Solar-Powered Water Desalination


Objective

The goal of this project is to build and test a solar-powered device for desalinating water.

Introduction

Nicholas Kinsman is interested in inventing solar-powered devices to reduce our dependence on other energy sources. He's also a winner of a Science Buddies Clever Scientist award for his 2007 California State Science Fair project (Kinsman, 2007). Nicholas set out to build a simple, inexpensive device to desalinate sea water, using readily available materials and easy construction methods.

Typical sea water contains dissolved salts at concentrations between 32 and and 37.5 parts per thousand. That means that if you started with one kilogram of sea water and then you allowed all of the water to evaporate, you'd be left with between 32 and 37.5 grams of salts (also called "total dissolved solids").

With all of that salt, sea water is not suitable for drinking nor for watering most plants. The fluid circulating in your body (blood plasma), contains much less salt than sea water (on the order of 9 grams of total dissolved solids). If you were to drink sea water, your body would actually lose water, because the high salt concentration of the sea water causes an osmotic pressure gradient which drives water out of your cells. Desalination is the process of removing the dissolved salts from water, making it pure enough for drinking or irrigation.

Nicholas's first design for a desalination device is shown in Figure 1. There are eight small bottles surrounding the large collection bottle. Each of the small bottles is filled with sea water. The small bottles have holes in their caps. One end of a flexible straw is inserted into the hole, and the other connects to the large collection bottle at the center. The idea is that the sea water in the small bottles heat up in the sun, the water vapor then condenses in the straws and flows down into the collection bottle. Unfortunately, the idea did not work. You can see in the picture that there is condensation on the inside of the top of the bottle, but there was very little condensation in the straws.

desalination device, first try
Figure 1. This is Nicholas's first try at a solar-powered desalination device. The idea was to add salt water to the eight small bottles. The condensation was supposed to drip down from the straws into the large collection bottle. Unfortunately, this design did not work. (Photo from Nicholas Kinsman's display board at the fair.)

Like any good inventor, Nicholas did not let an initial setback make him discouraged. He analyzed what was wrong with the design and set out to improve it. His second design (see Figure 2) still follows the same principles of using readily available materials and easy construction methods. This time, Nicholas has increased the surface area for collecting condensed water vapor and improved the efficiency of the device for collecting the condensate.

desalination device, first try
Figure 2. Here are two examples of Nicholas's second design for a solar-powered desalination device. The large bottle, laying on it's side, holds the sea water. The top side of the bottle has been cut out with a utility knife. Plastic cling wrap seals the top side, and a quarter is used as a weight to make a low point in the center. Beneath that low point, Nicholas placed a collector, made from the top of a small water bottle with a flexible straw inserted into a hole in the cap. The other end of the straw passes through the side of the large bottle, and then to a plastic cup where the condensate collects. The device on the right has no reflector, the one on the left has an aluminum foil reflector covering the back side and bottom of the large bottle. (Science Buddies photo of Nicholas Kinsman's display board at the fair.)

In the improved design a large bottle is used to hold the salt water. The bottle is laid on one side, and the top side is cut out, using a utility knife. Then the top is covered tightly with plastic cling wrap. The cling wrap provides a large surface area on which condensation can form. A quarter is used as a weight to make a low point at the center of the cling wrap. The drops of condensation will eventually flow down to this point and drip into the collector below. The collector is simply the cut-off top of a small water bottle, with a flexible straw inserted into a hole cut in the cap. The other end of the straw passes through a small hole in the large bottle, and then to a plastic cup (tightly covered with cling wrap to prevent evaporation).

For his science fair project, Nicholas tested the desalination devices with and without aluminum foil reflectors (you can see examples of each type in Figure 2). He made three devices of each type, so that he could test them all under identical weather conditions. For each device, he made several measurements so that he could compare the performance, including:

  1. Amount of condensate collected
  2. Temperature of the salt water
  3. Temperature of the collected condensate
  4. Conductivity of the salt water
  5. Conductivity of the collected condensate

The temperature and volume measurements told him how efficient his devices were at heating the salt water and producing desalinated water. The conductivity measurements told him how well the condensed water had been purified of dissolved salt.

When salts dissolve in water, they dissociate (split apart) into ions. Ions are atoms or molecules with a net charge. The charge can be positive or negative. An example is sodium chloride (NaCl), ordinary table salt. In water, sodium chloride dissociates into positively charged sodium ions (Na+) and negatively charged chloride ions (Cl−). Water that contains dissolved salt can conduct electricity. More salt in the water means more ions, and more ions means that it is easier for the electricity to flow. In other words, the more salt that is dissolved in the water, the higher the conductivity of the water. An easy way to measure conductivity is with a handheld meter that you dip into the water (see the Materials & Equipment section, below).

You can repeat Nicholas's experiment yourself (as described below), or you can try to improve his design even further. The Variations section below has some ideas to get your inventive imagination started.

Terms, Concepts and Questions to Start Background Research

To do this project, you should do research that enables you to understand the following terms and concepts:

  • Ions
  • Conductivity
  • Total Dissolved Solids
  • Parts per thousand (ppt)
  • Desalination
  • Evaporation
  • Condensation
  • Solar oven

Bibliography

Materials and Equipment

To do this experiment you will need the following materials and equipment:

  • Sea water
    • Alternatively, you can make your own salt water by adding 30 g of salt to each liter of water
    • If you don't have a scale for weighing out salt, one cup of salt is approximately 292 g (GourmetSleuth.com, 2006)
  • 6 large (1 gallon or 4 L size) water jugs
  • 6 water bottles (1 pint or 500 mL size) water bottles
  • Plastic cling wrap
  • Aluminum foil
  • 6 Flexible plastic straws
  • 6 Plastic cups
  • 6 Rubber bands
  • Modeling clay (small amount)
  • 6 quarters (as weights for plastic wrap
  • Metric measuring cup (or kitchen scale)
  • Conductivity meter
    • There are many conductivity meters on the market.
    • A relatively inexpensive and easy-to-use model is the Extech EC400 (also called an "Exstik® II.")
    • The EC400 measures conductivity, total dissolved solids (TDS)
    • You can find this from several different online retailers (do a web search for "Extech EC400"). Prices are in the $80–$90 range (as of June, 2007, not including shipping)
  • Utility knife with a fresh blade
  • Optional: hand drill and drill bit (for making holes in caps)

Experimental Procedure

  1. Do your background research so that you are knowledgeable about the terms, concepts, and questions, above.
  2. Build six identical desalination devices, using the illustrations and descriptions of the experimental apparatus from the Introduction as a guide. Here are some tips:
    1. Work carefully with the utility knife. Using a new, sharp blade will make the job easier.
    2. Be careful with your fingers around the cut edges of the bottle: they will be sharp!
    3. The plastic cling wrap covering the cut-out top of the large bottle needs to seal tightly. Otherwise water vapor will be able to escape, instead of condensing on the plastic.
    4. Remember to cover your collection cups tightly or the condensate will evaporate.
    5. To prevent evaporative losses, you can use modeling clay to seal the around the straws where they pass through holes.
    6. Add aluminum foil reflectors to three of the devices, and leave three without reflectors (see Figure 2, above).
    7. Cut a hole near the top of the side of each device for access for the conductivity meter. Cover the hole with a tightly balled-up piece of plastic wrap when you are not making measurements, to prevent evaporation.
  3. Test the performance of the devices.
    1. Set them up at the same time, in a location where they will all receive direct sunlight for the entire duration of the experiment.
    2. Use the same amount of salt water in each of the devices.
    3. Check the initial conductivity and temperature of the salt water in each device.
    4. Check the conductivity and temperature of the salt water at occasional intervals throughout the experiment.
    5. Measure the volume of the collected condensate at the end of the experiment by pouring it into a Pyrex® measuring cup or graduated cylinder.
    6. Check the conductivity of the collected condensate at the end of the experiment.
  4. How does the conductivity of the condensate compare to the conductivity of the starting salt water? To normal tap water?
  5. What was the average amount of condensate collected for each device?
  6. Did the reflector improve the performance of the devices in terms of amount produced?
  7. Taking 3 liters as the minimum required amount of drinking water per person per day (NAS, 2004), how many devices would you need to produce enough water for your survival needs? You can divide the volume of condensate by the total time of the experiment to get an average collection rate (mL/hour).

Variations

  • In this experiment you compared the performance of desalination devices with and without an aluminum foil reflector. What do you think would happen if you used a black wrapper (e.g., aluminum foil painted black) on the bottom side of the desalination device?
  • How does the collection rate change during the course of the day? For this experiment, it would be a good idea to have your collection container marked with graduated volumes. That way you can measure collection volumes easily without disturbing the collection system.
  • Solar ovens are easy to build and can be very efficient for heating water when well-designed. The Science Buddies project Now You're Cooking! shows how to build a solar oven. Can you adapt a solar oven to make a solar-powered desalination device? Is it more or less efficient than the plastic bottle desalination device from this project?

Credits

Andrew Olson, Ph.D., Science Buddies

Sources

This project is based on the following 2007 California State Science fair project, a winner of the Science Buddies Clever Scientist Award


Last edit date: 2007-10-12 15:00:00


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