I've worked with DC fans since they started. The standard Brushless DC fan will not work when the polarity is reversed. You may or may not kill it based on the manufacturer. I am not sure if you can buy a DC fan which uses a brush type DC motor, but that motor will reverse direction when polarity is reversed. You will also have the problem of finding a fanblade which blows effectively when reversed. One possibility is to buy two Brushless DC fans and mount them back to back. Then use a switch to turn on one and or the other. A single pole double throw (SPDT) switch will do. The voltage is not critical, but should not be very much higher than allowed by the fan "running" specs. ( A 12 VDC fan will run at 15VDC). Never violate the absolute max specs.