The performance enhancement of solar cooker integrated with photovoltaic module and evacuated tubes using ZnO/Acalypha Indica leaf extract: response surface study analysis

In this study, the effect of employing ZnO/Acalypha Indica leaf extract (ZAE) on the energy absorption of a coated portable solar cooker has been examined using an experimental setup. A prototypical model has been developed to corroborate in associating an investigative outcome per constituents of the experiments. The studied heat transfer process in ZAE is stable for harsh conditions. The design analysis and an estimation of the system performance were done given various parameters including the pressure of the vacuum envelope, bar plate coating digestion, emissivity, and solar rays. The fabricated solar was tested with and without ZAE to investigate the impact of this coating material on the solar cooker’s thermal performance. To observe the performance of the new design, two figures of merit (F1 and F2) have been introduced. The factual food cooking assessments were for a family of four people, which operated in ZAE coating (0.8, 1.0, 1.2 μm) of the solar cooker. The values of F1 and F2 for the proposed cooker were obtained as 0.1520 and 0.4235, respectively, which is intact with the BIS values. The results revealed that employing ZAE instead of a thermal NHC-PV solar cooker reduced the time required to boil 2 L of water for about 47 min. The overall thermal energy productivity of the solar cooker with electrical backup was obtained as 42.65%, indicating that the ZAE coating can improve the thermal efficiency by 10.35%.


Introduction
A solar cooking process is a method in which solar radiation is utilized to naturally cook substances. Research worldwide have found out that solar cookers have sustainability for cooking in rural and urban areas with reduced energy consumption. A solar cooker using sun radiation as a heat source, can be utilized on hot sunny days, but on rainy and winter days, it is useless. It has been found that a solar cooker with an electrical backup will overcome these inconveniences. An electric backup integrated with a solar cooker has shown better performance compared to the cooker supported with thermal energy alone. Moreover, selectively coated absorbers using nanoparticles play an important role in enhancing the performance of solar cookers. Several studies have assessed the effect of selective absorber coating using nanoparticles. Kaiyan et al. (2009) used a light funnel to congregate solar energy to achieve temperature suitable for industrial and domestic usage. It was identified that a temperature of 250 °C is achieved with a lighting area of 1.5 m 2 . Prasanna and Umanand (2011) experimented by transferring solar energy to the kitchen using fluid circulation to stimulate a cooker using the concept of maximum power point tracking (MPPT) for a thermal collector. Javadi et al. (2013) also reviewed the effect of using nanofluids on the performance of solar collectors. In their review, care was taken to highlight the impact of two-phase analysis of the nanofluid with more than one nanoparticle in the heat transfer fluid. Hussein et al. (2017) reviewed theoretical, numerical, and experimental studies of a heat pipe solar collector incorporating a nanofluid. They reported that various research works have proved the significance of nanofluid in improving the thermal performance and efficiency of solar collectors. Palanikumar et al. (2019aPalanikumar et al. ( , 2019b) fabricated a solar box-type cooker (SBC) including a phase change material (PCM) and nanoparticles coating in the design and evaluated its performance with thermal imaging of the structure. Then, they used a fuzzy logic algorithm to develop a model to predict the thermal performance of the solar cooker. The results of the study revealed that when eggs are boiled, the thermal performance of the solar cooker can reach 52.17% and 75.47%, utilizing PCM and nanoparticles, respectively.
Furthermore, Bhavani et al. (2019) designed an SBC and coated it with black paint and nanoparticles. Then the design was evaluated using fuzzy logic rules. From the results, it was found that using nanoparticles in the coating enhances the thermal efficiency of the solar cooker by 7.10%. The fuzzy logic rules have also been utilized in other studies to model solar cookers. Such as Palanikumar et al. (2021b) and Venugopal et al. (2012). Thamizharasu et al. (2022) developed a stepped SBC (SSBC) coated with silicon dioxide (SiO 2 ) and titanium dioxide (TiO 2 ) nanoparticles and analyzed them at different volume fraction ratios of nanoparticles between 5 and 25%. The findings indicated that the overall thermal efficiency of the solar cooker can be improved by 31.42% when using the SiO 2 /TiO 2 nanolayers at 15% volume fractions of nanoparticles. In another study by Thamizharasu et al. (2021), the solar cooker was analyzed using an adaptive control method which was modeled using the online sequential extreme learning machine (OSELM), and the heat analysis of the system was made in the Binary Search Tree data structure. From the results, the efficiency of the solar cooker was obtained as 49.21% for a 15% volume fraction of SiO 2 /TiO 2 . Palanikumar et al. (2021a) performed a comparative study on a fabricated SBC, this includes an SBC with PCM, and an SBC with PCM and a nanocomposite (NPCM). After analyzing the fuzzy logic and Cramer's rules, image processing techniques were applied. The results indicated that coating the bar plate absorber with nanoparticles and integrating a PCM increases the internal temperature of the SBC up to 164.12 °C, and enhance the overall thermal efficiency by 11% compared to the other tested configuration (2019). Palanikumar et al. (2020) studied an SBC coated with a nanocomposite film containing tantalum pentoxide (Ta 2 O 5 ) doped with stannic oxide/silver. The results indicated that the spectrally selective characteristic is improved and the bar plate temperature reaches 203.33 °C. Shinde et al. (2016) utilized preheated water to increase the thermal efficiency of a large-scale cooker with three levels of screw speed, solid flow rate, and liquid flow rate. The results indicated that the required time for batch cooking is equivalent to the minimum residence time. Cerium oxide (CeO 2 ) nanoparticles of 25 nm were prepared and combined with heat transfer fluid, including water in different concentrations by Sharafeldin and Gróf (2018). The obtained nanofluid was used in solar evacuated tube collectors (ETCs) to improve thermal performance. It was found that the thermooptical characteristics were increased by 34%. Khallaf et al. (2020) also fabricated a Quonset solar cooker with domeshaped polymeric glaze incorporated with internal reflectors and performed experimental and theoretical analysis. The study revealed that cooking fluid glycerine gives efficiency ranging from 9 to 92% during cooking hours. The solar cooker's energy and exergy efficiencies with multiwalled carbon nanotube-oil nanofluid were investigated with varying volumetric flow rates by Hosseinzadeh et al. (2021b). It was found that nanofluid-based solar cooker gives improved efficiency of 37.30% and 65.87% for 0.2% and 0.5% wt concentration in oil, respectively. Mallikarjuna et al. (2021) reviewed the effect of nanofluids to enhance the heat transfer rate in solar energy harvest. The results of different research works conducted by various researchers have been presented in more detail in their study. Liu et al. (2020) carried out studies using zinc oxide (ZnO), SiO 2 , copper oxide (CuO), aluminum oxide (Al 2 O 3 ), and carbon derivative nanomaterials on copper, silicon, SiO 2 , aluminum, and stainless-steel substrates and found the enhancement of thermal performance of the absorbers due to selective absorber coatings. A comprehensive review regarding the various kinds of nanofluids in ETCs was conducted by Vijayakumar et al. (2020), and conclusions were highlighted with the performance enhancement of the ETCs. Vengadesan and Senthil (2021) fabricated finned cooking vessels with a variable length of 25, 35, and 45 mm and evaluated them. It was found out that the fin length of 45 mm gives higher thermal efficiency of 50.03% with a heat transfer coefficient of 58.54 W/m 2°C . Hosseinzadeh et al. (2021a) used different nanofluids in a solar collector and cooking unit and performed energy and exergy analyses. The study indicated that employing silicon carbide (SiC) nanofluid in the system provides good performance in which 2 L of water boiled in 17 min. Shehayeb et al. (2021) employed cathodic electrophoretic deposition to enhance the optical properties of the CuO tandem solar absorber. It was observed that the fabricated tandem absorber provides enhanced efficiency.
Based on the reviewed literature presented supra, it is clear that a number of studies have been conducted to assess the impact of nanoparticles on the performance of various solar cookers. This study therefore uses ZnO/Acalypha Indica leaf extract (ZAE) and black mat paint on the sides and bottom of a cooking vessel to enhance its performance. A mathematical model has been proposed, and the validation of the analytically obtained results is obtained through experimental observations. Moreover, two figures of merit F 1 and F 2 were proposed to evaluate the solar cooker performance with and without using ZAE with black mat paint.
The rest of the study is presented in the following order, the materials and method used for the study are presented in ''Materials and methodology'', ''Results and discussion'' presents the results and discussion, whiles the conclusions made on the study is presented in ''Conclusion''.

Preparation of Acalypha Indica leaf extract
Fresh Acalypha Indica leaves were collected in March 2021 in Mellampude, Vijayawada -AP, India. The surface of the leaves was cleaned with running tap water to remove the dust and other contaminated organic compounds, followed by distilled water and dried at room temperature for 15 days (total count of 1000 leaves). The dried leaves were collected and then about 30 g of the collected leaves were taken and mixed with 300 mL of distilled water. The prepared sample was then heated and stirred at 80 °C for 3 h. The solution was boiled at 70 °C for 20 min. The color of the solution changed to green which indicates the formation of a leaf extract. The obtained solution was filtered with Whatman No. 1 sieve paper to obtain a clear extract. Then Acalypha Indica extracts were filtered two times and pure samples were collected and maintained at 5 °C. The extract samples were prepared to be directly utilized in synthesis experimentations as shown in Fig. 1a.

Preparation of ZnO using Acalypha Indica leaf extract
The experimental materials were prepared and all chemical materials were purchased from the company AR Scientific, Basaveshwara Layout, Bengaluru, and Karnataka 560094. Zinc acetate dihydrate (99.7%) and ethanol (99.5%) chemicals as standard cleaning agents in experimental works were obtained from the Research Centre for Solar Energy at Vijayawada civic KLEF India. A 2 g nickel nitrate (Ni (NO 3 ) 2 ) was added to 200 mL of distilled water and allowed to stir for 20 min. The solution was added in drops to the leaf extract under constant stirring for 2 h using a magnetic stirrer (Ghamsari et al.  Fig. 1b. The solution was then dried on a hot plate to obtain a powder form. Finally, the nickel oxide (NiO) sample was prepared. The powder was then placed in a muffle furnace at a temperature of 500 °C for 3 h in ZnO/ Acalypha Indica stem extract, as shown in Fig. 1c. The obtained sample can be used in further studies. Figure 1d shows this process and the establishment of chemical bonds structure and extracted with particles of ZnO using Acalypha Indica Leafs. Figure 2a shows the X-ray diffraction (XRD) pattern of NiO nanoparticles (NiO-Z) prepared using Acalypha Indica leaf and stem extract (AE). In the XRD analysis, the x-axis represents 2θ, and the y-axis indicate the intensity of the peak. The presence of 2θ for ZAE corresponds to (1 1 1), (2 0 0), (2 2 0), and (3 1 1) & (1 1 1), (2 0 0), (2 2 0), (3 1 1), and (2 2 2) orientation confirms the presence of NiO particles in a cubic structure. The grain size (D) of the nanoparticles was calculated using the Debye-Scherrer formula using the reflection form of the XRD pattern. Debye-Scherrer formula for grain size is presented in Eq. (1):

X-ray diffraction
where D is the grain size, λ is the wavelength of copper (1.54Å), β is the full width half maximum after correcting the instrument peak broadening (β expressed in radians), and θ is the Bragg's angle. In this XRD analysis, the grain size of NiO using ZAE was calculated as 20.61938×10 −9 m and 22.3126×10 −9 m, respectively. The Lattice parameter (a) was calculated using Eq. (2): where d is the interplanar spacing value; h, k, and l are Miller indices. The lattice parameter of ZAE has been calculated as 4.20825Å and 4.17629Å. The cell volume (a 3 ) is calculated from the value of lattice parameter (a). Therefore, the value of the cell volume for ZAE is calculated as 52.544 (Å 3 ) and 72.84033 (Å 3 ).

Fourier transform infrared spectroscopy analysis
Fourier transform infrared spectroscopy (FTIR) is a technique use to obtain an infrared spectrum of absorption or emission of a solid, liquid, or gas. An FTIR spectrometer simultaneously collects high spectral resolution data over a wide spectral range. The Fourier Transform Infrared Spectroscopy (FTIR) investigation was carried out using PERKIN ELMER (Spectrum RXI) spectrometer in 400 to 4000 cm −1 . The functional groups were identified using the peaks assignments. Figure 2b shows the FTIR spectrum of nickel oxide nanoparticles using ZAE in Table 1.

Analysis of UV spectrum
This study mixed the nickel nitrate source with the plant extract of the Acalypha Indica leaf and stem extract. The color of the AE changes from brown to dark green, the color of NiO (Z) also changes from yellow to light green. These color changes are due to the excitation of surface Plasmon vibrations. The UV-visible spectrum x-axis indicate wavelength whiles the y-axis indicate absorption. AE represents nickel oxide (Ni 2 O 3 ) nanoparticles using Acalypha Indica Leaf extract in the UV spectrum, and Z represents ZAE. In this spectrum, the range of Ni 2 O 3 nanoparticles using leaf and stem extract was reported in the range of 293 nm and 291.3 nm as shown in Fig. 2c. The corresponding absorptions are also identified. It indicates the possibility of the formation of nickel oxide nanoparticles, and in this spectrum, comparing the two ranges, we observed that the AE is better than ZAE.

SEM and EDAX analyses
The surface morphology of the resulting powder was examined using a scanning electron microscope. Figure 2d indicates that AE and ZAE were observed within the range of × 10,000 and × 10,000. The image shows that the morphology of Ni 2 O 3 nanoparticles is a spherical shape for AE and in coral reef structure for ZAE without any agglomeration in nano size. From the EDAX (energy dispersive X-ray analysis) spectrum image, elements in the Acalypha Indica leaf and stem extract are evident. The EDAX spectrum of nickel oxide nanoparticles was used, while that of ZAE shows only the peak of nickel (Ni) and oxygen (O 2 ) elements. ZAE shows only the peak of nickel (Ni) and oxygen (O) elements, which confirms that the prepared nickel nanoparticles are essentially free from impurities and are at the limit of EDAX. Identification lines for the significant emission energies for Ni, O 2 and thus correspond with the peak in the spectrum; thus, nickel has been identified correctly as shown in Fig. 2e

Design of the solar cooker
The compatible solar cooker has three components with evacuated tubes with a high vacuum (P < 5 × 10 −3 Pa) enclosed in a rectangular wooden box with parabolic trough reflectors (PTRs) Solar photovoltaic (PV) panel (2 × 100W); 12 V 75AH battery; Stove with two vessels for cooking.
A compatible solar cooker with a PV panel and evacuated tubes (Solar Chulha) was designed and fabricated. Evacuated tubes have a high vacuum (P<5 ×10 −3 Pa) used in the proposed cooker, and it is used to produce hot water of about 75°C for cooking. PTRs were designed, and the evacuated tubes were fixed on the focal line of the trough to receive maximum solar radiation. The copper tubes carrying the heat transfer fluid (water) were made to run through the evacuated tubes to extract the thermal energy received by the tube. The temperature of the heat transfer fluid at the outlet was high. A PV panel with a power output of 200 W was used to charge a 12V 75 AH battery. The electricity drawn from the battery was used to heat the heating filament (Nichrome) covering the cooking vessel. Hot water from the evacuated tubes was further heated up to the boiling point, and the food was cooked quickly. The DC power produced by the panel was stored in the battery during the daytime to be used at night. Fig. 3a and b show the solar cooker's photograph and its different components. Among the two cooking vessels, one vessel was coated with ZAE and mat black paint on the sides and base of the cooking vessel, and the other vessel with only black mat paint. Table 2 represents the specifications of the evacuated tubes used in the proposed solar cooker.

Experiment performance
Two cooking vessels are used with and without coated ZAE sides in which the vessel's bottom, was subjected to   Table 3 (summer) and Table 4 (winter) represent the variation of temperature at the base of the two cooking vessels with and without ZAE. The observed first figure of merit (F 1 ) for the cooking vessels with and without ZAE was 0.1520 and 0.1143, respectively. Due to the enhanced heat transfer from Nichrome heating coil to the sides and base of the cooking vessel with a coating of ZAE along with black mat paint, and the thermal stability of ZAE led to a good thermal conduction of the thermal energy transferred (summer and winter) to the cooking vessel, as shown in Fig. 4a and b.
From the graph, the temperature of the cooker's base with ZAE with black mat paint is noticeable throughout the path towards the stagnation temperature of 133°C compared to the vessels without ZAE. According to the Bureau of Indian Standard, the F 1 value of 0.12 km 2 /W is significant, and in the present cooker, the value is 0.1520 km 2 /W. A figure of merit (F 2 ) was found by experimenting with the proposed solar cooker with load (sensible heat material), i.e., 1 kg of water in the cooking vessels with and without a coating of ZAE on the sides of the bottom of the vessel.

Second figure of merit
The cooker performs based on the sensible heating of a load inside. The proposed cooker was filled with a sensible load of 1 kg of water in the two cooking vessels with and without a coating of ZAE and black mat paint on the sides and bottom of the vessel. Second figure of merit (F 2 ) can be calculated using the expression presented in Eq. (4): Measurements were done to find the second figure of merit, and they are: (i) Cooking fluid temperature at a regular time interval until the fluid temperature reaches a maximum of 95°C. (ii) Duration of time between the initial and final cooking fluid temperature (iii) The intensity of solar radiation and ambient temperature were measured using a solar radiation monitor and digital thermometer. Table 5 characterizes the data collected from the measurements made during the testing for the Second Figure of the Merit of the solar cooker.

Interval cooking power
It is also called average cooking power (P) which means the helpful energy available during the cooking period. It is defined as the ratio of increase in temperature of the cooking fluid for each interval of time multiplied by the mass and specific heat of the cooking fluid to the time interval specified, and the expression given as indicated in Eq. (5).

Standardized Cooking Power
Standard solar insolation of 750 W/m 2 for each interval has been used, and utilizing Interval cooking power (P), the standardized cooking power (P s ) can be evaluated using Eq. (6). Table 6 represents the variation of interval cooking power and standardized cooking power during the testing load for the second figure of merit.
From Fig. 4c, it is understood that the standardized cooking power is a function of interval cooking power (P) and is inversely proportional to the intensity of solar radiation during the interval of time. Interval cooking power is the

Thermal modeling
Since the cooker is incorporated with the evacuated tube, it can obtain preheated cooking fluid from the outlet of the evacuated tubes. The temperature of the cooking fluid at the outlet is considered the initial temperature of the cooking fluid in the cooking vessel. The base and sides of the cooking vessel are surrounded by the Nichrome heating coil, which receives the current from the solar panels of 200 W each. The base and sides of the cooking vessel are coated with ZAE to enhance the heat conduction of the vessel to deliver the auxiliary heat to the preheated cooking fluid. Since the nanoparticles play a vital role in transferring the heat from the nichrome heating coil to the cooking fluid, it will reduce the cooking time for the foodstuff reasonably. Here, energy balance equations have been written for the cooking fluid, base and sides of the cooking vessel and solved for the analytical solution.
The base of the cooking vessel: Sides of the cooking vessel: The outlet water from the evacuated tube into the cooking vessel absorbs energy given by the Nichrome heating coil through the base and sides of the cooking vessel. Since the base and sides of the cooking vessel are coated with ZAE, heat energy is convected to the preheated cooking fluid. Due to an effective heat transfer from the Nichrome heating coil to the cooking fluid through the ZAE-coated base and sides of the cooking vessel, the cooking fluid absorbs much more energy. The temperature of the fluid increases rapidly. As a result, the time for cooking the food in the proposed cooker is reduced.
The lid of the cooking vessel:

Cooking fluid
Eqs. (7), (8), and (9) have been solved for T b , T s, and T l, and substituted in Eq. (10). Rearranging Eq. (10) and can be written as where a and f(t) are constants, this can be obtained from the equations on temperature components of the evacuated tube collector, Nichrome heating coil, and cooking vessel. At t=0, T w = T w0, and due to the initial condition, we can write the solution of the equation as where α is a constant and depends on the different heat transfer coefficients of the cooker. The photovoltaic with evacuated tubes which connects the Nichrome heating coil utilized in the solar cooker's input and output has been calculated and the overall energy efficiency (ƞ ZAE ) can be written as presented in Eq. (13)

Results and discussion
This research work is focused on the effect of the presence of ZAE coating and its thickness on the thermal performance of a solar cooker.

The effect of ZAE coating thickness
An experiment was carried out with the proposed cooker during summer and winter days (June 2020 to June 2021) under local climatic conditions of Vijayawada, Andhra Pradesh, India. The analytical solutions of the energy balance equations for the temperature elements of the cooker were used to evaluate the temperature of the base, sides, and lid of the cooking vessel, followed by the temperature of the cooking fluid. The climatic parameters during the experiment were recorded using the solar radiation monitor and digital thermometer. Calibrated copper-constantan thermocouples were used to measure the temperature of the cooker.
The impact of varying the thickness of the ZAE coating on the performance of the solar cooker and the overall thermal efficiency was investigated, where tested thicknesses are 0.8 micron, 1.0 micron, and 1.2 microns. It was found that the coating thickness with the best absorption solar energy and ambient temperature on the design is 1 micron of the ZAE material. The performance of the solar cooker with a 1.0-micron coating was measured and analyzed from June 2020 to June 2021. The solar cooker with a 1-micron thickness coating of ZAE was absorbed in more solar radiation and ambient temperature during experimental average values compared to others with different thickness levels. KLEF climate conditions utilization of the solar cooker has improved performance in more absorption by solar radiation around 25W/m 2 and ambient temperature is 4°C, individually.

The effect of the presence of ZAE coating
An output thermal influence analysis of a design was utilized. We are focused on the best-performing thickness which was chosen as 1 micron as demonstrated in Fig. 5a. Cooking vessels with and without the ZAE coating (1 micron) were filled with 1 kg of water. The experimental work was performed until the temperature of the sides and base of the cooking vessel reached its stagnation temperature. Figure 5b represents the variation of the temperature of the cooking vessel's side, base, and lid. The base and sides of the vessels with and without ZAE were experimented for their activity to find the impact of ZAE coating. From the figure, it is observed that the temperature of the vessel with a coating of ZAE has dominated temperature at each measurement in regular intervals and proved that the ZAE enhanced the conduction of thermal energy from the Nichrome heating coil to the base and sides of the cooking vessel. Moreover, ZAE has a significant capacity to transfer heat energy to the cooking fluid through the base and sides of the cooking vessel. Furthermore, validation of the model was performed for the temperature of the base, sides, and lid of the cooking vessel with the analytical solutions obtained by solving the energy balance equations of the temperature elements of the cooker. Figure 5c shows the variation of the theoretical and experimental values of the temperature of the base, side, and lid of the cooking vessel with a coating of ZAE. It was observed that both the theoretical and experimental values have a conjoined trend with maximum coincidence. Standard deviation between the experimental and theoretical values

Experimental and theoretical cooking values analysis
The cooking fluid's temperature was used to find the theoretical values and compared with the experimental    Preheated water from the ETC with a temperature of 76°C entered the cooking vessel. The temperature of the preheated water was taken as the initial temperature of the cooking fluid in the cooking vessel with a coating of ZAE on the sides and the base. The preheated water reached a maximum temperature of 96°C within 15 min with the auxiliary heat obtained from the Nichrome heating coil through the base and sides of the cooking vessel. The energy balance equation for the cooking fluid was considered to evaluate the temperature of the cooking fluid during the cooking time. The theoretical values for the temperature of the cooking fluid were compared with the experimental observation by finding the standard deviation between them. From the graph, it is clear that both the experimental and theoretical values have conjoined trends throughout the working period. It was found that the average standard deviation between the experimental and theoretical values is 0.16122, which was expected.

Cooking performance
The new design was analyzed for different thicknesses of ZAE coating for a given water temperature inside the solar cooker as shown in Fig. 6a. The solar cooker's values were recorded during the heating of water (2 L), which was performed with coating with various ZAE thicknesses, the time required for cooking decreased from 102 to 58 min, 47 min, and 54 min. That is, 58 min (0.8 μm), 47 min (1.0 μm), and 54 min (1.2 μm) and without ZAE-coated test the obtained values are 102 min by the system. The analysis of the solar cooker with ZAE coated in 1.0 micron show that the average time to boil a 2 L water increased by 24.32 min (32.26%) from June 2020 to June 2021.
The absorbed energy by the 2 L of water can reduce the cooking time and obtain over a time intermission of 15 min. The performance of the solar cooker is established with output period intermissions, and the cooking ends at a constant temperature. As indicated earlier, ZAE coating in the solar cooker was found to enhance the cooking performance by absorbing more thermal energy from the bar plate. The PV power and nichrome heating coil were also connected with the solar cooker to increase the internal heat transfer modes, thereby increasing the thermal heat flow rate of the system. The experiments proved that the mathematical modeling for the proposed cooker could be utilized to optimize design parameters for largescale installation to serve large communities. Further, the model can be used to evaluate the proposed cooker in any other location with a similar climatic condition to study its performance.
The proposed cooker with cooking vessels with and without the ZAE coating (1 micron) was evaluated to cook various kinds of food stuffs. The time taken for cooking with both vessels is tabulated in Table 7. The cooking time for  Table 7 demonstrate that the cooking period for the vessel with ZAE coating reduced compared to that of the vessel without coating. Additionally, a cooker size of 24 × 7 can be used by a family of 4 members short of intermission. During the daytime, one can use the cooker by incorporating the thermal energy received from the evacuated tubes supported by electrical backup provided through the Nichrome heating coil. In the nighttime, the battery can be discharged to supply current through the Nichrome heating coil to fulfill the required energy for cooking in the vessel.

Design performance of ZAE mass fraction
The solar cooker with impact of using ZAE coated with 0.8 micron, 1.0 micron, and 1.2 microns on the overall performance of the solar cooker is assessed from the perspective of thermal energy. Also, the impact of the presence and absence of the ZAE coating (1 micron) on the solar cooker's performance was evaluated and discussed in the "The effect of ZAE coating thickness" section. It established that the solar cooker has been analyzed with coated samples in different micron levels including 0.8,

Efficiency analysis
One of the solar cooker vessels was coated with ZAE materials using different micron levels and the impact of this variable on the heat transfer and productivity of the solar cooker is shown in Fig. 6b. The coated section with ZAE increased the thermal energy absorption which in turn enhanced its heat transfer characteristics. This consequently increased the energy efficiency of the system. The average design energy efficiency is found to be around 39.11%, 48.31%, and 41.24%, for heat transfer with an efficient ZAE materials coating of 0.8-micron, 1.0-micron, and 1.2-micron thickness, respectively. The solar cooker performed with characteristics performance on ZAE as followed in Table 8 as comparative studies.

Conclusion
An overall performance of a solar cooker using ZAE coating was investigated from a thermal energy performance point of view. Also, the output of a cooker using PV and evacuated tubes with cooking pots with and without ZAE coating were estimated. Investigated parameters in this research include ZAE coatings with different thicknesses of 0.8 micron,  1.0micron, and 1.2 micron, which were used in the ZAE mass fraction of the system. The following conclusions are drawn from the current study: (i) The ZAE coating on the sides and the base of the cooking vessel were evaluated in terms of F 1 (0.1520) and F 2 (0.4230) which are below the BIS (Bureau of Indian Standards) values of Indian Standards. (ii) The mathematical model was validated with a negligible difference in the cooker's experimental and theoretical temperature which was proved by finding the standard deviation between the theoretical and experimental observations. (iii) For the temperature of the base, sides, lid, and cooking fluid, the standard deviation value between the experimental observation and theoretical values were 0.025, 0.109, 0.0416, and 0.16122. (iv) The cooking time for various foodstuff in the cooking vessel with ZAE coating reduced significantly to save time. (v) The thermal efficiency of the cooker was found to be 42.65% by incorporating the input energy with extra energy supplied through electrical backup. When ZAE coating was used, the efficiency was enhanced by 10.35% compared to the cooking vessel without the coating. (vi) The newly designed solar cooker with ZAE coating was used on a 24 × 7 basis as necessary to cook for a family of 4 which proves its reliability.
Consequently, the utilization of ZAE as a coating material for the solar cooker was found to be a suitable technique to improve the thermal performance of a solar cooker. In addition, phase change material (PCM)-based energy storage materials when integrated into a solar cooker can offer the possibility of cooking after sunset. Therefore, the upcoming study will be to investigate the consequences of simultaneous use of ZAE and PCM to improve the energy productivity of solar cookers.
Nomenclature A: area of the base and side of the cooking vessel (m 2 ); A sc : solar cooker area (m 2 ); C: specific heat capacity of water (J/kgK); E NHC, out, ave : average energy output in NHC (%); E p : energy pump; F 1&2 : first and second figure of merit; H: intensity of solar radiation (W/m 2 ); H dt: hourly variation of solar radiations (W/m 2 ); h cbcf : convective heat transfer coefficient from base of the cooking vessel to cooking fluid (W/ mK); h cscf : convective heat transfer coefficient from side of the cooking vessel to cooking fluid (W/mK); h cfl : convective heat transfer coefficient from cooking fluid to the lid of the cooking vessel (W/mK); h la : convective heat transfer coefficient from the lid of the cooking vessel to the ambient (W/mK); M: mass of water (kg); P : interval cooking power (W); P s : standardized cooking power (W); T a : ambient temperature (°C); t 1 : initial time; t 2 : final time; T w1 : initial water temperature (K); T w2 : final water temperature (K); T 1 : initial water temperature during the evaluation of interval cooking power (°C); T 2 : final water temperature during the evaluation of interval cooking power (°C); M w : mass of cooking fluid (kg); C w : specific heat capacity of cooking fluid (J/ kgK); T w : temperature of cooking fluid (°C); T b : temperaure of the base of the cooking vessel (°C); T s : temperature of the sides of the cooking vessel (°C); T l : temperature of the lid of the cooking vessel (°C); T a : temperature of the ambient (°C); T ps : stagnation temperature of the base of the cooking vessel without load (°C); ƞ pv : overall output of PV