AMP_04_May_June_2021_Digital_Edition

A D V A N C E D M A T E R I A L S & P R O C E S S E S | M A Y / J U N E 2 0 2 1 1 7 Although nickel-based superalloys are the only commercial materials ap- propriate for piping and other pres- sure-containing components at high temperatures, the highest-performing alloys are in the early stages of indus- try adoption, resulting in high cost and limited availability from manufacturing and fabrication suppliers. This immatu- rity of 740H and 282 requires continued research in manufacturing and fabri- cation technologies, both to develop a lower-cost supply chain and to increase the reliability of components made from these materials. DOE supports research on sev- eral of these alloy technologies. For example, researchers at the Electric Power Research Institute are working with Special Metals Corp. to develop a manufacturing process for seam-weld- ed 740H tubes, potentially offering up to 30% cost reduction relative to con- ventional production methods [10] . DOE also supports research on casting, dif- fusion bonding, welding, cladding, hot isostatic pressing, and additive manu- facturing of these alloys. The goals of this and other materials and manufac- turing research funded by DOE is to: integrate qualification considerations at an early stage of material develop- ment; jointly engage material suppliers and end users to enable rapid uptake of innovations by industry; incorpo- rate modeling, standardization, and Fig. 2 — The DOE SunShot loop was designed to demonstrate highly efficient turbine expander and recuperator performance for advanced supercritical carbon dioxide CSP power cycles. Courtesy of Southwest Research Institute. because they require high temperatures or have other challenging process char- acteristics. SETO aims to make solar in- dustrial process heat cost-competitive with fossil fuels to address high-priori- ty applications, including calcination to produce cement, thermochemical wa- ter splitting for producing solar fuels, and ammonia synthesis for producing fertilizer. The solid particle heat-trans- fer pathway being developed for Gen3 CSP may be particularly relevant for a number of these processes, which have solid-phase components or are mediat- ed by solid-phase catalysts. Three key components of the high-temperature system would great- ly benefit from the development of ad- vanced materials: sCO 2 power cycles, TES, and heat exchangers—including receivers and primary heat exchangers. These three subsystems are essential to unlocking higher-efficiency systems that would, therefore, lower the cost of energy from CSP. SUPERCRITICAL CARBON DIOXIDE POWER CYCLES Although the SETO 2030 cost tar- get is agnostic to the specific thermal- to-electric power cycle used, the sCO 2 Brayton cycle is of particular interest because of its potential for high effi- ciency and low capital costs. SETO and the DOE Offices of Nuclear Energy and Fossil Energy have collaboratively fo- cused on the development of the sCO 2 Brayton cycle [4] . Shared research goals have accelerated the development of critical components and broadened the foundational knowledge related to this cycle. DOE-funded research in SETO and the Office of Fossil Energy has pri- marily focused on developing and testing components for a sCO 2 cycle op- erating in a recompression Brayton cy- cle (RCBC) configuration at 715°C. SETO has funded the development and test- ing of expanders [5] and compressors [6] , which have operated at the 1 MW e scale (Fig. 2). Installation and testing were recently completed for an integrally geared compressor-expander [7] at sim- ilar temperatures. The Office of Fossil Energy’s Supercritical Transformational Electric Power (STEP) 10 MW e pilot plant is the next step to advancing sCO 2 tech- nology, which will increase the scale of demonstration and integrate com- ponents. STEP objectives include refining the sCO 2 power cycle, demon- strating component performance and scalability, and designing the facility to accommodate multiple new supporting technologies. Operating conditions for the most promising sCO 2 cycles present signif- icant challenges for materials. The RCBC configuration under develop- ment to be demonstrated at the STEP facility targets a turbine inlet tempera- ture of 715°C, with pressures poten- tially up to 300 bar, with some vendors targeting pressures up to 400 bar. Di- rect-fired oxy-combustion cycles may be designed for temperatures as high as 1500°C. Due to this extreme combination of temperature and pressure, research- ers and cycle designers have primarily focused on nickel-based alloys to meet the required strength at temperature. The highest strength and most recently developed alloys, 740H and 282, are the most attractive, because they can min- imize pipe wall thickness [8] . Fortunate- ly, sCO 2 -induced corrosion with nickel alloys does not appear to be a major concern [9] , although applications that introduce water, oxygen, or other impu- rities may require further study.