Solar Thermal Motor




This instructable describes the construction of a simple solar thermal motor, made from commonly available, low cost materials.
This device operates by harnessing the ability of certain polymers (in the case black plastic bags) to shrink when exposed to heat, and relax back to their original length 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 directionalizing the shrinkage.

The solar thermal motor operates by using bands of stretched black plastic bag to continually pull a flywheel off-center as it rotates on an axle. The strips are heated by sunlight on one side of a drum/flywheel assembly, pulling the flywheel toward the sun-side. As the strips rotate to the back, they cool in the shadow of the drum and relax. This makes the flywheel continuously off-center on the sun side, causing it to rotate.

Step 1: Tools and Materials

The following is a list of the materials you will need:

1x Black plastic trash bag

2x Styrofoam cups
1x 1/8" wooden dowel 12" long
1x Styrofoam freezer tray
2x Sewing pins (Forgot these in picture)
1x Plastic yogurt (or similar) lid approx. 4" dia.
1x Scotch tape
2x Tin cans

Tools used:

-Single edged razor or xacto knife
-Drawing compass
-Felt tip marker
-Ruler (forgot this in picture)
-Glue gun

Step 2: Cutting Bag

Unfold a bag and lay it out on your work surface. Cut the bag into 2-3 inch wide strips, the full length of the bag. You don't have to be perfect about making the cuts perfectly straight, just try not to make any jagged edges. If you have sharp sizers, you can start a cut, then slide them up the length of the bag.
You will probably need about 10-15 bag length strips, but it depends on how careful you are at stretching the material in the next step.
Cut each bag length strip into 8-12 inch sections.

Step 3: Stretching the Bags

Grip one of the strips between your fingers and the base of your thumb, ans shown in the picture.
Slowly, stretch the strip. It will begin to "neck out" at one location on the strip - the "neck" will then propagate up the length of the strip as you pull on it. See before and after picture.
Repeat the process with all your strips.
This might sound all very easy, but it takes practice. The strips frequently break before your done pulling them, so if it doesn't work at first, be patient and keep trying.
Cut the stretched strips into sections slightly longer than twice the length of the Styrofoam cups, leaving some extra length for error.

NOTE: Don't expose the stretched strips to direct sunlight, as it will per-shrink them. However, if the sun is out while you are working on this step, try laying laying one of the strips in the sun - watch it shrink!

Step 4: Makeing the Cylinder 1

Measure the diameter of one of the Styrofoam cups, and use your compass to draw an equal-sized circle on the freezer tray. You can trace the circle directly off the cup, but it will make it difficult to locate the center of the circle, which you will need to do.
Cut out the circle, and make a hole the size of your dowel at it's center.
I used my glue gun to melt the hole.

Step 5: Making the Cylinder 2

Make a hole in the center of one of the Styrofoam cups, and glue the Styrofoam disk to its bottom. place your dowel through both holes and glue it in place. You can also make a small (1") foam disk, and place it at the bottom of the cup, but this is not really necessary.
Make a smooth blob of glue around the dowel at the back of the cup. this will serve as a pivot for the second cup, which must be left free to wobble on its axis. You can also cut several 1/2" diameter Styrofoam disks and place them over the dowel instead.

Step 6: Gluing the Strips

Make a hole in the bottom of the second Styrofoam cup.
Apply glue to the inside lip of the cup and place the end of one of the stretched strips on top. You may need to let the hot glue cool for a few seconds to prevent the strip from melting, esp. if you are using a high temp. gun.
Repeat the process, gluing strips around the lip of the cup. Leave a 1/8" to 1/4" gap between strips.

Step 7: Attaching Cups

Slide the cup with the glued strips onto the dowel. It should be able to wobble freely on the pivot. Use scotch tape to center the open end of the cup to the dowel.
Begin gluing the strips to the Styrofoam disk on the other cup. Make the strips taunt when you glue them, but don't pull to hard or you will cause the wobble cup to be off center.
Continue with all the strips.

Step 8: Flywheel

Cut a 1-2 inch hole in the center of the plastic lid.
Remove the tape that you put on the wobble cup to center it and glue the lid to its rim, centering it on the dowel. This will act as a flywheel.
Stick a sewing pin into each end of the dowel, being careful to make them go in straight.

Step 9: Final Step!

Make a notch in the rim of the two tin cans (note: In the picture I used foam cups instead of cans, because I didn't have any cans, but this does not work as well.)
Rest the axle pins in the notches of the cans.

Although your solar motor might be ready to use now, it is a good idea to balance test it first. Give it a spin. If it looks like it if heave on one side, add weight (pins, small brads, etc) to the opposite side until it is balanced.
Note: add the weight to the Styrofoam Back disc, Not to flywheel.

You are done!
Put the whole assembly in the sun - if everything is built right, if will begin the rotate. The more balanced it is, the faster it will spin.



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    38 Discussions


    4 years ago on Introduction

    I posted a video of this type of motor, a couple different sizes, the largest is made with two 5 gallon buckets.


    Reply 4 years ago

    It was in an issue of Popular Science magazine '79-'83. Called a Solar Engine.


    Reply 4 years ago

    It was in an issue of Popular Science magazine '79-'83. Called a Solar Engine.


    5 years ago on Introduction

    if you extended the dowels past the ends of the cans you could stick a gear on it and connect it to a large mator and produce useful electricity from this motor


    5 years ago on Introduction

    Book marking,Im gonna take a stab at it maybe as a fan for a wood heater.


    6 years ago on Introduction

    Can you upload the video to YouTube or Vimeo?

    There's a DVIX restriction preventing it playing on my tablet.


    7 years ago on Introduction

    I made this for my science fair in 1980. Good learning tool! Keep on making stuff!


    8 years ago on Step 9

    I'd love to see a video of this in action.
    Would it be able to move air through a passive solar air heater? It could complement some of the other solar air projects that rely on thermosiphon effects to circulate air through them.

    1 reply

    Reply 7 years ago on Introduction

    I saw this just the other day on the PopSci archive, February 1980 (as referenced earlier), before I saw it here. It works...but after a few minutes quit working. After a good deal of tweeking it seems that the stretched plastic only goes through so many re-heatings until there is no stretch left. Re-stretching does not help. You have to make new strips. Or at least that has been my experience. Maybe too much sunlight or heat. As to power, it has very VERY little, just barely able to make the thing turn. That's why it has to be a lightweight as possible and in balance with friction from the axle points to a minimum, thus the styrofoam building material and straight pins. Maybe others have had better luck at making this thing operate for more than a few minutes. Fun and educational, but no practical application to power anything.


    11 years ago on Step 9

    This is a very clever concept. How did you discover the stretched black plastic shrinks in heat property?

    2 replies

    Reply 11 years ago on Introduction

    Most plastics tend to shrink in the presence of heat. Black is used here because it will absorb more heat than others. White or clear will either reflect or pass the heat rays more than absorbing them.


    10 years ago on Step 9

    This may sound like an unintelligent question but does the motor always rotate the same direction?

    1 reply

    11 years ago on Introduction

    how could you scale this up? could you use two bicycle wheels separated by a wood beam? there has to be a way to make power from this.

    1 reply

    Reply 11 years ago on Introduction

    Definetly. I built a 2ft diameter version - with intentions of submitting it for the 'go green' contest, but did not get it done in time. I used 2 plastic laundry baskits from the dollar store, a broomstick, and cardbord. It did not quite make the power that I hoped for, though. You could also link any number of these in series to increase the power.