NORFOLK, Va. (AP) -- Navy Secretary Ray Mabus defended the service's investment on biofuels Monday, saying that it is a "false choice" for critics to suggest the Navy should focus on building more ships instead of finding alternative fuels to power them.

Some Republicans in Congress are upset the Navy is spending money on expensive alternative fuels when several ships are being retired earlier than planned in an effort to cut costs.

The Navy, Department of Energy and Agriculture Department are spending $510 million to jump start commercial development of the advanced alternative fuels industry. By 2016, the Navy wants obtain 8 million barrels of biofuel to power a strike group that's being dubbed the "Great Green Fleet." It has already demonstrated that alternative fuels can work in some of its planes and ships without having to change out their engines.

The Navy wants the alternative fuel it purchases for that fleet to be available at prices that are competitive with conventional petroleum fuels. Mabus acknowledged that biofuels aren't that inexpensive yet, but said that investment by the Navy would make it affordable because of how much fuel the service consumes.

The military uses more than 90 percent of the energy consumed by the federal government. Alternative fuels burn cleaner than fossil fuels, require no drilling, and can be produced in the United States. Algae-based fuels are among those the Navy intends to use for its "Great Green Fleet."

If the Navy doesn't find alternative fuels, it may not be able to afford to build news ships or that it might not be able to get the fuel it needs for its ships and aircraft, Mabus said. Mabus noted that unrest in Libya and elsewhere around the world raised the price of oil $38 more a barrel than it budgeted, raising the fuel bill for the Navy and Marines by more than $1 billion.

"The only way we can afford to get the number of ships, the number of aircraft that we need is to change the way we get fuel," Mabus said.

Mabus made the made the comments aboard the USS Kearsarge at Naval Station Norfolk, where a field hearing was being held by a U.S. Senate Energy subcommittee. It was the first Senate committee hearing held aboard a Navy ship since 1960. The ship hosted numerous displays about how the Navy and Marines are becoming more energy efficient.

For the Marines, becoming more energy efficient can mean carrying less gear and making fewer convoy trips to refuel, which are often dangerous assignments. For every 50 convoys, the Marines experience one fatality.

"I don't understand how some members of Congress just don't seem to get it that this initiative is about saving money, it's about saving lives," said Sen. Mark Warner, D-Va.

The Kearsarge is an amphibious assault ship that has won numerous Navy awards for its energy initiatives.