Perhaps you should try some spelling variations:
"renewable energy resources" » "renewable energy resource", "renewable energy ressource", "renewable energy reresources"
"renewable energy sources" » "renewable energy resources"
"renewable energy source" » "renewable energy resource"

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 resources"))) OR ("renewable energy resources"))) OR ("renewable energy source")).
• The words AND, OR and NOT may confuse the search; try adding quotes: "((((((\"renewable energy sources\") OR (\"renewable energy resources\"))) or (\"renewable energy resources\"))) or (\"renewable energy source\"))".
• Removing quotes may allow a broader search: (((((( renewable energy sources ) OR ( renewable energy resources ))) or ( renewable energy resources ))) or ( renewable energy source )).
• Adding a wildcard symbol may retrieve word variants: (((((("renewable energy sources") OR ("renewable energy resources"))) or ("renewable energy resources"))) or ("renewable energy source"))*.