Did you know that solar heat can also be turned into electrical energy? You may have heard about solar cells, but what about a solar updraft tower? This very simple structure uses the sun to heat up air, which then powers a turbine within a large tower. In this activity, you will build one of these towers yourself. Do you think you can make a propeller spin with just hot air?

  • Black construction paper (about 20 x 20 inches)
  • Smaller piece of construction paper (any color)
  • Pencil
  • Scissors
  • Tape
  • Piece of modeling clay
  • Wooden skewer
  • Needle
  • Thermometer
  • Lamp with incandescent light bulb or heat lamp
  • Paper
  • Optional: sunshine, and a wind protected area outside
  • Optional: timer
  • Optional: white construction paper
    A lamp, black construction paper, scissors, pencil, thermometer, wood skewer, tape, modeling clay and strips of paper


Prep Work

  1. Roll the black construction paper into a cone with a smaller opening at the top (2-inch diameter) and a larger opening at the bottom (4-inch diameter).
  2. Use tape to hold the cone in place. Then, cut the top and bottom ends off to make them straight. The cone should be about 10-15 inches high and able to stand up by itself.
    A narrow cone made of black construction paper
  3. At the bottom of the cone, cut three equally distanced 2-inch by 0.5-inch arches out. These will be your air inlets. Make sure, the cone can still stand on the remaining rim.
    Rectangles cut from the base of a narrow paper cone
  4. Cut an about 3-inch diameter propeller out of the construction paper. Here is a propeller template you can download. Bend the blades of the propellers about a 45-degree angle downwards.
    A propeller with four blades is cut from a piece of paper
  5. With tape, attach the needle to the top of the wooden skewer with the pointy end facing upwards.
    A needle is taped to the end of a wooden skewer
  6. Make a ball out of modeling clay and place it on your workspace.


  1. Look for a wind-protected space indoors to set up your solar updraft tower. It is important that there is no external airflow happening during your experiment.
  2. Place the black paper cone over the clay so that it is located in the center of the cone.
  3. Stick the wooden skewer into the clay through the top opening of the tower, so that the skewer sticks out in the middle of the cone. The needle on top of the skewer should stick about 1.5-2 inches out of the cone as shown in the picture below.
  4. Take the propeller and place it on top of the needle as shown below. It should be balanced and able to turn freely. It might need some practice to make sure the propeller is resting on the needle correctly.
    What happens to the propeller resting on top of the needle? Does it move?
    A homemade updraft tower made of construction paper and a wooden skewer
  5. Measure the air temperature inside the tower and outside. Record the measured air temperatures on a piece of paper.
  6. Place the lamp next to the tower and direct it towards the base of the tower as shown in the picture below. Make sure you don’t leave the lamp unattended during the experiment.
    A small lamp next to a homemade updraft tower
  7. Switch the lamp on and observe the updraft tower for about two to five minutes.
    What happens when you switch on the lamp?
  8. After about five minutes, measure the air temperature inside and outside of the cone again. Record both temperatures.
    What do you notice about the temperatures? Did they change? If yes, how? Can you explain why?
  9. Switch the lamp off and continue to observe your updraft tower for five to 10 minutes.
    What happens when you switch off the lamp? How are your observations different than before?
  10. After another five minutes, measure the temperature inside and outside the cone one more time.
    How are the temperatures different now?


Carefully remove the needle from the skewer. You can reuse all the parts of your construction.

What Happened?

Did you get your propeller to spin? In the beginning, the propeller shouldn’t have moved. If it did, an external airflow from a door or wind was moving the propeller. The air temperatures inside and outside the tower should have been pretty similar. However, this should have changed when you switched on the lamp. The incandescent lamp emits light and heat, just like the Sun. You will notice this when holding your hand close to the light bulb. When the light shines onto the black construction paper, it absorbs most of the light and starts to heat up. The heat is trapped inside the cone, which increases the air temperature inside. As warm air is less dense than cold air, it starts to rise up the paper cone. This updraft makes the propeller on top of the cone spin, which you should have observed after a while.

The rising air reduces the air pressure inside the cone, which is why fresh air is sucked into the paper cone through the air inlets that you cut out at the bottom. This air is warmed up again, and a continuous updraft is created that keeps the propeller spinning. The cycle is only interrupted when you switch off the lamp. As there is no heat source, the air inside the cone slowly starts to cool down and after a while returns to the same temperature as the outside air. At that point, the propeller should have stopped spinning as there is no longer an updraft. You might have noticed that it took longer for the propeller to spin and that the propeller didn’t spin as fast with the white construction paper. This is because white paper cannot absorb the heat from the lamp as well as the black paper.