Effect of inclination angle and light source position on water temperature profile and productivity of solar still: an indoor experiment

Solar stills are generally used to obtain fresh and clean water from saline, brackish, and wastewater using solar energy. Many factors affect the performance of solar still, such as solar radiation, altitude, top cover inclination angle, etc. In this research, indoor solar still experiments have been carried out with different top cover inclination angles and light source positions with 7 hours of operating time. This research aims to study the effect of those two variables on water temperature profile and productivity of solar still. Halogen lamp was used as artificial light source. Three light source positions were selected based on the yearly solar azimuth and elevation angles data of Yogyakarta, Indonesia. The selected inclination angles were 20°, 30°, and 40°. Results demonstrated that light source position which related to incident angle of light at the top cover of solar still affected the water temperature profile in solar still. The water temperature profile decreased considerably when the angle of incident light was higher than 40°. Condensate productivity can be increased by using higher inclination angle of solar still top cover and highest condensate productivity of 518.75 mL/m2 can be obtained by using 40° of inclination angle. However, water temperature profile which affected by light source position must be taken into consideration when using higher inclination angle. Lower water temperature profile caused the decreasing of condensate productivity.


Introduction
Water and energy are important commodities for many countries in the world.The water crisis is already a major challenge faced by many countries.This problem is becoming even worse due to the increasing water demand caused by rapid population and economic growth [1].Meanwhile, the discharge of industrial and municipal wastewater effluent without proper treatment that caused serious pollution on freshwater sources has aggravated the problem [2].
Energy resources have been the essential element for economic growth.Economic development mainly depends on energy consumption, which is highly responsible for greenhouse gas (GHG) emissions, particularly CO2 [3].Primary consumption of non-renewable energy like fossil fuel generates CO2 emissions that drive global warming and climate change.Starting from this general environmental issue due to non-renewable sources, several countries have tried to make essential changes in production methods and energy consumption.Some governments have mainly switched to renewable sources, leaving fossil fuels to no longer be based on non-renewable energy sources [4].1239 (2023) 012006 IOP Publishing doi:10.1088/1755-1315/1239/1/012006 2 Solar energy is one of the renewable energy sources and this energy is the largest source of energy found on earth.The use of solar energy in technology is growing rapidly.Many companies engaged in technology use solar energy as their energy source, including automotive manufacturers, power plants and electronic equipment.One of the technologies that utilize solar energy is desalination system.Desalination is an artificial process to produce freshwater from saline water [5].
Solar still is one of the solar desalination technologies.It consists of various components, such as transparent top cover, water basin, absorber plate, insulation, and condensate collector channel.Solar still uses the principles of evaporation and condensation to separate salt and minerals from brine by utilizing the energy from the sun.The solar irradiation absorbed is used to drive the evaporation of freshwater from brine.As the water evaporates, the rising vapor condenses on a cooler transparent surface.The condensate formed on the cooler transparent surface rolls down the surface under the effect of gravity, along with adhesive and cohesive forces, and is then collected [6].Generally, solar still is used to produce freshwater from saline or brackish water for drinking, domestic and other purposes.However, several researchers have started to use solar still for wastewater treatment [2] [7].
Performance of solar still is influenced by several parameters.These parameters can be categorized into design, operational and climate parameters.These parameters include water depth, glass cover temperature, water temperature, wind velocity, ambient temperature, transparent top cover thickness and inclination.Several researchers have worked on these parameter variations to understand their optimum value at which maximum productivity is obtained.The design and operating parameters can be varied during experiments to obtain maximum distillate output.But climate-related parameters are uncontrolled and cannot be varied [8].Among these parameters, top cover inclination angle and weather condition have significant impact on solar still performance.The optimum solar still top cover inclination angle does not possess fix value for throughout the year [1].Some researchers concluded that inclination angle should be equal to latitude angle [9] [10] [11], some suggest higher than latitude angle [12] and some in the favor of lower tilt angle than latitude angle [13] [14].These phenomena occurred because the sun positions are different in every season thus affecting the amount of sun radiation reflected by the top cover of the solar still [15].
Many throughout year outdoor experiments for determining optimum top cover inclination angle of solar still and its effect on solar still performance have been carried out in several countries [9] [10] [11] [12] [13] [14], but the same experiment never been conducted in Indonesia.Moreover, there is no detailed research about the effect of sun position on solar still performance.In this research, a new approach has been proposed.Indoor solar still experiments have been conducted to investigate the effect of inclination angle and sun position on solar still performance and to give an early insight about optimum inclination angle in Indonesia.Indoor solar still experiments with different inclination angles and light source positions have been carried out using artificial light source.The different positions of light source which represent the sun positions have been chosen based on the yearly sun position data.

Indoor Solar Still Configuration
The type of solar still used in this work was single slope single basin solar still with an area of 0.16 m 2 .The solar still was insulated with syrofoam, while the still was made of acrylic with 5 mm thickness.Black acrylic was used for the heat absorber at the bottom part of the still.The type of water used in this work was tap water and the water level inside the solar still was maintained at 1 cm.The experiment was conducted continuously for 7 hours (09.00 AM to 04.00 PM GMT+7) using a 1000 Watt halogen spotlight and a fan which generate air at constant wind speed of 3 m/s.The water temperature data were collected using K-type thermocouple and were taken hourly.Solar intensity was measured using a solar power meter and condensate yields were measured by weighing, with data collected hourly.The configuration of solar still equipment can be shown in Figure 1.There were three different positions of light source and were chosen based on the yearly data of sun azimuth and elevation angle during midday at Yogyakarta, Indonesia with latitude of 7.7956 o .The yearly sun positions can be illustrated in Figure 2, in which the solar still is facing south.The light source positions can be shown in Figure 3.These positions were chosen assuming that solar still highest performances reached during midday (around 12.00 PM GMT+7) which affecting the overall performance of solar still.This assumption was supported by the daily sun radiation intensity data from which showed that the radiation intensity reached its highest amount during midday.Several researchers also reported that the highest productivity of solar still achieved during middays [16] [17].

Data Analysis
The hourly condensate yield data were used to calculate the cumulative yield value by using this formula Where Vc = cumulative yield of condensate (mL/m 2 ) V = hourly condensate yield (mL) A = basin area of solar still (m 2 ) t = operating time (hour) Vc and t were plotted on graph along with the water temperature (Tw) and t.The effect of inclination angle and light source position on solar still performance can be studied by analyzing the graphs.

Results and discussion
Indoor experiments with different top cover inclination angle and light source position have been conducted to study the effect of those two variables on solar still performance.There were two parameters observed in this indoor experiment.The first one was water temperature profile inside the solar still and the second one was condensate productivity which reviewed based on cumulative amount of condensate generated per hour.

Effect of Light Source Position on Solar Still Performance
The positions of light source affect the angle of incident light at the solar still top cover.The higher angle of incident light will result in increased intensity of reflected light and decreased intensity of transmitted light [18].The angle of incident light at the solar still top cover can be calculated using trigonometric principle by assuming that the radiation from halogen spotlights is a parallel type of radiation.The results of these calculations can be shown in Table 1.Other researchers also found that transmitted light intensity is a function of the angle of incident light.The transmitted light will heat up the water and enhance the evaporation rate [19].Figure 4, Figure 5 and Figure 6 show that the light source Position 1 and Position 2 were not giving the significant difference on water temperature profile (Tw).Significant decrease on water temperature profile observed when light source Position 3 was used and the inclination angle of 40 o had the lowest temperature profile among the others.These findings can be analyzed using the angle of incident light data, as shown in Table 1.The angles of incident light from solar still with light source Position 3 at different inclination angles were relatively higher than the angle of incident light from solar still with light source Position 1 and Position 2. The intensity of transmitted light will be lower if the angle of incident light is higher thus affecting the water temperature profile.
It was also observed that there is no direct correlation between light source position and cumulative yield of condensate (Vc).However, these light source positions will affect the angle of incident light and the intensity of transmitted light which affect the evaporation rate and condensate productivity, as shown in Figure 4, Figure 5 and Figure 6.However, the condensate productivity increased considerably to 518.75 mL/m 2 .These findings showed that higher inclination angles of solar still have the better potential to generate higher condensate productivity, which will be discussed in the next section.

Effect of Inclination Angle on Solar Still Performance
The top cover inclination angle is one of the factors affecting the performance of solar still.These inclination angles have the correlation with intensity of reflected light, heat transfer area at the top cover and gravitational force of condensate droplet [20].Figure 7 showed that inclination angle changes had almost no effect on water temperature profile.This means that the intensity of reflected light was relatively the same even though the inclination angle changed.The cumulative yield of solar still with 20 o of inclination angle was 240.62 mL/m 2 .The condensate productivity would increase to 409.37 mL/m 2 and 518.75 mL/m 2 when the inclination angle changed to 30 o and 40 o , respectively.These results showed that there were considerable differences of cumulative yield even though the water temperature profile was relatively the same.The top cover inclination angle had a big impact on the condensate productivity.The condensate droplets tend to easily roll down when higher inclination angle was used.Water temperature profile of solar still with different inclination angles and light source Position 2 also differ unsignificantly when the inclination angles were changed.A slight decrease on water temperature profile occured when the higher inclination angles were used, as shown in Figure 8.This water temperature profile decrease was caused by the increase of reflected light.The solar still with 20 o inclination angle had the highest water temperature profile but the cumulative yield was the lowest.This showed that higher inclination angle is necessary for producing higher condensate productivity.
The use of higher inclination angle would increase the productivity of condensate.However, Figure 9 shows that inclination angle of 40 o and light source Position 3 would generate the lowest cumulative yield of 175 mL/m 2 .The water temperature profile of solar still with that configuration was also the lowest after 7 hours of operating time.It can be observed from Table 1 that these configurations would result in 72 o of incident angle of light and causing the intensity of transmitted light to drop significantly.These results indicate that the condensate productivity can be increased by using higher inclination angle, but the water temperature profile of solar still with certain configuration must be considered as an impacful factor.

Conclusion
The performance of solar still with different top cover inclination angles and light source positions have been analyzed.From the analysis, it can be noted that light source positions affect the water temperature inside the solar still and there is no direct correlation between light source position and condensate productivity.However, the condensate productivity will increase as the water temperature inside the solar still increase.Condensate productivity can be increased by using higher inclination angle but there is also a possibility of condensate productivity decrease.Highest condensate productivity (518.75 mL/m 2 ) obtained from solar still with 40 o inclination angle and light source Position 1.The lowest condensate productivity (175 mL/m 2 ) also obtained from 40 o inclination angle when light source Position 3 were used.In addition, higher inclination angle is best used for the solar still with light source Position 1 which represent sun position during November until February and Position 2 which represent the sun position during March-April and September-October.However, the performance of solar still will decrease during May until August which represented by light source Position 3. From the results, it can be studied that the synergistic considerations between inclination angle and light source position are needed to obtain the high condensate productivity.

Figure 2 .
Figure 2. Illustration of yearly sun position at Yogyakarta, IndonesiaThis solar still constructed with adjustable inclination angle thus the angle can be configured flexibly as shown in Figure1.The inclination angles used in this work were 20 o , 30 o and 40 o .Plasticin clay was used to thighten the edge of the solar still top cover so that the water vapor would not leak.There were three different positions of light source and were chosen based on the yearly data of sun azimuth and elevation angle during midday at Yogyakarta, Indonesia with latitude of 7.7956 o .The yearly sun positions can be illustrated in Figure2, in which the solar still is facing south.The light source positions can be shown in Figure3.These positions were chosen assuming that solar still highest performances reached during midday (around 12.00 PM GMT+7) which affecting the overall

Figure 2 and
Figure 3 show that Position 1 represents the sun positions during November until February.The second light source position which represents the sun positions during March-April and September-October can be configured experimentally by implementing Position 2 configuration.The last configuration that is noted as Position 3, represents the sun positions during May until August.The elevation angle around 58 o is the lowest sun elevation angle achieved in June and December.

Figure 4 .Figure 4
Figure 4. Water Temperature Profile and Cumulative Yield from the Solar Still with Different Light Source Positions and Inclination Angle of 20 o Figure 4 shows that the condensate productivity of solar still with 20 o inclination angle would not increase considerably although the light source position and water temperature profile increased.However, different phenomena happened on solar still with inclination angle of 30 o and 40 o .The light

Figure 5 .Figure 6 .
Figure 5. Water Temperature Profile and Cumulative Yield from the Solar Still with Different Light Source Positions and Inclination Angle of 30 o

Figure 7 .Figure 8 . 8 Figure 9 .
Figure 7. Water Temperature Profile and Cumulative Yield from the Solar Still with Different Inclination Angle and Light Source Position 1

Table 1 .
Angles of Incident Light with Different Light Source Position and Inclination Angle