sCO2 Cycle Research Articles

Process Reduces Carbon Emissions From Natural Gas Compression and Production

_ This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 214974,“New Technology Reduces Carbon Emissions From Natural Gas Compression and Production Facilities,” by John Guoynes, SPE, David Stiles, SPE, and Cory Vail, Axip Energy Services, et al. The paper has not been peer reviewed. _ A chemical-free process has been developed to capture exhaust from natural gas drive compressors and supporting gas-fueled production equipment while featuring a small footprint. The process separates CO2 and nitrogen from exhaust, allowing the CO2 to be discharged at high pressure for transport, sequestration, or enhanced oil recovery. Introduction Over 50,000 compressors move natural gas in North America. Most are driven by either internal combustion engines or turbines. These compressors produce am estimate of more than 30 million tonnes of CO2 equivalent yearly, according to the US Environmental Protection Agency’s Greenhouse Gas (GHG) Reporting Program. The significance of GHG reduction policies has heightened with the passing of the Inflation Reduction Act, which provides support for companies investing in future carbon capture use and storage, along with methane reduction. Process Modeling Modeling and analysis of an exhaust-gas carbon capture system, and the design and optimization of a supercritical CO2 (sCO2) waste-heat-recovery power cycle, was performed. This cycle was chosen for its high efficiency and relatively compact design compared with competitive waste-heat-recovery technologies. The preliminary process-flow diagram was provided based on conditions for 30,000-hp large reciprocating gas engines. Several process configurations were investigated; these iterations are detailed in the complete paper. A summary of this development shows that Iterations 1 through 3 showed improvements in power through improving the sCO2 power-cycle efficiency. However, this came at the increase in high-pressure sCO2 components. A simplification of components returned the sCO2 cycle to a single cascaded cycle in Iterations 4 through 5. Temperature selection in the sCO2 cycle also improved its efficiency. Moving to Iteration 4 improved the system power requirement by 20.7%. Iteration 5 showed moderately reduced performance over Iteration 4. This was the result of shifting more duty to the ammonia chillers and a moderate worsening of performance in the gas compressor. Iteration 5 did not improve the power requirement of the system. Ultimately, Iteration 4 was the most-efficient cycle evaluated with the overall lowest power requirement. Exhaust-Capture Decarbonization System The system developed captures exhaust from compression and other internal combustion equipment commonly used in gas transmission, oil and gas production, and midstream applications. The system works with a chemical-free, cryogenic design with a small modular footprint to capture large volumes of exhaust gas at atmospheric pressure. This technology separates CO2 and nitrogen, generates enough power from waste heat to run the equipment, and discharges captured CO2 under high pressure at supercritical conditions for compressor stations of 5,000 to 20,000 hp. The design is fully self-contained and mounts on a compact skid package capable of handling up to 70 million scf/D of exhaust while extracting liquid CO2 from rich-burn driven compressors, lean-burn driven compressors, or turbines.

Numerical studies of sCO2 Brayton cycle

This work describes the one-dimensional, thermo-hydraulic model of the sCO2 cycle Sofia developed to investigate optimal control methods and the behaviour of the cycle during operation. This dynamic model includes all devices such as turbomachinery, heat exchangers, valves, and piping including heat loss, in line with concept of the 1 MWe sCO2 cycle, to be realised in the site of a fossil power plant in the Czech Republic.The model assembly and calculations were conducted using the commercial Modelica-based library ClaRa + using the simulation environment Dymola and in combination with another Modelica-based library, UserInteraction; the real-time simulations, with some parameter changes during the calculation, are made and described in this paper.Nominal parameters were achieved during the steady-state simulation, except for the lower mass flow of sCO2. Transient simulation of power turbine start-up from standby state and results are also presented in this paper. The nominal state is achieved with the semi-automatic procedure in approx. 3 h. The simulation results allow more detailed analyses of control methods and a better understanding of real device control and behaviour during start-up, shutdown, or other transients.Careful manipulation of turbine valves in coordination with the pressuriser operation was identified as crucial for optimal control of the system. Also, the initial amount of CO2 in the pressuriser affects its behaviour during transients.

Supercritical carbon dioxide cycle thermodynamic and exergoeconomic improvements using a bidirectional coupling strategy

Supercritical carbon dioxide (SCO2) combined cycle power system, one of the potentially effective ways to achieve energy grade utilization of the medium and high-temperature heat sources, is vulnerable to ambient temperatures as the system performance is susceptible to the compressor inlet temperature. So far, most studies have been focused on the system performance analysis and optimization under ideal operation conditions while the mitigation of unfavorable ambient temperature is missed. In this work, a bidirectionally-coupled system is proposed to reduce the compressor inlet temperatures in the original SCO2 cycle power system so that the adverse effects of ambient temperature are alleviated. The proposal utilizes the low-grade waste heat in a SCO2 recompression reheat Brayton cycle (i.e., topping cycle) to drive an absorption refrigeration cycle (i.e., bottoming cycle) which in turn precools the compressor inlet of the topping cycle. The thermodynamic and exergoeconomic analyses have been conducted for the proposed bidirectionally-coupled combined SCO2 RRBC and ARC system, with the attention to the effects of ambient temperatures on the system performance. Our results show that the combined SCO2 RRBC and ARC system increases the energy efficiency relative to the standalone cycle power system by over 5% at the ambient temperatures of 303.15 K or above, while the exergy efficiency improves by 1.124%–3.574%. In terms of the exergoeconomic factor, the combined SCO2 RRBC and ARC system also exhibits superiority against standalone cycle power system. The current work proves that a conventional recuperated SCO2 cycle system can be improved both thermodynamically and economically by coupling it to an absorption refrigeration cycle, despite of the additional upfront costs for the components. The discovery could further promote the commercial application of the SCO2 cycle power systems in new generation plants.

Parameters analysis and techno-economic comparison of various ORCs and sCO2 cycles as the power cycle of Lead–Bismuth molten nuclear micro-reactor

Organic Rankine cycle (ORC) and supercritical CO2 (sCO2) Brayton cycle are two key and competition technology routes as the power cycle of Lead–Bismuth micro-reactor (LBMR). However, few studies focus on the comparison of thermodynamic and economic performance between ORCs and sCO2 cycles in nuclear micro-reactor. The objectives in this study are to perform parameters analysis and techno-economic comparison in ORCs and sCO2 cycles with various configurations as the power cycle of LBMR, so as to provide the technical support for the selection of power cycle and its operating conditions in different scenarios. Several high/low-temperature working fluids are screened for ORCs in view to achieve high net efficiency (ηth) and low electricity production cost (EPC). Multi-objective optimization is performed to solve the trade-off between ηth and EPC, and then to compare the comprehensive performance of various cycles from thermodynamic, compact and economic aspects. Results show that, the ηth and EPC of ORCs are comparable or superior to that in sCO2 cycles when the two cycles have similar structures (simple/regenerative). Among the ORCs, the cascade ORC with two-side regeneration using N-Dodecane/Pentane as working fluid pair has the best performance. In all cycles, recompression sCO2 cycle performs best, but only when the heat source temperature is higher than 460 °C, the performance of the recompression sCO2 cycle are overall better than the cascade ORC.

Assessment of a sustainable multigeneration system integrating supercritical CO2 Brayton cycle and LNG regasification: Thermodynamic and exergoeconomic evaluation

This study proposes a cost-effective multigeneration system that integrates a supercritical carbon dioxide (sCO2) power cycle with a liquefied natural gas (LNG) regasification process. The system aims to deliver electricity, cooling, heating, and hydrogen simultaneously. Utilizing the temperature difference between the sCO2 cycle and the LNG thermal sink, an ammonia-water-based absorption refrigeration cycle (ARC) recovers waste heat from the sCO2 power cycle, producing cooling. The elevated ammonia concentration at the ARC's condenser outlet is leveraged by a heat pump system for heating production. Concurrently, cold energy from LNG is used to recover latent heat from the condenser and heat rejected from the sCO2 power cycle, providing inputs for additional electrical power through an NG turbine. The system incorporates a PEM electrolyzer (PEME) for hydrogen production. A comprehensive examination, including energy, exergy, and exergoeconomic analyses, evaluates system performance. A sensitivity analysis illustrates the impacts of key parameters. In the baseline scenario, the system achieves energy and exergy efficiencies of 62.79% and 54.89%, with a low product unit cost of 11.35 ($/GJ). The system demonstrates substantial net power output (259.184 MW), cooling capacity (88.938 MW), heating capacity (2.839 MW), and hydrogen production (391.68 kg/h). This robust performance, coupled with cost-effectiveness, positions the proposed system as a versatile and impactful solution for sustainable and environmentally friendly energy applications.

A universal ANN-based approach predicting PCHEs’ off-design performance across various operating conditions of sCO2 RCBCs

This paper proposes a universal approach that employs artificial neural networks (ANN) to predict the off-design performance of printed circuit heat exchangers (PCHEs) using supercritical carbon dioxide (sCO2) fluid. PCHEs are widely studied precoolers and recuperators of sCO2 power cycles, for which predicting their off-design heat transfer performance is challenging due to the fluid’s dramatic non-linear property variations. Due to their narrow valid range and constrained expressing ability, the existing empirical correlations might introduce significant out-of-scope errors that result in diverged simulation results, slow convergence and abnormal quitting. This work addresses the issue by proposing an ANN-based Nusselt number correlation, ANNu, to predict the off-design performance of PCHEs quickly and universally over the vast operating range of sCO2 cycles. Multiple techniques were adopted to promote ANNu’s prediction accuracy and generalization, such as consolidating 494 training data, data-driven input selection, and global optimization for the networks’ architecture and hyperparameters. This paper comprehensively evaluates ANNu’s performance by comparing its predictions with ten empirical correlations over four typical real-world scenarios, where ANNu features an overall MSE of 83.4222 and overall MAPE of 16.285 % that rank the second best and third best among eleven candidates (even overpassing two references), respectively. Moreover, ANNu exhibits good generalization and reliability, featuring MAPEs ranging from 9.89 % to 27.73 % when processing unmet experiment data from sCO2 PCHE applications.

Waste oxy-fuel combustion integrated with supercritical CO2 cycle and high-temperature electrolysis technologies for e-methanol production: A feasibility analysis

The municipal solid waste (MSW) oxy-fuel incineration power plant based on the supercritical carbon dioxide (sCO2) cycle integrated with high-temperature electrolysis technology has been developed with the aim of compensating the energy penalties of air separation unit (ASU), reducing the carbon emission and producing renewable methanol by CO2 hydrogenation process. Results shows that the suitable excess oxygen ratio, flue gas circulation ratio and methanol synthesis temperature are 1.1, 0.74 and 240 °C, respectively. The thermal efficiency of proposed system reaches 74.83%, above the that of traditional MSW incineration power plant. In addition, environmental analysis shows that the carbon retention index (CRI) is 97.76%, and the goal of near zero carbon emission can be achieved. Economic analysis demonstrates that the levelized cost of methanol (LCOM) of proposed system is 728.40 $/t, which is more than double the current market price of methanol. However, with the technological breakthrough and expansion of market scale in the further, e-methanol shows economic competitiveness when the cost of water electrolysis unit is reduced to 40 M$/250 MW and carbon emission credit is higher than 70 $/t. This work is innovative, and provides a high-efficiency and sustainable MSW disposal and energy recovering scheme for e-methanol production.

Transient simulation and analysis of a supercritical CO2 heat removal system under different abnormal operation conditions

The supercritical carbon dioxide (sCO2) heat removal system, which is based on multiple closed Brayton cycles with sCO2 as the working fluid, is an innovative, self-propelling and modular heat removal system for existing and future nuclear power plants. Previous studies analysed its design, layout, control and operation. In addition, this study considers different sudden failures during the accident progress, e.g. failure of single sCO2 cycles, control systems and valves. These abnormal conditions were investigated with the thermal-hydraulic system code ATHLET for a generic Konvoi pressurized water reactor. In most cases, the failure of a single sCO2 cycle can be compensated. On the one hand, failure of the fans of the gas cooler leads to a pressure increase which may be mitigated by an inventory control system or cycle shutdown. On the other hand, unintended fan speed-up can cause compressor surge without adequate countermeasures. Furthermore, the system can operate under the cyclic blow-off from the steam generator safety valves when the relief valves are not available. Finally, the unintended closure of the valve which controls the steam flow through the compact heat exchanger triggers a fast cycle shutdown but a subsequent restart might be possible.

Waste Heat Recuperation in Advanced Supercritical CO2 Power Cycles with Organic Rankine Cycle Integration & Optimization Using Machine Learning Methods

Supercritical CO2 (sCO2) stands out for concentrating solar power (CSP) due to its superior thermophysical and chemical properties, promising higher cycle efficiency compared to superheated or supercritical steam. Leveraging the waste heat from sCO2 cycles through the organic Rankine cycle (ORC) as a low-grade energy source enhances overall thermal efficiency. This research explores advanced sCO2 power cycles and introduces a novel approach by integrating machine learning and genetic algorithms for optimizing cycle performance. Utilizing a thermodynamic model-derived dataset, various machine learning algorithms, including Random Forest, XGBoost, and Artificial Neural Network are employed for prediction, evaluation and optimization. This innovative integration enables a comprehensive understanding of the complex dynamics of sCO2 power cycles. Subsequently, the study employs multi-objective optimization for the systematic evaluation and optimization of the combined power cycles, incorporating multiple bottoming cycles to maximize efficiency. The findings not only showcase the superiority of the unified sCO2ORC cycle but also emphasize the impact of integrating advanced computational methods in achieving optimal performance. The sCO2 cycle is explored in recompression, partial cooling, and main compression intercooling configurations. Recompression cycles utilize a single cooling system, while partial cooling and main compression intercooling layouts integrate a pair of ORCs at two precoolers. The ORC cycle enhances the recompression cycle through heat recuperation, extracting enhanced power originating from the bottom cycle. Critical parameters are taken into consideration to carry out optimization and performance evaluations. such as cycle temperatures, recuperator effectiveness, bottoming cycle pressure ratio, and condensation temperature. Results indicate that the combined partial cooling sCO2ORC cycle yields the optimal net output power of 7994.541 kW and thermal efficiency of 53.365 %. The findings highlight the capacity to propel and promote the utilization of waste heat recovery in energy technologies through enhanced and optimized power cycle designs, contributing to their advancement and widespread adoption.

Optimal design of a dual-pressure radial-inflow turbine for S-CO2 cycle based on constructal theory

Based on constructal theory, a dual-pressure turbine (DPT) model for supercritical CO2 cycle is established. Seven kinds of losses, such as the nozzle, incidence, passage, windage, tip clearance, trailing and exit velocity losses, are considered in the radial-inflow turbine. The volume fraction, wheel diameter ratio and inlet pressure of the DPT are optimized, and its power output (PO) is maximized. The parameter influences on constructal results are analyzed. It reveals that the total POs after primary and twice optimizations are 1.041 MW and 1.058 MW, and are augmented by 3.89 % and 5.62 %, respectively. The total PO after triple optimization is 1.148 MW, and the corresponding optimal volume fraction, wheel diameter ratio and inlet pressure are 0.194, 0.350 and 10600.2 kPa, respectively. The PO of DPT after optimization is improved by 14.57 %, and its performance is obviously improved by optimizing three variables. Among the discussed parameters, the total mass flow rate has a more significant effect on the triple maximum PO. The results supply valuable guidelines for the turbine designs of supercritical CO2 cycles.

Supercritical CO2 power cycle

The article is focused on summarizing the issue of energy circuits working with supercritical carbon dioxide sCO2. Carbon dioxide is currently coming to the forefront of many research institutions with regard to its possible use in its supercritical conditions, thanks to which higher efficiency and compactness of equipment can be achieved compared to conventional steam-water cycles. The article describes the basic properties of this medium, including a description of sCO2 cycles (Brayton, Rankine, etc.). Furthermore, the possible use of energy cycles with sCO2 is discussed, especially in solar and nuclear power plants and in power plants burning fossil fuels. In the case of solar power plants, the sCO2 Brayton cycle can offer higher cycle efficiencies than using super-critical steam cycles. The use of sCO2 in nuclear or coal power plants in turn increases efficiency and reduces water consumption. In addition, the use of the sCO2 circuit in new nuclear reactors of the IV generation is currently being considered, in which the operating temperatures are in the range of 500 - 900 °C, i.e. they are generally higher than operating water-cooled reactors. Even though the use of CO2 refrigerant in power systems has been known for a long time (e.g. use in the first Czechoslovak power plant in Jaslovské Bohunice), they are still not used commercially in the energy industry. One of the reasons is the ongoing research into the behavior of materials at high temperatures in sCO2, corrosion protection, or the effect of carbon dioxide purity on behavior in the supercritical region.

Multi-objective performance optimization & thermodynamic analysis of solar powered supercritical CO2 power cycles using machine learning methods & genetic algorithm

The present study is focused on multi-objective performance optimization & thermodynamic analysis from the perspectives of energy and exergy for Recompression, Partial Cooling & Main Compression Intercooling supercritical CO2 (sCO2) Brayton cycles for concentrated solar power (CSP) applications using machine learning algorithms. The novelty of this work lies in the integration of artificial neural networks (ANN) and genetic algorithms (GA) for optimizing the performance of advanced sCO2 power cycles considering climatic variation, which has significant implications for both the scientific community and engineering applications in the renewable energy sector. The methodology employed includes thermodynamic analysis based on energy, exergy & environmental factors including system performance optimization. The system is modelled for net power production of 15 MW thermal output utilizing equations for the energy and exergy balance for each component. Subsequently, thermodynamic model extracted dataset used for prediction & evaluation of Random Forest, XGBoost, KNN, AdaBoost, ANN and LightGBM algorithm. Finally, considering climate conditions, multi-objective optimization is carried out for the CSP integrated sCO2 Power cycle for optimal power output, exergy destruction, thermal and exergetic efficiency. Genetic algorithm and TOPSIS (technique for order of preference by similarity to ideal solution), multi-objective decision-making tool, were used to determine the optimum operating conditions. The major findings of this work reveal significant improvements in the performance of the advanced sCO2 cycle by 1.68 % and 7.87 % compared to conventional recompression and partial cooling cycle, respectively. This research could advance renewable energy technologies, particularly concentrated solar power, by improving power cycle designs to increase system efficiency and economic feasibility. Optimized advanced supercritical CO2 power cycles in concentrated solar power plants might increase renewable energy use and energy generation infrastructure, potentially opening new research avenues.

Constructal design of printed circuit recuperator for S-CO2 cycle via multi-objective optimization algorithm

Constructal design of printed circuit recuperator for S-CO2 cycle via multi-objective optimization algorithm

Supercritical carbon dioxide recovery system applied to cement industries

The paper addresses the potential heat-to-power application of supercritical CO2 (sCO2) plants to the cement industry, thereby reducing their electricity demand and improving energy efficiency. The research was conducted as part of the European project CO2OLHEAT (G.A. 101022831), which involves the installation of a 2 MW Waste-Heat-to-Power (WH2P) skid based on a sCO2 cycle in a cement plant, the first of its kind with a MW-scale power output. The paper summarizes technologies and processes employed in the Italian cement production sector, detecting where the waste heat can be successfully extracted to feed the recovery plant without compromising the industrial process. Moreover, the paper discusses the national cement market and explores the potential advantages and limitations of integrating sCO2 recovery plants within the national cement context, considering production and energy-related data. The final finding reveals the percentage of recoverable electricity per technological class for the cement production sector in Italy with a potential application of the sCO2 recovery plant, aiming at identifying the potential market penetration of the CO2OLHEAT installation.

WITHDRAWN: Integration of an O2 transport membrane and a biomass-fueled SCO2 cycle for liquid CO2/freshwater/power production: ANN-based optimization and techno-economic evaluation

This research presents a sustainable approach leveraging biomass as a renewable energy source and separation technology to produce liquid carbon dioxide for the food industry, alongside power and freshwater. The proposed system comprises five key sections. The biomass gasification section produces pure oxygen as a gasification agent using an oxygen transport membrane. The power generation section employs supercritical carbon dioxide oxy-fuel, enabling simultaneous power generation and carbon dioxide separation and purification. The freshwater production department uses a Multi-Stage Flash unit for desalination, drawing heat from the power generation cycle. The oxygen production section, based on oxygen transport membrane technology, supplies oxygen for the power generation cycle. The liquefaction section, utilizing the Claude cycle, facilitates carbon dioxide storage and transportation. The proposed system underwent rigorous techno-economic analysis and was further explored through multi-objective optimization based on artificial intelligence, aiming to minimize liquid carbon dioxide production costs and maximize net power output. Under pre-optimized conditions, the system exhibited a net power of 850 kW and a carbon dioxide price of $0.6/kg. Optimization results revealed a 12-year payback period, with a minimum carbon dioxide price of $0.51/kg and a maximum net power of 1692 kW. Comparative analysis under minimum carbon dioxide price conditions showed a 15% decrease in liquid carbon dioxide price and an 89% decrease in net power compared to pre-optimized conditions. Conversely, comparing the system's maximum net power after optimization to pre-optimized conditions demonstrated a 17% increase in liquid carbon dioxide price and a remarkable 99% increase in net power, highlighting substantial economic benefits.

A comparative study of combined cycles for concentrated solar power for efficient power generation using low Global Warming Potential (GWP) fluids to reduce environmental effects

AbstractA comparative analysis of a combined system comprising organic Rankine cycles (ORC) and supercritical CO2 (sCO2) cycles for concentrated solar power (CSP) applications was performed in the article. Low global warming potential (GWP) fluids were used to investigate strategies for reducing the effects on global warming. In the current study, four different combinations of the sCO2 cycles and ORC, including basic recuperated, pre‐compression, recompression with main compressor intercooling, and partial cooling, were taken into consideration, and their performances with regard to CSP application were compared. The integrated recompression with main compressor intercooling and ORC arrangement was seen to operate thermodynamically more efficient than the other considered configurations. This configuration's highest thermal and exergy efficiencies were discovered to be 51.9% and 55.84%, respectively, and R1243zf was found to be the best‐performing fluid among other considered low GWP fluids to reduce the environmental effects.

Performance analysis and multi-objective optimization of a novel poly-generation system integrating SOFC/GT with SCO2/HDH/ERC

The SOFC-based poly-generation system is an effective and environmental-friendly technology. To further improve energy efficiency of poly-generation system, a novel poly-generation system is developed based on solid oxide fuel cell/gas turbine (SOFC/GT) as the prime mover and supercritical carbon dioxide (SCO2) Brayton cycle, humidification and dehumidification (HDH) desalination and ejector refrigeration cycle (ERC) as the waste heat recovery units to produce power, cooling and freshwater. In the proposed system, a novel configuration of combining the SCO2 cycle with HDH is applied to recover the waste heat of SOFC/GT hybrid system. A mathematical model is developed to explore the system performances from the energy, exergy and economic perspectives. The effects of key parameters such as fuel flowrate, fuel utilization factor, SOFC operating pressure and temperature, and desalination top temperature on system performances are investigated. Multi-objectives optimization is conducted to obtain the optimal design parameters of the poly-generation system. The results show that the system net power output, cooling capacity and freshwater production are 273.769 kW, 12.997 kW and 24.23 g/s with the total cost being 14.01 $/h under the given condition, respectively. The electrical and exergy efficiencies and total gained output ratio of the system are 68.35 %, 67.23 % and 85.25 %, respectively. The parametric analysis indicates that an appropriate increase in fuel flowrate, fuel utilization factor and SOFC operating pressure and temperature can improve the electrical and exergy efficiency. The optimization results demonstrate that the system exergy efficiency and total cost are 67.48 % and 13.94 $/h.

The Allam Cycle: A Review of Numerical Modeling Approaches

In recent years supercritical CO2 power plants have seen a growing interest in a wide range of applications (e.g., nuclear, waste heat recovery, solar concentrating plants). The Allam Cycle, also known as the Allam-Fetvedt or NET Power cycle, seems to be one of the most interesting direct-fired sCO2 cycles. It is a semi-closed loop, high-pressure, low-pressure ratio, recuperated, direct-fired with oxy-combustion, trans-critical Brayton cycle. Numerical simulations play a key role in the study of this novel cycle. For this reason, the aim of this review is to offer the reader a wide array of modeling solutions, emphasizing the ones most frequently employed and endeavoring to provide guidance on which choices seem to be deemed most appropriate. Furthermore, the review also focuses on the system’s performance and on the opportunities related to the integration of the Allam cycle with a series of processes, e.g., cold energy storage, LNG regasification, biomass or coal gasification, and ammonia production.

Assessment of ternary CO2 mixtures as working fluids in supercritical Brayton cycles with floating critical points

The performance of the supercritical CO2 (sCO2) Brayton cycle deteriorates due to the mismatch between critical temperature and cold source temperature. An innovative solution based on ternary CO2 mixtures with adjustable compositions is proposed to expand the variation range of the critical temperature. An in-house MATLAB code is developed to investigate the design-point performance of the proposed system. The results indicate that the effect of improving cycle efficiency by using the modified floating critical point method becomes increasingly significant as the target critical temperature deviates from that of CO2. The CO2–H2S–Kr is screened out as the best ternary mixture, with the cycle efficiency being increased by over 8 % compared to the sCO2 cycle at very high (45 °C) and low (−15 °C) temperatures. The long-term performance is also evaluated based on the hourly historical weather data of two typical climate regions. The annual performance depends on both the average value and the distribution characteristics of the local ambient temperature. A relative improvement in annual average efficiency of 7%–9.56 % can be obtained depending on the regions. The present work is helpful to make the supercritical Brayton cycle give full play to its efficiency advantage in a wide cold source temperature range.

UTOP accident analysis and mitigation measures of lead-cooled fast reactor considering coupling effect with SCO2 Brayton cycle

The lead-cooled fast reactor (LFR) coupled with SCO2 Brayton cycle is an advanced nuclear power generation system which has high efficiency and high compactness. The purpose of this study is to investigate the impact of coupling effect between LFR and SCO2 cycle on the safety analysis of unprotected transient overpower (UTOP) accident, and to propose mitigation measures to UTOP accident with the aid of SCO2 cycle. Firstly, the dynamic simulation model of a 100 MW small modular natural circulation LFR coupled with SCO2 Brayton cycle was developed using APROS software, and was verified through the steady-state and transient simulations. Secondly, the UTOP accident of the LFR coupled with SCO2 Brayton cycle was analyzed, including a comparative case which simplified SCO2 cycle as a fixed boundary in the analysis. Finally, two measures to mitigate the UTOP accident by charging more SCO2 to the circulation loop from the storage tank and by maintaining constant SCO2 temperature at the inlet of in-reactor heat exchangers (IRHXs) were proposed. The results indicate that the conventional method of simplifying power generation system as the fixed boundary underestimated the severity of accident. Specifically, the maximum fuel cladding temperature was about 40 °C higher when considering the coupling effect of reactor and SCO2 cycle than that of assuming SCO2 cycle as the fixed boundary, which greatly exceeded the safety limit. Moreover, within the scope of present UTOP accident conditions, the measures proposed in this study could mitigate the severity of accident. The maximum cladding temperature was reduced by 15–23 °C in the condition of maintaining constant SCO2 temperature at IRHX inlet, and was reduced by 5–7 °C in the condition of charging more SCO2 to the circulation loop, compared with the condition not applying any measures.