Carbon-carbon composites as recuperator materials for direct GB systems

Of the numerous energy conversion options available for a space nuclear power plant, one that shows promise in attaining reliable operation and high efficiency is the direct gas Brayton (GB). In order to increase efficiency, the GB system incorporates a recuperator that accounts for nearly half the weight of the energy conversion system. Therefore, development of a recuperator that is lighter and provides better performance than current heat exchangers could prove to be advantageous. The feasibility of a carbon-carbon (C/C) composite recuperator core has been assessed and a mass savings of 60% and volume penalty of 20% were projected. The excellent thermal properties, high-temperature capabilities, and low density of carbon-carbon materials make them attractive in the GB system, but development issues such as material compatibility with other structural materials in the system, such as refractory metals and superalloys, permeability, corrosion, joining, and fabrication must be addressed.

The energy conversion concept chosen by the NRPCT for project Prometheus was a direct gas Brayton system utilizing a He-Xe coolant/working fluid. The closed Brayton cycle energy conversion system (ECS) for space applications has been studied extensively by NASA since the 1960s. The aerospace and commercial power industries commonly use Brayton systems, and there is a substantial amount of design and operational experience.

One of the critical components in the GB system is the recuperator, which is responsible for significant enhancement of the overall system efficiency and is typically the heaviest component in the Brayton ECS. The recuperator reduces entropy generation and increases cycle efficiency by transferring thermal energy between the hot and cold portions of the working fluid. In general, higher turbine inlet temperatures and colder compressor inlet temperatures improve system efficiency, becoming the key design drivers for the reactor and radiator temperatures, respectively.

Conventional recuperators are thin-sheet metallic heat exchangers made of 300-series stainless steels or high-temperature nickel-based superalloys such as Hastelloy X. Prometheus’ demanding performance targets for efficiency and weight reduction may exceed the limits of conventional ECS materials. As a result, development and design of the recuperator should be addressed. Some options include altering cycle state points to reduce performance expectations or incorporating advanced materials to reduce mass while improving heat transfer.

A promising advanced material candidate is a C/C composite. C/C is used in many engineering, especially aerospace, applications due to its high thermal conductivity, high-temperature capability, exceptional stiffness, low density and high strength-to-density ratio. Carbon carbon materials can withstand operating temperatures up to 2273K (2000C); however, at this temperature they require a non-oxidizing environment or protective coatings. C/C composites have demonstrated room temperature thermal conductivities in excess of 250 W/mK, which is over 10 times that of current heat exchanger materials. Finally, the thermal and mechanical properties of carbon-carbon composites can be tailored by fiber selection, fiber orientation, and/or processing conditions.

 

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