Experimental Study of Solar Still Under Influence of Various Conditions

In the present work, experimental tests was done to explain the effect of insulation and water level on the yield output. Linear basin, single slope solar still used to do this purpose. The test was done from May to August 2017 in Mosul City-Iraq (Latitude:35.866°N, Longitude:43.296°E, Elevation: 200 m, and 23° South-East face). Experimental results showed that the yield output of the still increased by 20.785% and 19.864% in case of using thermal insulation at 4cm and 5cm respectively, also the yield output decrease by 15.134% as the water level increase from 4 to 5cm, with the presence of insulation and 14.147% without it. It has been conclude that the insulation and water level play important role in the process of passive solar desalination, also the desalination process has significant value at night and cannot be ignored.


INTRODUCTION
Solar desalination is the process of utilizes solar radiation to produce fresh water Yousef, and Mousa, 2004 Solar desalination is simple and inexpensive method to transform brackish water into fresh water. There are two method used in the desalination process using solar radiation: direct method, using solar radiation directly to heat up the water, and this method used to produce small amount of fresh water, on the other hand indirect method, using pumps and modification equipment's with respect to solar radiation, and this method used to produce large quantities Hazim, and Fawzi, 2008.
Many researchers studied the behavior of solar still at various conditions, Akash, et al., 1998, showed the influence of adding some absorbing materials to the solar still to enhance system yield, results showed that the using of elastic carper and black ink may increase desalination rate by 38%, 45% respectively, and the best choice were black dye with desalination rate 60%. Khalifa, and Hamood, 2009, present experimental study to showed the effect of brine water depth on the system yield. Five water level (1, 4, 6, 8, and 10) cm have been tested in this case. Results shows that the productivity of solar still increased by about 48% in case of change water depth. Sengoltain, et al., 2010, studied the effect of adding square sponges to the solar still, two type of yellow and black color have been used in this case. Results showed that the system with black sponge better than that of yellow. Both of them enhance the productivity as compared to simple still. Hashim, et al., 2010, present experimental study and found the optimum still type, glass cover angle, and inner surface area to double slop, 45 o , and 1.1645 m 2 respectively. Agboola, and Egelioglu, 2011, studied the thermal performance of simple solar still under the influence of weather conditions in winter and summer seasons. Conclude that the using of wick plat produce 6.14 kg/m 2 daily compare to 3.327 kg/m 2 in case of without wick. Also the daily system efficiency were 52.4% and 43.6% in the two seasons. Khalifa, 2011, studied theoretically the effect of glass cover inclination, conclude that the glass angle must be perpendicular to incident solar radiation as possible to improve yield output. Panchal, and Shah, 2011, studied the effect of class cover thickness on the system yield. Three type of glass cover (4, 8, and 12 mm) have been used in the experimental study. Results showed that the glass cover thick of 4mm represent the best choice of them. Kabeel, et al., 2012, mentioned that it can be increase the yield output of solar still up to 25% with the aided of fan. Tenthani, et al., 2012, design two similar stills with white day on the internal surfaces, results showed that the empirical and classical type daily output were 2.55, and 2.38 kg/m2 respectively, and the efficiency increased by 6.8% in case of empirical type, conclude that the coating with color enhance the system productivity. Raed, 2012, showed the impact of various weather conditions on the yield, conclude that the productivity of the system changed directly to solar radiation, ambient temperature and wind speed, also using of PCM improve the productivity. Gnanadason, et al., 2013, conclude that the efficiency of the solar still improved by change the still material. Al-Mashat, and Dhurgam, 2014, Studied the effect of adding stearic acid and paraffin wax as phase change material (PCM) to improve the still productivity. Experimental results showed that adding PCM to the passive system enhance the productivity by 20.2% and 10% for sunny and cloudy days respectively. Gubta, et al., 2015, investigate experimentally the enhancement of the desalination in solar still by coupling evacuated tubes directly to the still and by adding float porous absorber inside basin also water high in the solar still was studied. Kabeel, et al., 2016, studied the possibility of increase the productivity of the system by blowing air over glass cover using Fan. Kumar and Rajesh, 2016, tried to enhanced the solar still productivity utilized the solar heater as well as evacuated tubes to increase the temperature of saline water and evaporate more water quantity.

THEORETICAL DESIGN OF THE SOLAR STILL
If the daily consumption for one person of drinking water are (2-3) Liter/day/one, so it has been required to design solar still produce this quantity of fresh water. The procedure of design as follow, Calbande, 2016.

EXPEREMINTAL SETUP
The solar still system was designed and constructed to verify the acceptable operational requirements. All the component were designed and selected according to either researches or studies and as on-hand in the markets, to get the optimum design to each component, to ensure that the system operation in best conditions with maximum productivity.

Still basin
This part of the system used as storage to store the saline water. It must be sealed and coating with black color as possible, in order to absorb the solar incident radiation, also it must be corrosive less, strong, smooth, cleanable and upload as possible. The present part constructed with the material of Galvanized Iron of (1.1 mm) thickness in order to be strength, no toxic, and non-corrosive and light weight. The dimension of the basin was (80cm x 50cm) and this dimensions have been selected according to specific design. Fig.1 shows evaporation and condensation process in the simple solar still, and Fig.2 shows the details of solar still. Fig.3 shows the experimental device used in the present work. The coating were dark black in base and brilliant weight color on the another sides of the still to absorb and reflect sunshine light. These procedures according to recent studies in order to increase the saline water temperature and maximize the useful solar incident radiation absorbed in the system.

Glass cover
In order to allow the incident solar irradiation to passes through, Glass cover have been used in the present work due to its advantages such as high transparency, rigid, easy fabricated, and inexpensive. Glass cover of thickness (4mm) are used due to its advantages over another high dimension, and its tilted at 35.866° (Zone latitude). Glass cover consider one of the most important parts because of its relation to energy source and condensation process.

Condensate accumulator (trough)
Its small passage used to collect and transfer the condensed water, its designed to be integrated with still body of the same material GI sheet of 1.1 mm thickness, with ignore area. In present work the trough design as triangular section of (3x3x50) cm. It must be small to prevent shading, on the other hand must be enough to collect and transfer the compensated water.

Insulation
In order to prevent heat loss from the still body to the environment, thermal insulation have been used, because of the heat loss decrease the still performance and minimizes the productivity. In the present work Cork Board insulation of 60 mm thickness, and 0.043 (w/m.k) thermal conductivity have been used and fabricated around the stills, the choice of insulation type are necessary to improve thermal performance and must be have low thermal conductivity, light weight, available, inexpensive and easy fabricated.

Seal
Its a metal that used to prevent any leakage to or from the still, also it used to connect any two part or more as one structure in our study this metal are used to prevent water vapor leakage from the stills inside space to the atmosphere, and water or dusty particles leakage also. It has been used Silicon rubber to fill the gaps between glass cover and still body, this type have been used because of low coast, inexpensive, easy to use, easy to remove without side effects when we need and controllable, two type have been used. Also window putty have been used to prevent leakage and this metal become hard after used.

Solar meter
The intensity of solar radiation have been measured using efficient solar meter of (TES-1333R) specifications as shown in the Fig.4.

Ambient conditions
The variable ambient conditions such (Ambient temperature and Wind speed and Relative Humidity) have been measured using (4 in 1 METER) portable measurement of (LM-8000A, Lutron Company-Taiwan) specifications, Fig.4.

Data logger
Temperatures of basin, saline water, Still's wall and glass cover have been recorded instantaneously using Data Logger of (BTM-4208SD, Lutron Company-Taiwan) specifications, with 8 point of thermocouples type-K, Fig.4.

Digital balance meter
The mass of desalinated water was measured hourly and accumulated using traditional type Digital balance meter. Fig.4. Glazing material Glass (0.485m 2 ) 8 Thickness of glass 4 mm 9 Inclination Angle of glass 35.866 o 10 Orientation of the Still South face 11 Trough Triangular (50x3x3) cm 12 Insulation Cork board (6 cm, 0.043 w/m. o C) 13 Solar still material Galvanized, 1mm

EXPERIMENTAL RESULTS AND DISCUSSION
The tests were take place during the interval from May to August month 2017, in Mosul City-Iraq. Figs. 5, 6, 7 shows weather conditions. Solar still (linear basin, single slope) of 0.4 m 2 area used in the experiment. The results have been recorded during tests, through 24 hour. The tests was done for different water levels (4cm and 5cm) with and without insulation.    Fig.8 to Fig.11, shows the variations of stills temperatures with weather conditions, and at local time. Its obvious that the solar radiation effect on temperature distribution, and the surface temperature be maximum with the presence of insulation that lead to produce more fresh water. Also it must be noted that the main effective temperatures are water temperature and glass temperature so the distillation process depends on them, and the productivity be maximum when the difference between water and glass temperatures be maximum because its represent the driven force.    Fig.12 and Fig.13, shows that the productivity of solar still without insulation be more than that of with insulation during the interval (12:30 a.m ‫ـــ‬ 8 a.m) at the two water levels, because of increase of water temperature quickly as the solar radiation strikes the still surface directly, on the other hand, insulation prevent this process in case of solar still with insulation. During the interval (8 a.m ‫ـــ‬ 5 p.m), sunshine pass through the glass and no or small heat loss in case of insulation, that lead to all heat absorbed used to evaporates the water and produce more fresh water. In the interval (5 p.m ‫ـــ‬ 12:30 a.m) insulation store energy and still evaporates the water more than the still without insulation.   Fig.14 and Fig.15, shows the percentage of water desalination during 24 hour interval, so its obvious that there are significant amount of water desalination during the interval (12 a.m-8 a.m), and this value be greater than at the presence of insulation in case of the two water levels because the effect of insulation on the heat storage, so insulation store the heat as possible with small heat loss, on the anther hand the ambient temperature reduced an night that lead to increase the temperature difference between water-glass temperatures and increase the productivity.

Effect of water level
From the Fig.16 and Fig.17, it has been noted that, when the water level be 4cm, the yield output in case of insulation and without insulation be more than that at 5cm level, this is logically result, because the small quantity of water absorb large amount of energy, and evaporates large water quantity.

Desalination Process at Night
At night (off Sunshine) there are no energy source to increase water temperature but depends on the heat storage only. In the present work the desalination process during (12 a.m-8 a.m) because of in summer season, sun rise at 5 a.m and the system absorbs the solar radiation and evaporate from sunrise until 8 a.m.

CONCLUSIONS
From the experimental study, it has been conclude that: 1. The yield output of the still increased by 20.785% and 19.864% in case of using thermal insulation at 4cm and 5cm respectively. 2. The yield output decrease by 15.134% as the water level increase from 4 to 5cm, with the presence of insulation. 3. The yield output decrease by 14.147% as the water level increase from 4 to 5cm, without insulation. 4. The desalination process before 8 a.m has significant amount, and cannot be ignored. With Insulation-4cm With Insulation-5cm