Energy Higher Efficiencies
The Power of Small Steps
The efficiency of power plant turbines continues to increase. Considering their output, even small steps can result in major savings for their operators.
Modern turbine blades are masterpieces of precision. Their three-dimensional shapes are optimized on computers. Tiny cooling channels and new coating materials or alloys offer protection against high temperatures
They look like an Oscar or one of the other numerous film, TV or music awards that are handed out each year: heavy and valuable, but somehow deformed. While these solid lumps of silvery metal have never won any prizes, they have played a crucial role in setting new technological records. Thousands of these turbine bladessome as small as a human hand, others the length of an adult's armare milled each year at a Siemens plant in Mülheim an der Ruhr, Germany. Threaded into rings on thick steel shafts, they propel Siemens turbines to unique levels of efficiency: 48.5 % for steam turbines on their own and 58 % for combined-cycle gas and steam turbines. And when the next generation of plants arrives, the company plans to top the 50 and 60 % marks, respectively.
While that doesn't sound like much of a difference, it represents a major step forward in a field where engineers measure progress in terms of a 0.1 % increase in efficiency. Such improvements are by no means only for the record books. In German combined-cycle power plantswhere one gas turbine and up to five steam turbines work together to exploit every last quantum of energy65 % of the total costs of procurement, maintenance and fuel are accounted for by natural gas. Improved efficiency means lower fuel consumptionand thus also a reduction in harmful emissions. "To fulfill the terms of the Kyoto agreement, we'll have to build as many combined-cycle plants as possible," says Bernard Becker, head of Gas Turbine Development at Siemens.
The blades play an important role in helping to boost efficiency. Their job is to ensure that as much energy as possible goes toward driving the generator. What's more, they have to be able to withstand unimaginably high temperatures. The special thing about Siemens' blades lies in their shape. Known as 3DS blades because of their distorted three-dimensional contour, they have nothing in common with their predecessors of 30 years ago, which basically looked like the blades found in a jet engine.
Some jobs still have to be done by hand. Here, hundreds of blades ranging from the size of a hand to the length of an arm are threaded onto the steel axes of a turbine
The blades' crooked outline is the product of computational fluid dynamicsCFD. Using powerful computers, engineers can simulate the flow of gas and steam through the turbine and calculate the thermodynamic variables and forces involved. In a new development, this can even be done separately for each blade ring in the turbine. "This means we can improve efficiency for individual turbine parts by up to 2 %," says Uwe Hoffstadt, who is responsible for Steam Turbine Development at Siemens. The latest generation of bladethe "3DV"will be used for the first time ever in a power plant in Niederaußem, Germany, which is scheduled to come on line this year.
CFD can also be used to optimize the aerodynamics of the piping and valves in a turbine system and thus reduce the frictional losses associated with moving gas or steam. This can be achieved using guiding plates and steam sieves. "We've improved aerodynamic efficiency to as much as 92 %," says Becker. "And we might be able to achieve a further 2 or 3 %."
Supercomputers simulate the flow of steam and gases through a turbine and the forces they exert on the blades. These computational fluid dynamics techniques can then be used to optimize turbines
In recent years, steady improvements in efficiency have also been made in other areas. For example, researchers have determined that steam for the turbine should be provided in a spiral flow, and that optimized sealing tips ensure reduced steam losses at the outer ends of the blades. In addition, research has revealed that a thermal jacket cuts heat losses and guarantees that heat-induced deformation of the turbine housing is rotationally symmetrical, while a device known as a twist cooler expands the steam before it hits the first row of blades. In short, turbine manufacturers exploit all the tricks they know to squeeze out a few tenths of a percent of extra efficiency.
Turbine efficiency depends primarily on thermodynamics. In simple terms, this means that the maximum amount of energy produced by the generator cannot exceed the difference in energy between the gas or steam when it enters and when it leaves the turbine. Were it possible to make the inlet temperature infinitely higher, you would eventually get a turbine with 100 % efficiency.
Why Gas and Steam Turbines Make a Top Team
Gas turbines: Fired with gas. Inlet temperature: 1,550 °C (blades must be cooled), pressure:17 bar, efficiency: 38 %.
Steam turbines: Fired with coal or oil. Inlet temperature: 600 °C, pressure: 270 bar. World record for efficiency: 48.5 % (power plant in Boxberg, Germany, output: 907 MW, Siemens turbine).
Combined-cycle power plants: Combination of a gas turbine, whose residual energy heats a boiler that supplies up to five steam turbines. World record for efficiency: 58.4 % (power plant in Mainz/Wiesbaden, Germany, Siemens).
In practice, engineers increase the inlet temperature in small stages. With gas turbines, it has risen from 1,100 to 1,230 °C (average of cooling air and hot gas) over the last nine years. As this also means an increase in the outlet temperatureand therefore no gain from a thermodynamic point of viewdesigners have incorporated a combined-cycle power plant that uses the excess energy to generate steam. The latter drives high-pressure, medium-pressure and multiple low-pressure steam turbines located downstream. By the end of the cycle, the steam has cooled to 45 °C. Efficiency has risen from 52 to 58 % since 1992. "But," says Becker, "over 70 % is mathematically impossible." In coal-fired plants, which only use steam turbines, the current record is 48.5 %. Around 25 years ago, it was only 40 %. "50 % is certainly attainable," says Hoffstadt. Indeed, this milestone should be reached later this year at the Niederaußem plant.
In regard to combined-cycle plants, not having to cool the combustion gas from 1,550 to 1,230 °C before it hits the blades of the gas turbine would represent a major step forward. Even at 1,000 °C, the glow of the blades is so intense that you could read by the light produced. Today, cooling is achieved using air fed in through small holes that subsequently covers the blades with a thin cooling layer. However, this air also reduces efficiency. General Electric is therefore concentrating on an alternative technique whereby steam diverted from the steam turbines is used to cool the gas turbinesa solution that Siemens is also working on, albeit only for cooling the blades of steam turbines.
Turbine producers are also trying out new materials that can withstand the enormous heat present inside gas turbines without any need for cooling. Today, nickel-based alloys comprising a wide range of metals are used. These are produced as single crystals and have to tolerate not only extreme heat, but also centrifugal forces some 10,000 times higher than the gravitational force. The only way of withstanding even higher temperatures would be to cover the metal with a ceramic layer. A layer only 0.2 mm thick can reduce the temperature of the underlying metal surface by 150 °C. However, currently available silicon nitrides and silicon carbides burn up over time and are too brittle.
These are also the biggest obstacles when it comes to making blades from solid, fiber-reinforced ceramicsa dream of all turbine makers. Compared to a conventional turbine of the same size, the output of such a non-cooled turbine would rise by 40 %, and the efficiency by 20 %.
Even if such a fantastic material existed, 1,550 °C still represents the upper limit. Otherwise, there would be environmental problems due to the fact that emissions of nitrogen oxides increase fivefold for every 100 °C increase in flame temperature.
Price also places a limit on improvements in efficiency. As Becker knows from experience, not everything that is feasible is affordable. Besides, the top priority for his customers is neither efficiency nor environmental compatibility. As he points out, "More than anything else, a power plant must be reliable."
Bernd Müller