Why hasn't this been thought of sooner?

To save money when cooling indoor commercial coolers and freezers in far northern and southern areas one could design a circuit controlling a mechanical door which would be situated outdoors connected to the inside of the cooler/freezer via a vent with a fan inside it. When the temperature in the cooler/freezer rises about the desired temperature and the outside temperature is below that level the circuit would open the door and turn on the fan which would draw the colder outside air, in the winter months , into the cooler/freezer saving about 10-15% of the annual cooling costs as opposed to running a condenser year round. Of course the vent would be filtered and the door would close when the temperature reaches the desired temperature. Feel free to use this idea in any way other then patenting it. I wish it to be public domain.