Experimental Study on Zeolite–Polyester-Coated Jute–Sisal Fibre Back Sheets for Improved Efficiency of Solar Panels: A Renewable Composite Material Strategy

Abstract

This study examines the potential of jute–sisal (JS) fibre, both coated and uncoated, as a sustainable alternative to solar panels with polyethylene terephthalate (PET) back sheets. The coated version was developed using a zeolite–polyester resin composite to enhance thermal performance. The investigation was carried out in two phases: controlled laboratory testing using a solar-cell tester and a 90-day real-world evaluation under natural environmental conditions. In controlled conditions, solar panels with coated JS (CJS) fibre back sheets exhibited improved electrical performances compared to PET panels, including higher current (1.23 A), voltage (12.79 V), maximum power output (14.79 W), efficiency (13.47%), and fill factor (94.03%). Lower series resistance and higher shunt resistance further indicated superior electrical characteristics. Under real-world conditions, CJS panels consistently outperformed PET-based panels, showing a 6% increase in current and an 8% increase in voltage. The model showed strong agreement with the experimental results. These findings suggest that coated JS fibre is a viable, eco-friendly alternative to PET for back sheets in solar panels. Further research should examine its long-term durability, environmental resistance, and commercial scalability.

1. Introduction

The increasing global population has made it challenging to meet the world’s electrical demand primarily by burning fossil fuels [1,2]. The International Energy Agency reported that in 2021, global CO₂ emissions increased by 1.5 billion tonnes compared to the previous year [3]. The continuous use of fossil fuels results in unintended consequences. It has significantly accelerated economic, technological, and sociological advancement, raising living standards. However, the extraction of fossil fuels has raised grave concerns, including heatwaves, droughts, and sea level rises [4]. To reduce greenhouse gas (GHG) emissions worldwide, research communities have strongly emphasised substituting sustainable energy sources for conventional ones, including solar, wind, hydroelectric, and bioenergy [5]. The Copenhagen Climate Change Conference (2009) and Paris Agreement (2015) [6], among other geopolitical decisions, have played a crucial role in achieving national promises to cut carbon emissions by supporting renewable energy sources and keeping the global temperature increase to less than 2 °C.

PV technology is predicted to significantly aid the economy’s transition from a conventional fuel-based to a renewable energy model. Crystalline silicon-based panels are the most common form of PV panels today. They are known for their dependability and high return on investment. Researchers have created substitute photovoltaic (PV) technologies, such as organic and hybrid solar cells for power production applications, as well as thin-film alternatives like amorphous silicon (a-Si), cadmium telluride (CdTe), etc. [7]. With improved materials, PV panels’ average 25-year lifespan can be increased to 30 years. Their lifespan might be shortened by damaging factors such as extreme weather, fires, natural catastrophes, poor maintenance, hotspots, and damage sustained during shipment and installation [8]. PV panels present problems at the end of their useful life (EOL) phase. After 25 to 30 years, or due to the circumstances above, they become hazardous waste for the biosphere. According to projections, by 2050, the amount of e-waste from silicon photovoltaic panels might range from 60 to 78 million tonnes [9]. Lead, cadmium, tin, silicon, and other elements risk human health and the environment, i.e., the biosphere may be harmed by improper disposal, contaminating water and soil.

Silicon-based photovoltaic panels are pivotal in generating electricity and significantly reducing carbon footprints. However, their lifecycle raises concerns due to the generation of electronic waste (e-waste) at the end of their life. The volume of e-waste from silicon PV panels is expected to surge dramatically, with projections indicating a peak between 2035 and 2040 [10]. Improper disposal methods, like landfilling, pose risks to ecosystems and human health and result in the loss of valuable metals. To prevent a scenario similar to the issues faced by plastic waste, effective management of PV panel e-waste is essential. According to the International Energy Agency (2022) [11] and IRENA (2016) [12], there were approximately 250,000 metric tonnes of solar PV panel waste by the end of 2016. This amount is anticipated to increase to 8 million tonnes by 2030, considering both early and routine losses, potentially reaching 78 million tonnes [13]. Additionally, the Government of Japan projected an increase in solar panel production from 10,000 tonnes in 2016 to 80,000 by 2040 [14].

The production and recycling of solar panels contribute to increased carbon emissions due to various industrial byproducts. Cadmium exposure, in particular, poses serious health risks, including cancer and other complications [15]. Researchers are now concentrating on creating more eco-friendly solar panels, emphasising the need for sustainability throughout both the manufacturing and usage stages. Older panels can sometimes be repaired and resold through recycling facilities [16]. When solar panels are beyond repair, they undergo a dismantling process in which components such as junction boxes, wiring, and aluminium frames are first detached. Specialised techniques are then used to separate the remaining materials, including polymer layers, metals, and glass. The recovered glass is frequently repurposed into glass wool for use in thermal insulation applications. Valuable metals, including lead, are extracted for recycling. In contrast, the plastic layers and films are typically incinerated in controlled facilities equipped with advanced filtration systems to minimise environmental impact [17]. Despite these efforts, recycling of these plastics still requires more improvement. Most components of solar panels can be recycled, but the special back sheets and transparent layers cannot be recycled and must be incinerated. This incineration process demands significant energy and results in CO₂ emissions, contributing to the overall carbon footprint [18].

Studies indicate that polyethylene terephthalate (PET) is widely utilised as a back-sheet material in solar panels. PET is synthesised by polymerising ethylene glycol (EG) with either terephthalic acid (TPA) or dimethyl terephthalate (DMT) [19]. Being a petroleum-derived plastic, its manufacturing and disposal pose significant environmental concerns, including greenhouse gas emissions and the generation of solid waste [20]. Moreover, PET is non-biodegradable, which means that if solar panels containing PET back sheets are not properly disposed of, they can contribute to the growing issue of plastic pollution. Another drawback of PET is its relatively low thermal resistance, making it susceptible to deformation or degradation during extended periods of high-temperature exposure, particularly in areas with strong solar radiation [21].

In spite of substantial progress in developing alternative PV technologies and recycling methods, several gaps must be addressed. While various types of PV panels have been explored, including organic and hybrid solar cells and thin-film alternatives, these materials’ specific environmental impacts and recyclability still require further investigation [22]. In particular, the challenge of managing e-waste from silicon PV panels still needs to be addressed, with projections indicating a massive increase in waste volume in the coming decades. Additionally, while some advancements have been made in improving the sustainability of back-sheet materials, PET remains a widely used option despite its drawbacks, such as non-biodegradability and heat sensitivity. Jute mixed with sisal fibre was employed as a sustainable alternative [23] to the toxic and non-biodegradable PET-based solar panels (back sheet) in this study.

Although natural fibres such as jute, flax, and sisal have been investigated in several engineering applications, their integration into photovoltaic (PV) back sheets remains very limited. Previous studies have primarily focused on epoxy- or flax-based composites; however, these materials have faced challenges such as weak fibre–matrix adhesion, hydrophilicity, and poor dimensional stability under environmental stress. More importantly, most research on natural fibre composites have been restricted to coupon-level or material property analysis, without progressing to module-level evaluation. As a result, the real-world feasibility of natural fibre back sheets in PV applications, particularly their long-term thermal and electrical performances, has not been adequately demonstrated. This gap leaves uncertainty about their potential to replace petroleum-derived materials such as polyethylene terephthalate (PET), which continue to dominate despite their environmental drawbacks.

The present study addresses these shortcomings by developing and testing zeolite–polyester-coated jute–sisal (CJS) fibre composites as sustainable alternatives to PET back sheets. The innovation lies in combining natural fibres with a zeolite–polyester coating, which enhances crystallinity, thermal conductivity, and interfacial bonding while reducing moisture sensitivity. Unlike earlier works, this study not only characterises the mechanical, thermal, and chemical properties of the proposed material but also integrates it into functional PV modules. These modules were rigorously evaluated under both controlled laboratory conditions and a 90-day outdoor field trial. The results demonstrated clear improvements in performance, with CJS modules achieving higher efficiency (13.47% compared to 11.96% for PET), greater maximum power output (14.79 W compared to 13.39 W), and a reduction in surface temperature of 1–3 °C. By linking material design with module-level validation, this work establishes the technical feasibility of natural fibre composites in photovoltaic applications while offering a sustainable solution to the environmental limitations of synthetic back sheets.

2. Materials and Methodology

Sisal fibre can regulate temperatures within solar panels, potentially enhancing thermal comfort by reducing heat transfer. Its ability to absorb solar radiation could mitigate heat transfer into the panel’s interior, lowering component temperatures [24]. Moreover, the inherent moisture-absorbing ability of jute-mixed sisal fibre may help regulate internal humidity levels in the panel, reducing the risk of condensation and protecting delicate electronic components. The manufacturing drawbacks of sisal fibre, such as its coarser texture, are effectively addressed by blending it with 20% jute fibre to enhance flexibility and softness. This blend offers a balanced combination of properties, improving tensile strength and durability compared to each fibre alone [25,26]. Despite challenges like degradation and manufacturing complexity, the balance of properties in the sisal–jute blend ensures its effectiveness. Proper treatment is needed to protect the fibres from environmental factors, and integrating fibre into the unique design of the panel may increase manufacturing complexity and costs. This study used jute-mixed sisal (JS) fibre as reinforcement mats for back sheets coated with thermal comfort materials like polyester resin and zeolite to prevent degradation. Through the vacuum infusion method, zeolite type 3A ([27,28]), unsaturated polyester (properties studied by [29,30,31]), and methyl ethyl ketone peroxide (MEKP) hardener types in coating enhance the material quality and prevents combustion. Figure 1 depicts the jute-mixed sisal fibre before and after zeolite–polyester resin coating.

Vacuum-Assisted Resin Transfer Moulding Technique

The vacuum-assisted resin transfer moulding (VARTM) process involves injecting resin into the fabric under vacuum pressure to create fibre-reinforced composites [32]. Initially utilised in the 1950s for boat hull production, VARTM has faced challenges leading to a trial-and-error approach [33]. Research has covered its specifications and applications, with studies discussing its history, development, and future trends. The pressure difference between the resin supply pipes and vacuum facilitates resin infusion into fabric layers. Preparation involves cutting the release spray, peel ply, high-permeable media (HPM), and a vacuum bag sealed with sealant tape (shown in Figure 2) according to the dimensions and placing them respectively. Vacuum generation occurs at one end, while resin supply occurs at the other. Techniques describe void content management and ensuring resin flow between and inside tows. Previous studies have investigated various factors impacting part quality.

The VARTM process begins with the application of wax on the surface of the manufacturing table to prevent sticking. The required dimensions are marked, and spiral pipes are positioned and fixed in place using sealant tape. A release film is then laid down to ensure a uniform surface on the back sheet. Fibre mats are carefully placed over the release film, with the overall thickness limited to two layers for the jute-mixed sisal fibre back sheet. Following this, a peel ply and a green flow mesh are added as the fourth layer. A T-connector is used to link the vacuum line to the spiral pipe, and two feed ports are placed diagonally to support resin infusion for the dual-layer fibre arrangement. The vacuum bag is sealed around the perimeter using the sealant tape, completing the VARTM setup as illustrated in Figure 2. This work employs a modified VARTM approach to uniformly coat jute-mixed sisal fibres with a mixture of zeolite and unsaturated polyester resin. The resin, combined with zeolite, is infused into the fibre layers under a vacuum pressure of approximately 1.3 to 1.6 kPa. The final back sheet dimensions are 20 × 20 cm, with a thickness ranging from 1.5 mm to 3 mm. Each fibre layer is infused using ninety grams of zeolite and one hundred and thirty-five grams of polyester resin. Resin is introduced from the centre, while the vacuum is applied from the edges inward to ensure optimal impregnation and mechanical performance.

The composite laminates were fabricated using the vacuum-assisted resin transfer moulding (VARTM) technique, with key process parameters carefully controlled to ensure repeatability. A constant vacuum of −0.8 bar (80 kPa below atmospheric pressure) was applied throughout resin infusion and curing, monitored using a calibrated vacuum gauge(Care Process Instruments, Gujarat, India). The unsaturated polyester resin, mixed with a catalyst, was injected at a controlled rate of approximately 2–3 mL/s to promote uniform wetting of the jute–sisal fibre preforms and minimise void formation. After complete impregnation, the lay-up was maintained under vacuum and allowed to cure at room temperature (27 ± 2 °C) for 24 h. The progression of the resin flow front was visually monitored to confirm complete infiltration before sealing. These detailed parameters are provided to ensure that other researchers can reliably reproduce the fabrication procedure.

3. Material Properties Assessment

This section initiates an initial exploratory examination of jute-mixed sisal fibre, focusing on fundamental physical attributes including tensile strength, specific gravity, modulus of elasticity, and water absorption. Chemical compositions were identified using techniques such as XRD (BRUKER, Billerica, MA, USA), EDS (Thermo Fisher Scientific, Waltham, MA, USA), and SEM (Thermo Fisher Scientific, Waltham, MA, USA) analysis. Temperature performance (thermal characteristics) was investigated utilising DSC (NETZSCH, Selb, Germany) analysis.

3.1. Physical Properties

Former investigations have explored the potential applications of natural fibres across several domains, accompanied by detailed examinations of their physical and chemical properties [34,35,36]. The specific gravity of jute-mixed sisal fibre measured 1.35, with a 1450 kg/m³ density. Balasubramanian et al. determined the Poisson ratio to range between 0.2 and 0.25. The thickness of six tensile coupons was measured at three locations on each sheet using a digital calliper. The per-ply thickness (PPT) and fibre-volume fraction (Vf) were then computed based on these measurements. The results of the thickness assessment are presented in

Table 1. Measured thickness values of the tensile specimens along with the corresponding average fibre-volume fraction results.

Sl.No	Description	Thickness of CJS
		Sample 3 Left Corner	Sample 4 Centre	Sample 5 Right Corner
1	Top section	1.83	1.89	1.87
2	Reduced area Middle section	1.9	1.95	1.92
3	Bottom section	1.86	1.91	1.84
4	Mean (mm)	1.89
5	CV	2.07
6	PPT (mm)	0.94

Note: CV stands for coefficient of variation and PPT (mm) for mean thickness of panel/number of layers.

, where the mean value for coated jute–sisal fibre was 1.89. Although the approximated specifications of a VARTM process were notably high, the coated jute–sisal fibre panel displayed more significant thickness variation, as expected, due to the fabric’s bidirectional waving pattern causing undulations.

3.2. Tensile Strength Testing (Mechanical Properties)

The specimens were subjected to tensile loading using a universal testing machine(Shimadzu Corporation, Kyoto, Japan), with tension applied until failure occurred within the grips, which were set at a fixed separation distance. In accordance with ASTM D 3822 [37], the tensile behaviour of jute-mixed sisal fibres was assessed using the universal testing equipment(Shimadzu Corporation, Kyoto, Japan) [38]. A total of three coated jute–sisal fibre specimens were tested, and their results were documented. A strain gauge (extensometer) was utilised to measure tensile modulus and elongation during the test. The tensile test enabled the determination of key mechanical parameters such as TS (in MPa), YM (in GPa), and FS (in percentage) [39]. The TS test coupons were accurately prepared by cutting from the composite sheets using a diamond-blade wet saw. Each coated jute–sisal (CJS) coupon had dimensions of 155 mm in length, 20 mm in width, and 1.9 mm in thickness. The tests were performed at a cross-head speed of 1 mm/min, using a gauge length of 55 mm and a width of 14 mm. Figure 3 presents experimental images captured during tensile testing. For each test specimen, 50 mm long fibreglass tabs were bonded to both ends, and a strain gauge was fixed at the centre of the coupon. The results for TS, YM, and FS for all three samples are summarised in

Table 2. Tensile strength results.

Sl. No	Description	Sample Name	TS Mpa	YM Gpa	FS %
1	Coated Jute–Sisal Fibre	Sample No 3	20.02	4.09	1.064
2		Sample No 4	20.21	3.64	1.055
3		Sample No 5	25.80	4.01	1.045

Note: TS stands for tensile strength; YM stands for Young’s modulus; and FS stands for failure strain.

. Based on the data, coated jute-mixed sisal fibre sheets demonstrate characteristics consistent with brittle materials, as indicated by a failure strain of less than 5%. In contrast, the uncoated jute-mixed sisal fibres are highly hydrophilic and readily absorb moisture. This inherent moisture absorption leads to poor interfacial bonding, dimensional instability, and premature failure during mechanical testing [26].

According to the literature, PET films typically exhibit tensile strengths ranging from 55 to 75 MPa, with a Young’s modulus in the range of 2.7–4.1 GPa [40]. Although PET demonstrates superior tensile performance, the coated jute–sisal (CJS) fibre sheets offer significant advantages, including eco-friendliness, lower thermal conductivity, and the use of renewable materials, all of which are critical attributes for solar back sheet applications.

3.3. Scanning Electron Microscope Analysis with Energy-Dispersive X-Ray Spectroscopy

In this study, the jute-mixed sisal fibre mat was treated for 12 h with 5% NaOH and oven-dried at 40 °C for 4 h per the standards [41,42]. The fibres were subjected to treatment using a 5% (w/v) aqueous sodium hydroxide (NaOH) solution. The solution was prepared by dissolving 5 g of sodium hydroxide in a total volume of 100 mL of distilled water, ensuring a uniform concentration for effective treatment. The SEM analysis was carried out individually for the three samples, which included jute-mixed sisal fibre mats before and after NaOH treatment and thermal materials-coated jute–sisal fibre mat. Field emission scanning electron microscopy (FE-SEM) (Thermo Fisher Scientific, Waltham, MA, USA) was utilised for this microstructural investigation. Micrographs were recorded and represented using backscattered electrons. Rougher surfaces and improved fibre separation were achieved by eliminating waxes, non-cellulosic content, and surface contaminants. Figure 4a illustrates that the jute-mixed sisal fibre surface appeared smooth due to the presence of waxes and other surface impurities. After treatment with NaOH, as depicted in Figure 4b, the fibre mat exhibited increased surface roughness and a more uniform texture. The alkali-treated jute–sisal fibre showed minimal impurities, and the fibre bundles appeared more distinct and separated, contributing to the enhanced surface roughness.

The SEM examination of zeolite–polyester-coated jute-mixed sisal fibre is shown in Figure 5a–c. It is evident from the analysis that the coated jute-mixed sisal has no porosity. A few regions, as shown in Figure 5a,b were rich in resin. The periodic “open” spots where the tows intersect with fabric preforms make this expected as the fibre mat was bidirectional. The resin-rich regions result from incomplete fibre plies nesting during the VARTM process or variations in resin flow. Additionally, with coated jute–sisal fibre, there were some differences in how the fibres were packed within the fibre tows. The overall quality of the microstructure was significant.

The EDS analysis was performed on three samples, as shown in Figure 6a–c. The energy levels of the observed X-rays are shown on the x-axis, typically expressed in keV or kilo electron volts. The X-rays’ intensity at each energy level is shown on the y-axis. This intensity indicates the amount of X-rays detected at a specific energy. The individual constituents of the sample are reflected in the peaks of the EDS plot. X-rays are released by each element at distinct energy levels. The graph displays these peaks as spikes or peaks, and the elements can be identified by the energy levels at which they occur. The peak height and the sample’s specific component’s abundance are correlated. The graphs show the corresponding weight proportion of each ingredient that is present.

The analysis reveals that the coated jute–sisal fibres contain characteristic signals of Si, Al, and Na, attributable to the zeolite–polyester coating, which are absent in both uncoated fibres and PET. These additional elements correlate with the observed improvements in thermal stability and tensile strength. PET spectra, by contrast, show only C and O, consistent with its polymeric structure, but lack inorganic reinforcement. This quantitative analysis clarifies the relationship between coating composition and enhanced performance, thereby strengthening the link between microstructural features and the experimental results.

The presence of Ti peaks in the EDS spectra of coated jute–sisal and PET samples indicates the inclusion of TiO₂, a common additive used as a pigment and UV stabiliser in polymer composites. While nanoparticulate TiO₂ powders may raise toxicity concerns in occupational settings, in the present case TiO₂ is encapsulated within the polymer matrix and therefore does not pose any risk of release during normal application.

Table 3. Quantitative EDS elemental composition (wt%).

Element	Uncoated JS (wt%)	Coated JS (wt%)	PET (wt%)	Key Observation
C	Present	Present	Present	Higher in PET due to polymer, reduced in coated JS due to zeolite presence
O	Present	Present	Present	High in natural fibres, but lower in PET
Si	Trace	Present (higher)	-	Characteristic of zeolite
Al	-	Present	-	From zeolite
Na, K	-	Trace	-	Zeolite components
Ca	Trace (from natural fibre ash)	May increase with coating	-	Mineral trace in natural fibres, absent in PET
TiO2	-	-	Present	Minimal amount

presents the quantitative EDS elemental composition (wt%) of uncoated jute–sisal fibre, coated jute–sisal fibre, and PET back sheet samples, highlighting characteristic element signals introduced by the zeolite–polyester coating and PET additives. The EDS quantitative analysis establishes a clear link between chemical composition and performance. Uncoated jute–sisal fibres mainly contain carbon and oxygen, reflecting their lignocellulosic structure, but these elements alone offer limited mechanical stability and high moisture sensitivity. In the coated jute–sisal fibres, additional signals of Si, Al, and Ca are detected, confirming the successful incorporation of the zeolite–polyester layer. The presence of Si and Al, characteristic of zeolites, enhances thermal stability and contributes to improved dimensional integrity. Similarly, Ca traces indicate mineral reinforcement from natural fibres and interaction with the coating. These elements collectively enhance fibre–matrix adhesion, reduce hydrophilicity, and contribute to the higher tensile strength and thermal resistance observed in the composites.

In contrast, PET samples exhibit only C, O, and TiO₂ signals, consistent with their polymeric structure and TiO₂ additives used as UV stabilisers, but lack the inorganic reinforcement provided by zeolite. This analysis highlights that while PET maintains excellent tensile strength, it exhibits lower thermal stability when compared to the coated natural fibre system. Overall, the EDS findings confirm that the modified elemental composition of the coated jute–sisal fibres underpins the improved mechanical and thermal performance demonstrated in the experimental results.

3.4. XRD X-Ray Diffraction Analysis

This study aimed to assess the different levels of crystallinity in jute–sisal fibre and coated jute–sisal fibre using X-ray diffraction with Cu Kα radiation (λ = 1.54). The diffraction intensity was measured across the range of 5° to 90° of 2θ, with a scanning speed of 0.02°/s. Figure 7 illustrates the X-ray diffraction analysis results for jute–sisal and coated jute–sisal fibres. The diffractograms display a prominent crystalline peak at 2θ = 22°, indicating varying levels of crystallinity in the coated jute–sisal samples. Moreover, in the case of jute–sisal, the primary crystalline peak is observed within the 2θ range of 26–27°, indicating varying levels of crystallinity. The analysis highlights that coated jute–sisal exhibits the highest peak intensity at 352 cps (see Figure 7). The XRD analysis determined the crystallinity index to be 95.83% for uncoated jute-mixed sisal fibre and 99.47% for zeolite–polyester resin-coated jute-mixed sisal fibre. This higher crystallinity index in the coated fibre indicates that the zeolite–polyester resin coating significantly enhances the fibre’s properties. This elevation in intensity suggests that coated jute–sisal surpasses jute–sisal in terms of crystallinity, likely attributable to the zeolite–polyester coating applied via the VARTM process. This augmentation in the crystallinity index is accompanied by a reduction in the amorphous phase, thereby improving the tensile strength of the fibres. Additionally, the enhanced interfacial bonding in coated jute–sisal can be attributed to the amplified content of crystalline cellulose.

3.5. Thermal Properties-Differential Scanning Calorimetry Analysis

This study is crucial to understanding the heat resistance, stability, and thermal properties of both jute–sisal and coated jute–sisal. Thermal analysis is vital in unravelling the relationship between structure and properties and advancing molecular design technology [43,44]. Additionally, it helps quantify volatile substances and moisture that can cause composite deterioration. Differential scanning calorimetry (DSC) analysis was employed in this research. As shown in Figure 8a, the uncoated (right) and coated fibre (left) structures differ significantly in surface texture and colour, indicating effective resin impregnation.

The endothermic peak temperature for the jute-mixed sisal fibre was observed at around 126 °C (Figure 8). This peak signifies a thermal transition involving the decomposition and degradation of the fibre’s structural components. However, the endothermic peak temperature rose to approximately 136 °C for coated jute–sisal, and the temperature drop-down remained consistent. The increased endothermic peak temperature and consistency in temperature indicate that the thermal stability of the coated jute–sisal fibre was enhanced due to the presence of the zeolite–polyester composite.

Table 4. Characteristics of individual components in polycrystalline solar panels.

Solar Panel Elements	TC (W/m·K)	Density (kg/m3)	SHC (J/kg·K)
Ethylene Vinyl Acetate (EVA) Film	0.3	930	2050
Polycrystalline Cells	142	2350	786
Polyethylene Terephthalate (Conventional) Back Sheet	0.2	1350	1350
Zeolite–Polyester Resin-Coated Jute-Mixed Sisal Fibre Back Sheet	0.25	1335	1150
Uncoated Jute-Mixed Sisal Fibre Back Sheet	0.05	1380	1300

provides a detailed comparison of the properties of various elements used in polycrystalline solar panels, including their thermal conductivity (TC), density, and specific heat capacity (SHC). The EVA film exhibits a thermal conductivity of 0.3 W/m·K, a density of 930 kg/m³, and a specific heat capacity of 2050 J/kg·K [45]. Polycrystalline cells have a significantly higher thermal conductivity at 142 W/m·K, with a density of 2350 kg/m³ and a specific heat capacity of 786 J/kg·K [46]. The conventional polyethylene terephthalate (PET) back sheet shows a thermal conductivity of 0.2 W/m·K, 1350 kg/m³ density, and a specific heat capacity of 1350 J/kg·K. For the innovative back-sheet materials, the zeolite–polyester resin-coated jute-mixed sisal fibre has a thermal conductivity of 0.25 W/m·K, a density of 1335 kg/m³, and a specific heat capacity of 1150 J/kg·K. In contrast, the uncoated jute-mixed sisal fibre back sheet displays much lower thermal conductivity at 0.05 W/m·K, with a 1380 kg/m³ density and a specific heat capacity of 1300 J/kg·K. Uncoated jute–sisal fibre has low thermal conductivity and is unsuitable for high-performance panels. The zeolite–polyester coating improves this, facilitating heat transfer from solar cells, maintaining optimal temperatures, and enhancing panel performance and lifespan. Low specific heat capacity is preferable for stable temperatures and efficient operation. The coated fibre has the lowest specific heat capacity, enabling quick temperature adjustments and preventing overheating. These properties help maintain ideal operating temperatures, reducing thermal stress and mechanical fatigue.

4. Manufacturing Process of Solar Panel with Jute–Sisal Fibre Back Sheet

The solar panels incorporating back sheets made from a blend of jute and sisal fibres were fabricated using the Ecolam Max 3 solar module automatic laminator(Ecoprogetti Srl, India), equipped with load and unload belts. The lamination process involved hot compression to securely attach the jute–sisal fibre back sheets to the solar cells. A collaborative effort with JP Solar Pvt. Ltd. in Chengalpattu, India, facilitated the lamination process for this research project. Polycrystalline solar cells were chosen for their resilience to temperature fluctuations. Natural fibres were utilised as back sheets to enhance the efficiency of these economical solar cells. Each solar panel comprised six polycrystalline solar cells (4.5 watts of capacity). The cells were organised into two vertical columns, with each column linked to a bypass diode to create separate cell strings. After compression, the solar panels were mounted on an aluminium frame, preparing the solar PV-module assembly for further testing. Figure 9 shows a graphical representation of the manufacturing process of solar panels with jute-mixed sisal fibre back sheets. For this study, two types of back sheets were provided for solar-panel manufacturing other than conventional PET back sheets. The coated jute-mixed sisal fibre sheet with a thickness of less than 3 mm was provided to manufacture the solar panels. Furthermore, two layers of the jute-mixed sisal fibre mats without zeolite and polyester coating were also provided as a back sheet. The optimal number of layers (two) was determined based on the thickness restriction, i.e., for the solar panel, the back sheet should be less than 3 mm to overcome the manufacturing difficulties.

5. Experimental Investigation

The mechanical, physical, thermal, and chemical characteristics of jute-mixed sisal and coated jute–sisal fibres demonstrate their potential as alternatives to PET sheets, which typically contain around 3% titanium.

Table 5. Influencing parameters of the experimental site and solar-panel samples used in this research work.

Description	Specifications
Panel Dimensions	20 × 20 cm
Thickness	Less than 0.3 cm
Type of Solar Cells	Polycrystalline
No of Solar Cells	Six (6)
Maximum Power	4.5 watt
Voltage	3 V
Current	1.5 A
Latitude	12.82
Longitude	80.04
Site Location	Kattankulathur, Chennai, India
Annual Air Temperature	25 °C to 35 °C
Annual Solar Irradiation	5.5 to 6.5 (kWh/m2/day)
Solar-Panel Tilt Angle	13°

presents the parameters influencing the experimental solar panels with conventional and natural fibre back sheets.

5.1. Techno-Thermodynamic Measurement Setup and System Architecture

To enhance understanding of the experimental procedures and facilitate a thorough technical evaluation, a techno-thermodynamic scheme of the measurement system is presented. This approach integrates the thermal and electrical domains involved in evaluating the performance of solar panels under both controlled and real-world conditions. The schematic layout of the measurement architecture is shown in Figure 10.

Energy Flow and Measurement Logic

The experimental setup for the energy flow and measurement logic consists of three primary functional domains. First, the solar energy absorption unit features solar panels (PET, JS, or CJS) that capture incident solar radiation or simulated light, converting it into electrical energy while also experiencing heat build-up from solar absorption, which can impact their efficiency and long-term performance. The second component, the thermal simulation and monitoring unit, evaluates the panels under two distinct thermal conditions: in a laboratory setting, all panels were subjected to controlled heating, achieving a target surface temperature of 60 ± 1 °C, with surface temperatures monitored using a Fluke infrared thermal imaging camera(Techforth Global Technologies, New Delhi, India) to compare heat dissipation and thermal resistance across the various back-sheet materials. In the second phase, the panels were installed on a rooftop, exposed to real-world weather, and surface temperature readings were recorded at regular intervals to assess their thermal regulation capability. The third component, the electrical measurement and data acquisition unit, also conducts evaluations in two phases to capture performance under controlled and field conditions. In the lab, a solar-cell tester measures the key performance indicators such as short-circuit current (I_SC), open-circuit voltage (V_OC), maximum power (Pmax), fill factor (FF), and efficiency, while interfacing with a data acquisition system for real-time monitoring. For field tests, the panels, connected in a standalone configuration on the rooftop, utilised a digital multimeter to manually record voltage and current values every 30 min throughout the day, alongside a decade resistance box (DRB)(Scientific equipment & Services, India) to simulate electrical loading and determine output power under varying resistive loads. This comprehensive approach provides an in-depth understanding of the panels’ performance across both controlled and real-world settings.

5.2. Experimental Investigation: Phase I

Phase I experiments were carried out using a solar-cell tester (Argus, British Colombia, Canada) (see Figure 11). Solar cells’ long-term performance and deterioration mechanisms can be studied with the aid of solar-cell testers. Researchers can determine the reasons for degradation, evaluate the stability of materials, and formulate strategies to increase the robustness and durability of solar cells by tracking changes in electrical characteristics over time. Essential electrical parameters of solar cells, including fill factor (FF), efficiency, short-circuit current (Isc), open-circuit voltage (Voc), maximum power point (Pmax), and current–voltage (IV) characteristics, are measured using solar-cell testers.

Manufacturers and researchers can evaluate the total performance and efficiency of solar cells by examining their electrical characteristics. Solar-cell testers can test variations in temperature, spectral distribution, and light intensity under varied climatic situations. This study analysed the solar panel’s temperature on different back sheets since temperature significantly impacts solar cells’ lifespan and performance. The solar panels were manually heated to 60 °C ± 1 °C and tested in a solar-cell tester. Figure 11 depicts the experimental setup: A is the solar-cell tester, B is the computer attached for generating and storing the results, and C is the three solar panels connected in series for testing.

Table 6. Details of solar-panel performance as measured by the solar-cell tester.

Description	PET Panel	Coated Jute-Mixed Sisal Fibre Panel	Uncoated Jute-Mixed Sisal Fibre Panel
ISC	1.15 A	1.23 A	1.17 A
VOC	12.48 V	12.79 V	12.68 V
Pmax	13.39 W	14.79 W	13.87 W
IPM	1.1 A	1.19 A	1.13 A
VPM	12.17 V	12.43 V	12.27 V
Light Intensity (Irradiance)	932.5 W/m2	914.8 W/m2	910.6 W/m2
Area	0.12 m2	0.12 m2	0.12 m2
Incident Solar Power on Panel	111.9 (W)	109.78 (W)	109.27 (W)
Efficiency (%)	11.96%	13.47%	12.69%
Fill Factor (FF)	93.28%	94.03%	93.46%
RS	6.2 Ω	9 Ω	10.25 Ω
RSH	249.6 Ω	355.28 Ω	317 Ω
P_Load	13.387 W	14.79 W	13.87 W
V_Load	12.17 V	12.43 V	12.27 V
I_Load	1.1 A	1.19 A	1.13 A
Temperature	60.25 °C	60.20 °C	60.42 °C

presents the specifications of the solar panels tested, comparing conventional PET panels, coated jute-mixed sisal fibre panels, and uncoated jute-mixed sisal fibre panels. The coated panels show superior performance with a short-circuit current (I_SC) of 1.23 A and an open-circuit voltage (V_OC) of 12.79 V, higher than both conventional PET (I_SC: 1.15 A, V_OC: 12.48 V) and uncoated panels (I_SC: 1.17 A, V_OC: 12.68 V). The maximum power output (Pmax) is also highest for coated panels at 14.79 W, compared to 13.39 W for conventional PET and 13.87 W for uncoated panels. Additionally, the efficiency of the coated panels is the highest at 13.47%, significantly outperforming conventional PET (11.96%) and uncoated panels (12.69%). The fill factor (FF) for coated panels is 94.03%, indicating better overall performance and efficiency. The coated panels also exhibit higher shunt resistance (R_SH) and lower series resistance (RS), suggesting improved electrical characteristics and reduced losses. These enhanced properties of the coated panels contribute to their superior performance in heat dissipation, operational efficiency, and lifespan. The irradiance during I–V testing was approximately 910–930 W/m², measured using a calibrated pyranometer (SSTC Met Solutions, Delhi, India). The “Incident Solar Power” values in Table 6 represent the total solar power falling on the panel’s surface area (0.12 m²), calculated as irradiance × area. Each experimental panel used for testing comprised three sub-modules of 20 cm × 20 cm each, resulting in a total active surface area of approximately 0.12 m². This total area value was used for calculating the incident solar power and efficiency parameters presented in Table 6.

The relatively high fill factor (FF) values of the three panel types, PET (93.28%), uncoated jute–sisal (93.46%), and coated jute–sisal (94.03%), despite measured series resistance (Rs) values between 6.2 and 10.25 Ω, can be explained by several factors. The small panel size (20 × 20 cm) results in minimal voltage drop at lower current outputs (1.1 to 1.23 A). Additionally, FF measurement techniques may yield slightly optimistic values due to idealised calculations, and the Rs readings may include external contact resistances that do not significantly affect the IV curve near the maximum power point. Moreover, the coated jute–sisal panel’s ability to maintain lower temperatures enhances charge carrier mobility, further supporting high FF performance. These insights suggest that optimising panel design and wiring could improve Rs in future prototypes. While it is well understood that PV-module voltage improves with lower operating temperatures due to reduced bandgap narrowing, the observed increase in short-circuit current (Isc) for the coated jute–sisal fibre panel (1.23 A) compared to PET (1.15 A) cannot be attributed solely to temperature reduction. The positive temperature coefficient for Isc is generally minimal (~+0.05%/°C), and in fact, Isc tends to decrease slightly or remain constant as temperature decreases. Therefore, the primary reason for the increased Isc is likely the result of enhanced light absorption and reduced internal optical reflection losses enabled by the natural fibre composite back sheet. The jute–sisal back sheet, especially in its coated configuration, may exhibit diffuse light scattering properties, which help redirect incident light within the panel, thereby increasing photogenerated carrier generation. Additionally, improved heat dissipation at the back sheet helps maintain better charge carrier mobility, reducing recombination losses and improving charge collection—indirectly contributing to higher Isc. Thus, the increase in Isc is the result of a combination of optical and material factors, not just thermal effects.

5.3. Experimental Investigation: Phase II

This study was conducted to evaluate the efficiency and temperature performance of solar panels. To enable individual performance analysis, each solar panel was connected separately using Tungsten Inert Gas (TIG) welding. TIG welding was chosen for its ability to deliver precise, high-quality joints, particularly suitable for solar panels and their aluminium framing. The solar panels were manufactured and temporarily installed on the rooftop of an institutional building, “Architectural Block, SRM Institute of Science and Technology, Kattankulathur, Tamil Nadu, India.” The solar panels were connected to a storage battery using a series configuration. Voltage, current, and temperature readings were recorded separately for each panel. To ensure accurate measurements, data was collected manually and individually by temporarily disconnecting the series connection for each panel type. The experimental study was carried out over a period of 90 days, from October to December 2023. According to previous research by Jacobson and Jadhav [47,48] in 2018, which utilised data from the National Renewable Energy Laboratory (NREL) PV Watts programme [49], the optimal tilt angle for solar panels in many regions was determined. For India, particularly in Chennai, the ideal tilt angle is 13°. Therefore, the test panels were positioned at a 13° tilt for the duration of the experimental analysis.

Voltage and current gains from the solar panels were manually recorded at 30 min intervals using a multimeter to measure both direct current and voltage. Based on the collected data, the solar panels with coated jute–sisal fibre back sheets exhibited higher voltage outputs compared to others. Notably, even the uncoated jute–sisal fibre panels showed significantly higher voltage than the conventional PET panels. To evaluate the performance of each panel, a decade resistance box (DRB) was employed to apply a resistive load. This resistive load mimics the type of resistance solar panels typically face in real-world applications [50]. The DRB was used to set varying resistance values, which were connected in series with each solar panel for performance testing. By measuring the voltage and current across the load using a multimeter (HTC Instruments, Mumbai, India), the power outputs under different load conditions were determined. Experimental results revealed that from 10:00 AM to 1:30 PM, both voltage and current gradually increased, while between 2:00 PM and 5:00 PM, they steadily declined. Figure 12a,b illustrate the open-circuit voltage and current trends of the solar panels, respectively. From the analysis, panels with coated jute–sisal fibre showed an 8% increase in voltage compared to conventional PET panels. Panels with uncoated jute–sisal fibre demonstrated a 4% voltage increase over conventional ones. The values presented are based on average open-circuit voltage and current measurements taken over a three-month period. Likewise, current output was 6% higher in coated jute–sisal fibre panels, and 3% higher in uncoated jute–sisal fibre panels, compared to the conventional PET type.

The surface temperatures of the solar panels were measured using a Fluke thermal imaging camera. Controlling the temperature around solar panels is essential, as excessive heat can negatively impact the efficiency and lifespan of the solar cells. Natural fibres coated with zeolite demonstrate potential for providing thermal regulation by minimising heat accumulation. Panels incorporating coated jute–sisal fibres exhibited lower surface temperatures [refer to Figure 13a], showing a reduction of approximately 1–3 °C compared to panels with conventional PET back sheets. Figure 14 displays thermal images captured by the Fluke camera along with their respective timestamps. The power output (in watts) was also calculated for each panel and presented in Figure 13b. Results show that panels with coated jute–sisal fibres produced 12% more power than conventional PET panels, while panels with uncoated jute–sisal fibres generated 6% more power compared to the traditional back sheet.

The experimental duration was 90 days. Equations from [28,51,52] were used to calculate the solar panels’ electrical power capacity, efficiency percentage, and solar-panel output power. The area of the panel is calculated as length × width (in m²). The factor 1000 is used to convert the power output per unit area from watts per square metre to a percentage [53]. SIr stands for solar irradiation, and a constant value of 5 kWh/m²/day was assumed for this analysis. The efficiency of the conventional PET solar panel was found to be 9.75%, while the coated jute–sisal fibre panel achieved 10.85% efficiency, and the uncoated jute–sisal fibre panel reached 10.2%. The slightly higher efficiency observed in natural fibre-based solar panels can be attributed to their improved thermal stability. Zeolite-coated fibres help minimise heat build-up, thereby protecting solar cells from excessive thermal exposure.

According to the power output calculations, the conventional PET panel produced 1.95 watts, the coated jute–sisal panel generated 2.17 watts, and the uncoated jute–sisal panel delivered 2.04 watts. Measurement uncertainty (MU) may occur in experimental values due to several factors, such as the limitations of the measurement instrument, variations in the quantity measured, environmental conditions, and human error [54,55]. The measurement uncertainties, the margin of error (MoE), and the confidence intervals (CI) were calculated for the experimental values and presented in

Table 7. Measurement uncertainties for the experimental results along with MoE and CI.

Description	Sample	Standard Deviation	MU	Average	Confidence Level: 0.95 and Z Score: 1.96
					MoE	CI
Current (A)	PET Solar Panel	0.10	0.0270	0.98	0.01	0.97 1.00
	CJS Solar Panel	0.11	0.0291	1.06	0.01	1.05 1.07
	JS Solar Panel	0.11	0.0293	1.02	0.01	1.01 1.04
Voltage (V)	PET Solar Panel	0.45	0.1164	2.88	0.06	2.82 2.94
	CJS Solar Panel	0.38	0.0991	3.20	0.05	3.15 3.25
	JS Solar Panel	0.41	0.1053	3.02	0.05	2.96 3.07
Temperature (C)	PET Solar Panel	10.04	2.5929	43.10	1.31	41.79 44.41
	CJS Solar Panel	9.84	2.5398	39.87	1.29	38.58 41.15
	JS Solar Panel	9.83	2.5380	41.66	1.28	40.38 42.95
Power (W)	PET Solar Panel	0.72	0.1861	2.87	0.09	2.78 2.97
	CJS Solar Panel	0.74	0.1903	3.43	0.10	3.33 3.52
	JS Solar Panel	0.73	0.1882	3.12	0.10	3.03 3.22

. The confidence level was fixed at 95% (i.e., 0.95) to calculate the margin of error and confidence.

6. Statistical Analysis

The Design Expert software (Version 13, Stat-Ease, Inc., Arden Hills, MN, USA) conducted the statistical analysis for conventional, uncoated jute-mixed sisal fibre and coated jute-mixed sisal fibre panels. This advanced statistical tool is widely recognised for its comprehensive capabilities in experimental design, data analysis, and identifying significant factors and their interactions [56,57]. The software’s intuitive interface facilitates the easy entry and management of experimental data, while its robust analytical tools provide thorough insights into the results. Among these tools, the analysis of variance (ANOVA) (Version 13, Stat-Ease, Inc., Arden Hills, MN, USA) is particularly valuable for assessing the statistical significance of the effects observed [58]. The ANOVA table for the performance metrics of the standard, coated, and uncoated jute-mixed sisal fibre panels is presented in

Table 8. ANOVA summary for conventional PET, CJS, and JS solar panels.

Panel Type	Source	Sum of Squares	df	Mean Square	F-Value	p-Value
PET Panel	Model	7.28	6	1.21	1.6348 × 106	<0.0001	Significant
	A—Open-Circuit Voltage (V)	0.0697	1	0.0697	93,948.57	<0.0001
	B—Current (A)	0.0398	1	0.0398	53,631.78	<0.0001
	C—Temperature C	1.332 × 10−6	1	1.332 × 10−6	1.80	0.2170
	AB	0.0007	1	0.0007	929.79	<0.0001
	AC	4.077 × 10−7	1	4.077 × 10−7	0.5495	0.4797
	BC	2.262 × 10−9	1	2.262 × 10−9	0.0030	0.9573
	Residual	5.936 × 10−6	8	7.420 × 10−7
	Cor Total	7.28	14
CJS Panel	Model	7.60	6	1.27	1.138 × 106	<0.0001	Significant
	A—Open-Circuit Voltage (V)	0.0322	1	0.0322	28,912.56	<0.0001
	B—Current (A)	0.0117	1	0.0117	10,514.89	<0.0001
	C—Temperature C	1.309 × 10−6	1	1.309 × 10−6	1.18	0.3099
	AB	0.0006	1	0.0006	545.62	<0.0001
	AC	3.577 × 10−8	1	3.577 × 10−8	0.0321	0.8622
	BC	2.899 × 10−7	1	2.899 × 10−7	0.2604	0.6236
	Residual	8.907 × 10−6	8	1.113 × 10−6
	Cor Total	7.60	14
JS Panel	Model	7.44	6	1.24	1.127 × 106	<0.0001	Significant
	A—Open-Circuit Voltage (V)	0.0343	1	0.0343	31,133.03	<0.0001
	B—Current (A)	0.0489	1	0.0489	44,413.06	<0.0001
	C—Temperature C	2.789 × 10−7	1	2.789 × 10−7	0.2535	0.6282
	AB	0.0005	1	0.0005	454.66	<0.0001
	AC	7.089 × 10−7	1	7.089 × 10−7	0.6443	0.4454
	BC	4.430 × 10−7	1	4.430 × 10−7	0.4026	0.5435
	Residual	8.803 × 10−6	8	1.100 × 10−6
	Cor Total	7.44	14

, providing a detailed comparison of their respective contributions to the overall performance of the solar panels. This rigorous statistical approach ensures that the findings are reliable and significant, underpinning the advantages of the zeolite–polyester coating in enhancing the efficiency and durability of solar panels.

The results of an ANOVA (analysis of variance) (Version 13, Stat-Ease, Inc., Arden Hills, MN, USA) are shown in Table 8 for three distinct types of solar panels: uncoated jute-mixed sisal fibre (JS) panel, coated jute-mixed sisal fibre (CJS) panel with zeolite–polyester resin, and conventional polyethylene terephthalate (PET) panel. With a p-value of less than 0.0001, the model for the PET panel is highly significant, suggesting that the response is substantially affected by the chosen components, including open-circuit voltage, current, temperature, and their interactions. The strong impact of open-circuit voltage and current is further confirmed by their significantly high F-values (93,948.57 and 53,631.78, respectively). There is also a significant interaction between open-circuit voltage and current (AB) (F-value of 929.79, p < 0.0001). However, the temperature interactions (AC, BC) are not statistically significant. Comparably, the model for the CJS panel has high F-values for open-circuit voltage and current (28,912.56 and 10,514.89, respectively) and is reliable (p < 0.0001). While interactions involving temperature (AC, BC) are not significant (p > 0.05), the interaction term AB is significant (F-value of 545.62, p < 0.0001).

The three solar panels’ performance parameters are revealed through statistical analysis. The CJS and JS panels have the lowest standard deviation (Std. Dev) at 0.0070 (see

Table 9. Statistical analysis summary of conventional PET, CJS, and JS solar panels.

Description	Conventional PET Panel	CJS Panel	JS Panel
Standard Deviation (Std. Dev)	0.0106	0.0070	0.0070
Mean	2.87	3.42	3.42
C.V.%	0.3700	0.2032	0.2032
R2	0.9999	0.9999	0.9999
Adjusted R2	0.9998	0.9999	0.9996
Predicted R2	0.9996	0.9998	0.9998
Adeq Precision	314.0081	473.7057	477.7057

), whereas the PET panel’s is 0.0106, suggesting that the previous panels’ results are more consistent. The CJS and JS panels have similar mean values of 3.42, which is greater than the PET panel’s mean of 2.87 and indicates better average performance. The CJS and JS panels exhibit less variability in their mean values, as seen by their lower percentages (0.2032%) in the coefficient of variation (C.V.%) as compared to the PET panel (0.3700%). All panels have extremely high R2 values (0.9999), indicating a perfect model fit. The corrected R2 and predicted R2 values, similarly near R2, further support the models’ accuracy and prediction ability. All panels have Adeq precision values significantly higher than 314.0081, with the CJS and JS panels having values of 473.7057 and 477.7057, respectively. High Adeq precision ratings indicate sufficient signal-to-noise ratio and model discrimination. Compared to the traditional PET panel, these data collectively show that the CJS and JS panels perform better overall in consistency, mean values, and precision.

Figure 15a displays the performance metrics for the conventional PET solar panel, showing the relationships between open-circuit voltage, current, and temperature. The data highlights the significant impact of voltage and current on efficiency, with temperature having a smaller influence. Consistency and repeatability of results are also emphasised. Figure 15b illustrates the results for the CJS solar panel with similar parameters. The CJS panel demonstrates higher mean values and lower standard deviations than the PET panel, indicating superior and more consistent performance. Voltage and current are vital contributors to efficiency, while temperature remains a minor factor. The interaction effects between these variables are also highlighted. Figure 15c presents the results for the JS solar panel, comparable to Figure 15a,b. The JS panel shows high mean values and low standard deviation, reflecting high consistency and efficiency. Voltage and current significantly impact performance, whereas temperature has a minor role. Interaction effects are notably favourable with those in the PET and CJS panels.

Figure 16a presents additional performance metrics for the conventional PET solar panel, focusing on specific efficiency rates and the influence of environmental conditions. This figure emphasises the significant impact of open-circuit voltage and current on the panel’s performance, with temperature playing a relatively minor role. Figure 16b offers more profound insights into the performance of the CJS solar panel, providing a detailed analysis of efficiency, durability, and performance under varying conditions. Similarly to Figure 16b, it demonstrates high mean values and low variability, reflecting the CJS panel’s superior efficiency and consistency.

The significance of open-circuit voltage and current is clear, with detailed interaction effects further enhancing overall performance. Figure 16c examines the JS solar panel’s performance, presenting data comparable to Figure 16b,c. This figure focuses on efficiency, consistency, and impact of environmental factors, confirming the JS panel’s high performance and low variability. The results indicate that open-circuit voltage and current significantly affect performance, while temperature has a minor impact. The detailed interaction effects in this figure highlight how different parameters influence overall efficiency.

In summary, Figure 16b,c (CJS and JS panels) consistently exhibits higher performance metrics and lower variability than Figure 16a (PET panel), indicating superior efficiency and consistency. Open-circuit voltage and current remain significant factors across all figures, with temperature showing less significance. Interaction effects between voltage and current are substantial and beneficial in the CJS and JS panels, enhancing overall performance. Each figure provides a detailed analysis of how specific operational metrics and environmental conditions affect the performance of the respective solar panels, offering a comprehensive comparison of their efficiencies and consistencies. These comparisons clearly illustrate the operational advantages of CJS and JS panels over conventional PET panels.

7. Discussion

The results of this study confirm that natural fibre composites, when appropriately modified, can serve as viable substitutes for petroleum-based PET back sheets. Under controlled testing, the coated jute–sisal (CJS) panels recorded a short-circuit current of 1.23 A and an open-circuit voltage of 12.79 V, compared to 1.15 A and 12.48 V for PET panels. This translated into a maximum power output of 14.79 W and an efficiency of 13.47%, which is higher than both PET (11.96%) and uncoated JS panels (12.69%). The high fill factor (94.03%) suggests that the CJS back sheet maintained low recombination and resistive losses.

A significant advancement of the CJS design lies in its enhanced thermal management, surpassing the capabilities of earlier studies (such as [59,60]). Thermographic analysis revealed that CJS panels operated at surface temperatures 1–3 °C lower than those of PET panels. Since the voltage in PV modules is highly temperature-dependent, this reduction contributed to maintaining higher open-circuit voltages and reducing performance losses at elevated irradiance levels. The improved thermal conductivity of the zeolite–polyester coating (0.25 W/m·K) compared to PET (0.20 W/m·K) supports faster heat dissipation, ensuring greater operational stability.

When compared with previous works, the present study demonstrates a significant advancement. Shalwan et al. [61] reported that sisal–epoxy composites provided good insulation properties but produced efficiencies only in the range of 10–11%, limited by weak fibre–matrix bonding and hydrophilicity. Similarly, bio-based flax–polyester composites studied by Walter et al. [29] achieved efficiencies of about 10–12%, but were restricted by dimensional instability under humid conditions. More recently, Sathyanarayanan et al. [28] investigated zeolite–polyester composites and reported surface temperature reductions of 2–4 °C in PV applications, but without showing efficiency gains beyond 12%. In contrast, the current study demonstrates that by integrating zeolite–polyester coatings with jute–sisal fibres, both thermal management and electrical efficiency are simultaneously enhanced, reaching an efficiency of 13.47%, which exceeds the typical values reported for natural fibre-based panels.

These findings suggest that natural fibre composites can offer more than just sustainability benefits. The zeolite–polyester coating improves crystallinity, interfacial bonding, and moisture resistance, while also enabling effective heat regulation. This combination allows for the CJS back sheet to surpass the electrical performance of PET panels, while offering the added benefits of biodegradability and a reduced carbon footprint. Therefore, the present study not only confirms the technical feasibility of natural fibres for PV back sheets but also establishes their competitiveness against conventional synthetic alternatives.

Similarly, the statistical analysis further supported these findings. The high R² values (0.9999) and adjusted R² values (0.9998) for all panels suggest that the models accurately represent the performance data, with the CJS and JS panels showing exceptionally high predictive capabilities. The Adeq precision values for the CJS and JS panels were also notably high (473.7057 and 477.7057, respectively), indicating a solid signal-to-noise ratio and excellent model discrimination. ANOVA results highlighted the significant impact of open-circuit voltage and current on panel performance. The statistical analysis revealed that the CJS panel’s open-circuit voltage and current had very high F-values of 28,912.56 and 10,514.89, respectively (dimensionless), demonstrating their strong influence on panel efficiency. Similar trends were observed for the JS panel, with significant F-values for open-circuit voltage and current. In contrast, the temperature interactions (AC and BC) were not statistically significant, suggesting that temperature had a minor role in affecting the panels’ performance. The interaction effects between voltage and current (AB) were significant across all panel types, further enhancing the overall performance of the CJS and JS panels compared to the PET panels. This interaction is crucial as it demonstrates these parameters’ combined effect on the solar panels’ efficiency.

8. Conclusions

This study demonstrated that zeolite–polyester resin-coated jute–sisal (CJS) fibre composites can serve as an effective and sustainable alternative to conventional PET back sheets in photovoltaic modules. The CJS panels achieved an efficiency of 13.47%, compared with 11.96% for PET and 12.69% for uncoated jute–sisal (JS), while the maximum power output increased to 14.79 W against 13.39 W for PET and 13.87 W for JS. An open-circuit voltage of 12.79 V further enhanced electrical performance and a short-circuit current of 1.23 A, corresponding to improvements of approximately 8% and 6% over PET panels. Thermographic measurements confirmed that the CJS panels operated at surface temperatures 1–3 °C lower than PET, thereby reducing heat-induced voltage drops and supporting stable operation under real-world conditions. In addition, the coated fibres exhibited higher tensile strength (up to 25.8 MPa) and a crystallinity index of 99.47%, ensuring better mechanical stability and resistance to degradation compared with uncoated fibres. These results indicate that the proposed back sheet not only surpasses PET in terms of efficiency and thermal regulation but also provides an eco-friendly solution that addresses the issues of non-biodegradability, limited thermal resistance, and high carbon footprint associated with synthetic polymers. Although this study demonstrated enhanced performance of coated jute–sisal fibre back sheets over a 90-day outdoor period, the long-term service life of solar panels typically exceeds 25 years. To bridge this gap, future work should focus on long-term durability assessments through accelerated ageing tests and the scaling of these materials for commercial photovoltaic applications.

9. Patents

Patent was published; application number: 202441029390 A; title of the invention: a vacuum-assisted resin transfer moulding method for manufacturing high-efficiency and non-toxic solar panels.