First make the turbine - use the mannequin by main map in the middle, the fan on the chair next to the work bench, and the plain wing next to the pay phone. You can also see the other cheat on this website for more detailed instructions.

After you build it buy the door (750pts) and go in between the gray door and the tree plant vase "thingy".

Then plant your turbine and the door should open. However, you now are glitched between the door and the vase.

Note that zombies will destroy your turbine. Don't worry about the door though - it won't close.