Off-design of a CO₂-based mixture transcritical cycle for CSP applications: Analysis at part load and variable ambient temperature

Abstract

This work focuses on the off-design analysis of a simple recuperative transcritical power cycle working with the CO₂ + C₆F₆ mixture as working fluid. The cycle is air-cooled and proposed for a state-of-the-art concentrated solar plant with solar salts as heat transfer fluid in a hot region, with a cycle minimum and maximum temperature of 51 °C and 550 °C at design conditions. The design of each cycle heat exchanger (primary, recuperator and condenser) is carried out in MATLAB with referenced models and the turbine designed in CFD, providing performance maps adopted by the cycle operating in sliding pressure. The off-design of the cycle is developed with a routine simulating the thermodynamic conditions of the cycle at variable ambient temperature and thermal inputs down to 40 % of the nominal value. The results show that the cycle can efficiently run in a wide range of part load conditions and ambient temperatures, from around 0 °C to over 40 °C, with net electric cycle efficiencies from 45 % to 36 %: according to the control philosophy proposed, the condenser fans are fixed at design speed, while the cycle operates in sliding pressure, when is possible. The results evidence the flexibility and good performances of the proposed system in various operating conditions.