1sockchuck writes: After you’ve built one of the most efficient data centers on earth, how do you make it even better? One refinement at a time, as Microsoft has found in its data center in Dublin, the primary hub that powers the company’s online services throughout Europe. Since 2009, the company has boosted server density while improving its PUE (a key measure of energy efficiency) in Dublin. It's found that it can rely entirely on fresh air to cool its servers, and has disconnected mechanical cooling systems held in reserve for hot weather. The refinements get as esoteric as switching from black cabinets to white ones, which reflect light better and can reduce the amount of overhead lighting required in the $500 million data center.