Helium Brayton Cycle and Water Rankine Cycle Combined System for Concentrating Solar Power Sustained by PCM Thermal Storage

This study proposes a combined thermal cycle by a Helium Brayton cycle at high temperatures (800-850 °C) on top of a water Rankine cycle. Thanks to its high thermal conductivity, helium is also an excellent heat transfer fluid so that jet impingement heat transfer and enhancement can obtain high energy efficiency at solar receivers. NaCl is considered as a phase change material (PCM) to meet the requirement of thermal storage, due to its suitable melting point of 801°C, cost effectiveness, and great reserves in nature. The studies focus on thermodynamic analysis of the power systems to obtain maximum energy conversion efficiency through optimizing system combinations and parameters of the power cycles. The studied systems with combined power cycles include: (1) simple Brayton and simple Rankine cycles (SBSR), (2) reheated Brayton and simple Rankine cycles (RBSR), (3) reheated Brayton and reheated Rankine cycles (RBRR), (4) reheated Brayton and reheated Rankine cycle with precooling in front of gas compressor (RBRR+P), and (5) reheated Brayton and reheated Rankine cycles with recuperation in Brayton cycle (RBRR+RE). The results demonstrate that the combined system by helium Brayton cycle and water Rankine cycle can attain a thermal efficiency in the range of 42% to 50%, when the supplied helium gas temperature and pressure approach 800 °C and 8 MPa, respectively.