Performance Comparison of single‐slope solar still loaded with various nanofluids

Nanofluids are great heat transfer carriers for collecting thermal energy in solar thermal applications. In the present study, a theoretical study of single‐slope solar still (passive type) has been carried out by incorporating CuO, Al2O3, Ag, Fe2O3, and SiC‐water nanofluids at different volume concentrations (0.02, 0.05, 0.08, 0.12, and 0.2). This analysis has been carried out with an optimum water depth of 0.02m as obtained from the experimental and theoretical studies. In order to validate the model, the experiments were conducted on solar still and then performance of still was compared. The analytical expression of the characteristic equation using Runga‐Kutta ODE, for passive single slope solar still was found to be in good agreement with experiments carried out in Patiala, India. The total deviation for both experimental and theoretical distillate output of a still for a day was found to be 12.24%. Daily production for Al2O3‐water‐based nanofluid was found to be (14.22%) higher than simple solar still without nanofluid, followed by CuO (10.82%), Ag (8.11%), Fe2O3 (7.63%) and SiC (7.61%).


| INTRODUCTION
Earth, known as the mother of life, has a nectar-like fluid called water. Two-thirds of the earth's surface is covered with water, 97% of which is salty and the remaining is fit for drinking. The quality of drinkable water is deteriorating due to regress development and industrial setups; so, to improve its quality, different water purification methods are investigated and desalination of water with the help of solar still 1 was found to be the most efficient and clean way. The illustrative diagram of solar still is shown in Figure 1. Solar still operates on the principle of conversion of solar radiation into heat. First, the impure water is filled into the basin of the still. A tilted glass has been kept on the top of the still, through which the solar radiation is passed to the black absorber lining. The impure water in the basin absorbs the heat which gets evaporated and the pure water in the form of vapors is stuck on the surface of the glass and gets condensed. The condensed vapors get collected in a container through a distillate collection channel of the still. Solar desalination systems are divided into two categories viz. passive solar stills and active solar stills. In the past few years, various researchers have studied the passive and active types of stills 2 and concluded that the performance of the passive type of still is better than that of the active type still. Dwivedi et al 3 investigate the performance of the double-slope passive-type solar still at three water levels. It was observed that in summers the performance of the double slope solar still was more, but the annual yield production of single slope solar still was higher than the double slope solar still. Xiao et al 4 reviewed the different types of solar stills and presented the fundamental heat and mass transfer process analysis, stated by Dunkle, Adhikari, Kumar, Elsafty, Tanaka, and Zheng. They have also integrated the solar reflectors in their studies and found the better performance of still for the regions having low solar incidence. Tiwari et al 5 analyzed the effects of orientation of still and glass cover inclination for the maximum yield both in summers and winters. Nafey et al 6 used a floating wick system in experiments and found some major enhancements in the productivity of still. Singh et al 7 studied the effects of some parameters like glass cover material, environmental conditions, insolation per day, the orientation of the still, wind speed, and inclination of the glass cover. Aboul-Enein 8 investigated the effect of water depth, the inclination of the glass cover, and optimum insulation for the still. Samee et al 9 studied various design parameters of single basin solar still and observed an optimum cover glass inclination and glass thickness of around 33.3° and 3 mm both in summer and winters for the southwest arid region. Abu-Hijleh et al 10 performed some experiments having water film cooling on the glass cover and the efficiency of the film cooling still was found to be non-sensitive to the wind speeds. Tiwari and Anil 11 analyzed the seasonal variation of distillate output at different water depths. Dunkle et al 12 correlate both convective heat transfer coefficient and evaporative heat transfer coefficient with experimental validation and it was found to be in good agreement of about 2% of the variation. Kumar et al 13 presented the annual performance of active solar still for the location in New Delhi. Singh et al 6 proposed an experimental and theoretical model of double slope solar still with an inclination angle of 55°.
Sakthovel et al 14 observed and proposed a mathematical model integrating jute cloth in the water medium, which maximizes the surface area of water and helps in getting more evaporation rates. Srivastava et al 15 proposed an experimental setup with multiple porous floating absorbers and studied its performance. Aboul et al 8 37 performed an experiment on a solar collector integrated with nanoparticles and observed a constant 10°C rise in the temperature of nanofluid with distilled water. Abujazar et al 38 worked with inclined stepped solar still with copper trays and found higher performance due to the higher conductivity of copper. Elashmawy et al 39 used a parabolic concentrator solar tracking system integrated with a simple solar still. It enhanced the performance of the solar still by 676%. Sahota et al 40 experimented on passive double slope solar still with water-based nanofluid (Al2O 3 , TiO 2 , and CuO) and observed an increase in (19.1%, 10.38%, 5.25%) in terms of productivity. Kabeel et al 41 used graphene oxide nanoparticles in the phase change material (PCM) to improve the thermal conductivity of nano-doped phase changed material by 52%. Tubular solar still loaded with PCM increases the water temperature by 7ºC and without phase change material it was found to be 3ºC. There was a 24% increase in temperature of nano-doped phase change materials as compared to phase change material without nanoparticles. Total yield for tubular solar still, tubular solar still with PCM, and tubular solar still with NPCM was found to be 2.59, 3.35, and 5.62 kg/m², respectively. Subhedar et al 42 performed an experiment in which a parabolic trough collector was integrated with conventional single-slope solar still. Water and Al 2 O 3 nanofluid was taken as working fluid with 0.05% and 0.1% volume fraction, respectively. The rise in productivity and thermal efficiency was found to be 66% and 70% with the use of Al 2 O 3 nanofluid in the complete integrated system.
The performance of the still depends upon some properties, like the volume fraction, particle size, and thermophysical properties like specific heat capacity, viscosity, and density. Nanoparticle generally is defined as the ratio of its surface area to the volume, if it is significant, then it is known as nanoscale material. This ratio increases the properties like thermal conductivity, thermal diffusivity, viscosity, electric conductance, and optical sensitivity changes. 7 Nanoparticles are tiny particles of size in the range of 1 to 100 nanometers (nm). As the particle size decreases, the transport and physical property of the particle change, which affects the performance of the still. Every nanoparticle size has a different wavelength at which it absorbs the maximum solar energy, which is known as resonant wavelength. So, the size of the nanoparticle is an important parameter. 7 Specific heat capacity of the fluid has a great effect on the performance of the solar still. When the solar radiation hits the surface, a portion of the sun's energy gets absorbed in sensible heating and the remaining goes into the latent heat storage. Thermal conductivity of the nanoparticle increases when the surface area to volume ratio increases, which means as size goes down, the performance of the solar still having nanofluid improves. 9 Using metallic nanoparticles of different sizes help solar still to capture all the incident range because of different resonant wavelength to each size. 7 According to the coined literature review, limited research has been carried out on developing a thermal model on single-slope solar still having nanofluids, with a comparison of its effects at different volume concentrations. Therefore, the main purpose of the present research is to analytically investigate the effects of nanofluid at different volume concentrations for various nanoparticles and validate it with the experimental result. Moreover, it can also help to fill the technological gap to compare the performance of solar still with different nanoparticles and their concentration.

| EXPERIMENTAL SETUP AND PROCEDURE
The simple single-slope solar still have been constructed, a basin was made of stainless steel-grade 304 in the shape of a rectangular tray having an evaporating surface area of 1 m 2 . The glass cover was inclined at 30° with the horizontal surface, which is almost equal to the latitude of the location. The sides of the tray were insulated with glass wool, rubber-type material was used as basin liner of thickness 5 mm to absorb the maximum solar energy and to transmit that energy to basin water. A constant water level is maintained by a constant head device arrangement. A sponge rubber gasket is installed between the glass and the tray, which helps to ensure there are no gaps between the glass panels. The tray is insulated from the ambient conditions as shown in Figure 2. The system was oriented toward the south. Window glass was used as a condensation surface and transparent cover from where the incident radiation enters into the still. To avoid some drops of distillate falling back to the evaporator surface, a rectangular plastic cross-sectional channel is fixed to the bottom of the glass cover.
In addition to this K-type, thermocouples were used to measure the temperatures and a data logger was employed to log the temperature data. The ambient temperature was observed between 31 and 41°C. Pyranometer was used to measure the direct radiation and diffused radiations incident on the surface. It has been found that the solar radiation varied from 12 to 830 W/m 2 . The wind velocity was varied from 0.1 to 2.5 m/s and was measured using the anemometer. The distillate was collected in a container at an interval of 1 hour and measured with a digital weighing pan.

| Mathematical model
The mathematical model attempts to describe the energy transition at every step of the still. Figure 1 shows the energy transfer involved in the still.

| Energy balance for basin liner
The solar energy falling on solar still was stored in the basin and remaining energy lost to the atmosphere through a convective heat transfer and energy balance can be written as: where Q cb convective heat transfer from basin liner to water, which can be calculated as and, Q W is the heat lost to ambient and can be calculated as T b , T a , T w are the basin temperature, ambient temperature, and water temperature, respectively. The energy balance of water can be represented as: Q u is the external heat supplied. For this passive type of still, the value of Q u is zero and Q cw is the convective heat transfer from the water to glass. 12 where P w and P g are partial vapor pressure at the water and the glass cover and can be determined as 12 Q rw is the radiative heat transfer between water and glass, ε eff is the effective emittance, and σ is the Stefan-Boltzman's constant and can be calculated as 12 Q ew is radiative heat transfer between water and glass and can be calculated as 12 If it is a passive solar still, Q u = 0 signifies the external heat transfer and it can be calculated as 10 where, I(t)' is the incident radiation on solar collector surface.

| Energy balance for glass cover
The energy balance of the glass cover can be represented by the following equation: Q cg is the convective heat transfer between the glass to ambient, and V is the wind velocity 3 Q rg is the radiative heat transfer between glass cover to the sky 3 To find the hourly distillate of the still, where LH is the latent heat of vaporization and can be determined as 3 On solving the energy balance Equations (3.1), (3.4), and (3.11), one can obtain the first-order differential equation to find the temperatures (T w , T g , T b ) after the time interval "Δt". 11 where Equation (3.17) can be written as T(w(i + 1)) is the water temperature after Δt time interval.
Mass of distillate m ew can be determined by only knowing the heat transfer by evaporation and the latent heat The efficiency of the still is defined as the ratio of the useful energy output to the total energy incident on the surface. The useful energy is defined as the product of distillate output to the latent heat absorbed by it.
where I is the solar incident radiation for t time.
Assumptions taken during the simulation where T w(i), is the temperature of water at t = 0, and T w(i+1) is the water temperature after Δt time. Now, for temporal discretization, Equations (3.1), (3.4), and (3.11) can be written for the time step of 0.1 seconds as, So, by knowing the initial temperatures T w , T g , T b , and T a , I(t) for every time interval Δt, we can estimate T w(i+1) , T g(i+1), T b(i+1) and further mass of distillate.

| Numerical Iterative method
This model is based on Runga-Kutta (ODE) method for an iterative solution using the functions given by Kumar and Tiwari. 11 With the time step of 0.1 seconds, data for 24 hours is being simulated. Equations where In order to validate with accuracy of the mathematical model, the experiment was conducted on solar still on 14 July 2019.

| METHODOLOGY
An experiment is carried out on single-glass solar still on 14th July 2019 at Patiala, India. The water level in the still is maintained at 3 cm. The thermal model is being validated with the corresponding results obtained by the experiment.
The flow chart of thermal modeling done using MATLAB software is shown in Figure 3. Initially, the temperature values of T w , T g , and T b were taken equal to the ambient temperature. Further, the metalogical data measured using various instruments have been taken and loaded to compute heat transfer coefficients. The energy balance equations were then solved for glass cover, water, and basin liner. After this, the next iteration of temperatures of T w , T g , and T b have been calculated. Finally, the distillate output has been calculated, and then the program stopped. During the simulation, first, the temporal discretization is being carried out, which is a FEM (finite element method) technique, to get a minimum deviation from the results. Runga-Kutta method is employed with the time step of 0.1 seconds, which generates a lower scope of error because of the closeness in the ambient temperature and intensities for the time gap. The perimeters of both the experimental model and thermal model are then compared for the hourly variation of distillate output and heat transfer coefficients.
After validation, the same thermal model is extended to determine the performance while using nanoparticles at different volume fractions. This model is carried out with an assumption that the value of ambient temperature and solar intensity falling on the surface is not changing for Δt time.

| RESULTS AND DISCUSSIONS
An experiment was performed on a single-slope solar still (passive type) with a water depth of 0.03 m and glass tilt angle of 30° on 14 July 2019, the total distillate output obtained was 3.327 kg/(day.m 2 ). The hourly variation in solar intensity and ambient temperature with respect to time are shown in Figures 4 and 5, it can be seen from the graphs that the solar intensity and ambient temperature are maximum around 12:00 PM-01:00 PM, respectively. The maximum value of solar intensity was 830 W/m 2 and for ambient temperature was 41°C.
In the present study, the hourly experimental and theoretical observations were compared. The various design parameters of solar still are presented in Table 1. The inclination of the glass cover was kept at 30°C, which is equivalent to the latitude of the equation. The While the glass area can be calculated using the geometry of solar still as mentioned in Table 1. The basin of still was designed to store water up to the depth of 0.03 m. The convective heat transfer coefficients and optical properties of solar still components are also mentioned in Table 1.

| Validation of Thermal model
Theoretical model (using Runga-Kutta ODE integrated with a analytical model by Tiwari 11 ) has been developed and compared with the experimentally obtained results. The predicted values of water temperature and glass temperature were in an average deviation range between 8% and 6% as shown in Figures 6 and 7. Figure 6 represents the hourly variation of theoretical and experimental water temperature and it has been observed a similar trend for both cases. The temperature of water starts rising as the solar intensity increased and tend to decrease during the later part of the day along with the solar intensity.
While Figure 7 represents the variation of glass temperature for 24 hours and it has been observed that variation in glass temperature of experimental and theoretical was more during the early part of the day. It is because the losses that occurred in actual condition were more than the theoretical loss considerations. From Figures 6  and 7, it has been observed that at the higher temperature, the ranges deviation from the experimental results are significant and it was because of the fact that the DHINDSA et al heat losses from the side insulations were more at higher temperatures.
The hourly variation of theoretical and experimental distillate output is shown in Figure 8. The productivity still follows the same trend as followed by the solar intensity. The experimental results are in good agreement with theoretical with a total deviation of 12.24% for both experimental and theoretical distillate output for a day and at higher temperatures, the range of deviation was found to be more. It is because of more losses from the still at higher temperature and solar intensity.

| Effect of water depth on the performance of solar still
To obtain optimum water level, a mathematical simulation was carried at different water depths (0.01, 0.02, 0.03, 0.04, 0.05 m). As illustrated in Figure 9, at 0.01 m water depth, there exists a maximum peak value for distillate produced during the time 12:00 pm− 2:00 pm. As water depth increases, the graph shifts toward the righthand side, which is because of the heat-storing capacity F I G U R E 7 Hourly variation of theoretical and experimental glass temperature F I G U R E 8 Hourly variation of theoretical and experimental distillate output F I G U R E 9 Hourly variation of distillate output at different water depths of water. The maximum distillate output was recorded for 0.02 m with 3.65 kg/(day.m 2 ) and minimum for 0.08 m with 3.10 kg/(day.m 2 ). As the depth of the basin water increased, the day distillate decreased but night distillate increased because heat storage takes place in the basin water at more basin water depth. Figure 10 shows the variations of evaporative heat transfer coefficients at various depths of basin water and a similar trend has been found as of hourly distillate output. The evaporative heat transfer coefficient for different water depths is maximum for the water level at 0.01 m having a peak during 2:00 PM, followed by 0.02 m and so on. The increasing trend of evaporative heat transfer slightly decreases at about noon at all depths of basin water because the solar intensity is almost at peak and temperature difference decreased.

| Variation in the performance of solar still with different Nanofluids
Seeding nanoparticles in the base fluid enhances heat transfer coefficients and results in higher performances. Also, increasing the volume fraction of the nanoparticle, the effective medium (surface area to volume ratio) increases, which contributes to higher efficiencies due to an increase in surface area.
Exceeding optimum levels of concentrations, there exists a noticeable change in flow resisting properties (with an increase in mass concentrations, the flow friction increases), and as a result, the viscosity increases. Increasing viscosity decreases the heat transfer efficiency. 40 With the maximum distillate output of 3.65 kg/(day.m 2 ) by simple solar still, 0.02 m was found to be the optimum water level to continue mathematical simulation for modified solar still, seeded with nanofluids.
The thermophysical behavior of a nanofluid depends on the particle size, volume fraction, and physical characteristics like density, thermal conductivity, and specific heat capacity. Also, the properties of nanoparticles are presented in Table 2. Figure 11 shows the distillate output for five different water-based nanofluids (CuO, Al 2 O 3 , SiC, Fe 2 O 3 , and Ag) that were simulated in MATLAB using Runga-Kutta numerical integration method. A higher yield of still was obtained for Al 2 O 3 nanofluid with a 14.22% increase in productivity at a volume fraction of 0.2 as compared to water. While the enhancement of productivity with CuO, Ag, Fe 2 O 3 , and SiC at 0.2 volume fraction was found to be 10.82%, 8.11%, 7.63%, and 7.61%, respectively.
From simulation results, the temperature of nanofluid and base fluid (water) has been calculated and the differences of these temperatures have been taken. The temperature gradient for Al 2 O 3 nanofluid was maximum because of the improved thermo-physical properties, as compared to the other simulated nanofluids. From Figure 12, it has been noticed that the peak temperatures for all the nanofluid were during the sunshine

| CONCLUSIONS
In the present study, the performance of five different nanofluids and base-fluid has been analyzed. On the basis of the present study, the following conclusions are drawn: 1. The optimum water depth for single-slope solar still having water as a base fluid was found to be 2 cm. It has been found that if we increase the basin water depth, the inertia of water increases which leads to a decrease in productivity of still.