Thermodynamic optimisation of supercritical CO2 Brayton power cycles coupled to Direct Steam Generation Line-Focusing solar fields

摘要

In this paper a new generation line-focusing solar plants coupled to a s-CO2 Brayton power cycles are studied. These innovative CSP will increase the plant energy efficiency, and subsequently optimizing the SF effective aperture area and SF investment cost for a fixed power output. Two SF configurations were assessed: the Configuration 1 with a condenser between the SF and the Balance Of Plant (BOP), for Turbine Inlet Temperatures (TIT) up to 400oC, and the Configuration 2, for higher TIT up to 550oC, with steam compressors in SF for pressure drop compensation. Both alternatives are interchangeable in the same CSP, and boosting with a backing boiler to warranty the plant performance. In relation to the BOP three configurations were studied the Recompression cycle (RC), the Partial Cooling with Recompression cycle (PCRC), and the Recompression with Main Compression Intercooling cycle (RCMCI), all these solutions without ReHeating. The methodology considered the thesis developed by Dyreby [1] as starting point, fixing the Brayton cycles recuperator conductance (UA), and optimizing the power cycles performance by means of the SUBPLEX [2] algorithm. The cycles optimal operating parameters were calculated with a “Windows” desktop application, called Supercritical_CSP (SCSP), calling the supercritical fluids properties database REFPROP, developed in C#, calling Fortran compiled dynamic linked libraries. The results obtained from the Brayton cycles optimizations were exported to Thermoflow [3] for SF simulation and design. The mathematical algorithms UOBYQA [4] and NEWOUA [5] were also integrated in the SCSP tool, for validating the SUBPLEX results. The HTF studied was Direct Steam Generation (DSG) in the SF, and the solar collectors simulated were PTC and LF. The plant net power output, the net efficiency, the SF effective aperture, were computed at DesignPoint. As main conclusion obtained it is confirmed minimum Pinch Point in heat exchangers is the main constrain, reaching a threshold in the net plant efficiency, when increasing the Low Temperatura Recuperator (LTR) and High Temperature Recuperator (HT) conductances UA. The shell-tubes heat exchanger types are the most suitable solution to couple the Balance Of Plant (BOP) and the SF. The target of future works will be aligned with the analysis of innovative linear solar collectors, as the Norwich Technologies company solution, for getting higher TIT as provided by Central Tower CSP. The s-CO2 BOP equipments detail design and detailed cost estimation are pending items under industrial development. Finally, the annual plant performance calculation, considering the variable ambient temperature and Direct Normal Irradiance (DNI), and the TES integration, are future researching works for calculating the Levelized Cost Of Energy (LCOE) in this new generation line-focusing solar power plants.