Return of the turbine

If you look at Princess Cruises' newer ships, the first thing that stands out are two cigar-shaped pods on either side of the funnel. "What are those for?" you may ask. They indicate that the stack contains a gas turbine, a rotary-type engine, which is part of a hybrid power system that also includes traditional diesel-electric engines.

It's an important technical advance, one that's made the cruise industry a better neighbor and a more welcome visitor in eco-sensitive cruise destinations. Several years ago, the impact of the cruise industry started to come under scrutiny in Alaska and elsewhere. Part of that focus was on visible emissions from cruise-ship stacks. At one point, federal regulators from the Environmental Protection Agency cited 13 cruise ships for exceeding federal limits on air pollution in Juneau, Seward, and Glacier Bay.

Since then, cruise lines operating in these waters have equipped their newest ships with the latest technology for treating onboard waste and reducing airborne pollutants. To help solve the latter problem, gas turbines have been the solution embraced by not only Princess but also Holland America Line, Royal Caribbean International, Celebrity Cruises, and Cunard (for the Queen Mary The gas turbine differs greatly from that other rotary-type engine we're familiar with--the steam turbine. In fact, the gas turbine is derived from an unexpected source--airplane engines, according to General Electric, which is responsible for adapting the system for maritime use and supplying the engines to the cruise operators.

"Very simply, the aircraft engines typically have a large fan that provides the majority of the thrust," explains Bruce Sanneman, program manager for commercial marine programs at GE. "This fan is driven by a power turbine. For marine applications, the fan is removed. The power turbine then drives a generator or a propeller," he says. "There's also some 'marine-ization' of materials, so they're more corrosion resistant."

However, there is an incidental connection to the steam turbines of old. Some power plants, like the ones used by Royal Caribbean and Celebrity, capture exhaust to drive a secondary steam turbine, which generates power for onboard applications, such as lighting and air-conditioning, and saves fuel consumption. This is known as the COGES system, Sanneman explains, which is shorthand for "combined gas turbine/electric steam."

The reduction of airborne emissions achieved by the gas turbines is substantial, Sanneman continues. "The gas turbines meet all the current and foreseeable emission regulations," he asserts, including stringent standards that have been advocated by the International Maritime Organization. These standards, which ship operators now abide by voluntarily, were proposed in 1996 and are now awaiting formal ratification.

Two of the biggest concerns when it comes to air emissions are the unseen nitrous oxide and sulfur oxide gases, which create acid rain and ozone pollution when released into the atmosphere. In both instances, gas turbines compare very favorably to diesel electric engines, according to GE. In the case of nitrous oxide, Sanneman says, "The IMO proposed limits are in the range of 10 to 17 grams per kilowatt hour. Current GE gas turbines emit five to six grams per kilowatt hour." Gas turbines perform even better in cutting sulfur oxides, he says. "With regards to sulfur oxides, they are almost entirely a function of the amount of sulfur in the fuel. Diesel fuels contain more than 10 times the sulfur of gas turbine fuels: four or five percent versus 0.3 percent for the gas turbine fuels."

As for visible emissions, "The gas turbines have invisible smoke," Sanneman adds. "Clearly that is an advantage, and one of the criteria that led operators to select gas turbines." Other benefits go beyond emissions. The gas turbines create less vibration. They are lighter and smaller than diesel-electrics and are more versatile in their placement. "They don't have to be put in the hull," he says. "Some ships have them located up by the funnel--that's relatively inexpensive real estate on the vessel, and they don't occupy interior volume."

Just about the only downside to the gas turbines seems to be their need for high-test fuel, which is costlier than that consumed by diesel electric plants. "The fuel is more expensive," Sanneman acknowledges. "It's a distillate fuel called marine gas oil," which contains fewer impurities than diesel. This accounts in part for the clean-running characteristics of the gas turbines. However, he says, "Other factors, such as maintenance costs, are comparable."

While gas turbines have been in use only a short time on cruise ships, their record for reliability has been impressive. "On the first ship that went into service, Celebrity Cruises' Millennium, one of the original turbines is still operating after 23,000 hours without major maintenance," Sanneman notes. "The other thing we are proudest of is that on the Royal Caribbean and Celebrity ships, since their entrance into service almost three years ago, never has the COGES plant been the cause of a missed arrival or departure into any port on any day on any ship