SUMMARYA supercritical carbon dioxide power cycle has been focused as a promising power cycle because of its compactness, a high efficiency, and a wide applicability. A 10-kWe-class simple unrecuperated supercritical carbon dioxide Brayton experimental loop including a turbo-alternator-compressor unit using a centrifugal compressor and a radial turbine was developed. A shrouded type of a compressor impeller and a turbine wheel with labyrinth seals were designed to overcome thrust balancing problems of the high-pressure fluid turbomachinery. In addition, this type has no issues on the thermal growth collision failure and clearance loss between a shroud and a wheel. Preliminary operation at 30,000 rpm, turbine inlet temperature of 83°C, and pressure of 8500 kPa was successful. It is founded that all states of the cycle existed in the supercritical region.

Three supercritical carbon dioxide (CO2) power cycle experimental loops have been developed in Korea Institute of Energy Research (KIER) from 2013. As the first step, a 10 kWe-class simple un-recuperated Brayton power cycle experimental loop was designed and manufactured to test its feasibility. A 12.6 kWe hermetic turbine-alternator-compressor (TAC) unit which is composed of a centrifugal compressor, a radial turbine and the gas foil bearings was manufactured. The turbine inlet design temperature and pressure were 180 °C and 130 bar, respectively. Preliminary operation was successful at 30,000 RPM which all states of the cycle existed in the supercritical region. Second, a multi-purpose 1 kW-class test loop which operates as a transcritical cycle at a temperature of 200 °C was developed to concentrate on the characteristics of the cycle, control and stability issues of the cycle. A high-speed turbo-generator was developed which is composed of a radial turbine with a partial admission nozzle and the commercial oil-lubricated angular contact ball bearings. Finally, a 60 kWe-class Brayton cycle is being developed which is composed of two turbines and one compressor to utilize flue-gas waste heat. As the first phase of development, a turbo-generator which is composed of an axial turbine, a mechanical seal and the oil-lubricated tilting-pad bearings was designed and manufactured.

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The development of a 60-kWe turbo generator that uses supercritical carbon dioxide (sCO2) cycle technology at the lab scale is described herein. The design concept for the turbo generator involved using commercially available components to reduce the developmental time and to increase the reliability of the machine. The developed supercritical partial-admission CO2 turbine has a single-stage axial-type design with a 73-mm rotor mean diameter. The design of the sCO2 turbine uses impulse and partial admission to reduce the axial force and rotational speed. We simulated the flow of the designed sCO2 turbine. To increase the simulation accuracy, a real gas property table is coupled with the flow solver. The turbine performance test apparatus and test results are described; then, the turbine is continuously operated for 44 min. The maximum turbine power is 25.4 kW, and the maximum electric power is 10.3 kWe.

In order to overcome reported failure problems of turbomachinery for the supercritical carbon dioxide power cycle induced by the high rotational speed and axial force, an axial impulse-type turbo-generator with a partial admission nozzle was designed and manufactured to reduce the rotational speed and axial force. The turbine wheel part was separated by carbon ring-type mechanical seals to use conventional oillubricated tilting-pad bearings. A simple transcritical cycle using a liquid CO2 pump was constructed to drive the turbogenerator. A 600,000 kcal/h LNG fired thermal oil boiler and 200 RT chiller were used as a heat source and heat sink. The target turbine inlet temperature and pressure were 200°C and 130 bar, respectively. Two printed circuit heat exchangers were manufactured for both sides of the heater and cooler. A leakage make-up system using a reciprocating CO2 compressor; CO2 supply valve-train to the main loop and mechanical seal; and an oil cooler for the bearings, load bank, and control systems were installed. Prior to the turbine power-generating operation, a turbine bypass loop was operated using an air-driven control valve to determine the system mass flow rate and create turbine inlet conditions. Then, 11 kW of electric power was obtained under 205°C and 100 bar turbine inlet conditions, and the continuous operating time was 45 min.