I'm not sure exactly when they were put in place, but those square panels attached to the top of the Xcel Energy Plant [I had to edit as I first called it the NSP plant as it was originally called before Xcel Energy bought them] had not always been there. I don't know what their purpose is, but they must do something. To this day I wonder what their function in life is. Exactly how were they put there? You'd need a very long ladder to reach to the top of the stack as it sits around 600 feet in the air. Perhaps someone can shed some light as to what they do up there but distract my driving as I pass by the High Bridge Energy Plant in St Paul.