Solar-Powered Water Desalination
AbstractHow can seawater from the oceans be turned into fresh water that is suitable for people to drink? Through a process called solar desalination! In this science project, you will make a solar desalination apparatus using readily available materials, and a power source that is free. How much water can the device produce, and is it still salty at all? What factors affect how effectively saltwater is turned into fresh water?
This project is based on the following 2007 California State Science fair project, a winner of the Science Buddies Clever Scientist Award
- Kinsman, N., 2007. Catching Rays: Reflections on Portable Desalination Devices. Retrieved March 21, 2018.
Recommended Project Supplies
Build and test a solar-powered device for desalinating water and investigate how the color of the bottom of the device affects its efficiency.
Nicholas Kinsman is interested in inventing solar-powered devices to reduce our dependence on other energy sources. He is 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 seawater, using readily available materials and easy construction methods.
Typical seawater contains dissolved salts at concentrations between 32 and 37.5 parts per thousand. That means that if you started with one kilogram of seawater (which is approximately one liter of seawater) and then you allowed all of the water to evaporate, you would be left with between 32 and 37.5 grams of salts (also called "total dissolved solids").
With all of that salt, seawater is not suitable for drinking nor for watering most plants. The fluid circulating in your body (blood plasma) contains much less salt than seawater (on the order of 9 grams of total dissolved solids). If you were to drink seawater, your body would actually lose water, because the high salt concentration of the seawater 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 below. There are eight small bottles surrounding the large collection jug. Each of the small bottles is filled with seawater. 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 jug at the center. When the entire device is set out in the sunlight, the seawater in the small bottles heats up, which causes the water to evaporate and fill the small bottles with water vapor. The idea was that as the water vapor increased, it would condense in the straws and flow down into the collection jug. Unfortunately, the idea did not work. You can see in the picture that there is condensation on the inside of the top of the bottles, but there was very little condensation in the straws.
Figure 1. This is Nicholas's first try at a solar-powered desalination device. The idea was to add saltwater to the eight small bottles. The condensation was supposed to drip down from the straws into the large collection jug. 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 discourage him. He analyzed what was wrong with the design and set out to improve it. His second design, shown in Figure 2 below, still follows the same principles of using readily available materials and easy construction methods, but also includes some important improvements. One important change is that this time Nicholas's design uses a container with a larger surface area to hold the seawater.
Two plastic jugs are used as desalination devices. The first jug lays on its side with the top facing wall cut out. Plastic wrap covers the gap in the jugs wall and a quarter is placed on the plastic wrap to create a low spot where condensation can collect and drip down into a funnel. The funnel underneath the quarter is made from a plastic bottle cut in half and feeds into a straw that goes through the cap of the funnel. The straw from the funnel passes through the plastic jug walls and empties into a collection cup. The entire plastic just is covered in aluminum foil except for the plastic wrap that covers the gap made in the plastic jug wall. The second plastic jug is setup in the same way but is not wrapped in aluminum foil.
Figure 2. Here are two examples of Nicholas's second design for a solar-powered desalination device. The large jug, laying on its side, holds the seawater. The top side of the jug 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 jug, and then to a plastic cup where the condensate collects. The device on the left has an aluminum foil reflector covering the back side and bottom of the large jug while the device on the right has no reflector.
(Science Buddies photo of Nicholas Kinsman's display board at the fair.)
Why is it important that in the improved design a large jug is used to hold the saltwater? The jug, which is laid on one side, lets the saltwater cover a relatively large area. Because water molecules can only evaporate from the surface of water, a body of water with a large surface area will have a greater rate of evaporation than a body of water with a smaller surface area (assuming all other conditions are the same). The top side of the jug is cut out, using a utility knife, and covered tightly with plastic cling wrap. The cling wrap covering the large opening provides a large surface area on which condensation can form, which is another reason why using a larger container is an important change in this design. A quarter is used as a weight to make a low point at the center of the cling wrap. When the device is heated up in the sunlight, the condensation that forms on the cling wrap eventually flows down to this low point and drips into a "funnel." In Nicholas's design, the funnel is simply the cut-off top of a small water bottle, which has 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 jug, and then to a plastic cup (tightly covered with cling wrap using a rubber band to prevent evaporation).
For his science fair project, Nicholas tested the desalination devices based on his second, improved design with and without aluminum foil reflectors (you can see examples of each type in Figure 2 above). For each device, he made several measurements so that he could compare the performance, including the amount of condensate collected, or his condensate yield, and the conductivity of the saltwater and condensate. The yield measurements told him how efficient his devices were at heating the saltwater and producing desalinated water. The conductivity measurements (which can be taken using a handheld meter) told him how well the condensed water had been purified of dissolved salt because water that contains dissolved salt can conduct electricity, and the more salt that is dissolved in the water, the higher the conductivity of the water.
In this environmental engineering science project, you will build desalination devices similar to Nicholas's second design and see how the design can be improved even more. Specifically, you will investigate how the color of the bottom of the device affects its efficiency. You will compare a device with a white-colored bottom to a device with a black-colored bottom. Something that is light-colored reflects more light than something that is dark-colored, which absorbs a lot of the light that hits it. Light is a form of energy and energy can be transferred to nearby objects (such as a body of water) in the form of heat, in a process known as heat transfer. Which colored bottom do you think will result in a more efficient desalination device?
Terms and Concepts
- Solar-powered devices
- Water purity
- Water vapor
- Surface area
- Reflection of light
- Absorption of light
- Heat transfer
- Why does condensation form when water is heated up?
- What are some factors that affect the rate of evaporation?
- Do you think a desalination device with a black-colored bottom would be more efficient than one with a white-colored bottom? Why? Can you relate your answer to the concept of heat transfer?
Do further research by visiting the following websites, which give information about salts in seawater, desalination, and rate of evaporation:
- Swenson, H. (n.d.). Why Is the Ocean Salty? U.S. Geological Survey Publication. Retrieved June 27, 2007.
- The Columbia Electronic Encyclopedia. (2007). Water, Desalination of. Pearson Education, publishing as Fact Monster. Retrieved June 28, 2007.
- Meng, A. and Meng, H. (n.d.). Factors Affecting the Rate of Evaporation. Interactive Assessment Worksheets. Retrieved September 27, 2012.
This project is based on the following 2007 California State Science fair project, a winner of the Science Buddies Clever Scientist Award:
- Kinsman, N. (2007). Catching Rays: Reflections on Portable Desalination Devices. California State Science Fair. Retrieved March 21, 2018.
Recommended Project Supplies
- Solar-Powered Water Desalination Kit, available from our partner
Home Science Tools.
You will need these items from the kit:
- Clear plastic rectangular containers with pre-drilled holes (2)
- Plastic cups with pre-drilled holes (2)
- 25 mL graduated cylinder
- 800 mL tripour beaker
- Modeling clay, 1 stick
- 5 mL funnels (2)
- Flexible straws (2)
- Steel washers, 7/16 size (2)
- Rubber bands (2)
- Black construction paper, 8 1/2 inch by 11 inch (3)
- White construction paper, 8 1/2 inch by 11 inch (3)
- You will also need to gather these items, not included in the kit:
- Plastic cling wrap
- Sunny location
- Lab notebook
- Optional, if you are doing this project during winter: heat lamp or high-wattage incandescent (not CFL or LED) light bulb, so you can do the project inside. See FAQ for more information.
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Making the Desalination Devices
In this part of the science project, you will make two desalination devices.
- Look at Figure 3 to get an overview of what you are making. Fit the stem of a funnel inside the short end of a straw. The easiest way to do this is to push the straw with a constant steady force while also twisting a little bit. Push the straw as far onto the funnel stem as it will go. Securely tape the straw to the funnel. Repeat for the second straw and funnel.
- Push a straw-funnel assembly through the hole near the bottom of each container so that the funnels are on the inside of the containers. Adjust the straw-funnel assemblies so that the funnels face up. Put some modeling clay around the hole, on the outside of the container, to hold the funnel in place. Your setup should now look similar to Figure 3. Do not worry if the funnel will not stay in place. The next steps will solve that.
Figure 3. After putting the straw-funnel assembly in the hole in the container and securing it using some modeling clay, your setup should look similar to the one shown here. The top picture shows a close-up from the side and the bottom picture shows an overall view from above.
- Look at Figure 4 to get an overview of what you are making in the next couple of steps. Start by putting the long end of each straw through the hole in a plastic collection cup. Adjust the straw so that the funnel faces up. Put some modeling clay around each hole, on the outside of each collection cup, to keep each cup in place.
- Now you will need to do some tinkering to get everything positioned correctly:
- The straw should slope down slightly from the box to the cup. This will allow gravity to help the water you collect flow from the straw to the cup. If there is no slope, the water will collect in the straw rather than in the cup.
- If the straw is too long for the funnel to face up and the straw to slope down towards the collection cup, cut a little bit off of the long end of the straw and test the setup again. Keep cutting a little bit of the straw off and re-assembling until it is right.
- Do not worry if the collection cups do not sit completely flat.
- Cover the opening of one container with a single large piece of plastic cling wrap. To seal the container closed, pull the wrap tightly over the opening and tape it in place at the four corners of the container. Repeat for the second container.
Set a washer on the plastic cling wrap, right above the funnel. Do this for each desalination device. Adjust as needed so that the washer creates a low point in the cling wrap right above the funnel, but make sure it is not so low that the cling wrap touches the funnel. For an example of this, see Figure 4.
- If the plastic cling wrap is touching the funnel, not all of the condensation will go down into the funnel. To fix this, either lower the funnel (such as by cutting the straw) or raise the cling wrap (by taping it tighter).
- If the cling wrap is so tight that it does not form a low point where the washer is, un-tape it in places and re-tape it more loosely.
Figure 4. Place the washer on the cling wrap, right above the funnel. Make sure that the cling wrap is not so low that it is touching the funnel. The top left picture shows the entire setup at this point. The top right picture shows a close-up of the collection cup and washer. The bottom picture shows a close-up of just the washer and funnel.
After you are done adjusting your setup, cover each collection cup with plastic cling wrap and secure the plastic tightly with a rubber band, as shown in Figure 5. This prevents your desalinated water from evaporating.
- Make sure that there are no gaps or holes in the cling wrap.
Figure 5. Secure a piece of cling wrap on to the top of each collection cup using a rubber band, as shown here.
Cover the outside bottom of one desalination container with black construction paper and cover the other one with white construction paper. Tape the paper in place.
- Arrange the construction paper so that it goes up about 2 to 3 cm on the sides of each container.
- You may need to cut a small slit in the construction paper for the straw to get through.
Make up a single batch of saltwater for both desalination containers.
- Add 1 tablespoon of salt to the tripour beaker and fill it with tap water to the 500 mL mark. Mix with a spoon until the salt is dissolved. Each of the desalination containers will need 250 mL of saltwater.
For each desalination container, remove the washer, gently remove the tape on one corner, lift the cling wrap, and pour in the saltwater. Add enough so that it just barely covers the bottom of the container, approximately 250 mL per container.
- Be careful not to let any saltwater spill into the funnel or onto the construction paper.
- Put the cling wrap back in place, making sure it is taped on all four corners of each container. Put the washer back on top of the plastic wrap directly above the funnel.
- Your desalination devices should look similar to the ones in Figure 6 below. They are now ready for testing!
A plastic container filled with salt water is covered with plastic wrap and a metal washer is placed in the center to create a low spot over a funnel. The funnel inside the plastic container feeds into a straw that runs through the wall of the plastic container into a collection cup. The collection cup is also covered with plastic wrap. Modeling clay is used to secure the straw and seal any holes made from the straw passing through the wall of the plastic contain and the collection cup.
Figure 6. When your desalination devices are ready for testing, they should look similar to the one in the top picture (except yours should have black or white construction paper on the bottom). The diagram on the bottom shows what the desalination devices should look like during testing as condensation is collected. (The construction paper is not shown in the diagram.)
Testing the Desalination Devices
In this part of the science project, you will test the performance of the desalination devices.
- Carefully take the desalination devices outside to an area that will receive direct sunlight for at least four hours.
Prepare your desalination devices for testing and do a final check to make sure that everything is in place and ready.
- One device should have black paper on the outside bottom of the container, the other should have white paper.
- Both devices should contain 250 mL of salt water.
- Plastic cling wrap should completely seal the top of the devices. If it does not, water vapor may escape and your results will not be reliable.
- On each desalination device, a washer should be sitting on top of the plastic wrap directly above the funnel. This should cause the plastic wrap to sag slightly such that water that condenses on the plastic wrap rolls down the plastic, towards the washer, and drips into the funnel.
- Get rid of any large wrinkles that do not flow down to the washer. Wrinkles can prevent the condensation from smoothly rolling down to the collection point.
- Make sure that the funnel in each device is facing up, directly below the washer, and not so high that it is touching the cling wrap. If the funnel is touching the cling wrap, either lower the funnel or raise the cling wrap by taping it higher.
- Make sure that the modeling clay has sealed the holes to prevent evaporative losses. Add extra modeling clay if needed.
- Check that the straws slope down towards the collection cups. Even a mild slope is enough to work.
- In your lab notebook, record the time. Measure and record the temperature near the desalination devices. You can use this information later to determine how temperature affects the condensation yield.
Check on the desalination devices after about 30 minutes. You may see condensation starting to form small drops on the cling wrap right below the washer. However, it may take longer, depending on how sunny and warm it is.
- If you see condensation forming small drops, do you see it on both desalination devices, or only one of them? Record your observations in your lab notebook.
If needed, adjust the washer and funnel-straw assembly on each device to make sure that the drops fall into the funnel. Make sure to record any adjustments you make in your lab notebook.
- You can arrange the washer so that one edge of it is the lowest point on the cling wrap, and this edge is positioned over the funnel so that condensation drips into it, as shown in Figure 7 below.
Figure 7. As shown in this close-up picture, you can arrange the washer so that its edge is the lowest point on the cling wrap and is positioned over the funnel, allowing condensation to drop into it.
Continue checking on your desalination devices every 30 minutes to make sure that they are still in the sunlight and that the condensation drops are falling into the funnel.
- If the desalination devices are not in the sun, gently move them to a sunny location.
- Does it look like one desalination device is making more condensation drops than the other? Record your observations in your lab notebook.
- If it is warm enough, the modeling clay may melt a little. If it does, just make sure that the holes are still sealed by the modeling clay. Add more clay if needed.
- If it is windy, you may want to check on your desalination devices more frequently to ensure that everything is still in place and functioning properly.
Leave your desalination devices in the sunlight for at least four hours before stopping your experiment.
- In your lab notebook, record the time when you stop your testing. How long were your desalination devices in the sunlight?
- Again, measure and record the temperature near the desalination devices.
- Open the large cling wrap covering on each device and try to get any condensate that is still in the straw to go out and into the collection cup. You can do this by gently blowing into the straw.
To determine the condensate yield of each desalination device, carefully disconnect the collection cup, remove its cling wrap covering, and pour the collected condensate into the 25 mL graduated cylinder.
- What was the condensate yield of each device? Record your results in your lab notebook.
- To determine whether the collected condensate is still salty, taste a little bit from each device. Record your observations in your lab notebook.
Repeat this experiment at least two more times on different days for a total of three trials. This will help ensure that your results are consistent and reproducible.
- Between trials, carefully rinse out each desalination device with tap water and let them dry along with all of the other desalination device components.
- Use the same amount of saltwater in each device and trial.
- For each trial, perform the testing for the same length of time.
- Between trials, carefully rinse out each desalination device with tap water and let them dry along with all of the other desalination device components.
After you have tested both devices in three trials, make a bar graph of your condensate yield results.
- On the x-axis of the graph, list your desalination devices. You can average the results for each device for the three trials, or you can show all three trials separately.
- On the y-axis, put the condensate yield in milliliters.
Analyze your results.
- Did one color of desalination device consistently have a higher condensate yield than the other? If so, why do you think this is? What does this tell you about the features an effective solar desalination device should have?
- Was the collected condensate ever salty?
- If the temperature near the desalination devices varied a lot between your three trials, do you see a correlation between the temperature and the condensate yield?
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 condensate yield by the testing time to get an average collection rate (mL/hour). You will need to think about how many hours of sunlight there are in your area. Would it change with the season?
For troubleshooting tips, please read our FAQ: Solar-Powered Water Desalination.
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This project explores topics key to Clean Water and Sanitation: Ensure access to water and sanitation for all.
- In this science project you compared the performance of desalination devices with a black-colored bottom and a white-colored bottom. What do you think would happen if you used an aluminum foil reflector, as Nicholas Kinsman did (as discussed in the Background)? How do you think you could set up such a reflector so that it heats up the water in the device as much as possible? Hint: research parabolic reflectors.
- How does the temperature of saltwater affect the rate of evaporation? You can try this science project again but this time fill one desalination device with ice-cold water and the other with hot water. Is one device much more efficient than the other?
- Surface area is a factor that affects the rate of evaporation. Think of a way to modify one of your desalination devices so that the same volume of saltwater takes up only half of the surface area of the bottom of the jug or container, such as by attaching a plastic divider to the bottom. Test the two desalination devices again, using the same volume of saltwater in each. Does the change in surface area correlate with a change in condensate yield?
- Saltwater has a higher boiling point than freshwater. Does this mean that you would get a higher condensation yield using freshwater than you did using saltwater? To find out, you can try this science project again but this time use saltwater in one desalination device and freshwater in the other. To make sure you are collecting "pure" water, you can add some food coloring to the initial water in each device. Are the condensate yields very different between the two devices? If you try even saltier saltwater than was used in this science project, is there a greater difference?
- How does the collection rate change during the course of the day? To investigate this, 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.
- In what other ways do you think you could change your desalination device to improve its efficiency? Find out what factors affect the rate of evaporation and how other desalination devices are designed. Figure out how you can use this information to modify your device, or design a completely new device, to improve efficiency.
- You can build other useful devices that use solar power. For example, the Science Buddies project Now You're Cooking! Building a Simple Solar Oven 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?
Frequently Asked Questions (FAQ)
- Make sure there are no large wrinkles in the plastic cling wrap, as this can trap condensation. To get rid of a large wrinkle, try taping up a little more cling wrap in that area. Also make sure that the corners and sides of the jug or container are taped to the cling wrap. The cling wrap should allow condensation to roll smoothly down to the collection point, below the washer or quarter.
- If you are using funnels made from the tops of 500 mL plastic water bottles, make sure that condensate is not pooling and getting trapped in the funnel. Make sure the straw is not sticking into the funnel, but is flush with the cap. You may want to try small, pre-made funnels as an alternative.
- You can try filling the jug or container with less salt water. You should fill it with enough to just barely completely cover the bottom of the jug or container, but do not add any extra salt water. All of the water needs to be heated up enough for water to start evaporating, so if you use more water then more energy will be required to heat it up.
- Watch to make sure that the condensation is actually dripping directly into the funnel, and make adjustments to the position of the funnel and washer or quarter if it is not.
- Make sure that all holes are sealed using modeling clay. If it gets very warm, the modeling clay may melt, so make sure that there is enough modeling clay that even if a little melts it is still sealing the holes.
- This science project will work best when it is warm and done in direct sunlight, so try to avoid testing it when it is cool and/or overcast.
- If you are not using the Science Buddies kit, selecting the right jug or container is important. Use a jug or container with a large surface area, since water molecules can only evaporate from the surface of water. It is recommended to use a jug or container with at least 1500 cm² of flat surface area on the bottom. Make sure the jug or container has straight sides that do not lean inwards and has no overhangs, as these features can trap condensation.
- If you need to change where the condensation is collecting, you can move the washer or quarter and/or adjust the saran wrap (by taping or re-taping it). You may need to move the washer or quarter so that the condensation collects along one edge, and position this edge over the funnel, as shown in Figure 7 in the project idea.
- Without taking apart your setup, you can try adjusting the position and angle of the funnel by moving the part of straw that is between the jug or container and the collection cup, or by moving the collection cup.
- The collection cup does not need to sit completely flat to function, but if it is windy and the collection cup is being knocked over, you can tape it to the jug or container.
- If the funnel seems too heavy to stay upright, try using a long, thin piece of tape to tape the funnel (on the outside rim) to the inside top edge of the jug or container to keep it upright. If you do this, a little condensation will be lost on the tape, but it will be much better than having a funnel that does not work.
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