System-level modeling of the storage unit and auxiliary components of a line-focusing concentrating solar collector-based power plant

The Concentrating Solar Power (CSP) technology converts the energy of the concentrated sun light in thermal energy at middle-high temperature, which can be stored in a thermal energy storage (TES) unit or directly used to drive a convectional thermodynamic cycle (power unit). The CSP systems are a promising solution for the generation of dispatchable renewable electricity because of the TES that allows decoupling the electricity generation from the intermittent solar source. However, CSP plants can also produce other assets, mainly hydrogen or fresh water, or they can make use of fossil fuels to increase its flexibility: this plant modification is called hybridization. The most diffuse concentration technology among CSP plants is the parabolic trough (PT) system. It adopts long strings of parabolic mirrors to focus the solar radiation onto a linear receiver, typically an encapsulated and evacuated tube, which transfers the energy of the concentrated sun light to a heat transfer fluid (HTF). An alternative line-focusing system is the Fresnel technology that uses a set of almost flat mirrors strips instead of the parabolic concentrator. The state of the art of the TES technology for line-focusing systems is the two-tank sensible heat thermal storage that uses molten salts as storage medium. The aim of this thesis consists of the development of a thermal-fluid-dynamic system-level model of a CSP plant based on the line-focusing technology using the Modelica (object-oriented) language. This thesis was performed in collaboration with the FATA E.P.C. Company, located in Italy (Torino), which kindly provided data about the line-focusing CSP plant of Partanna (Sicily). The model should include the solar field of mirrors, the receiver unit, the TES (in terms of two-tank technology), the pumping stations and the gas-fired unit that provides additional flexibility to the system, while the power unit is out of the scope of the work. The HTF and the storage medium are the same, namely molten salts. The solar field and the receiver model have been already developed at Politecnico, while the rest of system-level model is developed in the framework of this thesis. The final system-level model is capable of providing the thermal performance (i.e., the heat lost towards the environment) of the single components and of the plant as a whole, in a steady state and under various operational conditions.