High Temperature Materials

Virtually all of the technical innovations in power conversion and energy storage are expected to face serious materials/metallurgy limitations. Materials breakthroughs on several levels are required to bring to market some of the most potent technologies. Vapor cycle systems that can operate above 700°C and 35 MPa are but one example. Achieving efficiency levels of 45%, as a minimum, will require applications of specialty materials. Most of the supercritical plants built today use ferritic steels (with costs less than $1,000 per tonne of steel, see the top of Fig. 10.6). To reach ultra-supercritical conditions (those beyond 620°C) will require materials not widely used in the vapor power cycles nickel-based materials that can reach 70x the cost. Also revealed in Fig. 10.6, current technology (or perhaps better described as current affordable technology) has stayed below the critical 620°C threshold for a reason—cost being one of the biggest factors.

Many of these specialty materials are extensively used in gas turbines; hence, there is a good knowledge base to start with. However, gas turbines have several key attributes that make the use of specialized metallurgy possible—for example,

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Fig. 10.6 Material requirements for power generation. At temperatures above 620°C, material costs increase rapidly convective cooling of hot section components that are much smaller in size. This approach to thermal management is not practical in the vapor power cycle. While the highest-pressure ratio gas turbines operate at 20-30 atm, vapor power cycles are expected to operate in a range five to ten times greater to achieve the desired efficiency objectives. Specialty alloys will be required to continue to improve the thermal performance of the most widely used thermal cycles—the Rankine and Brayton cycle.

There are serious barriers to continuous operation above the 620°C limit for ferritic steels. Material properties such as bulk modulus, melting points, eutectics, conductivity need to be mapped out. Because of these limitations in the available materials, much of the world's steam fleet operates well below the 620°C limit, and nearly all of it operates below metal temperatures of 565°C.

Perhaps even more challenging than employing the right materials is to find materials that are affordable. Nickel based alloys could cost as much as 70x the current price of ferritic steels. HP and IP rotors weighing 30-40 tonnes would become very expensive if they were to be manufactured in the same configurations as today's high performance gas turbine units. Economic factors, namely the high price of super-alloys, will play a dominant role in the design approach of these advanced cycles. Piping headers and tubing would have to be made from similar materials, and require a greater level of skill in welding and assembly. All of these are acting in direction of substantial increases in plant costs, even above the seemingly inflationary rates for construction costs being experienced in 2007-2008.

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