Theory Of Finite Time Thermodynamics Research Articles

Optimizing Power and Thermal Efficiency of an Irreversible Variable-Temperature Heat Reservoir Lenoir Cycle

Applying finite-time thermodynamics theory, an irreversible steady flow Lenoir cycle model with variable-temperature heat reservoirs is established, the expressions of power (P) and efficiency (η) are derived. By numerical calculations, the characteristic relationships among P and η and the heat conductance distribution (uL) of the heat exchangers, as well as the thermal capacity rate matching (Cwf1/CH) between working fluid and heat source are studied. The results show that when the heat conductances of the hot- and cold-side heat exchangers (UH, UL) are constants, P-η is a certain “point”, with the increase of heat reservoir inlet temperature ratio (τ), UH, UL, and the irreversible expansion efficiency (ηe), P and η increase. When uL can be optimized, P and η versus uL characteristics are parabolic-like ones, there are optimal values of heat conductance distributions (uLP(opt), uLη(opt)) to make the cycle reach the maximum power and efficiency points (Pmax, ηmax). As Cwf1/CH increases, Pmax-Cwf1/CH shows a parabolic-like curve, that is, there is an optimal value of Cwf1/CH ((Cwf1/CH)opt) to make the cycle reach double-maximum power point ((Pmax)max); as CL/CH, UT, and ηe increase, (Pmax)max and (Cwf1/CH)opt increase; with the increase in τ, (Pmax)max increases, and (Cwf1/CH)opt is unchanged.

Performance Optimizations with Single-, Bi-, Tri-, and Quadru-Objective for Irreversible Atkinson Cycle with Nonlinear Variation of Working Fluid’s Specific Heat

Considering nonlinear variation of working fluid’s specific heat with its temperature, finite-time thermodynamic theory is applied to analyze and optimize the characteristics of an irreversible Atkinson cycle. Through numerical calculations, performance relationships between cycle dimensionless power density versus compression ratio and dimensionless power density versus thermal efficiency are obtained, respectively. When the design parameters take certain specific values, the performance differences of reversible, endoreversible and irreversible Atkinson cycles are compared. The maximum specific volume ratio, maximum pressure ratio, and thermal efficiency under the conditions of the maximum power output and maximum power density are compared. Based on NSGA-II, the single-, bi-, tri-, and quadru-objective optimizations are performed when the compression ratio is used as the optimization variable, and the cycle dimensionless power output, thermal efficiency, dimensionless ecological function, and dimensionless power density are used as the optimization objectives. The deviation indexes are obtained based on LINMAP, TOPSIS, and Shannon entropy solutions under different combinations of optimization objectives. By comparing the deviation indexes of bi-, tri- and quadru-objective optimization and the deviation indexes of single-objective optimizations based on maximum power output, maximum thermal efficiency, maximum ecological function and maximum power density, it is found that the deviation indexes of multi-objective optimization are smaller, and the solution of multi-objective optimization is desirable. The comparison results show that when the LINMAP solution is optimized with the dimensionless power output, thermal efficiency, and dimensionless power density as the objective functions, the deviation index is 0.1247, and this optimization objective combination is the most ideal.

Modeling and performance analysis of a combined thermal Brownian heat pump cycle

<p indent=0mm>Based on finite-time thermodynamics theory, this paper proposes a two-stage thermal Brownian heat pump cycle model combined with an intermediate reservoir. The model is formed by two Brownian heat pump cycles operating in series. Two heat pump cycles operate between three heat reservoirs with constant temperature. In this regard, finite-rate heat transfer processes exist among heat pump cycles and reservoirs. In consideration of the heat transfer losses among reservoirs and heat pump cycles, the expressions of heating load and coefficient of performance (COP) are derived. The influence of design parameters on system performance is analyzed by numerical calculations. Results show that heating load decreases monotonically with the increase in the barrier height of the potential and increases monotonically with the increase in the external load and the asymmetry of the potential. The COP reaches the maximum values with the barrier height of the potential, the asymmetry of the potential, and the external load. When the heat reservoir temperatures are constant, the closer the intermediate heat reservoir temperature is to the topping cycle heat reservoir temperature, the higher the heating load and the COP. Compared with one-stage thermal Brownian heat pump cycle, which works between the heat source and sink with the same temperatures, the combined Brownian heat pump cycle has a larger heating load but a reduced COP.

Performance Optimizations with Single-, Bi-, Tri-, and Quadru-Objective for Irreversible Diesel Cycle.

Applying finite time thermodynamics theory and the non-dominated sorting genetic algorithm-II (NSGA-II), thermodynamic analysis and multi-objective optimization of an irreversible Diesel cycle are performed. Through numerical calculations, the impact of the cycle temperature ratio on the power density of the cycle is analyzed. The characteristic relationships among the cycle power density versus the compression ratio and thermal efficiency are obtained with three different loss issues. The thermal efficiency, the maximum specific volume (the size of the total volume of the cylinder), and the maximum pressure ratio are compared under the maximum power output and the maximum power density criteria. Using NSGA-II, single-, bi-, tri-, and quadru-objective optimizations are performed for an irreversible Diesel cycle by introducing dimensionless power output, thermal efficiency, dimensionless ecological function, and dimensionless power density as objectives, respectively. The optimal design plan is obtained by using three solution methods, that is, the linear programming technique for multidimensional analysis of preference (LINMAP), the technique for order preferences by similarity to ideal solution (TOPSIS), and Shannon entropy, to compare the results under different objective function combinations. The comparison results indicate that the deviation index of multi-objective optimization is small. When taking the dimensionless power output, dimensionless ecological function, and dimensionless power density as the objective function to perform tri-objective optimization, the LINMAP solution is used to obtain the minimum deviation index. The deviation index at this time is 0.1333, and the design scheme is closer to the ideal scheme.

Power and Thermal Efficiency Optimization of an Irreversible Steady-Flow Lenoir Cycle.

Using finite time thermodynamic theory, an irreversible steady-flow Lenoir cycle model is established, and expressions of power output and thermal efficiency for the model are derived. Through numerical calculations, with the different fixed total heat conductances () of two heat exchangers, the maximum powers (), the maximum thermal efficiencies (), and the corresponding optimal heat conductance distribution ratios () and () are obtained. The effects of the internal irreversibility are analyzed. The results show that, when the heat conductances of the hot- and cold-side heat exchangers are constants, the corresponding power output and thermal efficiency are constant values. When the heat source temperature ratio () and the effectivenesses of the heat exchangers increase, the corresponding power output and thermal efficiency increase. When the heat conductance distributions are the optimal values, the characteristic relationships of and are parabolic-like ones. When is given, with the increase in , the , , , and increase. When is given, with the increase in , and increase, while and decrease.

Minimum Entropy Generation Rate and Maximum Yield Optimization of Sulfuric Acid Decomposition Process Using NSGA-II.

Based on the theory of finite-time thermodynamics (FTT), the effects of three design parameters, that is, inlet temperature, inlet pressure, and inlet total mole flow rate, of a tubular plug-flow sulfuric acid decomposition reactor on the total entropy generation rate (EGR) and SO2 yield are analyzed firstly. One can find that when the three design parameters are taken as optimization variables, the minimum total EGR and the maximum SO2 yield of the reference reactor restrict each other, i.e., the two different performance objectives cannot achieve the corresponding extremum values at the same time. Then, the second-generation non-dominated solution sequencing genetic algorithm (NSGA-II) is further used to pursue the minimum total EGR and the maximum SO2 yield of the reference reactor by taking the three parameters as optimization design variables. After the multi-objective optimization, the reference reactor can be Pareto improved, and the total EGR can be reduced by 9% and the SO2 yield can be increased by 14% compared to those of the reference reactor. The obtained results could provide certain theoretical guidance for the optimal design of actual sulfuric acid decomposition reactors.

Maximum energy output chemical pump configuration with an infinite-low- and a finite-high-chemical potential mass reservoirs

The chemical pumps widely exist in various chemical systems, such as mass exchangers, desalination systems, biological systems, electrochemical systems, photochemical systems, solid-state devices, fuel pumps for solar-energy conversion systems, absorption and carbon pumps for material capture systems, etc. Therefore, performance improvements of the chemical systems become an urgent and important problem, which can be solved by optimizing the cycle configurations of the chemical pumps. Based on this consideration, an isothermal endoreversible chemical pump model is built in this paper, which operates between an infinite-low- and a finite-high-chemical potential mass reservoirs. Based on finite time thermodynamic theory, two mass transfer laws, i.e., the linear law and diffusive law are considered. When the cycle work input and operating time are specified, the optimal cycle configuration with maximum energy output is gained. For the linear law, with the optimal chemical pump configuration, the chemical potentials in the finite-high-chemical potential mass reservoir and the working substance nonlinearly vary with the operating time, meanwhile their ratio stays invariant. When both the chemical potential mass reservoirs are infinite ones, the dimensionless energy pumping rate and dimensionless entropy generation rate under the optimal time decrease with the increases of coefficient of performance and exergy efficiency. For the diffusive law, the optimal cycle configuration is quite different with that gained from the linear law. For the linear or diffusive law, the optimal chemical pump configuration with infinite-chemical potential mass reservoirs is made up of two invariant chemical potential branches along with two instant invariant mass-flux branches. The gained results can offer new references for the practical chemical pump designs.

Power and efficiency optimization of open Maisotsenko-Brayton cycle and performance comparison with traditional open regenerated Brayton cycle

The search for higher power and efficiency has been the main purpose of studying the Brayton cycle. Maisotsenko-cycle is one of the most promising heat recovery technologies for improving cycle thermodynamic efficiency and reducing pollution of Brayton cycles at present. Finite time thermodynamics is a powerful tool for optimizing various processes and cycles. An open Maisotsenko-Brayton cycle (Maisotsenko air saturator combined with the Brayton cycle) model is established by using finite time thermodynamics theory and considering the size constraints of real plant in this paper. Some important parameter expressions such as power output and efficiency are derived by considering the seven pressure drop losses in the intake, compression, expansion, discharge processes, two sides of Maisotsenko air saturator and flow process in the piping caused by the working fluid in the circulation, heat transfer loss to the ambient, irreversible losses in the compressor and turbine, and irreversible combustion loss in the combustor. The power is optimized by adjusting the mass flow rate of the working fluid and the distribution of pressure drop loss along the flow path. All other relative inlet pressure drops is related to relative inlet pressure drop at the compressor inlet. There exists an optimal relative inlet pressure drop at the compressor inlet (it corresponds to an optimal mass flow rate of the working fluid) which leads to maximum power, which classical thermodynamics analysis does not find. The maximum power output and the corresponding optimal relative inlet pressure drop at the compressor inlet are obtained besides the optimal pressure ratio of compressor in the cycle, which is also found in classical thermodynamic analysis. Effects of the mass flow rate of the water injected to the cycle, the relative pressure drop at the compressor inlet, the cycle temperature ratio and the cycle pressure ratio on cycle performance are analyzed by using numerical examples. The performances of the open Maisotsenko-Brayton cycle and traditional open regenerated Brayton cycle are compared, and the results show that both the power and efficiency performances of open Maisotsenko- Brayton cycle are higher than those of traditional open regenerated Brayton cycle.

Multi-objective optimization for helium-heated reverse water gas shift reactor by using NSGA-II

Thermodynamic performance of helium-heated reverse water gas shift (RWGS) reactor is investigated by using the theory of finite-time thermodynamics. Taking into account both the radial temperature gradients and the diffusion-reaction phenomenon inside the catalytic pellets, a comprehensive two-dimensional pseudo-homogeneous mathematical model is established to represent the reactor. By using numerical calculations, the thermodynamic performance of reactors in the given conditions is analyzed, the influences of the effective parameters on reactor performance are also examined. Eventually, from the perspective of heat management and production improvement, a multi-objective optimization (MOO) procedure based on the non-dominated sorting genetic algorithm (NSGA-II) is applied to investigate the best working parameters considering the minimum radial temperature difference and maximum conversion rate as optimization objective functions. The results show that significant radial temperature gradients and the resulting radial gradients of apparent reaction rate can be observed. The thermodynamic performance of the counter-flow reactor is superior to that of the parallel-flow pattern. The MOO is an effective technique for selecting the best working parameters to reduce the radial temperature difference and improve the conversion rate simultaneously.

Maximum Hydrogen Production Rate Optimization for Tubular Steam Methane Reforming Reactor

Abstract The performance of a steam methane reforming (SMR) reactor is optimized by using the theory of finite time thermodynamics in this paper. The maximum hydrogen production rate (HPR) and the corresponding optimal exterior wall temperature (EWT) and the optimal pressure of the reaction mixture (PRM) profiles in the SMR reactor are obtained by using nonlinear programming method. In the optimization process, the fixed inlet mole flow rate of components, the thresholds of the state variables and the conservation equations are taken as the constraints. The performance of the optimal reactor is compared with that of the reference reactor with a linear EWT profile. The results show that the HPR of the optimal reactor increases by about 11.8 %. The optimal EWT profile is alike with the linear EWT profile. The HPR increases with the increase of the inlet temperature of reaction mixture and the decrease of the inlet PRM. The influence of the TRM on the HPR is smaller than that of the PRM. The results obtained herein are helpful to the optimal design of practical tubular reactors.

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.

Thermodynamic analysis and multi-objective optimisation of endoreversible Lenoir heat engine cycle based on the thermo-economic performance criterion

ABSTRACTA thermodynamic model of a steady-flow endoreversible Lenoir heat engine cycle (a ‘three-point’ cycle) coupled with constant-temperature heat reservoirs is established in this paper by using finite-time thermodynamic theory. The cycle consists of one isochoric heating branch, one adiabatic expansion branch and one isobaric cooling branch. A mathematical approach based on the finite-time thermodynamic is proposed to obtain thermal efficiency, the output power and the entropy generation rate throughout the Lenoir system. In this study, an irreversible Lenoir engine is analysed thermodynamically in order to optimise its performance. In this regard, the optimal values of the ecological coefficient of performance and dimensionless thermo-economic objective functions of the Lenoir heat engine are determined. Multi-objective evolutionary algorithms based on Non-dominated Sorting Genetic Algorithm II algorithm is applied to the aforementioned system for calculating the optimal values of decision variables. Decision variables considered in this paper include the ratio of fluid temperature (), the effectiveness of the cold-side heat exchanger, the effectiveness of the hot-side heat exchanger, the temperature of state 1 and relative investment cost parameter of the hot-side heat exchanger. Moreover, Pareto optimal frontier is applied and an ultimate optimal answer is chosen via three competent decision-making approaches comprising Linear Programming Technique for Multidimensional Analysis of Preference, fuzzy Bellman-Zadeh and Technique for Order of Preference by Similarity to Ideal Solution.

Thermodynamic performance of Dual-Miller cycle (DMC) with polytropic processes based on power output, thermal efficiency and ecological function

This study reports a new model of an air standard Dual-Miller cycle (DMC) with two polytropic processes and heat transfer loss. The two reversible adiabatic processes which could not be realized in practice are replaced with two polytropic processes in order to more accurately reflect the practical working performance. The heat transfer loss is taken into account. The expressions of power output, thermal efficiency, entropy generation rate (EGR) and ecological function are addressed using finite-time thermodynamic theory. Through numerical calculations, the influences of compression ratio, cut-off ratio and polytropic exponent on the performance are thermodynamically analyzed. The model can be simplified to other cycle models under specific conditions, which means the results have an certain universality and may be helpful in the design of practical heat engines. It is shown that the entropy generation minimization does not always lead to the best system performance.

Thermodynamic performance optimization for an irreversible vacuum thermionic generator

Theoretical model of an irreversible vacuum thermionic generator considering external and internal finite rate heat transfer is established in this paper. By assuming radiative heat transfer processes, the general expressions of performance parameters are derived based on non-equilibrium thermodynamics and finite-time thermodynamics (FTT). The thermodynamic performances of the irreversible thermionic device are further analyzed and optimized by using the FTT theory with multiple optimization criteria such as power output, efficiency, ecological function, and efficient power. The influences of design parameters, such as output voltage, collector work function and heat reservoir temperature, on optimal performance are analyzed in detail by numerical calculations. By properly choosing the work function and output voltage, the thermionic generator can be tuned to operate in the optimal condition with maximum power or efficiency. By comparing the device performance at different design points, the optimal operation regions of power and efficiency of the irreversible thermionic generator are determined. The obtained results are of theoretical significance for the optimal design of practical solar-powered thermionic generators.

Power and efficiency optimization for an energy selective electron heat engine with double-resonance energy filter

Theoretical modeling for a microscopic ESE (energy selective electron) heat engine with double-resonance energy filters is performed in this paper. The heat flow characteristics and performance parameters are discussed in two different cases according to the positions for central energy level of double resonances. It is found that the analytical expressions for performance parameter such as power output or efficiency in the two cases share the identical form. The optimal performance for the ESE heat engine system is analyzed by using finite time thermodynamic theory. The performance curves and fundamental optimal relation for the system's output power and efficiency are explored with numerical examples. The fundamental optimal relation of power and efficiency is an open loop-shaped curve. There exist a maximum efficiency and a maximum power output. The optimal operating regions of the power and efficiency are determined and the influences of the system's design parameters are discussed. Finally, a comparison of the ESE engine system with double- and single-resonance filters is carried out in order to show the performance differences and the effect of the newly adopted double resonances. The utilization of double-resonance energy filter leads to increased power output while decreased efficiency for the electron heat engine device.

三类微型能量转换系统有限时间热力学性能优化的研究进展

With the development of molecular biology technology, nanotechnology and micro electronic technology, more and more attentions have been paid to the microscopic energy conversion systems. The research on energy conversion mechanism and efficiency for micro-energy conversion system is a new subject concerning the interdisciplinary integration of thermodynamics, statistical mechanics, physics theory, etc. Performance optimization is one of the key science problems in revealing the mechanism of energy conversion and improving efficiency of energy utilization for micro-energy conversion system. On the basis of introducing the origin and development of the finite time thermodynamics theory, this paper reviews the recent advances of thermodynamic optimization for three kinds of typical microscopic energy conversion systems, i.e., the thermal Brownian motors, energy selective electron engines and the thermionic energy conversion devices, and proposes the future research prospects.

The optimal configuration of reciprocating engine based on maximum entransy loss

A class of two-heat-reservoir heat engine model with heat leakage, finite heat capacity high-temperature source and infinite heat capacity low-temperature heat sink, is researched with the finite-time thermodynamic theory and the entransy theory. The optimal configuration based on the minimum entropy generation and the maximum entransy loss under the given cycle is searched, which are compared with the optimal configuration based on the maximum output work, and all the heat transfer in the model is assumed to obey the Newton’s law. The results show that, for the infinite heat capacity high-temperature source, the optimal configuration does not change whether heat leakage exists or not; however, for the finite high-temperature reservoir, the optimal configuration is different among which based on the minimum entropy generation, the maximum entransy loss, and the maximum output work when the heat leakage exists.

The Combined Solar with a Dual Heat Source Heat Pump Applied in a Greenhouse Heating System - An Operation Optimization of Water Source Heat Pump

Based on the finite-time thermodynamic theory, an operation optimization, of water source heat pump in the combined solar with a dual heat source heat pump which is applied in a greenhouse heating system, is made. According to the ε-NTU method and entropy theory, heat exchange and balance equations are obtained. The function relationship between COP and the indoor temperature Tn, the ambient temperature Ta, low temperature heat source inlet temperature Tie and high temperature heat source inlet temperature Tic is also obtained. By means of programming, the impact of parameters on the COP and the way of regulating this water source heat pump system are presented in this article. The results show that: when a separate water source heat pump is running, by adjusting the hot water temperature and the match status of each indoor heating system, the energy-saving operation can be realized.

Heating load and COP optimization of a double resonance energy selective electron (ESE) heat pump

A model of a double resonance energy selective electron (ESE) heat pump in one-dimensional system with two idealized energy filters is established in this paper. The analytical expressions of heating load and coefficient of performance (COP) for the ESE heat pump are derived. The optimum heating load and COP performances of the double resonance ESE heat pump are analyzed by using the theory of finite time thermodynamics. The effective regions of central energy level and heating load are obtained. The influences of energy space as well as resonance width of the two resonances on the performance of the ESE heat pump are discussed by detailed numerical examples. The values of some important performance parameters are also obtained by numerical calculations. The performance of the double resonance device is compared with that of the single resonance device. It is found that the maximum heating load decreases with the increase of energy space or the decrease of resonance width, while the maximum COP decreases with the increase of energy space or resonance width. By reasonable choices of the first resonance energy level, the energy space and the resonance width, the heat pump can be controlled to operate under the maximum COP condition. The results obtained herein can provide some theoretical guidelines for the design of practical multi-barrier nanostructured devices.

Modelling and performance analysis for endoreversible Meletis-Georgiou cycle with non-linear relation between specific heat of working fluid and its temperature

Meletis–Georgiou (MG) vane rotor engine incorporates the exhaust gas recirculation process at the basis of Miller cycle, which produces a new cycle. An endoreversible MG cycle model is established using the finite time thermodynamic theory with considerations of heat transfer loss and the non-linear relation between specific heat of working fluid and its temperature. The analytical formulae of work output and efficiency are derived, and the cycle performance is analysed and optimised by detailed numerical examples. The effects of heat transfer loss, the non-linear relation between specific heat of working fluid and its temperature and design parameters on the cycle performance are discussed. The results obtained in this paper can provide some guidelines for the design of practical MG vane rotary engine cycle.