Multi-stage configurations for central receiver hybrid gas-turbine thermosolar plants

Abstract

[EN]A thermodynamic model for hybrid Brayton thermosolar plants is proposed with the aim to analyze possible configurations with improved performance. In these plants an array of mirrors with a two-axis tracking system gathers solar power and redirects it to a central receiver. In turn the receiver acts as a heat exchanger that heats up a gaseous working fluid that runs a Brayton-like cycle. These plants also include a combustion chamber that ensures an approximately constant power output even during night or in periods with poor solar irradiance. Throughout the last years it has been demonstrated by means of experimental projects and prototypes that this concept is technically feasible but still R+D+i efforts are required in order to reach commercial feasibility. From the thermodynamic viewpoint it is necessary to increase overall plant efficiency. The model proposed in this paper is an extension of previous studies from our group that takes into consideration multi-stage configurations with an arbitrary number of compression steps with intercooling and expansion stages with reheating between turbines. The model is comprehensive and includes the main sources of losses in real plants: pressure decays in heat absorption and release, losses in compressors, turbines and heat exchangers, non-ideal recuperators and, of course, losses in the solar subsystem and combustion chamber. A numerical application is done taking as reference the data from the project Solugas, developed by the Abengoa Solar at the south of Spain. Several plant configurations are analyzed and also different working fluids checked, including air, nitrogen, carbon dioxide, and helium at subcritical conditions. It is concluded that for air, nitrogen and carbon dioxide, plant configurations with 2-3 compression/expansion steps are capable of achieving improved overall plant thermal efficiency (about 25% above single step plants) and also fuel conversion efficiency, i.e., lead to a considerable increase in power output without an appreciable increase in fuel consumption.