Main Sources Of Irreversibility Research Articles

Exergy analysis of thermodynamic performance of a gas turbine unit

BACKGROUND: Gas turbine units are the most widespread type of power plants used in almost all industries. Such a wide distribution in combination with a high level of energy characteristics, high specific power and flexibility determine the relevance of research in the field of assessing their operating parameters.
 AIM: Determining the exergy characteristics (exergy efficiency and exergy dissipation factor) of a gas turbine plant and evaluation the efficiency of energy use in a gas turbine plant, as well as analysis of the influence of ambient temperature on the selected parameters.
 METHODS: The paper presents the results of an exergy analysis of the gas turbine plant as part of the Jandar Thermal Power Plant, Syria. To evaluate the described parameters, the method of exergy analysis of the thermodynamic characteristics of a gas turbine plant based on the first and second laws of thermodynamics in conjunction with the laws of conservation of mass and energy was used. An important feature of the study, which also emphasizes its novelty, is taking into account the chemical exergy of combustion gases during the exergy analysis.
 RESULTS: Based on the results of the study, it was found that the maximum exergy efficiency was 92.8% and was typical for a turbine as part of a gas turbine, while the highest exergy dissipation was observed in the process of fuel combustion in the combustion chamber and was equal to 80% (117.3 MW). The lowest exergy values corresponded to the compressor — 6.0% (9 MW). For the gas turbine unit as a whole, the total exergy dissipation was 147.3 MW and the exergy efficiency was 53.3%. In this case, the main sources of irreversibility in the gas turbine unit are the combustion chamber and combustion gases.
 CONCLUSION: Exergy analysis is a convenient method for evaluating the thermodynamic perfection of gas turbine units. As a further line of the study, it is necessary to evaluate the effects of various conditions, such as air humidity or compressor pressure ratio, on the exergy characteristics of the gas turbine fuller.

Process design, integration, and optimization of a novel compressed air energy storage for the coproduction of electricity, cooling, and water

The use of fluctuating renewable energy over a certain threshold may lead to an unmanageable mismatch between the electricity generation and demand profiles threatening the grid's stability. In this study, an innovative complex energy storage/conversion system is proposed for the cogeneration of electricity, cooling, and water by integrating the liquefied natural gas (LNG) regasification process, an organic Rankine cycle, a compressed air energy storage (CAES) system, and a multi-effect distillation unit. The study attempts to minimize the CO2 emission from the CAES technology while addressing interruptions and reductions in the grid upon the extensive use of intermittent renewables. In addition, the proposed system uses excess power and waste heat during the charging and discharging of the CAES to regasify LNG and produce fresh water. The reference system performance is analyzed considering thermodynamic, economic, and environmental perspectives. The multi-objective grasshopper optimization algorithm is used to make a trade-off between the technical, economic, and environmental performance factors of the system. The results show an exergy efficiency of 50.6 % and a total cost rate of 322.8 $/h for the proposed system at the TOPSIS optimal point. The Grassmann diagram indicates the combustion chamber is the main source of irreversibility, and the Chord diagram revealed the discharge unit was responsible for more than 55 % of the total cost.

Exergy and energy analysis of a novel solar powered fog desalination system

A novel system that applied fogging as a desalination technique was recently published, which could be considered a further development of the humidification-dehumidification desalination system (HDH), that provides an advantage through its ability to desalinate high salinity levels. This involves spraying hot saline water at high pressure vertically upwards, creating a mist instead of large water droplets. This increases productivity since the fine mist facilitates evaporation and the low-saline mist can go upwards more than the large high-salinity droplets. However, experiments showed that this methodology, in terms of productivity, performance and cost, was between other technologies. It was noted that the performance of this system peaked at ~39 %. Therefore, in this paper, an exergy analysis of the system is presented to determine the location of the highest exergy destruction and also highlight its comparative feasibility. The results indicated that the exergetic efficiency of the system reached 17.3 % while the main source of irreversibility is the fogging process itself (~75 %). Additionally, increasing the water temperature from 33 to 76 °C increased the exergetic efficiency of the fogger from 4 % to 20 %. Therefore, the authors recommend using a combination of humidification and atomization via nozzles of larger orifices with different spraying angles.

Increasing energy efficiency of hydrogen refueling stations via optimal thermodynamic paths

This work addresses the energy efficiency of hydrogen refueling stations (HRS) using a first-principles model and optimal control methods to find minimal entropy production operating paths. The HRS model shows good agreement with experimental data, achieving maximum state of charge and temperature discrepancies of 1 and 7%, respectively. Model solution and optimization is achieved at a relatively low computational time (40 s) when compared to models of the same degree of accuracy. The entropy production mapping indicates the flow control valve as the main source of irreversibility, accounting for 85% of the total entropy production in the process. The minimal entropy production refueling path achieves energy savings from 20 to 27% with respect to the SAE J2601 protocol, depending on the ambient temperature. Finally, the proposed method under near-reversible refueling conditions shows a theoretical reduction of 43% in the energy demand with respect to the SAE J2601 protocol.

Exergy and environmental assessments of the performance of a molten carbonate fuel cell cogeneration plant: External steam reforming against internal steam reforming

Two different configurations of a system including a cogeneration plant based on molten carbonate fuel cell (MCFC) with internal steam reforming (IR-MCFC) and external steam reforming (ER-MCFC) for producing power and hot water are modeled and investigated thermodynamically. Energetic, exergetic and environmental analyses are performed for the proposed systems and compared with each other from different viewpoints. Effects of various parameters, namely after-burner emissions recycling, fuel utilization ratio, operating temperature of stack and current density are investigated on the output potential voltages and any kind of voltage losses, net generated power, CO2 emission and energy and exergy efficiencies of two proposed cogeneration plants. The main sources of irreversibility are introduced for each system as well. The comparative analysis revealed that energy efficiencies of the IR-MCFC and IR-MCFC based cogeneration systems are about 14.85% and 4.82% larger than those of the ER-MCFC and ER-MCFC-based cogeneration systems, respectively. Also, exergy efficiencies of the IR-MCFC and IR-MCFC based cogeneration systems are about 14.46% and 11.08% more than exergy efficiencies of their external reforming types, respectively. The results indicated that CO2 emission of ER-MCFC system (0.18 kg/MW) is almost two times of IR-MCFC system (0.36 kg/MW).

Low perturbation limit decoherence analyzed by scaling the Double Quantum Hamiltonian

By varying the magnitude of the effective interaction between spins in relation to the perturbations, we study the decoherence behavior in a connected proton system. Making use of the Magnus expansion, we introduce a NMR pulse sequence that generates an average Hamiltonian with Double Quantum terms multiplied by a scaling factor, δ, with the possibility to take positive and negative values. The performance of the pulse sequence for different values of the scaling factors was validated in polycrystalline adamantane, by observing the evolution of the polarization. A time reversal procedure, accessible through the change of sign in the controlled Hamiltonian, was necessary to observe multiple quantum coherences. The spin counting develops a characteristic growth in two species of clusters for the scaled time. The influence of the scaling factor on the reversibility was observed through the behavior of the Loschmidt echoes, which decayed faster as the scaling factor increases. From the analysis of dynamics and its reversibility, we extracted characteristic times for the spin diffusion, T2δ and the intrinsic decoherence decay, T3δ for each scaling factor δ, and perturbation time scale, TΣ. Observing the dependence of reversibility vs. perturbation rates, both normalized with the spin diffusion rate, we find that in the limit of low perturbations, T2δ/T3δ deviates from the linear dependence on T2δ/TΣ that corresponds to strong perturbation. The asymptotic value T2/T3≈0.15 as T2δ/TΣ vanishes, gives evidence that the main source of irreversibility is the intrinsic decoherence associated to the chaotic many-body dynamics of the system.

A novel methanol-electricity cogeneration system based on the integration of water electrolysis and plasma waste gasification

A new methanol-electricity cogeneration system consisting of plasma gasification, alkaline water electrolysis, methanol synthesis and supercritical CO2 cycle was designed to achieve near zero emission resource utilization of medical waste. The newly designed methanol-electricity cogeneration system uses synthesis gas converted from medical waste by plasma gasification and hydrogen obtained by water electrolysis for methanol synthesis. It uses a supercritical CO2 cycle to improve its performance. In addition, the waste heat carried by each flow unit in the process is used to provide domestic hot water for residents. Based on a model built with Aspen Plus and Matlab software, the system's advantages are examined in terms of thermodynamics and economics, and the effects of the main parameters are studied using a 4.00 kg/s plasma gasifier as an example. The results show that the system can achieve a methanol yield of 6.32 kg/s, an energy efficiency of 67.42%, and an exergy efficiency of 68.60%. The main source of irreversibility can be traced to three components, the gasifier, the electrolyzer, and the methanol reactor, which account for 75.31% of the total exergy destruction. It only takes 3.53 years to recover the initial investment in the proposed system, and over a 20-year lifetime, the waste-to-energy project yields a net present value of 355,318.47 k$.

An innovative four-objective dragonfly-inspired optimization algorithm for an efficient, green, and cost-effective waste heat recovery from SOFC

This work proposes a novel yet practical dragonfly optimization algorithm that addresses four competing objectives simultaneously. The proposed algorithm is applied to a hybrid system driven by the solid oxide fuel cell (SOFC) integrated with waste heat recovery units. A function-fitting neural network is developed to combine the thermodynamic model of the system with the dragonfly algorithm to mitigate the calculation time. According to the optimization outcomes, the optimum parameters create significantly more power and have a greater exergy efficiency and reduced product costs and CO2 emissions compared to the design condition. The sensitivity analysis reveals that while the turbine inlet temperatures of power cycles are ineffective, the fuel utilization factor and the current density significantly impact performance indicators. The scatter distribution indicates that the fuel cell temperature and steam-to-carbon ratio should be kept at their lowest bound. The Sankey graph shows that the fuel cell and afterburner are the main sources of irreversibility. According to the chord diagram, the SOFC unit with a cost rate of 13.2 $/h accounts for more than 29% of the overall cost. Finally, under ideal conditions, the flue gas condensation process produces an additional 94.22 kW of power and 760,056 L/day of drinkable water.

Low-carbon biomass-fueled integrated system for power, methane and methanol production

A novel near zero-CO2 emission system based on gasification of municipal solid waste (MSW) is proposed and analyzed for waste-to-power and power-to-fuel purposes. The integrated system includes an externally fired gas turbine (EFGT), a molten carbonate fuel cell (MCFC), a Rankine cycle and an organic Rankine cycle (ORC), methane and methanol synthesis units and proton exchange membrane electrolyzer (PEME). Three different scenarios for synthetic fuel production (methane-only, methanol-only and dual-production) are investigated considering different operating loads of PEME, as green hydrogen route. For the case that 10% of produced power is consumed by PEME to generate the required hydrogen for methane and methanol synthesis, overall energy and exergy efficiencies are obtained 41 and 36%, respectively. In this case, the system consumes 1012 ton/yr of CO2 to produce 179 ton/yr methane and 393 ton/yr methanol with sustainability index of 1.57. The exergy analysis revealed that the employed EFGT unit is the main source of irreversibility with gasifier as the most exergy destructive component. In addition, the results of sensitivity analysis showed that increasing gas turbine inlet temperature improves both thermodynamic performance of the system and rate of synthetic fuel.

Inherently safety design of a dual-loop bi-evaporator combined cooling and power system: 4E and safety based optimization approach

In this article, a dual-loop bi-evaporator CCP (combined cooling and power) system is presented to simultaneously supply electricity, air-conditioning, and freezing demands. The system consists of two organic Rankine cycles (ORCs) for power generation and two ejector cooling cycles (ECCs) for cooling production which is adjusted at two different cooling temperature levels, using waste heat from the exhaust gases of a biogas plant as prime mover. The system is investigated from energy, exergy, exergoeconomic, exergoenvironmental, and safety standpoints. Later, the performance of the devised unit is optimized by simultaneously accounting the central metrics obtained from 4E + safety analysis. Four benchmarks with the same applications (i.e., cogeneration of power and cooling at two different temperature levels) are selected and the superiorities of the present CCP system over these previously available systems are investigated. At optimal point, the net power, air-conditioning, and freezing products are computed 96.59 kW, 302.4 kW, and 23.01 kW, respectively. Furthermore, the optimal energy efficiency, exergy efficiency, unit cost of product, environmental impact of product, and total risk are calculated 56.09 %, 25.45 %, 37.54 $/GJ, 4026 mPts/GJ, and 1716 $/Year, respectively. Moreover, the generator is recognized as the main source of irreversibility, while the pump used in the high-temperature circuit has the highest amount of risk level between all components. By rising the heat source temperature, the energy efficiency, unit cost of product, environmental impact of product, and total risk level are increased, while the exergy efficiency is reduced.

Techno-economic assessment of a conceptual waste-to-energy CHP system combining plasma gasification, SOFC, gas turbine and supercritical CO2 cycle

In this paper, a novel scheme consisting of plasma gasifier, solid oxide fuel cells (SOFC), gas turbine (GT), and supercritical CO2 cycle has been developed for power and heat cogeneration. Fed by syngas converted from medical waste through plasma gasification, the new design is a SOFC-GT hybrid system benefiting from supercritical CO2 cycle to enhance its performance. Besides, the waste heat carried by the low-temperature exhaust gasses and CO2 stream is further exploited for providing domestic hot water to residents. The benefits of the suggested system were examined based on a 3 t/h plasma gasifier in the thermodynamic and economic aspects, and the effects of the main parameters were also investigated. It is found that the net power output of the studied system could reach up to 14.02 MW with a net waste-to-electricity efficiency of 59.30% and an exergy efficiency of 57.56%. The main source of irreversibility can be traced to three components, gasifier, cell stacks, and afterburner, accounting for 62.45% of the total exergy destruction. Only 3.77 years is required to recover the initial investment of the proposed system and a net present value of 109815.39 k$ can be attained by the waste-to-energy project during its 20-year lifespan.

Exergy analysis of renewable light olefin production system via biomass gasification and methanol synthesis

The conceptual light olefin production system from biomass via gasification and methanol synthesis was simulated and its thermodynamic performance was evaluated through exergy analysis. The system was made up of gasification, gas composition adjustment, methanol synthesis, light olefin synthesis, steam & power generation and cooling water treatment. The in-depth exergy analysis was performed at the levels of system, subsystem and operation component respectively. The gasifier and the tail gas combustor were the main sources of irreversibility with exergy destruction ratios of 17.0% and 16.8% of the input exergy of biomass. The steam & power generation subsystem accounted for 43.4% of the overall exergy destruction, followed by 41.0% and 5.69% in the subsystems of gasification and gas composition adjustment respectively. The sensitivity evaluation of the operation parameters of gasifier indicates that the system efficiency could be improved by enhancing syngas yield and subsequent yield of light olefins. The overall exergetic efficiency of 30.5% is obtained at the mass ratios of steam to biomass and O2-rich gas (95 vol%) to biomass (S/B and O/B) of 0.26 and 0.14 and gasification temperature at 725 °C.

Thermoeconomic analysis of a novel combined cooling, heating and power system based on solar organic Rankine cycle and cascade refrigeration cycle

This paper proposes a solar organic Rankine cycle integrated cascade refrigeration system to meet a hospital’s heating, cooling (each in two temperature levels) and electricity demands. The system is analyzed from energy, exergy and exergoeconomic point of view. Energy and exergy efficiency of the system calculated 89.39% and 8.70%, respectively. Exergy destruction rates of the system components are carried out and results reveal that the solar collector is the main source of irreversibility. The value of exergoeconomic factor for total system shows that the capital investment cost is more than the cost of exergy destruction. Collector is the main component from exergoeconomic point of view. The effect of ORC working fluid change, collector type, solar irradiation and ambient temperature are investigated. Cyclohexane from energy and exergy point of view and octane from exergoeconomic point of view are more efficient. The system with parabolic through collector indicates better results compared to the use of linear Fresnel reflector and parabolic dish collector. Increasing solar irradiation causes reduction in solar field area, energy and exergy efficiencies and total cost of the system. Increasing ambient temperature reduces solar field area and exergy efficiency, and improves thermal efficiency and the total cost of the system.

Combination of passive and active enhancement methods for higher efficiency of waste-fired plants; flue gas and solar thermal processing

This article presents a novel and applied solution for increasing the efficiency and cost-effectiveness of waste-fired power plants and comprehensively analyzes the proposal from thermodynamic, economic, and environmental aspects. The idea is centered around the simultaneous use of passive (flue gas condensation) and active (solar thermal heaters comprising evacuated tube collectors and parabolic trough collectors) for feedwater preheating. In this way, the extracted heat from the turbines for this purpose could be used directly for power generation, and thus, higher waste-to-power efficiency of the cycle. The proposed configuration and the base-case power plant are both simulated in TRNSYS software for dynamic modeling and comparison of the results over an entire year of operation of the power plants. For making the results more reliable for real-life operation conditions, the simulations are run for a real case study in Qassim city, Saudi Arabia. The results of exergy analysis show that the waste incinerator with the annual exergy destruction of 128.3 GWh is the main source of irreversibility in both models. The results further indicate that the proposed novel system is the more suitable option with 11.36 GWh and 68.18 GWh more annual produced electricity and heat, respectively. With these numbers, although the unit product cost of the proposed novel system is 2.2 $/MWh higher than the conventional plant due to the high cost of solar systems, the new system results in a 205.08 ton/GWh lower CO2 emission index which is extremely important and even of high economic value preventing large emission taxes.

Nanoparticles for improving the efficiency of heat recovery unit involving entropy generation analysis

The present article aims to suggest new thermal unit as heat recovery system to augment the available energy. Hot gas flows inside the inner pipe and the operating fluid in annulus region is nanofluid (CuO-water). Helical tape with various revolution number (N) was employed to augment the fluctuations of fluid and reduce the boundary layer thickness. Range of Reynolds number (Re=4000 to 20,000) indicates turbulent regime. To involve such range of Re, K-ɛ approach has been utilized and its validity was reported. Achieving to greater useful energy happens if the irreversibility of system minimized. This function has two terms: frictional term (Sgen,f) and thermal term (Sgen,th). The thermal term is higher than frictional term and it should be considered as main source of irreversibility. With augment of rotational speed as a result of higher values of Re and N, velocity gradient goes up but thinner boundary layer will be generated. So, with rise of such parameters, temperature gradient declines and residence time enhances which results in better mixing of fluid. So, two scrutinized factors have favorite effect on Sgen,th and considering greater values of them leads to lower thermal irreversibility. Also, growth of these factors makes Sgen,f to increase which is unfavorable effect. As N augments from 3 to 5, the value of Sgen,f enhances about 47% but Sgen,th reduces about 42.47% when Re=5000. Also, with selecting higher N from 3 to 7 when Re=2000, frictional entropy augments about 128.95% while Sgen,th reduces by 35.97%. Sgen,th faces 39.14% decrement if Re goes up when N = 3 while with same change, the value of Sgen,f augments to 7.27 times greater value.

Numerical assessment on the hydrothermal behavior and irreversibility of MgO-Ag/water hybrid nanofluid flow through a sinusoidal hairpin heat-exchanger

The aim of this work is to study the hydrothermal and irreversibility characteristics in forced convection flow of Ag-MgO/water hybrid nanofluid through a sinusoidal hairpin heat-exchanger, numerically. The impact of nano-additive concentration (φ), Reynolds number (Re) and amplitude of the sinusoidal tube are investigated on the heat-exchanger performance form both the first law and second law points of view. The considered performance matrices are heat transfer rate, total heat transfer coefficient, heat-exchanger effectiveness, pressure loss, pumping power as well as the irreversibilities due to flow friction and heat transfer. The findings indicated that boosting the Re and φ causes an enhancement in the heat transfer, while the reverse is true about the remaining performance aspects. In addition, it was found that the irreversibility due to flow friction intensifies by boosting either Re or φ. Moreover, the outcomes revealed that the heat transfer is the main source of irreversibility in the flow of hybrid nanofluid (NF) inside a sinusoidal hairpin heat-exchanger. Furthermore, it was reported boosting the amplitude results in a decrease in the performance index of the heat-exchanger.

Entropy generation analysis of convective turbulent flow in alternating elliptical axis tubes with different angles between pitches; a numerical investigation

This paper presents a numerical analysis for studying entropy generation and the convective turbulent flow of an alternating elliptical axis (AEA) tube with different angles between the pitches at constant wall temperature boundary conditions. The turbulent flow of water fluid is simulated at Reynolds numbers of 10,000 to 60,000. The numerical results indicated that the multi-longitudinal vortices have a major influence on convective heat transfer, pressure drop and entropy generation. Two symmetrical vortices having the same direction of rotation close to the tube wall cause local wall shear stress and the local total entropy generation of the tube wall decrease. However, in some parts of the domain of the AEA tubes, these symmetrical vortices are absent. Therefore, there are different results for the local wall shear stress and the local total entropy generation in all domains of the AEA tubes. The Nusselt number and friction factor increases with an increase in Reynolds number and the angle between pitches of the AEA tube. The entropy generation analysis showed that the main source of irreversibility is by thermal entropy generation. The total entropy generation increases when there is an increase in the Reynolds number, as well as the angle between pitches of the AEA tube. Also, at Reynolds numbers of 20,000, the ratio of the total entropy generation of AEA tube to that of the circular tube is at its minimum.

Exergy analysis of hydrogen production via biogas dry reforming

Among the alternative pathways for hydrogen production, the use of biogas from organic waste via dry reforming of methane (DRM), water gas shift reaction and pressure swing adsorption (PSA) is often seen as an interesting option. In this work, the thermodynamic performance of this type of biohydrogen energy system –additionally including a combined-cycle scheme that satisfies the electricity and steam requirements of the process– is evaluated through exergy analysis. The main data needed for the analysis are acquired from a predictive simulation model implemented in Aspen Plus®. The system shows an exergetic efficiency of 55%, with the DRM and the power generation subsystems arising as the main sources of irreversibility. Furthermore, given the significant influence found for the PSA off-gas on the thermodynamic performance of the system, two alternative process configurations based on the use of this stream are evaluated. In this regard, full recirculation of the PSA off-gas to the DRM reactor is found to improve the system's exergetic performance.

Two-dimensional heat and mass transfer and thermodynamic analyses of porous microreactors with Soret and thermal radiation effects—An analytical approach

Transport of heat and mass and the thermodynamics of porous microreactors with thermal diffusion and radiation effects are investigated analytically. The examined configuration includes an axisymmetric, thick-wall microchannel with an iso-flux thermal boundary condition imposed on the external surfaces. The microchannel is filled with porous materials and accommodates a zeroth order homogenous chemical reaction. Internal radiative heat transfer is modelled in addition to heat convection and conduction, while the local thermal non-equilibrium approach is taken within the porous section of the system. The transport of species is coupled with that of heat via the inclusion of thermodiffusion or Soret effect. Two-dimensional heat and mass transfer differential equations are solved analytically. The results are subsequently used to predict the thermodynamic irreversibilities inside the reactor and a thorough analysis of local and total entropy generation rates is performed. Also, the changes in Nusselt number, calculated on the internal walls of the microreactor, versus various parameters are reported. It is shown that the radiation effects can impact the temperature of the solid phase of the porous medium and lead to alteration of Nusselt number. It is further observed that the transfer of mass is the main source of irreversibility in the system. The findings are of particular use for the design and analysis of the microreactors with homogenous chemical reactions and can be also used for the validation of computational models.

First and second laws of thermodynamics analysis of nanofluid flow inside a heat exchanger duct with wavy walls and a porous insert

This paper investigates the combined effects of using nanofluid, a porous insert and corrugated walls on heat transfer, pressure drop and entropy generation inside a heat exchanger duct. A series of numerical simulations are conducted for a number of pertinent parameters. It is shown that the waviness of the wall destructively affects the heat transfer process at low wave amplitudes and that it can improve heat convection only after exceeding a certain amplitude. Further, the pressure drop in the duct is found to be strongly influenced by the wave amplitude in a highly non-uniform way. The results, also, show that the second law and heat transfer performances of the system improve considerably by thickening the porous insert and decreasing its permeability. Yet, this is associated with higher pressure drops. It is argued that the hydraulic, thermal and entropic behaviours of the system are closely related to the interactions between a vortex formation near the wavy walls and nanofluid flow through the porous insert. Viscous irreversibilities are shown to be dominant in the core region of duct where the porous insert is placed. However, in the regions closer to the wavy walls, thermal entropy generation is the main source of irreversibility. A number of design recommendations are made on the basis of the findings of this study.