Multi-objective optimization and evaluation of supercritical CO_(2) Brayton cycle for nuclear power generation

The supercritical CO_(2) Brayton cycle is considered a promising energy conversion system for Generation IV reactors for its simple layout,compact structure,and high cycle efficiency.Mathematical models of four Brayton cycle layouts are developed in this study for different reactors to reduce the cost and increase the thermohydraulic performance of nuclear power generation to promote the commercialization of nuclear energy.Parametric analysis,multi-objective optimizations,and four decision-making methods are applied to obtain each Brayton scheme’s optimal thermohydraulic and economic indexes.Results show that for the same design thermal power scale of reactors,the higher the core’s exit temperature,the better the Brayton cycle’s thermo-economic performance.Among the four-cycle layouts,the recompression cycle(RC)has the best overall performance,followed by the simple recuperation cycle(SR)and the intercooling cycle(IC),and the worst is the reheating cycle(RH).However,RH has the lowest total cost of investment(C_(tot))of$1619.85 million,and IC has the lowest levelized cost of energy(LCOE)of 0.012$/(kWh).The nuclear Brayton cycle system’s overall performance has been improved due to optimization.The performance of the molten salt reactor combined with the intercooling cycle(MSR-IC)scheme has the greatest improvement,with the net output power(W_(net)),thermal efficiencyη_(t),and exergy efficiency(η_(e))improved by 8.58%,8.58%,and 11.21%,respectively.The performance of the lead-cooled fast reactor combined with the simple recuperation cycle scheme was optimized to increase C_(tot) by 27.78%.In comparison,the internal rate of return(IRR)increased by only 7.8%,which is not friendly to investors with limited funds.For the nuclear Brayton cycle,the molten salt reactor combined with the recompression cycle scheme should receive priority,and the gas-cooled fast reactor combined with the reheating cycle scheme should be considered carefully.

Design Optimization and Operating Performance of S-CO_(2) Brayton Cycle under Fluctuating Ambient Temperature and Diverse Power Demand Scenarios

The supercritical CO_(2)(S-CO_(2)) Brayton cycle is expected to replace steam cycle in the application of solar power tower system due to the attractive potential to improve efficiency and reduce costs.Since the concentrated solar power plant with thermal energy storage is usually located in drought area and used to provide a dispatchable power output,the S-CO_(2) Brayton cycle has to operate under fluctuating ambient temperature and diverse power demand scenarios.In addition,the cycle design condition will directly affect the off-design performance.In this work,the combined effects of design condition,and distributions of ambient temperature and power demand on the cycle operating performance are analyzed,and the off-design performance maps are proposed for the first time.A cycle design method with feedback mechanism of operating performance under varied ambient temperature and power demand is introduced innovatively.Results show that the low design value of compressor inlet temperature is not conductive to efficient operation under low loads and sufficient output under high ambient temperatures.The average yearly efficiency is most affected by the average power demand,while the load cover factor is significantly influenced by the average ambient temperature.With multi-objective optimization,the optimal solution of designed compressor inlet temperature is close to the minimum value of35℃ in Delingha with low ambient temperature,while reaches 44.15℃ in Daggett under the scenario of high ambient temperature,low average power demand,long duration and large value of peak load during the peak temperature period.If the cycle designed with compressor inlet temperature of 35℃ instead of 44.15℃ in Daggett under light industry power demand,the reduction of load cover factor will reach 0.027,but the average yearly efficiency can barely be improved.

Thermodynamic Performance Comparison and Optimization of sCO_(2)Brayton Cycle,tCO_(2)Brayton Cycle and tCO_(2)Rankine Cycle

In this paper,to further improve thermodynamic performance of supercritical carbon dioxide cycle,simple/recompression transcritical carbon dioxide Brayton cycle(STBC/RTBC)and simple/recompression transcritical carbon dioxide Rankine cycle(STRC/RTRC)are proposed.Thermal and exergy performance analysis and optimization for the above four transcritical CO_(2)cycles and simple/recompression supercritical cycle(SSBC/RSBC)are conducted.The effect of key thermodynamic parameters on those CO_(2)cycle performance is studied.Results indicate that the improvements of thermodynamic performance of CO_(2)cycle are obvious when transcritical Brayton and Rankine cycle are applied in it.Within the same range of optimization variables,the maximum thermal efficiency improvements of RTRC and RTBC are 4.98%and 3.6%,and maximum exergy efficiency improvements of RTRC and RTBC are 7.08%and 5.13%when compared with RSBC.Moreover,the thermodynamic performances of STBC and STRC are also outstanding than that of SSBC.This work provides a way to further improve the thermodynamic performance of CO_(2)power cycle.

Investigation on One-Dimensional Loss Models for Predicting Performance of Multistage Centrifugal Compressors in Supercritical CO Brayton Cycle

The main compressor in a supercritical carbon dioxide(SCO2)Brayton cycle works near the critical point where the physical properties of CO_(2)are far away from the ideal gas.To investigate the effectiveness of the conventional one-dimensional(1D)loss models for predicting the performance of compressors working in such nontraditional conditions,detailed comparisons of 1D predicted performance,experimental data and threedimensional CFD results are made.A 1D analysis method with enthalpy and total pressure based loss system is developed for multistage SCO2 centrifugal compressors,and it is firstly validated against the experimental results of a single stage SCO2 centrifugal compressor from the Sandia National Laboratory.A good agreement of pressure ratios with experiments can be achieved by the 1D method.But the efficiency deviations reveal the potential deficiencies of the parasitic loss models.On the basis of the validation,a two-stage SCO2 centrifugal compressor is employed to do the evaluation.Three-dimensional CFD simulations are performed.Detailed comparisons are made between the CFD and the 1D results at different stations located in the compressor.The features of the deviations are analyzed in detail,as well as the reasons that might cause these deviations.

Dynamic simulation of a space gas-cooled reactor power system with a closed Brayton cycle

Space nuclear reactor power(SNRP)using a gas-cooled reactor(GCR)and a closed Brayton cycle(CBC)is the ideal choice for future high-power space missions.To investigate the safety characteristics and develop the control strategies for gas-cooled SNRP,transient models for GCR,energy conversion unit,pipes,heat exchangers,pump and heat pipe radiator are established and a system analysis code is developed in this paper.Then,analyses of several operation conditions are performed using this code.In full-power steady-state operation,the core hot spot of 1293 K occurs near the upper part of the core.If 0.4$reactivity is introduced into the core,the maximum temperature that the fuel can reach is 2059 K,which is 914 K lower than the fuel melting point.The system finally has the ability to achieve a new steady-state with a higher reactor power.When the GCR is shut down in an emergency,the residual heat of the reactor can be removed through the conduction of the core and radiation heat transfer.The results indicate that the designed GCR is inherently safe owing to its negative reactivity feedback and passive decay heat removal.This paper may provide valuable references for safety design and analysis of the gas-cooled SNRP coupled with CBC.

Multiparameter optimization and configuration comparison of supercritical CO_(2) Brayton cycles based on efficiency and cost tradeoff

Supercritical CO_(2)Brayton cycle has high efficiency,compactness,and excellent power generation potential.In the design of the cycle,some parameters,such as recuperator pinch point temperature difference(ΔTrec,pp),turbine inlet temperature(Ttur,in),and maximum cycle pressure(pmax),are often preset without optimization.Furthermore,different preferences on efficiency and cost tradeoff can significantly affect the optimal design of the cycle,and the influence of different parameters on the design condition and the optimum cycle configuration becomes unclear as the preference changes.In this study,different preferences on efficiency and cost tradeoff are considered,and the effects of cycle configuration and optimization parameter addition on the tradeoff are investigated.In addition,four configurations under different preferences on tradeoff are recommended.Results show that the design condition parametersΔT_(rec,pp) decrease and T_(tur,in) and pmax increase as the preference of thermal efficiency(W_(th))increases.Different optimized parameters affect the results of the design point and cycle performance.In addition,the simple recuperative cycle and reheating cycle are recommended when low cycle initial cost dominates(W_(th)<0.598),and the recompression cycle and intercooling cycle are recommended when high cycle thermal efficiency dominates(W_(th)>0.701).The decision maker can select appropriate configuration according to specific preferences.

Review on heat-to-power conversion technologies for hypersonic vehicles

Hypersonic vehicles have enormous military and economic value,while their power and thermal protection demands will increase substantially with the rise in Mach number and duration.Converting the tremendous high-quality heat on the vehicle surface and engine wall into electrical energy through heat-to-power technologies will not only play a role in thermal protection,but also supply power for the vehicle.This paper provides a comprehensive review of heat-to-power conversion technologies on hypersonic vehicles,including the indirect conversion of Brayton and Rankine cycles,direct conversion of thermoelectric materials,and combined conversion.For the open Brayton cycle with hydrocarbon fuel as the working fluid,the Power-to-Weight Ratio(PWR)can achieve the highest,at around 1.8,due to the high PWR of the hydrocarbon fuel turbine and the few components of the system.However,its work capacity is limited by the flow rate of the supplied fuel.The closed Brayton cycle can maintain a relatively high PWR,ranging from 0.2 to 0.8,while achieving relatively high output power and conversion efficiency.The Rankine cycle has a higher PWR,its range is close to that of the closed Brayton cycle,peaking at about 0.88.The thermoelectric materials technology has a small power generation level,making it more suitable for scenarios with low power demand.This review provides a basis for selecting and developing heat-to-power conversion technologies on hypersonic vehicles.

Innovative power generation systems using supercritical CO_(2) cycles

Supercritical carbon dioxide(sCO_(2))power cycle is an innovative concept for converting thermal energy to electrical energy.It uses sCO_(2)as the working fluid medium in a closed or semi-closed Brayton thermodynamic cycle.The sCO_(2)power cycles have several benefits such as high cycle efficiency,small equipment size and plant footprint(and therefore lower capital cost)and the potential for full carbon capture.Achieving the full benefits of the sCO_(2)cycle depends on overcoming a number of engineering and materials science challenges that impact both the technical feasibility of the cycle and its economic viability.For example,the design and construction methods of turbomachinery,recuperator and high-pressure oxy-combustor pose significant technical challenges.Other R&D needs include material selection and testing,and optimized power cycle configuration.Over the years,particularly in the last decade,R&D efforts have been growing worldwide to develop sCO_(2)cycle technologies for power generation.Significant progress has been made in developing sCO_(2)cycle power systems.Some small,low-temperature sCO_(2)Brayton cycle power systems are starting to emerge in the commercial market,and a natural gas-fired demonstration power plant using a sCO_(2)cycle called the Allam Cycle is under construction.This article describes the sCO_(2)cycles for applications in power generation from fossil fuels and reviews the recent developments in sCO_(2)power cycle technologies.

Preliminary design of an SCO_(2) conversion system applied to the sodium cooled fast reactor

The supercritical carbon dioxide(SCO_(2))Brayton cycle has become an ideal power conversion system for sodium-cooled fast reactors(SFR)due to its high efficiency,compactness,and avoidance of sodiumwater reaction.In this paper,the 1200 MWe large pool SFR(CFR1200)is used as the heat source of the system,and the sodium circuit temperature and the heat load are the operating boundaries of the cycle system.The performance of different SCO_(2) Brayton cycle systems and changes in key equipment performance are compared.The study indicates that the inter-stage cooling and recompression cycle has the best match with the heat source characteristics of the SFR,and the cycle efficiency is the highest(40.7%).Then,based on the developed system transient analysis program(FR-Sdaso),a pool-type SFR power plant system analysis model based on the inter-stage cooling and recompression cycle is established.In addition,the matching between the inter-stage cooling recompression cycle and the SFR during the load cycle of the power plant is studied.The analysis shows that when the nuclear island adopts the flow-advanced operation strategy and the carbon dioxide flowrate in the SCO_(2) power conversion system is adjusted with the goal of maintaining the sodium-carbon dioxide heat exchanger sodium side outlet temperature unchanged,the inter-stage cooling recompression cycle can match the operation of the SFR very well.