Small scale solar thermal systems are increasingly investigated in the context of decentralized energy supply, due to favorable costs of thermal energy storage (TES) in comparison with battery storage for otherwise economical PV generation. The present study provides the computational framework and results of a one year simulation of a low-cost pilot 3kWel micro-Concentrated Solar Power (micro-CSP) plant with TES. The modeling approach is based on a dynamic representation of the solar thermal loop and a steady state model of the Organic Rankine Cycle (ORC), and is validated to experimental data from a test site (Eckerd College, St. Petersburg, Florida). The simulation results predict an annual net electricity generation of 4.08 MWh/a. Based on the simulation, optimization studies focusing on the Organic Rankine Cycle (ORC) converter of the system are presented, including a control strategy allowing for a variable pinch point in the condenser that offers an annual improvement of 14.0% in comparison to a constant condensation pinch point. Absolute electricity output is increased to 4.65 MWh/a. Improvements are due to better matching to expander performance and lower condenser fan power because of higher pinch points. A method, incorporating this control strategy, is developed to economically optimize the ORC components. The process allows for optimization of the ORC subsystem in an arbitrary environment, e.g. as part of a micro-grid to minimize Levelized electricity costs (LEC). The air-cooled condenser is identified as the driving component for the ORC optimization as its influence on overall costs and performance is of major significance. Application of the optimization process to various locations in Africa illustrates economic benefits of the system in comparison to diesel generation.
Dynamic control strategy of a distillation system for a composition-adjustable organic Rankine cycle