Comparison and assessment of the creep-fatigue and ratcheting design methods for a reference gen3 molten salt concentrated solar power receiver

Abstract

The Concentrating Solar Power (CSP) Gen3 Demonstration Roadmap has recognized Supercritical carbon dioxide (sCO2) Brayton Cycle as the best-fit power cycle as it provides thermal efficiency benefits relative to the conventional Steam-Rankine cycles. However, to enable the integration of sCO2 Brayton Power Cycle, it is necessary to increase the outlet temperature of current molten salt CSP systems from 565°C to approximately 720°C. Increasing the temperature of the salt brings significant material and engineering challenges. Creep-fatigue damage accumulation and ratcheting deformation are important design considerations for receivers operating at high temperature due to frequent transient loads caused by diurnal cycling and thermal shocks from transitions between sun and cloud cover. In this work, a reference thermomechanical model of a Gen3 CSP receiver is developed to evaluate different creep-fatigue and ratcheting design approaches identified in Section III, Division 5, of ASME Boiler and Pressure Vessel Code. The design methods are then ordered based on design margin and ease of use. Design by elastic perfectly-plastic analysis is found to be the most conservative among the methods. Elastic analysis is the easiest to perform but the design calculations are extremely complicated. In contrast, design calculations are straight forward in inelastic analysis method but it requires developing a sophisticated inelastic constitutive model describing the material behavior. Several recommendations are made for developing creep-fatigue and ratcheting design rules for CSP systems.