System Net Output Research Articles

Efficiency optimization of fuel cell systems with energy recovery: An integrated approach based on modeling, machine learning, and genetic algorithm

Maximizing the efficiency of fuel cell systems (FCS) with energy recovery capabilities is crucial for advancing high-efficiency FCS technology. This research aims to explore the maximization of FCS efficiency through the optimization of operational parameters and to elucidate the synergistic effects between parameter optimization and energy recovery. Employing an integrated approach that combines model simulation, experiments, machine learning, and genetic algorithm (GA), this work addresses key technical challenges in developing an efficient FCS model and a high-precision surrogate model. It also tackles the critical problem of achieving maximum system efficiency through the optimization of operational parameters and energy recovery. The findings indicate that the data-injection-based model simplifies the computational process and effectively validated over long time scales. Feature selection and network quality assessment ensure the precision and reliability of the long short-term memory (LSTM) network. Optimization using GA in conjunction with the LSTM surrogate model enables a rapid and accurate determination of the system's maximum net output power and corresponding operational conditions. The results demonstrate a 6 % increase in the system's net output power, with the system efficiency consistently surpassing 55 %. Moreover, energy recovery consistently boosts optimized system efficiency by about 1.6–1.7 %.

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.

Performance analysis of a novel multimode electricity-cooling cogeneration system (ECCS) driven by exhaust from a marine engine

Multiple operating modes and flexible outputs are typical characteristics of multimode electricity-cooling cogeneration systems (ECCS). The present study proposes an innovative multimode ECCS driven by exhaust gas from marine engines and presents a corresponding thermodynamic system model. The thermodynamic performance, distribution region of operating modes, and exergoeconomic performance of the system are comprehensively investigated under various operational conditions. A novel comprehensive evaluation index, namely, the system net output per unit total mass flow rate (NOPF), is introduced to ensure that the analysis of system performance remains unaffected by the variations in the mass flow rate of the working fluid. The findings indicate that the operational modes exhibit diverse responses to influencing variables, while the distribution region of the operational modes is more sensitive to the separation ratio of the working fluid flow rate (X). Under the electricity-cooling generation mode of the system, the maximum NOPF reaches 156.01 kJ⋅kg−1. Furthermore, the proposed system achieves a peak exergy efficiency (ηex) of 13.83 %, concurrently decreasing the energy production cost (EPC) to a mere 0.0981 Dollar·kWh−1. Finally, this study offers new research insights that can inform future regional analyses of other multimodal and multigeneration systems.

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.

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.

Performance analysis and optimization of a novel vehicular power system based on HT-PEMFC integrated methanol steam reforming and ORC

In this paper, a novel vehicular high temperature proton exchange membrane fuel cell (HT-PEMFC) power system integrated with methanol steam reforming (MSR) and Organic Rankine Cycle (ORC) is proposed. The system uses waste heat from the HT-PEMFC to provide the MSR subsystem for hydrogen production, and the ORC subsystem recovers the remaining heat to generate electrical power. The thermodynamic model of the system and multi-criteria evaluation are established. Numerical results show that increasing the current density and decreasing the cathode pressure and stoichiometry is favorable for improving the net output power of the system, but also increasing the levelized cost of energy and carbon mass specific emission of the system. Higher operating temperatures and anode pressures are beneficial to overall performance. Moreover, the system is optimized using the NSGA-II algorithm to obtain the three-dimensional (3D) Pareto solution and the optimal set of operating parameters. The optimized system net output power, levelized cost of energy and carbon mass specific emission at the Final Optimal Point are 36.98 kW, 0.2138 $/kWh and 0.5583 kg/kWh, respectively. Compared to the un-optimized system, the system working at optimal parameters has better thermodynamic, economic and environmental performance.

Proposal of a new low-temperature thermodynamic cycle: 3E analysis and optimization of a solar pond integrated with fuel cell and thermoelectric generator

The present research deals with a novel hybrid system that comprises of a salinity gradient solar pond (SGSP) integrated with a proton exchange membrane fuel cell (PEMFC) and a thermoelectric generator (TEG). The key novelty of this research is the use of PEMFC waste heat to increase the performance of the low-temperature heat source (solar pond). Also, a thermoelectric generator is employed in the proposed setup for complete waste energy recovery. In this paper, the proposed system's energy, exergy, and economic (3E) performance are investigated, as well as compared with two additional systems that were modelled by excluding TEG and PEMFC to showcase the benefits of the sub-systems in the final configuration. A parametric study is also conducted to investigate the effect of significant parameters on the overall system performance. The simulation results indicated that the integrated system's net output power is 2288.8 kW at a base case operating condition, with energy and exergy efficiency of 11.26% and 13.17%, respectively, and a system cost rate of 394 $/h. In comparison to other components in the integrated system, SGSP alone accounts for 75% of total exergy degradation and has the highest investment cost rate of 66.70 $/h. Further, to achieve the best system performance, a multi-criteria optimization is performed for the suggested system with net output power, energy efficiency, exergy efficiency, and system cost rate as objective functions. Besides, the multi-criteria decision analysis (MCDA) is also carried out on the obtained Pareto front using TOPSIS decision-maker to select the best operating condition that improves the output power, energy, and exergy efficiency by 52.6%, 49.6%, and 61.8%, respectively, at the expense of 5.2% increase in the system cost rate. The current investigation demonstrates how an appropriate optimized design of an integrated energy system using SGSP, PEMFC, and TEG can improve the overall performance with reasonable cost increment.

Simulation research of TEG-ORC combined cycle for cascade recovery of vessel waste heat

ABSTRACT The waste heat of ships has the characteristics of large temperature gradient, huge recoverable amount and various properties. Traditional technologies such as thermoelectric power generation (TEG) and organic Rankine cycle (ORC) are difficult to take into account the different characteristics of the various types of waste heat from ships. In order to realize the efficient cascade utilization of various waste heat from ships, improve the efficiency and meet the stringent requirements of the new international carbon emission regulations, a cascade recovery system of ship waste heat based on TEG-ORC (thermoelectric power generation/organic Rankine cycle) is proposed in this paper. After designing of the TEG-ORC combined cycle experimental system, the simulation research has also been carried out. The effects of TEG bottom cycle scale, ORC working medium flow rate and evaporation pressure on system net output power, system power generation cost and bottom cycle ratio (QTEG/QORC ) are studied. The results show that the TEG-ORC combined cycle overcomes the limitation of single waste heat utilization method, realizes multi-stage utilization of ship waste heat, as well as improves the utilization of waste heat. Of course, the stability and safety of ORC bottom cycle operation are also promoted. The simulation results show that when the TEG bottom cycle scale is 22 pieces, ORC working medium flow rate is 0.44 kg/s, and ORC working medium evaporation pressure is 2.2MPa, the system economy is optimal. At this time, the system power generation cost is 0.26 $/kW·h, the system net output power is 2520.3 W, and the thermal efficiency is 23.33%.

Correlation analysis based multi-parameter optimization of the organic Rankine cycle for medium- and high-temperature waste heat recovery

The present paper established a multi-parameter optimization model of the organic Rankine cycle (ORC) to recover medium- and high-temperature waste heat. The preliminary screening of 42 pure working fluids is completed according to the limitations of environment and safety requirements. Afterwards, the relationship between heat source temperature, working fluid physical properties, and system net output power are analyzed. It can be found that there is a parabola variation relationship between working fluid critical temperature and system net power output. When the net power output reaches its peak value, a further increase of working fluid critical temperature will causes the opposite effect. Furthermore, correlation analysis is introduce to investigate the variation relationship between system net output power and working fluid critical temperature, system evaporating temperature/pressure, and condensing temperature/pressure by different heat source temperatures is investigated in detail. Results show that it is not much effective to use a working fluid with great pressure ratio for the design of a high-efficiency ORC system. On the contrary, using working fluid with a high temperature ratio can always improve the system net power output. Besides, the working fluid specific heat difference/ratio, and the latent heat difference/ratio during the evaporation and condensation processes jointly influence the ORC output performance. As a result, the influence order of working fluid key properties to the system net power output is: temperature ratio＞pressure ratio＞latent heat＞specific heat.

Effect of blending hydrogen to biogas fuel driven from anaerobic digestion of wastewater on the performance of a solid oxide fuel cell system

Fuel composition is proved to have a significant influence on the performance of solid oxide fuel cell (SOFC) systems. In the present research, the effect of hydrogen injection to biogas-fed SOFC power systems is investigated. Hydrogen production is performed through implementing a Photovoltaic-based solar system to electrolyze water by means of a Proton Exchange Membrane (PEM) electrolyzer. In the present research, waste water as the primary source of energy is chosen to drive a novel integration of photovoltaic with SOFC systems. For comparison reasons, two cases depending on the hydrogen storage scenario or hydrogen utilization in the SOFC system are developed. Thermodynamics and exergoeconomic models have been developed to analyze the performance of proposed systems. Results indicate that increasing the molar ratio of hydrogen in the biofuel mixture up to 0.35 increases the SOFC cell voltage and the system net output power up to 4%. Also, due to the certain advantages of hydrogen, blending hydrogen with biogas results in a reduction of CO2 emission by 16%. The overall efficiency of the system, when the produced hydrogen is totally stored in the storage tank, would be higher than that of the system where the produced hydrogen is partially utilized in the SOFC system. The economic analysis revealed that the unit product cost of the system shows approximately a linear reduction with an increase in the hydrogen amount of the fuel.

Thermal performance evaluation of subcritical organic Rankine cycle for waste heat recovery from sinter annular cooler

The sinter cooling flue gas expelled from the end of an annular cooler was taken as the heat source of an organic Rankine cycle (ORC) system, and R123, R245fa, R600, R601 and R601a were selected as the working fluids of the ORC system. The effects of evaporation temperature and superheat degree of working fluid, as well as the pinch point temperature difference in the evaporator on the system thermal performance, were analyzed in detail. The results show that the system net output power and exergy efficiency for different working fluids first increase and then decrease with an increase in the evaporation temperature and decrease with an increase in the superheat degree and pinch point temperature difference. The change in pinch point temperature difference has no effect on the system thermal efficiency. For a given operational condition, the system thermal efficiency and exergy efficiency of R123 are the maximum, while the system total irreversible loss of R245fa is the maximum. When the evaporation temperature is greater than 110 °C, the system net output power of R600 is the maximum. The ORC system could obtain the maximum net output power and exergy efficiency through the adjustment of evaporation temperature of working fluid.

Simulation of CO2 Brayton cycle for engine exhaust heat recovery under various operating loads

A bottoming cycle system based on CO2 Brayton cycle is proposed to recover the engine exhaust heat. Its performance is compared with the conventional air Brayton cycle under five typical engine conditions. The results show that CO2 Brayton cycle proves to be superior to the air Brayton cycle in terms of the system net output power, thermal efficiency and recovery efficiency. In most cases, the recovery efficiency of CO2 Brayton cycle can be higher than 9% and the system has a better performance at the engine’s high operating load. The thermal efficiency can be as large as 24.83% under 100% operating load, accordingly, the net output power of 14.86 kW is obtained.

Discussion of the internal heat exchanger's effect on the Organic Rankine Cycle

This paper explores the performances of IHE (Internal Heat Exchanger) in ORC (Organic Rankine Cycle) systems. Although previous studies hold multitudinous opinions, this study gives clear statements of IHE in both subcritical and supercritical ORC systems by setting a new model taking pressure drop in loops and acid dew point into consideration. Commonly used working fluids R123 and R600 are chosen for subcritical and supercritical cases separately. The temperature of the heat source applied is 200 °C and the mass flow rate of it is 1 kg/s. The analysis is accomplished by program Engineering Equation Solver. A modified method of calculating maximum heat exchange in IHE is given when modeling a supercritical cycle, because of the momentously changing specific heat near the critical point. Besides, a new approach is put forward to calculate the outlet temperature of the heat source and find the location of pinch point in supercritical cases. The results provide that IHE is beneficial to a subcritical case, but it improves system performance only in part of the low pressure stage in a supercritical case. Moreover, after the acid dew point Tad is taken into account, it is found that IHE is able to enlarge euphemistically the maximum system net output in a subcritical case. And in a supercritical case, the original evaporation pressure which does not conform to the rule Th,out > Tad is available now. It is revealed that the utilization of IHE will strengthen the applicability of the system.

Impact of built-in and actual expansion ratio difference of expander on ORC system performance

Built-in expansion ratio is set with screw expander design, influenced by the geometric shape, size, and flow characteristic. Actual expansion ratio, which equals to the evaporation pressure divided by the back pressure of discharge line, is determined by operating conditions. In the previous ORC system studies, the efficiency of expander is usually assumed as constant. However, the built-in and actual expansion ratio difference affects the screw expander efficiency greatly in a practical application. Therefore, a correction factor is needed for characterization. The purpose of this paper is to study the impact on ORC system performance caused by the expansion ratio difference. Six working fluids (R123, R245fa, n-Pentane, Isopentane, n-Hexane and Cyclohexane) have been selected as the candidates and system net output and thermal efficiency have been chosen to evaluate the system performance. The test system is defined as a kilowatt class since the net output is about 1 kW. In the whole simulation, the heat source condition remains unchanged, the condensation temperature maintains 303.2 K and the evaporation temperature ranges from 333.2 to 393.2 K. The results show that the correction factor decreases faster in the under-expansion condition than it does in the over-expansion condition. Furthermore, the actual expansion ratio which results in the maximum net output is not equal to, but higher than the built-in one. After correction, all the optimal evaporation temperature, maximal net output and thermal efficiency have been reduced, as is meaningful in the system determination.

Optimized thermal coupling of micro thermoelectric generators for improved output performance

There is a significant push to increase the output power of thermoelectric generators (TEGs) in order to make them more competitive energy harvesters. The thermal coupling of TEGs has a major impact on the effective temperature gradient across the generator and therefore the power output achieved. The application of micro fluidic heat transfer systems (μHTS) can significantly reduce the thermal contact resistance and thus enhance the TEG's performance. This paper reports on the characterization and optimization of a μTEG integrated with a two layer μHTS. The main advantage of the presented system is the combination of very low heat transfer resistances with small pumping powers in a compact volume. The influence of the most relevant system parameters, i.e. microchannel width, applied flow rate and the μTEG thickness on the system's net output performance are investigated. The dimensions of the μHTS/μTEG system can be optimized for specific temperature application ranges, and the maximum net power can be tracked by adjusting the heat transfer resistance during operation. A system net output power of 126 mW/cm2 was achieved with a module ZT of 0.1 at a fluid flow rate of 0.07 l/min and an applied temperature difference of 95K.It was concluded that for systems with good thermal coupling, the thermoelectric material optimization should focus more on the power factor than on the figure of merit ZT itself, since the influence of the thermal resistance of the TE material is negligible.