The world's two largest energy consumers have clashed over their choices for fueling their economies, with solar industry subsidies and China's oil imports from Iran particular sore points.  But with Xi's stop in Washington an important one on his path to take over China's presidency later this year, and the two leaders hoping to highlight cooperation, energy offers an opportunity for the two nations to emphasize their collaboration: The deals they've struck on projects to clean up coal.

In China, coal is king. U.S. energy companies, from small start-ups to one of the nation's largest utilities, Duke Energy, have concluded that they must work with China to keep a hand in technology to reduce the greenhouse gas emissions of coal-fired electricity: