Perhaps you should try some spelling variations:
"renewable energie ressourcen" » "renewable energies ressourcen", "renewable energetic ressourcen", "renewable emerging ressourcen"
"renewable energy resource" » "renewable energy resources"
"renewable energy sources" » "renewable energy resourcess", "renewable energy resources"
"renewable energy source" » "renewable energy resources"

Try widening your search to All Fields.

You may be able to get more results by adjusting your search query.

• If you are trying to use Boolean operators, they must be ALL CAPS: (((("renewable energy sources") OR ((("renewable energie ressourcen") OR ("renewable energy resource"))))) OR ("renewable energy source")).
• Removing quotes may allow a broader search: (((( renewable energy sources ) or ((( renewable energie ressourcen ) or ( renewable energy resource ))))) or ( renewable energy source )).
• Adding a wildcard symbol may retrieve word variants: (((("renewable energy sources") or ((("renewable energie ressourcen") or ("renewable energy resource"))))) or ("renewable energy source"))*.