Supercritical CO2 Brayton Cycle Research Articles

Performance response to operating-load fluctuations for Sub-megawatt-scale recuperated supercritical CO2 Brayton cycles: Characteristics and improvement

The Sub-megawatt-scale supercritical CO2 Brayton cycles (SCBCs) have shown great potential in renewable energy and mobile power systems, but the response of technical performances to fluctuating operating load has not yet been completely known even for those basic cycle layouts. For this purpose, the present work investigated the performance characteristics of the global operating loads for the 0.1 and 1 MWe-scale recuperated SCBS systems. A thermodynamic model was proposed and demonstrated to be feasible to predict the cycle performance with 0.19%–9.59% relative error. Using the control-of-variables strategy, the response of cycle efficiency and net power were comprehensively examined to the 90%–110% reference load (namely ±10% fluctuations) of operating parameters, including the compressor inlet pressure, compressor inlet temperature, turbine inlet pressure, turbine inlet temperature, and mass flow rate. It was found that although the cycle performances depend upon the facility scale, the appropriate parameter matching can effectively improve the cycle efficiency and net power by 31.97% and 76.42% for the 0.1 MWe-scale cycle, and by 13.89% and 26.69% for the1 MWe-scale cycle respectively at a given the mass flow rate of CO2. The results provide a positive reference for the performance assessment and operation optimization of the small-scale recuperated SCBC systems.

Comparative Performance Evaluation of Gas Brayton Cycle for Micro–Nuclear Reactors

Gas Brayton cycles have been considered the next promising power cycles for microreactors. Especially the open-air and closed supercritical CO2 (S-CO2) Brayton cycles have received attention due to their high thermal efficiency and compact component sizes when compared to the steam Rankine cycle. In this research, the performances of the open-air and closed S-CO2 Brayton cycle at microreactor power range are compared with polytropic turbomachinery efficiency. When optimizing the cycle, three different optimization parameters are considered in this paper: maximum efficiency, maximum cycle specific work, and maximum of the product of both indicators. For the air Brayton cycle, the maximum of the product of both indicators allows to consider both efficiency and specific work while optimizing the cycle. However, for the S-CO2 Brayton cycle, the best performing conditions follow either maximum efficiency or the maximum cycle specific work conditions. In general, the S-CO2 power cycle should be designed and optimized to maximize the cycle specific work for commercial-scale application. The results show that the air Brayton cycle can achieve near 45% efficiency when it can couple with a microreactor with a core outlet temperature higher than 700 °C. However, the S-CO2 power cycle can still achieve above 30% efficiency when it is coupled with a microreactor with a core outlet temperature higher than 500 °C, whereas the air Brayton cycle cannot even reach breakeven condition.

Thermohydraulic Characteristics of Printed Circuit Recuperators in a Supercritical CO2 Brayton Cycle with Nonuniform Minichannels

Supercritical carbon dioxide (SCO2) technologies have been under the spotlight of developed countries because they believe that such applied sciences can be ingenious solutions to resolve energy and environmental problems. The SCO2 power cycle is one of the most promising technologies in this field due to its high efficiency and low expenditure. However, optimal designs of heat exchange devices are a vital issue in the SCO2 power cycle because they account for almost 80% of the capital costs. In this study, the comparison of diverse combinations of convergent and divergent minichannels inside the low temperature recuperator (LTR) and high temperature recuperator (HTR) is suggested to recognize the strengths and weaknesses of nonuniform minichannels at high operating pressures and temperatures. Three-dimensional conjugate simulations are carried out, and existing data in the literature are used to validate the obtained results. The comparison reveals a slight deviation (less than 10%) in temperature and pressure profiles. After the evaluation of the studied cases in terms of thermal and hydraulic characteristics, different criteria are employed to explore optimal combinations of the hot side and cold side minichannels. Totally, the minichannel convergence enhances thermal performance of recuperators (up to 26%), and their divergence improves the hydraulic behavior. Nonetheless, diverging the minichannels imposes greater positive effects than converging them. The overall analysis specifies that the most efficient LTR and HTR would be attained by diverging the hot side minichannel, while the cold side minichannel is kept unchanged. This combination of minichannels allows reaching the highest SCO2 temperature at the outlet of LTR and HTR (456 K and 656 K) with the minimum pressure drops (around 0.02% of inlet pressures) and achieving the best performance indexes (approximately 2). Finally, the comparative study discloses that at low Reynolds numbers (less than 14000 for the hot side and 10000 for the cold side), the nonuniform patterns of minichannels perform better than the complicated structures, such as zigzag, wavy, and other optimized geometries. The average performance enhancements for the hot and cold sides are 36.3% and 37.4%, respectively.

Thermodynamic and economic analysis of a supermarket transcritical CO2 refrigeration system coupled with solar-fed supercritical CO2 Brayton and organic Rankine cycles

Aiming to reduce the use of conventional energy sources, alternative methods should be applied in order to cover society’s needs. In this context, in the present study, the operation of a transcritical CO2 commercial refrigeration unit is investigated, coupled with two power production cycles. The first cycle is a supercritical CO2 Brayton system that is supplied with heat by a solar field including parabolic trough collectors, while the second one is an organic Rankine cycle, utilizing as a heat source the low enthalpy CO2 exiting the first cycle. The novelty of this system is attributed to the combination an advanced transcritical CO2 refrigeration system with power production cycles that operate under the optimal conditions, as well as to the reason that the operation of the refrigeration unit is independent from the solar energy, since it can provide refrigeration output regardless of the solar heat input. The results prove that the increased number of solar collectors leads to more advantageous power production results, although with higher capital costs. Furthermore, greater energy production is noted when the ambient temperature is lower and abundant solar power is available. For the case of installing five solar collectors and an organic Rankine cycle, it is found that the annual energy production of the total system is 173.72 MWh, while the annual energy consumption of the refrigeration unit is 466.66 MWh, meaning that 37.23 % of energy savings are achieved annually. For the same configuration, according to the life cycle cost analysis, the capital cost is found equal to 243.72 k€, leading to a payback period of 7.6 years. By the end of the project’s lifetime, the operation of the installed system returns 298.42 k€ to the investor.

Thermohydraulic experiments on a supercritical carbon dioxide–air microtube heat exchanger

Heat exchangers are critical components in supercritical CO2 Brayton cycles and provide necessary heat input, recovery, and dissipation. Tubular heat exchangers with unconventionally small tube sizes (tube diameters less than 5 mm) are promising components for supercritical CO2 cycles and potentially provide excellent structural stability with wide scope of application. This paper provides essential design and fabrication guidelines for a compact shell-and-tube heat exchanger with microtubes (with an inner diameter equal to 1.75 mm). A heat exchanger test rig is used to evaluate the thermohydraulic performance of this heat exchanger with supercritical CO2 and air as working fluids. Thermohydraulic data are reported for more than forty sets of experiments with varying Reynolds numbers for shell and tube flows. Critical performance metrics are calculated from the data and compared with predictions from a previously described numerical model. The average deviations between the experimental and model results fall within 10% for all critical metrics. This excellent agreement validates the numerical model for supercritical CO2 heat exchanger optimization and scale-up.

Performance analysis and multi-objective optimization for a hybrid system based on solid oxide fuel cell and supercritical CO2 Brayton cycle with energetic and ecological objective approaches

In this paper, an elaborate investigation for a hybrid system based on solid oxide fuel cell (SOFC) and supercritical CO2 (SCO2) Brayton cycle is implemented to enrich the performance design details. First, the hybrid system general performance characteristic is revealed and the power improvement rate comparison between the hybrid system and other typical SOFC-based hybrid systems is presented. Then, the combined effects of current density and six critical parameters are evaluated to show the interaction between the SOFC and SCO2 cycle. Based on above, the parameter sensitivity analysis is conducted and a comprehensive optimization considering both energetic and ecological performance is performed. The results show that the maximum total power output and ecological objective function of the hybrid system (Ptotal and Etotal) are respectively improved by 43.72 % and 45.70 % compared with those of an individual SOFC. The final decision of the multi-objective optimization is that Ptotal, Etotal and ηtotal are 2345.68 kW, 2014.52 kW and 64.35 %, respectively. This paper offers an effective parameter optimization strategy for the hybrid system based on the coupling mechanism study of the SOFC and SCO2 cycle, and hints the broad prospect of waste heat recovery for high-temperature fuel cells via SCO2 cycles.

An improved zigzag-type printed circuit heat exchanger for supercritical CO2 Brayton cycles

Printed Circuit Heat Exchanger (PCHE) is a kind of high performance compact heat exchanger. In this study, a zigzag-type PCHE with improved flow passage arrangement, i.e., cellular arrangement, is proposed considering the fin efficiency enhancement. And the new PCHE with channels cellular arrangement coupled with supercritical CO2 (sCO2) Brayton cycle is investigated numerically. Flow and heat transfer characteristics in cellular zigzag-type PCHE is compared with the traditional flat-plate zigzag-type PCHE. Results show that the structure of the flow field in zigzag channels promotes the mixing between the fluid near the wall and the bulk fluid. The process of mixing is beneficial to intensify the convective heat transfer capacity. A pronounced increase of frictional pressure drop is also observed in zigzag channels when compared with straight channel. The overall heat transfer coefficient of cellular zigzag-type PCHE is about 8.6% higher than that of traditional flat-plate zigzag-type PCHE with the same operating condition in this study. The reason for heat transfer enhancement in cellular zigzag-type PCHE is that the secondary heat transfer surface in flat-plate zigzag-type PCHE is transferred to be the primary heat transfer surface, which is highly efficient heat transfer surface. As just alternating the hot and cold channels in cellular arrangement when compared with traditional arrangement, the pressure drop does not increase much while heat transfer is increased, so thermal performance is improved in cellular zigzag-type PCHE. As a result, the fixed investment/operation cost of cellular zigzag-type PCHE can be decreased. In this paper, a possible manufacture design of the arrangement improved PCHE is also achieved with turn-over channels.

Thermoeconomic investigation of pressurized oxy-fuel combustion integrated with supercritical CO2 Brayton cycle

Pressurization is expected to increase the power efficiency of oxy-fuel combustion plant. Supercritical CO2 (sCO2) Brayton cycle could yield higher thermal efficiencies at lower capital cost than the state-of-the-art steam-based power cycles. The coupling of pressurized oxy-fuel combustion and sCO2 cycle could potentially offer higher net power efficiency over its counterparts with CO2 capture. In this paper, pressurized oxy-fuel combustion was integrated with three different sCO2 Brayton cycle layouts, i.e., single reheated recompression, double reheated recompression, and intercooled recompression. The systems were analyzed from the perspective of thermodynamics and economics. The net power efficiencies for the three systems were 39.4%, 40.2%, and 41.3%, respectively. The effect of boiler pressure on the latent heat recovery of the flue gas was analyzed. In the sensitivity analysis, the influence of turbine inlet temperature, turbine inlet pressure, turbine inlet temperature, turbine outlet pressure, recirculated fan pressure boost, and the high-temperature recuperator (HTR) split ratio on the plant net power efficiency was assessed. Finally, the economic analysis was conducted. The levelized cost of electricity (LCOE) of the three systems were 103.18 $/MWh, 108.45 $/MWh, and 110.07 $/MWh, respectively. The results showed that the integrated systems were attractive as potential candidates for the coal-fired power generation with CO2 capture.

A triple cascade gas turbine waste heat recovery system based on supercritical CO2 Brayton cycle: Thermal analysis and optimization

• A new triple cascade gas turbine waste heat recovery system based on supercritical CO 2 Brayton cycle. • Evaluated the performance of an integrated system combining a gas turbine and a triple cascade cycle. • Analyze and optimize the system's thermodynamic and economic performance. A new triple cascade waste heat recovery system coupled with a supercritical CO 2 power cycle and transcritical CO 2 regenerative cycle is proposed for the high temperature exhaust waste heat generated during the operation of gas turbine, and thermodynamic and economic models are established to analyze the performance of the system, with the following performance indicators: the net output power is 5.67MW, the thermal efficiency is 24.94%, the exergy efficiency is 46.08%, the total investment cost rate is 126.06$/h, and the per unit energy cost is 6.31$/GJ. The thermal performance of the integrated system consisting of the gas turbine unit and the proposed triple cascade waste heat recovery system is evaluated, and the results show that the integrated system effectively utilizes the waste heat compared with the gas turbine unit alone, while the net output power of the integrated system is 22.17 MW and the thermal efficiency is 47.7%. Each component's exergy and cost analysis is performed. The heat exchanger accounts for a large percentage of the exergy destruction, and the investment cost rate of the turbine, compressor and other work-making components is high, indicating the direction of system optimization. The sensitivity analysis of the system's key parameters was carried out through the control variables method to reveal the influence of individual setting parameters on the system performance. Based on the research range of the set parameters, the system is optimized with multiple objectives, and the net output power, exergy efficiency and per unit energy cost are selected as the objective functions. The optimization results show that the combination of exergy efficiency and per unit energy cost resulted in better economic performance, with exergy efficiency of 50.37% and per unit energy cost of 4.66$/GJ. Comparing the system with the other two configurations proves the superiority of the proposed system.

Selecting dry cooling system for supercritical carbon dioxide Brayton cycle: Thermoeconomic analysis

In this work, the thermoeconomic performance of the direct and indirect dry cooling systems for supercritical CO2 (sCO2) Brayton cycle are compared. The design and off-design performance of both cycles are analyzed, and the optimization methods of dry cooling systems under off-design conditions are proposed. A nominal 20 MW recompression sCO2 power cycle operates at the ambient temperature of 298 K and the turbine inlet temperature of 873 K is used for the case study. The results show that under design conditions, the net power generation of the direct dry cooling sCO2 power cycle is about 562 kW lower than that of the indirect dry cooling sCO2 power cycle. Based on the annual power generation in three different locations, the levelized cost of electricity (LCOE) is selected as the economic indicator for the economic comparison of the cooling system. LCOE of the direct cooling system is 34.8 ¢/MWh lower in places where the ambient temperature is low and has large fluctuation. By contrast, LCOE of the indirect cooling system is 25.4 ¢/MWh lower in places where the ambient temperature is mild and the variation of the ambient temperature is small. The results of this paper indicated that both thermal performance and the climatic condition of the location of the power cycle need to be considered for the selection of the dry cooling system for the sCO2 power cycle. This work could provide deeper insight into the design and control of the dry cooling systems for the sCO2 power cycle.

Performance Analysis of Heat Exchangers and Integrated Supercritical CO2 Brayton Cycle for Varying Heat Carrier, Cooling and Working Fluid Flow Rates

Supercritical CO2 power systems offer the potential of reduced system footprint and improved thermal efficiency, through the development and adoption of compact heat exchangers. Among these heat exchangers, the microtube, printed circuit, and plate heat exchangers are emerging as the most promising technologies for heat addition to the cycle, heat recuperation and heat rejection, respectively. To investigate the performance of supercritical CO2 recuperated Brayton cycle for heat to power conversion, simulation models of the heater, recuperator and cooler were developed using the distributed modeling approach and the ε-NTU method and then integrated with turbomachinery models to form the cycle model. The influences of flow rates of the heat carrier, cooling and working fluids on the heat exchanger performance and the integrated system were investigated. For the studied power system and under the off-design operating conditions, the net thermal efficiency of the cycle varies between 14.1% and 16.8%. Results show that increasing in the working fluid flow rate remains the net power output of the cycle but decreases the net cycle thermal efficiency, while increasing in the heat carrier fluid increases both, and the increase of cooling fluid increases the net power output but maintains the net thermal efficiency.

Dynamic characteristics and control strategies of the supercritical CO2 Brayton cycle tailored for the new generation concentrating solar power

The supercritical CO2 (sCO2) Brayton cycle has the advantages of high efficiency, good flexibility and compact equipment, and is widely regarded as the ideal power cycle for the new generation concentrating solar power (CSP). The application scenario of the CSP determines that the unit's fast peak shaving capability must be considered. In this paper, a dynamic simulation model was developed for an indirect-heated 800 °C/550 °C sCO2 CSP test plant, containing a particle heat storage system. The effects of control modes of heating power, cooling power, mass flow rate (m), turbine rotational speed and sCO2 inventory on the system dynamic characteristics were researched. It was found that the control mode of changing particle/water mass flow rate had a significantly faster response speed than changing temperature. Using the turbine optimal rotational speed control mode could improve the net cycle efficiency from 15.30% to 16.12%, in a continuous linear load reduction process. The sCO2 inventory control module played a crucial role in limiting the fluctuation of compressor inlet pressure. The regulation mode of changing m and turbine inlet temperature (T4) synchronously and that of changing m with constant T4 were all proved to achieve the goal of fast peak shaving well, but the latter one was safer, which could greatly limit the temperature change rate of sCO2 and equipment in the fast peak shaving process. Our work not only offers an operation solution for the sCO2 CSP test plant, but also gives guidance for the efficient, flexible and safe design of the third generation CSP plants.

Studies on thermal-hydraulic characteristics of supercritical CO2 flows with non-uniform heat flux in a tubular solar receiver

Supercritical CO2 (SCO2) Brayton cycle is one of the best approaches for concentrated solar power tower, with tubular solar receiver being one of the most attractive options to directly heat SCO2. The superposition of drastically variable properties of SCO2 and uneven distribution of solar irradiance flux poses a great challenge to the design and optimisation of solar receiver, thus the studies on thermal-hydraulic characteristics of SCO2 in solar receiver become crucial to deal with the challenge. In this study, the thermal-hydraulic characteristics of SCO2 in a vertical receiver tube under axially and circumferentially non-uniform heat-flux conditions are numerically studied with average heat flux ranging from 150 kW m−2 to 210 kW m−2 and mass flux ranging from 400 kg m−2 s−1 to 700 kg m−2 s−1, and the influences of buoyancy on the flow and heat transfer characteristics are evaluated. The non-uniform distribution of the axial heat flux leads to 1.2%–20.5% increase in the overall heat transfer coefficient, while more severe local heat transfer deterioration is observed. The buoyancy effect changes the flow structure, resulting in suppression of turbulence intensity and formation of thick momentum and thermal boundary layers, which is closely related to the severe heat transfer deterioration of SCO2 under non-uniform heat-flux conditions. Bo* could well predict the buoyancy effect in a vertical tube under non-uniform heat-flux conditions. Compared with upward flow, downward flow could effectively alleviate heat transfer deterioration and reduce wall temperature gradient especially for axially non-uniform heat-flux condition. The present work could provide guideline for the design and optimisation of tubular solar receiver and solar power system with SCO2 as heat transfer fluid.

Thermodynamic and economic investigation of a novel combined cycle in coal-fired power plant with CO2 capture via Ca-looping

CO2 capture from coal-fired power plants (CFPP) with low cost is crucial for China to achieve carbon peaks in 2030. The manuscript proposes a novel combined cycle of supercritical CO2 Brayton cycle (SCBC) and organic Rankine cycle (ORC) in CFPP with CO2 capture. The novelty of the proposed scheme lies on its high-level thermal integration which can efficiently complete the conversion of heat to electricity, waste heat recovery, and CO2 capture. Exergy analysis and levelized cost of electricity (LCOE) are employed to respectively evaluate the thermodynamic and economic performance. The simulation results show that the exergy efficiency of the combined cycle is 39.74% as considering CO2 capture, resulting in a 3.41% of efficiency penalty. The introduction of ORC improves the exergy efficiency by 1.05%. The LCOE of the combined cycle is 79.03$/MWh, almost 19.3$/MWh (32.29%) more expensive than the base plant, and the costs of CO2 captured and avoided are respectively 21.81$/tCO2 and 26.25$/tCO2. The results indicate that the proposed combined cycle has the potential to compete with other carbon capture technologies from the views of exergy efficiency and cost. Coal combustion in the S–CO2 boiler and the calciner is responsible for the maximum internal exergy destructions, and increasing the cycle maximum temperature may reduce the exergy destruction. The largest cost contributor to LCOE is associated with capital expenditures (41.55%), and the cost reduction efforts should be aimed at capital expenditures.

Effect of Actual Recuperators’ Effectiveness on the Attainable Efficiency of Supercritical CO2 Brayton Cycles for Solar Thermal Power Plants

One of the most promising concentrated solar power technologies is the central receiver tower power station with heliostat field, which has attracted renewed research interest in the current decade. The introduction of the sCO2 recompression Brayton cycles in the near future installations instead of the Rankine cycle is very probable, due to the prospects of a significant efficiency improvement, process equipment size and capital cost reduction. In this study, energy and exergy analysis of a recompression Brayton cycle configuration for a central receiver power station are performed. Special emphasis is given to the computation of actual performance for the High-Temperature Recuperator and the Low-Temperature Recuperator. The results define realistic thermal and exergetic efficiency limits for the specific cycle configurations applied on a central receiver solar power plant with variable turbine entry temperature. Thermal efficiency, predicted with the improved accuracy of heat exchanger computations, does not exceed the 50% target. Overall, a realizable total power plant efficiency of 37% at 900 K turbine entry temperature is predicted, which is a significant improvement on the current state-of-the-art with steam Rankine cycles.

Thermoeconomic optimization of a solar-assisted supercritical CO2 Brayton cycle, organic Rankine cycle and multi-effect distillation system

In this paper, the simulation and optimization of a combined supercritical carbon dioxide Brayton cycle, an organic Rankine cycle and multi-effect distillation system driven by solar energy have been applied for power and freshwater generation. In this cycle, the solar collector, the central receiver reflected the sun’s light by heliostats, enters the storage system and then enters the fluid stream according to the amount of heat required to initiate the cycle. The working fluid of solar receiver is a mixture of the 60% NaNO3 and 40% KNO3, supercritical carbon dioxide is working fluid of the Brayton cycle and R600 is the working fluid of the organic Rankine cycle. The innovation of this article is using power and fresh water cycle without fuel consumption (with solar system and storage tanks). The simulation of this combined cycle was carried out by engineering equation solver software and energy and exergy efficiency changes in terms of different parameters are obtained. Then, a multi objective optimization of this system considering exergy efficiency and cost of system as objective functions is performed by genetic algorithm in Matlab software. Decision variables of the whole cycles are including Compressor inlet temperature, Turbine inlet temperature, Number of MED effect, The temperature of the water fed the desalination, Evaporator pinch point, Mass flow (Critical Carbon Dioxide), Turbine inlet pressure, Compressor inlet pressure and Pressure drop. The two objective functions optimization including exergy and economic parameters of this cycle is carried out for achieving reduction of electricity generation cost and increase of the exergy efficiency. The results of this optimization showed, the maximum exergy efficiency of this combined system is 61.78% and the minimum cost of electricity production is 0.2617 $/kWh. In this regard, the multiple effect distillation system produces 530.9 KgS freshwater in 15 stages.

Assessment on energy and exergy of combined supercritical CO2 Brayton cycles with sizing printed-circuit-heat-exchangers

The supercritical carbon dioxide Brayton cycle is one of the alternative thermal schemes for advanced energy systems. In this study, ammonia absorption Refrigeration-reheat supercritical carbon dioxide Brayton combined cycle is constructed, and the thermodynamic performance of reheat supercritical carbon dioxide Brayton cycle with ammonia absorption refrigeration cycle are analyzed. The characteristics of Brayton cycle and ammonia absorption refrigeration cycle configuring with Printed Circuit Heat Exchanger are analyzed. The result shows that under controlled freezing-point storage condition, optimized combined cycle energy and exergy efficiency is 46% and 64.98%, electricity output and refrigeration capacity is 11.4 MW and 1.78 MW. Under air-conditioning refrigeration condition, optimized combined cycle energy and exergy efficiency is 50.28% and 65.04%, and electricity output and refrigeration capacity is 11.4 MW and 3 MW. Compared with combined cycle performance before optimization, average improvement of cycle energy efficiency, exergy efficiency and exergy destruction is 4.19%, 0.73% and 3.64%, which shows obvious improvement of cycle performance after optimization. Comparative analysis of printed circuit heat exchanger applied at recuperator and subcooler shows that heat transfer performance of single-faced etched printed circuit heat exchanger in straight channel is better than that of double-faced etched printed circuit heat exchanger for same heat transfer and pressure drop.

Performance enhancement of a C-shaped printed circuit heat exchanger in supercritical CO2 Brayton cycle: A machine learning-based optimization study

The present work is focused on enhancing the overall thermo-hydraulic performance of a previously proposed C-shaped printed circuit heat exchanger (PCHEs) using Machine Learning (ML) Algorithms. In this context, CFD analysis is carried out on 81 different channel configurations of the C-shaped channel geometry, and computed data is used to train three ML algorithms. Later, C-shaped channel geometry is optimized by coupling the trained ML model with the multi-objective genetic algorithm (MOGA). Finally, the optimized channel geometry (called optimized ML ) is investigated numerically for a wide range of Reynolds numbers. Its performance is compared with the zigzag geometry, C-shaped base geometry, and previously optimized C-shape channel geometry using response surface methodology (RSM). The findings showed that the multilayered approach combining MOGA, CFD, and machine learning techniques is beneficial to accomplish a robust and realistic optimized solution. Comparing the thermo-hydraulic characteristics of the optimized ML channel geometry with zigzag channel geometry shows that the former is up to 1.24 times better than the latter based on the performance evaluation criteria (PEC). Furthermore, the overall performance of the optimize ML channel geometry was found up to 21% and 16% higher than the optimized RSM geometry on the cold and hot sides, respectively.

Optimization of zigzag parameters in printed circuit heat exchanger for supercritical CO2 Brayton cycle based on multi-objective genetic algorithm

The printed circuit heat exchanger (PCHE) shows promise for applications in the supercritical carbon dioxide (S-CO2) Brayton cycle due to its great resistance to high temperature and pressure. The zigzag channel is the typical channel configuration of PCHEs containing structural parameters such as pitch, bend angle, and bend radius that can significantly influence flow and heat transfer performance. In this study, the pitch, bend angle, and bend radius are selected as the design variables, while the pressure and heat exchange capacity are considered as the optimization objectives. Based on the non-dominated sorting genetic algorithm-II (NSGA-II), optimization work is conducted to improve the performance and size of the zigzag channel over a wide range of structural parameters. To predict local flow and heat transfer performance, the surrogate model embedded in the one-dimensional analysis code is proposed using the artificial neural network (ANN) method and trained by 144 groups of three-dimensional simulation results. The results calculated by the one-dimensional analysis code show that both the bend angle and bend radius can significantly affect the performance of the printed circuit heat exchanger, while the bend radius plays a more important role on the pressure drop. However, the effect of the pitch on the zigzag channel is relatively small. The optimization results indicate that the appropriate core length of the zigzag channel varies with the design of the outlet temperature. For the 0.4 m core length suitable for the 306 K outlet temperature, the size is 20% less than for a 0.5 m core length with similar flow resistance, and the pressure drop is 66.3% less compared to that of the 0.3 m core length. In addition, the zigzag channel with hybrid structural parameters is proposed and compared to the conventional zigzag printed circuit heat exchanger. The optimization results indicate that the maximum difference between the two objectives is only 4%. Thus, the zigzag channel with constant structural parameters can satisfy the demand for optimization with a simpler design.

Performance Analysis and Optimization of a Novel Combined Cooling, Heating, and Power System‐Integrated Rankine Cycle and Brayton Cycle Utilizing the Liquified Natural Gas Cold Energy

For realizing efficient utilization of liquified natural gas cold energy and high‐temperature flue gas waste heat, herein, a combined cooling, heating, and power system using Peng–Robinson property package in Aspen HYSYS software, which includes a three‐stage parallel Rankine cycle (RC) in series with single‐stage RC and a supercritical CO2 (S‐CO2) Brayton cycle, is proposed. The system performance is evaluated using the first law of thermodynamics, second law of thermodynamics, specific energy costing method, and exergoenvironment analysis method. The influences of the pump 1 outlet pressure, turbine 3 inlet temperature, working fluid R23 mass flow rate, compressor 1 outlet pressure, CO2 liquefaction pressure, and ambient temperature on system performance are investigated. The nondominated sorting whale optimization algorithm and technique for order preference by similarity to ideal situation are applied to optimize the multiobjective system performance, and the payback periods of system before and after optimization are calculated. The analysis indicates that under the initial conditions, the thermal efficiency, exergy efficiency, total product unit cost, and sustainability index of the system are 41.68%, 48.50%, 112.293 $ GJ−1, and 1.75, respectively. After optimization, exergy efficiency increases by 2.4% and exergoeconomy decreases by 8.277 $ GJ−1.