In this video I rush through playing the Earth levels, making fairly extensive use of the TAB key to speed up time (the video is shot in realtime). Notice how the powerups remain for as long as time is speeded up, and are collected automatically at the end of the level.
As you can see I'm being fairly reckless, placing my refineries before I bother placing any defences down. In the end my blasé approach gets the better of me and the boss and his friends on level 10 defeat me. In fact the game has decided by then that I know what I'm doing, so the replay of level 10 sees me fighting for my life again, but I make it through by the skin of my teeth. I should probably not have squandered my Bezerk and Smartbomb powerups on the easy earlier levels.
So notice how I place tons of turrets down along the roadside, and get them firing as fast as possible with 4 cooling towers. And I don't mind losing the odd turret here or there.
That's how it's meant to be played :)
