Solar Ferris Wheels are desktop toys which rotate slowly in the sun, and are meant for gentle amusement rather than active play. They are close philosophical cousins of the famous "dunking bird", radiometer and Gallilean thermometer. The actual toy can take the shape of a flipping whale, a rotating yin-yang symbol, circus acrobats, etc. It may use sunlight or a desk lamp for power. A large solar Ferris wheel (about a foot long) generates enough power to be coupled to a cart and propel its wheels, though not very fast or over a bumpy surface.

The illustration below is a somewhat coarse and schematic, but does convey the general principles, though a practical toy incorporates other design details for reliable operation.

How it works: The Ferris wheel consists of an upper and lower sealed chamber, connected together by a thin hollow pipe. The entire apparatus rotates freely around the center axle, shown end-on above as a black dot in the middle of the image.  A liquid (shown in red) can flow from one chamber to the other through the pipe.

Each chamber is painted white on one side, and black on the other. The white side (the thin blue line above) reflects sunlight, while the black side absorbs sunlight. So, if the black side is on the lower chamber, it absorbs light and heats the air within the lower chamber. This hot air expands, pushing the red liquid into the upper chamber (for greater thermal insulation, the actual design supports the black layer inside the chamber, and a 30 c rise in temperature is typical).

With more liquid now in the upper chamber than the lower, the apparatus is unbalanced and flips over. The cycle repeats- once or twice a minute under normal conditions. The solar Ferris wheel is self-starting, whisper quiet, and mesmerizing.

Click the picture to see an actual working model of the solar Ferris wheel in operation. The movie is in flash, and is about 3 MByte in size. For scale, the spheres are about 3 inches in diameter. The ability to blow such beautiful glass objects is a dying art- this example was fabricated by the excellent team at Vitri-Forms, Inc.

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CAUTION!! This is a toy, not a perpetual motion machine or mechanism which can practically be scaled in size to operate a car or run a pump.

A slightly more sophisticated rocker uses "flipping" vanes in each sphere. The vanes are black on one side and silver on the other. They are attached to the spheres at two bearing points along a straight axle, and one side of the vane is weighted so gravity always tips that side toward the ground. When the rocker is placed in the sun, the black vane is in the bottom sphere facing the sun. A reflecting vane is in the upper sphere. So, the bottom sphere heats up, pumping liquid to the upper sphere, the rocker tips, AND THE VANES ROTATE, so the black side is again in the bottom sphere, with the shiny surface again in the top sphere facing the sun. Fascinating to watch- tips about once a minute in full sun.

A third example of a solar ferris wheel is shown below. This device does not rely on gas expansion or liquid pumping. Instead, the ferris wheel is decorated with a number of black plastic strips along the perimeter. These strips are bilayers made of two plastics with different expansion coefficients. When heated by the sun or even a table lamp, the difference in thermal expansion causes the strip to "bow" (note the strips near the lamp are more curved than those shaded by the wheel). Since they bow away from the center of rotation, the center of mass is moves toward the lamp, and the wheel - now unbalanced- rotates in response.

You can view an even simpler, two strip device called "scoop" at talusfurniture.

The Solar Ferris Wheel is very, very slow example of a Stirling Engine. Because the temperature differences from direct solar illumination are small, its efficiency is similarly reduced and produces very little torque. Commercial solar Stirling engines use a lens or reflector to concentrate sunlight, resulting in a much higher temperature within the chamber.

Naturally, a larger wheel creates a larger force, but a low temperature hot air engine is never very efficient. Indeed, the brilliant engineer John Ericsson invented a hot air engine to propel a cargo ship in 1851- but to make it powerful enough  to sail the oceans the pistons were almost 15 feet in diameter and rose and fell 6 feet with every stroke. So gentle was the motion, that Ericsson placed a dining room table inside the piston chamber and held dinner parties atop the pistons as they cycled up and down! Unfortunately, the engines were so huge, there was no room left for cargo.

If you would like to purchase a Solar Ferris Wheel, see the Talus Lamp at TalusFurniture.com

Some other interesting solar wheels:

The Minto Wheel is really a flock of "dunking bird" toys, arranged in a circle. Enjoy the article on the wheel, but be very skeptical about "free energy" claims, and the efficiency of the machine.

Some Stirling engines use liquid for the piston, such as this seesaw engine, or a solar variation based on the dunking bird.

And rising hot air can spin a turbine, (often called a solar chimney) if you build one large enough.