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Sun 4 SchoolsSun4Schools Curriculum

The Power of Solar Energy
Lesson 4 ~ How Is Solar Energy Used to Heat Water?

Activity 1: Showing the Movement of Water as it Heats
Activity 2: Making a Simple Solar Water Heater


  • Students will understand and demonstrate how solar water-heating systems work.


Just as solar energy can be used to heat air in buildings, it also can be used to heat water for bathing and laundry. Most solar water-heating systems have two main parts: the solar collector and the storage tank. The solar collector heats the water, which then flows to the storage tank. The storage tank can be simply a modified water heater, but ideally it should be a larger, well-insulated tank. The water stays in the storage tank until it is needed for something, such as a shower or to run the dishwasher.

A common collector - called a flat-plate collector - is usually mounted on the roof. This collector is a rectangular box with a transparent cover that faces the sun. Small tubes run through the box, carrying the water or other fluid, such as antifreeze, to be heated. The tubes are mounted on a metal absorber plate, which is painted black to absorb the sun's heat. The back and sides of the box are insulated to hold in the heat. Heat builds up in the collector, and as the fluid passes through the tubes, it heats up.

Like solar-designed buildings, solar water-heating systems can be either active or passive. The most common systems are active, which means they use pumps to move the heated fluid from the collector and into the storage tank. While a solar water-heating system can work well, it can't heat water when the sun isn't shining. For that reason, homes also have a conventional backup system that uses fossil fuels.

Activity 1: Showing the Movement of Water as it Heats

(Source: Australian Academy of Science)

Hot water weighs less than cold water. This is because a substance occupies more volume, and is therefore less dense, when it is heated. In gases and liquids, a light substance will rise to the top, since hot water rises above cold water. Observe how this principle is used in solar water heaters.


  • Wide test tube (about 4 inches in diameter)
  • Glass tube about the same width as the test tube, or a second test tube with the bottom cut off
  • Two lengths of glass tubing, long enough to fit inside your glass test tubes, as shown in the graphic below
  • Two corks with two holes cut the size of the glass tubing in each
  • Cold water
  • Food coloring
  • Thermometer


  • Set up the apparatus as shown in the diagram to the right
  • Fill the bottom tube with colored water, and the upper tube with clear water. Note: don't fill the tubes quite full to allow for expansion as the water heats.
  • Place the apparatus in direct sunlight.
  • Record the temperature of the water in the lower tube every two minutes until there is no further evidence of the movement of water through the system.
  • Ask your students to try to describe what happened to bring about the changes in the appearance of the water. These changes occur because, as water heats, it expands, forcing the colored water in the lower tube up through the glass tubing in the upper tube.


Activity 2: Making a Simple Solar Water Heater

(Source: Australian Academy of Science)

Illustrate how solar water heaters take advantage of the fact that hot water rises.

Teacher's Notes: Ideally, this experiment is conducted by breaking your class into two groups, having each group build two model solar water heaters (four total). The experiment will take three class periods to complete. You can reduce the time to two days by eliminating the third-day experiment.


  • Four large card board tray, approximately 2 feet x 1 foot x 6 inches (made by cutting down a cardboard carton)
  • Four sections of plastic tubing or garden hose, approximately 20 feet long
  • Four 2-liter plastic bottles
  • Four cardboard cartons (large enough to hold the plastic bottle)
  • Four thermometer
  • Scissors with pointed ends
  • Clay or similar product to hold thermometer in place while sealing bottle opening
  • Sticky tape
  • Water
  • Black and white poster paints
  • Two sheets of clear plastic, larger than 2 feet x 1 foot
  • Insulating material such as sawdust, plastic foam, crumpled newspapers


  • Day 1: Each group constructs two model solar water heaters as shown in the diagram below
  • One group paints the bottom of both cardboard trays white. The other group paints the bottom of each tray black.
  • Day 2: Fill each 2-liter bottle and the plastic tubing with water. Note: don't fill the bottle quite full; the water will need a few inches at the top for expansion as it heats.
  • Each group records the initial temperature of the water in each bottle.
  • Place each unit in a sunny location for 20 minutes and then record the water temperature of each.
  • Each group then changes the tilt angle of one of their units by tilting the cardboard tray to a different angle than the other tray.
  • Leave the units in the sun for 20 minutes. Record the tilt angle (in degrees) and water temperature of each.
  • Day 3: Each group records the initial water temperature of both units. Then place the units in the sun for 20 minutes and record the temperature again.
  • Each group then covers one of its trays with the plastic. Record the temperatures after 20 minutes.



Lead a class discussion on how different factors affect water temperature. Your students might observe that darker surfaces collect more heat, that different tilt angles capture more energy, or similar things.

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