Grey literature is typically unpublished or informally published works from the government, research groups, think tanks and professional associations and may include all kinds of sources, including:
National Renewable Energy Laboratory. Building and calibration of a FAST model of the SWAY prototype floating wind turbine preprint. By J.H. Koh. (NREL/CP-5000-60096). September 2013. (Conference Paper). Available at: http://purl.fdlp.gov/GPO/gpo47393; Accessed: 05/12/2014.
"Energy integrative building with ecological environmental protection" by Xudong Liu, Geng Liu, and Ziyan Liu. U.S. Patent 8,615,940 (Dec. 31, 2013). Available at: Google, Patents, http://www.google.com/patents/US8615940. Accessed: 12/09/2016.