This is a neat heat engine that looks like a flower with petals that open and close in the heat. The neat thing is that if you heat only one side of the "flower" then only the petals on that side open, resulting in an unbalanced wheel. That side falls, bringing fresh petals into the heat. Meanwhile, the previously heated ones are rotated to the cool side and close.
Video - Sunflower Heat Engine Demos/How it Works
Here's a video showing some demonstrations along with an explanation of how it works.
How to make the sunflower heat engine
Here's how to make the sunflower heat engine.
Step 1. Get a coffee bag. You'll need one that has a silver colored metal foil on the inside, very likely aluminum. Luckily I could tell which one was like this in the store by looking at the top where some of the metal was visible. Not all coffee bags have this foil inside. The foil is laminated with the plastic outside.
Step 2a. Make some sort of stand for holding it. The vertical parts of the stand should be made of something fairly thin and that don't deform easily (they'll hold their shape without bending easily.) In my case I had some scrap aluminum bars. The reason for having them be thin is that in step 3 you'll put magnets on the vertical parts and the magnetic field will have to extend through them. For that reason they should not be iron or steel since those will absorb magnetic fields. Basically, if the magnets don't stick to the parts then they're good to use.
Step 2b. As shown in the photo below put a small indentation near the top of one vertical support. You can do this by putting a nail against it and hitting the nail once or twice with a hammer. It shouldn't be deep. If you're using wood for your vertical parts then get a small piece of metal, a metal coin will do, and make the indentation in that. Then tape that to the vertical part.
Step 3a. Put magnets on the vertical parts as shown in the photos below. I'm using neodymium magnets bought from Home Depot. As shown in the first photo below, on one part put magnets on both sides. Their attraction is what'll hold them in place.
Step 3b. On the other part the magnets are one one side only so you'll need to tape them in place. The magnets should be placed opposite the indention that's on the other side of the part.
Step 4. Cut a piece of plastic or sturdy cardboard in the shape of circle. Mine is around 1 3/8" (45mm) in diameter. Push a needle through the exact center of it.
Step 5. Suspend the disk on the stand as shown in the photo below. The sharp point of the needle should go in the indentation you made in one vertical part. Both ends of the needle will be attracted to the magnets you'd put on both parts. There should be a gap between the blunt end of the needle and the magnets nearest it. You'll need to play around with how many magnets you use and what sized ones until you get the needle suspended as shown.
Step 6. This is the first of a few balancing steps. Rotate with disk and whatever orientation you rotate it to, it should stay that way. If it doesn't, and instead rotates and always stops in the same position, then make a note of which part of the edge is lowest and sand away some of the material from that edge. Keep doing this until the disk stays at whatever orientation you rotate it to.
Step 7. Cut eight strips from the coffee bag around 1/2" (13mm) x 2 1/8" (55mm). Try to make them all close to the same dimensions.
Step 8. Use a small piece of tape to tape two of them to the disk. Use a ruler to test that their outer edges are both the same distance from the needle.
Step 9. As shown below, suspend what you have so far on the stand. Again, do some balancing. If the same strip always wants to hang down then trim it a tiny amount. Another thing you can use is trim a bit of the tape for that one. You're basically trying to get the mass on both sides to be the same. Keep working on it until the disk stays in whatever orientation you put it, which at this point is ideally with both strips horizontal. As you can see in the photo below, I got it fairly close.
Step 10. Add all the rest of the strips, as shown below. I used a piece of paper with eight lines coming out of the center, meaning that each two consecutive lines are 45 degrees apart, to help space them out evenly.
Step 11. Suspend the result on the stand. You may have to add more magnets, as I did, due to the added weight.
Step 12. For the last time, you need to do some more balancing. One particular section of petals will again rotate to the bottom always. To counteract that cut a small piece of tape and tape it somewhere to the opposite side of the disk. You'll have to experiment with how much tape to put and exactly where to put it. Work on it until the whole thing sits in whatever orientation you put it.
Step 13. Carefully curl each petal. The photo below shows when I did it while there were only two petals. I do this using the blunt side of a pair of scissors. Hold the petal between your finger and the scissors and gently squeeze as you pull along the length of the petal from the inside to the outer edge. The scissors should be on the metal foil side, resulting in the petal being curled toward that side.
Step 14. Try it out. You want to heat one side of the petals while either cooling the other side or just letting that side cool by not being in the heat.
If you don't have sunlight then you can use glasses of hot and cold water as shown below and at the top of this page. Just have the petals be as close to the glasses as possible without touching. The hot and cold air surrounding the glasses will do the work.
But if you do have sunlight, as shown below put one side in sunlight and the other side in shade. The result looks awesome!
Video - How to Make Sunflower Heat Engine using Coffee Bag
The following video shows step-by-step how to make this sunflower heat engine using a coffee bag.