PHILADELPHIA — Comcast on Monday pledged to double the energy efficiency of its vast nationwide broadband system that channels a torrent of cable and internet data into millions of homes. The global media and technology company with headquarters in Philadelphia said it plans to double its network energy efficiency by 2030, cutting in half the electricity consumed per terabyte of data delivered. The promise comes on top of Comcast’s goal set last year to reduce its net greenhouse gas emissions to zero by 2035 for the direct and indirect emissions it owns and controls. The pledge was timed to co…