Innovative cooling concepts – including transpiration via "pores" in porous structures using laser drilled hole fields, metallic foams and sintered materials – combined with novel thermal barrier coatings have the potential to increase the efficiencies of combined cycle plants to unprecedented levels, according to an R&D project currently underway in Germany.
The collaborative research activity known as SFB 561 – “Thermally highly loaded, porous and cooled multi-layer systems for combined cycle power plants”, to give it its full title – was established in 1998. It brings together specialists in aero-thermo-mechanics, structural mechanics, materials science and production technologies, with the aim of establishing the basic knowledge base for creating a combined cycle power plant with an efficiency of 65%. The programme is funded by the German Research Council (DFG), with a current budget of about 9 million euros distributed over three 3-year periods. It presently consists of 15 sub-projects and involves 14 institutes located within RWTH Aachen University, Research Center Jülich and the German Aerospace Center, Cologne.
Compared with the present state-of-the-art – which is typically an ISO turbine inlet temperature of 1230°C in the case of gas turbines and live steam temperature of 580-620°C for steam turbines – the operating parameters of these future plants will have to be upgraded considerably. To achieve a total efficiency of 65% it will be necessary to employ gas turbines with combustion chamber outlet temperatures of 1520°C at a pressure of 17 bar. To reach such process parameters will require the development of new materials, including thermal barrier coatings, in combination with improved cooling technologies. But steam cooling of gas turbine components is not needed, and indeed the avoidance of the need for steam cooling, of either rotating or stationary gas turbine components, is a key feature of the SFB 561 effort. Instead SFB 561 is focused on innovative air cooling concepts and novel thermal barrier coatings for gas turbines, while steam cooling is applied to steam turbine components.
Improvements to the gas or the steam turbine will only achieve their full potential if the machines are fully matched to each another in terms of process parameters.
There are basic limits on the possibilities of increasing operating fluid temperatures solely by developing advanced structural materials, such as 9-12% Cr steels (for steam turbines) and single crystal nickel alloys (for gas turbines) – ie without improved cooling and thermal barrier coatings (see Figure 1).
The anticipated stages in the development of the new technologies required for a future combined cycle power plant with an efficiency of 65% were projected at an early stage of the SFB programme (see Figure 2).
In the first research grant period it was shown that the production of open porous structures using drilled hole fields, metallic foams and sintered materials for application in combined cycle power plants is possible in principle. The aim of the second period was further development of the basics of production techniques for these porous materials. The requirements, in terms of geometries and materials properties, that are dictated by the application of the new technologies in power plants were given special attention.
During the present phase design and manufacturing methods for the new technologies are being validated by meaningful experimental data.
The production technologies need to be developed in a direction that makes it possible to manufacture porosities with a specified gradient. The manufactured parts need to be proven in a subsequent phase under conditions close to reality (as far as the cooling fluids are concerned) to check that the newly developed material systems can fulfill their tasks of cooling and suction. After the conclusion of this fourth phase the insights gained and technologies developed will be placed at the disposal of the industry.
The new technologies being developed under SFB 561 are intended for use in the combustion chambers and blading of the gas turbine, in the hot steam section of the HP and IP stages of the steam turbine and in the water draining section of the LP stage of the steam turbine. In all these regions the plan is to use multi-layer materials to either achieve more effective cooling of highly loaded parts, ie with less cooling fluid, or to reduce aerodynamic losses and thus raise the total efficiency of the power plant.
New gas turbine technologies
With film cooling, which is the state-of-the-art technology in modern gas turbines, the cooling fluid exits from single holes. It can suffer from local lift-off of cooling jets, leading to a reduction in cooling effectiveness and, thus, a higher thermal load on the cooled surface. These effects, which reduce the life span of a part significantly, can potentially be avoided by adopting effusion cooling. In effusion cooling the cooling fluid is blown out of small holes (“pores”), distributed over the entire surface of a thermally highly loaded component, thus increasing the effectiveness of cooling and helping to save cooling fluid and increase efficiency (Figure 3). In the future transpiration cooling (as used in the pores of the skin) is contemplated.
The insights gained so far in the SFB 561 programme attest that the application of porous cooled structures shows great promise.
Theoretically, applying ZrO2 thermal barrier coatings would make it possible to increase the turbine inlet temperature by 150 K, but this potential cannot be exploited at present because the life span of these coatings has not yet been accurately assessed. By changing to effusion cooling the tendency for blistering to occur should be reduced and at the same time the amount of cooling fluid required should be decreased. Both will help to increase power plant efficiency. The envisaged efficiency will require the combination of thermal barrier coatings with effusion and/or transpiration.
Figure 4 shows a schematic of a transpiration cooled gas turbine combustion chamber wall and blade. The open porous intermediate layer is supplied with cooling fluid through a number of chambers in the structural material. The intermediate layer is protected from the hot gas by a permeable thermal barrier coating. At the surface, ie, on the hot gas side, the cooling fluid escapes over the whole of the surface, thus ensuring the development of a homogeneous cooling film.
The SFB 561 programme is investigating two different ways of doing this: fields of laser-drilled, contoured holes; and open porous metallic foams, perhaps made of oxidation resistant NiAl alloys.
A homogeneous cooling film is achieved on the surface by the precise contouring of the holes and their geometrical distribution. A laser drilling procedure which succeeds in driving the molten material from the drilled hole has been applied to the manufacturing of cooling holes through multi-layer systems, consisting of a substrate, a bondcoat and a ceramic thermal barrier coating.
The reduction of the amount of cooling fluid in the combustion chamber and first stage guide vane will lead to increased temperatures especially for the first stage blade. To avoid a big increase in cooling effort for the gas turbine blades, a new approach to blade design is being developed as part of the SFB 561 programme, in particular a manufacturing method is being elaborated for including Al2O3 fibres in a NiAl matrix for the load-bearing core of the blades (see Figure 5). The first investigations into the required fibre coatings and boundary layer reactions have been concluded. In the field of investment casting for the manufacturing of gas turbine components from new materials, a major effort has been put into the development of new mould ceramics and core materials and have been applied to the manufacture of samples.
New steam turbine technologies
As the cycle analysis results of the SFB 561 programme have shown the steam inlet temperatures for the HP and IP stages of the steam turbine needs to be raised to about 690°C (at 30 MPa), but without application of a highly efficient cooling technique the 9-12%Cr steels, which are used in this region in modern steam turbines, are no longer applicable due to corrosion and oxidation. And use of nickel alloys in this application would increase manufacturing costs dramatically.
Therefore, a cooling technology based on the use of porous materials is being developed for these regions. The porous material consists of welded multi-layer sheets consisting of woven wire mesh and a covering plate. Steam, as the cooling fluid, is led through the woven mesh region to cool, in a first step, the casing of the steam turbine in the inlet region. By application of multi-material mesh and plate construction, the thermal durability of the structure can be adopted to the local requirements.
In the wet steam region of the LP stage of the steam turbine the condensing water has to be removed from the flow to reduce damage to vane surfaces due to erosion by impinging droplets. The water is directed through hollow vanes and through ducts in the casing out of the flow path. Finally, it joins the main flow of condensed water again so that the cycle is closed. By application of open porous inserts both in the casing and in the vanes, near their roots, aerodynamic losses attributable to the water draining system could by reduced compared with conventional slot technology due to draining of the water over a wider area and the smoother surfaces achieved. In Figure 6 a sample can be seen with open porous insert that was sintered into the surrounding plate. With this technique, three dimensionally curved surfaces may be realisable in the future so that the casing and vane contours can be reproduced.
Evaluation of the results
To evaluate potential efficiency increases attainable by the technologies being developed under the SFB 561 programme, the Tapada do Outeiro combined cycle power plant in Portugal was selected as a reference point reflecting current state-of-the-art. The power plant is of the single shaft configuration, with unfired heat recovery steam generator, three-pressure evaporation and simple reheating.
A model for the process analysis was developed on the basis of this reference power plant. This model is detailed enough to contribute to the evaluation of the overall efficiency increase from both the gas turbine components and the steam turbine components of the SFB 561 programme. The steps needed to attain the goal of a total efficiency of 65% can be analysed and, through close co-operation with industry advisors, the focus for future work specified.
In order to pass on the goals that have to be achieved to other sub-projects and to be able to process the new insights from the different sub-projects, a flowchart for formalising the information flow was developed and copied to all sub-projects. The items which need to be exchanged to achieve smooth co-operation, such as process and flow field data, coated and uncoated material samples, material data and manufacturing parameters, were co-ordinated according to the work and time schedules among the co-operating sub-projects.
In all three project areas challenging targets, for example high material and fluid temperatures, must be dealt with. The current investigations show that the gas turbine inlet temperature must be increased by about 300 K compared with today’s state-of-the-art, while at the same time the cooling fluid supply must be more than halved. The material temperatures in the steam turbine will also increase, despite application of the improved cooling techniques, due to the increased steam temperatures.
Table 1 summarises the goals for the first three phases of the SFB 561 programme.
On the basis of progress with the individual projects during the first phase of SFB 561 (ie three years in) a combined cycle efficiency of 61.7% is calculated to be attainable. This is attributable to advancements in transpiration cooling of the gas turbine and improved steam turbine performance thanks to modifications in steam cooling, water removal and materials.
At present (after completion of phase 2) an efficiency of up to 63.3% is estimated to be achievable – due to better understanding of flow conditions, improvements in manufacturing procedures and an increase in material durability (see Figure 7). With further improvements – in particular introduction of porosity grading – the new concepts are judged to be capable of delivering further efficiency increases. A reduction in cooling fluid flow and an increase in permissible material temperatures can lead to the desired efficiency of 65%, it is estimated.
Aiming for 70%
But this is not necessarily the end of the story. The SFB 561 programme concentrates primarily on the improvement of the flow and component technology, with the primary goal of achieving better cooling. But when this is coupled with measures elsewhere in the plant, such as innovative process control, there exists the possibility of increasing the efficiency of combined cycle power plants to over 70%.