sCO2 Brayton Cycle Research Articles

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.

Thermodynamic Analysis of a Combined Recompression Supercritical Carbon Dioxide Brayton Cycle With an Organic Flash Cycle for Hybrid Solar-Geothermal Energies Power Generation

Exploring renewable energy is beneficial for ameliorating the energy crisis and reducing environmental emissions. The hybrid utilization of solar and geothermal energies is an effective way to improve the existing energy consumption structure dominated by fossil energy. This paper proposes a novel power generation system composed of a topping recompression supercritical carbon dioxide (sCO2) Brayton cycle and a bottom organic flash cycle (OFC) driven by the hybrid solar-geothermal energies. The sCO2 Brayton cycle is driven by the heat from the solar tower system, and the OFC is driven by a part of the heat from CO2 in the sCO2 Brayton cycle and another part of the heat from the geothermal water. The corresponding energy and exergy analyses of the proposed combined system are presented. The effects of the five main parameters on the system thermodynamic performance are carried out, which are direct radiation intensity, concentration ratio, sCO2 pressure ratio, preheater terminal temperature difference, and flash temperature. Results show that the OFC with R245ca has the highest exergy efficiency among the different four fluids. The energy efficiency and exergy efficiency of the total system are 26.03% and 33.38%, respectively, since the energy losses exist in the heliostat field and central receivers. There observes that through the parametric study the parameters of direct radiation intensity and concentration ratio are larger causing better system thermodynamic performance. Through the thermodynamic analysis, there observes the power cycle subsystem has the largest energy loss, while the central receiver possesses the highest among other subsystems.

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.

Completely Recuperative Supercritical CO2 Recompression Brayton/Absorption Combined Power/Cooling Cycle: Performance Assessment and Optimization

Excessive heat losses and water consumption in cooling units are significant constraints restricting the application circumstances and performances for the SCO2 Brayton cycle, and the heat exchange capacity in the precooler (PRC) is typically 1.5 times that of power generation. Therefore, this research offers a high-integrated combined power/cooling system in which two waste heat exchangers (WHEs) and a rectifier (RET) are used instead of the PRC to achieve 100% exhaust heat recovery. Each component’s energy and exergy models are developed, and the operational characteristics, coupling relationships, and exergy destruction distribution are examined. Results indicate that, when compared to the Brayton cycle, the thermal and exergy efficiency is considerably increased, and the concentration difference and WHE1 pitch point difference have significant influences on system performance. Further exergoeconomic and optimization analysis reveals that the superior exergy case is mostly recommended for relevant thermal and exergy efficiency increasing rates of 13.7% and 9.17%, respectively, and the unit cost is 81.33% that of the base case. Turbine 1 (TUR1) and main compressor (MCP) are the first and second highest cost rates, respectively, and RET and generator (GEN) account for roughly 34% exergy destruction rate and 20% exergy destruction cost rate, respectively. In addition, reducing heat transfer differences in relevant equipment can further promote system performance.

Comparative analysis on design and off-design performance of novel cascade CO2 combined cycles for gas turbine waste heat utilization

To provide a higher energy conversion efficiency for gas turbines (GT), this paper proposes a novel two-stage cascaded supercritical CO2 and transcritical CO2 (sCO2-tCO2) power cycle for waste heat recovery (WHR) of GT exhaust. A comparative study on system design and off-design performance is conducted between the typical GT-sCO2 cycles and the novel GT-cascaded CO2 (GT-CCO2) cycles based on the detailed steady-state mathematical models and self-built simulation platform. The simple layout (SSBC) and recompression layout (RSBC) are chosen to represent the typical sCO2 Brayton cycle configurations. The simulation results show that the SSBC-tCO2 cycle is more suitable than the RSBC-tCO2 cycle to work as the bottoming cycle for GT since the SSBC-tCO2 cycle can produce higher power with a simpler configuration. Compared with the traditional GT-RSBC and GT-SSBC, the optimal GT-CCO2 cycle (GT-SSBC-tCO2) gains an improvement of 5.32% and 4.32% for the thermal efficiency, and a decrement by 4.08% and 2.42% for the Levelized cost of electricity respectively. The combined GT-CCO2 cycles show superior performance with the variable inlet guide vane modulation during the off-design conditions because this control strategy increases the GT exhaust gas temperature and thus brings about a large improvement of the bottoming cycle output power. These findings could provide references for high efficiently utilizing gas turbine exhaust and verify the commercial viability of the CO2 system.

Analysis and Development of a Small-Scale Supercritical Carbon Dioxide (sCO2) Brayton Cycle

Carbon dioxide’s (CO2) ability to reach the supercritical phase (7.39 MPa and 304.15 K) with low thermal energy input is an advantageous feature in power generation design, allowing for the use of various heat sources in the cycle. A small-scale supercritical carbon dioxide (sCO2) power cycle operating on the principle of a closed-loop Brayton cycle is currently under construction at The University of Texas at San Antonio, to design and develop a small-scale indirect-fired sCO2 Brayton cycle, acquire validation data of the cycle’s performance, and compare the cycle’s performance to other cycles operating in similar conditions. The power cycle consists of four principal components: A reciprocating piston compressor, a heating source, a reciprocating piston expander to produce power, and a heat exchanger to dissipate excess heat. The work explained in the present manuscript describes the theory and analysis conducted to design the piston expander, heating source, and heat exchanger in the cycle. Theoretical calculations indicate that using sCO2 for the Brayton cycle generates 4.5 kW of power with the inlet pressure and temperature of 17.23 MPa and 358.15 K to the piston expander. Based on the fully isentropic conditions, the thermal efficiency of the system is estimated to be 12.75%.

Multi-objective performance optimization of regenerative S-CO2 Brayton cycle based on neural network prediction

• A finite time thermodynamic model for regenerative S-CO 2 Brayton cycle is set up. • Artificial neural networks and multi-objective optimization are used. • Four different optimization objectives are considered. • The thermal efficiency and net power output are increased by 31.15% and 43.29%. • The exergy efficiency is increased by 33.75%. The regenerative supercritical CO 2 Brayton cycle (RSCBC) has great development potential in waste heat recovery and utilization, and it is necessary to carry out performance analysis and optimization. This article first applies the theory of finite time thermodynamics to establish a RSCBC model with finite temperature difference heat transfer, irreversible compression, irreversible expansion and other irreversible factors under variable temperature heat source conditions, and then uses the data samples to construct the corresponding neural network model. Based on the NSGA-Ⅱ algorithm, the working fluid mass flow rate, the pressure ratio, the heat conductance distribution ratios of the regenerator and the heater are chosen as optimization variables, multi-objective optimization is carried out with the goals of cycle thermal efficiency, net power output, ecological function and exergy efficiency. The results show that the use of neural network models to predict cycle performance can save a lot of calculation time compared to traditional calculation methods; after optimization, the positive ideal point is not on the Pareto front, which shows that the four optimization objectives are mutually restricted and affect each other. The results by using Shannon Entropy method for decision-making have a lower deviation index, and those by using TOPSIS and LINMAP methods for decision-making are consistent with each other; for the results by using Shannon Entropy method for decision-making, the cycle thermal efficiency and net power output can reach 38.4% and 12.047 MW respectively, which can be increased by 31.15% and 43.29% compared to those for the initial design point respectively, and the ecological function and exergy efficiency can reach 7.6274 MW and 73.2% respectively, which are 4.2588 times and 33.75% higher than those for the initial design point respectively. The obtained results can provide some guidance for the optimal design of the RSCBC in real engineering.

Three-dimensional shape optimization of fins in a printed circuit recuperator using S-CO2 as the heat-transfer fluid

Printed circuit heat exchangers (PCHEs) are among the most promising candidates for recuperators in the S-CO2 Brayton cycle owing to their high-pressure resistance and high compactness, where the fins are usually installed in channels to improve the thermal-hydraulic performance. The fin shape optimizations have been performed in several previous studies for enhancing the heat transfer and reducing the pressure drop. However, most of them are limited to two-dimensional optimization where the shape variation along the height direction was ignored. In the present study, three-dimensional fin shape optimizations are performed for the PCHE recuperators using S-CO2 as heat transfer fluid to further improve the thermal-hydraulic performance. The heat transfer and pressure drop characteristics in channels with optimized fins are comprehensively evaluated by comparing against conventional cylindrical fins and airfoil fins. The results indicate that the optimized three-dimensional fins feature variable cross-sections along the height direction. Such three-dimensional fins can significantly diminish the impact area of fluid, which is conducive to reducing the local flow resistance caused by high-velocity gradient and consequently improving the comprehensive heat transfer performance. Under the typical working condition studied in this paper, the heat transfer capacity in hot channels of recuperators with the optimized three-dimensional fins is 21.2% greater than the cylindrical fins, and 17.3% greater than the airfoil fins when the identical pressure drop is fixed; while the heat transfer capacity in cold channels with the optimized three-dimensional fins is 24.3% greater than the cylindrical fins, and 29.3% greater than the airfoil fins. In addition, the robustness analysis indicates that the optimized three-dimensional fins can yield more excellent thermal-hydraulic performance than conventional cylindrical fins and airfoil fins over a wide range of working conditions.

Analysis of Turbomachinery Losses in sCO2 Brayton Power Blocks

Abstract This paper analyzes the contribution of different turbomachinery loss mechanisms to the overall efficiency of a simple recuperated supercritical carbon dioxide (s-CO2) Brayton cycle for output capacities ranging from 100 kW to 1 GW. The optimum turbomachinery specifications suitable for the specified powers are retrieved using a standard design tool that provides information on various turbomachinery losses. The losses are influenced by operating pressures and mass flowrates, which are unknown a priori. An iterative approach is used to arrive at the turbomachinery efficiency and mass flowrate. Earlier studies have shown the dependence of optimal pressures on heat source and sink temperatures alone. This analysis reveals that design-point optimal cycle pressure ratios differ with varying power outputs due to differences in realizable turbomachinery efficiencies. The information on dominant loss mechanisms provides insights on a viable scale of power generation at which s-CO2 Brayton cycles become worthwhile. Poor turbomachinery efficiencies (less than 80%) render the s-CO2 technology commercially unviable at the sub-MW scale. For higher power scales (10 MW and above), axial machines are found to be appropriate, with corresponding turbomachinery efficiencies greater than 85%. The dominant loss mechanisms also help identify issues related to improving turbomachinery efficiencies at the sub-MW power levels, where the cycle efficiencies are not competitive.

Solid particle solar receivers in the next‐generation concentrated solar power plant

AbstractSolid particles are generally considered to be the most suitable heat transfer fluid (HTF) and thermal energy storage (TES) materials for the next‐generation concentrated solar power (CSP) plant. The operating temperature of the solar receiver can be raised to exceed 800°C by the application of appropriate solid particles. In this way, power conversion efficiencies greater than 50% can be achieved with the supercritical carbon dioxide (sCO2) Brayton cycle. Solid particle solar receiver (SPSR) is the key equipment to absorb the concentrated solar flux, and its thermal performance is remarkably affected by receiver system designs, particle flow characteristics, and properties of solid particulate materials. This paper provides an in‐depth review of various SPSR technologies, as well as pertinent solid particle selections, optimization of the receiver system structures, particle flow characteristics, and heat transfer characteristics. The technical drawbacks, the large‐scale development prospects, and the potential optimization strategies of the various SPSR designs are highlighted by the comparative analysis of multiple parameters.image

Dynamic Characteristic Study of Supercritical CO2 Recompression Brayton Cycle System

The supercritical carbon dioxide (SCO2) Brayton cycle has been regarded as the main development direction of future nuclear power generation by more and more scholars, due to its high environmental efficiency and high thermoelectric conversion rate. However, due to fluctuations in the operation of the primary loop of the system with nuclear energy, parameters such as the power of the heat source and the mass flow of the working medium in the system will change, which will affect the dynamic performance and operation of the SCO2 Brayton cycle system. Therefore, it is necessary to study the dynamic response of the system performance under disturbance conditions, analyze the operating characteristics of the SCO2 Brayton cycle system. In this paper, a comprehensive dynamic model of SCO2 recompression Brayton cycle, which analyzes the response curves of critical parameters under the disturbance of heat source heating power and system mass flow rate, is accurately developed based on Simulink software. In order to verify the validity of the proposed model, the simulation results are compared with the experimental results conducted by Sandia Laboratory under the same conditions. The results show that the model has high accuracy, and can reflect the dynamic response of system performance under parameter perturbation. In this paper, the closed-loop simulation is innovatively performed to show the dynamic response to step-change in the heat source power and mass flow rate. And the thermal efficiency is about 31.85%, when the system operates stably at the design point of working condition. If a disturbance is applied to the system, the temperature change will be mainly concentrated near the heat source of the cycle, and the change near the precooler will be relatively small. The change of the heat source power will lead to a large monotonic variation of cycle efficiency. By contrast, an inflection point in cycle efficiency will be resulted in by changing the system mass flow rate. The results of this paper would provide good approaches for the design, control, and improvement of the SCO2 Brayton cycle.

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.

Comparison of different load-following control strategies of a sCO2 Brayton cycle under full load range

The supercritical CO2 Brayton cycle has been regarded as a promising next-generation power conversion system owing to its flexibility and high efficiency. Dynamic performance and control strategy are essential research topics when systems are subjected to various load demands. Inventory control has been proven as a very effective load control method and valve controls have the potential to meet wider load demands. While few studies have focused on the differences in efficiency and system performance under different control strategies. In this study, a dynamic model of recompression supercritical CO2 Brayton cycle is proposed, and its components are carefully validated. Moreover, an inventory and anti-surge coupled control strategy is proposed to achieve better control performance. Under the premise of considering system security, various control strategies meeting 0%–100% load range are compared. The primary objective of this study is to reveal the differences in efficiency and dynamic performance of various control strategies in the full load range, to provide a basis for the selection of control strategies in off-design conditions. The results demonstrate that inventory and anti-surge coupled control allow safe tracking loads as low as 0%, and it provided an absolute advantage in terms of efficiency compared to valve controls.

Power conversion from spherical tokamak test reactor with helium-cooled and water-cooled blanket

Possible configurations for Power Conversion System (PCS) for a futuristic, small, pulsed Spherical Tokamak (ST) fusion reactor are presented. For the typical blanket coolant choices namely, helium and water, an intermediate heat-exchanger (IHX) cum buffer energy storage system (ESS) with Molten Salt (MS) is introduced to examine Rankine and Brayton cycle’s effectiveness. Preliminary thermal sizing of the involved heat-exchangers and parameters of the optimized cycle are discussed. S-CO2 Brayton cycle seems to be a promising candidate for efficient power conversion for a pulsed plasma reactor with efficiency of 42.75%. Rankine cycle yields efficiency of 35–42% depending on water/helium as the choice of the blanket coolant.

Optimal Design of Airfoil Fin Microchannel Heat Exchanger for the S-Co2 Brayton Cycle of Generation IV Nuclear Reactors

Optimal Design of Airfoil Fin Microchannel Heat Exchanger for the S-Co2 Brayton Cycle of Generation IV Nuclear Reactors

Impact of Startup Strategy and Buffer Tank on the Compressor Inlet State for a 5mwth Sco2 Brayton Cycle

Impact of Startup Strategy and Buffer Tank on the Compressor Inlet State for a 5mwth Sco2 Brayton Cycle

Development of a sustainable multi-generation system with re-compression sCO2 Brayton cycle for hydrogen generation

Current research aims to develop, design, and analyze a novel solar-assisted multi-purpose energy generation system for hydrogen production, electricity generation, refrigeration, and hot water preparation. The suggested system comprises a solar dish for supplying the necessary heat demand, a re-compression carbon dioxide-based Brayton cycle, a PEM electrolyzer for hydrogen generation, an ejector refrigeration system working with ammonia, and a hot water preparation system. The first law and exergy analyses are implemented to determine the performance of the multi-generation plant with various outputs. Besides, the exergo-environmental evaluation of the plant is conducted for the environmental impacts of the plant. Furthermore, parametric analyses are executed for investigating the system outputs, exergy destruction rate, and system efficiencies. According to the results, the rate of hydrogen generated by means of the multi-generation power plant is determined to be 0.062 g/s which corresponds to an hourly production of 0.223 kg. Besides, with the utilization of the supercritical closed Brayton cycle, a power generation rate of 74.86 kW is achieved. Furthermore, the irreversibility of the overall plant is estimated as 535.7 kW in which the primary contributor of this amount is the solar system with a destruction rate of 365.5 kW.

A potentially non-contact monitor method for CO2 at the pseudo-critical region using infrared spectrometer

Supercritical carbon dioxide (sCO2) Brayton cycle is a promising choice for thermal power generation with the advantages of high efficiency, compactness and low cost. There is a very sensitive part for monitor and control in the cycle, i.e. CO2 at the critical region, which affects the efficiency of compressor greatly and maybe brings safety problems due to the rapid and great changes in the properties of CO2 near the critical point. A non-contact measurement method is proposed to monitor the transient state of CO2 at various operating conditions using an infrared spectrometer. Experimental investigations were carried out to verify the adaptability of the method at dynamic processes in transcritical CO2 loop tests. We found the density change of CO2 fluid can directly impact the absorption spectrum near the wavelength of 3400 nm, which could serve as an efficient signal of CO2 state changes. In a steady state, a dramatic reduction of the density (− 70 %) and the absorption ratio (− 60 %) were observed between 30 and 34 ºC. In the dynamic processes, density-related scattering phenomena were observed near the pseudo-critical points, which strengthened the non-transmissive ratio (up to 0.95) of CO2 and gave a quick signal (<1 s) whenever abnormal condition occurred. Density stratification was further observed in the heating process at 32.5 ºC and 7.5 MPa, which was close to the critical point. Moreover, the gasification time was more than five times longer than the liquefaction time, which caused a stronger scattering phenomenon in depressurization processes. This work is expected to serve as the foundation work of the potentially non-contact monitor in the sCO2 Brayton cycle or other applications.

Design and Performance Analysis of a Solar-Coal-Fired Complementary Power System Based on the S-CO2 Brayton Cycle

Abstract To make solar energy conversion more effective and enable effective complementary utilization of multiple energy sources, two types of solar-coal-fired complementary power (SCCP) systems, which use the supercritical CO2 Brayton cycle, are investigated and their layouts are improved. In addition, a thermodynamic performance analysis is carried out. The results show that, as the amount of work done by the solar energy module increases, the coal saving rate increases linearly and proportionally in both SCCP systems. Also, the supplementary electric power generated by the solar field increases. The two improved layouts increase the net efficiency of the SCCP systems significantly (SCCP1: from 43.60% to 47.65%, SCCP2: from 43.60% to 47.67%). More specifically, the net efficiency of the improved layout for SCCP2 increases faster than that for SCCP1 (with its improved layout), when the second split ratio (SR2) exceeds 0.031. When the net efficiency remains unchanged, the SR2 for SCCP2 improved layout has a wide range. Furthermore, both the operation performance and operating mode conversion of the basic system are studied for varying sunlight conditions. The simulation results are consistent with the expectations, which underlines the development potential of the system to a certain extent.

Molecular dynamics investigation on isobaric heat capacity of working fluid in supercritical CO2 Brayton cycle: Effect of trace gas

Supercritical CO2 Brayton cycles have become a research hotspot in high-efficiency power production. Adoption of other gas into CO2 is proved to be an effective way to improve the performance of supercritical CO2 cycle. In practice, it is inevitable to mix impurity gas into CO2. The additive gas would have significant effects on isobaric heat capacity of CO2 system. However, the lack of isobaric heat capacity of CO2 mixture gas over supercritical regions of CO2 poses a challenge in the design and optimization of supercritical CO2 Brayton cycle.In this work, N2 and Xe, whose mole fraction ranges from 1% to 50 %, is considered as an additive gas in CO2. The isobaric heat capacity of pure CO2, CO2/Xe and CO2/N2 binary mixture is calculated via molecular dynamics simulation. Compared to the commonly utilized database REFPROP, it is found that molecular dynamics simulation exhibits comparable and acceptable performance in prediction of the isobaric heat capacity of the mixed CO2 system. The absolute average deviate of isobaric heat capacity of pure CO2 and CO2/N2 binary mixtures is less than 1% and 1.3 % compared to the REFPROP database, respectively. Then, the effect of the Xe and N2 on the isobaric heat capacity is discussed. The results demonstrated that the isobaric heat capacity would decrease continuously when CO2 is mixed with Xe and N2. The prediction models and data put forth in this paper offer great values for practical application system involving S-CO2 Brayton cycle.