AC not rectified to DC properly
Your grounding is messed up.  You only show one of the transformer outputs, labeled "12 V 50 Hz AC", so the other is implied to be connected to ground.  However, the "+15V" node is then not 15 V with respect to ground.

You should consider the negative side of the full wave bridge to be ground, then NOT ground either transformer output.

This would have been more obvious if you had drawn the schematic properly.  Try to put high voltages at top, low voltages at bottom, and logical flow left to right.  Here is the circuit you really want, properly shown to be as little confusing as possible:



Your overall circuit could be better too:



Instead of a emitter follower, I'd use a low side NPN in common emitter switch configuration.  Since the 741 opamp can't get all the way to the negative supply, use a voltage divider to drive the transistor base.  At 15 V, there is plenty of voltage available.  Arrange the resistors so that the transistor doesn't come on until the opamp output is at least 5 V or so above the negative supply.

The 741 opamp is not a good choice for comparator, but can be made to work in this situation.

As WhatRoughBeast mentioned in a comment, a little hysteresis would be good too.  You want to make sure the fan is driven either fully on or fully off, not in-between due to noise when the opamp inputs are nearly equal.  A high-value resistor from the opamp output to its positive input will provide some hysteresis.  This will have the effect of "snap action".  That keeps both the fan and the transistor from being run partly on, which could cause a lot of dissipation and damage them.