Referencing images found in public domain
The images may still be under copyright. I think you have to contact NREL. Here's their copyright policy which states:


  Copyright Status
  
  NREL-authored documents are sponsored by the U.S. Department of Energy
  under Contract DE-AC36-08GO28308. Accordingly, with respect to such
  documents, the U.S. Government and others acting on its behalf retain
  a paid-up nonexclusive, irrevocable world-wide license to reproduce,
  prepare derivative works, distribute copies to the public, and perform
  publicly and display publicly, by or on behalf of the Government. Use
  of documents available from or referenced by this server may be
  subject to U.S. and foreign Copyright Laws.


So you need to identify the source and copyright status of the document that contains the image.