Key concepts
Physics
Energy
Density
Temperature
Greenhouse effect

Introduction
Have you ever had a sunburn? If so, you know how powerful the sun's heat can be! Did you know this heat can also be turned into electrical energy? You might 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?

Background
The sun is a very powerful source of energy. In fact, in less than 15 seconds it can provide as much energy to Earth as humans use in one day! Most of this energy reaches Earth in the form of light and heat. It's no wonder people have long considered how this energy can be harvested and put to good use! During the past two centuries scientists researched and developed technologies to convert the sun's energy directly into electrical energy. Today many different methods of solar power generation exist, such as solar cells, solar-thermal collectors and solar-concentration systems.

One of these systems, called the solar-updraft tower, is a solar-thermal collector technology. A solar-updraft tower can generate electricity from low-temperature solar heat. The science concepts behind this idea are based on the greenhouse effect and the chimney, or stack, effect. These power plants consist of a large collector structure and a very tall and slender tower. The collector structure resembles a gigantic greenhouse and consists of a massive, transparent canopy that is suspended six to 65 feet (two to 20 meters) off the ground. The tower is located in the middle of the canopy structure and has large air inlets at its base. Inside the tower are big wind turbines to produce electricity.

When the sun shines onto the collector structure, its heat gets trapped underneath as in a greenhouse, which warms the air and ground underneath the canopy structure. As hot air is less dense than cold air, it starts to rise, causing an airflow, also called thermal convection. The air has no other way to escape than through the tower. The rising air in the tower creates low pressure at the bottom of the tower, and more hot air from the collector is sucked inside the tower through its air inlets at its base. As a result solar radiation causes a continuous updraft in the tower. The wind turbines installed inside the tower convert the energy contained in the updraft into mechanical energy by moving its blades. In the next step this movement is converted into electrical energy by generators.

Although the technology seems pretty simple and can generate lots of energy, no full-scale units are in operation—yet. Several prototype models, however, have been built and studied. In this activity you will build your own very simple solar-updraft tower prototype—but without the collector structure. Can you use the energy created by the updraft to power a propeller? Get started to find out!

Materials

  • Black construction paper (about 20 by 20 inches)
  • Pencil
  • Scissors
  • Tape
  • Piece of modeling clay
  • Wooden skewer
  • Needle
  • Thermometer
  • Lamp with incandescent lightbulb or a heat lamp (An LED bulb is too cool to produce enough heat.)
  • Sunshine and a wind-protected area outside (optional)
  • Stopwatch (optional)
  • White construction paper (optional)


Preparation

  • Roll the black construction paper into a cone with a small opening at the top (two-inch diameter) and a larger opening at the bottom (four-inch diameter).
  • 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 to 15 inches high and able to stand by itself.
  • At the bottom of the cone cut three equally distanced two-inch by half-inch arches out. These will be your air inlets. Make sure the cone can still stand on the remaining rim.
  • Cut a three-inch-diameter propeller out of the construction paper. You can find propeller templates online. Bend the blades of the propellers at about a 45-degree angle downward.
  • With tape, carefully attach the needle to the top of the wooden skewer with the pointy end facing upward.
  • Make a ball out of modeling clay, and press it on your workspace.


Procedure

  • Look for a wind-protected space indoors to set up your solar updraft tower. It is important there is no external airflow happening during your test. Why would your workspace need to be wind-protected?
  • Place the black paper cone over the clay so that it is located in the center of the cone.
  • Stick the wooden skewer into the clay through the top opening of the tower so the skewer sticks out in the middle of the cone. The needle on top of the skewer should stick about one and a half to two inches out of the cone.
  • Take the propeller and push it onto the point of the needle. After you mount the propeller, it should be balanced and able to turn freely. You 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?
  • Measure the air temperature inside the tower and outside. What inside air temperature do you measure compared with the outside temperature?
  • Place the lamp next to the tower and direct it toward the base of the tower. Make sure you don’t leave the lamp unattended during the test. What do you think the lamp will do?
  • Switch on the lamp and observe the updraft tower for a while. What happens?
  • After about five minutes measure the air temperature inside and outside of the cone again. What temperatures do you measure this time?
  • Switch the lamp off and continue to observe your updraft tower for five to 10 minutes. What happens when you switch off the lamp?
  • After another five minutes, measure the temperature inside and outside the cone one more time. Did the temperatures change? If yes, how?
  • Extra: How fast can you make your propeller spin? Mark one of your propeller blades with a pencil and then count the number of turns in 15 seconds using a stopwatch. Multiply the counted turns by four and you get the rotation speed of your propeller in rpm (rotations per minute).
  • Extra: Repeat the test, but this time use white construction paper to build your solar updraft tower. Does the color of the paper make a difference? Why or why not?
  • Extra: Go outside and let the sun power your solar-updraft tower instead of the lamp. Make sure you find a wind-protected spot. Can the sun power your propeller, too?

Observations and results
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 likely moving the propeller. The air temperatures inside and outside the tower should have been pretty similar. This should have changed, however, 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 lightbulb. When the light shines onto the black construction paper it absorbs most of the light and starts to warm. The heat is trapped inside the cone, which increases the air temperature inside. Because 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 new air is warmed, and a continuous updraft is created that keeps the propeller spinning. The cycle is only interrupted when you switch off the lamp. Because there is no heat source, the air inside the cone slowly starts to cool, and after a while returns to the same temperature as the outside air. At that point the propeller should have stopped spinning because there was no longer an updraft. You might have noticed it took longer for the propeller to spin and the propeller didn’t spin as fast with the white construction paper. This is because white paper does not absorb the heat from the lamp as well as the black paper can.

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

More to explore
Simmering Science: Can Your House’s Color Reduce Your Summer Energy Bill?, from Scientific
American

Are You in Hot Water? Use the Sun’s Energy to Heat Your Own Water, from Science Buddies
Sunny Science: Build a Pizza Box Solar Oven, from Scientific American
Create a Sea Breeze, from Scientific American
Science Activity for All Ages!, from Science Buddies

This activity brought to you in partnership with Science Buddies

Science Buddies