Heat and Mass Transfer and Thermo Physical Analysis of Water Distillation Using Passive Solar Still

The demand of fresh water is becoming an increasingly important issue across the world. To alleviate this problem, a single slope single basin solar still was designed and tested. Top glass cover 9 ̊ slope ensures a very good transmission of solar radiation inside the still. In this paper a review of factors that improve the performance of the still is presented. Energy balances are made for each element of the still; solar time, direction of beam of radiation, clear sky radiation, optical properties of the cover, convection outside the still, convection and evaporation inside are accounted. Theoretical analysis of the heat and mass transfer mechanisms inside the solar still has been developed to find out heat transfer coefficients such as internal and external heat transfer modes and thermo physical properties such as dynamic viscosity, density, thermal conductivity, latent heat of evaporation and saturated vapor pressure for passive solar distillation system. Heat transfer coefficients were estimated for the present system since the still efficiency depends on heat transfer modes. It is also suggested that the water temperature plays a vital role in the distillate output since it increases significantly with the rise of basin water temperature. It is also found that the rise in water temperature increases the evaporative heat transfer coefficient value significantly.


Introduction
In many areas of the world especially in developing countries, the shortage of drinking water for livelihood is an important problem. An immediate attention is required as the drinking water resources are being exploited and polluted rapidly by human beings [1]. The potable water is very scarce and the establishment of a human habitat in these areas strongly depends on how such water can be made available in arid regions. The importance of supplying potable water can hardly be over stressed [2]. Many techniques are available for purification of saline water among them solar still is simple and economical [3] [4]. Two major advantages favor the use of solar stills-clean and free energy and friendly to environment [5] [6]. Also the availability of clean water is a necessity for reducing the spread of diseases in these countries [7] [8]. Solar still is the widely used solar desalination device. Single slope solar still is the basic model of solar still and it is shown in Figure 1. The solar still at its lower part consists of basin which is insulated with insulating material to reduce the thermal losses to the surroundings.
An inlet is provided for the access of saline water. Condensing covers are located on the top of the still. An outlet is provided to collect the distilled water [9].

Parameters of Solar Still Influencing the Performance
The solar still productivity and efficiency depends on the various parameters like solar radiation intensity, temperature, basin water depth, glass cover material thickness, and its inclination, wind velocity and the heat capacity of the still [9].
The parameters such as solar radiation intensity, temperature and wind velocity are uncontrolled as they are metrological parameters. Other parameters such as collector area, aperture ratio, water depth, glass thickness and insulation are controlled since they are managed by humans. Omar et al. [10] revealed that the productivity and efficiency of the solar still improves with increase in intensity.
Also due to higher solar intensity the overall heat loss coefficient increases until it reaches the maximum. Kalidasa Murugavel et al. [11] suggested that the temperature difference between the water and glass is high, the productivity increases.  Ahmed et al. [12] conducted experimental analysis on double slope solar still based on temperature and also verified its numerical results. The productivity is enhanced by increasing the temperature of water usingwater heater in the base of solar still [13] [14]. Velmurugan et al. [15] inferred that an increase in collector area results in more productivity. Bilal et al. [16] concluded that the productivity of the solar still decreases linearly with increase in depth of water at the rate of 0.4 lit./cm. and found that the optimum tilt angle for more production as 35˚ during the month of May in Jordan, where the latitude angle is 31˚. Singh and Tiwari [17] and Velmurugan et al. [18] found that the annual yield of the solar still is maximum when the condensing glass coverinclination is equal to the latitude of the place. Solar still with glass cover plate with 3 mm thickness gives 15.5% more production than the cover of 6 mm glass plate thickness [19]. erly oriented, and directly exposed to the solar radiation. The measured parameters and quantities were: the solar radiation intensity (I), the glass temperature (T g ), the basin water temperature (T w ), and the ambient air temperature (T a ).

Experimental Procedure
The measurements of the solar intensity, wind velocity, ambient, glass inside, glass outside, vapor, water, basin temperature and distillate yield are taken hourly to study the effect of each parameter on the still productivity. The water level in the solar basin is maintained to a level of 1 cm for constant basin water mass of 2.7 kg. Readings are taken at an interval of one hour till the sun sets in the evening. The solar intensity, wind velocity, wet bulb temperature, dry bulb temperature, basin plate temperature, basin water temperature, glass temperature, vapor temperature and distilled water productivity are measured every 1 hour. The same measurement process is repeated by varying the water level inside the solar still to 3 cm and 5 cm.

Heat Transfer
The energy balance equations at various portions of the solar still [20] [21] are described as follows: The rate of energy gained by the glass and the rate of energy gained from the water surface to the glass by radiation, convection and evaporation are equal to the rate of energy lost to air.

Outside Surface of Glass Cover
Rate of energy received from inner glass cover by conduction = Rate of energy lost to the ambient by convection and radiation Equation (1) for the energy balance equation of the outside surface glass cover becomes, By simplifying Equation (2), the glass outside temperature go T is written as,

Inner Surface of Glass Cover
Rate of energy absorbed from solar radiation + Rate of energy received from water mass by convection, evaporation and radiation = Rate of energy transferred to glass outside surface Equation (3)-(4) for the energy balance equation of the inside surface of the glass becomes, Smart Grid and Renewable Energy By substituting Equation (3) into Equation (5), the temperature of the inside surface of the glass is written as, By rearranging Equation (6), By simplifying the above equation, we get,

Basin Liner
Rate of energy absorbed from solar radiation + Rate of energy lost to water mass by convection = Rate of energy lost to the ambient by conduction and convec- After simplifying Equation (10), the basin temperature of the solar still is,

Water Mass
Rate of energy absorbed from solar radiation + Rate of energy received from basin liner by convection + Rate of energy received from external devices = Rate of energy stored + Rate of energy lost to the glass inner surface by convection, evaporation and radiation The energy balance equation of the water mass in the solar still becomes, Smart Grid and Renewable Energy By substituting values of T gi and T b from Equations (8) and (11) into Equation , , In the case of a passive solar still, additional thermal energy becomes zero, 0 u Q = .
The above equations can be expressed in simplified form as follows: where, The following assumptions have been made to find the approximate analytical solution: 1) The time interval ∆t (0 < t < ∆t) is small; 2) The value of "a" is constant during the time interval ∆t; 3) The function "ƒ(t)" is constant for the time interval between 0 and t; By using the boundary condition, , the solution for above first order differential equation is The hourly yield is given as follows, The total daily yield is given as follows,

Overall Thermal Efficiency of the Olar Still
The instantaneous thermal efficiency is calculated as Smart Grid and Renewable Energy where, m ew = mass of distillate collected in kg/h; A = Area of the basin in m 2 ; I(t) = Solar radiation with respect to time W/m 2 ; and L = Latent heat of vaporization in kJ/kg.

Results and Discussion
The experimental study on single basin single slope passive solar still with black coated copper sheet is conducted at Nazareth, Tamil Nadu, India (Latitude: 9˚N; Longitude: 78˚E and an altitude of 9 m above sea level). Experiments are carried out from 7:00 h to 17:00 h during clear sunny days. During the experiments, hourly variation of solar intensity, wind velocity, ambient, glass inside, glass outside, vapor, water, basin temperature and distillate yield are recorded. The average solar radiation is 755 W/m 2 . It has been observed that solar radiation varies parabolically with time, with its maximum value at 13.00 h. In the morning, the temperature of water is low; therefore it needs high energy to change its phase from saturated liquid to saturated vapor phase. The ambient temperature reached the minimum value in the early morning and it is gradually increased after sun rise. The results show that temperature and required heat are inversely proportional. In the early afternoon the temperature of water reaches the maximum so it needs less heat to vaporize and vice versa in the late afternoon. The average wind velocity on test day was 3 m/sec. The wind velocity is an important parameter which affects the glass outside temperature of the solar still due to which condensation at higher rate is attained and ultimately it is expected to give higher yield.
The hourly variation of measured parameters such as ambient temperature, basin, water temperature, vapour temperature, glass inside temperature and glass outside temperature are shown in Figure 3. The maximum water temperature of 77˚C and glass inside temperature of 70˚C was observed at 14:00 h.
The hourly variation of yield is shown in Figure 4. Copper has higher thermal conductivity and coating of black paint absorbs all the incident radiation so that the basin water temperature is increased and hence the increased in the evaporation rate. It was found that the higher distilled water yield was obtained with higher basin water temperature. It revealed that there is proportionality increase in distilled water production with respect to the basin water temperature. Figure 5 shows the yield of solar stills in summer and winter for the PSS-Cu, B. It is observed that the maximum yields of 0.19 kg for the PSS-Cu, B was obtained at 13:00 h during the summer. This was due to the higher water temperatures in the basin of the solar still which increases the evaporation rate and in turn increases yield. It also shows that maximum yield of 0.13 kg for the PSS-Cu, B at 12:00-13:00 h during the winter. Daily climatic conditions have resulted in slight variations in water evaporation rates under low radiation conditions. By comparing the operation of the solar stills in the summer and winter, lower yield was observed for the solar still during the winter as a result of lower radiation and ambient temperatures. However, the percentage increase in yield was as high as it was during the summer. This can be attributed to less thermal losses under low temperature operating conditions resulting in an increase in yield. Figure 6 shows the comparison of convective heat transfer coefficient in summer and winter. The convective heat transfer coefficient is not varied drastically and it is dependent on wind velocity. The maximum convective heat transfer coefficients of 2.91 W/m 2 K and 2.41 W/m 2 K for the PSS-Cu, B were obtained for the summer and winter respectively. The convective heat transfer coefficient is PSS-Cu, B is lower due to the vacuum present inside the solar still.
The temperature difference between (ΔT) evaporative surface (water) and condensing surface (glass inside surface), which plays significant role in raising the yield and in evaporative heat transfer coefficient as well.      Figure 7 shows the comparison of evaporative heat transfer coefficient in summer and winter. The evaporative heat transfer coefficient increases with time and reaches a maximum at 14:00 h during the summer and 15:00 h during the winter prior to decreasing with time. The maximum value of the evaporative heat transfer coefficient measured during the summer and the winter was 75.13 W/m 2 K and 42.52 W/m 2 K respectively. The evaporative heat transfer coefficient is much higher than the convective and radiative heat transfer coefficients. The evaporative heat transfer coefficient is temperature dependent and is higher in the summer due to higher water temperature in the basin and lower in the winter. This is due to the higher thermal conductivity of copper sheet. Figure 8 shows the comparison of radiative heat transfer coefficient in summer and winter. The radiative heat transfer coefficient did not vary much compared to the evaporative heat transfer coefficient and it was independent of temperature under normal operating conditions (<80˚C). The maximum radiative heat transfer coefficients of 7.96 W/m 2 K and 6.91 W/m 2 K were measured during the summer and winter respectively. Further it is also noticed that the convective and radiative heat transfer coefficients remain independent of the operating temperature. Figure 9 shows the comparison of total heat transfer coefficient insummer and winter. The maximum total heat transfer coefficients were 110.62 W/m 2 K and 65.43 W/m 2 K in the summer and winter respectively, which can be attributed to the higher operating water temperature in the basin. The total heat transfer coefficient was much lower. The maximum values of total heat transfer coefficient 85.01 W/m 2 K and 52.10 W/m 2 K in the summer and winter respectively. Figure 10 shows the hourly variation of the convective, radiative, evaporative and total heat transfer coefficients. It also shows that the evaporative and total heat transfer coefficients were much higher than the convective and radiative heat transfer coefficients. The evaporative heat transfer coefficient increased with time until 14:00 h and then its value decreased with time. The maximum values of the total and evaporative heat transfer coefficients were 58.6 W/m 2 K and 50.4 W/m 2 K respectively. The higher value of the total and evaporative heat transfer coefficients are due to the increase in the basin water temperature. The convective heat transfer coefficient did not vary much and mainly depended on the wind velocity.
The maximum thermal efficiencies achieved by passive solar still with black coated copper solar still is 40% as shown in Figure 11 and 22% for the summer and winter respectively. The yield of the still was higher when compared to the still made up of galvanised iron sheet in both the summer and winter.

Conclusion
The performance of single basin passive solar still is analysed on normal sunny days. The radiative heat transfer (Q r ), convective heat transfer (Q c ) and evaporative heat transfer (Q e ) for internal and external heat transfer rates inside the still are predicted. The instantaneous efficiency and overall efficiency are calculated for all the studies. Heat transfer coefficients predicted under internal and external heat transfer modes for still performance. The thermal conductivity and dynamic viscosity increase with respect to time and possess almost the same trend.
The variation of density of water is found for the still under these experiments and it is observed as 1.094133 to 1.1025812 kg•m −3 . It is observed that the density decreases with respect to increase in water temperature and it starts to increase with the decrease in water temperature. The amount of daily distillate yield collected is 1.09 kg/day. The instantaneous efficiency is increased according to the time.