Indoor and Outdoor Performance Study of Metallic Zinc Particles in Black Paint to Improve Solar Absorption for Solar Still Application

: In this study, the effects of metallic zinc (Zn) particles were studied to increase surface temperature on a solar-still absorber, which is a major component of increased production. Various concentrations of Zn particles were mixed in black paint and applied to the absorber plate. SEM and XRD were used to examine and conﬁrm the surface morphology and phase identiﬁcation of as-received powder. UV-Vis spectroscopy was used to examine light-absorption properties. Finally, extensive indoor testing (using an improvised solar emulator) and outdoor testing were conducted to optimize the concentration. The specimens containing 10 wt% Zn in black paint showed the highest increase in temperature, i.e., 103.53 ◦ C in indoor conditions at 1000 W/m 2 irradiation, which is 59.17% higher than a bare aluminum plate and 17.57% higher than an only black-paint-coated aluminum plate. On the other hand, specimens containing 10 wt% Zn reached just 87.53 ◦ C, compared to 80.00 ◦ C for an only black-paint-coated aluminum plate and 60.62 ◦ C for bare aluminum.


Introduction
Freshwater is essential for human beings' survival, and it plays a crucial role in a sustainable environment. Therefore, the demand for potable water is increasing gradually every year due to population enlargement. Although 70% of the earth is covered with water, most of the accessible water cannot be utilized for drinking due to salinity and water contamination [1]. Various techniques, such as membrane desalination [2][3][4] and dialysis [5,6] are used in the purification of water. One of the potential alternatives to produce fresh water and decrease the cost of distillation plants is the utilization of solar energy directly using flat collections of concentrated collectors [7]. Solar distillation is one technique to produce fresh water at a lower cost than the other possible seawater desalination processes [8,9]. Solar distillation using solar stills is the most feasible and environment-friendly way to supply fresh water in arid and remote regions [10].
Solar stills work on evaporation and condensation processes and generally consist of a water basin and a transparent cover that allows sunlight penetration and stimulates the evaporation process. The evaporated water condenses on the backside of the top cover and Various concentrations of Zn were mixed with black paint and applied to the aluminum plate. Indoor and outdoor testing was conducted using an improvised apparatus for concentration optimization.

Samples Preparation
The test samples were prepared using a simple chemical mixing method. Black paint was used as a base matrix to paint onto an aluminum plate, and Zn nanoparticles in the various fractions 0.5, 1, 3, 5, 7, 10, 15, and 20 wt% were mixed in black paint. In brief, the nanoparticles were suspended in isopropyl alcohol (5 mL), sonicated for 1 h and stirred for 5 h into a proper suspension. The suspension was mixed with black paint using a mechanical mixer, and the now-formed formulation was applied to the aluminum plate using a hand brush. The samples were dried at room temperature for 24 h. The same procedure was repeated for each concentration. A clean black-painted aluminum plate and a bare aluminum plate of the same dimensions were also prepared for comparative purposes. Figure 1 shows the prepared samples for testing.
Coatings 2021, 11, x FOR PEER REVIEW 3 of 14 studied to improve the absorption property of an absorber plate for solar-still applications. Various concentrations of Zn were mixed with black paint and applied to the aluminum plate. Indoor and outdoor testing was conducted using an improvised apparatus for concentration optimization.

Samples Preparation
The test samples were prepared using a simple chemical mixing method. Black paint was used as a base matrix to paint onto an aluminum plate, and Zn nanoparticles in the various fractions 0.5, 1, 3, 5, 7, 10, 15, and 20 wt% were mixed in black paint. In brief, the nanoparticles were suspended in isopropyl alcohol (5 mL), sonicated for 1 h and stirred for 5 h into a proper suspension. The suspension was mixed with black paint using a mechanical mixer, and the now-formed formulation was applied to the aluminum plate using a hand brush. The samples were dried at room temperature for 24 h. The same procedure was repeated for each concentration. A clean black-painted aluminum plate and a bare aluminum plate of the same dimensions were also prepared for comparative purposes. Figure 1 shows the prepared samples for testing.

Indoor Setup and Experimental Procedure
The indoor investigation was carried out at the solar thermal laboratory, UMPEDAC, the University of Malaya, using a solar emulator. The room temperature was set to 25 °C using a central air conditioning system under virtually still air. There were many components and instruments used in this experiment to fulfill the required indoor conditions. For providing the required irradiation, a solar emulator comprising 120 halogen bulbs (brand: OSRAM) with power, voltage, and current capacity of 90 W, 12 V, and 7.5 A, respectively, was used. The solar emulator was controlled by three variable control AC power-supply transformers that could simulate solar radiation from 100 W/m 2 to 1200 W/m 2 [26]. Furthermore, a few other standard measuring instruments were used to record data. Some essential instrumentation and sensors used were a pyranometer (brand: LI-COR, LI200R), data logger brand: Data Taker (DT80), and thermocouples (K-type). The measuring and accuracy range of all the instruments and sensors is given in Table 1. The temperature of the room was set to 25 °C using a central air conditioning system.

Indoor Setup and Experimental Procedure
The indoor investigation was carried out at the solar thermal laboratory, UMPEDAC, the University of Malaya, using a solar emulator. The room temperature was set to 25 • C using a central air conditioning system under virtually still air. There were many components and instruments used in this experiment to fulfill the required indoor conditions. For providing the required irradiation, a solar emulator comprising 120 halogen bulbs (brand: OSRAM) with power, voltage, and current capacity of 90 W, 12 V, and 7.5 A, respectively, was used. The solar emulator was controlled by three variable control AC power-supply transformers that could simulate solar radiation from 100 W/m 2 to 1200 W/m 2 [26]. Furthermore, a few other standard measuring instruments were used to record data. Some essential instrumentation and sensors used were a pyranometer (brand: LI-COR, LI200R), data logger brand: Data Taker (DT80), and thermocouples (K-type). The measuring and accuracy range of all the instruments and sensors is given in Table 1. The temperature of the room was set to 25 • C using a central air conditioning system. For the indoor experiment procedure, the samples of size 6 × 6 were placed on a cardboard in the solar emulator. The samples with different concentrations of Zn particlecoated plates were placed in two rows with displacement 3 inches apart on the cardboard. A bare aluminum plate and an only black-paint-coated aluminum plate were also placed on the same board for comparative purposes. The K-type thermocouple was placed on each plate by placing fiberglass wool (insulating material) in between the thermocouple bed and adhesive tape to avoid the thermocouple's direct heating. The experiments were conducted at various irradiation levels in indoor working conditions. Five samples for each concentration were tested, and the average maximum temperature is presented. The standard deviation for all the readings ranged between ±2% and ±3%. It was observed that it took about 30 min in each irradiation level of an experiment to reach stable conditions. Then the solar emulator was switched off for about 15 min to record all plates' decreasing temperature. The arrangement of the indoor experiment is systematically depicted in Figure 2.

Instrument
Measuring Range Accuracy For the indoor experiment procedure, the samples of size 6″ × 6″ were placed on a cardboard in the solar emulator. The samples with different concentrations of Zn particlecoated plates were placed in two rows with displacement 3 inches apart on the cardboard. A bare aluminum plate and an only black-paint-coated aluminum plate were also placed on the same board for comparative purposes. The K-type thermocouple was placed on each plate by placing fiberglass wool (insulating material) in between the thermocouple bed and adhesive tape to avoid the thermocouple's direct heating. The experiments were conducted at various irradiation levels in indoor working conditions. Five samples for each concentration were tested, and the average maximum temperature is presented. The standard deviation for all the readings ranged between ±2% and ±3%. It was observed that it took about 30 min in each irradiation level of an experiment to reach stable conditions. Then the solar emulator was switched off for about 15 min to record all plates' decreasing temperature. The arrangement of the indoor experiment is systematically depicted in Fig

Outdoor Setup and Experimental Procedure
The outdoor experimental setup has been installed in the solar garden of the Higher Institution Centre of Excellence, UM Power Energy Dedicated Advanced Centre (UM-PEDAC), Kuala Lumpur, Malaysia. Experiments were carried out from 8:00 a.m. to 5:00 p.m. The same standard devices and instruments were used to record the data. The measuring and accuracy ranges of all the instruments and sensors are given in Table 1. Sensors were placed in the system at the necessary locations for recording the data. Other meteorological data, such as wind speed, humidity, etc., were measured using the weather station installed at the solar garden.
The same arrangement of samples, i.e., zinc-coated nanoparticles, the plate coated with black paint, and the aluminum plate, was attached to a wooden sheet with a thickness of 1" and mounted at an optimum inclined angle of 15 degrees. It is worth mentioning here that only the best performing and uniformly coated samples for each concentration

Outdoor Setup and Experimental Procedure
The outdoor experimental setup has been installed in the solar garden of the Higher Institution Centre of Excellence, UM Power Energy Dedicated Advanced Centre (UMPEDAC), Kuala Lumpur, Malaysia. Experiments were carried out from 8:00 a.m. to 5:00 p.m. The same standard devices and instruments were used to record the data. The measuring and accuracy ranges of all the instruments and sensors are given in Table 1. Sensors were placed in the system at the necessary locations for recording the data. Other meteorological data, such as wind speed, humidity, etc., were measured using the weather station installed at the solar garden.
The same arrangement of samples, i.e., zinc-coated nanoparticles, the plate coated with black paint, and the aluminum plate, was attached to a wooden sheet with a thickness of 1" and mounted at an optimum inclined angle of 15 degrees. It is worth mentioning here that only the best performing and uniformly coated samples for each concentration (i.e., 0.5 wt%, 1 wt%, 3 wt%, 5 wt%, 7 wt%, 10 wt%, 15 wt%, and 20 wt%) under indoor conditions were tested in outdoor conditions and compared with an only black-paint-coated aluminum plate and a bare aluminum plate. The layout of the outdoor experiment, including the position and connectivity of all instruments and components, are systematically represented in Figure 3. The optimum angle was calculated using Cooper's Equation (1) to intercept the maximum radiation throughout the day.
where δ is the inclination angle, and n 1 is the day of the year. coated aluminum plate and a bare aluminum plate. The layout of the outdoor experiment, including the position and connectivity of all instruments and components, are systematically represented in Figure 3. The optimum angle was calculated using Cooper's Equation (1) to intercept the maximum radiation throughout the day.
where is the inclination angle, and 1 is the day of the year.  Figure 4 shows the scanning electron micrographs (SEM) (model: Tescan Mira3, Tescon, Kohoutovice, Czech Republic) along with X-ray diffraction (XRD) (Bruker D8 discover, Bruker, Germany) and UV-visible spectroscopy (model: UV-3600i Plus UV-Vis-NIR Spectrophotometer, the range set to 200−1100 nm, Shimadzu, Japan) to understand the surface-morphology, composition, and light-absorption characteristics of as-received Zn particles. Powder samples were used for all three characterizations. For SEM, the powder was dispersed in ethanol and sonicated for 30 min. After that, the suspension was dropcasted on an aluminum stub covered by carbon double-tap. For Uv-Vis-Nir spectroscopy, the sample suspension used neat ethanol as a reference in reference cuvettes. To measure the UV-Vis spectra of black paint, 0.30 mL of black paint was mixed with 5 ml of ethanol, and the diluted solution was tested. It is evident from Figure 4a that the particles are spherical with a rough surface texture. The size of the particles ranged from micron to submicron size, but the majority of particles are in the micron-sized range. Figure 4b shows the XRD pattern of the as-received Zn particles with miller indices. The diffraction pattern was matched with JCPDS number 00-004-0784 for confirmation. Little to no zinc oxide peaks have been detected. Figure 4c shows the UV-Vis spectra of as-received particles and black paint. A sharp absorption at around 350 nm in the UV region was identified, followed by a reduction in absorption. A second sharp absorption was identified in the near-infrared region with onset starting at the visible region of the solar spectrum. On the other hands, black paint shows light absorption only in the visible region. This high absorption in the UV (230−350 nm) and near-infrared (990−1030 nm) regions indicates high sunlight-absorption properties that helped improve the surface temperature of the absorber plate.  Figure 4 shows the scanning electron micrographs (SEM) (model: Tescan Mira3, Tescon, Kohoutovice, Czech Republic) along with X-ray diffraction (XRD) (Bruker D8 discover, Bruker, Germany) and UV-visible spectroscopy (model: UV-3600i Plus UV-Vis-NIR Spectrophotometer, the range set to 200−1100 nm, Shimadzu, Japan) to understand the surface-morphology, composition, and light-absorption characteristics of as-received Zn particles. Powder samples were used for all three characterizations. For SEM, the powder was dispersed in ethanol and sonicated for 30 min. After that, the suspension was drop-casted on an aluminum stub covered by carbon double-tap. For Uv-Vis-Nir spectroscopy, the sample suspension used neat ethanol as a reference in reference cuvettes. To measure the UV-Vis spectra of black paint, 0.30 mL of black paint was mixed with 5 ml of ethanol, and the diluted solution was tested. It is evident from Figure 4a that the particles are spherical with a rough surface texture. The size of the particles ranged from micron to submicron size, but the majority of particles are in the micron-sized range. Figure 4b shows the XRD pattern of the as-received Zn particles with miller indices. The diffraction pattern was matched with JCPDS number 00-004-0784 for confirmation. Little to no zinc oxide peaks have been detected. Figure 4c shows the UV-Vis spectra of as-received particles and black paint. A sharp absorption at around 350 nm in the UV region was identified, followed by a reduction in absorption. A second sharp absorption was identified in the near-infrared region with onset starting at the visible region of the solar spectrum. On the other hands, black paint shows light absorption only in the visible region. This high absorption in the UV (230−350 nm) and near-infrared (990−1030 nm) regions indicates high sunlight-absorption properties that helped improve the surface temperature of the absorber plate.

Results and Discussion
The notable outcome of the performance of the absorber plates is properly analyzed from the evidence and presented in detail with results and insightful discussion. In the following sections, the indoor and outdoor experiment results are presented, demonstrating complete graphical information about the beneficial effects of using various Zn concentrations with black paint.

Kuala Lumpur Weather Profile
The variation of maximum and minimum ambient temperatures, relative humidity, wind speed and direction, precipitation, and cloud cover across the year from December 2019 to 2020 is shown in Figure 5. It can be seen that the profile is almost consistent, with an average temperature of 25 • C throughout the year. However, a maximum and minimum temperature of 34 • C and 23 • C was also seen in March. Kuala Lumpur is located in the equatorial region with two monsoon seasons in a year, namely the northeast and southwest.  The notable outcome of the performance of the absorber plates is properly analyzed from the evidence and presented in detail with results and insightful discussion. In the following sections, the indoor and outdoor experiment results are presented, demonstrating complete graphical information about the beneficial effects of using various Zn concentrations with black paint.

Kuala Lumpur Weather Profile
The variation of maximum and minimum ambient temperatures, relative humidity, wind speed and direction, precipitation, and cloud cover across the year from December 2019 to 2020 is shown in Figure 5. It can be seen that the profile is almost consistent, with an average temperature of 25 °C throughout the year. However, a maximum and minimum temperature of 34 °C and 23 °C was also seen in March. Kuala Lumpur is located in the equatorial region with two monsoon seasons in a year, namely the northeast and southwest.

Indoor Analysis
The indoor experiment was conducted by varying irradiation from 100 to 1000 W/m 2 . However, for analysis purposes, the effect of temperature on the absorber plates under the increment of 200 W/m 2 and the rate of drop in temperature is shown in Figure 6 (adi). The left-hand side of Figure 6 illustrates the effect of 200, 600, 800, and 1000 W/ m 2 on the absorber plate temperature, and the right-hand side illustrates the rate of temperature drop (cooling curve) in 15 min intervals. The initial increase in the temperature is directly related to thermal properties, i.e., absorption and uniform distribution of the heat in the material. After that, the rise in temperature slows down and becomes almost linear. This linearity shows that the maximum temperature has been achieved. Furthermore, thermal convection is the only mode of heat transfer to the surrounding environment, which caused linearity (balance) in the temperature at the peak point. An increase in temperature with an increase in irradiation was observed, as shown in Figure 6a−d. Furthermore, increments in the concentration of particles in black paint also increased the surface temperature. This is due to the ability of Zn particles to absorb UV irradiation. The optimum concentration of Zn has been observed to be 10 wt% in black paint. The maximum temperatures 55.26, 82.12, 94.67, and 103.56 °C were observed for specimens containing 10 wt% Zn in black paint at 200, 600, 800 , and 1000 W/m 2 respectively. Further increments in concentration did not increase the surface temperature, which can be explained based on

Indoor Analysis
The indoor experiment was conducted by varying irradiation from 100 to 1000 W/m 2 . However, for analysis purposes, the effect of temperature on the absorber plates under the increment of 200 W/m 2 and the rate of drop in temperature is shown in Figure 6 (a-di). The left-hand side of Figure 6 illustrates the effect of 200, 600, 800, and 1000 W/ m 2 on the absorber plate temperature, and the right-hand side illustrates the rate of temperature drop (cooling curve) in 15 min intervals. The initial increase in the temperature is directly related to thermal properties, i.e., absorption and uniform distribution of the heat in the material. After that, the rise in temperature slows down and becomes almost linear. This linearity shows that the maximum temperature has been achieved. Furthermore, thermal convection is the only mode of heat transfer to the surrounding environment, which caused linearity (balance) in the temperature at the peak point. An increase in temperature with an Coatings 2021, 11, 536 7 of 12 increase in irradiation was observed, as shown in Figure 6a−d. Furthermore, increments in the concentration of particles in black paint also increased the surface temperature. This is due to the ability of Zn particles to absorb UV irradiation. The optimum concentration of Zn has been observed to be 10 wt% in black paint. The maximum temperatures 55. 26, 82.12, 94.67, and 103.56 • C were observed for specimens containing 10 wt% Zn in black paint at 200, 600, 800, and 1000 W/m 2 respectively. Further increments in concentration did not increase the surface temperature, which can be explained based on cooling curves. Figure 6ai−di shows the cooling curves of the specimens. It can be observed that, with an increase in the concentration of Zn particles in black paint, the cooling rate increased. This increase in cooling rate is due to the high thermal conductivity of Zn particles, which causes excessive convective losses when the concentration exceeded the optimum value. All the cooling curves of specimens at each irradiation level follow the same trend.  Table 2 summarizes the temperatures recorded for each concentration at 1000 W/m 2 irradiation level. The maximum temperature values of all the samples, along with mean value and standard deviation, are presented in Appendix A. The bare aluminum plate's surface temperature was recorded to be 65.04 °C, whereas the aluminum plate coated with only black paint shows an 83.80 °C temperature, which is around 28.25% higher. This is due to the high irradiation-absorption property of black color and low thermal conductivity of paint. The introduction of 0.5 wt% Zn particles in black paint increased the temperature to 88.06 °C, which is a 5.06% increase compared to the only black paint and 35.37% higher compared to the bare aluminum plate. At 1 wt% concentration, the recorded temperature reached 90.14 °C, which is 7.56% higher than only black paint and  Table 2 summarizes the temperatures recorded for each concentration at 1000 W/m 2 irradiation level. The maximum temperature values of all the samples, along with mean value and standard deviation, are presented in Appendix A. The bare aluminum plate's surface temperature was recorded to be 65.04 • C, whereas the aluminum plate coated with only black paint shows an 83.80 • C temperature, which is around 28.25% higher. This is due to the high irradiation-absorption property of black color and low thermal conductivity of paint. The introduction of 0.5 wt% Zn particles in black paint increased the temperature to 88.06 • C, which is a 5.06% increase compared to the only black paint and 35.37% higher compared to the bare aluminum plate. At 1 wt% concentration, the recorded temperature reached 90.14 • C, which is 7.56% higher than only black paint and 38.57% higher compared to bare aluminum. At 3 wt% concentration, the recorded temperature reached 96.01 • C, which is 14.57% higher than only black paint and 47.62% higher compared to bare aluminum. Further, the temperature increments with concentrations up to 10 wt% reached a maximum temperature of 103.56 • C, which is 23.58% higher than only black paint and 59.22% higher compared to bare aluminum.

Outdoor Analysis
The weather of Kuala Lumpur is an equatorial climate featuring hot and humid weather throughout the whole year. The southwest monsoon takes place from April to September, and the northeast monsoon occurs from October to March. The southwest monsoon highlights a drier climate with less precipitation contrasted with the northeast monsoon, which brings more rainfall [28]. In this research study, the outdoor experiments were conducted from 8:00 a.m. to 5:00 p.m. for about five days in the last week of September, and the data for the best sunny day is presented in Figure 7. It was noted that tropical thunderstorms and rainfall usually occurred in the afternoon, although the weather conditions were generally good in the morning during the experiments.

Outdoor Analysis
The weather of Kuala Lumpur is an equatorial climate featuring hot and humi weather throughout the whole year. The southwest monsoon takes place from April t September, and the northeast monsoon occurs from October to March. The southwes monsoon highlights a drier climate with less precipitation contrasted with the northeas monsoon, which brings more rainfall [28]. In this research study, the outdoor experiment were conducted from 8:00 a.m. to 5:00 p.m. for about five days in the last week of Septem ber, and the data for the best sunny day is presented in Figure 7. It was noted that tropica thunderstorms and rainfall usually occurred in the afternoon, although the weather con ditions were generally good in the morning during the experiments.  Figure 7a,b shows the outdoor temperature of specimens under investigation wit time duration, solar irradiation levels and wind speed (being recorded by a weather sta tion) on a particular day. It was observed that the outdoor data follows the same trend a indoor experimental data. The maximum recorded temperature for each specimen is sum marized in Table 3. The highest temperature, 87.50 °C, was observed for a specimen con taining 10 wt% Zn particles, which is 9.37% higher than only black paint and 44.38% higher than a bare aluminum plate. This is due to optimum light absorption and therma conductivity. Further increments did not improve the surface temperature mainly due t convective heat losses caused by higher thermal conductivity, which was observed durin indoor cooling.  Figure 7a,b shows the outdoor temperature of specimens under investigation with time duration, solar irradiation levels and wind speed (being recorded by a weather station) on a particular day. It was observed that the outdoor data follows the same trend as indoor experimental data. The maximum recorded temperature for each specimen is summarized in Table 3. The highest temperature, 87.50 • C, was observed for a specimen containing 10 wt% Zn particles, which is 9.37% higher than only black paint and 44.38% higher than a bare aluminum plate. This is due to optimum light absorption and thermal conductivity. Further increments did not improve the surface temperature mainly due to convective heat losses caused by higher thermal conductivity, which was observed during indoor cooling. After carefully analyzing indoor and outdoor performance, a significant difference was observed. For instance, under an indoor environment, the performance increase for 10 wt% specimens was 23.85 and 59.22% compared to the black paint and bare aluminum plate. Whereas in outdoor conditions, though the trend was the same, the noted increase in the performance was 9.37 and 44.38% for the black paint and bare aluminum plate, respectively. This behavior is due to high convective losses due to wind and solar intermittence under uncontrolled outdoor conditions. The difference between indoor controlled and outdoor uncontrolled conditions for specimens containing optimum concentration and black-paint-coated samples is much less than the difference between the highest performing specimens and the bare aluminum plate under indoor and outdoor conditions. This is because of the much lower thermal conductivity of black paint compared to a bare aluminum plate. It is worth mentioning that, like all incandescent light bulbs, a halogen lamp produces a continuous spectrum of light, from near-ultraviolet to deep into the infrared. This makes halogen lamps the only option for consumer light sources with a black-body radiation spectrum like the sun. With this information, the spectrum in indoor and outdoor conditions will remain the same with minimal deviation.

Conclusions
Solar-still technology is improving at an incredible pace to avoid the future scarcity of fresh drinking water. In this study, Zn particles were investigated to increase the sunlight absorption of a solar-still absorber plate. Various concentrations of metallic zinc particles were mixed with black paint and applied to the absorber plate. SEM and XRD were employed to examine surface morphology and composition, respectively. UV-Vis spectroscopy was conducted to study the optical absorption properties of Zn. Indoor and outdoor tests were conducted using an improvised apparatus. With the introduction of Zn particles in black paint, a temperature rise was observed, which can be associated with the ability of zinc to absorb sunlight in UV and the near-infrared range of the solar spectrum. The highest temperature, 103.53 • C, was recorded for samples containing 10 wt% Zn in indoor conditions at 1000 W/m 2 irradiation levels, whereas the same samples, with 10 wt% Zn, performed best in outdoor conditions, achieving a maximum of 87.53 • C. Further increments in concentration did not effectively improve absorption characteristics.

Institutional Review Board Statement: Not applicable.
Informed Consent Statement: Not applicable.

Data Availability Statement:
The prepared samples to support the findings of this study are available from the corresponding authors upon reasonable request.

Conflicts of Interest:
The authors declare no conflict of interest.