Mikel Bilbao/Getty ImagesNegative energy prices are becoming more common across Europe.The latest instance occurred in Germany on Tuesday.Prices generally slip into the red when both solar and wind generation is strong on the same day.Europe's energy prices are trading in negative territory with increasing frequency, and it's because of strong solar and wind generation in the region.In the latest example, Bloomberg reported that power prices in several markets slid below zero for Wednesday.