Comparison between s-CO2 and other supercritical working Fluids (s-Ethane, s-SF6, s-Xe, s-CH4, s-N2) in Line-Focusing Solar Power Plants with supercritical Brayton power cycles

Thermosolar power plants with linear solar collectors and Rankine or Brayton power cycles are maturing as a competitive solution for reducing CO2 emissions in power plants as an alternative to traditional fossil and nuclear fuels. In this context, nowadays a great effort is being invested in supercritical Carbon Dioxide Brayton (s-CO2) power cycles for optimizing the line-focusing solar plants performance and reducing the cost of renewable energy. However, there are other working fluids with similar properties as s-CO2 near critical point. This researching study was focused on assessing the solar plants performance with alternative supercritical working fluids in the Balance Of Plant (BOP): Ethane, Sulfur Hexafluoride, Xenon, Methane and Nitrogen, see [1, 2, 3]. The integration between linear solar collectors (Parabolic or Fresnel), Direct Moten Salt (MS) as Heat Transfer Fluids (HTF) and a Simple Brayton cycle with Recuperation and ReHeating were studied in this paper. Main innovation in this researching study is the Brayton power cycle parameters optimization at Design-Point via the Subplex algorithm as proposed in John Dyreby Thesis [4]. After obtaining the optimum reheating pressure, compressor inlet pressure, recompression fraction, and other optimized variables, the solar power plants performances were simulated and detail designed with Thermoflow software [5], providing a first approach about the Solar Fields (SF) effective areas and investment costs. As main conclusion, we deducted the importance of heat exchangers conductance (UA) for increasing the Brayton power plants efficiency and reducing the SF effective area and investment cost. The pinch point at recuperators exit is the main constrain for increasing the UA in s-CO2 cycles. This limitation is overcome with the other working fluids proposed in this study providing higher plant efficiency but requiring higher UA in the recuperators. In future studies the heat exchangers detailed design constitute a great challenge for increasing the UA and optimizing these equipments cost. The material corrosion and equipments dimensions and cost is another key issue discussed for selecting the optimum energy transfer fluid in Brayton power cycles