Bristol Hospital is preparing to embark on the next phase of a $15-million expansion and renovation that will roughly double the size of its emergency department. The design calls for the new and old square footage to be one unified space, said project manager Tom Roche. The hospital is financing the project through the Connecticut Health and Educational Facilities Authority, which in June issued $34.6 million in bonds for the project as well as refinancing of prior hospital debt. Mary Lynn Gagnon, executive director of the Bristol Hospital Development Foundation, said she is aiming to raise between $4.5 and $5 million in donations. Since the campaign launched in April, the foundation is up to $3.8 million, she said.