NOVEL DESIGN SOLAR STILL for Fulfilling Potable Water Requirements

Availability of potable water is one of the major problems in the contemporary world. Solar still is a simple device to convert available brackish or saline water into drinkable water by use of solar energy. Many researchers from all around the world have worked on solar still for the increment in distillate output. But the solar still with lower capital cost and minimum payback period are the prime requirements. Hence, a new design of novel solar still was developed and tested in climate conditions of Mehsana, Gujarat, India. Presently, the cost of solar still is approximately Rs 19,640 and gives average distillate output of about 25 litre. It has energy payback time of around 82 days. So, it could be the best option to fulfill potable water requirements of small families. Also, the water obtained from solar still also fits into the standards of pH and total dissolved solids (TDS) suggested by Government.

The Operating Principle of the Novel Solar Still

During day time solar radiation falls on solar still as well as on evacuated tubes. Lower basin of solar supplies cold water into the evacuated tubes, hence it will undergo hot water from the evacuated tubes and hot water would go to the lower basin. This process will continue for producing hot water in lower basin.

The water in the lower basin evaporates into water vapour and condenses in lower stacked tray to produce potable water. Latent heat of lower basin will evaporate water inside middle basin and water vapour of middle basin condenses on lower portion of upper stacked tray.

Some amount of latent heat will evaporate water inside upper basin, in which solar rays are also entrapped for producing steam and potable water. In all the basins, the raw water quantity remains the same at all times. Hence, here distillate output is obtained from all the three basins. Schematic diagram and experimental set-up are shown in Figure 1 and Picture 1.

Structure of Solar Still

The schematic diagram of the novel design of solar still is shown in Figure 1. The seawater in the solar still is heated by two kinds of heat sources. One is the heat pipe vacuum tube collector installed on the bottom with dimensions of ∅0.1 m × 1.88 m, and the other is the black titanium alloy coating on the upper surface of the top stacked tray which can absorb sunlight and heat the seawater. The inclinations of the vacuum tube collector and glass cover are all 23°, which is applied for low latitude zone. The stacked trays are made of stainless steel with thickness of 1 mm, and are folded into corrugated shape (isosceles right triangle) with right side length of 3 cm. The number of stacked trays is three for this solar still. The V-shape troughs downside the stacked trays are used to collect the freshwater condensed on the lower surface of the stacked trays. The surroundings of the solar still are insulated with polyurethane foams (7 cm thick) to reduce heat loss. The average height between the water surface and the lower surface of the stacked trays is 6 cm, and the condensation area is 1.7 m2. For the bottom stacked tray, the seawater storage capacity is about 100 kg, while for other stacked trays the capacity is about 30 kg. So, the total seawater capacity of the solar still is about 200 kg. The saline or brackish water first goes through this pipe and then goes into the top stacked tray.

Novel Design of the Solar Still

In any research work, novelty is a crucial part. The set-up discussed above has the following features with novel value:

ƒ The wastage of latent heat of condensation to atmosphere in traditional solar still is utilized to evaporate water in subsequent stages of a solar still contained corrugated tray for condensation purpose.

ƒ The produced potable water can quickly trickle because the flow distance of the condensed water on the condensation surface is very low and the tilt of the condensation surface is more.

ƒ The latent heat of condensation is released from the lower basin and utilized for increment in distillate output of remaining middle and upper basin for increment in distillate output.

ƒ The total distillate output (lower basin + middle basin+ top basin) is higher as compared to any conventional solar still.

Cost Analysis of the Solar Still

Economic Analysis

Article courtesy : Dr Hitesh Panchal, Assistant Professor, Department of Mechanical Engineering, Government Engineering College, Patan, Gujarat, India. Email: engineerhitesh2000@gmail.com.