beneficial for specific applications.

The VI-BiPV system displays a noticeably superior performance during numerous days in May, June, and July. This enhanced efficiency is explicitly illustrated in the power profile samples, as indicated in Figure 6. Further details regarding the daily performance can be found in Appendix (A.1) of the supplementary material. However, during winter, the horizontally installed BiPV (HI-BiPV) system proves more effective than the VI-BiPV system. This superior winter performance of the HI-BiPV system is well-demonstrated in Figs 6(e) and (f). Therefore, even though the VI-BiPV system can outperform the HI-BiPV during certain months, the overall performance is contingent on seasonal changes. As expected, the daily performance varies from fully sunny to partially cloudy days. Moreover, as expected, the performance on sunny days also varied between the seasons. A critical aspect to consider is the differential energy yield of the Vi-BiPV and HI-BiPV panels during the summer and winter. As indicated in Figure 6 and Appendix (A), the energy yield of the VI-BiPV panels is equivalent to or sometimes surpasses that of the HI-BiPV panels during the summer.

Conversely, during winter, the energy yield of the VI-BiPV panels is lower than that of the HI-BiPV panels. Although it might at first seem like a performance shortfall, it is crucial to recognize that the increased energy output in the summer is due to particular reasons, detailed in the subsequent section.

The incident angle of light, frame shading, and bifaciality factor emerge as pivotal physical determinants that render VI-BiPV systems inferior to HI-BiPV in wintertime. The VI-BiPV’s susceptibility to suboptimal light angles and potential shading, coupled with the nuanced implications of BF on energy yield, collectively compromise its comparative performance, especially in contexts where optimal, direct sunlight exposure is paramount for maximizing photovoltaic efficiency.

Despite these factors, in winter, shorter daylight hours, lower solar irradiance, and frequent overcast skies can distort the energy yield percentage, making it an unreliable overall performance measure. Therefore, a comprehensive understanding of these seasonal variations is crucial to assess the performance of the BiPV systems.

4.1.2 Monthly energy yield and performance

The cumulative monthly energy output is depicted in Figure 7 below. The legend in the graph shows the total energy yield over the whole month in kWh. In late spring, summer, and early autumn, the energy yield of the VI-BiPV panels is close to that of the HI-BiPV panels, and sometimes it surpasses it. This is due to the VI-BiPV panels harnessing the direct solar rays more effectively during the early morning and late afternoon. After all, the early morning and late afternoon solar irradiance is directly incident on one of the panel surfaces. On summer days, the HI-BiPV can capture direct sunlight on its rear side in the early morning and late afternoon. However, this solar irradiance would be less effective than that in the VI-BiPV panels because of the low solar irradiance and the far normal incidence angle on the rear side of the panel. This phenomenon can be seen in May and June, as shown in Figure 7, where samples of the monthly energy yield for the winter, spring, summer, and autumn are shown. The total energy yield over each month is inserted as an inset in the plot with a vertical-to-horizontal yield percentage. Appendix (A.2) of the supplementary material contains the monthly performance over the experiment period.

Figure 7

Comparative analysis of seasonal energy yield: this figure illustrates the energy output during summer and winter months across various years, providing a representative sample of seasonal variations in energy production.

Table 2 compares the monthly and specific energy yields of two installed systems. It reveals that both systems perform comparably, with the HI-BiPV exhibiting performance nearly equivalent to that of the VI-BiPV from May through September. The energy yield of the VI-BiPV is remarkably high, reaching up to 100% of the HI-BiPV’s daily yield in May through October. However, during winter, the VI-BiPV’s yield declines to as low as 62% compared to the HI-BiPV. This decrease is anticipated and can be attributed primarily to the shading of the frame panels on the rear side. Additionally, the sun’s lower elevation and azimuth angles during winter in the experiment’s region exacerbates the shading effect on the VI-BiPV panel. Notably, the panels’ frame also contributes to shading, even in summer, but with a less pronounced impact on the VI-BiPV’s overall performance.

Table 2

Comparison between the performance of the vertical and horizontal BiPV systems.

Month	The ratio of energy yield vertical/tilted	The mean daily specific yield [kWh/kWp] VI-BiPV	The mean daily specific yield [kWh/kWp] HI-BiPV
June/2021	~93%	6.23	6.70
July/2021	~97%	6.51	6.70
Aug/2021	~99%	5.78	5.83
Sep/2021	~93%	5.17	5.60
Oct/2021	~79%	4.22	5.60
Nov/2021	~68%	3.33	4.90
Dec/2021	~ 65%	2.47	3.82
Jan /2022	~63%	2.43	3.90
Feb/2022	~70%	3.11	4.47
Mar/2022	~68%	3.31	4.88
April/2022	~80%	4.00	4.95
May/2022	~88%	5.79	6.63
June/2022	~96%	4.81	6.52
July/2022	~99%	6.23	6.31
Aug/2022	~90%	5.97	6.66
Sept/2022	~90%	4.67	5.16
Oct/2022	~82%	3.66	4.48
Nov/2022	~63%	2.83	4.45
Dec/2022	~65%	2.72	4.18
Jan/2023	~65%	2.59	4.00
Feb/2023	~68%	3.26	4.83
Mar/2023	~75%	4.19	5.59
April/2023	~82%	4.99	6.07
May/2023	~90%	5.82	6.48

4.1.3 Yearly energy yield

Figure (9) presents the energy yield accumulated over a two-year experimental period. The energy generation profile throughout the year is consistent with expected patterns. The data, spanning July 2021 to July 2023, offers definitive insights into the system’s long-term performance.

The VI-BiPV panels generate higher energy yields during summer than the HI-BiPV panels, as depicted in Figure 8. Specifically, in May, June, and July, the energy yield of VI-BiPV panels exceeds that of HI-BiPV panels. In contrast, VI-BiPV panels yield less energy than HI-BiPV panels during winter.

Figure 8

Monthly energy yield analysis of HI-BiPV and VI-BiPPV systems: this figure presents a month-by-month breakdown of the energy yields for both the HI-BiPV and VI-BiPPV systems, spanning the duration of the experimental period, 24 months.

Over the 24 months, the VI-BiPV panels achieved a relative energy yield of approximately 82% compared to HI-BiPV panels. This yield is comparable to, or even surpasses, monofacial PV panels. The lower energy yield of VI-BiPV relative to HI-BiPV is attributed to bifacial factors and self-shading from the panel frames.

This in-depth analysis provides a basis for making informed decisions on deploying bifacial PV panels. It emphasizes the significance of installation configurations in optimizing energy yields.

The VI-BiPV panels exhibit an annual specific yield of ~1673 kWh/kWp, which is on par with the typical yield of monofacial PV panels in Jordan [36]. Notably, in this study, the optimally installed HI-BiPV panels achieve an annual specific yield of ~2000 kWh/kWp, marking a 20% improvement over the VI-BiPV. This enhancement, relative to VI-BiPV panels, is ascribed to the self-shading caused by the panel frames and a bifacial factor of 75%, as previously discussed. Based on Equations (6 and 8), PR and CUF for the VI-BiPV were determined to be approximately 81% and 19%, respectively. These values are derived from two years of long-term experimental data.

4.2 Inverter loading

The inverter loading ratio (ILR), also called the DC/AC ratio, indicates the relationship between a PV system’s DC nameplate capacity and its associated inverter’s AC nameplate capacity. Implementing bifacial modules can increase energy yield, which elevates the DC/AC ratio due to more significant DC power generation per module compared to their monofacial counterparts possessing equivalent nameplate power ratings.

However, it is critical to note the downside of this enhanced efficiency, which includes increased clipping losses or potential wasted energy during peak production periods. In most instances, the benefits of the additional yield from bifacial production offset these losses, validating the shift to bifacial technology.

Peak clipping in PV power profiles denotes a phenomenon that transpires when the power generated by a PV system surpasses the inverter’s capacity. Inverters are engineered to convert the DC power produced by solar panels into AC power appropriate for grid or domestic consumption. However, each inverter possesses a maximum power threshold it can manage. If the PV system’s power output exceeds this threshold, the inverter will limit or ‘clip’ the output power to its maximum capacity. This creates a flat peak on the power output curve, hence the term ‘peak clipping’. The power generated by the PV system that is not transformed by the inverter is effectively wasted, which can diminish the total efficiency of the PV system.

Therefore, the potential increase in energy output from HI-BIPV panels may be limited due to the operational characteristics of the inverter, which can result in power clipping, especially during peak energy production periods. Figure (9) visually depicts the increased energy yield from bifacial modules contrasted with clipping losses. The total energy clipped is represented by the area under each respective curve. Fundamentally, a bifacial system with a lower DC/AC ratio could achieve equivalent performance to a monofacial system with a higher DC/AC ratio, implying that bifacial modules provide a superior return on investment.

Figure 9

Comparative analysis of power profiles in bifacial and monofacial PV systems over time. This figure illustrates the normalized power outputs from (a) inclined bifacial and equivalent tilted monofacial systems and (b) inclined and vertically positioned bifacial systems, highlighting the occurrence of clipping loss in each setup.

In the case of the VI-BiPV panels, inverter overloading and peak clipping are circumvented, as multiple peaks manifest below the clipping threshold. This is evident in Figure 9 and is further corroborated by empirical testing, which shows no peak clipping for VI-BiPV panels throughout the study, especially during summer.

4.3 Anti-soiling property

An exciting and significant feature of the VI-BiPV is the anti-soiling property. Figure 10 presents images captured for the VI-BiPV and the HI-BiPV panels at the exact location six months post-installation. The unambiguous contrast in soiling accumulation between the two systems is a striking observation from the images. The VI-BiPV panels exhibit remarkable resilience to soiling, appearing almost pristine, whereas the HI-BiPV panels are visibly marred by heavy soiling.

Figure 10

Photographic depiction of panel soiling in both systems: (a) displays the soiling on the HI-BiPV system with tilted panels oriented southward; (b) showcases the soiling on the VI-BiPV system (right side of the image) alongside the tilted panels (left side of the image).

The implications of this observation are profound within the realm of photovoltaic energy generation. Soiling, which refers to the accumulation of dirt and debris on the surface of photovoltaic panels, is a well-documented impediment to optimal energy yield. It attenuates the incident solar radiation, thereby curtailing the panels’ electrical output. Consequently, frequent cleaning interventions are necessitated to sustain the projected energy yield. However, these cleaning processes are not without cost and contribute to the overall levelized cost of electricity of a photovoltaic system, which is a critical metric for evaluating the economic feasibility of solar installations.

In light of these considerations, the inherent resistance of VI-BiPV panels to soiling emerges as a highly advantageous attribute. By substantially reducing, if not altogether eliminating, the need for regular cleaning, the adoption of vertically installed configurations can yield significant operational efficiencies. This can positively influence the levelized cost of electricity, enhancing the economic feasibility of solar setups. Moreover, it alleviates the logistical challenges associated with maintenance, particularly in regions where accessibility is a constraint.

In conclusion, the implementation of VI-BiPV panels represents a promising avenue for mitigating the detrimental effects of soiling on photovoltaic systems and warrants further investigation and consideration in the design and deployment of future solar installations.

4.4 Implications for BiPV panel installation

Bifacial photovoltaic (PV) panels represent a significant advancement in solar technology, primarily due to their ability to capture sunlight on both their front and back sides, leading to increased energy production compared to traditional monofacial panels. Nevertheless, the way these BiPV panels are installed plays a crucial role in tapping into these benefits.

It's crucial to understand that if not installed correctly, the dual-sided nature of these panels might not offer the expected advantages. Thus, careful planning and design are essential when setting up BiPV panels to make the most out of bifacial technology. One effective approach is mounting the panels vertically. This setup helps avoid the problem of shadowing from the structure itself, which can significantly reduce energy generation, thereby making the most of the bifacial panels' capabilities.

However, opting for a vertical layout comes with its own set of challenges, such as ensuring the panels can withstand wind and maintain structural stability. Addressing these issues requires detailed technical advice and the creation of thorough guidelines for installation, maintenance, and safety. When planning the installation, factors like local wind conditions, structural strength, and maintenance ease should be carefully considered.

Beyond performance enhancements, vertically mounted BiPV panels can also improve the visual appeal of the site. Their sleek, contemporary look can be seamlessly integrated into architectural designs or used in practical applications like fencing. This aspect is especially useful in areas where maximizing land use is crucial, and blending functionality with design appeal is greatly appreciated.

In summary, although bifacial PV panels promise higher energy outputs, achieving these gains largely depends on how they're installed. Vertical mounting stands out as a feasible strategy, but it demands careful attention to details such as wind resistance, structural stability, and upkeep. Moreover, the aesthetic and functional flexibility of vertically installed panels offers additional benefits, enhancing their value beyond just energy efficiency.

4.5 Future research

While bifacial PV panels and their vertical installation present promising opportunities for enhancing energy yield, certain limitations and areas warrant further research.

One of the primary limitations is the absence of mature technical guidelines for installation companies. There is a noticeable lack of clarity regarding the dos and don’ts of installing bifacial PV panels, particularly in vertical configurations. This lack of standardized instructions can lead to inconsistencies and inefficiencies in installation practices.

A specific area that requires attention is the study of wind loading on the VI-BiPV panels and the design of appropriate support structures. While this study does not delve into the intricacies of wind loading, future research must develop comprehensive guidelines and formulas that can aid installers in designing robust and reliable support structures.

Another limitation pertains to regulatory constraints, especially concerning the height of vertically installed panels on the rooftops of residential apartments. Vertical installation is an attractive solution for deploying solar PV systems in apartments with limited space. However, in some jurisdictions, regulations may restrict such installations due to aesthetic considerations, particularly in urban areas. Understanding local regulations clearly and working towards regulatory frameworks that balance aesthetic concerns with the benefits of renewable energy deployment is essential.

Vertical installation is more feasible for individual residential houses as it is less likely to be constrained by stringent regulations or aesthetic considerations.

In conclusion, while the VI-BiPV panels hold promise, addressing the technical and regulatory limitations through rigorous research and collaboration with stakeholders is essential. Future research should focus on developing technical guidelines, studying wind loads, and engaging with regulatory bodies to create an enabling environment for the widespread adoption of this technology.

The energy yield from VI-BiPV panels is presently less than that of HI-BiPV panels. To enhance energy yield, two primary methods can be employed: first, by using frameless BiPV panels, which eliminates the shading from frames and thereby increases energy output, and second, by adopting a higher BF, a change that can notably boost the energy yield, as inferred from Figure 7.

5 Conclusions and recommendations

The exploration of building-integrated photovoltaic (BiPV) panels, specifically focusing on vertical integration (VI-BiPV) and horizontal integration (HI-BiPV) configurations, has unveiled a spectrum of findings that not only underscores the potential of these technologies but also illuminates pathways for their optimized deployment in various operational settings. This study, spanning a period of two years, meticulously analyzes the performance, energy yield, and additional properties of the two BiPV systems, one vertically aligned and the other horizontally inclined to the south, in real-world operational settings. The findings and observations derived from this study are pivotal, providing practical solutions to prevalent challenges in deploying PV systems, such as peak loading, soiling, and applications in limited-space scenarios.

5.1 Key findings

The findings of this study illuminate the multifaceted performance and additional properties of VI-BiPV and HI-BiPV panels, providing a comprehensive understanding of their respective efficiencies, anti-soiling characteristics, and installation prerequisites. The key findings are as follows:

• Energy yield and performance:

• VI-BiPV panels exhibit multiple peak power profiles, enhancing load distribution and grid stability.

• VI-BiPV panels demonstrate superior performance in the summer, while HI-BiPV panels prevail in winter.

• Seasonal changes and weather conditions influence the overall performance of both systems.

• VI-BiPV panels mitigate inverter overloading and peak clipping, particularly during summer.

• Anti-soiling property: VI-BiPV panels demonstrate notable resistance to soiling, thereby minimizing maintenance requirements.

• Proper installation: the efficacy of BiPV panels is significantly influenced by their installation, emphasizing the necessity of adherence to optimal installation practices.

• Vertical installation: vertical installation offers benefits such as reduced holding structure shading but necessitates mechanical stability and wind tolerance considerations.

In summary, VI-BiPV panels, characterized by their anti-soiling property and distinctive power profile, emerge as a promising frontier in solar energy generation.

5.2 Limitations and future research

Despite the promising findings, the study acknowledges several limitations and areas warranting future research:

• The absence of technical guidelines for installing VI-BiPV panels poses a significant challenge.

• Regulatory constraints in certain regions may impede the widespread adoption of BiPV technologies.

• Future research avenues may explore potential enhancements, such as using frameless BiPV panels and augmenting the bifaciality factor.

5.3 Recommendations

In light of the findings and acknowledged limitations, the following recommendations are proposed:

• The performance of BiPV panels can potentially be enhanced by using frameless panels coupled with a high bifaciality factor.

• There is a palpable need to develop comprehensive technical guidelines and robust stakeholder engagement to foster the adoption and optimized deployment of BiPV technologies.

In conclusion, the insights derived from this study pave the way for not only harnessing the potential of VI-BiPV panels but also for navigating the challenges and limitations inherent in their deployment, thereby contributing to the advancement of sustainable energy solutions.

Author contributions

Omar AL-Zoubi (Conceptualization [Lead], Data curation [Lead], Formal analysis [Lead], Funding acquisition [Lead], Investigation [Lead], Methodology [Lead], Project administration [Lead], Resources [Equal], Software [Lead], Supervision [Lead], Validation [Equal], Visualization [Equal], Writing—original draft [Lead], Writing—review & editing [Lead]), A.H. AL-Zoubi (Data curation [Equal], Software [Equal], Validation [Equal]), Hamza Al-Tahaineh (Supervision [Supporting], Validation [Equal], Visualization [Equal], Writing—original draft [Equal]), Rebhi Damseh (Validation [Equal], Visualization [Equal], Writing—original draft [Equal]), S. Odat (Visualization [Supporting], Writing—original draft [Supporting]), and B. Shbool (Visualization [Supporting], Writing–original draft [Supporting])

Appendix A: comprehensive analysis of daily power profiles, monthly, and annual energy yield

Appendix A: comprehensive analysis of daily power profiles, monthly, and annual energy yield

A.1. The daily power profile, with daily energy yield for both systems as inset in the plot

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

A.2. The monthly energy yields

 

 

 

 

 

A.3. The yearly energy yield

 

B. Appendix (B): the specifications of the pyranometer

Appendix (B): the specifications of the pyranometer

Table 3

The pyranometer specifications .

Specification	Description
Model	CMP11
Spectral range	285 to 2800 nm
Temperature dependence (-10-40 |$ ^{\textrm{o}}$|C)	|$ < $| 1%
Uncertainty (W/m2)	|$ < $| 5
Non_linearity (100 to 1000 W/m2)	|$< $| 0.5%
Response Time (seconds)	5

C. Appendix (C): data acquisition system

Appendix (C): data acquisition system

C.1. Image of the National-Instrument Compact Roi data logging system

 

C.2. Sample of the LabVIEW program code implemented for data logging, saving, and monitoring

 

C.3. Image of the front panels showing the real-time monitoring of system parameters