The over 48,000 orange trees that permeate all corners of Seville, Spain, not only fill the city's air with the pleasant smell of azhar, or orange blossoms, in spring; they also yield over 16,500 tons of fruit every winter. Though that gives the capital of southern Spain’s Andalusia region the bragging rights of being Europe's top orange-producing city, the fruit is too tart to be consumed fresh. While some of the produce is used to make marmalade and orange liqueur, most of it ends up in Seville's landfills. However, that may change soon thanks to an ingenious idea to use the oranges to produce clean energy.