Perhaps you should try some spelling variations:
"renewable energy ressource" » "renewable energy resources", "renewable energie ressourcen", "renewable bioenergy resource"
"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 energy ressource") OR ("renewable energy resource"))))) OR ("renewable energy source")).
• The words AND, OR and NOT may confuse the search; try adding quotes: "((((\"renewable energy sources\") or (((\"renewable energy ressource\") OR (\"renewable energy resource\"))))) or (\"renewable energy source\"))".
• Removing quotes may allow a broader search: (((( renewable energy sources ) or ((( renewable energy ressource ) OR ( renewable energy resource ))))) or ( renewable energy source )).
• Adding a wildcard symbol may retrieve word variants: (((("renewable energy sources") or ((("renewable energy ressource") OR ("renewable energy resource"))))) or ("renewable energy source"))*.