In the past years, the fact that fossil energy resources are limited has stimulated research in several directions. These include the investigation and the development of alternative and sustainable energy sources, but also increasing the efficiency of both the generation and the use of energy.
Regarding the energy efficiency of power plants hydro and tidal power plants are the most efficient technologies arriving at efficiencies of around 90 %. These are followed by large gas fired combined cycle gas turbine (CCGT) plant which have proven efficiencies of 59 %. These combine a gas turbine together with a steam turbine. Energy in form of heat present in the gas turbine’s exhaust gases is used to produce steam which allows the creation of additional energy with a steam turbine, thus increasing the plant’s overall efficiency. This technology is mainly dominated by players such as Alstom Power, Mitsubishi, General Electric and Siemens. Actually, the latter one has recently claimed to have even reached the threshold of 60 %.
Ten years ago Siemens started to aim on improving the efficiency of the CCGT technology above a threshold of 60%. On the 11th of May 2011 the German Technische Überwachungsverein TÜV Süd (Technical Inspection Association ) participated in the control of the performance of a natural gas power plant unit in Irsching, close to Ingolstadt, Germany. The scope of the test was to determine an official figure of the efficiency of unit 4 of the Irsching power plant belonging to E.ON group. Indeed, the test revealed that the envisioned goal was achieved . In July 2011 Siemens will hand over the plant to E.ON whose normal operation power equals 550 MW.
To achieve their goals Siemens invested about 500 Million of Euro and employed 250 engineers and 500 technical personal. In particular, a key to the success was based on the research of new materials allowing that the gas turbines can deal with gas-air mixture temperatures of 1500 °C which is essential in the efficiency increase. The exhaust gas has a temperature of 625 °C heating water which evaporates and which then drives a steam turbine with a temperature of 600 °C and a pressure of 170 bar.
The critical aspect regards the gas turbine blades which are composed of a nickel-alloy. They are protected by a layer of cobalt, nickel, chromium, aluminum and yttrium, so called MCrAlY mixtures, together with about 1% -2% of rhenium. Rhenium is one of the rarest elements present in the earth’s crust and it has a melting temperature of 3186 °C, thus, Rhenium only tungsten and carbon have even higher melting temperatures.
At high temperatures an aluminum oxide layer forms on the MCrAlY layer protecting the turbine blade material from the oxide present in the burning gas. Hereby, the rhenium increases the mechanical properties of the protection layer and prevents that the aluminum diffuses into the turbine blade base material. The protection layer has a size of 300 µm and, in addition to its protection function, it acts as a bonding agent for the heat protection ceramics which is essential to reduce the temperature on the blade’s metal base to a figure of 950 °C.
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