Preliminary Design and Projected Performance for Intercooled-Recuperated Microturbine

Abstract

The Inter-Cooled-Recuperated (ICR) cycle is recognized for its high efficiency potential in gas-turbine applications. This paper reports on a proposed implementation of the ICR cycle in a microturbine setting, using a three-spool configuration incorporating a variable-geometry nozzle on the low-pressure ‘free’ power turbine. Hardware specified for the high-pressure turbine is an existing ceramic rotor fabricated and spin-tested in connection with a prior DOE-sponsored program. Rated engine design-point power and efficiency are projected at 378kWe and 39.5% (net LHV), under realistic prescriptions for component efficiencies and parasitic losses, and with TIT = 1366K (2000°F) specified for the ceramic rotor. Detailed off-design performance projections are carried out, demonstrating exceptional range and part-load efficiency. A key attraction of the ICR compared to a non-intercooled recuperated cycle is its compatibility with high cycle pressure ratio, making for dramatic size and cost reductions for high-pressure components, most importantly the recuperator. A related advantage is reduced ceramic-turbine rotor diameter for a given power level, extending the applicability of ceramic components under conservative manufacturability limits. Engine layout and preliminary mechanical designs for the major subassemblies are developed for application to a forty-foot transport bus with hybrid-electric drive. Further applications under evaluation for the proposed microturbine are stationary power generation, and in a hybrid powerplant setting using a solid-oxide fuel cell.

Model of a novel pressurized solid oxide fuel cell gas turbine hybrid engine

“…Figs. 7 and 8 as well as the omitted turbine performance maps closely resemble those published by Brayton Energy for the non-hybrid version of this engine [36]. At 20% turndown a surge margin of 5% is held by both compressors, however if turndown continues to 16% the surge margin approaches zero, thus a 5:1 turndown is deemed safe.…”

“…100% of rated system power corresponds to 640.74 kW of produced power. compressors, TIT not exceeding 1093.3 • C (2000 • F) on the highpressure ceramic turbine, and TIT not exceeding 927 • C (1700 • F) on the low-pressure turbine [36]. Diamonds are plotted on all plots in order that a correlation can be made between figures based upon the turndown percentage.…”

“…Intercooler: The intercooler is placed between compressor stages and is simply modeled as decreasing the gas stream temperature to 3.9 • C above that of the ambient air temperature [36]. It is assumed that a blower, moving ambient air, will be able to meet the required heat exchanger effectiveness.…”

“…1 minus the SOFC) has been proposed as a diesel bus engine replacement, and as such, is designed so that the physical volume is quite compact [36]. The compact design minimizes the plenum volume, which is expected to decrease both transient instabilities and the amount of expensive high temperature piping.…”

“…The goal of this study is to allow a large turndown while maintaining the SOFC stack exit temperature at 1000 • C, using constant fuel utilization of 85%, and without the need for cathode bypasses and/or bleeds. This is accomplished using individual fuel controllers for both the SOFC and the auxiliary burner and incorporating two novel, yet commercially viable components [36]. First, a ceramic turbine is placed on the high-pressure spool, allowing higher turbine inlet temperatures (TIT) (controlled by the auxiliary burner).…”