An ambitious expansion of Bradley International Airport, with an additional terminal and parking garage, would benefit the region's economy while making travel smoother for Connecticut residents and visitors, according to a detailed new plan.

The plan - parts of which may not come about until demand materializes, which could take many years - includes a 19-gate terminal at the location of the old Murphy Terminal, which closed in 2010 as the oldest terminal at any major U.S. airport.

A parking garage with consolidated car rental facilities would rise where the surface lot is now located in front of the Murphy Terminal, and the airport would have its own power plant on site.

The project, as laid out in a report for the state Department of Transportation and the Federal Aviation Administration, would nearly double the number of gates now available. The first phase - demolition of Murphy, also known as Terminal B; site preparation; and part of the new terminal - would cost as much as $650 million, using a combination of federal and state funds.

The report, issued July 2, relies on growth projections of about 5 percent a year from 2009 to 2013. But a top airport official said Tuesday that those projections are overly optimistic.

Instead, he said, it's the garage and demolition project that are viewed as the more solid parts of the plan for the near term.

"The passenger forecast, we recognize, needs to be updated, and we've actually started that process. That'll help us determine at what point we actually need new terminal construction," said Mark Daley, Bradley's chief financial officer. "What we're doing is preparing the airport for the future. We need to be in a position to respond to demand when we see it, and we won't be able to do that unless we take down the old terminal."

The most likely immediate benefit for travelers would be a move of the rental car lots to a new garage at the airport. For the last two years, rental car customers have been paying a $3.50 surcharge, and $10 million has been put into escrow for that project.

Daley said that once the garage opens, that fee will be raised so it covers the full cost of the debt service on the bonds that will pay for the construction. But he did not estimate how much the garage would cost, and the plan is silent on that.

University of Hartford economics Professor Jeffrey P. Cohen, a Federal Reserve visiting scholar who studies the economics of airport expansion, said spending the money to bring rental cars on the property is a good idea because it benefits customers, and they're the ones who will bear the costs.

The plan makes it clear that the garage will happen even if the terminal expansion never does.

Charles Gray, member of the airport authority's board, highlighted the need to tear down the Murphy Terminal even without a demonstrated need for more gates. "Removing the terminal opens the ability to add additional development for the space: roadway realignment, parking garage and consolidated car rental," he said. "The first major puzzle piece has been: Can we get rid of the building?"

The state Department of Transportation is overseeing the project in its early stages, but many people expect that it will be turned over to the newly formed Connecticut Airport Authority, which oversees Bradley and other state-owned airports.

The airport has 23 gates on two concourses of Terminal A, which was built in several stages. Under the long-term plan, Terminal A would be reduced to 20 gates to allow for larger planes, and the new terminal would have 19 gates in two concourses.

The forecasts that will be replaced project that passenger trips will grow by 44 percent from 2009 to 2028.

The proposed U-shape design for the new terminal will feature two gates among the 19 for international gates for wide-body aircraft, an asset Bradley lacks. A utility plant, to be built west of new terminal, would meet the facility's power, heat and cooling demands.

A 2005 study considered the possibility of rehabilitating the Murphy Terminal, but found that it was "less expensive and more prudent" to replace the terminal than renovate and rehabilitate it.