whaase... you got things a bit confused with the closed loop and return thing
but anyways, yeah, you could use them to replace powerheads. I have one of mine running on a closed loop with a velocity T4 pump. The pump pulls water directly from my tank, and returns it thru the scwd back into the tank, creating a wave action in the tank. I like it.
I also have one scwd hooked up to one of my return pumps (I have two). This setup pulls the water from my sump, back up to the tank thru the scwd, creating even more wave action (actually, creates some nice turbulence because the two scwds always seem to alternate).