System Net Power Research Articles

Optimization Potential for Operating PEMFC Systems Under High-Altitude Conditions

The PEMFC technology is on the rise to be implemented as climate neutral power source in next generation aircraft. Despite the PEMFC’s high efficiency at ground level, it suffers from severe power losses when it is operated under high-altitude conditions, i.e. low ambient pressure and temperature.In this study, a fuel cell system consisting of a 120 cells PEMFC stack, an air blower, an anode recirculation pump and a cooling unit is investigated experimentally inside an altitude test chamber, where the ambient pressure and temperature can be controlled. In an extensive parameter study, the fuel cell’s cathode stoichiometry and the stack temperature are varied while the system is operated at different average cell voltage levels under high-altitude conditions. The ambient pressure is varied between 1000 and 500 hPa, the latter corresponding to an altitude of 5600 m. The air blower inflow temperature is varied between 40 and -30 °C. By means of statistical design of experiment methods an empirical model is derived from the measurements, which represents the system performance depending on the five-dimensional parameter space, i.e. cathode stoichiometry, stack temperature, average cell voltage, ambient pressure and blower inflow temperature.The model is evaluated in order to identify the optimal parameter configurations for stoichiometry and stack temperature control, yielding the maximum system net power at high-altitude operation. The results show that the negative effects in terms of power losses due to low ambient pressure prevail over low air temperature. However, the low temperature is non-negligible in terms of the air density, affecting the required blower power to maintain a certain stoichiometry and consequently also affecting the system net power. When operating at high-altitude conditions with optimized control parameters, the resulting net power output is significantly higher compared to the net power output obtained with the default control parameters suggested by the manufacturer. The main reason for this power increase can be found in the optimized membrane humidification. Under high-altitude conditions, humidification becomes more sensitive to deviations from the identified optimal control parameters. The results highlight the large optimization potential through altitude-adaptive control of fuel cell systems for aviation applications.

Optimizing of working conditions of vanadium redox flow battery based on artificial neural network and genetic algorithms

The integration of electrode compression in a vanadium redox flow battery (VRFB) with optimized operating conditions is essential for achieving the maximum net discharge power. This study presents the development of a 3D steady-state model for VRFB aimed at maximizing net discharge power (Pnet) by optimizing key working conditions, including electrode compression ratio (CR), applied current density (Iapp), and electrolyte inflow rate (Qin). The comprehensive impacts of CR, Iapp, and Qin on VRFB charging and discharging curves, energy efficiency (EE), and system efficiency (SE), and Pnet were investigated. Subsequently, the optimization of CR and Qin under a fixed applied current density range of 20–400 mA/cm2 was carried out employing a hybrid methodology integrating artificial neural network (ANN) and genetic algorithms (GA). Results reveals that increasing CR does not linearly improve system efficiency and net discharge power, with an optimal CR identified to maximize net discharge power. The optimal CR varies with operating conditions and is influenced by factors such as Qin and Iapp. Notably, both CR and Qin negatively impact net discharge power due to increased pump losses, while Iapp and state of charge (SOC) positively affect net discharge power through enhanced electron transfer and higher active substance concentration, respectively. The optimal CR is identified around 65 % for Iapp exceeding 60 mA/cm2. Furthermore, the optimal Qin correlates directly with increasing Iapp, as higher Iapp rates necessitate greater Qin to sustain active species supply. This study offers an optimization approach for addressing coupled operating conditions and provides insights for achieving high net discharge power in VRFB operation.

Design and optimization of power conversion system for a steady state CFETR power plant

A promising magnetic fusion steady state reactor is Chinese Fusion Engineering Testing Reactor (CFETR) and a different power conversion system is required than tokamak reactor. The control and utilization of high outlet temperature and thermal power generated by the fusion reactor efficiently are the main challenges in the field of thermo-fusion technology. An efficient and optimized power conversion system is an urge for the thermo-fusion energy system with a candidate working fluid under high temperature and pressure. Five cycle topologies based on the Brayton cycle have been designed such as multi stage expansion and recompression cycle with intercooling, recompression cycle with intercooling, partial expansion with multistage compression, partial expansion and partial recompression with cooling across range of turbine and compressor inlet temperatures. Brayton cycle with different system configurations have been simulated with He-gas and supercritical Carbon Dioxide (CO2) to conceive high outlet temperature of CFETR about 500 °C and thermal power of 200 MWth for better thermal performance by using REFPROP, EES and analytical model has been developed with MATLAB. The supercritical CO2 has better thermal performance about 36.18 % as compared to He-gas has 29.81 % under same thermal conditions. The Schematic-I with addition of preheat and recompression enhance the thermal performance about 2 % and has been proposed for CFETR. The effect of change in pressure of the system, effect of the working fluid, isentropic efficiency, heat rate and back to work ratio on the thermal performance and network output have been analyzed and optimized based on analytical model with the MATLAB. The system net power is increased from 69.20 MW to 79.71 MW, heat rejected by the system is decreased from 134.46 MW to 127.06 MW and thermal performance is increased about 34.62 %–39.85 % with pressure from 24 MPa to 28 MPa. Using multi stage expansion and recompression cycle with intercooling instead of other topologies can improve the thermal performance up to 6.1 % depends upon the operation conditions and working fluid. The heat rate of the system decreased while back work ratio increased with the pressure and it reveal that calculations are correct.

Performance evaluation and economic analysis of integrated solid oxide electrolyzer cell and proton exchange membrane fuel cell for power generation

In order to generate clean electricity from thermal energy, a hybrid electrochemical system is conceptually developed by coupling the proton exchange membrane fuel cell (PEMFC) and solid oxide electrolyzer cell (SOEC). For evaluating the proposed hybrid system, firstly, the two subsystems are modeled numerically and then they are merged into an integrated SOEC-PEMFC system. Moreover, the SOEC-PEMFC is analytically modeled for further evaluation. The effects of important operational parameters are examined. The outcomes show that when the SOEC operating temperature increases from 823 to 1273 K, the efficiency increases from 18.7 % to 38 % and the net output power improves about 36 % while cost per unit of power of hybrid system decreases about 80 %. Furthermore, by increasing the PEMFC operating temperature from 323 to 348 K, the system net output power and efficiency increase about 16.7 % and 10 %, respectively, whilst the cost per unit of electricity decreases about 19 %. In addition by increasing operating pressure of system, the net output power and efficiency are also improved. The proposed system has maximum output power density of 3.9 kW.m−2 and maximum efficiency of 38 %. In addition, the SOEC-PEMFC system is compared with the previously studied proton exchange membrane electrolyzer cell-proton exchange membrane fuel cell (PEMEC-PEMFC) system. In comparison with the previous PEMEC-PEMFC system, the present system's cost per unit of power and efficiency are about 16 % and 17 % higher, respectively; while the output power density is about double that of the PEMEC-PEMFC system. Generally, because hydrogen-powered systems offer reliable operation from an economic and energetic perspective, the SOEC-PEMFC system represents a promising technological solution to the clean energy demands.

Comprehensive performance assessment and multi-objective optimization of high-power proton exchange membrane fuel cell system under variable load

Due to the advantages of zero emission, high efficiency and low noise, Proton Exchange Membrane Fuel Cell (PEMFC) has grown to be the spotlight in the field of new energy in recent years. In this work, a high-power PEMFC system model is designed, which consists of the fuel feed Subsystem, the air feed subsystem, the thermal management subsystem and the stack. Among them, the material diffusion in the catalyst layer of the stack is intensively studied and derived. For the comprehensive system performance assessment, the thermodynamic, economic and environmental evaluation models have been developed. The mapping relationships between the operating parameters, the structural parameters and the system comprehensive performance under variable load conditions are obtained. The results of the study prove that increasing the operating temperature and decreasing the membrane thickness under high load current conditions are conducive to increase the system net power output, but at the same time increase the levelized cost of energy and the carbon mass specific emission. In addition, a multi-objective optimization strategy based on the NSGA-Ⅱ is designed to obtain the optimal performance of the system under different operating conditions. The optimized net power, the levelized cost of energy and the carbon mass ratio emission of the system for the final optimal operating point are increased by 40.5 %, 10.3 % and 33.3 % respectively. The aforementioned evidence substantiates the effectiveness of the proposed multi-objective optimization strategy in enhancing the overall performance of the PEMFC system.

Efficiency improvement strategy of fuel cell system based on oxygen excess ratio and cathode pressure two-dimensional optimization

Improving the commercial fuel cell system efficiency is critical to reducing hydrogen consumption and improving the operational economy. This paper studies the power consumption pattern of the air supply subsystem, the most power-consuming auxiliary components, aiming to improve the system’s efficiency. A method was proposed to get the extremum of the system net power by optimizing the oxygen excess ratio and cathode pressure. Mathematical models describing the fuel cell stack and air supply subsystem power characteristics were developed based on the mechanism analysis and estimation of unknown parameters. Then, a system net power model for energy efficiency optimization was developed. The operating conditions for optimal efficiency at a specific current density can be obtained by numerical calculation. To verify the method’s effectiveness, a series of experiments were conducted, and the results show that the proposed method can maximize system net power output at various current density conditions. Compared with the conventional oxygen excess rate based one-dimensional optimization, the two-dimensional method can improve system efficiency by 2.06%, offering a good application prospect.

Targeting zero energy increment for an onboard CO2 capture system: 4E analyses and multi-objective optimization

In this paper, a novel zero energy increment onboard carbon capture (ZEIOCC) system was proposed. With LNG cold energy and waste heat recovery, the ZEIOCC system integrated organic Rankine cycle (ORC), CO2 liquification, and CO2 capture unit. The ZEIOCC system performances were evaluated via energy, exergy, economic, and environmental (4E) analyses. Parametric studies were performed to investigate the effects of the flue gas mass flow rate (mfg), liquid to gas ratio (L/G), and the pinch point temperature difference at the outlet of the ORC condenser (Tpin) on the system performance indicators. Furthermore, multi-optimization was conducted with the product of energy-exergy efficiencies (ηen,sys*ηex,sys), the total capital investment cost (CI,tot), and the carbon reduction capacity (ξ) as objective functions. Two decision methods TOPSIS and LINMAP were introduced to find the most optimal point from the Pareto front. Under the basic condition, the proposed system proved its advantages of high efficiency, diversified energy output, fast return on investment, and efficient CO2 capture capacity. The results of optimization showed that the system performances meet all the requirement of the design goal. The optimal energy-exergy efficiencies is 10.06 %, the carbon reduction capacity is 28.01 %, and the total capital investment cost is 7.06 × 106 $. Under the optimal condition, the system net output power (Wnet), energy efficiency, and exergy efficiency are 12 kW, 20.53 %, and 49.02 %, respectively. Meanwhile, the payoff period (POP) and the CO2 capture cost (CCC) are 12.0 year and 99.9 $/tonCO2, respectively.

Estudio comparativo de un ciclo transcrítico de dióxido de carbono alimentado por un único ciclo geotérmico flash en los modos de funcionamiento con/sin economizador

Renewable energy, particularly geothermal energy, is on the rise globally. It has been demonstrated that recovering heat lost during geothermal cycles is essential due to the inefficiency of these cycles. This paper pproposes a combined power generation cycle using EES software to model a single-flash geothermal cycle, and a trans-critical carbon dioxide cycle. The study compares the system's performance during its "Without Economizer" and "With Economizer" operational stages. The impact of the economizer on the system's output metrics, including the net power output, energy efficiency, and exergy efficiency, was examined. The results show that the "With Economizer" system's net power output increased from 451.3 kW to 454 kW. The energy efficiency difference between the two systems is based on the first law of thermodynamics, where the value ofthe "Without Economizer" system is 6.036%, and the "With Economizer" system is 6.075%. The system without an economizer had an exergy efficiency value of 26.26%, whereas the system with an economizer reached 26.43%, based on the second law of thermodynamics. Installing the economizer increased the total economic cost rate of the system from 0.225M$/Year to 0.2294M$/Year, which increased the product cost rate from 15.82$/GJ to 16.02$/GJ.

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

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

Enhancing photovoltaic performance through solar radiation splitting: A beam splitter-assisted hybrid approach with 2-D tracking and PCM integration

Solar energy is an abundant and environmentally friendly power source; however, its conversion efficiency is limited by the mismatch between the response of photovoltaic (PV) and the solar spectrum. This paper proposes a novel hybrid system that splits solar radiation into visible and thermal components using a beam splitter. Through experimental and theoretical analyses, the study explores the integration of a phase change material (PCM) packed bed with a PV cell as a hybrid system to optimize energy conversion. The experiment is carried out using a 2D tracker. The concentration ratio, determined based on the maximum concentration ratio (MCR) supported by the PV cell, is established as the optimum point. For Silicon PV, the maximum concentration ratios are determined to be 8 and 16 sun for the cases of using and not using the beam splitter, respectively. The ratios of thermal and visible energy, measured at a wavelength of 700 nm, are observed to be 60.66 % and 39.33 % respectively. Under MCR conditions, in the PV system alone, the generated power reaches approximately 850 W/m2, while in the hybrid splitting configuration, it increases to around 1200 W/m2, resulting in a 43 % increase in the system's net power when employing the beam splitter. Furthermore, the study reveals that the utilization factor remains constant at 0.13 across all irradiation ranges for PV-alone systems. In contrast, for the PV/PCM hybrid splitting system, the utilization factor exhibits an increasing trend with higher solar irradiation intensities, reaching values of 0.31, 0.41, and 0.46 for irradiation ranges of 900, 950, and 1000 W/m2, respectively. The findings of this study suggest the potential of using a beam splitter in combination with a 2-D tracker and PCM-packed bed to increase PV efficiency. The proposed configuration demonstrates a substantial enhancement in total net power output, offering a promising avenue for advancing solar energy conversion technologies. Adding PCM to the system has proven to be beneficial, as it addresses the fluctuations in solar radiation and may enhance the total system efficiency.

Proposal of a proton exchange membrane fuel cell-based hybrid system: Energy, exergy and economic analyses and tri-objective optimization

This article presents energy, exergy and economic analyses and tri-objective optimization of a hybrid system that comprises a proton exchange membrane fuel cell and a recuperative-regenerative organic Rankine cycle. The novelty of this study is the utilisation of a recuperative-regenerative organic Rankine cycle rather than the basic configuration of the organic Rankine cycle to recover waste heat from the fuel cell. The simulation results showed that the proposed system generates a net power of 1195.6 kW, with an exergy efficiency of 44.37% and a product cost rate of 160.78 $/h at the base condition. The hybrid system is found to be more efficient and cost-effective as compared to a standalone fuel cell with 15.05% and 15.06% improvements in energy and exergy efficiencies, respectively, and a 9.28% decrement in unit product cost. The fuel cell is the component with maximum exergy destruction rate of 636.72 kW, accounting for 90.62% of the total exergy destruction. Further, it is also observed that the fuel cell accounts for 89.2% of the total levelized cost rate of the hybrid system. Then a tri-objective optimization is carried out on the system using Pareto Envelope-based Selection Algorithm-II with net power output, exergy efficiency, and product cost rate as the objective functions. The optimization results showed that the proposed system generates a net power of 1271.52 kW, with an exergy efficiency of 49.48% and a product cost rate of 152.62 $/h. It implies that at the optimal conditions, the hybrid system's net power and exergy efficiency increased by 6.35% and 11.51%, respectively, and the product cost rate was reduced by 4.98%. Finally, this study showed that the inclusion of recuperative-regenerative organic Rankine cycle as compared to other organic Rankine cycle layouts improved the performance of the suggested hybrid system compared to similar other systems reported in the literature.

Thermodynamic design and analysis of a multigeneration system to support sustainable dairy farming

Continued growth in the global human population and the high per capita food consumption renders it necessary to provide secure and sustainable food production systems. As current food production systems are vast users of energy and water resources, one of the most important goals achieved in this study is to contribute towards the development of sustainable and resilient energy systems for dairy farming. In this context, a novel multigeneration system is proposed in this study utilising renewable energy (solar and waste biomass) within the dairy farm. Accordingly, a comprehensive thermodynamic analysis of energy and exergy is conducted to assess the cycle's and component’s performances. The subsystems used in the system are; biomass gasifier, parabolic trough collectors, reheat organic Rankine cycle, Brayton cycle, water treatment unit and LiBr-Water absorption cooling system. Gasifier and solar parabolic trough collectors are integrated into a multigeneration system, which generates several outputs (electricity, hot water, hot air, milk evaporation, and a water filtering). The system's net power production and desalinated water can satisfy the farm's necessary energy and water requirements and provide for at least nine-hundred neighbouring houses. The system's overall energy and exergy efficiencies are calculated to be 81.6% and 42.75%, respectively.

Thermodynamic performance optimization and environmental analysis of a solid oxide fuel cell powered with biomass energy and excess hydrogen injection

Recently the energy systems' developers focused on the efficient exploitation of renewable resources and novel technologies to decrease fossil fuel consumption and environmental damage. The SOFC units are known in this regard as high-efficiency and versatile fuel-driven systems, for which the biofuel from biomass gasification is an abundant and clean fuel source. The current article investigates a biomass-driven SOFC that is hybridized with wind turbines in order to increase the hydrogen concentration of intake fuel. To do so, the generated electricity by wind turbines is given to a polymer electrolyte membrane electrolyzer for hydrogen generation and injection into the SOFC. Feasibility studies of the proposed framework are conducted based on thermodynamic laws, and its performance enhancement is appraised using three exergy-based environmental indices. A parametric study is carried out to indicate the influence of operating variables on system performance in terms of power output, exergy efficiency, and environmental indices, after which a multi-objective optimization is implemented. The results indicated significant efficiency enhancement of biomass-driven SOFC via integration with wind turbines for hydrogen injection. It is found that more power from wind turbines leads to more hydrogen input to the SOFC and increases net power as well as efficiency. Increasing the fuel utilization factor for the basic structure and the structure with HI, the exergetic efficiency decreases by 5% and 4%, respectively. Under optimal point, the overall system's environmental damage factor and net output power are found to be 0.0092 kW and 322 kW, respectively.

Analysis of main engine various waste heat cascade recovery systems under different evaporation pressure

In this paper, a novel technology, which is the thermal power generation (TEG)-organic Rankine cycle (ORC) combined cycle system, is proposed to recycle cascade utilisation of main engine waste heat. Evaporation pressure of working fluid (P) is an important parameter affecting the combined cycle, but its performance in the TEG-ORC combined cycle is unknown. Therefore, the TEG-ORC experimental device is built in this paper, and the environmentally friendly R245fa is employed to study the variation of the main parameters such as the system net power output (Wnet), system thermal efficiency (ηs), power-production cost (Cg), and waste heat utilisation of main engine flue gas (fg) under different evaporation pressure (P). When the P is 0.75 Mpa, the output performance of the system parameters is optimal. The Wnet, ηs, waste heat utilisation power (Wp), Cg, and rate of fg, is 483.25 W, 8.34%, 5796.72 W, 0.3464 $ /kWh, and 69.05 %, respectively.

CO2-plume geothermal: Power net generation from 3D fluvial aquifers

Previously CO2, as a heat-extraction fluid, has been proposed as a superior substitute for brine in geothermal energy extraction. Hence, the new concept of CO2-plume geothermal (CPG) is suggested to generate heat from geothermal aquifers using CO2 as the working fluid. In January 2015, a CPG-thermosiphon system commenced at the SECARB Cranfield Site, Mississippi. By utilizing CO2, the demand for the pumping power is greatly reduced due to the thermosiphon effect at the production well. However, there are still parameters such as aquifer thermal depletion, required high injection rates, and CO2-plume establishment time, that hinder CPG from becoming viable. Moreover, the fluvial nature of sedimentary aquifers significantly affects the heat and mass transfer inside the aquifer, as well as the system performance. In the present study, a direct-CO2 thermosiphon system is considered that produces electricity from a 3D braided-fluvial sedimentary aquifer by providing an excess pressure at the surface that is used in the turbine. The system performance and net power output are analysed in 15 3D fluvial heterogeneous – with channels’ widths of 50, 100, and 150 m – and three homogeneous aquifer realizations with different CO2 injection rates. It is observed that the presence of fluvial channels significantly increases the aquifer thermal depletion pace (22%–120%) and therefore, reduces the system’s performance up to about 75%. Additionally, it is found that the CPG system with the CO2 injection rate of 50 kg/s and the I-P line parallel to the channels provides the maximum cycle operation time (44 years), as well as the optimum performance for the heterogeneous cases of the present study by providing about 0.06–0.12 TWh energy during the simulation time of 50 years. Also, to prevent rapid drops in excess pressure, a system with a yearly adjustable injection rate is implemented, which prevents the production well bottomhole temperature to fall below 80 °C.

Sensitivity Analysis of Transcritical CO2 Cycle Performance Regarding Isentropic Efficiencies of Turbomachinery for Low Temperature Heat Sources

The transcritical CO2 (T-CO2) power cycle using low temperature waste heat is a promising technique for energy saving and environmental protection. However, according to the literature, there is no commercialized unit in service yet. This study provides developers a reference to shorten the design phase of the T-CO2 cycle commercialization process. A sensitivity analysis of the system performance, i.e., thermal efficiency and net power output, regarding the isentropic efficiencies of pump (ηp) and expander (ηe) and the heat source temperature (Th,in) has been carried out using MATLAB and NIST REFPROP database. Simple and recuperative configurations are investigated based on their own optimal working pressures. The results show that the enhancement of ηe has a greater influence on improving the system performance, but the improvement will diminish as ηp, ηe, and Th,in increase. Although better system performance can be achieved with higher ηp, ηe, and Th,in, the cost of the system equipment will also increase due to the higher optimal working pressure. In addition, increasing ηp and ηe will negatively affect the effectiveness of the recuperator. Therefore, the turbomachinery efficiencies and the heat source temperature should be considered simultaneously for the most cost-effective system design.

Optimal coupling design for organic Rankine cycle and radial turbine rotor using CFD modeling, machine learning and genetic algorithm

The organic Rankine cycle (ORC) as a potential technology can be used to recover low-grade waste heat. This study provides a rapid and systematic methodology to design ORC system coupling with the radial turbine rotor. Firstly, the coupled model of ORC and the radial turbine is established, and the thermodynamic and economic performance is obtained to optimize the preliminary design parameters. Then, the CFD analysis for the turbine rotor is performed to generate the databases, and the machine learning algorithms (random forest, XGBoost, decision tree, and support vector machine) are used to train the surrogate models. And finally, the optimal shape of the meridional surface is obtained to improve ORC’ power output. The results indicated that the system net power output and the cost rate at the decision point achieved 393.06 kW and 13.59 $/h, respectively. The support vector machine exhibited the highest value of R2 (coefficient of determination) among the regression algorithms. The optimized results showed a more uniform pressure distribution is founding on the suction surface of the optimized structure compared with the basic structure’s, leading to the power output was increased by 7.5 %. The findings can provide references for the rapid coupling design and optimization of ORC and the turbine.

Thermodynamic investigation of a single flash geothermal power plant powered by carbon dioxide transcritical recovery cycle

The use of new energy sources, such as geothermal energy, is rapidly expanding worldwide, and it is a viable alternative to fossil fuels. This paper proposes a geothermal power generation system (combined single flash geothermal cycle with transcritical carbon dioxide recovery cycle). To improve system performance, a recuperator is used to recover some of the heat loss. The proposed system simulation is performed in EES software and then validated by previous research. The proposed system is analyzed from the perspective of energy and exergy. The effects of the proposed system separator pressure, carbon dioxide turbine inlet pressure and carbon dioxide condenser outlet temperature on energy efficiency, exergy efficiency and system net power output have been investigated. Findings show that the proposed system in recovery mode has a net power output of 401.40 kW, energy efficiency of 53.6 percent, and exergy efficiency of 46.32 percent, which are significantly higher than the basic mode’s values of 149.9 kW, 2.39 percent, and 12.12 percent, respectively.

Dynamic response and emergency measures under failure conditions of sCO2 Brayton cycle

AbstractThe sCO2 Brayton cycle has gained interest because of its flexibility and ability to provide higher thermomechanical conversion efficiency. Studies on failure conditions and corresponding emergency measures are of great significance to ensure the safety of the system, while there are limited studies related to the sCO2 Brayton cycle because the test circuit has not been applied in practice at present. In this study, we have developed a dynamic model of the CO2 Brayton cycle, and carefully validated its components against experimental data. Preliminary safety assessment and dynamic response under loss of cooling water (LOCW), loss of heat source (LOHS), and pipeline leakage have been analyzed. Emergency strategies are proposed to maintain the safe operation under failure conditions. The primary objective of this study is to reveal the dynamic performance and emergency measures under failure conditions of the sCO2 Brayton cycle. The results demonstrate that the system net power decreases at rates close to 10%/s and less than 0.1%/s under LOCW and LOHS, respectively. The system responds more dramatically under LOCW compared with LOHS. The system needs emergency measures under LOCW, while it has sufficient time to realize a normal slow shutdown under LOHS. The leakage rate is at least five times higher in the high‐pressure pipeline compared with the low‐pressure pipeline for the same area of the leakage hole. Under pipeline leakage, the heat source should be cut‐off slowly, and the rotation speeds of the main compressor and recompressor should be reduced simultaneously to protect the system.

Design and experimental research of a novel droplet flow field in proton exchange membrane fuel cell

The flow field plays a vital role in the performance and cost of the proton exchange membrane fuel cell (PEMFC). However, the traditional flow field structure cannot meet the high-power density requirements of PEMFC commercialization due to poor mass transport capacity. In addition, new flow field designs have some problems, such as complicated structure, high pressure drop and high manufacturing cost, which make them unsuitable for large-scale applications. In this paper, a novel droplet flow field (DFF) structure is proposed to further improve cell performance and reduce manufacturing cost. In particular, in-situ electrochemical measurement and printed circuit board segmented fuel cell technology are used to investigate the performance of cells with different DFFs. Based on the experimental results, the mass transport mechanism of DFF and its influence on the cell performance and current density distribution are deeply analyzed. The results indicate that DFFs have excellent oxygen transport and water management capabilities, thus improving the cell performance, especially at high current density. By optimizing the droplet placement direction in DFF, we found that the structure with the gas flow direction in contact with the droplet tail (DFF(a)) has the highest cell performance. Compared with the conventional parallel flow field, the maximum power density of the DFF(a) can be increased by 23.7%. Also, the introduction of droplet structure improves the current density distribution uniformity. Furthermore, DFFs have a lower pressure drop, which reduces the pump parasitic power and increases the PEMFC system net output power. Considering the cell performance, current density distribution uniformity and pressure drop, we think that the DFFs (especially the optimized DFF(a)) are more suitable for the practical application of PEMFC.