Comparative Study of Solar Thermal Brayton Cycles Operated With Helium or Argon

Concentrating Solar Power (CSP) plants often use Rankine cycles operated with water/steam as energy conversion cycles. Since the solar central receiver technology could provide receiver fluid outlet temperatures higher than 900°C, open and closed gas turbine technologies become a promising alternative. Closed solar Brayton cycles operating with appropriate fluids can reach similar or higher thermal efficiencies than water/steam Rankine cycles but have the advantage of less consumption of fresh water. This paper presents the results of a comparative thermodynamic and process study of closed solar thermal Brayton cycles operated with Helium or Argon as working fluids. The main components of the cycles are two axial compressors with an intercooler, a recuperator and one axial turbine. The solar heat is fed in by a central receiver technology. It is assumed that the transferred heat to the cycles is constant and the turbine inlet temperature is 900°C. A first one-dimensional design approach for both cycles is performed based on the results of the thermodynamic considerations. The major parameters like stage types, number of stages, rotational speed, etc. are determined and discussed. The thermodynamic and process investigation results for the described closed Brayton cycles show that thermal efficiencies over 46% can be established for both fluids. The design considerations show that both cycles are feasible, but with respect to design dimensions the Argon based cycle can be built up with fewer stages and more compact, if compared to the Helium cycle.