The spinning fans use energy to move air, a (slightly) viscous fluid, which eventually encounters turbulence and friction and turns its kinetic energy into heat

The spinning fans use energy to move air, a (slightly) viscous fluid, which eventually encounters turbulence and friction and turns its kinetic energy into heat. Similar to discs that use energy to overcome friction in bearings and generate noise, which is a kinetic energy in the form of vibrations that also eventually dissipates into heat.