External Irreversibilities Research Articles

Techno‐economic and carbon footprint analyses of steam Rankine cycle

AbstractSteam Rankine cycle (SRC), which is mainly utilised in power generation sector, faces external irreversibility in its daily operation causing inefficiency in the system. To address this issue, reheat Rankine cycle (RHRC) and regenerative Rankine cycle (RRC) have been widely studied and implemented in power plants to improve thermal efficiency and reduce external irreversibility of Rankine cycle. This study investigates the implementation of different RRC configurations in a combined heat and power plant, including RRC with modified thermal deaerator, RRC with open feed water heater (OFWH) and closed feed water heater (CFWH). A base case simulation model was first constructed using commercial simulation software Aspen HYSYS for the basic SRC system based on actual plant data. Various scenarios were then evaluated for their profitability and sustainability through techno‐economic analysis (TEA) and carbon footprint analysis (CFA). From both analyses, the scenario of RRC with CFWH showed the greatest long‐term potential, generating the highest annual profit of $ 771 691 and carbon footprint reduction of 14.63%, while RRC with modified thermal deaerator showed the greatest potential in the short run with the highest return of investment (ROI) of 201.51% and shortest payback period (PBP) of 0.50 year.

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.

Synergizing perovskite solar cell and thermoelectric generator for broad-spectrum utilization: Model updating, performance assessment and optimization

Hybridization of perovskite solar cell with thermoelectric generator is a promising broad-spectrum harvesting strategy. However, the existing modeling studies omit the thermal effects within the hybrid system, leading to inaccuracy of available results. Herein, an updated photovoltaic/thermoelectric system model incorporating a perovskite solar cell, a solar selective absorber, and a thermoelectric generator is theoretically developed to more accurately predict its real performance, in which the external heat transfer irreversibility between subsystems as well as the Thomson effects within thermoelectric generator are considered, and the fundamentally physical processes within the perovskite solar cell are included. It is found that P-type BiTe and n-type BiTe are the best compatible thermoelectric materials for the thermoelectric generator. Extensive parametric studies identify that the operating temperature, operating voltage and two interface defect densities of perovskite solar cell, and two structure parameters of thermoelectric generator are optimizable. A series of optimizations predict that the hybrid system has a maximum energy efficiency of 22.36 %, which is 13.68 % and 12.59 % higher than that of the unoptimized single perovskite solar cell and that of the unoptimized system, respectively. The obtained results are helpful to evaluate and optimize the performance of such an actual system.

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.

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.

Thermal Brownian heat pump with external and internal irreversibilities

Thermal Brownian heat pump with external and internal irreversibilities

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.

EN Thermodynamic analysis of a real marine refrigeration system for cruise ship "Scarlet Lady"

This paper presents an energy analysis of the marine refrigeration system as a component of the machi­nery system of the cruise liner “Scalet Lady”. This type of ship is one of the most complex types from an energy point of view, which is associated with a wide variety of energy consumers. The study uses two methods of thermodynamic analysis of real cycles: the method of cycles and entropy-statistical method. For the analysis, the experimental data on the operating modes of the marine refrigeration system has been used. According to the results of processing the experimental data, a thermodynamic cycle has been formed and an energy calculation of the cycle has been performed using classical methods for two-stage refrigeration machines. With the help of the cycle model, the external irreversibility is determined by successively reducing the energy efficiency of the ideal cycle. The Lorentz cycle has been chosen as an ideal model, taking into account the variable temperatures of heat supply and removal during heat exchange “heat source – phase transformations of the refrigerant R407f”. Along with the evaluation of efficiency, the distribution of losses by the components of the refrigeration system has been established, energy weaknesses that require improvement has been identified, and the most effective ways to reduce energy costs have been outlined. Analytical conclusions have a graphical interpretation of the thermodynamic efficiency assessment for the real refrigeration cycle. It has been established that in the considered real cycle of a marine refrige­ration system, the greatest influence on energy efficiency is exerted by external irreversibility in the condenser, in particular, in the overheating zone (25.64% of the total energy consumption of the plant). The effect of irreversibility in the compressor is not fixed

Thermodynamic optimization of parallel and spiral plate heat exchangers for modified solar thermal Brayton cycle models

The receiver and heat exchangers in a Solar Thermal Brayton Cycle (STBC) have been the main sources of exergy loss. Duct profiles used in the heat exchange process have been observed to possess varying degrees of heat transfer effectiveness. To this end, the effects of the elliptical, circular and rectangular absorber tubes are investigated on three variants of the dual serial-regenerative STBC models employing reheater, intercooler, or in a combined arrangement. Also, the impact of the parallel plate heat exchanger (PPHE) and spiral plate heat exchangers (SPHE) on irreversibility is investigated. The particle swarm algorithm (PSA), a stochastic optimization tool is used for the minimization of irreversibilities within the cycle and optimization of the geometric parameters of the STBC components. The largest irreversibility loss on a component-basis is observed on the receiver. The rectangular absorber system for the receiver has the least irreversibility loss compared to other profiles studied, though, a higher internal to external irreversibility ratio was noticed. Improved exergy use via the dual regenerative system was observed on all models with reductions of 22% and 15% in irreversibility obtained from the receiver and recuperator respectively. In addition, the SPHE produced less irreversibilities compared to the PPHE system and this could be attributed to its large surface area available for heat transfer. An optimal second law efficiency of 62% and 74% on the PPHE and SPHE STBC systems, respectively is achieved at around a pressure ratio of 2.2. The dual serial-regenerative system without reheats and intercooling has the advantage of optimal energy available and efficient exergy use followed by the combined system.

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.

An application of conventional and advanced exergy approaches on a R41/R1233ZD(E) cascade refrigeration system under optimum conditions

Painstaking adjustment of an optimum low-temperature cycle (LTC) condenser temperature allows cascade refrigeration system (CRS) to operate at maximum performance. This study exhibits an original approach because, for the first time, advanced exergy analysis is implemented under an optimum LTC condenser temperature of CRS operating with R41/R1233zd(E) as an environmentally-friendly refrigerant pair. Under the auspices of advanced exergy analysis, there is endogenous exergy destruction of 50.43% and exogenous exergy destruction of 49.57% within total exergy destruction. It is pointed out that the interactions between the CRS components (external irreversibilities) are partly less than exergy destruction that occurs within components (internal irreversibilities). The avoidable part within total exergy destruction, which is greater than the unavoidable part, indicates that components have a high improvement potential with a value of 56.31%. Furthermore, LTC compressor depends significantly on other components, as it has the largest exogenous part of exergy destruction with 75.82%. The results indicate that the CRS’s exergy efficiency, which can be determined based on conventional exergy analysis, is only 36%. However, this increases to 68% with the improvements needed for the components.

Thermodynamic Performance of a Brayton Pumped Heat Energy Storage System: Influence of Internal and External Irreversibilities.

A model for a pumped thermal energy storage system is presented. It is based on a Brayton cycle working successively as a heat pump and a heat engine. All the main irreversibility sources expected in real plants are considered: external losses arising from the heat transfer between the working fluid and the thermal reservoirs, internal losses coming from pressure decays, and losses in the turbomachinery. Temperatures considered for the numerical analysis are adequate for solid thermal reservoirs, such as a packed bed. Special emphasis is paid to the combination of parameters and variables that lead to physically acceptable configurations. Maximum values of efficiencies, including round-trip efficiency, are obtained and analyzed, and optimal design intervals are provided. Round-trip efficiencies of around 0.4, or even larger, are predicted. The analysis indicates that the physical region, where the coupled system can operate, strongly depends on the irreversibility parameters. In this way, maximum values of power output, efficiency, round-trip efficiency, and pumped heat might lay outside the physical region. In that case, the upper values are considered. The sensitivity analysis of these maxima shows that changes in the expander/turbine and the efficiencies of the compressors affect the most with respect to a selected design point. In the case of the expander, these drops are mostly due to a decrease in the area of the physical operation region.

Multicriteria optimization of Brayton-like pumped thermal electricity storage with liquid media

A multi-objective and multi-parametric optimization of a Pumped Thermal Electricity Storage system based on Brayton cycles is presented by the calculation of different Pareto fronts and the associated Pareto optimal sets for energetic and design analysis, respectively. A large range of internal and external irreversibilities and the thermodynamic properties of the storage media are taken into account. The analysis shows that the heat capacity of the working fluid and the heat capacity of the storage media should be the same in the contact with the hot reservoirs and in the contact with the cold reservoir in the heat pump, but in the contact with the cold reservoir for the heat engine the ratio should be 0.33, this offers information regarding the mass flow increasing significantly the achievable values for the round-trip efficiency, power output and the heat engine efficiency in the discharge process. Optimal values are given in terms of the degree of irreversibilities in the system and a comparison is made with extreme cases of infinite and minimum sizes for the storage system. Round-trip efficiencies in the so-called optimum scale/mass-flow-ratio design point exhibits noticeably larger values compared to previously reported results including the so-called endoreversible limit, where no internal irreversibilities are considered and where the improvement can achieve 49% over the endoreversible case in the most ideal scenario. Explicit numerical values of the maximum round trip efficiency, power output, and efficiency are given for a broad range of both internal and external irreversibilities.

Exergetic optimisation of a four-temperature-level absorption heat pump based on finite-time thermodynamics

ABSTRACT A finite-time thermodynamics study and an exergetic optimisation for a four-temperature-level absorption heat pump have been performed. The maximum of the exergetic-performance coefficient (EPC) and the corresponding optimal ecological coefficient of performance (ECOP), ecological objective function (E), exergetic efficiency (ex), coefficient of performance (COP), specific entropy generation rate and heating load have been obtained. The effects of internal irreversibilities, external irreversibilities, heat leakages, ratio of the rejected heat of the absorber to the condenser and the heat-transfer coefficients of different components on the system performance have been analysed and discussed. It was noticed a greater harmful impact of heat losses in the generator-absorber assembly. Comparisons of EPC criterion with ECOP, COP, E and ex criteria have been provided. The results demonstrate that the four-temperature-level absorption heat pump working at maximum EPC conditions has a significant advantage in terms of entropy generation rate and coefficient of performance.

Open Dual Cycle with Composition Change and Limited Pressure for Prediction of Miller Engines Performance and Its Turbine Temperature

An improved thermodynamic open Dual cycle is proposed to simulate the working of internal combustion engines. It covers both spark ignition and Diesel types through a sequential heat release. This study proposes a procedure that includes (i) the composition change caused by internal combustion, (ii) the temperature excursions, (iii) the combustion efficiency, (iv) heat and pressure losses, and (v) the intake valve timing, following well-established methodologies. The result leads to simple analytical expressions, valid for portable models, optimization studies, engine transformations, and teaching. The proposed simplified model also provides the working gas properties and the amount of trapped mass in the cylinder resulting from the exhaust and intake processes. This allows us to yield explicit equations for cycle work and efficiency, as well as exhaust temperature for turbocharging. The model covers Atkinson and Miller cycles as particular cases and can include irreversibilities in compression, expansion, intake, and exhaust. Results are consistent with the real influence of the fuel-air ratio, overcoming limitations of standard air cycles without the complex calculation of fuel-air cycles. It includes Exhaust Gas Recirculation, EGR, external irreversibilities, and contemporary high-efficiency and low-polluting technologies. Correlations for heat ratio γ are given, including renewable fuels.

Optimization Modeling of Irreversible Carnot Engine from the Perspective of Combining Finite Speed and Finite Time Analysis.

An irreversible Carnot cycle engine operating as a closed system is modeled using the Direct Method and the First Law of Thermodynamics for processes with Finite Speed. Several models considering the effect on the engine performance of external and internal irreversibilities expressed as a function of the piston speed are presented. External irreversibilities are due to heat transfer at temperature gradient between the cycle and heat reservoirs, while internal ones are represented by pressure losses due to the finite speed of the piston and friction. Moreover, a method for optimizing the temperature of the cycle fluid with respect to the temperature of source and sink and the piston speed is provided. The optimization results predict distinct maximums for the thermal efficiency and power output, as well as different behavior of the entropy generation per cycle and per time. The results obtained in this optimization, which is based on piston speed, and the Curzon–Ahlborn optimization, which is based on time duration, are compared and are found to differ significantly. Correction have been proposed in order to include internal irreversibility in the externally irreversible Carnot cycle from Curzon–Ahlborn optimization, which would be equivalent to a unification attempt of the two optimization analyses.

Stirling Refrigerating Machine Modeling Using Schmidt and Finite Physical Dimensions Thermodynamic Models: A Comparison with Experiments

The purpose of the study is to show that two simple models that take into account only the irreversibility due to temperature difference in the heat exchangers and imperfect regeneration are able to indicate refrigerating machine behavior. In the present paper, the finite physical dimensions thermodynamics (FPDT) method and 0-D modeling using the Schmidt model with imperfect regeneration were applied in the study of a β type Stirling refrigeration machine.The 0-D modeling is improved by including the irreversibility caused by imperfect regeneration and the finite temperature difference between the gas and the heat exchangers wall. A flowchart of the Stirling refrigerator exergy balance is presented to show the internal and external irreversibilities. It is found that the irreversibility at the regenerator level is more important than that at the heat exchangers level. The energies exchanged by the working gas are expressed according to the practical parameters, necessary for the engineer during the entire project. The results of the two thermodynamic models are presented in comparison with the experimental results, which leads to validation of the proposed FPDT model for the functional and constructive parameters of the studied refrigerating machine.

The Exergy Cost Theory Revisited

This paper reviews the fundamentals of the Exergy Cost Theory, an energy cost accounting methodology to evaluate the physical costs of products of energy systems and their associated waste. Besides, a mathematical and computationally approach is presented, which will allow the practitioner to carry out studies on production systems regardless of their structural complexity. The exergy cost theory was proposed in 1986 by Valero et al. in their “General theory of exergy savings”. It has been recognized as a powerful tool in the analysis of energy systems and has been applied to the evaluation of energy saving alternatives, local optimisation, thermoeconomic diagnosis, or industrial symbiosis. The waste cost formation process is presented from a thermodynamic perspective rather than the economist’s approach. It is proposed to consider waste as external irreversibilities occurring in plant processes. A new concept, called irreversibility carrier, is introduced, which will allow the identification of the origin, transfer, partial recovery, and disposal of waste.