Assessment of Recuperator Materials for Microturbines

Microturbines in production (or nearly in production) use metal recuperators with gas inlet temperatures of less than 700 C to raise their efficiency to about 30%. To increase their efficiencies to greater than 40% (which is the DOE Advanced Microturbine Program goal) will require operating at higher gas inlet temperatures, if the compression ratio remains less than 6. Even at higher compression ratios, the inlet temperature will increase as the efficiency increases, necessitating the use of new materials of construction. The materials requirement for recuperators used in microturbines may be categorized by their maximum operating temperatures: 700, 800, and {approximately}900 C. These limits are based on the materials properties that determine recuperator failure, such as corrosion, oxidation, creep, and strength. Metallic alloys are applicable in the 700 and 800 C limits; ceramics are applicable in the 900 C range. Most of the heat exchangers in the current microturbines are primary surface recuperators (PSR), compact recuperators fabricated in 347 stainless steel by rolling foil that is a few (>5) mil thick into air cells; the metal recuperators are operated at temperatures below 650 C. Preliminary research indicates that the use of 347 stainless steel can be extended to 700 C. However, additional directed research is required to improve the current properties of 347 stainless steel and to evaluate extended demonstrations on recuperators fabricated from it. Beyond 700 C and up to about 800 C, advanced austenitic stainless steels or other alloys or superalloys become applicable. Their properties must be measured in the expected operational environment, and recuperators fabricated from them must be evaluated for an extended period. Temperatures beyond 900 C exceed the limits of metals, and ceramic materials will be needed. The relevant properties of Si{sub 3} N{sub 4} and SiC, (creep, corrosion, and oxidation) have been studied extensively. Prototype ceramic recuperators have been fabricated from both cordierite and RBSN; consequently, their properties and those of other low-cost applicable ceramic materials need to be investigated further. Because no ceramic microturbine recuperators are in production, it will be necessary to fabricate prototype units and evaluate their properties over an extended demonstration period. A comprehensive workshop for those involved in recuperators for microturbines is recommended to determine how the technology can be accelerated to support the development of ultra-efficient microturbines. The immediate emphasis should be on the cost-effective manufacture of higher-temperature metallic recuperators; the development of ceramic recuperators should be considered a long-term objective.