CO2 Turbine Research Articles

Design of an efficient space constrained diffuser for supercritical CO2 turbines

Radial inflow turbines are an arguably relevant architecture for energy extraction from ORC and supercritical CO2 power cycles. At small scale, design constraints can prescribe high exit velocities for such turbines, which lead to high kinetic energy in the turbine exhaust stream. The inclusion of a suitable diffuser in a radial turbine system allows some exhaust kinetic energy to be recovered as static pressure, thereby ensuring efficient operation of the overall turbine system. In supercritical CO2 Brayton cycles, the high turbine inlet pressure can lead to a sealing challenge if the rotor is supported from the rotor rear side, due to the seal operating at rotor inlet pressure. An alternative to this is a cantilevered layout with the rotor exit facing the bearing system. While such a layout is attractive for the sealing system, it limits the axial space claim of any diffuser. Previous studies into conical diffuser geometries for supercritical CO2 have shown that in order to achieve optimal static pressure recovery, longer geometries of a shallower cone angle are necessitated when compared to air. A diffuser with a combined annular-radial arrangement is investigated as a means to package the aforementioned geometric characteristics into a limited space claim for a 100kW radial inflow turbine. Simulation results show that a diffuser of this design can attain static pressure rise coefficients greater than 0.88. This confirms that annular-radial diffusers are a viable design solution for supercritical CO2 radial inflow turbines, thus enabling an alternative cantilevered rotor layout.

Thermodynamic and exergy analysis and optimization of a transcritical CO2 power cycle driven by geothermal energy with liquefied natural gas as its heat sink

The main objective of this research is to study a transcritical CO2 cycle via geothermal resources to produce electrical energy. Heat sink of this cycle is Liquefied natural gas (LNG) to drop back pressure of the CO2 turbine greatly. It is presumed that the system works under steady state situations to establish the mathematical model of the transcritical CO2 geothermal power generation system. To evaluate the impacts of different main thermodynamic parameters in the performance of the system a parametric investigation is employed. Furthermore, to determine an optimum system performance from an economical and thermodynamic point of view, a multi-objective optimization jointed to NSGA-II algorithm is employed. To calculate the final solution three decision makers comprising TOPSIS, LINAMP and FUZZY were use. Moreover, sensitivity analysis and error examination were performed on the solutions gained from the decision makers. In conclusion, outputs achieved from this investigation were compared to the other related studies and this comparison reveals that the solutions gained in this study are satisfactory compared to the previous works.

Thermodynamic analysis and optimization of a waste heat recovery system for proton exchange membrane fuel cell using transcritical carbon dioxide cycle and cold energy of liquefied natural gas

Hybrid systems are receiving more and more attention due to their higher efficiencies compared with standalone systems. Higher efficiencies mean lower consumption of fuels and as a result lower environmental problems. In this paper, a hybrid system consists of a transcritical carbon dioxide cycle and a liquefied natural gas cycle is proposed to recover the waste heat produced in a proton exchange membrane fuel cell. A complete sensitivity analysis is performed on the system considering five key parameters, namely operating temperature of fuel cell, CO2 turbine inlet temperature, CO2 turbine inlet pressure and pinch temperature of condenser and preheater. Also to achieve the highest possible energy efficiency, an optimization technique is applied on the system. The results showed that using the proposed system, generated power of the system increases by 39% compared with a standalone fuel cell. Also its efficiency reached to 72% after optimization, while it was equal to 39% before using the waste heat recovery system.

Oxidation and carburizing of FBR structural materials in carbon dioxide

A key problem in the application of a supercritical CO2 turbine cycle to a fast breeder reactor is the corrosion of structural materials brought about by supercritical CO2 at high temperatures. In this study, metallurgical analysis of the ferritic–martensitic steels (12Cr-steel and T91) exposed at 400–600 °C for up to 8000 h in supercritical CO2 pressurized at 20 MPa and at 550 °C for up to 5000 h in CO2 gas at atmospheric pressure has been carried out.Two successive oxide layers formed on the steels were kept under the maximum testing conditions (at 600 °C for 8000 h at 20 MPa), and no breakaway corrosion was observed. Each oxide layer thickness increased with exposure time, due to parabolic oxide growth. By using the electron backscattering diffraction technique, the outer oxide layer could be separable into thin Fe2O3 and thick Fe3O4, and the formation behavior was clarified. Carburizing was observed in the base metal just under the oxide layer, and the crystal structure was mainly M23C6.

Thermodynamic analysis and optimization of a transcritical CO2 geothermal power generation system based on the cold energy utilization of LNG

This paper investigates a transcritical CO2 cycle using geothermal resources to generate electricity. Liquefied natural gas (LNG) is employed as heat sink to drop the CO2 turbine back pressure sharply. The mathematical model of the transcritical CO2 geothermal power generation system is established for system simulations under steady-state conditions. A parametric analysis is conducted to evaluate the effect of several key thermodynamic parameters on system performance. Additionally, a multi-objective optimization using NSGA-II method is carried out to find the optimum performance of system from both thermodynamic and economic aspects. The results show that there is an optimal CO2 turbine inlet pressure that yields the maximum exergy efficiency. A higher CO2 turbine inlet temperature or a lower CO2 turbine back pressure brings about a higher exergy efficiency. In addition, an optimal CO2 turbine inlet pressure obtains the minimum required heat exchange area per net power output. The lower the CO2 turbine inlet temperature or the CO2 turbine back pressure is, the smaller the required heat exchange area per net power output is. By the multi-objective optimization, a Pareto optimal solution is obtained, which shows that an increase in exergy efficiency would increase the required heat exchange area per net power output.

Thermodynamic analysis and optimization of a solar-powered transcritical CO2 (carbon dioxide) power cycle for reverse osmosis desalination based on the recovery of cryogenic energy of LNG (liquefied natural gas)

A solar-powered transcritical CO2 (carbon dioxide) power cycle for reverse osmosis desalination based on the recovery of cryogenic energy of LNG (liquefied natural gas) is proposed. The system consists of a solar collector subsystem, a transcritical CO2 power cycle subsystem, a LNG subsystem and a RO (reverse osmosis) desalination subsystem. A thermal storage unit is introduced into the system to guarantee continuous and stable operation of the system. A mathematical model is developed to simulate the system based on several assumptions. The effects of several key thermodynamic parameters on the system performance are examined based on the performance criteria, including daily exergy efficiency, daily mechanical work output and daily fresh work output. Parametric optimization is conducted by genetic algorithm to maximize the daily fresh water output. The results show that the CO2 turbine inlet pressure has an optimal value to reach the daily maximum exergy efficiency under the given conditions. The daily exergy efficiency could decrease with an increase in condenser temperature, and increase with an increase in mass flow rate of oil and NG turbine inlet pressure. Through parametric optimization, the system can reach the daily exergy efficiency of 4.90% and provide 2537.33 m3 fresh water per day under the given conditions.

Design and evaluation of an IGCC power plant using iron-based syngas chemical-looping (SCL) combustion

Chemical-looping combustion (CLC) is a novel and promising combustion technology with inherent separation of the greenhouse gas CO2. This paper focuses on the design and thermodynamic evaluation of an integrated gasification combined-cycle (IGCC) process using syngas chemical looping (SCL) combustion for generating electricity. The syngas is provided by coal gasification; the gas from the gasifier is cleaned using high-temperature gas desulfurization (HGD). In this study, the oxygen carrier iron oxide (Fe2O3) is selected to oxidize the syngas in a multistage moving-bed reactor. The resulting reduced iron particles then consist of FeO and Fe3O4. To create a closed-cycle operation, these particles are partially re-oxidized with steam in a fluidized-bed regenerator to pure Fe3O4 and then fully re-oxidized in a fluidized-bed air combustor to Fe2O3. One advantage of this process is the co-production of hydrogen diluted with water vapor within the steam regenerator. Both the HGD and CLC systems are not under commercial operation so far. This mixture is fed to a gas turbine for the purpose of generating electricity. The gas turbine is expected to exhibit low NOx emissions due to the high ratio of water in the combustion chamber. Cooling the flue gas in the HRSG condenses the water vapor to yield high-purity CO2 for subsequent compression and disposal. To evaluate the net efficiency, two conventional syngas gasifiers are considered, namely the BGL slagging gasifier and the Shell entrained-flow gasifier. The option of using a CO2 turbine after the SCL-fuel reactor is also investigated. A sensitivity analysis is performed on the SCL-air reactor outlet temperature, this being a key design parameter. It was found that the best net efficiency of 43% (based on HHV) can be obtained using a BGL gasifier without a CO2 turbine at an air reactor temperature of 1000°C including CO2 compression for transport and storage. Simulation data are based on software Aspen Plus® and EES (Engineering Equation Solver).

Real-Gas Effects in Foil Thrust Bearings Operating in the Turbulent Regime

In this study, an elastohydrodynamic model was created for predicting the pressure field in a compliant thrust bearing assembly lubricated by high pressure CO2. This application is of significance due to ongoing research into the closed-cycle supercritical CO2 turbine as a high-efficiency alternative to steam turbines. Hardware development for this concept has been led by Sandia National Laboratories, where turbomachinery running on gas foil thrust and journal bearings is being tested. The model accounts for the fluid velocity field, hydrodynamic pressure, and frictional losses within the lubrication layer by evaluating the turbulent Reynolds equation coupled with an equation for structural deformation in the bearings, and the fluid properties database RefProp v9.0. The results of numerical simulations have been compared with empirical correlations, with reasonable agreement attained. Of particular interest is the contrast drawn between the performance of high pressure CO2 as a lubricant, and ambient pressure air. Parametric studies covering a range of fluid conditions, operating speeds, and thrust loads were carried out to illustrate the value of this model as a tool for improved understanding and further development of this nascent technology.

Conception of a Pulverized Coal Fired Power Plant with Carbon Capture around a Supercritical Carbon Dioxide Brayton Cycle

Power generation efficiency penalty is the main concern about carbon capture technologies. Improvement of power plant efficiency due to higher steam condition is frequently foreseen to outbalance the carbon capture penalty. This work focuses on the conceptual design of two coal-fired power plants using CO2 as working fluid: one with post- combustion MEA-based CO2 capture and one with oxy-combustion CO2 capture with cryogenic air separation. Two maximal supercritical CO2 temperatures are investigated (620 and 700°C). The combination of a Brayton supercritical CO2 power cycle, a coal boiler and a capture process allows significant increase of the overall plant efficiency. Both post-combustion and oxy-combustion capture processes lead to a very high overall plant efficiency: approximately 41.5% for a 620°C power cycle and 44.5% for a 700°C power cycle. Oxy-combustion seems best fitted for supercritical CO2 Brayton cycle due to a simpler thermal integration and the CO2 purification devices already integrated in the CO2 processing unit. Main resulting technological challenges are the very large heat exchanger needed in the CO2 cycle in order to achieve high power cycle efficiency and the development of supercritical CO2 turbine significantly different than steam or gas turbine especially because of the very large effort on wheel and the small size of the equipment. Numerous technological developments on process component will be necessary and a representative CO2 cycle pilot plant will be needed to assess CO2 leakage, corrosion and flexibility issues.

Optimal Cycle Scheme of Direct Cycle Supercritical CO2 Gas Turbine for Nuclear Power Generation Systems

A supercritical CO2 turbine cycle can achieve a considerably high cycle thermal efficiency at medium turbine inlet temperatures of 500-650°C at high pressure such as 20 MPa, which is too high to produce a reactor pressure vessel within the existing fabrication limits. To solve this problem, a dual expansion turbine cycle is effective; its application was examined for both the fast reactor (FR) of 527°C and 12.5 MPa and a high-temperature gas-cooled reactor (HTGR) of 650°C and 8 MPa. Results showed that, in the case of FR, the cycle thermal efficiency became 42.6%, 44.0%, and 45.1%, respectively, for the 12.5 MPa cycle, the dual expansion cycle, and the 20 MPa cycle. Therefore, the dual expansion cycle is effective. On the other hand, for HTGR, the cycle thermal efficiency became 47.5%, 48.5%, and 50.3%, respectively, for the 8 MPa cycle, the dual expansion cycle, and 20 MPa cycle. In this case, the cycle efficiency advantage becomes smaller than that for the FR, but a 1.0% advantage is obtainable.

Multi-stage chemical looping combustion (CLC) for combined cycles with CO 2 capture

This paper presents application of the chemical looping combustion (CLC) method in natural gas-fired combined cycles for power generation with CO 2 capture. A CLC combined cycle consisting of single CLC-reactor system, an air turbine, a CO 2-turbine and a steam cycle has been designated as the base-case cycle. The base-case cycle can achieve net plant efficiency of about 52% at an oxidation temperature of 1200 °C. In order to achieve a reasonable efficiency at lower oxidation temperatures, reheat is introduced into the air turbine by employing multi CLC-reactors. The results show that the single reheat CLC-combined cycle can achieve net plant efficiency of above 51% at oxidation temperature of 1000 °C and above 53% at the oxidation temperature of 1200 °C including CO 2 compression to 110 bar. The double reheat cycle results in marginal efficiency improvement as compared to the single reheat cycle. The CLC-cycles are also compared with a conventional combined cycle with and without post-combustion capture in amine solution. All the CLC-cycles show higher net plant efficiencies with close to 100% CO 2 capture as compared to a conventional combined cycle with post-combustion capture, which is very promising.