sCO2 Cycle Research Articles

Study on supercritical carbon dioxide recompression Brayton cycle system integrated with thermoelectric generator

Sharing advantages of high compactness, SCO2 Brayton cycle and thermoelectric generator are both considered promising energy conversion solutions and have attracted great attention due to the increasing energy shortage and global climate change. The integration of thermoelectric generators in the SCO2 Brayton cycle will remain the inherent advantages and further optimize the system performance, especially on special occasions with high requirements of compactness such as heat recovery systems in the marine environment. The presented study proposed a novel type of SCO2 recompression Brayton cycle coupled with a thermoelectric recuperator for the marine environment and combined thermoelectric generators inside the SCO2 Brayton cycle. The influence of the thermoelectric recuperator on the performance of the overall system and other components is discussed, and the optimized design parameters of the thermoelectric recuperator are explored. The results show that the thermoelectric recuperator in the SCO2 cycle system improves the cycle efficiency from 32.9 % to 34.7 % and increases the cycle output power from 261 kW to 276 kW. Moreover, the transient response characteristics of the thermoelectric recuperator under disturbing signals and the working performance of the system under variable working conditions are examined, and the economic analysis is developed as well.

High-efficient and carbon-free biomass power generation hybrid system consisting of chemical looping air separation and semi-closed supercritical CO2 cycle with a bottoming organic Rankine cycle

Herein, a novel biomass power generation system, comprising of CO2-assisted gasification unit, chemical looping air separation (CLAS) unit, semi-closed supercritical CO2 (sCO2) cycle with the bottoming ORC unit, was proposed to overcome the problems of low-efficiency and high-pollution rendered by traditional biomass utilization method. And the heat integration between those subunits was well designed based on the principle of energy cascade utilization. The sensitive analysis was carried out to clarify the reasons behind thermodynamic performance variation, and the optimized net power efficiency reached 38.76% when the CO2/C ratio was 0.25, gasification temperature was 830 °C, sCO2 turbine inlet temperature and pressure were 1200 °C and 300 bar respectively, which was superior to 28.01% of biomass integrated gasification combined cycle system. In addition, the exergy analysis found that modification efforts should be exerted on CLAS and sCO2 compression pump, whose exergy destruction were 7.58 MW and 5.15 MW, respectively. Besides, its environmental and economic merits were obvious with the CO2 specific emission at 0.31 kg/MWh, and the levelized cost of electricity at 57.38 $/MWh. Furthermore, three typical biomasses were examined for the proposed system, the results showed that the dryer and carbon-richer biomass could benefit system thermodynamic and economic performance.

Thermodynamic analysis of CO2–SF6 mixture working fluid supercritical Brayton cycle used for solar power plants

To overcome the poor cycle thermal efficiency of supercritical CO2 Brayton cycle caused by the high rejection temperature used for solar power plants, mixture of CO2–SF6 as working fluid for the Brayton cycle system is proposed and evaluated. Detailed thermodynamic analysis is conducted to the CO2–SF6 mixture Brayton cycle compared with the referenced s-CO2 cycle. Influence of MC inlet temperature and mole fraction of SF6 on the thermodynamic performances is discussed. Moreover, exergy efficiencies and exergy loss distribution of the components in the CO2–SF6 mixture Brayton cycle are analyzed. The results suggest that the system cycle thermal efficiencies of CO2–SF6 mixture cycle and s–CO2 cycle both decrease with the increase of the MC inlet temperature, however, the thermal efficiency of CO2–SF6 mixture cycle is significantly higher than that of s-CO2 cycle. An optimal mole fraction of SF6 exists in the CO2–SF6 mixture for the CO2–SF6 mixture cycle. The optimal value is 0.356 in this context. The heliostat field and solar receiver has the lowest exergy efficiency among all the components of the CO2–SF6 mixture cycle, which is as low as 31.59%.

Turbomachinery design of an axial turbine for a direct fired sCO2 cycle

This paper presents the details of the design process of a 950 MW rated sCO2 turbine for a direct fired sCO2 power plant. The design is made by using a meanline turbomachinery design code, which is then used in a design optimization process using mixed integer distributed ant colony optimization (MIDACO) algorithm. Turbine blade and disk designs and key turbomachinery design metrics are optimized by using a three-step optimization procedure. A single-flow and a double-flow turbine design were made by using the design optimization procedure. The two turbine designs made in this work represent the first publicly available information on detailed turbine parameters for a large-scale sCO2 turbine, which has unique temperature and pressure design requirements compared to the turbine design conditions used in comparable sCO2 cycle literature.

Investigation of thermodynamics of the supercritical CO2 Brayton cycle used in solar power at off-design conditions

The performance degradation of solar-power used SCO2 Brayton cycle under off-design condition is always a serious problem for the solar power station. However, very rare investigations have paid attention on it. The paper focuses on the behavior of SCO2 (supercritical carbon dioxide) Brayton cycle used for solar power generation under off-design condition using MATLAB code. Both the regenerative and recompression models, which are believed to have more promising performance, are established and compared. The variation of cycle performance on both annual and daily scales are investigated. It has been observed that the recompression model with specific bypass fraction always has superior performance under off-design condition. Meanwhile, for both models, the compressor is presented as an important component which limits the cycle performance under the off-design condition. Moreover, to keep with the reality, the weather factors of cloud coefficient (CC) and cloud cover coefficient (CCF) are firstly led into the off-design analysis of cycle. The relationship model between the SCO2 cycle performance and the CC, characterized by an approximate parabolic equation, has been developed.

How to identify trade-off of thermodynamic and economic performance of the S-CO2 cycle caused by the cycle structures?

Generally, the improvement of thermodynamic performance of supercritical CO2 Brayton cycles is realized by more complex cycle structures at the expense of expensive system cost. How to identify trade-off of the thermodynamic and economic performance of the S-CO2 cycle is a challenging issue because system structure is hard to be optimized with its nature of discrete variable. Therefore, taking the net power output and cost of electricity as the objective functions, this study tries to solve this issue by multi-objective optimization of superstructure of supercritical CO2 Brayton cycle for gas turbine waste heat recovery. Besides discussing the effect of heat source temperature and cooling temperature, the optimization results of the superstructure are compared with the traditional structures. The results show that at different heat source temperatures, the distribution regularities of the cycle structures on the Pareto-front are the same, and the structure of the trade-off solution tends to be the same. With the increase of heat source temperature, the costs of trade-off solutions are reduced by 8.86%, 11.16% and 16.51%, respectively. The change of cooling temperature has a great effect on the structure of the trade-off solution. In addition, superstructures have obvious advantages over the traditional structures in terms of trade-off solution performance and structure selection.

Ecological Function Analysis and Optimization of a Recompression S-CO2 Cycle for Gas Turbine Waste Heat Recovery.

In this paper, a recompression S-CO2 Brayton cycle model that considers the finite-temperature difference heat transfer between the heat source and the working fluid, irreversible compression, expansion, and other irreversibility is established. First, the ecological function is analyzed. Then the mass flow rate, pressure ratio, diversion coefficient, and the heat conductance distribution ratios (HCDRs) of four heat exchangers (HEXs) are chosen as variables to optimize cycle performance, and the problem of long optimization time is solved by building a neural network prediction model. The results show that when the mass flow rate is small, the pressure ratio, the HCDRs of heater, and high temperature regenerator are the main influencing factors of the ecological function; when the mass flow rate is large, the influences of the re-compressor, the HCDRs of low temperature regenerator, and cooler on the ecological function increase; reasonable adjustment of the HCDRs of four HEXs can make the cycle performance better, but mass flow rate plays a more important role; the ecological function can be increased by 12.13%, 31.52%, 52.2%, 93.26%, and 96.99% compared with the initial design point after one-, two-, three-, four- and five-time optimizations, respectively.

Cooling analysis of an axial turbine for a direct fired sCO2 cycle and impacts of turbine cooling on cycle performance

In this paper, the details of the cooling analysis and design process of a 950 MW rated sCO2 turbine for a direct-fired sCO2 power plant are presented. An analytical cooled turbine algorithm for combustion gas turbines is modified for sCO2 coolant and mainstream flows to determine the coolant fractions. Inputs of the cooling technologies were determined from internal NETL studies and publicly available data for prototype axial turbines. A simplified thermal stress analysis method is used to down-select and determine the cooling configurations to be used by considering the disk and material properties and candidate stream coolant temperatures. The cooled turbine algorithm was first used to eliminate configurations that would require too much coolant from the cycle. The best cooling configurations for a single flow and a double flow turbine were determined by testing in a direct-fired sCO2 power plant performance model in Aspen Plus. A weighed scoring criteria was developed to help select amongst closely related performance outcomes. Results of the scoring gave the best option as a single flow turbine with a calculated turbine efficiency of 88%, and a resulting cycle LHV efficiency of 65.73% and plant LHV efficiency of 54.43% for a 601.58 net MW direct-fired sCO2 power plant (952 MW turbine).

Study on configuration of gas-supercritical carbon dioxide combined cycle under different gas turbine power

In order to obtain the optimal configuration scheme of gas-supercritical carbon dioxide (SCO2) combined cycle system under different gas turbine capacities, the main parameters of each gas-SCO2 cycle scheme were optimized to maximize the cycle efficiency of the gas-SCO2 cycle system, and the scheme with the highest cycle efficiency was selected. The gas cycle power is 10 MW, 100 MW, 300 MW and 600 MW respectively. SCO2 cycle adopts cascade cycle, which is configured with different cooler series, number of flue gas-SCO2 heat exchanger and number of shunt pipe respectively. Twelve SCO2 cycle comparison schemes are designed. The results show that with the increase of gas turbine power, the cycle efficiency of gas-fired SCO2 cycle system also increases. The highest cycle efficiency configuration scheme under different gas turbine capacities is three-stage SCO2 cycle with three-stage cooling, and the cycle efficiency is 0.508, 0.562, 0.591 and 0.622 respectively. From increasing device for gas-SCO2 combined cycle system, the influence of the cycle efficiency of gas-SCO2 circulation system with the cooler-SCO2 sets, flue gas heat exchanger sets and increase with the increase of the shunt pipe number, increase equipment to improve the cycle efficiency level is: increase the flue gas heat exchanger > SCO2 increasing bypass line > cooler.

Analysis and Performance Optimization of Supercritical CO2 Recompression Brayton Cycle Coupled Organic Rankine Cycle Based on Solar Tower

Abstract Supercritical carbon dioxide (SCO2) Brayton cycle has been proved to be an efficient power cycle to replace the traditional steam Rankine cycle. The thermal efficiency of SCO2 cycle can be further improved by coupling another type of cycle (the bottom cycle) at the waste heat end. A supercritical carbon dioxide recompression Brayton cycle (SCRBC) coupled organic Rankine cycle (ORC) based on solar tower is designed and established. According to the requirements of the waste heat temperature range of the top cycle, R600 is selected as the working medium of ORC. Under the design conditions, the effects of split ratio on the net power, the thermal efficiency, and the exergy loss of the combined cycle are studied. The variation of thermal efficiency of each part of the system with split ratio under different turbine inlet pressures and temperatures is further analyzed, and the influence of turbine inlet pressure and working fluid mass flow ratio ε (mass flow ratio of CO2 to R600) on the system performance is analyzed. Genetic algorithm-based multiobjective optimization is used to obtain the Pareto solution set for the thermal performance and unit investment cost of the system. The results show that the thermal efficiency of the combined cycle can be increased by more than 2% compared with that of a single top cycle. There is an optimal split ratio to maximize the thermal efficiency of the combined cycle, and the positions of the optimal split ratio are different for different turbine inlet pressures. Finally, through the multiobjective optimization method, several groups of Pareto solutions can be found, which can provide some reference for engineering design.

The comprehensive solution to decrease cooling wall temperatures of sCO2 boiler for coal fired power plant

Compared with water-steam cycle, supercritical carbon dioxide (sCO2) cycle has higher efficiency when applied in coal fired power plant. However, it also introduces challenges in boiler: because of the higher working fluid temperature and the lower convective heat transfer coefficient in boiler tubes, the cooling wall is more prone to overheating and bursting due to higher wall temperatures. Here, based on fundamental consideration of the thermal coupling between 3D radiation heat flux in furnace side and CO2 fluid in cooling wall tubes, we propose a comprehensive solution to decrease cooling wall temperatures. The solution includes four consecutive techniques: improved coupling in furnace width direction (CWD), flue gas recirculation for heat flux reduction (FGR), improved coupling in furnace height direction (CHD), and enhanced heat transfer in cooling wall tubes (EHT). A comprehensive thermal-hydraulic model is developed for a 1000 MWe power plant. It is found that the new solution can reduce the cooling wall temperatures from 670.5 °C to 635.0 °C, among which CWD, FGR, CHD and EHT contribute to the decrement of cooling wall temperatures by 13.3 °C, 4.4 °C, 6.8 °C and 11.0 °C, respectively, concluding that CWD and EHT are more effective than other techniques.

Techno-economic investigations of supercritical CO2-based partial heating cycle as bottoming system of a small gas turbine

Supercritical CO2 (sCO2) cycles represent an innovative technology especially if applied for waste heat recovery from gas turbines. This work investigates the performance of the partial heating sCO2 cycle as the bottoming cycle of a 5 MW-class gas turbine. Particular attention is paid to the selection of both minimum CO2 pressure and temperature and use of the non-dimensional criterion named Acceleration Margin to Condensation is made to prevent the formation of liquid CO2 droplets at the inlet of the compressor. Moreover, in order to avoid heavily loaded turbomachinery, considerations about the machine Mach numbers result in possible limits for the maximum cycle pressure. Single-stage radial turbomachines are selected and their efficiency calculated according to Aungier's correlations by taking actual size and running conditions into account. Focusing on the net electric power produced by the bottoming cycle and on the corresponding specific cost, a number of investigations has been carried out. After setting a limit for the compressor Mach number, around 1600 kW can be recovered by the sCO2 bottoming cycle, though a techno-economic optimization would guide towards an optimum point with slightly more than 1500 kW of power output and a specific cost for the technology of around 2000 $/kW.

Design optimization of an additively manufactured prototype recuperator for supercritical CO2 power cycles

Supercritical CO2 (sCO2) power cycles are being developed due to their potential for high efficiency and reduced capital cost. It is necessary that these recuperators operate at high pressures and temperatures, up to 30 MPa and 900 K, with effectiveness values > 95% and pressure drops <1% to achieve high cycle efficiencies. Moreover, it is also necessary to have reasonable cost recuperators to control the capital costs of the sCO2 power cycles. In this study, a Plate Pin-Fin (PPF) heat exchanger has been proposed as an sCO2 recuperator. This preliminary recuperator design leverages capabilities enabled by additive manufacturing. Although the PPF design has characteristics similar to those of a plate heat exchanger, small diameter and relatively long fins are used to increase surface area, enhance heat transfer, and provide structural support for the partition plates that separate the fluid streams. Existing correlations for heat transfer and pressure drop were adapted for the PPF heat exchanger. These correlations were implemented in a 1D analytical model and used for the optimization of a 5-kWth high temperature recuperator for an indirect sCO2 cycle by varying the design parameters to minimize the quantity of material required. A 3D conjugate heat transfer numerical simulations were conducted to validate the heat transfer and pressure loss correlations. A steepest descent method was used to minimize heat exchanger mass for a 5-kW prototype recuperator subject to a maximum specified pressure drop. The design analysis indicated that an optimum PPF recuperator would be attained for the minimum allowable pin transverse spacing, minimum pin width, minimum pin height and near maximum cell aspect ratio. At a low material requirement of 0.216 kg/kW and a pressure drop, which is almost five times lower than the allowable pressure drop design target, the optimized PPF heat exchanger has the high potential to be an alternative to a printed circuit heat exchanger, which is a conservative design basis for the current state-of-the-art sCO2 recuperators.

Systematic analysis of additives on the performance parameters of sCO2 cycles and their individual effects on the cycle characteristics

Compared to existing technologies, thermodynamic cycles based on supercritical carbon dioxide (sCO2) are leading to higher efficiencies and reduced component sizes. However, it is possible to further improve the performance of sCO2 power cycles by using mixtures of CO2 with suitable additives, as also discussed in the literature for different applications.This work investigates the potential to optimize the characteristics of sCO2 power cycles by selectively adding different substances in varying amounts to CO2. A new methodology is proposed: By using the reference equation of state for CO2 in combination with a multi-fluid mixture model, a theoretical screening of suitable additives was done. In the literature, studies were mainly limited to mixtures for which adjusted mixture models are available. By contrast, in this work the use of a predictive mixture model, which was recently developed at our institute, also allows to include fluids, for which no adjusted models are available. Applied to two thermodynamic cycles, changes in efficiency compared to the use of pure CO2 have been evaluated. Several promising mixture candidates have been identified. Additionally, individual effects on the cycle characteristics as well as shifts of the critical points have been investigated and are discussed.

Techno-economic investigation and multi-criteria optimization of a novel combined cycle based on biomass gasifier, S-CO2 cycle, and liquefied natural gas for cold exergy usage

It has long since been recognized that biomass energy can be used as a renewable source to convert chemical energy into power. In this research, a novel combined cycle is proposed for generating competent power from biomass chemical energy. Since the closed Brayton cycles have many advantages over open ones, the system is composed of two closed Brayton cycles with working fluids of nitrogen and supercritical carbon dioxide. Furthermore, to cool down the N2 compressors inlet temperatures to below 0℃, the technique of utilizing the liquefied natural gas is used. A comprehensive thermodynamic and economic study is performed, and the effect of key design variables is out posted by parametric evaluation. In a valuable procedure, the cost of the pollutants (NOx, CO, and CO2) emitted to the environment is found. Finally, to seek the best solution from the exergoeconomic aspect, the multi-criteria optimization is carried out, in which the results show that exergetic efficiency of 43.51% with a power cost of 19.78 $/GJ can be reached at the optimal point. Also, results indicate that the capital investment cost of gas turbines is higher than other elements; on the contrary, the destructed cost of the elements with chemical reactions like gasifiers and combustion chambers is the highest.

Optimization strategies of different SCO2 architectures for gas turbine bottoming cycle applications

Cycle architecture, fluid parameter selection, and component design of an exhaust/waste heat recovery cycle require an integral approach. The exhaust/waste heat shall be utilized to a maximum, at minimum costs. The bottoming cycle needs to be aligned with the topping cycle regarding operational behavior, especially for a part load. To analyze potentials of exhaust heat recovery in a combined gas turbine sCO2 cycle, the bottoming cycle's optimum cycle architecture and fluid parameters have to be determined. A thermo-physical model of the sCO2 bottoming cycle, including knowledge of component design, component behavior, and costs, is based on the optimization procedure. As part of the CARBOSOLA project, techno-economic optimizations for a use case of exhaust heat recovery have been carried out. The paper aims to present the optimization methodology followed by the specific use case's boundary conditions, investigated sCO2 cycle architectures, and results of optimum cycle architecture and fluid parameters for maximum heat recovery and minimum costs. Attention will also be paid to accurate modeling of heat exchangers operating near the critical point.

Comparison of a solar thermal tower power plant based on LiF-NaF-KF, NaCl-KCl-ZnCl2, and NaNO3-KNO3 as HTF under different DNI conditions for different power cycles

This work introduces the use of three different types of HTFs (NaNO3-KNO3, NaCl-KCl-ZnCl2, LiF-NaF-KF) and three different energy cycles (sCO2 Recompression, sCO2 Partial cooling, and Rankine) in two different locations (Seville and Dubai) for the solar thermal tower power plant. Detailed simulations have been carried out to calculate the solar to electric conversion efficiency and optimization is carried out to achieve maximum performance. The results obtained showed that the performance of the plant that is based on NaCl-KCl-ZnCl2 as HTF is least with the sCO2 cycles and only suitability is with the Rankine cycle. Plants based on NaNO3-KNO3 as HTF have shown similar results and it also has more suitability with the Rankine cycle. The plant with LiF-NaF-KF as HTF performed better with both sCO2 Recompression and sCO2 Partial cooling cycle but it is not recommended with the Rankine cycle due to poor performance. For all three HTFs, the sCO2 Partial cooling cycle is the best performance cycle for the optimized plant. The placement of the Seville plant in Dubai and after optimization increased efficiency by 4.29%, capacity factor by 81.48%, and electricity by 81.41%. The power factor was increased by 30.8, and an additional 76.86 GW of electricity was generated.

Comparison of sCO2 and He Brayton cycles integration in a Calcium-Looping for Concentrated Solar Power

Concentrated Solar Power is expected to play a crucial role to reach a sustainable power generation. Efficiency increase and development of long-term storages are the main aspects that could be further addressed to enhance the Concentrated Solar Power diffusion. In this context, the Calcium-Looping process is a promising opportunity for the ThermoChemical Energy Storage. Previous studies showed that, between the indirect integrations, Brayton cycles operating with supercritical CO2 or Helium are an interesting alternative. However, since their discussion has been developed in separate works, a detailed comparison of these integrations is lacking. The purpose of the present study is to find the differences, similarities and important aspects that characterize the two systems, performing a comparison in energy, economic and technical terms. Results show that choosing the He power block allows to reach the highest performances, while the cheapest alternative is the sCO2 cycle. Compression, regeneration, and high-temperature heating are the most important aspects for the He integration, differently from the case with sCO2, where only the regeneration process determines changes of the plant benchmarks. The comparison does not highlight univocally a better option, but according to the criteria required is possible to evaluate the most suitable alternative.

Adoption of the CO2 + SO2 mixture as working fluid for transcritical cycles: A thermodynamic assessment with optimized equation of state

This paper focuses on the use of the CO2 + SO2 binary mixture as innovative working fluid for closed transcritical power cycles with a minimum temperature above 50 °C. Starting from a literature review of the available experimental data on the mixture, the PC-SAFT EoS is identified as a suitable model to characterize the mixture behavior. Once the proper thermodynamic model is selected for this mixture, a comparison between the innovative transcritical cycle and the sCO2 cycle is proposed for various plant layouts in order to find out the advantages of the innovative mixture. The analysis is presented fixing the cycle maximum temperature at 700 °C and the maximum pressure at 250 bar: the results depict an increment in cycle electric efficiency and cycle specific work, along with a lower temperature of heat introduction in the cycle for any considered configuration of transcritical CO2 + SO2 cycle, when compared to pure sCO2.An economic analysis of the power block is then performed to support the selection of the innovative working fluid. Two of the most promising plant layouts are evidenced: the recompression layout is selected for highly efficient power blocks, while the dual recuperated layout works effectively in applications characterized by higher hot source exploitation. The recompression layout adopting the CO2 + SO2 mixture presents a power block electric efficiency of 48.67% (2.33% higher than the respective sCO2 cycle) and a reduction of the power block CAPEX from 1160 $/kWel to 1000 $/kWel when compared to the sCO2 configuration for a 100MWel size, while the dual recuperated layout exploiting the CO2 + SO2 mixture shows a power block electric efficiency of 39.58% (0.69% above the same sCO2 cycle), a decrease of power block CAPEX from 795 $/kWel to 718 $/kWel and 70 °C of additional heat recovery from the hot source with respect to the analogous sCO2 cycle.

Performance investigation of the solar power tower driven combined cascade supercritical CO2 cycle and organic Rankine cycle using HFO fluids

ABSTRACT Current study examined the effect of solar power tower (SPT) design parameters (solar emittance, concentration ratio and heat transfer fluid velocity, solar irradiation) on SPT-integrated combined cascade sCO2 (CSCO2) cycle and organic Rankine cycle (ORC) using ultra-low global warming potential (GWP) hydro fluoro olefins (HFO) fluids. Exergy efficiency, thermal efficiency and net output power were considered as performance parameters. A computational technique was used for the analysis. It was investigated that thermal and exergy efficiencies of the standalone (SPT+ CSCO2) cycle improved by 2.36% and 2.41%, respectively, by the incorporation of the ORC as bottoming cycle. Highest exergy efficiency, thermal efficiency and net output power were increased with solar irradiation, concentration ratio, heat transfer fluid velocity while decreased with solar emittance. Highest performance were found with R1224yd(E) while lowest with R1234yf among other considered low GWP fluids at current input conditions.