Design of 20 kW Turbomachinery for Closed Loop Supercritical Carbon Dioxide Brayton Test Loop Facility

Abstract

Indian Institute of Science, Bangalore in collaboration with Sandia National Labs has developed a 140kW (thermal) simple recuperated supercritical CO2 (s-CO2) test facility to enable power generation of up to net 20 kWe output using turbomachinery components. The primary intent of the test loop is to understand the design and operational aspects of an s-CO2 Brayton cycle for distributed power generation. This paper describes the development of suitable turbomachinery to be deployed in the test loop. Turbomachinery design study is primarily performed using a commercial design tool AxStream® for both design and off-design operating conditions with a maximum cycle temperature limit of 525°C and a pressure of 145 bar. Present design considers a decoupled turbine and compressor driven independently by an electrical motor and a generator pair. This arrangement provides flexibility to independently assess compressor and turbine prototypes and also helps establish stable operation of the s-CO2 Brayton test loop. A range of single stage compressor and turbine geometries are independently evaluated considering un-coupled shafts and appropriate loss models using the above boundary conditions. Specific geometries are filtered based on total-to-total efficiency for a given shaft speed. The speed of the turbo-machinery is restricted to 40,000 rpm to enable independent testing and characterization using direct drive high-speed Switched Reluctance (SRM) motor-generator pair that is being developed in-house for this purpose. The investigation reveals the absence of a suitable compressor and turbine geometry at desired operating speed, hence, to circumvent the problem of low blade heights in the preliminary impeller design at 40,000 rpm, the turbomachinery is designed for 65,000 rpm and the off-design condition is taken for study.