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Using solar energy to purify polluted or salt water

Using solar energy to purify polluted or salt water


Introduction: (Initial Observation)

With the increase of population in the world, shortage of potable water is becoming more noticeable. Just hundred years ago water was free and you couldn’t imagine that some day you will pay one dollar for a bottle of water. In the past few years some large bottling companies have tried to dominate the water resources of the word by purchasing natural springs and other water rights in many different countries.

Soon many people in the world can not afford to purchase fresh water or drinking water. When we run out of fresh water resources, we will need to purify water for our consumption.

Distillation is the most reliable and efficient method of water purifications. All other methods including de-ionization and filtration can only remove parts of impurities and often introduce new impurities to the water. Distillation is often performed by heating water to it’s boiling point. At that temperature water evaporates. Water vapors are then directed to condensers where liquid water will form again. Can we use solar energy to distill water? Heat energy from the sun can warm up water, but it can not bring it to the boiling point, however we have seen that water evaporates and wet objects dry even without heat or sunlight. Can we use the natural evaporation of water to purify polluted water? We have seen that hanging cloths under the sun will make it dry faster. Can we use the same method to evaporate and re-condense pure water?


If you have any questions, click on the help button at the top of this page to send me your questions. I may respond by email, but often I update this page with the information that you need.

Project Advisor

Information Gathering:

Find out about what you want to investigate. Read books, magazines or ask professionals who might know in order to learn about the effect or area of study. Keep track of where you got your information from.

Search the Internet for The Hydrologic Cycle to see how water is being recycled or purified in the nature. You may also find some pictures and illustrations that you can use for your report.

Do another search about water evaporation. You will need to know what factors affect water evaporation.

Following are some information that you may find.

To dispel a common belief, it is not necessary to boil water to distill it. Simply elevating its temperature, short of boiling, will adequately increase the evaporation rate. In fact, although vigorous boiling hastens the distillation process it also can force unwanted residue into the distillate, defeating purification. Furthermore, to boil water with sunlight requires more costly apparatus than is needed to distill it a little more slowly without boiling.

Many levels of purification can be achieved with this process, depending upon the intended application. Sterilized water for medical uses requires a different process than that used to make drinking water. Purification of water heavy in dissolved salts differs from purification of water that has been dirtied by other chemicals or suspended solids.

The present dollar cost of solar-distilled drinking water is several times that of water provided by most municipal utilities, but it costs less energy-wise. On the other hand, solar-distilled water is much less expensive than bottled water purchased in the store.

For people concerned about the quality of their municipally-supplied drinking water and unhappy with other methods of additional purification available to them, solar distillation of tap water or brackish groundwater can be a pleasant, energy-efficient option.

Solar distillation systems can be small or large. They are designed either to serve the needs of a single family, producing from ½ to 3 gallons of drinking water a day on the average, or to produce much greater amounts for an entire neighborhood or village. In some parts of the world the scarcity of fresh water is partially overcome by covering shallow salt water basins with glass in greenhouse-like structures. These solar energy distilling plants are relatively inexpensive, low-technology systems, especially useful where the need for small plants exists.

Solar distillation of potable water from saline (salty) water has been practiced for many years in tropical and sub-tropical regions where fresh water is scare. However, where fresh water is plentiful and energy rates are moderate, the most cost-effective method has been to pump and purify. Continue…

You can also visit Delphion.com website for other solar distillation designs registered as invention.

Question/ Purpose:

What do you want to find out? Write a statement that describes what you want to do. Use your observations and questions to write the statement.

The purpose of this investigation is to design and construct a water distillation apparatus that can work with solar energy. We will try to employ every possible factor that can contribute to the evaporation and condensation in our design. The effectiveness of one of such factors may be investigated as an experimental science project. Following are sample questions to choose from. Each of these questions may be used as the start point for a science project. In our examples bellow, we take the question 1 for our study.

1. How does the color of a solar distillation unit affect the production of distilled water? (This is the main Question of this project. Variables and hypothesis bellow are based on this question.)

2. How does the slope of the condenser glass affect the production of distilled water?

3. How does the size of a solar distillation unit affect the production of distilled water?

Identify Variables:

When you think you know what variables may be involved, think about ways to change one at a time. If you change more than one at a time, you will not know what variable is causing your observation. Sometimes variables are linked and work together to cause something. At first, try to choose variables that you think act independently of each other. Variables that can affect the rate of evaporation are color, surface area, heat, air currents and humidity. You may consider one of these variables as your design factor. In this example, color is considered as a variable for this study.

Independent variable (also known as manipulated variable) is the color of heat absorbing surface (inside the solar distillation unit). Possible values are white, green, blue, red and black. (You may skip some of these colors or you may add more colors)

Dependent variable (also known as responding variable) is the daily production of distilled water.

Controlled variables are sunny hours of the day.

Constants are the size, shape, structure and position of the solar distillatory.


Based on your gathered information, make an educated guess about what types of things affect the system you are working with. Identifying variables is necessary before you can make a hypothesis. This is a sample hypothesis:

My hypothesis is that a black color will heat up faster and produces more water vapors and more distilled water. This hypothesis is being tested in Experiment 1.

Another sample hypothesis (Related to surface area and surface material):

My hypothesis is that a black cotton fabric can act as a wick to suck the polluted water and distribute it in a wide area for quick evaporation. Vapors can condense on a slanted clear glass and slide down toward a storage tank.

Experiment Design:

Design an experiment to test each hypothesis. Make a step-by-step list of what you will do to answer each question. This list is called an experimental procedure. For an experiment to give answers you can trust, it must have a “control.” A control is an additional experimental trial or run. It is a separate experiment, done exactly like the others. The only difference is that no experimental variables are changed. A control is a neutral “reference point” for comparison that allows you to see what changing a variable does by comparing it to not changing anything. Dependable controls are sometimes very hard to develop. They can be the hardest part of a project. Without a control you cannot be sure that changing the variable causes your observations. A series of experiments that includes a control is called a “controlled experiment.”


Obtaining clean drinking water is a constant challenge in many countries. Often the only water available is rife with disease-causing bacteria, and must be disinfected to make it safe. Conventional methods for disinfecting drinking water, including boiling and adding chlorine compounds, can be time-consuming and expensive. In some regions, it is difficult to ensure a reliable supply of chlorine, which can also give the water an unpleasant taste. In many areas, there is little fuel available for boiling water.

In this experiment we intend to make a low-cost, practical means to provide safe drinking water in rural and urban areas where needed. The key to this method lies in the ability of direct sunlight to destroy bacteria. The treated water is expected to be suitable for drinking. Solar radiation is a form of renewable energy that is abundant and accessible in many areas in the world.

Sunlight with wavelengths of 315-400 nanometers (nm) on the ultraviolet (UV) range of the electromagnetic spectrum is most effective at destroying bacteria. Since colorless glass or plastic can transmit light in this near ultraviolet range, they are the best materials for disinfections. Visible light (400-750 nm) next in terms of efficiency, with the visible band of violet and blue light (400-490 nm) is the most useful within this range. As a result, violet, blue, and very light green-tinted glass follow colorless glass or plastic in order of suitability.

How do you purify water using solar energy?

Solar energy (Heat of sunlight) evaporates pure water. Impurities such as salt and sand do not evaporate.

Water vapors are then condensed on the clear cover and slide down to the section where you collect pure water.


  • Containers made from colorless or tinted glass, or transparent plastic
  • Adequate sunshine (optimum regions for solar radiation lie between latitudes 35°N and 35°S).

Potential users

The system can potentially be used in small communities, refugee camps, institutions, and in disaster situations where water supply is interrupted.

Design: Water Purification by Solar Distillation

Every solar water distiller has a tank of polluted water covered by a clear glass or plastic, slanted or curved such that the condensed water vapors can slide down into the distilled water storage. Any of the following designs can procedure a working model of a solar water distiller. For your experiments you may use any of the following ideas to construct your water distillator or you may come up with your own design.

Design Activity 1:

Use any wide container (round or rectangular) such as a plastic tank or aluminum pan. Fill it up with one inch water. Place a glass cup in the center and cover it with a sheet of clear plastic. Used adhesive tape to secure the clear plastic cover from all sides so it stays stretched and flat. Place a weight on the plastic above the cup so the plastic film will be bent toward the center of glass cup. Vapors from all over the tank will condense on the clear plastic and move down toward the glass where they drip. Some black fabric on the tank can absorb more solar energy and accelerate the evaporation.

A container about 7 to 10 inches deep and about 2 feet wide is a perfect choice for this experiment. A glass cup is used because it is heavier and does not float on top of polluted water. Clear plastic cover can be any flexible sheet of plastic such as those used in plastic bags. (Polyethylene or polypropylene). Painting the outside or inside of the container with black color can accelerate evaporation.

Design Activity 2:

In this design instead of a flexible sheet of clear plastic, a sheet of glass or clear hard plastic will be used. This clear cover and the tank itself are installed in an angle to force condensed water to move to one side where it will be collected.

The tank is divided in two pieces. One side (A) holds the polluted water. The other side (B) holds the distilled water.

Clear glass or hard plastic is sealing the tank preventing any water vapors to evade. Side A also has a lining of water absorbent fabric to extend the area of polluted water to the walls of the tank.

Each section of the tank can be connected to a pipe. One for entering polluted water and the other for the distilled water to exit. A trial model can be made of plastic or wood, however most plastics can give an unpleasant odor to the water. Actual working models should be made of glass and stainless steel. If you make your model from wood, use enough glue and sealing material to prevent any water leakage. Also paint the wood with black paint a few days before your test.

If you make this model from wood, make sure that the wall separating two sections is shorter than other walls so it will not touch the glass and allow some water vapors to pass. If you are making your model from hard plastic or glass, use silicon glue to join the parts.

Design Activity 3:

In this design instead of a flexible sheet of clear plastic, a sheet of glass or clear hard plastic will be used. This clear cover is installed in an angle to force condensed water to move to one side where it will be collected.

Brackish water is carefully placed inside solar vessel via an inlet near the base of the vessel.
As sunlight warms the black silicone bottom and heat is transferred to the water,
it evaporates and condenses on to the inside of the glass cover, which is tilted toward the fresh water drain.
approximately 8 square feet (of glass cover) will distill around 1 gallon of water per day, over five hours of full sunlight.

Use black Silicon to completely coat the inside bottom of distillation module.

You can find most of the parts in a hardware store. Professional solar vessels are generally made of glass over formed sheet metal. But the base can be made of anything that will hold up outdoors.
The most important elements of the design are the sealing of the base with black, high temperature silicone rubber;
(spread it on with a Bondo squeegie) and creating a good seal between the glass cover and the bottom of the box.

Details are optional:

Design and construction of this evaporation chamber does not require any specific procedure. All sizes and materials are quite optional as long as what you make serves the purpose. Obviously the list of materials varies depending on the design and materials that are available to you. When I made one of these, I made it with wood, nails and wood glue. I covered inside with a layer of tar to make it black and water resistant, but instead of tar you may use black paint. I also use a cut to size piece of Plexiglas to cover it.

Experiment 1:

Effect of color on the rate of production in a solar water purification system

Introduction: The color of the solar distillator may affect the amount of heat energy it absorbs from the sunlight and it may affect the amount of distilled water you produce each day. In this experiment you will make 5 identical solar distillation units with 5 different colors. You will then put them to test and record their daily production of distilled water. Since you are testing all different colors at the same time, you will not need a control experiment. In other words any possible environmental factor such as wind will affect all distillators, not some of them. (This experiment is recommended for students who need to do an experimental project. If you are just doing an engineering project, you will not need this experiment.)


  1. Use any of the recommended designs to make 5 water distillator units all with the same size and materials.
  2. Use silicon glue to completely seal the container to prevent any leak.
  3. Get 5 different colors (Both Latex and Oil based colors work fine) and paint the inside and outside of the units. Let them dry and repeat the paint 2 more times. This will provide a strong and continuous layer of paint on your device.
  4. Add a small amount of food coloring or a small amount of burned sugar to water to make enough polluted water for your experiments. The amount of water you need depends on the size of distillators you make.
  5. Set all distillators in a place where they get equal amounts of sunlight. It is best if you do this in the morning.
  6. Place equal amounts of polluted water in each distillator and cover the distillators with clear plastic or glass.
  7. The next day, early in the morning, remove the distilled water from the containers and record the amount of water distilled by each color distillator. Then refill the polluted water containers and make the system ready for the next production.
  8. Repeat this for seven days and record the results in a table like this:

Daily production of distilled/ purified water in distillators with different colors

Experiment Date

Distillator Color

White Red Blue Green Black

Make a graph:

Make a bar graph to visually present your results. Make one vertical bar for each color. The height of each bar will be the average daily production or weekly production of the distillator with that color.

Data table

(If you do this as an engineering project)

Engineering projects like this may have a data table to show the productivity of your device in a period of about 7 days. Measure and record the amount of distilled water in each day, as well as average temperature and day length for each day. You may also use your data table to draw a bar graph with one vertical bar for each day. The height of the bar will represent the amount of distilled water produced on that day.

This is a sample data table structure:

Day Average Temperature Day length (day hours) Production of distilled water

Materials and Equipment:

List of material can be extracted from the experiment design and it depends on your choices and the availability of materials in your area. Parents with tools and experience with sheet metal working may help their children in building distillators using aluminum, copper, or galvanized iron sheet metals. Some others may construct the entire distillator using Plexiglas. Wood and plastics are the most widely available materials for this project. It is also possible to make the distillator with cardboard, but it will not last long and the risk of failure is high. In either case after you complete your project, you must write a list of materials you have used in your report.

Results of Experiment (Observation):

Experiments are often done in series. A series of experiments can be done by changing one variable a different amount each time. A series of experiments is made up of separate experimental “runs.” During each run you make a measurement of how much the variable affected the system under study. For each run, a different amount of change in the variable is used. This produces a different amount of response in the system. You measure this response, or record data, in a table for this purpose. This is considered “raw data” since it has not been processed or interpreted yet. When raw data gets processed mathematically, for example, it becomes results.


If you do any calculations for this project, write your calculations in your report.

Summary of Results:

Summarize what happened. This can be in the form of a table of processed numerical data, or graphs. It could also be a written statement of what occurred during experiments.

It is from calculations using recorded data that tables and graphs are made. Studying tables and graphs, we can see trends that tell us how different variables cause our observations. Based on these trends, we can draw conclusions about the system under study. These conclusions help us confirm or deny our original hypothesis. Often, mathematical equations can be made from graphs. These equations allow us to predict how a change will affect the system without the need to do additional experiments. Advanced levels of experimental science rely heavily on graphical and mathematical analysis of data. At this level, science becomes even more interesting and powerful.


Using the trends in your experimental data and your experimental observations, try to answer your original questions. Is your hypothesis correct? Now is the time to pull together what happened, and assess the experiments you did.

Related Questions & Answers:

What you have learned may allow you to answer other questions. Many questions are related. Several new questions may have occurred to you while doing experiments. You may now be able to understand or verify things that you discovered when gathering information for the project. Questions lead to more questions, which lead to additional hypothesis that need to be tested.

Possible Errors:

If you did not observe anything different than what happened with your control, the variable you changed may not affect the system you are investigating. If you did not observe a consistent, reproducible trend in your series of experimental runs there may be experimental errors affecting your results. The first thing to check is how you are making your measurements. Is the measurement method questionable or unreliable? Maybe you are reading a scale incorrectly, or maybe the measuring instrument is working erratically.

If you determine that experimental errors are influencing your results, carefully rethink the design of your experiments. Review each step of the procedure to find sources of potential errors. If possible, have a scientist review the procedure with you. Sometimes the designer of an experiment can miss the obvious.


List of References



The following is a selected list of publications on the technologies and issues related to the use of solar energy to distill, desalinate, or pasteurize water. Unless otherwise specified, all reports are available from the National Technical Information Service (NTIS). Other publications may be obtained from the publisher or source as indicated, or possibly from a local library through its inter-library loan system. This list was revised in November 2001.

Books, Pamphlets, and Plans

How to Make a Solar Still (Plastic-Covered), A. Whillier and G. Ward, Brace Research Institute, 1973. Available from Brace Research Institute (see Source List below). 9 pp., $1.75, Do-It-Yourself Leaflet L-1.

Plans for a Glass and Concrete Solar Still, T. Lawand and R. Alward, Brace Research Institute, 1982. Available from Brace Research Institute (see Source List below). 9 pp., $4.75, Technical Report T58.

Simple Solar Still for the Production of Distilled Water, T. Lawand, Brace Research Institute, 1967. Available from Brace Research Institute (see Source List below). 6 pp., $2.00, Technical Report T17.

Small-Scale Desalination For Remote Areas, Ayoub, Brace Research Institute, 1994. Available from Brace Research Institute (see Source List below). 29 pp., $3.25, Technical Report T178.

Solar Distillation, M. Malik et al., Elsevior Publishing, 1982. 75 pp. Out of print.

Solar Distillation of Water, R. McCluney, Florida Solar Energy Center (FSEC), 1981. Available from Florida Solar Energy Center, 1679 Clearlake Road, Cocoa, FL 32922; Phone: (321) 638-1000; Email: info@fsec.ucf.edu; World Wide Web: www.fsec.ucf.edu. 4 pp., Free, Energy Note No. 3.