Energy giant Uniper warns that Germany could face natural gas shortages in the winter if it doesn't accelerate the rate of filling gas storage sites. Uniper's chief executive Michael Lewis called for incentives for companies to stock up gas in storage in an interview with Germany's business daily Frankfurter Allgemeine Zeitung. Germany's gas storage sites were only 30.6% full as of May 27, according to data from Gas Infrastructure Europe. That's well below the 38.65% filled storage at the same time last year. “If we don’t fill the gas…