**SPOILER ALERT** For Portal Room 18, if its not that then don't read below.
Use the energy balls to knock over the turrets by making portals. Then line up the energy ball with the door (to the left of where you entered the room), there's a button you need to time and open for the ball to reach the receiver.
then....
use the side-lift to get to the lower ledge, press the button, noticing it opens the wall you came in from, and noticing the angle the wall makes for a small amount of time. The rest i'm sure you can piece together