Green energy is a very broad subject, and I can imagine many different projects that might be developed that could engage students in learning more about this.
For example, you might have them design a home, a school or some other building using green energy. They would need to investigate building methods, materials and technologies that conserve energy, and means of collecting energy, such as solar electricity and wind.
Here is a project from High Tech High focused on Urban Ecology: http://www.hightechhigh.org/projects/?name=Urban Ecology&uid=0a55f414711512b34a41f04abb94546f
Here is a page with a nice collection of alternative energy resources;
http://www.mrsoshouse.com/pbl/energy/resources.htm
Here is an activity focused on a town: ShoULd yoUR Town BEComE A gREEn Town? yoU dECIdE
http://www.msta-mich.org/index.php/publications/journalArticle/185
Here is an example of a student proposal for a “Green Energy Rmada.” I could imagine this coming from a design challenge sort of driving question, that might be something like: “Can you design something for the school that would save energy?”
http://21k12blog.net/2011/03/10/design-build-class-project-proposal-green-energy-ramada/
Those are some ideas to work with. Let me know if any of them might help!