The growth of U.S. wind power has begun to create operating challenges for nuclear and coal plants that must be ramped up and down as wind speeds vary, panelists at a Massachusetts Institute of Technology energy conference reported last week. The MIT Energy Initiative symposium on integrating large-scale wind and solar power attracted executives of utility and transmission companies, senior government officials and academic researchers, whose comments were off the record. Some papers prepared for the conference were made public by their authors, and they define a growing challenge of matching the current U.S. mix of power plants with new requirements to respond quickly to changes in wind and solar resources.