Turbines do have a simple but crucial role in power generation; they are converting a chemical (fuel) or physical (from steam) energy into a kinetic one, the rotation needed to drive a generator to convert it into electrical power.
While in smaller systems – depending on the application – there is the alternative to burn fuel in reciprocating engines or fuel cells, there is no other way to do fuel or steam based power conversion in large scale systems.
The IEA estimates that of all efforts required to deliver a 50% reduction in global emissions by 2050 24% will need to come from end use fuel efficiency, 12% has to come from end use electricity efficiency and a further 7% will need to come from power generation efficiency. There is substantial potential for improving thermal efficiency of Europe’s power plants. Our coal plants operate at an average 38% (BAT - Best Available Technology - on new coal plants delivers 46%). Our gas plants operate at an average of 52% efficiency (BAT- Best Available Technology - on new gas plants delivers more than 60%). Due to the age of the installed base, the average efficiency of Chinese coal plants is now higher than in Europe.
Advanced flow path design
The potential for improvement of gas and steam turbine efficiency by aerodynamic optimisation adds to the contributions of cooling and sealing air reductions. In the high pressure end of the turbine with short blades and vanes the secondary losses constitute a large portion of the total losses. These losses can be reduced by introduction of advanced endwall shaping, 3D features in the aerodynamic design of airfoils and clearance control. Also the detailed design of the endwalls, steps between platforms and interaction with purge flows from rotor stator cavities can be improved
Next generation hot gas path concepts
Increased gas turbine performance is very much related to an increase of the Turbine Inlet Temperature (TIT). But the coolant mass flows will need to be minimised to achieve as high performance benefits as possible. Advanced cooling systems concepts should be developed for engine first stage components and tested experimentally in test rigs (existing and new rigs, like liquid crystal rig, thermal imaging rig, film cooling rig as required, and in new, cold flow, low cost, component internal cooling test rigs). New cooling surfaces as well as new cooling schemes should be studied by CFD modelling to use the coolant in the best possible way before it will leave the component. To improve turbine efficiency today’s levels of cooling air, leakage air and sealing air (air to keep the hot gas in the flow path) must be heavily reduced
A phenomenon that must be further addressed in this context is hot gas ingestion that can cause unacceptably high rotor temperatures. We need to develop more advanced technologies that will handle hot gas ingestion to make sure that the hot gases will be confined in the cavity without reaching the rotor itself. To use new sophisticated cooling methods based on porous structures a new thinking is necessary. Analysis methods and design concepts and criteria must be developed and tested for such structures in order to optimize the design for components with porous structures.
High temperature materials in gas turbines have properties that change significantly during the expected life of the component due to thermal exposure, mechanical load and the combination of the two. Issues that affect lifing and reliability and can cause serious problems are e.g. crack propagation commonly due to creep or fatigue. In many components early TBC (Thermal Barrier Coating) spallation will increase the material temperature of the component and reduce the safe life for which the component can be used. This makes it difficult to utilize existing materials in a safe and efficient manner, which implies that we now suffer from too low efficiency (CO2 penalty) and too high costs. One measure to improve crack propagation estimates in high temperature components and that will be even more pronounced for new materials like composites is to develop ‘long test time’ rigs with a low cost per test hour, run qualification tests, perform basic Modelling and engine testing and compare with engine tests. This will also make it possible to improve and assure the quality of ‘safety factors’ in the real design of turbine components
New combustor designs for advanced gas turbines
In combustors of premixed flame type pressure oscillation phenomena often referred to as ‘combustion dynamics’ are common. Combustion dynamics often start in the linear regime but if coupled to the heat release of the premixed flame it might cross over into the non-linear regime and very rapidly reach damaging levels. Typical amplitudes of such pressure oscillations may easily reach 5% of the mean pressure level and are highly destructive. These difficulties are typical for low emission combustion systems where they often may be avoided by adjusting the ratio of fuel to the pilot passage as compared to the main passage. Increased ratio leads to increased NOx emissions and reduced ratio may lead to increased CO emissions and unburnt hydrocarbons.
Therefore combustion stability is utterly important for low emission systems. The overall objective for research work in this field should be to improve knowledge and Modelling capabilities (both flow and acoustics) in the field of combustion related pressure dynamics and emissions, and also on measures to damp the combustion dynamics by for instance using liner walls, i.e. porous walls of various types, which gives potential of emission reduction. Modelling efforts should include investigations of the merits of using an approach based on the Navier-Stokes’ equations (NSE), either through linearised NSE or unsteady approaches and to evaluate and further develop existing eigenmode extraction tool(s) for realistic combustor cases.
Flexibility – enabling intermittent renewables
Highly flexible power generation will enable all energy sources to become partners. Electric power generation and load demand (including grid losses) constantly require balance. Any imbalance between power generation and load demand results in a system-wide deviation of the frequency from its nominal value (50 Hz). The necessary control and balancing power is provided mainly by power generating facilities by means of primary, secondary and tertiary control reserve.
To maintain the voltage in acceptable ranges throughout the network and to prevent the transmission systems from voltage collapses, the Generation Units have to be able to provide reactive power to the network within a definite range. Shortage of reactive power can lead to unacceptably low voltage levels and finally to a voltage collapse of the system
Advanced large Nuclear Power Plant Steam Turbines
Until recently, large power plants including NPPs have been expected to cover mainly the base load electricity generation. The recent developments have shown the necessity to change this approach – load changes with short reaction times are required even from medium and all large power plants gradually shifting such requirements also into the NPP segment. This is predominantly due to the increasing volume of the energy produced from the renewable sources. (cf. chapter on flexibility)
While innovative concepts of reactors for so called Generation IV are currently underway, fast load change capability and flexibility will be required both for existing and newly built nuclear power plants with reactors of the Generation III and III+. This means that the future development of turbines, their auxiliaries, control and diagnostic systems for future NPPs must reflect this changing environment. Reliable turbines with flexible characte-
ristics and long operational life are needed for current and future NPPs.
In fact, the flexibility is expected from existing turbines while maintaining or even extending their original design lifetime.