Designing an Evaporation Chamber

(from Global Lab: Water)

The Need

Many very arid countries do not have enough drinking water to support their populations. Saudi Arabia expects that it will run out of water in the near future and plans to purchase water from Turkey. Many many countries are projected to have water shortfalls soon, and pollution in countries with enough water will cause their reserves to run out as well. But if these countries are near the ocean, they are close to a great deal of water! How can ocean water that is high in salinity become usable freshwater?

With a small investment, one can build a solar still that will purify saltwater using little or no external power. There is a lot of competition to see who can design the most efficient and effective solar still.

The science of a solar still is simple: when saltwater evaporates, it leaves the impurities and salt dissolved in the water behind. When the vapor condenses again it is pure and without salt. The idea is to evaporate lots of saltwater and capture the condensation to use as fresh drinking water.

Your Challenge:

Design and build a solar still out of inexpensive parts. Mix your own sea water (add 35 grams of salt to 1 liter of water) and test your design.

Measure how much freshwater your design produces each hour on a sunny day.

Figure out how much your design would cost to build if it were scaled up to produce enough water.

Things to Understand:

• To get the water to evaporate, it needs to be hot. The hotter it is, the faster it will evaporate.

• Water vapor condenses best on cool surfaces.

Things to Consider:

• One way to heat the sea water is to let a thin film of it flow over something black that is exposed to the sun.
• For efficiency, allow the air to flow between the evaporating saltwater and the condensing pure water.
• Evaporation and condensation are more efficient when protected from the wind and from other matter that might drop into the pure water.
• Be sure the freshwater falls into a collector so it flows out of the solar still and does not fall back into the salty water. Also make sure that the salty water cannot contaminate your freshwater.
• The evaporating sea water will leave salt deposits behind. Design a way to flush this back into the sea so the salt does not accumulate.
• Consider using wicks made of cloth to move the water rather than pumps. They cost less and will allow your still to work without electricity.

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