Turbines power on

Having scored a couple of noteworthy successes, turbine manufacturers are keeping up the momentum.

Gas turbines compare favourably against diesel engines on weight, size, power and NOx/SOx emissions ? where they suffer is in fuel consumption, especially at part load. It was the fuel consumption problem that prompted the US Navy to fund Rolls Royce?s development of the recuperated WR21. This is nearing the end of its development verification testing in the US and will start endurance testing in the second half of the year in France. This will involve a 3,150-hour run and shock testing aimed at exposing the engine to an environment like that on-board a ship ? but in an accelerated form. It will follow a running schedule giving extended periods at maximum power, idle, and regions that exercise the variable area nozzles. A representative fuel will be used and salt will be injected into the air intake to simulate a marine installation.

While the WR21 has been primarily developed for military operations, Gordon Price, principal marine applications engineer at Rolls Royce, points out that the operating profile of a cruise ship is not unlike that of a battleship. They both spend much of their time at part load interspersed with short periods at full power. At the lowest part of the fuel consumption curve, just below maximum power, the 25MW (34,000 bhp) unit is delivering a fuel economy of 200 g/kWh. Although simple cycle turbines used in a COGES configuration are excellent at full load, he points out that in part load conditions the economy suffers. Part load is where the recuperator comes into its own. Mr Price puts it more forcefully, saying: “The whole issue of WR21 is part load fuel efficiency.”

With its flatter fuel consumption characteristics, light weight and low installed space, Mr Price says, “preconceptions of machinery selections have to be thrown out of window.” In diesel-electric installations the turbine can run at either 3,000 or 3,600 rev/min (50 or 60Hz) and directly couple to the alternator. Rolls Royce estimates the weight of the generating package would be around 120t. This includes the turbine, recuperator and control gear with a directly coupled 58t generator all mounted on a common bedplate.

As to emissions, the unit produces NOx at a rate of around 4 g/kWh but there is the option of DLE (Dry Low Emission) combustors that would take the figure to 1 g/kWh or below. These are already used in land-based turbines simple cycle but as the standard system produces emissions far below the IMO?s limits, there is no pressure to fit such nozzles and they have not been used in cyclical operation.

The idea of recuperation has been around for almost as long as the gas turbine itself, but only with the latest technology and control has it been possible to produce a viable version in terms of size, weight and form. It is at the lower pressure ratios that the recuperator shows the greatest benefit, and by 20:1 it shows no advantage over a simple cycle turbine. The reason is that as the pressure ratio rises, so does the temperature of the compressed inlet air. This lessens the temperature difference between the inlet air and exhaust, thereby reducing the heat transfer within the recuperator. This can only be regained at the expense of an unacceptable increase in back pressure.

Manufacturers have increased the pressure ratios in larger simple cycle turbines to improve efficiences ? for instance the LM2500+ has a 23.1:1 pressure ratio against 18.8:1 in the LM2500. This means the smaller turbines would benefit most from recuperation, but they cannot withstand the additional on-cost. To counter this, Rolls Royce is introducing a spiral-wound recuperator manufactured from two sheets of high temperature alloy sandwiching a third piece that is concertinered. This sandwich is then simply wound into a scroll with the hot gasses passing through the concertina and cold air running through pre-set gaps between the plain sheets. As the design lends itself to automated manufacturing, the unit price will be dramatically lower than the individually manufactured units on the WR21.

It is envisaged that the spiral recuperator will be of pre-set sizes with multiple units arranged to accommodate the maximum gas flow of each installation. A test unit at Rolls Royce weighs around 500kg and would provide 1MW of heat transfer. To put this in perspective, about 16 of the spiral-wound units would be required to replace the single recuperator on a WR21. However, it is turbines in the 4 to 20MW (5,450 to 27,200 bhp) range that are likely to benefit from this technology.

Meanwhile GE Marine remains the only turbine manufacturer with products aboard cruise ships. GE?s director of marine sales and marketing, Mike McGuire, sees Royal Caribbean?s move to exercise options on more turbine-powered vessels as confirmation of its confidence in the product. Ever since the initial announcement Mr McGuire says there has been a high volume of enquiries. He is hopeful of an order from P&O, which has also confirmed an order with Chantiers de L`Atlantique although the decision of which turbine to select has not been finalised. Having lost out to Rolls Royce for the FastShip order he is taking nothing for granted. The marinised LM6000 (45MW/ 61,180 bhp) that it offered for FastShip remains available. Mr McGuire says currently there are no orders for the engine although there have been other enquiries.

There are now over 60 of GE?s LM range turbines operating or on order for marine use ? the largest number being for the LM2500. By adding an extra compression stage the LM2500+ was created to develop 25MW (34,000 bhp) with a fuel consumption figure of 221.4 g/kWh (0.359 lbs/hp/hr). While the LM2500 has a slightly higher fuel consumption and lower power (22MW/29,900 bhp), Mr McGuire says the two appear to be complementary as enquiries are running at a similar level for both. He believes that one reason may be that the current generation of fast ferries have been designed for engines of 22MW (29,900 bhp), and the LM2500+ would be too powerful.