(745a) Optimal Start-up Policies for a Nanofluid Based Solar Thermal Power Plant

Continuous increment in human population, along with the depletion of conventional fossil fuels and global warming, have propelled the development of renewable and sustainable technologies for energy production such as photovoltaics technologies and solar thermal plants, which allow recovering the solar energy. In this sense, parabolic trough solar power plants use a thermal fluid to transfer thermal energy from solar radiation to a Rankine cycle in order to drive a turbine that, coupled to an electrical generator, produces electricity. In order for these technologies to truly compete with conventional energy systems, research has been focused on the improvement of their performance either through closed-loop control of thermal solar plants or through the employment of different working fluids such as nanofluids, produced by the addition of nanoparticles to a base fluid. However, optimal start-up procedures of such plants, and an operational analysis when using diverse nanofluids (Al₂O₃-water and TiO₂-water) compared against the base fluid, are yet to be addressed.

Thus, this work seeks a highly sturdy approach for the optimal start-up procedure of such-mentioned plants, focused on power delivery and the time required for the system to reach target conditions. Proposed approach consists on energy and mass balances for all components of the system, constraints for flow through the collector and for maximum filling volume of storage tanks, as well as an evaluation of the power delivered by a coupled ORC when certain conditions are reached. Also, this work presents a comparison of various start-up scenarios when alternating between nanofluids and pure water as the working fluid on the collector. In addition, mathematical model was formulated as a Mixed Integer Nonlinear Programming problem where diverse case studies were solved off-line using GAMS software for their later implementation using feedback control systems.

Results show how optimal control of the working fluid volumetric flow aids the system to reach normal operation in a short period of time, maximizing the power delivered in the time range of study and preserving energy on call stored for the plant to keep producing energy afterwards. Regarding the working fluid, results illustrate that TiO₂-water nanofluid is the most suitable among the ones studied, since its heat transfer characteristics allow for a higher amount of energy recovery at the solar collector and its exchange at the boiler of the ORC, leading to both minimum start-up time and maximum power delivery, and in some cases, achieving more energy stored at the tanks. It is worth noting that saturation of the fluid might occur if optimal start-up policies are not tracked, bringing out deposition of nanoparticles and, consequently, damaging the equipment. Therefore, the importance of deploying these policies is key for an optimal and reliable operation, not only in early stages of operation, but also through the entire lifetime of the system.