WASHINGTON (Reuters) - The U.S. Energy Department on Wednesday unveiled a plan to offer up to $4 billion in loan aid to renewable energy projects, opening up another round of funding for a program that faced harsh political attacks over past government-backed failures.

The draft plan would provide loan guarantees for innovative projects that limit or avoid greenhouse gas emissions.

It will specifically focus on advanced electric grid technology and storage, biofuels that can be used in conventional vehicles, energy from waste products and energy efficiency improvements.

This latest loan offering follows a round of funding from the 2009 economic stimulus that backed solar, wind and geothermal projects, including now bankrupt solar panel manufacturer Solyndra.

Despite the high-profile collapse of Solyndra, the Obama administration has stressed that most of its energy investments have done well and it credits the program with helping the U.S. solar industry go from panels on the roof of a house to industrial-scale power plants.

"We want to replicate that success by focusing on technologies that are on the edge of commercial-scale deployment today," Energy Secretary Ernest Moniz said in a statement.

The department issued a plan last year to offer up to $8 billion in loan assistance for fossil fuel projects that reduce greenhouse gas emissions.

No loan guarantees have been issued under that proposal yet.

Critics of the loan program have accused the administration of favoring political allies. They argue that the government should not be picking winners and losers in the private sector.

A 2005 law established the multibillion dollar Energy Department loan program to help finance ground-breaking energy technologies.

(Reporting by Ayesha Rascoe; Editing by Lisa Shumaker)