Picture of 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

Picture of 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
BruceY17 months ago

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

flyingpuppy2 years ago
How did you THINK of this?
It was in an issue of Popular Science magazine '79-'83. Called a Solar Engine.
It was in an issue of Popular Science magazine '79-'83. Called a Solar Engine.

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

Awesome ! No video showing the contraption moving ??… aaah !

spylock2 years ago
Book marking,Im gonna take a stab at it maybe as a fan for a wood heater.
Kiteman2 years ago
Can you upload the video to YouTube or Vimeo?

There's a DVIX restriction preventing it playing on my tablet.
kaiphas633 years ago
I made this for my science fair in 1980. Good learning tool! Keep on making stuff!
DanYHKim4 years ago
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.
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.
This is a very clever concept. How did you discover the stretched black plastic shrinks in heat property?
The first time I saw this motor was in the 70's in popular science magazine.
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.
do you need to drill a hole in the dowel before inserting the needle
Mr.NHRA6 years ago
This may sound like an unintelligent question but does the motor always rotate the same direction?
It probably depends on which way the light hits it.
nerissa6 years ago
JOEDUPONT7 years ago
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.
erikstrahl (author)  JOEDUPONT7 years ago
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.
how many brake power come up with your 2ft diameter solar thermal motor that u made???.......
I'm going to have to risk sounding like Mr. Spock... Fascinating!
budhin7 years ago
its cool.
Reminds me of my (Crookes) radiometer, before the kitties got at it (sigh).
tintin7 years ago
hi! i think this project will be best for my elementary pupils, and for future teachers in elementary education. kids love stuffs like this. please post a video to let subscribers see the actual results.. more power DIY genius!!
luv4sun7 years ago
What does the motor do? Like once you build it, what function does it do? I'm trying to inflate a balloon for my science class and I was thinking this might work.
kaiphas7 years ago
I built this very solar engine in 1980 and won my high school competition for a science project. Went to state competition and someone moved the sun lamp too close to my motor and the bands melted. I guess they wanted to see how fast it would spin. Hint... if you make the bands thin, the motor will have better balance.
jarney17 years ago
My high-school physic teacher had the class build these to demonstrate thermodynamics and the thermal expansion/contraction of plastics. I thought it was brilliant then and I still do. Thanks for sharing it!
jhonny7 years ago
where did you come up with this idea?
I recall seeing this very mechanism described in an old Popular Science article from the 1980. A search of the H.W. Wilson database (the Reader's Guide to Popular Literature folks) turned up this reference, which I believe is the article I vaguely remember -

Title: Build a sun tracker and a solar engine with solar muscle
Personal Author: RAY, Edward D.
Journal Name: Popular Science
Source: Popular Science v. 216 (February 1980) p. 126-8
Publication Year: 1980
Physical Description: Illustration
ISSN: 0161-7370
Subject(s): Solar engines
Historical Subject(s): Solar engines
Document Type: Feature Article
Database: Readers' Guide Retrospective
Accession Number: 198003201152027

Check you local library's microfilm department for a copy.

(German genes, Irish whiskey --- what could possibly go wrong?)
erikstrahl (author) 8 years ago
OK, I added a video, plus a CAD drawing. The particular Motor in the video is an extra long one that I built - I found there isn't much of an advantage to the extra length unless you add a heavier flywheel - especially if the device is off-balance (mine was). If you watch the video you will see it almost come to a stop as the heavy side reaches the bottom, then start spinning again. Initially, as the strips heat up for the first time, they will tighten, and that sometimes changes the balance of the device.
I'm looking forward to seeing more of your work, really nice.
Gene Hacker8 years ago
That is a cool/hot heat engine. Sorta reminds me of the Feynman rubber band heat engineFeynman rubber band heat engine. How fast does it spin in rpm? I would love to see how this baby compares with Carnot efficiency. If you can't get any sun to run your engine use a halogen lamp or a heater. If it doesn't spin fast enough with these use some ice to cool the dark side.
erikstrahl (author) 8 years ago
I got the idea from my Dad, who read about the concept in some scientific paper back in the 70's. I'll try to get a video of the device spinning soon - tomorrow if its a sunny day.
This is brillinat, How'd you come up with it?.......I'd love to see how fast it spins
Kiteman8 years ago
Excellent idea - would probably look good on a sunny windowsill. I agree with the others, though: post a video, please!

"Use scotch tape to center the open end of the cup to the dowel." (Step 7)

> This instruction needs a bit of clarification, and a better photo, please.
Yes video please. Coincidentally I had been talking to a professor this weekend about shape memory alloy motors, this the same concept but much more doable at home, great job.
rimar20008 years ago
That's interesting. It would be pretty to see it in movement.