DEVELOPMENT OF THE SUPERCRITICAL CARBON DIOXIDE POWER CYCLE TEST LOOP WITH THE TURBO-GENERATOR USING THE PARTIAL ADMISSION NOZZLE

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1 Proceedings of the Asian Conference on Thermal Sciences 2017, 1st ACTS March 26-30, 2017, Jeju Island, Korea ACTS-P00317 DEVELOPMENT OF THE SUPERCRITICAL CARBON DIOXIDE POWER CYCLE TEST LOOP WITH THE TURBO-GENERATOR USING THE PARTIAL ADMISSION NOZZLE Junhyun Cho 1, Hyungki Shin 1, Ho-Sang Ra 1, Gilbong Lee 1, Chulwoo Roh 1, Beomjoon Lee 1, Young-Jin Baik 1* 1 Korea Institute of Energy Research, 152 Gajeong-ro, Yusung-gu, Daejeon, South Korea Presenting Author: jhcho@kier.re.kr * Corresponding Author: twinjin@kier.re.kr ABSTRACT The supercritical carbon dioxide power cycle has been considered as a promising power generation cycle, recently. The development of the turbo-generator for this cycle has technical challenges such as selection of rotating parts, design of the turbine wheel, design of axial force balance and rotor dynamics due to its high rotational speed induced by low expansion ratio through the turbine and by small mass flowrates condition in the lab-scale experimental test loop. A 1 kwe small-scale experimental test loop was manufactured to investigate characteristics of the supercritical carbon dioxide power cycle, a high speed turbo-generator was also designed and manufactured. The designed rotational speed of this turbo-generator is 200,000 rpm. By using only one channel of the nozzle, partial admission method is adapted to reduce a rotational speed of the rotor. The cold-run test using a nitrogen gas in an atmospheric condition was conducted to observe effect of the partial admission nozzle on the rotor dynamics. The vibration level of the rotor was obtained using a gap sensor and results shows that effect of the partial admission nozzle on the rotor dynamics was allowable. KEYWORDS: Supercritical Carbon Dioxide Power Cycle, Turbo-generator, Turbine, Partial Admission, Power Cycle 1. INTRODUCTION A turbo-alternator-compressor unit (TAC) using a radial compressor is used in a several published kwclass supercritical carbon dioxide power cycle test loop to configure a Brayton power cycle [1-3]. Due to a high energy density near the critical point, a size of the TAC unit becomes very small in a lab-scale test loop. Therefore, the TAC unit has a minimum size to obtain proper efficiency and manufacturability. Then, system components such as a heater, the heat exchangers, a chiller, valves, pipes and other parts have to be large to build a test loop, thus it is hard to build, control and handle the system. In this study, using a small size of piston-type carbon dioxide pump, a multi-purpose 1 kw-class supercritical carbon dioxide power cycle test loop which operates as a simple recuperated Brayton cycle at a temperature of 500 o C and at a pressure of 135 bar, and as a transcritical cycle at a temperature of 200 o C is designed to concentrate on the characteristics of the cycle, control and stability issues of a supercritical carbon dioxide power cycle [4]. 1

2 2. DESIGN OF THE EXPERIMENTAL TEST LOOP Figure 1 shows a full schematic of an experimental test loop and its transcritical operating conditions which turbine inlet temperature is a 200 o C. Two piston-type carbon dioxide pumps (Catpump, USA) which mass flow rate are kg/s and kg/s respectively were used to pressurize liquid carbon dioxide at a temperature of 20 o C and at a pressure of 5729 kpa up to kpa which is a supercritical state. An immersion type electric heater heats supercritical carbon dioxide up to 200 o C and then, hot CO2 drives a radial type turbine. After expansion at a turbine, a supercritical carbon dioxide is cooled to a liquid state by coolant water at a brazing plate heat exchanger (BPHE). When system is operated as a simple recuperated Brayton cycle, a turbine inlet temperature goes up to 500 o C and a printed circuit heat exchanger (PCHE) type recuperator is used to preheat a working fluid using remained heat after a turbine outlet. Instead of a BPHE, a PCHE type cooler is used because a pressure of a carbon dioxide is still high at a cooling process (above 7400 kpa). Using several valves and loop, these two cycles are configured by one test loop facility [4]. air # 19 * # 4~9 : High Temperature s-co2 T=200 C s-co2 T=20 C Syringe pump P/Ts View port # 20 # 16 Expansion Tank # 18 Water jacket CO2 fill liquid pump Vacuum pump CO2 bombe Mass flow control P/Th,o Electric Heater P/Th,i # 4 T=34 C Rupture disc # 3 # 6 P/Tex # 5 Expansion or MFC P/Tt,i CO2 bombe Oil Tank Oil Filter Turbine Check Regulator Relief Tt,i Tt,i P/Tt,o Expansion Oil pump # 7 Power meter W G Tbearing 2EA Tt,o 2EA T=157 C # 8 P/Trh,i # kg/s Pump 1 M Motor P/Tp,o1 Damper P/Tp,i P/Tp,o2 T=34 C P/Trc,i # 2 # 11 # 17 Seperator M Motor P/Tp,o kg/s Pump 2 Damper Relief # 12 # 1 Expansion P/Tc,i Compression loop P/Tc,o # 10 Precooler 1 PCHE 10kW Cooling water T=157 C Rupture disc # 13 # 14 P/Tw,i # 21 # 15 # 22 FMw Precooler 2 BPHE 20kW # 25 # 26 # 28 P/Tw,o Water pump # 24 Water Tank # 29 Water pump * # 23, 27 : Turbine cooling # 30 Chiller 15RT P/Trc,o Recuperator P/Trh,o Fig. 1 Schematic diagram of an experimental test loop (200 o C operating condition) [4] 3. DESIGN OF THE TURBO-GENERATOR As a first step, a transcritical carbon dioxide power cycle is designed which a turbine inlet temperature is 200 o C. Because a mass flow rate is so small (0.07 kg/s), it is difficult to design radial-type turbine. In our operating condition, an optimal radial turbine has a diameter of 22.6 mm and a rotating speed of 800,000 rpm. Since it is nearly impossible to drive 800,000 rpm turbine, a partial admission nozzle is adopted to manufacture and operate a turbo-generator in an experimental test loop. By using only one channel of a nozzle, 200,000 rpm rotating speed condition is designed as shown in Figure 2 [4]. 2

3 Fig. 2 The turbo-generator layout with the partial admission nozzle [4] In particular, a commercial angular contact ball bearing (SKF) is used to overcome technical problems of the gas foil journal and thrust bearings shown in results of the advanced research groups such as Sandia National lab. Due to thrust balancing and high temperature operation, a gas foil bearing has operation limits. Thus, in this study, bearing room is separated from a turbine room by a several labyrinth seal in order to make atmospheric pressure condition to operate oil-lubricated ball bearing. In this design, inevitable leakage loss through the labyrinth seal occurs, so a compensation loop of the carbon dioxide from the additional CO2 tank is also designed in the test loop. 4. PRELIMENARY OPERATION After assembling each component, as a first step, balancing process of the turbo-generator was conducted. In wideopen atmospheric condition called as cold-run, a turbo-generator was driven by external electric power source. A level of vibration of the rotor and temperatures of the bearings were monitored. As a second step, by supplying a compressed air and a nitrogen into the turbine nozzle, effect of the partial admission nozzle on the vibration of the rotor was monitored. 3

4 As a preliminary operation test, before using a supercritical carbon dioxide, a refrigerant R134a was used as a working fluid to test each component such as pumps, separator, valves, heater, safety and so on and also to experience closed Rankine cycle operation and to obtain various troubleshooting procedure. Because of the characteristics of the R134a, low pressure turbine operation is possible, so it is convenient to check the experimental loop system before testing the supercritical carbon dioxide cycle which the maximum pressure is 130 bar. Figure 3 shows preliminary test results of the experimental loop using a R134a as a working fluid. A pressure of 29.5 bar and a temperature of 110 o C turbine inlet conditions were obtained by controlling the closed Rankine cycle operation strategy. An expansion ratio was 3.0 and a rotational speed of the turbo-generator was 90,000 rpm at the highest power operation condition. Through the preliminary test, operation and control strategy were developed and tested. After experiencing the closed power cycle loop operation, a working fluid will be substituted by a carbon dioxide. Fig. 3 Preliminary test results using a R134a as a working fluid 5. CONCLUSIONS A small-scale, multi-purpose supercritical carbon dioxide power cycle experimental test loop was developed with a high-speed radial type turbo-generator. A partial admission nozzle was designed to obtain proper turbine sizing and operational speed to manufacture under very small mass flow rate condition. A commercial oil-lubricated angular contact ball bearing was used to avoid bearing failure problems discovered by prior research groups. A preliminary experimental test was conducted by using a R134a as a working fluid to experience operation characteristic of the closed Rankine cycle. 4

5 ACKNOWLEDGMENT This work was conducted under the framework of Research and Development Program of the Korea Institute of Energy Research (KIER) (B6-2415) In addition, this work was supported by the On Demand Development Program of Core Technology for Industrial Fields ( , Engineering Technique for Power Generation System Design using Industry Waste Heat) funded by the Ministry of Trade, industry & Energy (MI, Korea) REFERENCE [1] Wright, S. A., Radel, R. F., Vernon, M. E., Rochau, G. E. and Pickard P. S., 2010, Operation and Analysis of a Supercritical CO2 Brayton Cycle, Sandia National Laboratories, available at: (accessed in Jan. 2016) pdf [2] Convoy, T., Pasch, J. and Fleming, D., 2013, "Control of a Supercritical CO2 Recompression Brayton Cycle Demonstration Loop," ASME Journal of Engineering for Gas Turbines and Power, Vol. 135, [3] Cho, J., Choi, M., Baik, Y-J., Lee, G., Ra, H-S., Kim B. and Kim, M., 2016, Development of the turbomachinery for the supercritical carbon dioxide power cycle, International Journal of Energy Research, Vol. 40, No. 5, pp 587~599. [4] Cho, J., Shin, H., Ra, H-S., Lee, G., Roh, C., Lee, B. and Baik, Y.-J., 2016, "Development of the supercritical carbon dioxide power cycle experimental loop in KIER, ASME TurboEXPO 2016, GT