What it shows
In coastal and desert regions with salty groundwater, solar stills give drinking-quality distilled water without fuel. The project builds a basin-type still, measures the daily yield of distilled water from salty water and compares cover angles and basin colours.
The science
Sunlight passes through the cover and heats the black water basin. Water evaporates, leaving salt behind, and the vapour condenses on the cooler cover and runs down into the channel. A black basin absorbs more heat, and a cover angle near the latitude lets sunlight enter with less reflection and lets droplets slide off. Thin water layers heat faster than deep ones.
Materials
Cost: under Rs 1,500. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.
- Three identical wooden or plastic trays 50 x 40 cm with a collecting channel
- Window glass or clear polythene sheets cut at angles of 15, 30 and 45 degrees
- Black paint (a water-safe, non-toxic one) and a second tray left unpainted
- Salt water (2 spoons of salt per litre) and a measuring cylinder
- Silicone sealant
- A thermometer and a pyranometer app or sunlight timing record
- Collection bottles
Safety
For this project: Handle glass with gloves and have an adult cut it. Keep away from the hot water and steam. Never drink the collected water unless its safety is tested by a lab, because paint or sealant can contaminate it.
Never use mains electricity: power models with cells, batteries or a low-voltage adapter. Sharp and hot tools, flames and lab chemicals need an adult or your teacher beside you. Keep magnets, button cells and small parts away from young children, and wash your hands after making.
All the safety rulesStep by step
11 steps. Do them in order, and test each part before you go on to the next. Write what you notice in a notebook as you go.
Calculate the expected yield from the solar radiation, basin area and latent heat of vaporisation to set a target.
Paint the inside of the trays black and let them dry and air for a few days, and keep one tray unpainted for comparison.
Fit the collecting channel along the lower edge of the cover and make the still airtight with sealant.
Fill each basin with 1 cm depth of salt water and record the mass or volume.
Place the stills outdoors facing south with the three cover angles and record the start time.
Record the water, cover and air temperatures every hour from 9 am to 5 pm.
Measure the distilled water collected every hour and at the end of the day.
Test the conductivity of the product with a simple conductivity tester or TDS meter and compare it with the feed water.
Repeat for five sunny days, then repeat with 3 cm depth of water in one still.
Calculate the daily yield per square metre and the efficiency as energy used for evaporation divided by the sunlight received.
Write the report with graphs of yield and temperature, the best design and an estimate for a family of five.
What you should see
A black basin still with a cover around 20 to 30 degrees gives the largest yield, typically 1 to 3 litres per square metre on a sunny day. The TDS of the product is far lower than the salty feed.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
1Why is the basin black?
Black surfaces absorb most of the sunlight and convert it into heat.
2Why does the salt stay behind?
Only water evaporates, and the dissolved salts do not.
3Why does a deeper water layer give lower yield?
More mass must be heated, so the water takes longer to reach its evaporation temperature.
4What limits the efficiency of the still?
Heat loss from the cover and sides, and the energy lost when vapour condenses and releases heat that is not re-used.
Take it further
Add a mirror reflector on one side or a second condensing surface and measure the gain in daily yield.
Change one thing, measure what happens, and add a table or chart of your readings: that is what turns a model into your own investigation.
Free ebooks for this project
Science projects
Investigatory Projects for Class 11 and 12
Topics, method and report structure for physics, chemistry and biology