Internal Irreversibility Research Articles

Thermodynamic theorical and parametric analysis of a photovoltaic-photo/thermochemistry synergistic conversion system with full spectrum cascading utilization

Solar spectral splitting and cascade utilization is an efficient way to improve solar energy conversion efficiency. This study analyzes the photovoltaic-photo/thermochemistry synergistic conversion system from a thermodynamic perspective. The photo/thermochemistry conversion combined with photochemistry and thermochemistry can effectively utilize the residual spectrum that photovoltaic cannot efficiently use. Through the established thermodynamic model, the thermodynamic limitation of the photovoltaic-photo/thermochemistry synergistic conversion system is calculated. The analysis examines the effects of parameters such as cutoff wavelengths, concentration ratios, and internal irreversibility on system performance, including photovoltaic output voltage, available energy per absorbed photon of photocatalysis, conversion temperature, and selective absorption cutoff wavelength. The results indicate a high thermodynamic limitation of 47.1% at one sun and 68.56% at full concentrations. Energy and exergy flow analysis identifies the proportion of system energy and exergy loss. Compared with other spectral splitting systems, the proposed system demonstrates a significant advantage in solar cascading utilization. This study provides a thermodynamic foundation for the development and design of photovoltaic-photo/thermochemistry synergistic conversion systems.

Energy and exergo-environmental analysis of a refrigerator-Stirling/Photovoltaic system for cold production

The Stirling refrigerator is a device capable of using green energies such as solar power. In this work, a refrigerator-Stirling/photovoltaic system is proposed to harness solar energy for cold production. The system consists of photovoltaic (PV) panels that capture solar energy and convert it into electricity, and a transmission belt linking the motor to the Stirling beta refrigerator to use the electricity for cooling purposes. The system is modeled on the basis of finite-dimensional thermodynamics, taking into account the refrigerator's internal and external irreversibilities. In addition, the wiring loss, dust coverage and shading of the photovoltaic system and the mechanical power loss of the Stirling refrigerator are taken into account in this model. Power equality has been considered as a system constraint. Finally, the effects of operational and design parameters on system performance are investigated. A numerical code is written in Matlab. The effects of PV module efficiency, Stirling regenerator efficiency and PV derating factor on system performance are quantified, also the optimum operating parameters obtained. The refrigerator-Stirling/photovoltaic has a high COP and a low cooling capacity, in contrast to the refrigerator-Stirling/Dish-Stirling. The optimum solar electric COP and maximum cooling rate are 0.7063 and 1413 W respectively at an optimum panel area of 2 m2. Optimum exergy efficiency is 38.5 % at fv = 0.1, ecological coefficient of performance (ECOP) is 20.3 % at N = 48 rot.s−1, optimum exergy destroyed is 33.31 W at K1 = 21.5 W.K−1. This system can be used to preserve vaccines in hospitals.

An Effective Flux Framework for Linear Irreversible Heat Engines: Case Study of a Thermoelectric Generator.

We consider an autonomous heat engine in simultaneous contact with a hot and a cold reservoir and describe it within a linear irreversible framework. In a tight-coupling approximation, the rate of entropy generation is effectively written in terms of a single thermal flux that is a homogeneous function of the hot and cold fluxes. The specific algebraic forms of the effective flux are deduced for scenarios containing internal and external irreversibilities for the typical example of a thermoelectric generator.

Exergoeconomic Analysis of a Mechanical Compression Refrigeration Unit Run by an ORC.

To improve the efficiency of a diesel internal combustion engine (ICE), the waste heat carried out by the combustion gases can be recovered with an organic Rankine cycle (ORC) that further drives a vapor compression refrigeration cycle (VCRC). This work offers an exergoeconomic optimization methodology of the VCRC-ORC group. The exergetic analysis highlights the changes that can be made to the system structure to reduce the exergy destruction associated with internal irreversibilities. Thus, the preheating of the ORC fluid with the help of an internal heat exchanger leads to a decrease in the share of exergy destruction in the ORC boiler by 4.19% and, finally, to an increase in the global exergetic yield by 2.03% and, implicitly, in the COP of the ORC-VCRC installation. Exergoeconomic correlations are built for each individual piece of equipment. The mathematical model for calculating the monetary costs for each flow of substance and energy in the system is presented. Following the evolution of the exergoeconomic performance parameters, the optimization strategy is developed to reduce the exergy consumption in the system by choosing larger or higher-performance equipment. When reducing the temperature differences in the system heat exchangers (ORC boiler, condenser, and VCRC evaporator), the unitary cost of the refrigeration drops by 44%. The increase in the isentropic efficiency of the ORC expander and VCRC compressor further reduces the unitary cost of refrigeration by another 15%. Following the optimization procedure, the cost of the cooling unit drops by half. The cost of diesel fuel has a major influence on the unit cost of cooling. A doubling of the cost of diesel fuel leads to an 80% increase in the cost of the cold unit. The original merit of the work is to present a detailed and comprehensive model of optimization based on exergoeconomic principles that can serve as an example for any thermal system optimization.

Finite time thermodynamic optimization for performance of absorption energy storage systems

Absorption energy storage (AES) has attracted worldwide attention due to the high energy storage density and environmental friendliness. To optimize the performance of the AES system, a finite time thermodynamic (FTT) model considering some influencing factors such as time, heat transfer area, heat transfer temperature difference, internal friction and dissipation has been developed in this paper. A new concept of ratio of charging and discharging time is proposed. The general relation of the energy storage efficiency, energy storage rate and energy release rate are induced based on FTT analysis. The distribution of the total thermal conductivity was optimized under the condition of a certain total thermal conductivity and the optimized maximum cooldown release rate has been increased by 14%. The optimal operating conditions of the AES system are obtained. Furthermore, the influences of the internal irreversibility, ratio of charging and discharging time and heat source temperatures on the performance of the AES system are analyzed. The research results can provide theoretical basis and guidance for the AES system design and optimization.

A metaheuristic approach for multi-objective optimization of the Stirling cycle with internal irreversibilities and regenerative losses using artificial bee colony algorithm

This study aimed to investigate the multi-objective optimization of a Stirling cycle with a mathematical model that takes into account the dead volumes, effects of regenerative losses, and internal irreversibilities on the thermodynamic performance of the cycle. The optimization was carried out using the Artificial Bee Colony algorithm, which is a population-based metaheuristic method that mimics the foraging behavior of honeybees. The developed algorithm employs the concepts of Pareto frontier and ε-dominance to find the optimal solutions in the multi-objective space which is obtained with Artificial Bee Colony algorithm. The input parameters of the system are the maximum and minimum temperatures, compression and expansion volumes, and charge pressure which affect the thermodynamic variables of the cycle. The optimization process consisted of two stages: firstly, single-objective optimization was performed separately for each objective function to obtain the baseline results. Then, three different sets of triple-objective function groups were used to perform multi-objective optimization. These are Case 1 (net work output, thermal efficiency, irreversibility parameter), Case 2 (net work output, thermal efficiency, 2nd law efficiency), and Case 3 (net work output, thermal efficiency, entropy generation). The obtained results from the single and multi-objective optimizations were compared and analyzed. Since multi-objective optimization involves conflicting objectives, it does not result in a single optimal solution, but rather a set of optimal solutions that represent different trade-offs among the objectives. In order to achieve the optimum results with a good trade-off between solutions Pareto frontier method is used. In addition, to obtain a good distribution of solutions and filter very similar solution points in the solution space, ε-dominance was used to filter them. Finally, to select the final optimal solution from the Pareto frontier solution set, LINMAP was used as a decision-making tool which is a linear programming technique that assigns weights to each solution. The weights of the solutions are achieved according to the relative distance between objectives with their single optimal values achieved by single optimization. The best solution based on the net work output is achieved with Case 3 which includes entropy generation as different from other cases. Also, Case 3 has the lowest irreversibility parameter value even though the irreversibility parameter was optimized in Case 1. Entropy generation and mean effective pressure are quite sensitive due to the multiple solutions they have at each solution step when entropy generation is not an objective function.

Performance investigation of a concentrated photovoltaic-thermoelectric hybrid system for electricity and cooling production

To fully exploit inlet sunlight, a novel hybrid system comprising a concentrated photovoltaic cell, a thermoelectric generator, and a thermoelectric cooler is established. The thermoelectric generator generates electricity by thermally harnessing the solar spectrum energy unavailable to the concentrated photovoltaic cell and then drives the thermoelectric cooler to produce additional cooling. With full consideration of internal and external irreversibility, the numerical model of the hybrid system is developed, where comprehensive heat transfer equations are constructed by the effectiveness-number of transfer units method. Subsequently, the performance characteristics of the system are evaluated by energetic and exergetic analysis. Over the operating range of the thermoelectric unit, when the performance enhancement of the hybrid system is maximized, the associated power density, energy efficiency, and exergy efficiency are, respectively, 42.828 Wm-2, 0.0251, and 0.0255, which are improved by 3.53%, 3.72%, and 3.66% in comparison to the stand-alone concentrated photovoltaic system. Meanwhile, the exergy destruction rate density is reduced by 0.49%. Furthermore, comprehensive sensitivity analyses are undertaken to derive the effects of critical designing parameters and operating conditions on the overall performance. The obtained outcomes may contribute to the design and operation of such a cogeneration system.

Finite time and finite speed thermodynamic optimization for an irreversible Atkinson cycle

Combining the finite piston speed thermodynamics and finite time thermodynamics and considering the irreversible losses caused by heat transfer, friction, finite piston speed and internal irreversibility, a more practical irreversible Atkinson cycle model is founded in the paper. When the finite piston speeds in cycle processes are not equal, choosing the ecological coefficient of performance, thermal efficiency, power output and ecological function as performance parameters, and choosing the piston speed ratio and finite piston speed as the design parameters, the relationships among the performance parameters and design parameters are obtained, and the influences of the finite piston speed and piston speed ratio on the performance parameters are presented. The research results show that it is necessary to consider the different piston speeds in each process. The designer can choose the corresponding ranges of design parameters to make the corresponding performance parameter reach the maximum value in practice. This work has a certain theoretical significance and can guide the practical design.

Cycle architectures for two-door refrigerators: Performance breakdown

In this study, four different refrigeration cycle architectures for two-door applications were evaluated and compared: (i) fan-and-damper single-evaporator cycle, (ii) serial/parallel (hybrid) dual-evaporator architecture, (iii) parallel dual-evaporator architecture, and (iv) two independent single-evaporator cycles. The performance analysis was carried out by means of quasi-steady mathematical models that combine steady-state submodels for the refrigeration loop with transient submodels for the refrigerated compartments. The numerical calculations were conducted in such a way that layers of internal and external irreversibility could be introduced into the analysis, from a fully-reversible Carnot cycle to the cycling behavior imposed by the control logic. The dual-evaporator cycles, which are far more complex than single-evaporator ones, were prototyped and tested in a climate chamber to gather data used to calibrate and validate the mathematical models. A methodology was devised on thermodynamic grounds to allow a fair comparison between the architectures. The system with two independent single-evaporator cycles performed the best, followed by parallel and hybrid dual-evaporator loops, and the single-evaporator fan-and-damper solution. Also, the replacement of isobutane with n-butane in the fresh-food branch of the two independent cycles led to an extra 5% energy consumption reduction under the same conditions, providing better matching of the compressor capacity to the thermal loads.

Energetic optimization and local stability of heliothermal plant models under three thermo-economic performance regimes

Solar power plants have the advantage of not producing greenhouse gases into the atmosphere. In their operation, some design and construction parameters can be identified for improving their performance. In this work within the context of Finite-Time Thermodynamics (FTT), we analyze a Müser–Curzon–Ahlborn heat engine model by maximizing economic-objective functions which not only provide optimal operation modes, but also they can reduce the energy dissipated in the form of heat towards the atmosphere. In our thermo-economic study we take into account both investment as well as operation and maintenance (O&M) costs. From operation data of three commercial tower-power plants: Eurelios, Gemasolar and Ivanpah, we find optimal values of solar concentration (Ci) for each economic-operation regime, accounting for the effect of internal irreversibilities (through a lumped parameter R). The Ci-values that we found are in the range of current reported values for the three power plants aforementioned. Besides, we present a local stability analysis for the three aforementioned solar plants and we show that for large size plants, disturbed isothermal branches return faster to their respective steady states, in less dissipative regimes, (e.g. the ecological regime).

Restrictions on the thermodynamic efficiency of the detonation cycle

In this study, an attempt was made to comprehensively analyze the detonation cycle by comparing it with the Brayton and Otto cycles with six variants of restrictions on the main thermodynamic characteristics. As a tool, we used a set of necessary and sufficient evaluation criteria introduced by the authors, taking into account equivalent Carnot cycles. It is shown that the thermal efficiency does not give a complete picture of the thermodynamic perfection of a detonation engine. In the considered variants of restrictions, the thermal efficiency of the detonation cycle exceeds (variants 1 and 5 of restrictions) or is equal (variants 3, 4 and 6 of restrictions) to the Otto and Brayton cycles. The exception is the second variant of restrictions (Tmin = const, Tmax = const), when the thermal efficiency of the detonation cycle is 3 % lower than the Otto cycle and 27 % lower than the Brayton cycle. In the first variant of restrictions (ε = const), the detonation cycle exceeds the thermal efficiency of the Otto and Brayton cycles by 1.42 and 1.21 times, and in the fifth variant of restrictions (Tmin = const, ηi = const), by 1.12 times 1.06 times, respectively. At the same time, under these restrictions, the detonation cycle is inferior in terms of the degree of perfection of the actual cycle to the relative theoretical cycle: the relative efficiency of the Otto and Brayton cycles exceeds the detonation cycle by 1.09 times and 1.05 times, respectively, in the first variant, and by 1.11 times and 1.07 times, respectively, in fifth variant of restrictions.In the third case of restrictions (Tmin = const, Tmax = const, ηt = const), the detonation cycle is inferior in some criteria to the Otto cycle (specific volumetric work, indicator and relative efficiency), but surpasses the Brayton cycle in almost all criteria. In the fourth variant of restrictions (Tmin = const, ηt = const), the detonation cycle is less effective in terms of indicator and relative efficiencies than the Otto cycle (by 11 %) and the Brayton cycle (by 6–7 %). And, in terms of specific volumetric work, the detonation cycle exceeds the Brayton cycle by 1.27 times, but is inferior to the Otto cycle by almost 70 %. In the fifth variant of restrictions (Tmin = const, ηi = const), the detonation cycle has the lowest relative efficiency ηr (11 % and 7 % less than the Otto and Brayton cycles, respectively), despite its superiority in thermal efficiency. Under the sixth variant of restrictions (Tmin = const, ηt = const, ηi = const), the detonation cycle exceeds the Otto and Brayton cycles in terms of internal irreversibility criterion β. At the same time, in terms of specific volumetric work, the detonation cycle is 64 % inferior to the Otto cycle, but exceeds the Brayton cycle by 1.29 times.

Efficient Power Characteristic Analysis and Multi-Objective Optimization for an Irreversible Simple Closed Gas Turbine Cycle.

On the basis of the established irreversible simple closed gas turbine cycle model, this paper optimizes cycle performance further by applying the theory of finite-time thermodynamics. Dimensionless efficient power expression of the cycle is derived. Effects of internal irreversibility (turbine and compressor efficiencies) and heat reservoir temperature ratio on dimensionless efficient power are analyzed. When total heat conductance of two heat exchangers is constant, the double maximum dimensionless efficient power of a cycle can be obtained by optimizing heat-conductance distribution and cycle pressure-ratio. Through the NSGA-II algorithm, multi-objective optimizations are performed on the irreversible closed gas turbine cycle by taking five performance indicators, dimensionless power density, dimensionless ecological function, thermal efficiency, dimensionless efficient power and dimensionless power output, as objective functions, and taking pressure ratio and heat conductance distribution as optimization variables. The Pareto frontiers with the optimal solution set are obtained. The results reflect that heat reservoir temperature ratio and compressor efficiency have greatest influences on dimensionless efficient power, and the deviation indexes obtained by TOPSIS, LINMAP and Shannon Entropy decision-making methods are 0.2921, 0.2921, 0.2284, respectively, for five-objective optimization. The deviation index obtained by Shannon Entropy decision-making method is smaller than other decision-making methods and its result is more ideal.

Thermal Brownian heat pump with external and internal irreversibilities

Thermal Brownian heat pump with external and internal irreversibilities

Four-Objective Optimization for an Irreversible Porous Medium Cycle with Linear Variation in Working Fluid's Specific Heat.

Considering that the specific heat of the working fluid varies linearly with its temperature, this paper applies finite time thermodynamic theory and NSGA-II to conduct thermodynamic analysis and multi-objective optimization for irreversible porous medium cycle. The effects of working fluid’s variable-specific heat characteristics, heat transfer, friction and internal irreversibility losses on cycle power density and ecological function characteristics are analyzed. The relationship between power density and ecological function versus compression ratio or thermal efficiency are obtained. When operating in the circumstances of maximum power density, the thermal efficiency of the porous medium cycle engine is higher and its size is less than when operating in the circumstances of maximum power output, and it is also more efficient when operating in the circumstances of maximum ecological function. The four objectives of dimensionless power density, dimensionless power output, thermal efficiency and dimensionless ecological function are optimized simultaneously, and the Pareto front with a set of solutions is obtained. The best results are obtained in two-objective optimization, targeting power output and thermal efficiency, which indicates that the optimal results of the multi-objective are better than that of one-objective.

Thermal Brownian refrigerator with external and internal irreversibilities

In this paper, a new model of irreversible thermal Brownian refrigerators considering external and internal irreversibilities is established by using finite time thermodynamics. The analytical formulas of cooling load and coefficient of performance (COP) are derived with Newtonian heat transfer law. The operating temperatures of the refrigerator are obtained by solving equations, and the basic performance characteristics and valid working areas are studied. The optimal performance is achieved by optimizing the design parameters and the heat transfer process. The effects of key parameters, temperature ratio and irreversible factors on optimal performance are investigated. The new model performance is compared with that of previous models. Results show that the new model provides a unified description for thermal Brownian refrigerators. The cooling load and COP exist maximal values about thermal conductance ratio between the two heat exchangers. When the thermal conductance ratio and other design parameters are optimal, the corresponding operating temperatures of the thermal Brownian refrigerator reach the optimal values. The results are universal, and it contains the results of previous models with different irreversible factors.

Thermodynamic evaluation of two portable cooler running with reciprocating and mini-rotary compressors

Small-capacity refrigeration has got an increasing attention of the consumermarket. In addition to the efficiency of electric equipment, the demand forcompactness has also become a relevant requirement. Among thealternatives available to cool down small cabinets, mechanical vaporcompression refrigeration remains prominent due to its satisfactoryperformance when compared to other cooling technologies. Compressorindustries have invested in the development of compact and efficientproducts, exploring different compression principles for small-capacityapplications, as is the case of mini-rotary compressors. In this context, theobjective of the present work is to assess the thermodynamic performanceof a 38-liter portable vapor compression refrigeration system running withtwo different compressors designed for small-capacity applications: minirotaryand reciprocating. Experimental tests were carried out at threedifferent ambient temperatures (16, 25 and 32 °C) in order to obtain the keyperformance parameters for each compressor (e.g., power consumption,cooling capacity, internal air temperature, and the condensing andevaporating temperatures). Finally, thermodynamic analyses wereconducted to account for the internal and external irreversibilities by meansof second law efficiencies, allowing for a comparison of the systemperformance running with both compressors in the same thermodynamicgrounds. Albeit a refrigeration (second law) efficiency by 25% higher wasobserved for the reciprocating compressor, it provided a smaller coolingcapacity and, therefore, led to a higher pull-down time.

Thermal Brownian heat engine with external and internal irreversibilities

A finite time thermodynamic model of irreversible thermal Brownian heat engines with various irreversibilities is built. The analytical expressions of power and efficiency are derived. The working temperatures are solved by combining equations, the essential performance characteristics of heat engine are further analyzed. The relationships among optimal performance indicators and design parameters are also studied. The optimal performance characteristics are obtained by optimizing the internal parameters and thermal conductance ratio of heat exchangers. The optimal working regions and optimal working temperatures are also given. Finally, the new model performance is compared with that of the non-equilibrium thermodynamic one, and the significance of the new model is illustrated. The results show that the new model can be simplified to the existing models, and the optimal performances of those existing models with different irreversible factors can also be derived through the optimization relation in this paper. The power and efficiency have extreme values about barrier height, thermal conductance ratio and external load, respectively. When those parameters are optimized simultaneously, the corresponding values of the working temperature are optimal for the heat engine. The kinetic energy will reduce power and efficiency, which is different from the result of the non-equilibrium thermodynamic models.

Performance analysis and optimization of an irreversible Carnot heat engine cycle for space power plant

In this paper, an irreversible Carnot heat engine cycle model for space power plants is built by using finite time thermodynamics. A radiator panel is considered between low temperature heat sink and space environment, expressions for power output and thermal efficiency of the plant are derived. Influences of internal irreversibility effect and thermal leakage loss on the optimal power output performance are analyzed when thermal conductivity of hot- and cold-sides heat exchangers are given. For the fixed total area of three heat exchangers the double-maximum power output and double-maximum thermal efficiency are obtained further by optimizing area distributions of hot- and cold- side heat exchangers and radiator, and by optimizing temperature of low-temperature heat sink. Influences of internal irreversibility effect and thermal leakage loss on the optimal performances are investigated. The results show that when taking power output as optimization objective, if considering thermal leakage and irreversibility effect, internal irreversibility effect increases from 1.1 to 1.3, double maximum power output decreases from 216.38 kW to 139.85 kW and decreases by 35.37%, the optimal the temperature the low-temperature heat sink decreases from 556K to 501.1K and decreases by 9.87%, and the optimal area allocations of hot- and cold-side heat exchangers decrease from 0.213 to 0.182 and decreases by 14.55%, from 0.213 to 0.207 and decreases by 2.82%, respectively, and that of radiator increases from 0.563 to 0.611 (an increase of 8.35%). Such temperature and area distribution condition ensure the future design of plant conversion system that aligns better performances and compactness.

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.

Thermoelectric generator in endoreversible approximation: The effect of heat-transfer law under finite physical dimensions constraint.

We revisit the optimal performance of a thermoelectric generator within the endoreversible approximation, while imposing a finite physical dimensions constraint in the form of a fixed total area of the heat exchangers. Our analysis is based on the linear-irreversible law for heat transfer between the reservoir and the working medium, in contrast to Newton's law usually assumed in literature. The optimization of power output is performed with respect to the thermoelectric current as well as the fractional area of the heat exchangers. We describe two alternate designs for allocating optimal areas to the heat exchangers. Interestingly, for each design, the use of linear-irreversible law yields the efficiency at maximum power in the well-known form 2η_{C}^{}/(4-η_{C}^{}), earlier obtained for the case of thermoelectric generator under exoreversible approximation, i.e., assuming only the internal irreversibility due to Joule heating. On the other hand, the use of Newton's law yields Curzon-Ahlborn efficiency.