Internal Irreversible Losses Research Articles

Thermoradiative coupling graphene-based thermionic solar conversion

Thermionic conversion, due to its simple solid-state structure capable of converting heat to electricity directly, is promising for concentrated solar power. However, because of the extremely high cathode temperature, a large portion of the heat is lost to the environment. The paper introduces a novel concept of selective thermoradiative-graphene thermionic conversion (STR-GTI) that involving a combined control of photon and electron emission to modify the radiation dissipation. A detailed thermodynamic model is developed to evaluate the energy transfer irreversibility of STR-GTI solar conversion. The results demonstrate that the selective themoradiative photon emission and graphene thermionic electron emission effects synergistically reduce internal irreversible losses in the STR-GTI system, leading to a maximum energy efficiency of 34.34 % at a concentration ratio of 425, cathode work function of 1.8 eV and thermoradiative bandgap of 0.2 eV. The STR-GTI system outperforms both individual GTI and STR converters in terms of exergy efficiency and entropy production minimization. It exhibits a remarkable 102.03 % increase in exergy efficiency compared to the STR & GTI system at a thermoradiative voltage of −0.14 eV, accompanied by a 28.56 % reduction in exergy loss and a 37.83 % decrease in entropy production. The combination of narrowing the spectral radiation bandwidth and significant electron emission capabilities of graphene contribute to the system’s resilience against solar radiation fluctuations.

Thermodynamic optimization criterion for practical Meletis–Georgiou cycle

A new engine patent was awarded to Meletis and Georgiou in 1998. The engine is with vane rotor and the rotor is fully balanced and symmetric. It also has an exhaust gas recirculation (EGR) process. Symmetric rotor mechanism leads much smother running, and EGR process makes working fluid (WF) preheated before entering to compression stage which provides very high temperature after compression so that the compression ignition can be implemented. An irreversible (practical) Meletis–Georgiou heat engine cycle (IMGC) model with irreversible heat transfer loss (IHTL) as well as internal irreversibility loss (IIL) is built by applying finite-time thermodynamic theory. A non-linear relation between specific heat (NLRSH) of WF and WF’s temperature is considered. Analytical expressions of output work and thermal efficiency versus compression ratio (CPR) of the IMGC are derived. Performance analyses and optimizations of the IMGC are done using numerical calculations. Both of output work and thermal efficiency of the IMGC with NLRSH of WF and its temperature are larger than those with constant specific heat of WF at the same CPR. There are an optimal CPR make output work approaches maximum and another optimal CPR make thermal efficiency approaches maximum. The range of maximum output and maximum thermal efficiency gives the design optimization criterion for the IMGC, which means the thermodynamic trade-off optimization with the two performance indicators for the IMGC. Effect features of IHTL, IIL, NLRSH and other parameters on the optimal performances and optimization criterion are analyzed.

Study on the internal irreversible losses and process exponent of single screw expanders

Study on the internal irreversible losses and process exponent of single screw expanders

Power density analysis and multi-objective optimization of an irreversible Otto cycle

<p indent="0mm">Based on the finite-time thermodynamic theory and NSGA-II, thermodynamic analysis and multi-objective optimization of an irreversible Otto cycle are performed. Further, based on the irreversible Otto model established in previous literature, the expression of cycle power density is obtained. Additionally, the effect of the maximum cycle temperature ratio, heat transfer loss, friction loss, and internal irreversibility loss on the power density performance of the cycle is analyzed. The characteristic relationships among the cycle power density versus compression ratio and power density versus thermal efficiency are obtained. Moreover, the thermal efficiency, maximum specific volume ratio, and maximum pressure ratio of the cycle are compared under the maximum power output and maximum power density criteria. Using NSGA-II, single-, bi-, tri-, and quadru-objective optimizations are performed for the irreversible Otto cycle by taking power output, thermal efficiency, ecological function, and power density as optimization objectives. The optimal design plan is obtained using three decision-making methods: LINMAP, TOPSIS, and Shannon entropy, by comparing the deviation indexes under different objective function combinations. When power output and ecological function are taken as objective functions of bi-objective optimization, the LINMAP solution is used to obtain the minimum deviation index, and the design scheme is close to the ideal scheme.

Ecological optimization of an irreversible Diesel cycle

Applying finite-time thermodynamics and air standard assumption, the irreversible Diesel cycle model is established with friction loss, heat transfer loss and internal irreversibility loss considered. Calculating the entropy generation rate by loss items, the cycle ecological function performance is optimized. The performance characteristics of ecological function and entropy generation rate are derived, and the impacts of three losses on ecological function performance are examined by the numerical method. The work in this paper can provide some guidelines for designers and manufacturers to assess the practical Diesel cycle performance.

Power and efficiency optimizations of an irreversible regenerative organic Rankine cycle

Rational and efficient uses of low temperature waste heats have become an important research topic, and organic Rankine cycle is one of effectual methods for the recoveries of waste heats. The performances of simple and regenerative organic Rankine cycles have been analyzed and optimized by using classical thermodynamics and finite thermodynamics. Based on finite time thermodynamics, an irreversible regenerative organic Rankine cycle model is built in this paper. With the fixed total heat transfer area of all heat exchangers (evaporator, condenser and regenerator) in the cycle, the power output and thermal efficiency of the cycle are maximized with the changes of heat transfer area ratios of the heat exchangers, mass flow rate of the working fluid and superheat. The influences of the internal irreversible losses on the power output and thermal efficiency of the cycle are analyzed. The results illustrate that there exist an optimal evaporator heat transfer area, an optimal mass flow rate and an optimal superheat to make the power output reach its triple maximum. There also exist an optimal evaporator heat transfer area ratio, an optimal regenerator heat transfer area ratio and an optimal superheat to make the thermal efficiency reach its triple maximum. The power output and thermal efficiency after triple optimizations are increased by 25.22% and 12.16%, respectively. Moreover, the internal irreversible losses have similar effects on the power output and thermal efficiency. Useful guidelines for the designs of the practical regenerative organic Rankine cycles can be offered from the gained results of this paper.

Performance of Universal Reciprocating Heat-Engine Cycle with Variable Specific Heats Ratio of Working Fluid.

Considering the finite time characteristic, heat transfer loss, friction loss and internal irreversibility loss, an air standard reciprocating heat-engine cycle model is founded by using finite time thermodynamics. The cycle model, which consists of two endothermic processes, two exothermic processes and two adiabatic processes, is well generalized. The performance parameters, including the power output and efficiency (PAE), are obtained. The PAE versus compression ratio relations are obtained by numerical computation. The impacts of variable specific heats ratio (SHR) of working fluid (WF) on universal cycle performances are analyzed and various special cycles are also discussed. The results include the PAE performance characteristics of various special cycles (including Miller, Dual, Atkinson, Brayton, Diesel and Otto cycles) when the SHR of WF is constant and variable (including the SHR varied with linear function (LF) and nonlinear function (NLF) of WF temperature). The maximum power outputs and the corresponding optimal compression ratios, as well as the maximum efficiencies and the corresponding optimal compression ratios for various special cycles with three SHR models are compared.

Visualization experimental study of the condensing flow regime in the transonic mixing process of desalination-oriented steam ejector

There are few available related studies about condensing flow regime in the transonic mixing process of steam ejector due to its extreme complexity, although it is of great importance for improving ejector entrainment performance and broadening its application. In this paper, a visualization experimental platform of desalination-oriented steam ejector is designed, and the condensing flow regimes under various entrainment pressures pH are captured from multiple perspectives, by means of direct photography approach. The results reveal that the supersonic jet flow is full with massive amounts of condensation droplets whose particle size distribution is not uniform at various vertical positions, and some easily lose their dynamic equilibrium. There exists a slant reverse condensing flow cross-section at the mixing chamber end, which moves upstream as pH rises and where great numbers of condensation droplets constantly generate around. The condensate quantity of each observation surface is different and increases with pH. The reverse condensing flow regime formed is very complex and accompanied by flexible swirls, clockwise circumferential flow and condensate dynamic accumulation. Then the generated condensate significantly reduces after multiple evaporations downstream the reverse cross-section. It is also found that the flow regime is similar at a same field of view, but diverse at the different fields of view. These meaningful results could serve as a good start point for reducing internal irreversible losses and tailoring ejector design, as well as refining and evaluating physical and mathematical two-phase flow ejector models.

Optimum ecological performance of irreversible reciprocating Maisotsenko-Brayton cycle

Based on the established irreversible reciprocating Maisotsenko-Brayton cycle (RMBC) model with heat transfer loss (HTL), piston friction loss (PFL) and internal irreversible losses (IIL), this paper derives important parameter expressions such as entropy generation rate (\( \sigma\)) and ecological objective function (E), and studies the optimal ecological performance of the irreversible RMBC by using the finite-time thermodynamics (FTT) theory. The influences of the outlet temperature of the air saturator (AS), the maximum cycle temperature, the outlet exhaust gas temperature and the mass flow rate (MFR) of the water injected to the cycle on the cycle ecological performance are analyzed by using numerical examples. The performances of the irreversible RMBC and traditional irreversible reciprocating Brayton cycle (RBC) are compared, and the results show that the performance of the M-Brayton cycle is more superior to that of the Brayton cycle.

Near-field thermionic-thermophotovoltaic energy converters

We proposed a model of the coupling system composed of a vacuum thermionic power generator and a near-field thermophotovoltaic cell. The coupling system can simultaneously convert electrons and photons into electricity. Expressions for the efficiency and power output density of the coupling system are analytically derived. The influences of external and internal irreversible losses on the performance of the coupling system are discussed. The working temperature of the anode is calculated through an energy balance equation. The maximum efficiency and power output density of the coupling system are found to be obviously larger than those of a single vacuum thermionic power generator or a near-field thermophotovoltaic cell. Some parametric selective criteria are supplied and the boundaries of optimized parameters are determined. The results obtained here will be helpful to the optimal design and actual development of near-field thermionic-thermophotovoltaic cells.

Optimal Performance Characteristics of Subcritical Simple Irreversible Organic Rankine Cycle

Based on the theory of finite time thermodynamics, a subcritical simple irreversible organic Rankine cycle (SSIORC) model considering heat transfer loss and internal irreversible losses is established in this paper. The total heat transfer surface area is taken as a constraint, and R245fa is adopted as working fluid of the cycle in the performance optimization. The evaporator heat transfer surface area and mass flow rate of the working fluid are optimized to obtain the maximum power output and thermal efficiency of the SSIORC, respectively. In addition, the influences of the internal irreversibilities on the optimal performances are also investigated. The results show that when the evaporator heat transfer surface area is varied, the relationship between power output and thermal efficiency is a loop-shaped curve, and there exist maximum power output and thermal efficiency points, respectively. However, the two maximum points are very close to each other. When the mass flow rate of the working fluid is varied, the relationship between power output and thermal efficiency is a parabolic-like curve. With the decreases of expander and pump irreversible losses, the performances of the irreversible SSORC are close to those of the endoreversible SSORC with the only loss of heat transfer loss.

Optimization of the power, efficiency and ecological function for an air-standard irreversible Dual-Miller cycle

This paper establishes an irreversible Dual-Miller cycle (DMC) model with the heat transfer (HT) loss, friction loss (FL) and other internal irreversible losses. To analyze the effects of the cut-off ratio (ρ) and Miller cycle ratio (rM) on the power output (P), thermal efficiency (η) and ecological function (E), obtain the optimal ρopt and optimal rMopt, and compare the performance characteristics of DMC with its simplified cycles and with different optimization objective functions, the P, η and E of irreversible DMC are analyzed and optimized by applying the finite time thermodynamic (FTT) theory. Expressions of P, η and E are derived. The relationships among P, η, E and compression ratio (e) are obtained by numerical examples. The effects of ρ and rM on P, η, E, maximum power output (MP), maximum efficiency (MEF) and maximum ecological function (ME) are analyzed. Performance differences among the DMC, the Otto cycle (OC), the Dual cycle (DDC), and the Otto-Miller cycle (OMC) are compared for fixed design parameters. Performance characteristics of irreversible DMC with the choice of P, η and E as optimization objective functions are analyzed and compared. The results show that the irreversible DMC engine can reach a twice-maximum power, a twice-maximum efficiency, and a twice-maximum ecological function, respectively. Moreover, when choosing E as the optimization objective, there is a 5.2%of improvement in η while there is a drop of only 2.7% in P compared to choosing P as the optimization objective. However, there is a 5.6% of improvement in P while there is a drop of only 1.3% in η compared to choosing as the optimization objective.

Thermodynamic optimization for an air-standard irreversible Dual-Miller cycle with linearly variable specific heat ratio of working fluid

This paper establishes an air-standard irreversible Dual-Miller cycle (DMC) model with the specific heat ratio (SHR) of working fluid (WF) linearly varying with its temperature. Because the specific heat (SH) of WF varies with combustion reaction in actual internal combustion engine (ICE), the SHR of WF should be a function of temperature but not a constant. In order to accurately reflect the practical characteristics of DMC engine, performance of DMC with linearly variable SHR, and with heat transfer (HT) loss, friction loss (FL) and other internal irreversible losses (IILs) is analyzed and optimized by applying finite-time thermodynamics. Analytical formulae of the power output (P), efficiency (η), entropy generation rate (EGR) and ecological function (E) are derived. Relationships among P, η, E and compression ratio are obtained via numerical calculations. Effects of the design parameters, cycle temperatures and linearly variable SHR of WF on P, η and E are investigated. Performance differences among the DMC and its simplified cycles, including Otto cycle (OC), Dual cycle (DDC) and Miller cycle (OMC) are compared. Performance characteristics of the DMC with different optimization objective functions (OOFs) are analyzed. The results indicate that the maximum power output (MP), maximum efficiency (MEF) and maximum ecological function (ME) of the DMC are superior to those of OC, DDC and OMC, and optimizing E is the best compromise between optimizing P and optimizing η. The presented results may be helpful to optimize the performance of practical DMC engines.

Thermodynamic Analysis of an Irreversible Maisotsenko Reciprocating Brayton Cycle.

An irreversible Maisotsenko reciprocating Brayton cycle (MRBC) model is established using the finite time thermodynamic (FTT) theory and taking the heat transfer loss (HTL), piston friction loss (PFL), and internal irreversible losses (IILs) into consideration in this paper. A calculation flowchart of the power output (P) and efficiency (η) of the cycle is provided, and the effects of the mass flow rate (MFR) of the injection of water to the cycle and some other design parameters on the performance of cycle are analyzed by detailed numerical examples. Furthermore, the superiority of irreversible MRBC is verified as the cycle and is compared with the traditional irreversible reciprocating Brayton cycle (RBC). The results can provide certain theoretical guiding significance for the optimal design of practical Maisotsenko reciprocating gas turbine plants.

Effect of specific heat variations on irreversible Otto cycle performance

Considering internal irreversibility loss, friction loss and heat transfer loss, an irreversible Otto cycle model is built up by using finite-time thermodynamics with air standard assumption. Using the various irreversible losses in the cycle to compute the entropy generation rate, the optimal ecological function performance of the cycle is studied when the specific heats of working fluid are nonlinear relation with its temperature. Some important expressions, including ecological function, entropy generation rate, efficiency and power output, are obtained. The cycle ecological function performances with constant specific heats, specific heats changed with linear and nonlinear relations of its temperature are compared. Moreover, the impacts of internal irreversibility loss, friction loss and heat transfer loss on ecological function performance are analyzed. The results show that optimization of the exergy-based ecological function not only represents a compromise between the power output and the rate of entropy generation but also represents a compromise between the power output and the thermal efficiency, the specific heat models have no qualitative effect and only have quantitative effect on the performance characteristics of ecological function versus power output and ecological function versus efficiency, and the ecological function, power output and efficiency decrease with the increase of heat transfer, friction and internal irreversibility losses.

Evaluation of the waste heat and residual fuel from the solid oxide fuel cell and system power optimization

A hybrid system including the solid oxide fuel cell (SOFC), Carnot cycle, and Brayton cycle is proposed, where the high-grade waste heat and residual fuel from the SOFC is utilized and transformed into power. Based on electrochemistry and thermodynamics, the electric power, flow rate of waste heat, and composition of residual fuel from the SOFC are calculated, respectively, under a given flow rate of natural gas into the anode of SOFC. The working substance of Carnot cycle absorbs heat in the cooling-tube of SOFC stack and the efficiency of the cycle is derived by considering the finite-rate heat transfer and internal irreversible losses. The working substance of Brayton cycle absorbs high-temperature heat from the combustion products of the residual hydrogen and the efficiency of Brayton cycle is obtained by considering the finite-rate heat transfer and irreversible compression and expansion processes. To achieve optimal power of SOFC or the maximum power of the hybrid system, the favorable working temperature, mole flow rate of natural gas, and hydrogen utilization factor in SOFC are calculated by using MATLAB software.

Parametric optimum design criteria of a thermophotovoltaic cell

A new model of the thermophotovoltaic cell (TPVC) including internal and external irreversible losses is proposed. Expressions for the efficiency and power output of the TPVC are analytically derived. The curves of the temperatures of the emitter and the photovoltaic (PV) cell, two heat flows of the TPVC, and current density of the PV cell varying with the voltage output are presented through numerical calculation. The effects of the voltage output of the PV cell and the band gap energy of the semiconductor material in the PV cell on the efficiency and power output are discussed in detail. The maximum efficiency and power output density are calculated. The choice criteria of some key parameters are supplied. The influence of the temperature of the heat source on the performance of the TPVC is revealed. The lower and upper bounds of the optimized parameters are determined. The results obtained here may provide some theoretical guidance for the optimal design of practical TPVCs. © 2017 American Institute of Chemical Engineers Environ Prog, 2017

Exergy-based ecological performance of an irreversible Otto cycle with temperature-linear-relation variable specific heat of working fluid

Considering internal irreversibility loss (IIL), friction loss (FL) and heat transfer loss (HTL), an irreversible Otto cycle (IOC) model is built up by using air standard (AS) assumption. Based on finite-time thermodynamics (FTT), computing entropy generation rate (EGR) by using the irreversible losses in the cycle, the ecological function (EF) performance of the cycle is optimized when the specific heat (SH) of the working fluid (WF) varies with temperature with linear relation. Some important expressions, including efficiency, power output, EGR and EF, are obtained. Moreover, the effects of variable SH of WF and three losses on cycle performance are investigated. The research conclusion can provide some guidelines for the actual Otto cycle engine performance optimization.

Power, efficiency, ecological function and ecological coefficient of performance of an irreversible Dual-Miller cycle (DMC) with nonlinear variable specific heat ratio of working fluid

Finite time thermodynamic (FTT) theory is applied to perform performance analysis for an air-standard irreversible Dual-Miller cycle (DMC) based on the power output (P), thermal efficiency (\(\eta\), ecological function (E) and ecological coefficient of performance (ECOP) criteria by considering nonlinear variable specific heat ratio, piston friction loss, heat transfer loss and other internal irreversible losses. Relationships between different performance characteristics are obtained via numerical calculations. Effects of pressure ratio and stroke length on each criterion are analyzed, and performance characteristics with different optimization objective are compared. The results show that pressure ratio has little influence on performance characteristics, but stroke length has great influence on performance characteristics. Moreover P, \(\eta\), E and ECOP decrease with increasing stroke length, but when stroke length increases to a certain value, E is less than 0 whatever value of compression ratio takes. Choosing the E and ECOP as optimization objectives is more significant by comparing with other performance indexes.

Performance evaluation and parametric choice criteria of a Brayton pumped thermal electricity storage system

A more realistic thermodynamic model of the pumped thermal electricity storage (PTES) system consisting of a Brayton cycle and a reverse Brayton cycle is proposed, where the internal and external irreversible losses are took into account and several important controlling parameters, e.g., the pressure ratio and heat flows of the two isobaric processes in the Brayton cycle, are introduced. Analytic expressions for the round trip efficiency and power output of the PTES system are derived. The general performance characteristics of the PTES system are revealed. The optimal relationship between the round trip efficiency and the power output is obtained. The influences of some important controlling parameters on the performance characteristics of the PTES system are discussed and the optimally operating regions of these parameters are determined.