System Net Power Output Research Articles

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.

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.

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.

Influence study of bottom cycle ratios and superheat for vessels waste heat cascade recovery based on TEG-ORC combined cycle system employing R245fa

ABSTRACT The vessel’s waste heat has the characteristics of a large temperature gradient, huge recoverable volumes, and various properties. TEG-ORC combined cycle is a promising method for multiple vessels waste heat cascade utilization. Keeping the evaporation pressure at 0.7Mpa, R245fa of low toxicity and environmental friendly is employed for the research under the conditions of different bottom cycle ratios(₣) and superheating. The interaction between essential parameters and system performance is explored. As the ₣ gets larger, the system net power output (W net ), the system thermal efficiency (η s ) and the waste heat utilization of main engine flue gas (f g ) gradually increase, the power-production cost (C g ) of the system gradually decreases. When the degree of superheat increases from 3.36K to 9.23K, the output power of the expander increases synchronously. The results show that superheating improves the combined cycle W net and f g while reducing theC g . The stability and safety of the ORC unit are further enhanced by expanding the ₣. When the ₣ is 0.885 and superheat is 11.28 K, W net is 965.18 W, η s is 8.74%, and the C g of the system is 0.5363 $ k Wh−1, and the rate of f g is 85.07%.

Thermoeconomic assessment of a geothermal based combined cooling, heating, and power system, integrated with a humidification-dehumidification desalination unit and an absorption heat transformer

Population growth, economic challenges, and the environmental crisis force scientists and designers to pay more attention to clean, sustainable, and renewable-based energy systems. In this regard, due to the unlimited geothermal potential in many countries, the geothermal energy resource can be an economical alternative. Therefore, in the present work, a novel multi-generation system, based on a 100% geothermal resource, for power, cooling, heating, and desalination has been designed and analyzed, thoroughly. The system is evaluated from the energy, exergy, and thermo-economic viewpoints, and providing high heating/cooling potentials while reducing the thermoeconomic indexes is the major achievement of this study. The results demonstrate that the system's net power output, freshwater production rate, heating, and cooling capacities are 78.47 kW, 92.1 m3/day, 6251 kW, and 4991 kW, respectively. Moreover, the highest amount of exergy destruction occurs in the absorption heat transformer (42%) and the absorption chiller (35%), respectively. In addition, the chiller’s absorber has the highest cost rate of exergy destruction, and the turbine and the evaporator of the organic Rankine cycle have the highest investment costs. It is found that energy and exergy efficiencies are 60.55% and 17.05%, respectively for summer, and 70.58% and 43.59%, respectively for winter, and the system's total cost rate is 44.12 $/h with the payback period of 5.63 years. Furthermore, the parametric study shows that increasing the ambient temperature and decreasing the terminal temperature difference of the heat transformer’s evaporator lead to higher exergy efficiency and lower total system cost rate.

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.

Constructal thermodynamic optimization for dual-pressure organic Rankine cycle in waste heat utilization system

Dual-pressure organic Rankine cycle (DPORC) is a technology with great potential to solve energy problems. This paper performs constructal thermodynamic optimization (CTO) for the DPORC based on a combination of constructal theory and finite-time thermodynamics. The tube outside diameters of the heat exchangers, such as high- and low-temperature evaporators (HTE and LTE) and condenser, and the volume ratio of the high-pressure turbine are the design variables, the system net power output (POW) is the optimization function, and the total volume of the turbines and total heat transfer area (HTA) of the heat exchangers are the constraints. The influences of some parameters, such as the HTA ratios of the HTE and LTE, and the total mass flow rate (MFR) of the working medium on the CTO results are studied. The performance comparison between the DPORC and single-pressure ORC (SPORC) is carried out under the same conditions. The results demonstrate that the system net POWs after the four stepwise CTOs are improved by 1.70%, 3.01%, 5.80% and 8.84% over against the initial system net POW, respectively. Reducing the HTA ratios of the HTE and LTE and increasing the total MFR of the working medium can increase the system net POW. Compared with the performance of the SPORC, the system net POW and system net thermal efficiency of the DPORC are increased by 4.91% and 5.51%, respectively. The optimization results can be used to guide the optimal designs of the low-temperature waste heat utilization systems.

Off-Design Analysis of a Supercritical CO2 Brayton Cycle with Ambient Air as the Cold Source Driven by Waste Heat from Gas Turbine

Utilizing energy efficiently could greatly relieve current energy crisis. Therefore, in this article, a simple supercritical CO2 Brayton cycle for waste heat recovery from gas turbine is presented. The system uses ambient air as its cold source, which is more convenient and economic. Considering temperature difference between heat source and turbine inlet, it is of great significance to research the effects of turbine inlet temperature on the system performance due to the high temperature range of the exhaust gas from gas turbine. For this purpose, a control strategy is proposed and a mathematical model is established on off-design conditions to predict system performance when turbine inlet temperature increases from 673.15 K to 773.15 K. Results indicate that both turbine power output and system net power output increase with the rise of turbine inlet temperature, as well as total heat transfer in heater and cooler. Furthermore, the system thermal efficiency also increases until it becomes maximum on design point. In addition, the efficiency of turbomachinery varies slightly.

Thermo-economic analysis and multi-objective optimization of S-CO2 Brayton cycle waste heat recovery system for an ocean-going 9000 TEU container ship

Supercritical CO2 recompression Brayton cycle (S-CO2 RBC) can be used as waste heat recovery system in a ship because of the advantages of high efficiency, compact size and lower cost. This paper proposes a modified S-CO2 RBC system, namely the dual turbine-alternator-compressor (TAC) S-CO2 RBC system, which to be used to recover the waste heat from the main engine exhaust gas of an ocean-going 9000 TEU container ship. Mathematical and simulation models for the modified system are established to investigate the improvement in the performance. Parametric study is conducted to analyze the effects of key thermodynamic parameters on the system performance. The Imperialist Competitive Algorithm - based multi-objective optimization is carried out to get the modified system’s optimal operating parameters, aiming to maximize the system net power output, energetic efficiency and exergetic efficiency as well as to minimize the heat exchanger area per unit power and the levelized cost of energy. The results show that the performance of the modified system is strengthened significantly, whose highest net power output, energetic efficiency and exergy efficiency are 452.2 kW, 24.53% and 41.47%, approximately 26.58%, 16.67% and 16.90% higher than that of the S-CO2 RBC system. Compared with the S-CO2 RBC system, the modified system is more compact because its heat exchanger area per unit power decreased by 44.08%. The modified system can contribute to decreasing the ship auxiliary engine fuel consumption and the Energy Efficiency Design Index by about 1.01% and 1.02%, and improving the thermal efficiency of the ship main engine system by 3.23%. The results of this study can provide theoretical support for the application of the dual TAC S-CO2 RBC waste heat recover system on ships.

Fixed air flow-rate selection by considering the self-regulating function of low power air-cooled PEMFC stack

In this study, it is found that the air-cooled proton exchange membrane fuel cell (PEMFC) has a self-regulating function in terms of its thermal and water balances within the fuel cell. Such a function enables the fuel cell's operation to be stable and efficient without any electronic controlling circuit to control the operating parameters. A small air-cooled PEMFC stack (15 cells with a rate power of 60 W) integrated with an axial fan is used for this experimental study to verify the self-regulating feature. The results show that the air-cooled open-cathode PEMFC stack can run stably and efficiently under a wide range of load currents at a fixed air flow rate. The best fuel cell performance and maximum system net power output are achieved at an air flow of 55.7 SLPM. To further validate the results, the temperature distribution and cell voltage distribution of the stack are simultaneously measured under different loads. The results demonstrate that the temperature and cell voltage of the stack are evenly distributed at the air flow of 55.7 SLPM. The present study may provide guidance in the design of a cooling subsystem of a small and compact fuel cell system incorporating an air-cooled open-cathode PEMFC.

Proposal and assessment of a novel carbon dioxide energy storage system with electrical thermal storage and ejector condensing cycle: Energy and exergy analysis

The intermittent nature of renewable energy leads to significant issues when increasing the penetration level of renewable power. Energy storage system is generally considered to be an efficient measure for stabilizing the fluctuations of renewable power. A creative liquid carbon dioxide energy storage system integrating with transcritical Brayton cycle, electrical thermal energy storage and ejector condensing cycle is kindly proposed in this paper. To evaluate the feasibility of the developed system, the system thermodynamic model is clearly established and verified. The parametric analysis is performed based on energy and exergy methods to assess the relationships between key functioning variables and the system performance. The analysis results indicate that the ejector condensing cycle can be introduced as a green solution to the dilemma of carbon dioxide condensation during discharging stage. A higher turbine inlet pressure and temperature both bring about a promotion of the system net power output as well as the round trip efficiency. The ejector entrainment ratio gives a decrease with the rise in ejector back pressure, and the decreasing trend is first steep until the ejector back pressure being at 8.2 MPa in the considered conditions and then becomes distinctly moderate. Meanwhile, the optimal system efficiency is always located at the constant ejector back pressure (8.9 MPa in the considered conditions). The exergy destruction within the ejector condensing cycle decreases substantially with the low pressure storage temperature, increases monotonically with turbine inlet pressure, keeps unchanged with turbine inlet temperature and decreases with ejector back pressure.

Exergoeconomic and exergoenvironmental analysis of a combined heating and power system driven by geothermal source

This paper develops a geothermal driven combined heating and power system to obtain multi-generation, in which power is generated by Organic Rankine Cycle and heating is supplied by radiant floor heating system. Comprehensive thermodynamic, exergoeconomic and exergoenvironmental models of the system are performed and the capital cost and environmental impacts of the system components are analyzed. Superheat degree of the organic working fluid and inlet pressure of the Organic Rankine Cycle turbine are selected as variables to assess the system performance. Multi-objective optimization is employed to obtain the maximum system net power output, minimum levelized cost per exergy unit and minimum levelized environmental impact per exergy unit of the system. Results show that the increase of the working fluid superheat degree and the Organic Rankine Cycle turbine inlet pressure would cause the decrease of the Organic Rankine Cycle turbine power output and the increase of the heat supplying. Cooling water holds the highest levelized exergy cost and geothermal water heat exchanger has the highest environmental impact reducing potential. Under the condition of 11 °C superheat degree and 833 kPa Organic Rankine Cycle turbine inlet pressure, the system obtains a better performance with system net power output 1.19 MW, levelized cost per exergy unit 4.80 $/GJ and environmental impact per exergy unit 16.0 mpts/GJ.

Thermodynamic analysis and multi-objective optimization of a novel power generation system driven by geothermal energy

A novel combined system integrated with a Kalina and a transcritical CO2 (T-CO2) cycle is proposed and investigated for making effective use of geothermal resources. Geothermal energy is first utilized by Kalina sub-cycle and then deeply recovered by T-CO2 sub-cycle. A steady-state mathematical model is developed to further study the novel combined system. The result shows that under the given conditions, the system net power output is up to 2808 kW, which is higher than both the single Kalina cycle and single T-CO2 cycle. The exergy analysis shows that the evaporator II is the component with the largest exergy destruction. The parametric analysis shows that increasing the basic ammonia-water solution concentration and turbine I inlet temperature, or decreasing the turbine I inlet pressure and condensation pressure of condenser I have significant effects on system performance. Furthermore, a multi-objective optimization considering both thermodynamic and economic aspects is carried out to obtain the Pareto frontier solutions, and the decision-making analysis shows that the two-stage recycling system could greatly improve the geothermal water utilization degree compared to the conventional system. The findings could provide a useful reference for system design and operation to make better use of geothermal energy.

Entropy, Entransy and Exergy Analysis of a Dual-Loop Organic Rankine Cycle (DORC) Using Mixture Working Fluids for Engine Waste Heat Recovery

The exergy, entropy, and entransy analysis for a dual-loop organic Rankine cycle (DORC) using a mixture of working fluids have been investigated in this study. A high-temperature (HT) loop was used to recover waste heat from internal combustion engine in 350 °C, and a low-temperature loop (LT) was used to absorb residual heat of engine exhaust gas and HT loop working fluids. Hexane/toluene, cyclopentane/toluene, and R123/toluene were selected as working fluid mixtures for HT loop, while R245fa/pentane was chosen for LT loop. Results indicated that the variation of entropy generation rate, entransy loss, entransy efficiency, and exergy loss are insensitive to the working fluids. The entransy loss rate and system net power output present the same variation trends, whereas a reverse trend for entropy generation rate and entransy efficiency, while the exergy analysis proved to be only utilized under fixed stream conditions. The results also showed that hexane/toluene is the preferred mixture fluid for DORC.