Website : www . citigroup . com Citigroup ’ s data centre makes the list after enormous progress . Its giant data centre in Frankfurt started at 2.8 PUE and has been brought down by nearly half to 1.5 , and the company is still on a mission to get it to 1.0 . Awarded with a platinum rating from LEED in 2009 , this building is covered in vegetation . Climbing greenery covers each wall of the 230,000 sq-ft building , which also features a roof garden . The plants absorb the heat to lower energy use and purify the air both in and around the buildings . 10 22 November 2017