Performance Investigation of Tempered Glass-Based Monocrystalline and Polycrystalline Solar Photovoltaic Panels

Solar photovoltaic (PV) converts sunlight into electricity and is an appropriate alternative to overcome the depletion of conventional fuels and global warming issues. The performance of a PV panel may vary with respect to PV cell technology, fabrication methods, and operating conditions. This research aims at performing an experimental study to investigate the electrical performance of novel tempered glass-based PV panels using two different types of solar cells: monocrystalline and polycrystalline. Tempered glass-based panels are modified forms of commercial PV panels, in which ethylene-vinyl acetate (EVA) and Tedlar are not utilized. This new fabrication method was carried out in this research. Real-time data recordings regarding the PV electrical characteristics (I-V curve) and solar irradiance were conducted under Malaysian weather conditions on clear sunny days. Results indicated that, at solar irradiance of 900W/m, the outputs from the fabricated polycrystalline and monocrystalline PV panels were 67.4W and 75.67W, respectively. However, at the highest average solar irradiance (634.61W/m), which was obtained at 12:30 PM, the outputs from both panels were 47.87W and 54.89W. An I-V curve was obtained for the real-time weather. The electrical efficiencies of the two PV panels were analyzed to be 10.54% and 12.23%.


Introduction
Global warming is currently one of the most concerning issues caused by greenhouse gas (GHG) in the atmosphere. Generally, there exists a direct relationship between global warming and energy consumption. As stated by the U.S. Energy Information Administration 2017, world energy consumption would escalate by 5% between 2016 and 2040 [1]. Energy can be produced from different means, yet the most compromising ones are renewable energies. Renewable energies are considered clean and sustainable energy sources that are utilized to minimize both secondary waste and environmental impacts by providing an excellent opportunity for GHG mitigation via substituting conventional fossil-burning energy [2].
The adoption of clean, alternative, and sustainable energy sources is being driven by energy demand and environmental concerns in the globe [3]. Solar energy is one of the examples where energy is free and direct and that both sunlight and heat are absorbed and employed by humans and the environment. Solar energy can be applied with various benefits. In solar thermal collector (STC) systems, the energy from the sunbeam is right transformed into heat or thermal energy, one form of which can be applied as industrial process heat [4,5]. In solar photovoltaic (PV) systems, moreover, solar energy is generated to electricity via solar PV cells, which are the minimal part of solar PV panels. Solar PV module or panel produces direct current (DC) electricity when the solar rays strike the surface of the panel.
The photovoltaic cell, which is the fundamental component of PV panels, may be categorized as crystalline silicon, thin-film, organic and polymer, hybrid PV cell, dye-sensitized, and new technologies including carbon nanotubes (CNT), quantum dots, and hot carrier solar cells. In the crystalline silicon group, cell technologies include monocrystalline, polycrystalline, and gallium arsenide (GaAs). In the amorphous silicon group, however, the cell technologies consist of single, double, and triple junction types. Figure 1 shows the chart of different PV cell technologies.
At the time of their review, Tyagi et al. [6] concluded that monocrystalline and polycrystalline PV technologies were the most popular, having more than 40% share of the market with an efficiency range of 15%-17%. In terms of cost, polymer solar cells were considered as a predominant challenger in production compared to crystallinebased solar cells. Not only it has a lower cost but it could also be produced at a massive rate [7]. Besides the cell technologies, operating temperature (cell temperature), solar irradiance, humidity, and dust accumulation also significantly impact the performance of the PV panels [8,9]. A review of polymer materials for PV in which epoxy was considered was also studied by Gorter and Reinders [10]. In their research work, Gorter and Reinders mentioned that epoxy and glass fiber-reinforced polymer (GFR) epoxies were great alternatives for reducing the weight and cost of PV panels. These polymers offer a high strength to the panel fabrication when the thickness is lower than 4 mm. In a recent research, Suman et al. [11] have grouped the PV cell technology into four generations such as first-, second-, third-, and fourth-generation PV cells. They have illustrated the best research cell efficiencies based on solar companies and solar cell technologies represented by the National Renewable Energy Laboratory (NREL). To date, the efficiencies of the solar cells just before the year 2020 and the major recordings are as follows: (i) The highest efficiency obtained from multijunction cells was 46.0% by Fraunhofer (ii) Efficiencies obtained from single-junction GaAs were in the range between 27.5 and 29.1% by LG Electronics, Alta Devices, and FhG-ISE (iii) Efficiencies obtained from crystalline Si were between 21.2 and 27.6% (iv) Efficiencies of the thin-film technology were in the range of 13.6 to 23.3% (v) Efficiencies of emerging PV cells were recorded between 10.6 and 22.1%. The perovskite cell made by KRICT/UNIST is the most outstanding in this cell type with an efficiency of 22.1% Solar PV modules are the combination of solar cells connected in series. The module of a PV system is a box-like component that is packaged, protected, and assembled by multiple solar cells in series to deliver a specific electrical output power. Figure 2 illustrates the components and layers of a PV module.
The conventional PV module glass-to-Tedlar fabrication method suffers many problems that affect the PV electrical performance. These drawbacks include electrical power reduction because EVA becomes yellow after years of usage (reducing the amount of solar irradiance striking onto the PV cells) and moisture gain inside the module through EVA and Tedlar sheets. A study was carried out to identify the impact of encapsulating materials, processing, and testing of the PV panel [13]. It was observed that different types of encapsulating materials provide different PV 2 International Journal of Photoenergy performances. The encapsulation failures including encapsulation breakdown, delamination, corrosion, and discoloration and the impact of these problems on the electrical performance reduction have been studied [14][15][16]. The discoloration is normally reported as an EVA color change (to yellow or sometimes brown) between the glass and the cells. Owing to the reduction of optical propagation and sunlight hitting the cells, this phenomenon causes a depletion of the power produced by reducing the photogenerated current of the panel [17]. To overcome these issues, the glass-toglass PV module was proposed [18]. The literature review reveals that, despite extensive research on the electrical efficiency of PV panels, accomplishment on the performance investigations to overcome the drawbacks of EVA and Tedlar is yet to achieve. EVA and Tedlar sheet traps the heat and reduces the efficiency of the PV panels. Therefore, this study aims at investigating the electrical performance analysis of tempered glass-based solar PV panels that are modified forms of PV panels where EVA and Tedlar are not utilized like commercial PV panels. The tempered glass-based panels are of the same concept with the glass-to-glass PV panels.

Methodology
2.1. Onsite Experimental Setup. The experimental setup was performed using the main components as illustrated in Figure 3. In this whole setup, newly fabricated polycrystalline PV (Poly-PV) and monocrystalline PV (Mono-PV) panels have been introduced. All panels were supported by supporting structures made of aluminum profiles. One pyranometer was attached to the top of one of them to measure the intensity of solar irradiance. The slope of the supporting frames was set to 15°following the recommended installation slope in Malaysia [19]. All temperature and solar irradiance data were real-time recorded by DataTakers (DT80), which are located inside a sealed box below the panels. Moreover, an IV tracer was used to show the electrical output of each panel. At the end of the day (at 17:00), all recordings were retrieved using a PC provided by the laboratory. Figure 3 shows the experimental setup with the data acquisition facility. The instruments have been calibrated following standard procedures. Table 1 shows the model and range of measurements of different instruments used in the experimental study. Table 2 shows the specifications of the tempered glass-based PV panels.
The layout of the tempered glass-based PV panels is indicated in Figure 4, where the solar cells were placed beneath the tempered glass having a thickness of 3 mm and sealed by encapsulation tape and an epoxy layer of 4 mm.     3 International Journal of Photoenergy solar irradiance readings were recorded by DataTakers modeled DT80. All data was collected from 8:30 in the morning until 17:00, before being retrieved into a PC for data analysis purposes.

Mathematical Formulation.
To understand the performance of a system and how much it differs from another, efficiency calculation and comparison are the most crucial identification. Malaysia is situated in the slightly northern hemisphere of the globe. Therefore, solar modules have been installed facing south. The angle of the PV and modules can be calculated using δ = 23:45 sin 360 365 d + 284 ð Þ , where δ is the solar declination, d is the day of the year, φ is the latitude of the experimental site, and β is the angle of the module towards the equator.
The electrical efficiency of the system is given by where the maximum electrical power output is calculated as follows:

Results and Discussion
3.1. Malaysian Weather Conditions. The annual variation of maximum and minimum solar radiations and ambient temperatures across Malaysia is shown in Figure 5. Malaysia is situated at the equatorial region, and it observes two monsoon seasons annually. Average solar radiation increases from November to March, and it decreases from May to September. In Figure 5, average solar radiation is about 3420 kJ/hr·m 2 which is around 950 W/m 2 from January to March, and then, average solar radiation reduces to 3060 kJ/hr·m 2 which is around 850 W/m 2 from May to

International Journal of Photoenergy
August; afterwards, it again rises to 3240 kJ/hr·m 2 which is around 900 W/m 2 from September to December [20].

Electrical
Characteristics. Voltage and current are the key electrical properties of PV panels. The intensity of the solar irradiance entering the cell regulates the current, while its voltage is decreased by the elevation of cell temperature. The I-V curves in Figure 6 represent the electrical characteristics of the PV panels experimented within this research work. According to the graph, the short-circuit current (I sc ) obtained from the maximum solar irradiance (900 W/m 2 ) of the panels was 6.42 A and 4.4 A, respectively, for polycrystalline and monocrystalline PV, while the current at maximum power (I mpp ) was 5.83 A and 3.9 A, respectively. The voltages at maximum power obtained from the same irradiance were 11.57 V and 19.41 V, while the open-circuit voltages (V oc ) were 17.06 V and 27.24 V. Moreover, the maximum power (P max ) obtained from these panels at 900 W/m 2 was 67.4 W and 75.67 W, respectively.

Electrical Output
Power. Solar photovoltaic is the concept of converting sunlight into electricity. Therefore, the amount of solar irradiance received by the PV panels is the key and an impactful parameter to determine the output. From the graph in Figure 7, the electrical output power of both panels followed the trend of solar irradiance. As the solar irradiance increased to the maximum averaged value of 634 W/m 2 between 12:00 PM and 12:30 PM, the output power of the panels also increased to their peaks. The electri-cal output powers obtained in this period were 67.4 W and 75.67 W, respectively, for the polycrystalline and monocrystalline PV. The monocrystalline PV offered a higher output than the polycrystalline PV. At the beginning of the day (at 8:30 AM), the solar irradiance was only 100.75 W/m 2 , while the output power of the polycrystalline and monocrystalline PV was only 4.37 W and 5.10 W. All values increased to their peaks between 12:00 PM and 12:30 PM before experiencing a dramatic decrease. A substantial drop in solar irradiance occurred after 12:30 PM, as the values of the output power of the panels also followed accordingly. The trend experienced a significant drop after this time due to the shading from the building near which the experimental setup was located.

Electrical Efficiency.
Electrical efficiency is the main characteristic to determine the electrical performance of the systems installed in this experimental study. In electrical efficiency analysis, the current and voltage at maximum power, area of the PV panel, and the amount of solar irradiance are the major parameters to be considered. Generally, more solar irradiance would increase the current of the cells in the PV system. However, the higher the solar irradiance, the warmer the PV cells would become. The increase in cell temperature would result in a reduction of the voltage of the cell as well as the panel as a whole, therefore affecting the performance of the system. Figure 8 illustrates the changes in electrical efficiency of the polycrystalline and monocrystalline PV panels in this experimental study, with respect    International Journal of Photoenergy to time and solar irradiance. In this graph, it could be seen that, despite sudden changes in solar irradiance, the efficiencies of the panels followed the same trends. The efficiency of the panels fluctuated quite significantly, but the overall trends show increasing efficiency from the morning at 8:30 AM until the afternoon before experiencing a sudden drop at 3:30 PM. The maximum obtainable efficiency for the polycrystalline and monocrystalline PV was 13.25% and 15.55%, respectively. The average electrical efficiencies of the four panels are 10.54% and 12.23%.

Conclusion and Future Work
The purpose of this study was to investigate the electrical performance of the tempered glass-based polycrystalline and monocrystalline PV panels under typical Malaysian weather. The real-time outdoor experiment was successfully achieved. The electrical performance analysis was conducted to compare the two fabricated panels. An I-V curve was also obtained to identify the electrical characteristics of the monocrystalline and polycrystalline panels. For such, it is concluded that the polycrystalline PV provided a higher current compared to the monocrystalline PV. In contrast, the monocrystalline PV produced a much higher voltage compared to the polycrystalline PV. At solar irradiance of 900 W/m 2 , the electrical power produced by the polycrystalline PV and monocrystalline PV was 67.4 W and 75.67 W, respectively. The electrical output is affected by the amount of solar irradiance. The higher the solar irradiance, the more the electrical output power is obtained from the PV panels. At an average solar irradiance of 634.61 W/m 2 , the electrical output from the polycrystalline and monocrystalline PV was 47.85 W and 54.89 W. The average electrical efficiency of the polycrystalline and monocrystalline PV was 10.54% and 12.23%, respectively. Furthermore, in solar photovoltaic (PV) systems, the performance or electrical efficiency of solar PV panels is affected by their surface temperature. The excessive temperature in solar PV panels can cause severe degradation to the solar cell material and reduce the lifespan of the panel itself. Alternatively, a possible performance improvement by employing cooling technologies to reduce the surface temperature of the panel should be studied [21,22]. In this regard, solar thermal collectors with the solar PV panel can play a significant role in both producing thermal energy and in cooling down the PV panel surface temperature. The combination of STC and solar PV is called a solar photovoltaic-thermal (PVT) system. For future work, the authors would like to continue this work and compare these results with tempered glass-based PV/T systems using novel flat-plate absorber designs.

Data Availability
Data are available upon request to the corresponding author.

Conflicts of Interest
The authors declare that they have no conflicts of interest.