The U.S. power generation fuel mix continues to shift, driven in part by low natural gas prices, state renewable mandates, tightening environmental regulations, and lower installed costs of some generation technologies such as solar. On June 18, 2014, the U.S. Environmental Protection Agency (EPA) issued its proposed Clean Power Plan (CPP), a proposed greenhouse gas emissions regulation scheme for existing plants, promulgated under section 111(d) of the Clean Air Act.