Solar water heater
Black-painted bottles or pipe in the Sun heat water. Measure the temperature as it rises.
The big idea
Dark things soak up sunlight and turn it into heat. A black bottle full of water in the Sun is the simplest solar water heater there is.
You will need
Tick things off as you gather them.
Step by step
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Step 1: Paint the bottles
Paint most of the bottles black and let the paint dry. Keep one clear bottle, unpainted, as your comparison.
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Step 2: Fill with water
Fill every bottle to the same level with tap water. Record the starting temperature of each one.
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Step 3: Lay out the board
Cover the board with foil and place the bottles side by side on top of it. The foil reflects extra sunlight onto them.
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Step 4: Put it in the Sun
Tilt the board towards the Sun and leave it in direct light. Place a layer of thermocol underneath if you have it, to keep heat from escaping downwards.
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Step 5: Measure every 15 minutes
Take each bottle's temperature every 15 minutes for 2 hours. Use the same thermometer each time.
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Step 6: Plot a graph
Plot time along the bottom and temperature up the side, one line per bottle. Which line rises fastest?
What to look for
- The black bottles heat up faster than the clear one.
- The temperature climbs quickly at first, then more slowly.
- On a cloudy day, the rise is much smaller.
An example of the pattern you should see, not measured data. Your numbers will differ, and that is fine.
Why it works
Black paint absorbs almost all the sunlight that hits it and turns it into heat. The heat passes into the water. Foil sends extra light onto the bottles, and thermocol slows the heat from leaking away. Real rooftop solar water heaters use the same recipe: dark pipes, a reflective back and good insulation.
Try this next
- Add a second layer of plastic over the bottles to trap more heat.
- Try a coil of black garden pipe in place of bottles.
- Work out how many bottles it would take to heat a bucket of bath water.
Stay safe
Water in a black bottle can get hot enough to burn. Test it with a thermometer before you touch it.
Link to the grid
Using the Sun for heat saves electricity that would otherwise run a geyser. Every unit saved at home is a unit the grid does not have to make.
Learn how the grid worksMore sun power experiments
Mini solar panel circuit
A small solar panel lights an LED or spins a toy motor. Test it in sun, in shade and at different angles.
Solar cooker
A cardboard box, foil and plastic wrap trap the Sun's heat. Melt chocolate, warm water or cook rice.
Shadow-tracking solar map
Track shadows on the school roof through the day to find the best place for solar panels.