Regulatory Assistance Project (RAP), Jun. 2014.
http://www.raponline.org/document/download/id/7175

"A new policy brief from the Regulatory Assistance Project (RAP), Clean Energy Keeps the Lights On, dispels the myth that electricity portfolios with high penetrations of variable renewable resources threaten reliability. The authors review eight recent studies commissioned by utilities, governments, and non-governmental organizations to address this issue, and find that none suggest insurmountable reliability problems."