Optimization of a supercritical carbon dioxide solar thermal power system
Date
2020-12
Authors
Journal Title
Journal ISSN
Volume Title
Publisher
Stellenbosch : Stellenbosch University
Abstract
ENGLISH ABSTRACT: A new procedure for the optimal design of a solar thermal power system that uses a supercritical carbon dioxide (sCO2) Brayton cycle is developed. The design procedure is compatible with different types of component models, solution methods and design constraints. The variables of the system, and the objectives and constraints of the system design, are managed through a comprehensive computational architecture. Multi-objective optimization of 23 thermodynamic-, geometric- and performance design variables of the system is achieved.The design procedure is based on a specific series of design decisions that continually reduce the design spaces of the turbo machinery and heat exchanger sub-systems, in such a way that Pareto-optimality ofthe final system design is ensured. For computational expediency, initial design decisions are made based on the analysis of a thermodynamic model. It is demonstrated that the optimal thermodynamic design of the system is influenced by the performance values of the turbo machinery and recuperator. Subsequent design decisions are made based on the independent analyses of detailed turbo machinery and heat exchanger models.The turbo machinery is modeled in Matlab® with a mean-line analytical approach that uses specified performance coefficients. Explicit constraints ensure that the turbo machinery designs are within established limits.The heat exchangers are modeled in Flownex® using a control-volume-based convection-diffusion approach that can accurately represent the internal pinch-point of the recuperator. All models make use of realistic thermodynamic properties for supercritical carbon dioxide and are extensively validated with published data.A formal derivation shows that there are two distinct operating regions for the heat exchangers of the system. A successful system design depends on the region in which the heat exchangers function. This region can be controlled by changing the value of the nominal flow area, which is considered the most important design variable of the system.
Six designs of the same basic system, but with different objectives and constraints, are presented. These designs are evaluated and compared to each other through a detailed quantitative investigation that highlights which factors contribute most to the inefficiency of each design. The best design achieves a thermal-to-mechanical efficiency of 40% at a turbine inlet temperature of 550◦C. This efficiency is demonstrated to be near the practical maximum for an sCO2system that employs the recuperated cycle configuration with a linear solar receiver. Future developments of the design procedure could consider the addition of a financial model as well as constraints to account for the structural integrity of the system.
AFRIKAANSE OPSOMMING: Raadpleeg teks vir opsomming
AFRIKAANSE OPSOMMING: Raadpleeg teks vir opsomming
Description
Thesis (PhD)--Stellenbosch University, 2020.
Keywords
Brayton Cycle, Supercritical Carbon Dioxide (sCO2), Multidisciplinary Design Optimization (MDO), Concentrated Solar Power (CSP), UCTD