🌀 Chapter 6: Improvement of Closed Cycle Gas Turbines

Closed-Cycle Gas Turbines: Toward 50% Efficiency?

  • Basic Cycle:
    • Helium turbine (HTR) → 22.85% efficiency.
  • Progressive Improvements:
    • Regeneration: Waste heat recovery → 40.41%.
    • Intercooled compression + reheat → 47.4%.
  • Supercritical CO₂ Cycles:
    • Advantages: Reduced compression work (CO₂ > 78 bar, 31.1°C).
    • Challenges:
      • Cp variations near critical point → Heat exchanger complexity.
      • Temperature crossover risks.
      • Need for cold sources < 25°C.

Technology Readiness Level (TRL):

  • Turbomachinery: TRL 6-7 (CO₂), < 7 (helium).
  • Materials: Limits at extreme temperatures.

Abstract

This chapter examines thermodynamic cycles where the working fluid remains gaseous throughout, focusing on two main topics: closed-cycle gas turbines and supercritical CO₂ systems. Beginning with a simple helium cycle for High Temperature Reactors achieving 22.85% efficiency, the chapter systematically demonstrates progressive improvements through exergy analysis. The regenerative cycle recovers waste heat from turbine exhaust, nearly doubling efficiency to 40.41% by preheating compressed gas before reactor entry. Further enhancements include intercooled compression reducing compression work, staged expansion with reheat, and combined configurations achieving 47.4% efficiency. The second section explores supercritical CO₂ cycles, leveraging reduced compression work when CO₂ operates above its critical point (78 bar, 31.1°C). Four cycle variants are analyzed: simple regeneration (34% efficiency), pre-compression with split compression, recompression achieving 45% efficiency through optimized flow distribution, and partial cooling reaching 43.7% efficiency with higher power output. Critical technical challenges are identified, including significant Cp variations near the critical point causing heat exchanger design complexities, temperature crossover risks, and requirements for cold sources below 25°C. The final section evaluates Technology Readiness Levels of high temperature reactors, concluding that while thermodynamically attractive, gas cycles face major barriers in turbomachinery development (TRL 6-7 for sCO₂, below 7 for helium) and materials limitations at extreme temperatures.