Supercritical CO2 Recompression Cycle Research Articles

Dynamic performance and control analysis of supercritical CO2 brayton cycle for solid oxide fuel cell waste heat recovery

Due to the high-temperature operation of solid oxide fuel cell, recovering its exhaust waste heat is worthwhile. The supercritical CO2 cycle features low compression power consumption and high thermal efficiency, making it a promising power cycle. Therefore, this paper adopts the supercritical CO2 recompression cycle as the bottom cycle, combined with solid oxide fuel cell to form a joint system, further improving energy utilization. Given the long temperature response time of the solid oxide fuel cell and the variability of its exhaust waste heat with load changes, this paper innovatively proposes establishing an integrated dynamic model of the joint system, instead of directly providing heat source flow and temperature changes as in traditional research, to more accurately study the dynamic response of the cycle during solid oxide fuel cell startup and load variations. Based on this, a control strategy utilizing inventory control is proposed, and the feasibility of the control effect is verified. The research results indicate that the uncontrolled cycle may lead to safety issues with a significant increase in turbine inlet temperature when solid oxide fuel cell load increases. Under the control strategy, the turbine inlet temperature and main compressor inlet temperature are rapidly stabilized at the target values. The controlled cycle achieves higher efficiency compared to the uncontrolled cycle, particularly prominent during solid oxide fuel cell load reduction, with an efficiency increase of approximately 5.23%.

A recompression supercritical CO2 cycle using heat source storage to achieve low-frequency reactor regulation: Analysis and optimization of load capacity under safe compressor operation

This study focuses on proposing a heat source storage mode for the indirect recompression supercritical CO2 cycle (RSCC) coupled with a sodium-cooled fast reactor, aiming to achieve low-frequency regulation of the reactor and enhance the load capacity of system. A deep neural network-based turbomachinery performance prediction approach is employed to assess the system's part-load and overload performance under several typical control strategies. The turbine inlet temperature of the system under all control strategies except the intermediate heat exchanger (IHX) bypass rises with decreasing load. The system's part-load performance is positively correlated with its corresponding overload power. The findings reveal that the adjustable load ranges of the heat source storage tanks under inventory, turbine bypass, turbine throttle, and IHX bypass controls are 10%–156.10%, 10%–156.10%, 30%–147.30%, and 10%–146.88%, respectively, along with the safe compressor operation. As the near-choke operation of the compressor limits the rise in overload power, a turbine fitted with variable inlet guide vanes is tried. The optimization results demonstrate that the largest overload power of the system can reach 161.18 % without reducing the thermal efficiency. This study provides a heuristic insight into the variable load operation for indirect nuclear-powered Brayton cycles under low-frequency reactor regulation.

Investigation on the integration of supercritical CO2 cycle with natural gas oxy-fuel combustion power plant

The utilization of LNG cold energy is promising to offset the energy penalty of oxy-fuel combustion technology and achieve efficient carbon emissions reduction. In this paper, natural gas oxy-fuel power systems utilizing LNG cold energy are investigated. Supercritical and transcritical CO2 cycles are adopted to deeply recover flue gas waste heat. Results show that the thermal efficiency of the supercritical CO2 recompression cycle is 6% higher than that of the regenerative cycle, whereas the net power output of the recompression cycle is lower. With transcritical CO2 cycle as the secondary subsystem, the efficiency of both integrated systems is enhanced by approximately 9%. Meanwhile, 95% CO2 is captured with the power consumption of 0.07 kWh/kg-CO2. The total investment costs of the integrated regenerative and recompression cycle are 53.96 M$ and 54.62 M$. The levelized cost of electricity of the two integrated systems is 3.21 $/MWh and 3.24 $/MWh, indicating great financial availability. Therefore, applying supercritical and transcritical CO2 cycles contributes to efficient energy conversion and carbon capture. The research could provide meaningful information on the oxy-fuel combustion power system integrated with supercritical and transcritical CO2 cycles from thermodynamic and economic insights.

Energy and exergy analyses of a recompression supercritical CO2 cycle combined with a double-effect parallel absorption refrigeration cycle

In this study, a new cogeneration system is presented that can make optimal use of the waste heat from the sCO2 power cycle using a double-effect parallel absorption refrigeration cycle (DEARC), generating electricity and refrigeration simultaneously. A comparative study between the proposed (sCO2/DEARC) system and the stand-alone sCO2 power system is conducted to show its advantages and potential. The performances of the sCO2 and sCO2/DEARC systems are evaluated from the energetic and exergetic points of view using an in-house developed thermodynamic simulation platform via Engineering Equation Solver (EES) software. Variables such as pressure ratio, turbine inlet temperature, and evaporator temperature are also used for parametric analysis. The results indicate that the proposed system can generate of 132.72 MW cooling capacity. Also, the results show that the proposed sCO2/DEARC system can achieve a 55.8% and 5.2% improvement in thermal efficiency and exergy efficiency, respectively, with a negligible decrease in net power output (Wnet) compared to the stand-alone sCO2 power plant.

Experimental study of the startup of a supercritical CO2 recompression power system

The supercritical CO2 Brayton cycle is a revolutionary power conversion technology. However, due to the lack of experimental research, the cycle's dynamic characteristics and control rules are still unclear. This study presents experimental research of a 300 kW supercritical CO2 recompression cycle power system based on two integrated high-speed turbo-alternator-compressor units, and characteristics of dynamic control, flow transition, and thermal inertia of the cycle are presented. The difficulty of generator startup increases with initial pressure, therefore a low-pressure startup strategy is proposed and verified. During startup, the closing time of turbine bypass value and control performance of the main compressor inlet temperature are found to have great influence on the stable operation of the cycle, and relevant control strategies are verified and discussed. The study also presents the existence, transformation mechanism, and flow characteristics of four flow modes during the startup of recompression cycle. These findings are of great help to judge the flow pattern of CO2 in the complex piping system and improve the transparency and safety level of system operation. Test results also show that despite the large thermal inertia, the load of recompression cycle can be quickly adjusted.

Multimode operation control strategy for improving part-load performance of supercritical CO2 Brayton cycle

The recompression supercritical CO2 (S-CO2) Brayton cycle is regarded as one of the most promising energy conversion systems owing to its high efficiency and compactness. Because the recompressor impairs the performance of a system at low loads, the concept of multimode operation by switching the recompressor on or off is proposed to improve the part-load performance. In this study, a dynamic model of the recompression S-CO2 cycle with reheating is developed, and three control strategies, including turbine throttle valve control, turbo-alternator-compressor (TAC) speed control, and inventory control, are selected to follow the part-load. Subsequently, the action mechanism of the multimode operation and the effects of different control strategies on the part-load performance of the system under multimode operation is investigated and compared. Simulation results indicate that the improved thermal efficiency by switching operation modes is due to improvement in the thermal match inside the high-temperature recuperator and reduced degradation of the recompressor. Moreover, switching the system to Mode 2 by switching off the recompressor at 20% load under TAC speed control, turbine throttle valve control, and inventory control can improve thermal efficiency by 1%, 3.5%, and 5.4%, respectively. This result can serve as a guideline for the efficient operation of the recompression cycle under part-load.

Design and Off-design Performance Analysis of a Zigzag Channeled Precooler for Indirect Cooling System of Supercritical CO2 Recompression Cycle Incorporated with a Flow-Bypass System

The employment of a Printed Circuit Heat Exchanger (PCHE) as a Precooler in an indirect cooling system of a supercritical CO2 (sCO2) power block is the prevailing trend in research since cooling the working fluid with water offers superior cooling performance compared to an air-cooled system. The enhanced cooling performanceofthe Printed Circuit Heat Exchanger offers the desired sCO2 compressor inlet temperature close to the pseudo-critical point, maximizing the thermodynamic efficiency of the cycle. The improbable fluctuating properties of sCO2 near the critical region make it arduous to design a component that operates near the critical region. However, the high heat transfer coefficient in the near-critical region increases the power cycle’s performance and reduces the cycle components’ size, making it cost-effective. In the indirect cooling system of the sCO2 recompression cycle, the working fluid (sCO2) is cooled by cooling water in a Printed Circuit Heat Exchanger known as the Precooler. Thus, the heat gained by the cooling water is cooled by the air-cooled heat exchanger unit inside a Natural draft dry cooling system. In this study, an iterative nodal approach is implemented to develop a Python code for designing a Zigzag channeled Printed Circuit Heat Exchanger as the Precooler for the indirect cooling system of the sCO2 recompression cycle for the Concentrated Solar Power application. The off-design performance study is conducted under different sCO2 inlet temperatures (338 K–378 K), inlet pressure (7 MPa–10 MPa), and a wide range of cooling water inlet temperatures (283 K–318 K), suggesting the heat transfer rate can be enhanced by lowering the water inlet temperature and sCO2 working pressure. Nevertheless, the variation in sCO2 input temperature has little influence on the precooler heat transfer rate. The result also demonstrates that the pressure drop on the sCO2 side decreases upon increasing these parameters. The off-design performance improvement during low ambient temperature is achieved by introducing a flow bypass in the power block, which optimizes the precooler sCO2 mass flow to maintain the desired compressor inlet temperature.

Optimization and performance analysis of a near-zero emission SOFC hybrid system based on a supercritical CO2 cycle using solar energy

Renewable energy represented by solar energy has the characteristics of intermittency and fluctuation. To ensure that the grid can operate safely and stably after the large-scale grid connection of renewable energy power, a new hybrid system consisting of a solid oxide fuel cell (SOFC), a solar power tower plant, and a supercritical CO2 (S-CO2) Brayton cycle is proposed in this study from the perspective of energy, exergy, and economy. By utilizing the cold source loss of the S-CO2 cycle to provide part of the energy required for the alkaline electrolyzer, it can efficiently produce and store hydrogen, achieving zero greenhouse gas emissions while increasing efficiency. Meanwhile, the high-temperature exhaust gas from SOFC is recovered by the S-CO2 cycle. The system considers two operation modes, switching two types of S-CO2 cycle configurations to efficiently utilize solar energy according to the variation of solar radiation energy. The thermal efficiency of the proposed system is improved by 16.61% and 6.88% compared with the other two typical S-CO2 cycle power generation systems, and the net power output reaches 8.0 MW. A parameter study shows that increasing the split ratio is beneficial to improve the thermodynamic performance of the system. However, at a split ratio less than 0.76, it is beneficial to the total cost rate of the system. The results of multi-objective optimization show that the exergy efficiency of the combined system and the total system cost rate reach 56.86%, 513.10$/h and 56.45%, 481.59$/h for the system with an additional simple S-CO2 cycle and the system with an additional recompression S-CO2 cycle, respectively, and the optimum point is selected by the TOPSIS and LINMAP methods.

Integrated design and off-design hybrid control strategy of supercritical CO2 recompression cycle for nuclear power

The supercritical CO2 power cycle has attracted extensive attention for its potential application in next-generation nuclear reactors. In this study, the integrated design and off-design control strategies of supercritical CO2 recompression cycle for a small-scale lead-cooled fast reactor were investigated. After determining the optimal design conditions of the system through thermodynamic and exergoeconomic models, the off-design operating states of the compressors of the system controlled by four strategies were evolved. According to the compressor operation map under each control strategy, a series of hybrid control strategies based on inventory control were proposed. The design results indicate that the system has the highest efficiency of 42.12% and the lowest total production unit cost of 11.67 $∙GJ−1 when the split ratio is 0.37. The off-design results indicate that the re-compressor of the turbine bypass-controlled systems faces chock with decreasing load due to the increase of the optimal split ratio, which can be alleviated by adopting an inventory-bypass hybrid control strategy. Moreover, this hybrid control strategy both improves the thermal efficiency of the bypass-controlled system and protects the re-compressor of the inventory-controlled system away from the stall boundary. Because of the analogous operation footprints on the compressor map, a hybrid strategy consisting of inventory control and turbine inlet temperature control can achieve a similar effect as inventory-bypass hybrid control. The analysis results provide theoretical guidance for the selection of off-design control strategies for the supercritical CO2 recompression cycles applied in various energy fields.

Dynamic characteristics of a recompression supercritical CO2 cycle against variable operating conditions and temperature fluctuations of reactor outlet coolant

The transient performance of an inventory-controlled supercritical CO2 recompression cycle under synchronous adjustment of parameters has rarely been studied. In this study, a one-dimensional design of a supercritical CO2 recompression cycle with a 10 MW output power was completed for a lead-cooled fast reactor. The dynamic characteristics of the system power, which decreased from full load to half load under three ramp signals, were compared. Furthermore, the effect of the fluctuating reactor outlet coolant temperature on the transient performance of the system was investigated. Steady-state analysis revealed that the maximum design efficiency of the system was 43.40% at a split ratio of 0.37. The dynamic results showed that the shorter the ramp signal for parameter adjustment, the more significantly the transient performance of the system degraded. When the load decreased significantly, a secondary efficiency trough appeared in the system. These poor phenomena were improved by appropriately extending the action time of the ramp signal. Additionally, the influence of coolant temperature fluctuation on the temperature at both ends of the recuperator differed owing to the difference in the heat capacity. Moreover, the reduction in the target load of the system made the fluctuation range wider for efficiency but narrower for power.

Multi‐objective optimization of thermoeconomic and component size of supercritical carbon dioxide recompression cycle based on small‐scale lead‐cooled fast reactor

When the supercritical CO2 power cycle is employed in confined spaces, such as nuclear-powered ships and spacecraft, its size should be given priority. To estimate the component size, the one-dimensional heat exchanger model developed in this study is used in a recompression supercritical CO2 cycle integrated on a small-scale lead-cooled fast reactor. A parameter analysis was performed to study the influence of several key parameters on the levelized cost of electricity, thermal efficiency, and system size. Moreover, four types of multi-objective optimizations were conducted to provide optimization schemes for different scenarios. Results indicated that the increased turbine inlet temperature improved the thermoeconomic but augmented the system volume. The size of the low-temperature recuperator was observably enlarged near the optimal flow split ratio, thereby increasing the system volume. Pareto optimal solution of bi-objective optimization based on the levelized cost of electricity and system size reached the lowest system volume of 3.71 m3. Additionally, the best trade-off result of three-objective optimization was a thermal efficiency of 42.14%, a levelized cost of electricity of 56.30 $∙MWh−1, and a system volume of 4.43 m3. Meanwhile, maximal and minimal pressure drops of CO2 appeared in the cooler and intermediate heat exchanger, respectively.

Design and Off-Design Performance Analysis of a Zigzag Channeled Pre-Cooler for Indirect Cooling System of Supercritical Co2 Recompression Cycle

Design and Off-Design Performance Analysis of a Zigzag Channeled Pre-Cooler for Indirect Cooling System of Supercritical Co2 Recompression Cycle

Off-design operation of the dry-cooled supercritical CO2 power cycle

The supercritical CO2 recompression cycle (the ‘sCO2 cycle’) is highly efficient, scalable, simple, and compatible with dry cooling. Two scenarios where dry-cooled sCO2 cycles may operate off design are (1) during periods of high ambient temperature, and (2) when operating at part-load output (in load-following roles). Understanding sCO2 cycle off-design performance is essential for financial feasibility assessment, detailed design, and providing control system setpoints. Off-design performance can be maximised by exploiting the cycle’s pressure, temperature, and turbomachinery speed degrees of freedom. Several previous studies arbitrarily constrain these degrees of freedom, giving sub-optimal performance predictions. Other studies fail to consider CO2’s steep and non-linear property variations, leading to inaccurate results. To address these limitations, this work presents a detailed sCO2 cycle model, which captures property changes and component off-design behaviour. The model is used to systematically explore the off-design operating space, thus showing novel off-design performance trends and best operating strategies. Elevated ambient temperatures reduce power output more than efficiency, but this effect can be mitigated with appropriate operating strategies. At an ambient temperature of 40 °C (10 °C above design), design-point power output can be maintained with an 8.7% minimum turbine pressure increase, alongside compressor speed control. Alternatively, at this same temperature, design-point power output can be maintained with only a 4.4% (2 percentage point) efficiency reduction and a slightly higher (13.7%) turbine pressure increase. During turn-down, design-point efficiency can be maintained at 44% of design-point power output by using pressure and compressor speed control. Main compressor speed control alone is effective for above-design ambient temperatures, while dual compressor speed control provides significant benefits during turn-down. Compared to past sCO2 cycle studies, this work reports better off-design performance, which demonstrates the importance of off-design operating strategies and highlights the flexibility and potential of the cycle.

Size optimization of heat exchanger and thermoeconomic assessment for supercritical CO2 recompression Brayton cycle applied in marine

When the state-of-the-art supercritical CO2 cycle is applied to the marines, priority should be given to the feasibility of its component size. In this study, a thermodynamic model of the marine supercritical CO2 recompression cycle was developed. To evaluate the thermoeconomic and component size of the system, the recuperator and precooler with a microtube structure were designed. After quantitatively discussing the impact of the two-stage compression mode on the component scale, the sizes of the heat exchangers were optimized based on the maximum allowable pressure drop. The parameter analysis demonstrates that increasing the speed of the power turbine is beneficial to the system thermoeconomic. Not only can an efficiency improvement of 1.73% and a cost saving of 1.77 $/MWh be achieved, but also the implementation of two-stage compression reduces the volume of the high-temperature recuperator, low-temperature recuperator, and precooler by 4.62%, 6.33%, and 37.4%, respectively. The size optimization analysis suggests that the direction with the longest distance should be regarded as the length constraint of the heat exchanger design in the absence of multi-objective optimization. Moreover, this study has guiding significance for the size optimization of the supercritical CO2 cycle used in a limited space.

Optimization of the supercritical CO2 power conversion system based on the net efficiency under conditions of the pulse-operated fusion power reactor DEMO

Fusion power plants represent a new energy technology with specific features including multiple heat sources with different outlet temperature and power, fluctuating high self-consumption, and in the case of the demonstration fusion power plant, the pulse operation of the heat sources. The supercritical CO2 (S-CO2) Brayton cycle was applied and optimized under DEMO conditions with the nuclear net efficiency as the main optimization criterion. The optimization was performed for helium-cooled and water-cooled reactor blanket concepts, and for simple and recompression S-CO2 cycle layouts with four heat sources. The optimization process used a brute-force search problem-solving technique applied to multiparametric space. The model data was taken from the European model of the fusion power plant DEMO1 2019. When applying the simple S-CO2 cycle, the net efficiency of the model power plant with the helium-cooled and water-cooled blanket was found to be 19.5% and 12.3%, respectively, whereas when applying the recompression S-CO2 cycle, the net efficiency was found to be 16.4% and 8.1%, respectively. The simple S-CO2 cycle provided higher net efficiency for the model fusion power plants with the given configuration of multiple heat sources than the recompression S-CO2 cycle, and the model power plants using the helium-cooled blanket achieved higher net efficiency compared to the power plants using the water-cooled blanket despite significantly higher self-consumption. The application of the simple S-CO2 cycle allowed the model fusion power plant with the helium-cooled blanket to achieve the net efficiency higher than the reference net efficiency of 17.9% of the current DEMO model using the steam cycle. For the power plants with the water-cooled blankets, S-CO2 cycles were found to be less efficient than the steam cycles.

Energy and exergy analyses of an improved recompression supercritical CO2 cycle for coal-fired power plant

An improved recompression S–CO2 cycle for coal-fired power plant is proposed in order to improve boiler thermal efficiency, increase cycle exergy efficiency, and resolve issues of regulating the working fluid temperature of the boiler rapidly and effectively. The models for energy and exergy analyses are built. Systematic comparisons between the improved S–CO2 cycle system and the original S–CO2 cycle system are carried out based on the detailed energy and exergy analyses. The results suggest that the improved S–CO2 cycle is excellent in both energy and exergy efficiencies, the thermal efficiency and exergy efficiency of the boiler in the improved S–CO2 cycle system are 94.68% and 53.41%, respectively, which are 1.04% and 2.76% higher than those of the original S–CO2 cycle system. The power efficiency of the improved S–CO2 cycle system is higher than that of the original one, which are 48.06% and 47.89%, respectively. The exergy efficiency of the entire cycle system of the improved S–CO2 cycle is 48.24%, which is 2.32% higher than that of the original S–CO2 cycle. In addition, the spray attemperator performance is investigated. The boiler in the improved S–CO2 cycle can regulate the working fluid temperature effectively.

Development and comparison of control schemes for the off-design operation of a recompression supercritical CO2 cycle with an intercooled main compressor

Four control schemes with different main compressor outlet pressure control modes and/or parametric optimization options are developed for the off-design operation of a supercritical carbon dioxide cycle with an intercooled main compressor. The cycle performance under four control schemes is first evaluated under possible off-design conditions and then compared with two case studies on annual performance. The off-design results indicate that the parametric optimization can simplify the main compressor control during the off-design operation regarding the surge risk control. The results also indicate that parametric optimization under different pressure control mode results in significant differences regarding the cycle performance improvement. The parametric optimization under the variable pressure control mode leads to more noticeable cycle efficiency improvement (0.02%–23.1%) than under the fixed pressure mode (0.02%–16.7%), but also larger reductions in net output power and temperature differential compared to those with the latter scheme (8.4–47.7 MW and 6.1–26.1 °C). The hybrid scheme of these two modes is recommended to accommodate different operating scenarios. The optimum annual cycle efficiency with an efficiency improvement up to 11% is obtained with the parametric optimization under the variable pressure control mode at the design-point main compressor inlet temperature of 41 °C.

Thermodynamic design and Off-design investigation of nuclear power supercritical CO2 recompression cycle

The off-design behavior of sCO2 recompression cycle caused by adjusting the CO2 flow to meet the production remains to be one of the most concerned problems. In this paper, a thermodynamic model of sCO2 recompression cycle based on sodium-cooled fast reactor was established. And the sensitivity analysis of inlet conditions at turbomachinery and cycle pressure ratio (PR) was conducted in combination with the flow split ratio (FSR). Based on the highest cycle efficiency, the performance analysis of the main compressor was performed by employing a similar method, which was extended to the system with the aim of investigating the off-design behavior. The sensitivity analysis results indicate that enlarging p1 can reduce the sensitivity of T1, and when the compressor inlet fluid approaches the critical state (304.12 K 7.38 MPa), the cycle efficiency reaches maximum at FSR = 0.31 and PR = 2.9. Then the off-design performance of the system in the case of flow adjustment is successfully predicted, during which excessive flow is found to reduce the output power of the system and lead to a maximum efficiency difference of about 13%. Also observe that the efficiency of the re-compressor in single-stage recompression mode is significantly lower than that of the main compressor and turbine. Finally, the mechanism of two-stage recompression to improve the compactness and off-design performance of the system is clarified and the applicability of the two-stage turbine without intermediate reheat is discussed.

Simulation of a Supercritical CO2 Recompression Cycle with Zero Emissions

AbstractThe NET power cycle (NPC) offers an alternative way to inherently capture combustion derived CO2 with low efficiency penalty. This manuscript improves the NPC by recompressing a portion of ...

Advanced power cycles for coal-fired power plants based on calcium looping combustion: A techno-economic feasibility assessment

Carbon capture and storage is crucial to decarbonising the power sector, as no other technology can significantly reduce emissions from fossil fuel power generation systems. Yet, the mature CO2 capture technologies result in net efficiency penalties of at least 7% points. Emerging technologies, such as calcium looping combustion, can reduce the net efficiency penalty to 2.4% points. Further reductions can be achieved by replacing the conventional steam cycle with advanced power cycles. This study aimed to assess the techno-economic feasibility of the coal-fired power plant based on calcium looping combustion with different advanced Brayton cycles. These included single power cycles, such as recompression supercritical CO2, simple supercritical CO2 cycle, and xenon cycle, as well as combined power cycles based on helium, nitrogen and recompression supercritical CO2 cycles. The net efficiency and break-even electricity price, which was estimated using the net present value method, were used as the key techno-economic performance indicators. A parametric study was also conducted to assess the impact of the key thermodynamic parameters. This study showed that the case based on a single recompression supercritical CO2 cycle had the best overall techno-economic performance, while the recompression supercritical CO2 combined cycle case had the best techno-economic performance among combined cycle cases. The former was characterised with a net efficiency of 38.9%, which is higher than that of the reference coal-fired power plant without CO2 capture (38.0%). Such performance was achieved at a break-even electricity price of 71.2 €/MWel,neth, corresponding to a cost of CO2 avoided of 16.3 €/tCO2.