MIT, Boston Medical Center, and Post Office Square Redevelopment Corporation have formed an alliance to buy electricity from a large new solar power installation, adding carbon-free energy to the grid and demonstrating a partnership model for other organizations in climate-change mitigation efforts.The agreement will enable the construction of a roughly 650-acre, 60-megawatt solar farm on farmland in North Carolina. Called Summit Farms, the facility, the largest renewable-energy project ever built in the U.S. through an alliance of diverse buyers, is expected to be completed and to begin delivering power into the grid by the end of this year.