By incinerating waste to produce electricity and heat, the 4.4 million-square-foot power plant releases less carbon dioxide into the atmosphere than it would if it burned fossil fuels.
Copenhagen hopes to become the world's first zero-carbon capital city by 2025.
Some environmental groups say waste-to-energy plants still aren't exactly "clean," since burning waste emits carbon dioxide, as does transporting that waste to the plant. CopenHill also produces a lot less energy than fossil-fuel power plants.
But the trash the plant burns would otherwise end up in a landfill, where it could emit methane — a greenhouse gas that's up to 35 times as potent as carbon dioxide.