Viridity Energy, the Philadelphia smart-grid and energy optimization company, said Monday it had received a $15 million equity investment from Mitsui & Co. Ltd., the Japanese conglomerate. Viridity is giving Mitsui a 15 percent equity investment. Viridity, whose projects include a battery system to capture and reuse electricity from SEPTA subways, said the financing would support the development of distributed energy projects that feature storage as a significant component. The investment also includes a cooperative agreement to work with Viridity to deploy its proprietary software platform in Asia. The investment requires the approval of the Federal Energy Regulatory Commission.