Solar Distillation

        Distillation in general is a process of separating the component substances from a liquid mixture by selective evaporation and condensation. Distillation may result in essentially complete separation (nearly pure components), or it may be a partial separation that increases the concentration of selected components of the mixture. Solar distillation is simply the usage of solar energy for the process of distillation. Solar stills are in general used for solar distillation.

PRINCIPLE OF SOLAR DISTILLATION:

      The basic principle of solar water distillation is simple and effective. The sun's energy heats water to the point of evaporation. As the water evaporates, water vaporises, condensing on the glass surface for collection. This process removes impurities such as salts and heavy metals.

CONSTRUCTION:

      The construction is simple. It requires a transparent glass or polycarbonate sheet which acts as a white body and allows maximum sun’s rays to pass through it. In addition to this, the surface should also act as a green house, reflecting the short wave radiations and holding back the long wave radiations. The base of the still should be coated black or covered with EPDM or other materials with high absorptivity. This acts as a black body capturing and retaining maximum heat.

WORKING:

      Water or the liquid to be distilled should be poured over the black surface. The heat absorbed by the black body transfers it to the liquid by conduction. The liquid evaporates and the vapour is arrested by the top cover. It then condenses and the condensed water trickles down the inclined glass cover to an interior collection trough and out to a storage bottle. There are no moving parts in Solar still and only the sun's energy is required for operation. and slides over the surface of the top cover and the distillate is collected. 

Working of a solar still

PARAMETERS AFFECTING THE PERFORMANCE OF A SOLAR STILL:

      Wind velocity, ambient air temperature, water depth, gap distance, material of construction, humidity, slope angle, latitude and longitude of the location etc. play a major role in affecting the performance of a solar still.

DESIGN OF SOLAR STILLS:

      Three types of solar stills viz. sloping roof type, pitched roof type and trapezoidal type were designed to test their performance efficiencies and find out the best type.

Design calculations

            The design for the three types of stills proposed was finalized as per the following calculations:

Daily available solar radiation = 6 kWh/m²-day

            Efficiency of solar still = 30%

            Useful solar radiation = 6 * 0.30

                                                =1.8 kWh / m²-day

                                                =6480 kJ/m²-day

The capacity was arrived through the following calculation:

            Latent heat of the sample = 2437 kJ/kg

            Quantity of distilled water produced per m² per day

                                                       = 6480/2437

                                                       = 2.65 litres / m²-day

            Length of the square base plate = 0.508 m

            Available area exposed to solar insulation = L²

                                                                                = 0.2581m²

From the yield per m² area, litres of distilled water that can be produce = 0.2581*2.65

                                                                                                                   = 0.6916 litres.

CAD designs

CAD designs were made to fabricate the three types of solar stills viz., Roof type, reflector type and trapezoidal type. All the three types are designed with same dimensions (24” * 18”) in order to compare the working.  They differ only in the slope angle. The pitched roof type solar still has a slope of 60° while the sloping and trapezoidal type stills have 42° and 70° respectively. All dimensions are marked in inches in the CAD design.                               

         

                                   

Fabrication of solar still

All the three types of solar stills were fabricated using same materials to maintain uniformity. The stills differ only in the shapes in which the polycarbonate sheet is bent, to study the effect of various angles on the efficiency of the still.

Structural frame

            Stainless steel's resistance to corrosion and staining, low maintenance, relatively low cost and familiar luster make it an ideal base material. Due to this reason, this material is used for making the base plate and dome support structure. The legs of the frame are also made using stainless steel with a height of 16cm.

Collector

            The collector is made of polycarbonate sheet. The transmissivity of the material is around 98 - 99 %. Since this is flexible formation of collector is easy and its life is 10 years with very less maintenance required. There would be no contamination since it is UV coated on both the sides.

Absorber plate

            The absorber plate needs to be made of material that has high heat absorption and heat retention. Any black material that does not react with the urine to be distilled serves the purpose. Black mulching sheet of 500µ thickness was used to make the absorber plate. The absorber plate is made in the shape of a square trough to fit into the base plate.

EXPERIMENTAL PROCEDURE

The experiment is quite simple. The still has to be filled the liquid to be distilled through the inlet. The sample is given to a laboratory and tested for its constituents. Experiment should be started early in the morning so that there is enough time for the liquid to be distilled. Between twelve and three is the peak time for distillation to take place. Time taken to get the quantity of distillate is noted to test the efficiency. The sample is usually allowed to distil throughout the day. The obtained sample is then tested for its constituents. The initial sample and final sample are compared for its constituents. The weather parameters are monitored using an automatic weather station.

PERFORMANCE EVALUATION OF THE SOLAR STILLS

The analysis was done for a period of eight days. On comparison, the pitched roof type solar still yields more distillate and is highly efficient when compared to the sloping and trapezoidal type stills since it has the optimum slope angle. The pitched roof type solar still has more sloping surface and hence the efficiency is high. The lower efficiency of the sloping type still is that the parallel sides are at right angles. The major drawback of the trapezoidal type still is the flat surface on the top where the condensate directly falls by gravity. 

          

 EFFICIENCY CALCULATION

The still efficiency and the performance conversion ratio was calculated using

                                                Ƞ = (m_d*h_fg)/(A*G*τ*α*Δt)

m_d =  mass of distillate output

h_fg = latent heat of vaporization of cow’s urine

A= absorber area

G = Solar intensity

τ*α = transmittivity -absorptivity product of still cover

Δt = time interval

h_fg =2437 kJ /kg K

A= 0.2581 m²

G = 0.7534 kJ/m²

τ*α= 0.91*0.92

Δt = 8 hrs = 28800 s

The efficiency of pitched roof type is 32% while that of sloping roof and trapezoidal types are 24% and 12% respectively.

Hence proved that the pitched roof type solar still is the most efficient one