Experimental Investigation of Double Slope Solar Still for the Climatic Condition of Sultanpur

Raj Vardhan Prasad Patel, Anil Kumar # Master Scholar, Department of Mechanical Engineering KNIT Sultanpur, U.P. (India) Pin-228118 Tel: +91 9458581003 1 rvpatel.knit@gmail.com * Assistant Professor Department of Mechanical Engineering KNIT Sultanpur, U.P. (India) Pin-228118 Tel: +91 9453290647 2 anilap.knit@gmail.com Abstract— A single basin double slope solar still of 1 m basin area is fabricated from an acrylic sheet of 3.5 mm. The condensing glass covers of 3.5 mm thickness with 30° tilt angle are used. In the present work, an attempt has been made to investigate the effect of the various parameters on the productivity of solar still like water depth, wind velocity, solar radiation, etc. The solar still experimentally tested under the climatic condition of Sultanpur (26.26° North, 82.07° East) in the month of March and April. For the present study, the experiments are conducted for both orientation East-West and North-South of solar still. The hourly temperature has been recorded for water, basin liner, and glass surfaces. It is seen that the production rate increases with increase in wind velocity and cooling of glass covers. The production of 75 ml (from16:30 hrs. to 17:00 hrs.) found for the temperature difference of 9.2°C after cooling the covers. The yield is 20.46% higher when basin water depth is 2 cm as compared to 4 cm basin water depth from 8:00 hrs. to 17:00 hrs. The results indicated that the production of distilled water increases with the increase in wind velocity, cooling the glass covers and the decrease in water depth. Keyword Solar still, acrylic, solar desalination, East-West and North-South orientation.


II. EXPERIMENTAL A. Experimental Setup
The acrylic-based double slope passive solar still is designed and fabricated for the experimental investigation of the climatic condition of Sultanpur. The schematic diagram of the experimental setup is shown in Fig.1 and the real experimental setup is shown in Fig.2. The black acrylic is chosen for the basin material so that it can absorb maximum solar radiation. The single basin, double slope, solar still is made airtight and the area is taken as 1 m2 for a basin and the height of the basin is taken as 10 cm. The overall size of the inner basin of the experimental setup is 1m×1m×0.01m. Plywood of 12 mm thickness is used for the insulating the solar still so that there is a maximum utilization of solar radiation in the solar still. Therefore, the outer basin size is 1.31m ×1.31m× 0.0255m. A glass of 4mm thickness is used to cover the top of the basin and glasses are placed at the 30˚ angle with the horizontal. The V-shaped drainage made from aluminium is provided for the collection of distilled water from the solar still and the water is collected into a jar. The temperature sensors are used to measure the temperature at the different places of a solar still like basin liner temperature, basin water temperature, water vapour temperature, inner glass temperature, etc. The solar radiation data are taken from SRRA station which is installed at KNIT, Sultanpur and anemometer are used for taking the wind velocity data. The Pyranometer of SRRA station is shown in Fig.3. Building. Among all the reading some typical readings are taken for clear days. The readings are taken during the day as well as night-time. The solar still are placed both orientations for reading, i.e. E-W and N-S. The basin water depth also changes in experiments. The solar radiation data are taken from SRRA station Sultanpur which is installed at Knit, Sultanpur academic building where the experimental setup is placed. The basin water, ambient, glass cover, vapour and basin liner temperature are recorded hourly with the help of temperature sensors. There are seven temperature sensors are used in the experimental setup. The wind velocity is also recorded by an anemometer. Hourly and daily distillate output is measured with the help of measuring jar. C. Measuring Devices 1) Measurement of Temperature: The Digital Mini LCD Temperature Thermometer with Probe sensors are used to measure the temperatures at various locations of the still. There are 7 temperature sensors are used for taking the readings of temperature at the different location in the experimental setup. The two sensors are placed at the top of the glass covers for measuring the temperature of the outer surfaces of glass covers and two sensors are placed inside the casing on the glass for measuring the temperature of the inside surfaces of the glass. There is one sensor used to measure the temperature of water present in the basin and one sensor is used to measure the temperature of basin liner. For measuring the water vapour temperature, the one sensor is used. The range of this temperature sensor is from -50°C to 110°C. 2) Measurement of Wind Speed: The Generic GM816 LCD Digital Wind Speed, Temperature Measure Gauge Anemometer is used to measure the wind velocity. The wind speed plays a vital role in cooling of glass covers of solar still which affect the productivity of solar still. The anemometer which is used for measuring the 32 wind speed has a range of 0-30 m/s with an accuracy of ± 5%. The threshold value of this anemometer is 0.1 m/s. The velocity reading is also taken from SRRA station. pH (for operating temperature 0°C -50°C). A digital TDS Meter of Range 0-9,990ppm with Accuracy of ±2% has been used for measuring the TDS of basin water and distillate water.

III. THERMAL MODELING A. Heat Transfer in Solar still
The heat transfer in solar distillation system can be classified in terms of internal and external heat transfer modes. The different heat interactions in the solar distillation unit have been explained below.
1) Internal heat transfer: The internal heat transfer mode, i.e. the heat exchange from water surface to glass cover inside the solar still distillation unit is governed by radiation, convection and evaporation.  Convective heat transfer: The heat transfer is taking place across the air, which is inside the solar still. As our system is airtight, therefore, there is no external velocity provided to the inside air for causing heat transfer. The air is humid because of vapour evaporating from the water surface, the heat transfer is due to the buoyancy only i.e. free convection heat transfer occurs inside the still casing.
The rate of convective heat transfer ( cw q  ) from water surface to condensing glass cover is given by- The value of convective heat transfer coefficient is depending upon the following parameters- Operating temperature range of still and physical properties of the fluid at this operating temperature.  Condensing cover geometry.  Flow characteristics of the fluid.  Evaporative heat transfer: The evaporative heat transfer occurs between the water surface and the glass inner surface of the solar distillation unit.
The rate of evaporative heat transfer ( ew q  ) from water surface to glass cover surface is given by-  Radiative heat transfer coefficient: The rate of radiative heat transfer ( rw q  ) from water surface to glass cover for these infinite parallel surfaces is given by- The rate of radiative heat transfer is also given by- The ( rw h ) is the radiative heat transfer coefficient from water surface to the glass cover and it's given by- (by comparing above equations) Where eff  = Effective emissivity of water and glass surface  = Stefan-Boltzmann constant = 5.67 External heat transfer: The external heat transfer is primarily governed by conduction, convection and radiation process, which are independent of each other. These heat transfers occur outside the solar distiller, from the glass cover and the bottom and side insulation.
 Top loss coefficient: Due to the small thickness of glass cover the temperature in the glass may be assumed to be uniform. The external heat transfer radiation and convection losses from glass to ambient are expressed as- Comparing the above equations, we get- Where sky T = 6  a T And the rate of convective heat transfer from the glass surface to ambient is given by- On substituting the value of ( rg q  ) and ( cg q  ) in equation (9) we get- Bottom and side loss coefficient: Heat is also lost from the water in the basin to the ambient through the insulation subsequently by convection and radiation from the bottom or side surface of the basin. The bottom loss coefficient ( b U ) can be written as- Here SS A is the surface area in contact with water and S A is the area of the basin of the distiller. SS A is very small in comparison to S A , for small water depth.
The rate of heat loss per m 2 from the basin liner to ambient can be written as,

B. Dunkle's model
Dunkle has developed an equation for evaluation of the internal heat transfer coefficient which is quite popular and is given below - The convective heat transfer coefficient is given by And the evaporative heat transfer from water surface to the glass surface is given by- Similar equation has also been derived by Cooper, and is given by- The above equation can be rearranged as- Where w P and g P are partial saturation pressures and given by- IV. RESULTS AND DISCUSSIONS The temperature variation for 2 cm water depth for North-South as well as East-West orientation shown in Fig. 6. The basin temperature has been found higher for North-South orientation. The cumulative yield from the double slope, solar still is shown in Fig. 7. The yield is shown for 2 cm water depth (8 April) and 4 cm water depth (14 April) in the basin from 8:00 hrs. to 17:00 hrs. It is clear from Fig. that the yield is better for 2 cm as compared to 4 cm water depth in the basin. The production of distilled water is started earlier when depth is 2 cm because the basin water heating is faster than when depth is 4 cm. The Fig. 8 shows the temperature variation of the glass inner surfaces, basin water and vapour temperature for 2 cm water depth on 8 April when still is placed E-W orientation. The ambient temperature is also shown in the Fig. It can be observed in Fig. that the water temperature is more than the inner surface of east glass after 12:30 hrs. due to this, the production of water from the east side is better than the west side.
The Fig. 9 shows the temperature variation of the glass inner surfaces, basin water and vapour temperature for 4 cm water depth on 14 April when still is placed E-W orientation. The sun rays only incident on the east side, up to 11:30 hrs. not on the west side, therefore, the temperature of east glass i.e. Tgi (E) is greater than Tgi (W) and after that west side temperature increases when sun rays directly incident on west glass only. The rate of increase of Tgi (E) is decreased after 11:00 hrs. because of the temperature increases due to condensation of water on the glass surface. The Tgi (W) remains higher than Tgi (E) after 11:30 hrs. because there is heat addition by condensation (basin water is heated up to a temperature from starting of experiment time to 11:00 hrs. for evaporation) as well as direct sun radiation. It is observed that at 12:30 hrs. There is no temperature difference between water and glass inner surfaces to maintain the temperature difference there is a spray of water on both glasses, therefore at 13:30 hrs. there is a decrement in glass surface temperature and due to this, the production rate is high at this point.   Fig. 10 shows the cumulative water production from one side of double slope solar still. It can be noted that the production rate is increased from 12:30 hrs. to 14:00 hrs. and also from 16:00 hrs. to 17:00 hrs. because water sprays on the glass to maintain the good temperature difference between the glass surface and basin water. The spray starts at 12:40 hrs. for maintaining temperature difference and after maintaining a temperature difference of 11.3°C at 13:00 hrs. the condensation rate is good and there is a production of about 120 ml between 13:00 hrs. and 13:30 hrs. Between 15:30 hrs. to 16:00 hrs. the production of water is very less, about 40 ml because there is a very less temperature difference between the glass surface and basin water also there is no spray of water. After that, there is a continuous spray of water from 16:00 hrs. to 16:30 hrs. to maintain temperature difference, due to the spray of water the temperature difference is of 9.2°C at 14:30 hrs. then there is a production of 80 ml of water in next 30 minute. Though the temperature of the water is higher at 15 Distillate Water Fig. 10. Cumulative water production from one side of double slope, solar still and effect of water spray on production rate Fig. 11 shows the variation of the temperature of the upper surfaces of glass with the variation of solar radiation and wind velocity. It can be seen from the Fig. that when the wind velocity increases the upper surfaces temperature of glass decreases. Between 11:00 hrs. to 11:30 hrs. as the wind velocity increases the south glass temperature decreases though the solar radiation is increased. The glass temperature is also decreased from 12:30 hrs. to 13:00 hrs. because of wind velocity increases, but the temperature decreases slightly because solar radiation is maximum during this time of period. The glass temperature also decreases from 16:00 hrs. to 16:30 hrs. for a small increment of velocity because solar radiation is minimum during this time. Due to this the water and glass temperature increases and production rate of solar still unit increase.
Thus, the wind velocity and solar radiation are the major parameters for temperature variation of the glass surface. The temperature of the glass surface is more affected after afternoon period because solar radiation is minimum and it is less affected before afternoon period because solar radiation is maximum during this period. The Fig. 12 and 13 shows the Comparison of calculated and measured distillate output at 2 cm and 4 cm water depth respectively. (Dunkle's model is used for theoretical calculations and it is given in Appendix-A) For same water depth, the North-South orientation gives the maximum temperature for basin water. It is found to be 15.61% higher temperature of basin water for North-South orientation of still as compared to East-West orientation when water depth is 2 cm (from 8:00 hrs. to 17:00 hrs.). The East-West orientation production of solar still is found 51.54% higher than North-south orientation for 4 cm water depth. 2.
The production of solar still increases with the decrease in water mass (depth of water) in the basin. It is found that the production of distilled water is 20.46% higher for 2cm water depth as compared to 4 cm water depth from 8:00 hrs. to 17:00 hrs. 3.
The rate of production increases with an increase in solar radiation and also increase in wind velocity up to critical value.
4. The rate of production increases with proper water spray on glass surface because it maintains a good temperature difference between water and glass surface, due to this the rate of condensation increases. It has been found that there is a production of 40 ml for 3°C temperature difference (from 15:00 hrs. to 15:30 hrs.) and after spraying water there is a production of 75 ml when temperature difference is of 9.2°C ( from16:30 hrs. to 17:00 hrs.).
ACKNOWLEDGMENT This work was supported in part by a grant from the TEQIP II at KNIT Sultanpur.