How to make a solar thermal motor step by step

This tutorial describes how to build a simple solar thermal engine, made from inexpensive and commonly available materials.

This device works by utilizing the ability of certain polymers (in this case black plastic bags) to contract when exposed to heat and relax to their original size when cooled.

Normally this shrinkage occurs in all directions within the material. However, the material can be stretched causing its polymer strands to line up and direct such shrinkage.

The solar thermal engine works by using strips of stretched black bagged plastic to continuously move a flywheel, pushing outward from the center, while rotating on an axis. The belts are heated in sunlight with a compound between the drum and the flywheel, which is also pushed in the sunlight. As its plastic bands rotate backwards, they cool in the shadow of the drum, causing them to loosen.

This causes the steering wheel which is in the sunlight to continuously drift, causing it to spin constantly.

Tools and materials.

How to make solar thermal engine materials

The following list shows the materials you will need:

  • 1 black plastic bag.
  • 2 polystyrene cups.
  • 1 wooden dowel 12 ”long.
  • 1 polystyrene tray .
  • 2 sewing pins.
  • 1 plastic container lid for yogurt (or similar), approximately 4 ”in diameter.
  • 1 adhesive tape.
  • 2 aluminum cans.


  • A single-edged X-ACTO knife.
  • The compass.
  • Conical tip marker.
  • Rule.
  • The scissors.
  • Glue gun.


Step 1: Cut the bag.

Stretch the bag out on your work surface. Cut into strips 2 to 3 inches wide, lengthwise. They don’t have to be perfect cuts or completely straight, just try not to have jagged edges. If you have sharp scissors, you can make a first cut, then slide them to the end of the bag.

You might need 10 or 15 of these cuts, but it all depends on how carefully you stretch the material in the next step.

Cut each bag strip into 8 to 12 inch sections.

Step 2: stretch the bags.

Firmly grasp the end of one of the strips using force with your thumb as shown in the photo.

Slowly stretch this piece of plastic and it will start to create a “neck” in the middle of the strip. This “neck” will create more length in the strap as you stretch it. Look at the before and after photo.

Repeat the process with all the strips.

It may sound very simple, but it takes practice. The bands break easily before you’re done removing them, so if that doesn’t work for you at first, be patient and keep trying.

Cut the stretched strips into sections a little more than twice the length of the styrofoam cups, leaving a small extra piece, in case there is any room for error.

NOTE: Do not expose stretched plastic to direct sunlight, as it may shrink. If you are working in the sun, try leaving one of the strips exposed to see how it shrinks.

Step 3: Create cylinder 1.

Measure the diameter of one of the styrofoam cups and use the compass to draw a regular circle on the board. You can draw this circle directly on the glass, but it will be difficult to locate the center, which you will need to do.

Cut out the marked circle and drill a hole the size of the wooden dowel in the middle.

Use the glue gun to melt the hole area.

Step 4: Create cylinder 2.

Make a hole in the middle of one of the styrofoam cups and glue the disc you cut from the tray down. Put the dowel in the two holes and glue them in place. You can also make a smaller 1-inch disc from the same material to place it at the bottom of the glass, but that’s not completely necessary.

Put glue around the dowel, placing it on the back of the cup. This will serve as a pivot for the second cup, which must be left free to move on its axis. You can also cut several 1/2 ” diameter polystyrene discs and place them on the dowel.

Step 5: Glue the strips.

Drill a hole in the bottom of the second Styrofoam cup.

Apply silicone to the inside of the rim of the glass and place the end of one of the stretched pieces of the bag on top. You may need to cool the hot glue for a few seconds to keep the plastic strip from melting, especially if you are using a high heat gun.

Repeat the process by gluing the strips around the edge of the glass. Leave about 1/8 “to ¼” of space between each of the strips.

Step 6: Glue the glasses.

Slide the glass with its respective strips glued to the edge so that it fits the dowel. It must be able to swing freely on the pivot. Use the masking tape to center the open side of the glass with the dowel.

Start by gluing the strips of the polystyrene disc to the other glass. Heat the strips as you glue them, but don’t stretch them too much as you will slide the glass out of its center.

Continue with the rest of the strips.

Step 7: The steering wheel.

Cut a 1 or 2 inch hole in the center of the plastic cap.

Remove the tape you placed on the glass to center it, and glue the cover to its edge, centering it on the peg. This will serve as a leaflet.

Glue a sewing pin to each end of the dowel, making sure they are in a straight line.

Step 8: The last step.

Make a notch on the rim of both cans (note: foam cups were used in the photo instead of cans, as I didn’t have any cans, but it doesn’t work as well).

Replace the axle pins in the notches of the cans.

While the new solar motor might be ready to go, it would be a good idea to do a balance test first. Give it a ride. If it’s okay with you, lift the weight on one side, add some weight (pins, small snaps, etc.) on the opposite side until it is well balanced.

Note: Add this weight to the polystyrene disc, not the flywheel.

You’re ready!

Put the whole structure under the sun: if everything is built correctly, the rotation will start without problems. The more balanced it is, the faster it spins.