Enhancing photo voltaic solar panel efficiency by using a combination of silica based and phase change material coating

This study attempts to enhance the overall efficiency of a photo voltaic solar panel by employing a dual-coating approach. The initial coating consists of a Silica-based anti-reflective material synthesized through an acid-catalyzed sol–gel process, utilizing cetyltrimethylammonium bromide as a template. Subsequently, the second coating was formulated using a phase change material, such as expanded graphite infused with paraffin jelly. The coating materials were characterized using Ultraviolet-Visual ray spectroscopy, Fourier Transform-Infrared Ray spectroscopy, Field Emission-Scanning Electron Microscopy, x-ray Diffraction sepctroscopy, and Thermogravimetry analysis. The panels’ performance had been investigated under three conditions: uncoated, single-coated, and double-coated. These panels were subjected to both indoor and outdoor experiments. Indoor tests were conducted in a laboratory with induced irradiance levels of 350, 600, and 850 W/m2. The corresponding variations in maximum power output, maximum surface temperature and peak current were recorded. For the outdoor experiments, two sets of panels were installed on a rooftop. One set underwent daily surface cleaning, while the other was left untouched, allowing dust accumulation. Over a 45-day period, outdoor experiments were carried out (daily cleaned and unclean panels) to examine the impact of dust accumulation on power loss, current, voltage, surface temperature, energy generation and panel efficiency. Upon comparing the performance of uncoated, single-coated, and double-coated solar panels, it was observed that photovoltaic solar panels coated with both silica-based anti-reflective coating and paraffin jelly-infused expanded graphite coating exhibited superior performance when compared to other coating options.


Introduction
Photovoltaic (PV) solar panels serve as an effective tool in harnessing renewable energy and confronting climate change.They offer cost-effective solutions for both residential and commercial energy requirements, contributing to reduced carbon emissions [1].However, challenges in using solar panels include inconsistent performance and susceptibility to environmental factors [2].The current market offers PV panels with 8%-12% conversion efficiency, leaving much sunlight untapped [3].Regular maintenance is essential as panels are being exposed to harsh weather [4], shading, snow, dust [5], rain and smoke [6].To address these challenges, antireflective coatings (ARCs) are applied, reducing light reflection and enhancing efficiency [7], with self-cleaning ARCs showing a 5%-6% efficiency improvement even in adverse weather conditions [8].
Incorporation of plasmonic nanoparticles into ARCs resulted in higher panel efficiency [9].Dual-layer ARC coating with SiO 2 and TiO 2 optimized panel output and increased erosion resistance [10].Panels exposed to dust and extreme weather conditions exhibited low panel output (50% loss) [11].Researchers identified diverse ARC materials.A hybrid SiO 2 , Al 2 O 3 , and TiO 2 coating effectively reduced reflection and had a high refractive index [12].In ARC, blend of ZnO and Ta 2 O 5 achieved high refractive index and panel durability, resulting in 19.2% power conversion efficiency [13].Choosing ARCs depends on the technology, manufacturing process, performance criteria and cost.Phase change materials (PCMS) help in optimizing solar panel output and temperature control [14].PCM coatings assist in heat dissipation, panel temperature reduction and improving efficiency [15].Paraffin Jelly Infused Expanded Graphite (PJ-I-EG) are PCMs, offering excellent anti-reflective properties.Expanded Graphite's high thermal conductivity lowers panel temperature, enhancing durability in diverse conditions and increasing solar ray absorption for higher energy conversion efficiency [16].
A lot of investigations have been conducted in the recent times regarding ARCs and phase change materials.Law et al (2023) [17] conducted a comprehensive evaluation of anti-reflection coatings for solar module shield glass, examining their relevance in mitigating reflection losses in photovoltaic solar panels.Materials, deposition methods and durability concerns pertaining to solar panels were investigated, while proposing future recommendations like diversification for sustainability.Womack et al (2019) [18] conducted studies on evaluating single-layer sol-gel anti-reflection coatings for solar panel glass modules.While these coatings significantly minimized reflection losses, their long-term endurance, particularly in hot and humid environmental conditions, raised concerns.Intensified weathering and durability experiments disclosed some resilience, but exposure to dampness and acid posed complications.Ultimately, this study advocated that sol-gel anti-reflection coatings might not be the best option for extended use in photovoltaic applications, especially those requiring a long lifespan.
Zimmermann et al (2023) [19] conducted studies on sol-gel based coatings for solar panels, analyzing coating structure's contribution in dust deposition and elimination.Hydrophilic sol-gel coatings demonstrating a variety of surface morphologies determined by colloidal silica particles were analyzed for anti-soiling behavior.Tests indicated that coatings considerably minimized dust accumulation over 10 μm.Coatings with minimal structures aided in the smoother removal of dust by the wind, on the other hand, coatings with more pronounced structures (>40 nm) facilitated the removal of larger, round particles.Chauhan & Singh (2023) [20] conducted research to improve solar cell performance through Anti-Reflection Coating.By using high-index silicon as the substrate and accurate film thickness, it sought to reduce surface reflectance and increase optical energy utilization in photovoltaic modules.The research included coating films of different dielectrics with refractive indices estimated using the 'Root-Principle' and assessing the spectra of reflected visible rays in oblique and normal incidence, applying six layers of antireflection coatings, to reach close to zero reflectivity.Zeng et al (2023) [21] performed a comparative study on the sustainability of antireflection coatings with internal pores and substitutes for dense photovoltaic module coatings.These coatings are essential to resist wear and hostile environment.Different commercial solar glass specimens were studied for their optical properties, wear resistance, and outdoor durability.Dense coatings exhibited enhanced durability, yet it marginally reduced optical properties.Double-layer dense coatings were examined to balance durability and optical enhancement.
Luo et al (2023) [22] researched a bio-friendly, transparent superhydrophobic film for coating photovoltaic glasses, avoiding hazardous fluorosilane methods.This non-fluorinated dip-coating method combined antireflective and superhydrophobic layers, improving transparency.The film demonstrated UV resistance, high transmittance, self-cleaning aspects, tolerance to acidic and deionized water exposure.Aldawood et al (2023) [23] improved Myristic Acid (MA) Phase Change Material (PCM) for photovoltaic temperature reduction using vulcanization and elastomeric coating.0.5 phr sulfur-cured Nitrile Butadiene Rubber (NBR) coating improved heat resistance and prevented leakage.It was better than prior works, reducing PV module temperature by 17 °C, enduring 1000 thermal cycles and increasing voltage production by 7.92%.This study advances PCM-enabled PV cooling.
Maghrabie et al (2023) [24] studied the performance improvement of PV panels using paraffin wax RT-42 Phase Change Material (PCM).Outdoor tests with different panel tilt angles and PCM thicknesses exhibited higher electrical efficiency of PV-PCM panels, peaking at a 14.4% increase over the standard panel at a 30°tilt angle.PraveenKumar et al (2023) [25] conducted experiments using PCMs for enhancing solar PV panel efficiency.Economical aluminum sinks, aluminum reflectors and PCM with ZnO based nanoparticles reduced energy costs, improved electrical output, cooling, and CO 2 emission avoidance rates.
Even though numerous investigations have been conducted on ARCs (Anti-Reflective Coatings) for solar panels, there is still scope for reducing temperature fluctuations, dust accumulation, and improving absorption efficiency on the PV panel surfaces.Hence, in this study, an attempt has been made to enhance the overall effectiveness of PV solar panels by using a combination of coatings.SiO 2 nanoparticles-incorporated coating material was employed to improve the efficiency of photovoltaic solar panels by reducing the reflection of incident light.To decrease the operating temperature of the panels and increase their performance, phase change material-based coating, such as Petroleum Jelly-incorporated Expanded Graphite, was utilized.Variations in peak current, surface temperature, power, and energy generated by the panels using these coatings were analyzed.Outdoor experiments were conducted on the panels under both clean and unclean conditions to assess the effect of dust accumulation on panel performance.

Photovoltaic solar panel test kit
In this study, an indigenously made PV solar panel experimental kit (Make-ITPL) was used.The key aspects and characteristics of the PV test kit are shown in table 1.
The PV solar panel was fixed on the roof top of Research & Development Lab, M/s.Vikram Engineering Industry, Tiruchirappalli, Tamil Nadu, India (Latitude − 10.81855, Longitude − 78.7769, Altitude-88 m above sea level).To measure the current and voltage, a digital multimeter (Make: SG Flash 9205) was used.For evaluating and plotting the current-voltage (I-V) characteristics of the PV solar panel and to provide a graphical representation of the panel's performance under different operating conditions, an I-V curve tracer (Make: SGDA 15 M) was used.Pyranometer (Make-SUBITEK) was used for measuring the total solar irradiance (W/m 2 ) reaching the PV panel.Data loggers with software-enabled computers were used to evaluate and store the values of power, current and voltage developed in the solar panels.A temperature transducer (Make -CHINO, range −30 °C to + 80 °C was used to measure the temperature of the PV solar panel.One transducer was fixed on the top of the solar panel kit to measure the temperature on the top surface, while another transducer was fixed at the bottom of the solar panel kit to measure the temperature on the rear side.The experiments were conducted in two types.One set of experiments were conducted indoors, in laboratory and another set of experiments were conducted outdoors, in external environment.

Indoor experiments
Indoor laboratory experiments were also carried out by subjecting the panels to three specific irradiances of 350, 600, and 850 W m −12 .Irradiance experiments under each irradiance values were conducted for duration of 6 h.For each irradiance value, important panel output characteristics such as peak current (I peak ), maximum power (P max ) peak power and peak temperature (T max ) was identified.In laboratory studies, contact angle variations due to coatings were also studied.For each condition, the measurement was done thrice and its average was taken as the final value.

Outdoor experiments
Outdoor experiments were conducted in broad daylight, from approximately 1:00 PM to 3:00 PM, when the sky was clear and without clouds.The outdoor experiments were conducted for duration of 45 days using two sets of panels (clean and unclean).
(i) Clean panels -One set of outdoor panels were surface cleaned daily.The surfaces of the panels were cleaned by using an air blower to remove debris, and a gentle, soft brush was used to remove and wipe away smallsized dust.
(ii) Unclean panels -Another set of experiments was conducted on panels without surface cleaning for 45 days.
For the unclean panels, dust accumulation was measured daily and the daily average surface dust accumulation density was used to identify the variations in panel outputs.For both clean and unclean panels, the energy, open circuit voltage, short circuit current and the power generated were compared.The solar panel test kit used for conducting the experimental studies is shown in figure 1.
For minimizing the errors in data measurement and other variations, the uncertainty of the equipments used in this experiment were considered.The sum of highest percentage of uncertainty of used equipments was calculated by equation (1) [26].
In the above equation, TU is the total uncertainty and IU is the individual equipment uncertainty of the different factors which includes open circuit voltage, short circuit current, power and energy developed, peak current, maximum and minimum panel surface temperatures.The equipment uncertainties are shown in table 2. The sum of maximum percentage of uncertainty found to be about ±0.75.

Preparation of silica based anti reflective coating
In this research, SiO 2 nanoparticles incorporated Anti-Reflective Coating (ARC) was prepared using cetyltrimethylammonium bromide (CTAB) under acidic conditions.For ARC preparation, 99.9% pure SiO 2 nanopowder (15-30 nm in size), CTAB, ethanol (EtOH), hydrochloric acid (HCl) (40% by weight), distilled water (H 2 O), and tetraethylorthosilicate (TEOS) were purchased from M/s. Sigma Aldrich Chemicals Pvt.Ltd, Bengaluru, India.As the purchased chemicals were of high purity, they were used directly in the experiments without the need for further purification.The chemicals, such as EtOH, CTAB, TEOS, HCl, and H 2 O, were mixed in the molar ratio of 46:0.2:2:0.008:24 to prepare the ARC solution [10].
For a duration of 6 h, the solution was continuously stirred at a temperature of 70 °C.Then, it was allowed to sit in a dark and cold place for 24 h.After 24 h, it was filtered using a Millipore filter.SiO 2 nano powder was then added to the prepared solution at a concentration of 50 mg L −1 and ultrasonicated for 15 min.The prepared ARC solution was stored in an airtight container to prevent contamination.For coating silica sol based ARCs, spray coating technique was found to be economical, easy to use and quicker coating without complicated  procedures [17].Using the spray coating method, the prepared SiO 2 -incorporated Anti-Reflective Coating (SiO 2 -I-ARC solution) was deposited onto borosilicate glass (which was placed over the panel surface).
Cleaned borosilicate glass sheets were stored in containers filled with ethanol.Prior to spray coating, the glass sheets were heated to 80 °C using a box furnace.The Anti-Reflective Coating (ARC) was sprayed onto the glass at 38 psi pressure, using a 1.25 mm nozzle and maintaining a 10 mm distance between the nozzle and the glass substrate throughout the spraying process.
As the coated panels were subjected to dusty environments and measurements were done in clean and unclean surfaces with dust accumulation, 250 micro meter thick coatings were preferred from trial experiments and previous literatures [27,28].From trial experiments, thicknesses lower than 250 μm did not contribute much for improving the output.Increase of coating thickness beyond 250 μm increased the opacity of the surface and affected the transmittance.Thus, coating was done to achieve a thickness of 250 μm.Subsequently, the coated glass surface was baked at 200 °C for duration of 45 min and tempered at 700 °C for 5 min to remove CTAB [10].The prepared SiO 2 -incorporated Anti-Reflective Coating Solution (SiO 2 -I-ARC) was used as the first coat.

Preparation of paraffin jelly infused expanded graphite for coating
In this investigation, a phase-change material, such as Paraffin Jelly Incorporated Expanded Graphite, was indigenously used as the second coat.High-quality expanded graphite powder (size: 12-18 μm) and soft paraffin jelly were purchased from M/s. BRM Chemicals, Jaipur, Rajasthan, India.Initially, the paraffin jelly was melted in a container.An electric heater was used to slowly raise the temperature to 65 °C while continuously stirring.Once the Paraffin Jelly had completely liquefied, an equal quantity of Expanded Graphite powder (at a 1:1 ratio) was gradually added, and stirring was maintained to achieve a homogeneous mixture.After agitating the mixture for 15 min, the contents were transferred into a vacuum chamber.Before transferring the contents into the vacuum chamber, the chamber was preheated to 80 °C using hot water.In the vacuum chamber, the pressure was maintained at 0.01 MPa, and the mixture was allowed to set for 45 min.After removing the contents from the vacuum chamber, they were allowed to settle for 2-3 h [29].After complete absorption of the Paraffin Jelly into the pores of Expanded Graphite, Paraffin Jelly-Infused Expanded Graphite (PJ-I-EG) was formed.This prepared PJ-I-EG was used as the top coat for PV solar panels.From literature studies [30,31] and trial experiments the thickness of the coating was fixed at 200 μm.During trial experiments, thinner films (less than 200 μm) did not contribute in maintaining lower panel surface temperatures and thicker films (greater than 200 μm) induced opacity due to its high porous nature.Using a pressured dispensing unit, a thin coat of PJ-I-EG (200 μm coating thickness) was applied over the glass substrate.The variations in the performance of the PV solar panel were measured in the following conditions: uncoated, with a single coat of SiO 2 -I-ARC, with a single coat of PJ-I-EG, and with a double coat of SiO 2 -I-ARC and PJ-I-EG.

Characterization techniques used for identifying the characteristics of the prepared coating materials
The prepared SiO 2 nanoparticle-incorporated Anti-Reflective Coating solution (SiO 2 -I-ARC) and Petroleum Jelly-Infused Expanded Graphite (PJ-I-EG) were subjected to characterization tests.

Ultra violet visual ray spectroscopy testing of SiO 2 -I-ARC and PJ-I-EG coating
The prepared SiO 2 -I-ARC and PJ-I-EG coating materials, in liquid form, were subjected to Ultraviolet-Visible (UV-vis) Ray Spectroscopic testing using equipment (Make -AVI Scientific).For UV-vis testing, samples with appropriate dilution were prepared, and variations in absorbance peaks were identified within the range of 300 nm to 650 nm.

Fourier transform infrared ray spectroscopic analysis of SiO 2 -I-ARC and PJ-I-EG coatings
The transmittance spectrum, indicating the bond growth and constrictions in the chemical bonding of SiO 2 -I-ARC and PJ-I-EG, was evaluated using Fourier Transform Infrared (FTIR) Spectroscopy (Equipment Make -M TEK Power Analytical Services).FTIR analysis of the coating materials was conducted from 4000 cm −1 to 400 cm −1 .An average of 6 scans was taken at intervals of 1.0 cm −1 .After acquisition, data normalization was performed, setting the maximum absorbance to 1.0 for comparative analysis.

FE-SEM analysis of SiO2-I-ARC and PJ-I-EG coatings
The modifications in surface, upon coating with SiO 2 -I-ARC, PJ-I-EG, and double coating, were evaluated using Field Electron Scanning Electron Microscopy (FE-SEM) equipment (Make -JEOL).The evaluation was conducted under a nitrogen atmosphere.Under high magnification, the microstructural variations of SiO 2 -I-ARC, PJ-I-EG, and the double-coated (SiO 2 -I-ARC + PJ-I-EG) surfaces were assessed.

X-Ray diffraction testing of SiO2-I-ARC and PJ-I-EG coatings
The chemical aspects of SiO 2 -I-ARC and PJ-I-EG were studied using x-ray Diffraction (XRD) spectroscopy (Make-Malvern Panalytical).XRD analysis was performed with a Cu target, a step count of 0.002, and a range from 0 to 100 degrees two theta.

Thermogravimetric analysis of SiO2-I-ARC and PJ-I-EG coatings
The effect of temperature variations on SiO 2 -I-ARC and PJ-I-EG was evaluated using Thermo-Gravimetric Analysis (TGA) equipment (Make-TA Equipment).These coatings were designed to enhance the performance of PV solar panels.The properties of the coating material were analyzed in the temperature range of 15 °C to 90 °C.TGA experiments were conducted under a nitrogen atmosphere.The temperature of the samples was increased at a rate of 10 °C min −1 , and the N 2 flow rate was maintained at 25 ml/min.For testing, 5 mg powder samples were taken and sealed in aluminum containers.The changes in material properties were recorded with each degree increase in temperature.The precision of the calorimeter and the temperature transducers used were within ±0.1% and ±2.5 °C.

Testing on the coated solar panels
To evaluate the performance of SiO 2 -I-ARC, PJ-I-EG, and double-coated (SiO 2 -I-ARC + PJ-I-EG) PV solar panels, the following tests were conducted.

Irradiance testing
Irradiance testing was conducted on both individual and double-coated solar panels.An artificial indoor radiance setup (Make-AN Electronics) was used to generate artificial irradiance.Experiments were conducted using three irradiance values: 350 W /m 2 , 600 W m −2 , and 850 W/m 2 .For solar panels coated with SiO 2 -I-ARC, PJ-I-EG, and double coating (SiO 2 -I-ARC -Inner, PJ-I-EG-Outer), IV and PV characteristics were identified.For each irradiance value, important parameters such as maximum power (P max ), peak current (I peak ) and peak temperature (T max ) were identified.

Contact angle variations
Using a contact angle goniometer (Make-Kyowa), the contact angle of water droplets on SiO 2 -I-ARC, PJ-I-EG, and the double-coated solar panels was measured.Generally, if the contact angle was less than 30°, it was termed as hydrophilic.Surfaces that exhibit a contact angle near 90°are termed hydrophobic.An increase in the contact angle beyond 90°improves the hydrophobic nature of the surface.When the contact angle nears 150°, the surfaces become superhydrophobic.

Effect of dust accumulation
To study the effect of dust accumulation, the performance of two sets of uncoated, single-coated, and doublecoated panels was evaluated under daily cleaned and uncleaned conditions.The delivered energies of the uncleaned panel kits were measured daily and recorded.To record the variations, reference values of clean solar panels (cleaned daily) were used for each coating.The energy, current, voltage, and power generated by the daily cleaned and unclean panels were recorded for 45 days.Readings were taken around 2:00 PM.The loss in power due to dust accumulation was calculated using the following equation In the above equation, ΔPD denotes the power loss due to dust accumulation, P Cl indicates the power output of a clean panel, and P Dt indicates the power output of the dusty panel.The dust accumulated over the coated panel surface was evaluated per square meter as the 'average dust density' and denoted as f Dt .After completing every five days, the power loss was graphically compared with the average dust density.The energy (E) delivered by the panels after each day was identified.Variations in short-circuit current (I sc ) and open-circuit voltage (V oc ) for the uncoated, single-coated, and double-coated panels were identified due to dust accumulation.

Panel temperature and average power production
To assess the effect of dust accumulation on varying panel temperatures and average power production of uncoated, single-coated, and double-coated photovoltaic solar panels, experiments were conducted over a period of 45 days.Each day, measurements were taken for the minimum and maximum temperatures on both clean and unclean panel surfaces.In figure 2 (a), a specific UV-vis absorbance at 417 nm was observed for the SiO 2 -I-ARC sol.There was a noticeable shift in the absorption peak, attributed to the partial collapse of the porous structures, according to Chorbadzhiyska et al (2023) [32].This shift was a result of the mesoporous walls becoming thinner due to excessive condensation between silanol groups, a consequence of ARC preparation under acidic conditions [33].The primary goal of ARCs is to enhance the transmission of radiation towards solar cells.This enhancement is associated with the refractive index of the coating, which is inversely related to its porosity, as per the Lorentz-Lorenz equation [34].Fresnel reflection losses occur because of the refractive index mismatch between the glass and the surrounding medium.The observed shift in the absorption peak suggests a reduction in porosity, which, in turn, leads to lower optical losses and decreased Fresnel reflection losses [35].This reduction in losses results in a higher refractive index for the coated surface.

Results and discussion
The interaction between SiO 2 nanoparticles and ARC sol resulted in an alteration of the light scattering behavior, primarily due to reduced optical confinement [36].SiO 2 nanoparticles in ARC sols can create a modified light pathway characterized by more total internal reflections [37], stemming from the differences in refractive indices between the nanoparticles and ARCs.
An increase in total internal reflections induces the occurrence of surface plasmon resonance [38].In an ARC, chemically stable nanomaterial like SiO 2 exhibits small particle size, a large specific surface area, and substantial volume voids [39].These characteristics enable the adsorption of a greater number of coating material molecules.These optical properties of SiO 2 nanoparticles contribute to a shift in surface plasmon resonance [40].
In figure 2 (b), the absorption peak at 316.3 nm signifies the presence of Expanded Graphite in the test sample solution, suggesting the integration of Expanded Graphite [41].Van der Waals forces, which are weak non-covalent interactions occurring between molecules and atoms, are responsible for both attraction and repulsion among the particles.They play a crucial role in maintaining the cohesion between the layers of graphite [42].The presence of this peak is attributed to the Van der Waals interactions between the dispersed layers of Expanded Graphite [43].FE-SEM image of SiO 2 -I-ARC coated surface (figure 4 (a)) indicates dense arrangement of SiO 2 particles in TEOS layer.At regions where presence of TEOS was greater, SiO 2 agglomeration was more.This resulted in minimization of pores, reducing scattering and thereby increasing transmittance [53], which is favorable for  This indicated that the force between materials was physical adsorption and only intermolecular forces existed.The surface morphology was similar to observations conducted by Fang et al (2021) [56].Such morphology was due to the surface tension and capillary force of expanded graphite under negative pressure [57].Petroleum Jelly was found to be absorbed on the macroporous walls of Expanded Graphite surface.This pattern of single-layer and multilayer adsorption was found in the observations conducted by Yasmin et al (2006) [58].This pattern of Expanded Graphite distribution ensured better thermal dissipation [59].

FTIR results of
FE-SEM image of double coat (figure 4 (c)) indicates partly fused SiO 2 particles in Expanded Graphite matrix.Thicker agglomerations of SiO 2 nanoparticles were identified due to its better surface activity [60].Despite the coating of PJ-I-EG over SiO 2 -I-ARC, we observed protruding SiO 2 nanoparticles within wormshaped graphite layers.In specific regions, expanded graphite appeared as blocky patches [61].The overall shape and dimensions of both the Expanded Graphene particles and SiO 2 nanoparticles resembled those found in single coats, indicating the stability of ARCs [62].These nanostructured ARC surfaces were found to enhance transmittance and improve the efficiency of photovoltaic power conversion [63].

X-RD evaluation of SiO 2 -I-ARC and PJ-I-EG
X-RD spectrum of SiO 2 -I-ARC and PJ-I-EG are shown in figure 5 (a) and (b).From Scherrer formula, the size of the crystals of SiO 2 -I-ARCs was found to be around 4 nm [64].This indicated that the refractive index of SiO 2 nanoparticles was adjusted with a reduced packing density in an amorphous structure.This improvement enhanced transmittance [65].X-RD diffraction of PJ-I-EG (figure 5 (b)) indicates diffraction peaks at 26.81°and 55.31°.At these two theta values, sharp and high intensity were observed.These observations indicate that the crystallinity of the Expanded Graphite was very high [66].Similar peaks indicated reduced damage of expanded graphite.This indicates improved interfacial adhesion of expanded graphite particles [67].This attributes to better performance and durability when expanded graphite is applied in combination with SiO 2 incorporated silica sols [68].On evaluating the thermal behavior of SiO 2 -I-ARC, PJ-I-EG and combination (SiO 2 -I-ARC+PJ-I-EG), defined exothermic peak for the heating curves and defined endothermic peak for the cooling curves were observed.From DSC heating curves, initial heating temperature T HI (°C), maximum heating temperature T HMAX (°C), heating end-set temperature T HEND (°C), heating curve enthalpy ΔH HEnt (J/g) were evaluated.Similarly, from DSC cooling curves, initial cooling temperature T CI (°C), maximum cooling temperature T CMAX (°C), cooling end-set temperature T CEND (°C) and cooling curve enthalpy ΔH CEnt (J/g) were evaluated.The corresponding values are shown in table 3.

Thermogravimetry analysis
From the values indicated in table 3, it was found that for PJ-I-EG material, the values of T HI and T CI were lower than that of SiO 2 -I-ARC.On combining PJ-I-EG with SiO 2 -I-ARC, distribution of SiO 2 nanoparticles in the phase change material matrix caused better heat dissipation.This attributed to lower T HI and T CI values for the combined coating samples, compared to individual PJ-I-EG and SiO 2 -I-ARC material.In DSC studies, compared to the heating curve enthalpy of PJ-I-EG, heating curve enthalpy ΔH HEnt 116.4 (J/g) for SiO 2 -I-ARC was less.This was due to movement restraining aspect of the SiO 2 nanoparticles [69].Similar enthalpy characteristics were observed in studies conducted by Cai et al (2011) [70] which ensures a thick non porous coating layer, ensuring better transmittance.T HMAX of 27.6 °C and ΔH HEnt 176.3 J g −1 indicates that Expanded Graphite requires less heat energy to overcome weaker non-graphitic hybridized carbon after oxidation reaction [71].This property improves better heat dissipation [72], favorable for reducing the overall surface temperature of the coated panels.Combination of the two coating sols was found to be favorable with ΔH HEnt 207.For each experiment, the peak current I peak (A), maximum power developed P max (W) and maximum surface temperature (T max ) was recorded and the values are shown in table 4.
On conducting experiments using different irradiance values on the uncoated, single coated and double coated photo voltaic solar panels, increase in I peak , P max and T max was observed while increasing the irradiance values from 350-850 W/m 2 .At a lower irradiance value of 350 W m −2 , double coated panels exhibited greater I peak and I max , compared to single and uncoated panels.Yet, T max of double coated panels at 350 W/m 2 was lower than that of the uncoated and single coated panels.On comparing the temperature ranges of SiO 2 -I-ARC and PJ-I-EG coated panels for all three irradiance values such as 350, 600 and 850 W m −2 , the operating temperature was found to be consistently lower in PJ-I-EG coated panels.Lower operating temperatures were due to the presence of phase change material in PJ-I-EG coated panels.On comparing the performance of double coated panels with single coated panels, double coated panels exhibited better I max , P max and lower T max .This was due to the presence of dispersed SiO 2 nanoparticles in Expanded Graphite matrix, which ensured faster heat dissipation [75].

Variations in contact angle
The contact angle on the surface of the un-coated coated and double coated panels was evaluated and the results are shown in table 5. Compared to uncoated panel (contact angle − 63.6°), SiO 2 -I-ARC and PJ-I-EG panels exhibited greater hydrophobicity.Contact angle of PJ-I-EG (92.4°) was greater than the contact angle of SiO 2 -I-ARC (86.3°) coated panels.This indicated that water retention of SiO 2 -I-ARC was greater than PJ-I-EG coated panels.On combining SiO 2 -I-ARC and PJ-I-EG, the shift towards super hydrophobicity was observed, as the contact angle of double coated panels was found to be as high as 113.6°.
The contact angle of the combined double coat (SiO 2 -I-ARCs + PJ-I-EG) in the present investigation was compared with the results of other reported literatures and the comparative study is shown in table 6.From  table 6, it is observed that the present research was helpful in developing a combination of SiO 2 -I-ARC and PJ-I-EG coating, which exhibited appreciably high contact angle.

Effect of dust accumulation
From figure 9 (a), it can be observed that f Dt consistently increased as the days progressed.Compared to coated surfaces, f Dt was greater on uncoated surfaces.The f Dt of coated surfaces was lower, which can be attributed to their lower water retention.This can be attributed to the presence of SiO 2 nanoparticles and infused phasechange material.As the days progressed, power loss (ΔPD) consistently increased for all panels, regardless of the coatings (figure 9(b)).A consistent increase in ΔPD was observed from the first to the 20th day.After the 20th day, the increase in ΔPD was minimal until the last day, which was the 45th day.
The ΔPD in the uncoated panel was consistently higher than in single and double-coated panels.When compared to uncoated panels, SiO 2 -I-ARC and PJ-I-EG coated panels exhibited lower ΔPD.When comparing the  power losses of double-coated panels with those of uncoated and single-coated panels, ΔPD in the double-coated panel was consistently lower.This was due to improved anti-reflection properties.On coated surfaces, the reflection losses and surface opacity were reduced.The transparency of the surface for solar light to pass through them was better maintained with double-coated panels compared to the uncoated and single-coated panels.
When comparing the variations in open-circuit voltage (V oc ) with dust accumulation density f Dt (figure 9(c)), a slight increase in Voc was observed in all panels.V oc was found to gradually increase at higher values of f Dt .Compared to uncoated and single-coated panels, double-coated panels exhibited lower V oc .Figure 9(d) shows the variations in short-circuit current (I sc ) with f Dt for the uncoated and coated panels.Isc reduced gradually as f Dt increased.When comparing the double-coated, single-coated, and uncoated panels, I sc variations for double-coated panels were consistently found to be greater, even at higher f Dt .

Variations in solar panel output characteristics
The variations in minimum panel temperatures (T min ), maximum panel temperatures (T max ), I max and V max , P max , Energy Delivered (ED) and obtained efficiency obtained (η) for the uncoated, single coated and double coated panels in daily cleaned and unclean conditions, for 45 days are shown in figure 10.   [80] Sodium alginate based 40°Hydrophillic Figure 10(a) and (b) show the minimum temperatures on the panel surfaces of the cleaned and unclean panels.Minor fluctuations in temperature variations were observed for both the uncoated and coated panels.However, the minimum daily temperatures were consistently lower for both double and single coated panels compared to uncoated panels.When comparing the temperatures of clean panels, the minimum temperatures of unclean panels were found to be 2.3% to 3.5% lower.Upon observation, this phenomenon was attributed to the increased quantity of moisture combined with the dust present on the unclean panels.
Similarly, variations in the maximum surface temperature on the uncoated, single-coated, and doublecoated surfaces of the clean and unclean panels over a 45-day period are shown in figure 10(c) and (d), respectively.The variations in maximum temperatures were greater than that in minimum temperatures.The uncoated panel experienced significant thermal fluctuations compared to the single-coated panels.Due to the presence of phase-change material (Expanded Graphite) in PG-I-EG coatings, faster heat dissipation occurred, resulting in consistently lower maximum surface temperatures compared to SiO 2 -I-ARC coated panels.When compared to single-coated panels, double-coated panels exhibited lower surface temperatures in all readings.Maximum surface temperatures in unclean panels were 2.3% to 5.5% greater than the surface temperatures of clean panels.
The variations in maximum current (I max ) for the clean and unclean panels are shown in figure 10(e) and (f), respectively.In both clean and unclean panels, I max increased due to the coatings.Consistently higher I max values were recorded in the double-coated panels throughout the 45-day period.
For the dusty, uncleaned panels, a reduction in T max was observed as the days proceeded.Variations in maximum voltage (V max ) for the clean and unclean sets of panels are shown in figure 10(g) and (h).Coatings caused a reduction in V max , enabling better output current.Fluctuations in V max were lower in cleaned panels, whereas uncleaned panels exhibited greater variations as the days progressed.
Variations in maximum power (P max ) for the clean and unclean panels are shown in figure 10(i) and (j).Although coatings improve P max in clean panels, a consistent decrease in P max was observed for the unclean panels as dust accumulation increased.
The energy (E) delivered by the clean panels (figure 10(k)) was consistent, while that of the unclean panels (figure 10(l)) consistently decreased, regardless of coatings.Similarly, stability in output efficiency in clean panels (figure 10(m)) and gradual reduction in output efficiency was observed for the unclean panels (figure 10(n)).This observation was attributed to the presence of dust and dirt on the surfaces of the unclean panels, which acted as thermal barriers to the coatings, hindering the rate of heat dissipation by the top coat (PJ-I-EG).When comparing the uncoated panels with single-coated panels, the energy delivered and output efficiency of the SiO 2 -I-ARC coated panel were greater than those of the uncoated panels.Furthermore, PJ-I-EG panels exhibited better performance compared to uncoated and SiO 2 -I-ARC coated panels.However, doublecoated panels consistently delivered higher energy throughout the experimentation process.The recorded values of average daily solar irradiance in outdoor experiments for the 45 days duration are shown in figure 11.
From figure 11, fluctuations in average daily solar insolation received on the panel surface were observed.In spite of the fluctuations in solar insolation, double coated panels delivered consistently greater than the single and uncoated panels (in both clean and unclean conditions).Variations in average power production (P avg ) for  Throughout the experimentation process, the average power (P avg ) of double-coated panels was consistently higher than the P avg developed by uncoated and single-coated panels.This was achieved due to a reduction in the loss of solar rays through reflection by the coated panels.Double-coated panels exhibited better P avg compared to single-coated and uncoated panels.P avg of the clean panels was 4% to 7% greater than the P avg of the unclean panels.This was due to the drop in efficiency owing to dust accumulation on the unclean panels.The reduction in power developed due to dust accumulation on the double-coated panels (14%) has been compared with previous investigations that reported on power losses due to dust accumulation.The comparison is shown in table 7.
From this comparison, it can be observed that the panels coated with a double coat, prepared in this investigation, performed better in a dusty and unclean environment, as power loss due to dust accumulation was lower.
This study has been aimed to improve the performance of photovoltaic solar panels using a dual-coating using silica-based anti-reflective material (SiO 2 -I-ARC) and Paraffin Jelly-Infused Expanded Graphite (PJ-I-EG).Different techniques such as UV-vis, FTIR, X-RD, FE-SEM and DSC studies were used to characterize SiO 2 -I-ARC and PJ-I-EG.Indoor experiments were performed at irradiance levels of 350, 600, and 850 W/m 2 .Outdoor experiments were conducted on two sets of panels for 45 days: one set was cleaned daily, while the other accumulated dust throughout the test duration.
When comparing the performances, the dual-coated panels (SiO 2 -I-ARC and PJ-I-EG) were better than single and uncoated conditions.Dual-coated panels exhibited superior power output, lower surface temperatures, and higher efficiency.Under different irradiance levels the double-coated panels exhibited higher peak current, maximum power, and lower surface temperatures compared to uncoated and single-coated panels.
The contact angle measurements indicated that the combined double coating (SiO 2 -I-ARC + PJ-I-EG) had a significantly high contact angle at 116°, indicating super-hydrophobic behavior.Dust accumulation density (f Dt ) consistently increased with time, with uncoated surfaces accumulating more dust than coated surfaces.Power loss (ΔPD) increased continuously for all panels, with a significant increase from the 1st to the 20th day and a minimal increase thereafter.
Uncoated panels experienced higher ΔPD compared to single and double-coated panels, showcasing the benefits of coatings, while double-coated panels exhibited lowest ΔPD.T max on double and single-coated panels were consistently lower than those on uncoated panels.Coatings led to increased I max in both clean and unclean panels, with double-coated panels consistently greater than others.
V max decreased with coatings, leading to better output current.V max fluctuations were lower in clean panels.P max improved with coatings in clean panels but decreased in unclean panels due to dust accumulation.Energy delivered (E) was stable in clean panels but consistently decreased in unclean panels, primarily due to dust acting as a thermal barrier.Efficiency (η) was maximum in double-coated panels, followed by single-coated and uncoated panels in both clean and unclean conditions.

Conclusions
Studies on performance improvement of solar panels with advanced coatings such as SiO 2 -incorporated Anti-Reflective Coating (SiO 2 -I-ARC) and Petroleum Jelly-infused Expanded Graphite (PJ-I-EG), were conducted and the following conclusions were drawn.
• On conducting characterization studies, SiO 2 -I-ARC was found to have reduced porosity, leading to lower optical losses and improved refractive index.PJ-I-EG exhibited high crystallinity and demonstrated its ability to absorb water molecules and repel water, making it suitable for hydrophobic coatings.[81] Dust prone environment, Transparent borosilicate glass covered panels 34% Katoch et al (2021) [82] Unclean condition with sand dust (12.5 g m −2 ) covered panels 38% Hussain et al (2017) [83] Unclean environment with Badarpur sand covered panels 55.6% • Combining the two coatings (SiO 2 -I-ARC + PJ-I-EG) resulted in better heat dissipation and thermal stability, reflected by lower enthalpy values.
• Performance studies indicated that double-coated panels, combining SiO 2 -I-ARC and PJ-I-EG, outperformed both single-coated and uncoated panels by exhibiting superior power output, lower surface temperatures, higher efficiency levels, higher peak current, maximum power, lower surface temperatures and lower dust accumulation.
• The combination of SiO 2 -I-ARC and PJ-I-EG coatings showcases a significantly high contact angle of 116°i ndicating super-hydrophobic behavior, which is advantageous for applications requiring water resistance and least reduction in power due to dust accumulation (14%).
• This coating combination resulted in improved energy output and dust tolerance of solar panels in a variety of environmental conditions, ultimately contributing to the advancement of solar energy technology.

Figure 1 .
Figure 1.Photovoltaic solar panel test kit used in the experimentation process.

3. 1 .
UV-vis evaluation UV-vis absorbance graph of SiO 2 -I-ARC sol is shown in figure 2 (a) and that of PJ-I-EG is shown in figure 2 (b).
SiO 2 -I-ARC and PJ-I-EG has been shown in figure 3 (a) and (b), respectively.In FTIR spectrum of SiO 2 -I-ARC (figure 3 (a), an extra band near 1264 cm −1 was observed, indicating the presence of nominal attachment between the incorporated SiO 2 nanoparticles and the prepared TEOS ARC sol.It indicates the
The Differential Scanning Calorimetric (DSC) heating curves of SiO 2 -I-ARC, PJ-I-EG and combined (SiO 2 -I-ARC + PJ-I-EG) samples are shown in figure 6 (a), figure 6 (b) and figure 6 (c) respectively.Also, DSC cooling curves of the three coating materials (SiO 2 -I-ARC, PJ-I-EG and combined SiO 2 -I-ARC + PJ-I-EG) are shown in figure 6 (d), figure 6 (e) and figure 6 (f) respectively.
3 ensuring better stability.The DSC curve patterns of the combined (SiO 2 -I-ARC + PJ-I-EG) coat were similar to the individual DSC curves of SIO 2 -I-ARC and PJ-I-EG.The convoluted DSC patterns were similar to DSC curves of ARCs in Liu et al (2021) [73], Tao et al (2015) [74] ensuring retention of hydrophobic properties.3.6.Irradiance testing IV characteristics curves of photo voltaic solar panels are shown in figure 7. IV curves were plotted for the uncoated, SiO 2 -I-ARC, PJ-I-EG, double (SiO 2 -I-ARC+PJ-I-EG) coated PV solar panels under irradiance of 350 W/m 2 , 600 W/m 2 and 850 W/m 2 .PV characteristics curves of photo voltaic solar panels are shown in figure 8. PV curves were plotted for the un-coated, SiO 2 -I-ARC, PJ-I-EG, double (SiO 2 -I-ARC+PJ-I-EG) coated photo voltaic solar panels under irradiance values of 350 W/m 2 , 600 W/m 2 and 850 W/m 2 .

Figure 7 .
Figure 7. IV curves of photovoltaic solar panels with different coatings at three irradiance values such as 350 W/m 2 , 600 W/m 2 and 850 W/m 2 .

Figure 8 .
Figure 8. PV curves of photovoltaic solar panels with different coatings at three irradiance values such as 350 W/m 2 , 600 W/m 2 and 850 W/m 2 .

Figure 9 .
Figure 9. Variations in (a) dust accumulation density, (b) power loss, (c) open circuit voltage and (d) short circuit current.

Figure 11 .
Figure 11.Average daily solar insolation received near the panel surface.

Figure 12 .
Figure 12.Variations in average power production.

Table 1 .
Characteristics of PV solar panel used in the investigation.

Table 3 .
Thermal properties of the coating materials evaluated using DSC.

Table 4 .
Irradiance test results for different coating conditions.

Table 7 .
Reduction in power developed due to dust accumulation.Unclean environment, dust prone panels coated with SiO 2 -I-ARC and PJ-I-EG 14% Chen et al(2019)