Research and development activities in electric power generation have three principal goals:
Strategic Matrices for Materials Development
The relative significance of materials development goals can be derived from a two-axis representation covering overall R&D priorities in materials research as derived from scientificmeetings, publications, and funding information, and the strategic importance of materials forproducts. Figs. 1 and 2 present such diagrams for metals and inorganic non-metallic materials. The two above-mentioned axes define four strategically relevant areas for electrical engineeringcompanies involved in power generation technology. Starting clockwise from the upper left,these are:
The first step in attaining higher steam temperatures will nevertheless continue to depend onimproved ferritic steels that can be pushed to their design limits. The reliability of such materialscan be improved by optimizing fabrication sequences, especially for large components such asvalve casings and turbine shafts. Materials development in this area is therefore based to a largeextent on the cooperation of steel manufacturers.
Materials testing is an integral part of development design, fabrication and operation ofcomponents in power generation. The major areas to be studied pertaining to steels up to 600 degrees Celciusare:
Since the results of such investigations are of general interest, associated studies should bepredominantly conducted by public R&D institutions.
Advanced Materials Development for Gas Turbines
Interest in the development of gas turbines that can operate at above 1000 degrees Celcius is giving impetus tothe development of materials with suitable creep rupture strengths. Such materials could increasegas turbine efficiency by allowing higher gas inlet temperatures. While the gas temperature at theburner is limited only by NOx formation and can easily attain 1500 degrees Celcius, the creep rupture strengthof the first row blades limits the maximum operating temperature to 850 degrees Celcius at the material's surface. Advanced high temperature materials may be able to obviate internal blade cooling,which reduces efficiency, and to double the temperature gradient over the turbine.
This would in turn permit increased outlet temperatures -- a highly desirable outcome whenapplied to combined-cycle plants. Figure 1 provides a development timetable for such materials.
The first step in this process -- the development of materials that can withstand surfacetemperatures up to 1100 degrees Celcius -- is being carried out in cooperation with precision casting andpowder metallurgy companies. Turbine manufacturers are setting specifications for materialssuppliers. In this connection, the following areas are being studied:
Step two, which involves surface temperatures above 1100 degrees Celcius, will require considerable basicresearch to understand the structure and properties of new intermetallic compounds and possiblybulk ceramics, as well as to determine how to attain better creep rupture strength, toughness andcorrosion resistance at high temperatures.
Both step one and step two could benefit from R&D collaboration with the aircraft industry, asadvanced jet engines often provide insights into the development of stationary turbines.
High Temperature Superconductors
A second major research area in materials development for the generation of electric power isceramic high temperature superconductors (HTSCs). Such materials will make it possible toreplace liquid helium cooling with liquid nitrogen, which would result in reduced insulation andoperating costs.
The prerequisite for applications in this area is an improvement of the maximum current density ofthe conductor under a magnetic field. Obviously this will require intense basic research regardingmaterials processing and future manufacturing methods. For example, the stages of developmentof an HTSC cable are given in Fig. 5. The ideal type of conductor material is still beingresearched, as is the microstructure of the compounds. The single conductor with filaments havebeen realized, but the manufacturing process for it is still being debated. The successfulcompletion of these projects will lead to lighter, more efficient transformers and generators.
Another HTSC example is a current limiting device, which relies on the suppression ofsuperconductivity in a strong external magnetic field, and does not require any moving parts orswitch contacts, thus being free of wear. High-field magnets may find applications in electric loadleveling systems, where they could store energy in a magnetic field, offering fast retrieval and lowloss.
High Temperature Fuel Cells
Although low temperature fuel cells and, to a lesser extent-medium temperature types, haverecently found application areas, high temperature solid oxide fuel cells (SOFCs) are still in theirinfancy. Some of the major areas that remain to be answered in connection with SOFCs are:
In connection with these research efforts, interdisciplinary investigations in materials science,electrochemistry, and chemical engineering must be coordinated with studies of other types of fuelcells. Furthermore, these studies should consider SOFCs' potential as a "topping" element whenintegrated into a combined-cycle gas and steam power plant. In such plants, the hot gas exhaustcan be fed into the gas turbine; thus allowing the SOFC to replace the burner unit, and -- as adevice outside the Carnot efficiency-to improve the efficiency of the whole system.
Materials for Nuclear Power Plants
Even if technological progress in nuclear power generation is retarded as a result of reducedpublic acceptance, cooperative efforts in terms of developing materials for this energy area areessential. This is becoming all the more true as resources become scarcer and as environmentalrestrictions are tightened. In this connection, the industry is concentrating principally oncommercial PWR and BWR plants by: refining methods for life time assessment with regard tothe properties of materials and interactions caused by operating conditions; developing on-sitemonitoring techniques for an accompanying verification system; and pushing materials resistance,design and operational safety margins even further.
On the other hand, the results of R&D studies pertaining to advanced nuclear reactors such as fastbreeders and high temperature reactors should be pushed forward, or at least maintained byresearch institutes, which are in a position to apply this information to solving materials researchquestions, such as the problem of inclastic material propertics, e.g. stainless steels; modelling ofcreep-fatigue processes, and verification of conformity with regulatory values for long-termoperation above 105 hours.
Summary
Improvement of the long-term stability of system components under high temperature is one ofthe main challenges to materials development in electric power engineering.
Breakthroughs in this area improve the efficiency of power plants, and thus save resources whilehelping to protect the environment. In this connection, power plant suppliers are called upon toset ambitious goals for development and realize them in close cooperation with materialsproducers and research institutions.
Article reprinted from Siemens Review, Spring issue.