Description
Science rarely gets cooler than this. This windmill does not run on wind or water. It runs on sunlight! The solar windmill is a radiometer that spins when it is put in the windowsill.
But photons don't have mass, you might think. And you are absolutely right. The small vanes in the rotor are black on one side and white on the other. The black side will thus be heated up more by the sunlight. For a while researchers thought that the air, when heated and expanded, gave enough push to start the mill. They were wrong.
The effect is probably due to so-called thermal transpiration. Because the black and shiny side of each rotor blade have different temperatures when the solar turbine is placed in sunlight, the air molecules will constantly migrate from the shiny to the black side. This air flow from the cold to the warm side is what pushes the mill around.
Want to read the whole explanation, you say? You want to go to the original article by Osborne Reynolds from 1879? Maybe use it as wallpaper in the room where the solar windmill will be? Here you go: On certain dimensional properties of matter in the gaseous state
Dimensions: 15.2cm x 11.5cm x 11.5cm