Low-Leakage Shaft End Seals for Utility-Scale Supercritical CO₂ Turboexpanders

Supercritical carbon dioxide (sCO₂) power cycles could be a more efficient alternative to steam Rankine cycles for power generation from coal. In this paper, the end seal layout for a nominally 500 MWe sCO₂ turbine is presented and the shaft end sealing requirements for such utility-scale sCO₂ turbines are discussed. Shaft end leakage from a closed-loop sCO₂ cycle and the associated recompression load can result in net cycle efficiency loss of about 0.55% points to 0.65% points for a nominally 500 MWe sCO₂ power cycle plant. Low-leakage hydrodynamic face seals are capable of reducing this leakage loss (and net cycle efficiency loss), and are considered a key enabling component technology for achieving 50–52% or greater thermodynamic cycle efficiencies with indirect coal-fired sCO₂ power cycles. In this paper, a hydrodynamic face seal concept is presented for end seals on utility-scale sCO₂ turbines. A 3D computational fluid dynamics (CFD) model with real gas CO₂ properties is developed for studying the physics of the thin fluid film separating the seal stationary ring and the rotor. The results of the 3D CFD model are also compared with the predictions of a Reynolds-equation-based solver. The 3D CFD model results show large viscous shear and the associated windage heating challenge in sCO₂ face seals. Following the CFD model, an axisymmetric finite-element analysis (FEA) model is developed for parametric optimization of the face seal cross-section with the goal of minimizing the coning of the stationary ring. A preliminary thermal analysis of the seal is also presented. The fluid, structural and thermal results show that large-diameter (about 24 inch) face seals with small coning or out-of-plane deformations (of the order of 0.0005 inch) are possible. The fluid, structural and thermal results are used to highlight the design challenges in developing large-diameter and high-differential-pressure face seals for the operating conditions of utility-scale sCO₂ turbines.