On what principle does a solar fan work?

The first law of thermodynamics. In brief, energy can neither be created nor destroyed. However, energy may be converted from one form to another.

Explanation:

A basic electrical solar fan comes in two parts: a solar panel connected to an electrical motor. The solar panel converts solar energy to electricity; the electrical motor converts electricity to mechanical energy, which causes the blades to turn.

This energy conversion is possible because of the first law of thermodynamics. This law states that the total energy of the universe is constant. Energy can neither be created nor destroyed. However, energy may be converted from one form to another.

For example, the solar panel does not destroy the solar energy; it simply converts that energy to electrical energy. Note that a fraction of that energy absorbed was lost as thermal energy. Similarly, the motor merely converts electrical energy to mechanical energy. It might also lose some thermal energy as it heats up.