Robust Ceramic/Metal Composites for High-Temperature Heat Exchangers for Concentrated Solar Power

Concentrated solar power (CSP) plants utilizing thermal energy storage could provide large-scale renewable and dispatchable electricity, with an associated significant reduction in greenhouse gas emissions, if such electricity production were to become cost competitive with fossil-fuel-based power plants. The solar thermal-to-electrical conversion efficiency of CSP plants may be appreciably increased, with a commensurate reduction in the levelized cost of CSP-derived electricity, by operating such power plants at higher temperatures. By increasing the turbine inlet temperature of the working fluid to <u>&gt;</u>750^oC and by using power cycles with high-pressure, supercritical carbon dioxide (sCO₂) as the working fluid (instead of conventional Rankine cycles at <u>&lt;</u>550^oC with subcritical steam as the working fluid), the relative heat-to-electricity conversion efficiency may be increased by &gt;20%. However, effective heat transfer to sCO₂ at <u>&gt;</u>750^oC and <u>&gt;</u>20 MPa requires compact heat exchangers capable of operating under such extreme conditions. The significant decrease in mechanical performance of conventional stainless steels and nickel-based superalloys at such high temperatures and pressures inhibits their use in such heat exchangers.<br/> Co-continuous ceramic/metal composites can provide attractive combinations of properties for use in compact heat exchangers operating under extreme mechanical, thermal, and chemical conditions [1-4]. The interconnected ceramic phase can provide such composites with high-temperature stiffness and creep resistance, whereas the metallic phase can provide high-temperature ductility for enhanced resistance to fracture as well as enhanced high-temperature thermal conductivity. Ceramic and metallic phases in such composites can also exhibit desired thermal and chemical compatibility at high temperatures; that is, properly-selected ceramics and metals can possess similar thermal expansion coefficients (to allow for resistance to thermal cycling) and can retain a high solidus temperature with limited mutual solid solubility. The ceramic/metal composites may either be inherently resistant to high-temperature corrosion or corrosion-resistant coatings may be utilized. Dense co-continuous ceramic/metal composites can also be generated in desired geometries (e.g., thin plates with tailorable channel patterns) for compact heat exchangers via combined use of scalable, low-cost ceramic forming (e.g., tape casting, pressing) followed by shape-preserving reactive liquid metal infiltration (e.g., the patented Displacive Compensation of Porosity process [1]). The net-shape processing, microstructures, and attractive properties of several ceramic/metal composites (carbide/metal, oxide/metal) for use in advanced, high-temperature heat exchangers for CSP plants will be discussed.<br/> <br/>[1] M. Caccia, M. Tabandeh-Khorshid, G. Itskos, A. R. Strayer, A. S. Caldwell, S. Pidaparti, S. Singnisai, A. D. Rohskopf, A. M. Schroeder, D. Jarrahbashi, T. Kang, S. Sahoo, N. R. Kadasala, A. Marquez-Rossy, M. H. Anderson, E. Lara-Curzio, D. Ranjan, A. Henry, K. H. Sandhage, “Ceramic–Metal Composites for Heat Exchangers in Concentrated Solar Power Plants,” Nature, 562 (7727) 406 (2018).<br/>[2] Q. Zhu, X. Tan, B. Barari, M. Caccia, A. R. Strayer, M. Pishahang, K. H. Sandhage, A. Henry, “Design of a 2 MW ZrC/W-Based Molten-Salt-to-sCO₂ PCHE for Concentrated Solar Power,” Applied Energy, 300, 117313 (2021).<br/>[3] T. D. Nguyen, M. Caccia, C. K. McCormack, G. Itskos, K. H. Sandhage, “Corrosion of Al₂O₃/Cr and Ti₂O₃/Cr Composites in Flowing Air and CO₂ at 750°C,” Corrosion Science, 179, 109115 (2021).<br/>[4] T. D. Nguyen, G. D. Scofield, S. Hwang, M. D. Sangid, G. Itskos, M. Caccia, K. H. Sandhage, "Corrosion of a Dense, Co-Continuous, SiC/Si Composite in CO₂ and Synthetic Air at 750^oC," Journal of Materials Research and Technology, in press.