Internal Irreversibility Research Articles

A novel hybrid system consisting of a dye-sensitized solar cell and an absorption refrigerator for power and cooling cogeneration

To improve the energy conversion efficiency of a dye-sensitized solar cell (DSSC), a novel hybrid system representatively consisting of a DSSC and an absorption refrigerator (APR) is put forward, in which the APR is used to thermally harness the part of high wavelength sunlight that not converted by the DSSC. The amount of heat from the DSSC is calculated, and mathematical formula for energy conversion efficiencies and output powers of the DSSC, APR and hybrid system are derived. Numerical calculations show that the maximum output power density and maximum energy conversion efficiency of hybrid system are enhanced by 9.93% and 9.92% in comparison with that of the single DSSC, respectively. Integration of APR to DSSC is a feasible route to effectively improve the energy conversion efficiency. Comprehensive sensitivity analyses are conducted to explore how the whole hybrid system performance depends on various working conditions and designing parameters, including porous nano-TiO2 film thickness, photoelectron absorption coefficient, internal irreversibility factor, heat transfer coefficient and various temperatures. The research results can provide some new understandings for further enhancing the DSSC performance.

THERMODYNAMIC OPTIMIZATION OF AN IRREVERSIBLE REGENERATED BRAYTON HEAT ENGINE USING MODIFIED ECOLOGICAL CRITERIA

The modified configuration of regenerated Brayton heat engine along with supplementary addition of heat in its irreversible mode is thermodynamically investigated and optimized. The definite temperature differential between system/reservoir is the source of external irreversibility and the losses because of rubbing/friction in turbine/compressor, regeneration heat losses and losses due to pressure drop are the internal irreversibilities considered in this analysis. The difference of output power and the exergy destruction rate, termed as ecological function, is thermodynamically optimized. It is found that regenerative effectiveness plays a vital role in obtaining maximum possible ecological function whereas output power and 1st law efficiency predominantly depends on the cold side effectiveness in the system. It is also observed that the thermodynamic performance of proposed system/device is prominently depends on the efficiency of the turbine and consequently less dependent on compressor efficiency. The major outcome of this analysis is that with the inclusion of additional thermal heats at constant temperature conditions, various performance parameters i.e., output power (about 13%) and 1st law efficiency (about 9%) of the model get improved significantly in comparison with the conventional gas power plant. Moreover, the model investigated in this study yields lesser output power, first law efficiency and ecological function and exactly follows the results/outcomes presented in the available literature at α1=α2=1, which are the pressure recovery coefficients at two ends.

Exergetic, ecological and thermo-economic (3E) optimization of an absorption heat pump with heat resistance, heat leakage and two internal irreversibilities: Comparison

The multi-objective optimization of a three-heat-reservoir absorption heat pump (THR AHP) is used mainly considering three objective functions: exergetic performance criterion (EPC), exergy-based ecological criterion (E) and thermo-economic criterion (F). Indeed, the exergy, ecological and thermo-economic criteria are highlighted analytically. This is done by using the law of convective heat exchanges between the working fluid and the different components and finite time thermodynamics analysis. This is to determine the optimum operating points of the system by minimizing exergy destruction rate, the environmental impact and the costs of capital and energy consumed. Also, the parameters taken into account influencing the performance of the absorption heat pump are the heat resistances, the heat leakages, the thermo-economic parameter and the two internal irreversibility factors in particular that between the evaporator-condenser assembly. However, the Flowchart optimization methodology made it possible to deduce that the system presents a significant advantage at the maxima of the F and the EPC criteria in terms of coefficient of performance. Nevertheless, the THR AHP presents a significant advantage at the maximum of the EPC in terms of exergy destruction rate and at the maximum of the E in terms of specific heating load and exergy output rate.

Progress in Carnot and Chambadal Modeling of Thermomechanical Engine by Considering Entropy Production and Heat Transfer Entropy

Nowadays the importance of thermomechanical engines in recognized worldwide. Since the industrial revolution, physicists and engineers have sought to maximize the efficiency of these machines, but also the mechanical energy or the power output of the engine, as we have recently found. The optimization procedure applied in many works in the literature focuses on considering new objective functions including economic and environmental criteria (i.e., ECOP ecological coefficient of performance). The debate here is oriented more towards fundamental aspects. It is known that the maximum of the power output is not obtained under the same conditions as the maximum of efficiency. This is shown, among other things, by the so-called nice radical that accounts for efficiency at maximum power, most often for the endoreversible configuration. We propose here to enrich the model and the debate by emphasizing the fundamental role of the heat transfer entropy together with the production of entropy, accounting for the external or internal irreversibilities of the converter. This original modeling to our knowledge, leads to new and more general results that are reported here. The main consequences of the approach are emphasized, and new limits of the efficiency at maximum energy or power output are obtained.

Comparative performance evaluations of various optimization functions for irreversible Otto cycles

A comparative performance analysis for irreversible Otto cycles has been carried out with consideration of overall and effective power loss for various optimization functions available in the literature. Basically, maximum power, maximum power density, ecological coefficient of performance and the ecological function are taken into consideration, and their optimal operating conditions were compared with respect to each other in terms of various engine parameters. The irreversible cycle model operating between two isothermal high and low temperature heat reservoirs is considered and combined with various irreversibilities for power loss analysis. In the irreversible model, non-isentropic compression and expansion, heat leak through the cylinder head and power loss due to friction are taken into account. Effects of engine volumetric efficiency, engine speed, friction and extreme cycle temperature on the overall performance for the respective optimal operating conditions have been investigated. It is seen that the overall power loss increases for increasing engine speed, decreasing volumetric efficiency, increasing friction coefficient and decreasing high temperature of the cycle. It is also observed that the maximum power conditions have higher overall power loss but lower internal irreversibility losses among the considered optimization functions.

Multiobjective optimisation of absorption heat pump performance with double internal irreversibility based algorithm NSGAII: case of three-heat-source model

The optimisation of an irreversible absorption heat pump based on the new thermo-ecological criterion is presented. We mathematically modelled the ecological coefficient of performance (ECOP), and the entropy generation rate (S) of an absorption heat pump with three heat sources and double internal irreversibility. Internal irreversibility for generator – absorber assembly and the one for condenser – evaporator assembly were considered. S and ECOP were assumed as objective functions in the multi-objective optimisation process. The Multi-Objective Optimisation Algorithm (MOEA) enhanced by the NSGAII algorithm was used to optimise the two functions. In order to obtain a better operation system, the ECOP was maximised and S was minimised thanks to the Pareto front established using the NSGAII algorithm. From the variations of ECOP and S according to two irreversibility parameters, it was found that those irreversibilities have a strong influence on the ECOP (lowering) and S (increasing).

Efficiency optimisation of advanced gas turbine recuperative-cycles

The paper presents a theoretical analysis of three advanced gas turbine recuperative-cycles, that is, the intercooled, the reheat and the intercooled and reheat cycles. The internal irreversibilities, which characterise the compression and expansion processes, are taken into account through the polytropic efficiencies of the compressors and turbines. As customary in simplified analytical approaches, the study is carried out for an uncooled closed-circuit gas turbine without pressure losses in the heat exchangers and using a calorically perfect gas as working fluid. Although the accurate performance prediction of a real-gas turbine is prevented by these simplifying assumptions, this analysis provides a fast and simple approach which can be used to theoretically explain the main features of the three advanced cycles and to compare them highlighting pros and contra. The effect of the heat recuperation is investigated comparing the thermal efficiency of a given cycle type with those of two reference cycles, namely, the non-recuperative version of the analysed cycle and the simple cycle. As a result, the ranges of the intermediate pressure ratios returning a benefit in the thermal efficiency in comparison with the two reference cycles have been obtained for the first time. Finally, for the sole intercooled and reheat recuperative-cycle, a novel analytical expression for the intermediate pressure ratios yielding the maximum thermal efficiency is also given.

Thermoelectric generator at optimal power with external and internal irreversibilities

The exact power optimization of a thermoelectric generator is performed under the simultaneous presence of internal and external irreversibilities—modeled as nonideal thermal contacts and Joule heating, respectively. We consider a modification of the constant properties model whereby thermal conductivity of the thermoelectric material has spatial dependence, giving an asymmetric fraction of the Joule heat dumped in each reservoir. In particular, with a linear form of this dependence, the heat leakage term drops out as well as Joule heat is dumped only on one side. Exact expressions for efficiency at maximum power in each of these regimes are derived and are compared at thermal impedance matching and close to equilibrium, where interesting, stepwise changes in efficiency at maximum power are observed. The present analysis leads to the inference that higher values of efficiency at maximum power are obtained when both internal and external irreversibilities are taken on the hot side.

Comparative performance analysis of various optimization functions for an irreversible Brayton cycle applicable to turbojet engines

A comparative performance analysis for various optimization criterion functions has been carried out for an irreversible Brayton cycle applicable to turbojet engines; the cycle includes the engine core and nozzle and diffuser. Newly defined parameters are introduced as power loss parameter (PLOS), effective power loss parameter (EPLOS) and Carnot-Brayton shape factor (CBSF) for a better assessment of the performance and power losses throughout the operation of the engine cycle. In addition, optimization functions, such as maximum power (MP), maximum power density (MPD), ecological coefficient of performance (ECOP) and ecological function (ECOL) are considered and their optimal operation conditions are compared with respect to each other. Some important evaluations are made for the optimal operation conditions with respect to the ideal Carnot cycle case and also with respect to the ideal Brayton cycle case. Power losses due to internal irreversibilities, as a result of the second law of thermodynamics and due to shape of the cycle model are classified and evaluated. In addition, this paper introduces an assessment of the performance of the turbojet engines in terms of a mass flow rate, gas generator speed and shaft force models as well as considerations for variations of heat leakage. Various parameters affecting the performance analysis are also considered such as compressor pressure ratio θc, compressor and turbine efficiencies (ηc and ηt respectively), altitude and flight Mach number M∞. The comparisons show that the maximum power conditions have the lowest EPLOS but the highest PLOS values with respect to the other optimal function criterions, indicating the effect of the internal irreversibilities at the lowest but exergy destruction for the environment at the highest.

Comparison and sensitivity analysis of the efficiency enhancements of coal-fired power plants integrated with supercritical CO2 Brayton cycle and steam Rankine cycle

The supercritical CO2 (SCO2) power cycle, which is highly efficient and cost effective and features a compact structure, is expected to replace steam Rankine cycle and bring technological revolution to coal-fired power plants. To obtain the quantitative energy saving potentials of the SCO2 power cycle, this study compared the thermodynamic performances of coal-fired power plants with SCO2 cycle and 10 in-service coal-fired power plants with steam as the working fluid. The efficiencies of 538 °C, 566 °C, and 600 °C typical power plants were increased by the SCO2 cycle by 2.52%, 2.39%, and 2.84%, respectively. Exergetic analysis revealed that the decrease in heat transfer irreversibility in the boilers mainly caused the efficiency enhancement. Then, the main devices’ performance parameters, including the isentropic efficiency of the compressor, isentropic efficiency of the turbine, pressure drops of the heat exchangers, regenerator terminal temperature difference, and leakage ratio, were analyzed in terms of their sensitivity to the efficiency enhancement of coal-fired power plants integrated with the SCO2 cycle. The influence of these parameters on the internal irreversibility of coal-fired power plants was also studied. With a 600 °C power plant as an example, the energy saving limits of the main devices’ parameters of SCO2 cycle in comparison with the steam Rankine cycle are as follows: the compressor isentropic efficiency exceeded 75%, the turbine isentropic efficiency exceeded 86%, the pressure drop of the heat exchanger was less than 0.35 MPa, the temperature difference of the regenerator terminal was less than 21 °C, the leakage ratio was less than 2.5% when the leakage was not recovered, and the leakage ratio was less than 16% when the leakage was recovered.

Thermo-ecological analysis and optimization of a three-heat-reservoir absorption heat pump with two internal irreversibilities and external irreversibility

The investigations carried out in this work concern the optimization and the thermodynamic analysis of an absorption heat pump with multi-irreversibilities and provided with three-heat-reservoir mainly using the thermo-ecological criterion as objective function. The thermodynamic analysis of the first and second laws of thermodynamics and the linear law of heat-transfer allowed us to evaluate and optimize the objective functions of performance such as: the specific heating load, the coefficient of performance, the specific entropy generation rate, the ecological coefficient of performance and the heat-transfer areas of the various components. These allowed us to study the variations of these criteria according to the design parameters and the temperatures of the working fluid in the components. From this study, it follows that the optimal system implies a maximum thermo-ecological coefficient, a minimum entropy production rate corresponding to a certain value of the specific heating load, while maintaining a system which limits the losses between the condenser and the evaporator. Moreover, the consideration of the two internal irreversibilities, the external irreversibility and the losses of the heat resistances is very close to a real absorption heat pump and allows a more precise and objective study rather than in the optimization and the design of an absorption system like an endoreversible system.

Work and efficiency optimization of advanced gas turbine cycles

The paper presents a theoretical analysis of advanced gas-turbine cycles. Specifically, three cycles are investigated, that is the intercooled, the reheat, and the intercooled and reheat cycles. The internal irreversibilities, which characterise the compression and expansion processes, are taken into account through the polytropic efficiencies of the compressors and turbines.New analytical formulations for the overall and intermediate pressure-ratios which maximise the net work of the three aforementioned cycles are proposed along with an order relation between these optimum pressure-ratios. Moreover, the thermal efficiency of these cycles is also analysed providing, among other findings, the ranges of the intermediate pressure-ratios returning a benefit in the thermal efficiency in comparison with the simple cycle. Finally, for the sole intercooled and reheat cycle, a novel analytical expression for the maximum point of the thermal efficiency is given. It is also shown that, for the intercooled and reheat cycle, there is a unique value of the overall pressure-ratio which simultaneously maximises the net work and the thermal efficiency.To give some quantitative information, consider a maximum to minimum cycle-temperature ratio equal to 1573/300 and a compressor (resp. turbine) polytropic-efficiency equal to 0.8 (resp. 0.88). The net work and the thermal efficiency are maximised by a set of overall pressure-ratios obeying an order relation. The simple, the reheat, the intercooled, and the intercooled and reheat cycles reach the maximum net-work (resp. thermal efficiency) for increasing values of the overall pressure-ratio, that is 8.550 (resp. 18.260), 14.950 (resp. 25.039), 20.846 (resp. 39.984), and 73.109 (resp. 73.109). The reheat cycle achieves a 34.899% (resp. 6.077%) gain in the net work (resp. thermal efficiency), while the intercooled cycle returns a 31.477% (resp. 15.970%) increment. The maximum net-work of the intercooled and reheat cycle exactly doubles that of the simple cycle. Finally, the maximum thermal efficiency of the intercooled and reheat cycle yields a 24.966% improvement in comparison with the simple one.

Thermodynamic analysis of endoreversible six-stroke Otto cycle with respect to equivalence ratio, residual gas fraction and mean piston speed

The analysis of the six-stroke Otto cycle was performed using the finite-time thermodynamics dependence on mean piston speed, equivalence ratio and residual gas fraction. In the analysis, friction loss, internal irreversibilities, heat transfer losses were taken into account by empirical correlations. Relations between power output, thermal efficiency, and compression ratio were obtained by detailed numerical examples. The results were presented as comparative for four- and six-stroke Otto cycle. It was assumed that the cycles operate with the same mixture flow per second so that both performances of cycles can be compared. The study showed that more fuel flow can be supplied to the six-stroke Otto cycle for the same mixture flow. However, there was a slight increase in the maximum cycle temperature. For this reason, the results are crucial in providing a good guideline for evaluating and improving the performance of real six-stroke Otto engines.

A novel solar assisted vacuum thermionic generator-absorption refrigerator cogeneration system producing electricity and cooling

A novel solar assisted cogeneration system composed of a parabolic dish collector (PDC), a vacuum thermionic generator (VTIG), and an absorption refrigerator (APR) is proposed, in which the APR harvests the rejected heat from the VTIG for further cooling production. The equivalent power output and efficiency of the proposed system are obtained by considering various irreversible losses including radiation and convection heat losses of the PDC, radiation heat losses between the cathode and anode of the VTIG, heat transfer irreversibility, irreversible effects inside the working fluid of the APR. The maximum attainable efficiency increases from 17.21% to 27.06% as the bottoming APR is hybridized into the system. Meanwhile, the corresponding power density gains a 13.47% increase. Comprehensive sensitivity analyses are conducted to examine the dependence of the proposed system performance on some decisive design parameters and operating conditions such as absorber temperature of the solar collector, solar concentration ratio, solar irradiance, work function and anodic temperature of the VTIG, heat transfer area, heat transfer coefficient, and internal irreversibility factor of APR. The obtained results may provide some new insights into development and operation of such a solar powered cogeneration system.

Entropy generation minimization of thermoelectric systems applied for electronic cooling: Parametric investigations and operation optimization

Two categories of thermoelectric systems, i.e. thermoelectric active cooling (TAC) and thermoelectric self-cooling (TSC), were broadly applied for electronic cooling. Energy efficiency maximization and entropy generation minimization will be simultaneously achieved for both thermoelectric systems. Thermodynamic models of TAC and TSC systems are established to clarify the effects of main influencing parameters (i.e., applied current, thermal conductance in hot and cold ends) on the energy and entropy performances. Subsequently, under the situation of electronic cooling with constant material properties, a comprehensive performance comparison between TAC and TSC has been conducted, including decision targets, irreversible characteristics and environmental impacts. Furthermore, in order to improve the heat dissipation potential, the effects of material properties on the system performance are also sensitively identified for both TAC and TSC systems. For both systems, internal irreversibility is more pronounced than external one. Numerical results demonstrated that system efficiency of TAC could be higher than that of TSC; the operating mode for TAC could be completely switched to that for TSC when the input power was turned into zero. Present investigation could give suitable guidelines for thermoelectric systems applied for electronic cooling.

Performance Analysis of a Hybrid System Consisting of a Molten Carbonate Direct Carbon Fuel Cell and an Absorption Refrigerator

By integrating an Absorption Refrigerator (AR), a new hybrid system model is established to reuse the waste heat from a Molten Carbonate Direct Carbon Fuel Cell (MCDCFC) for additional cooling production. Various irreversible losses in each element of the system are numerically described. The operating current density span of the MCDCFC that allows the AR to work is derived. Under different operating conditions, the mathematical expressions for equivalently evaluating the hybrid system performance are derived. In comparison with the stand-alone MCDCFC, the maximum attainable power density of the proposed system and its corresponding efficiency are increased by 5.8% and 6.8%, respectively. The generic performance features and optimum operating regions of the proposed system are demonstrated. A number of sensitivity analyses are performed to study the dependences of the proposed system performance on some physical parameters and operating conditions such as operating temperature, operating current density, and pressure of the MCDCFC, cyclic working fluid internal irreversibility inside the AR, thermodynamic losses related parameters and the anode thickness of the MCDCFC. The obtained results may offer some new insights into the performance improvement of an MCDCFC through a reasonable heat management methodology.

Enhanced exergy analysis of a bubble-pump-driven LiCl-H<SUB align="right">2O absorption air-conditioning system

A thermally-driven bubble pump can replace the electrical pump in the absorption refrigeration cycle for lifting solution from the absorber to the generator and producing the required refrigerant vapour for the cooling effect. However, the lower efficiency of bubble-pump-driven absorption refrigeration cycle hinders its widespread application. The potential of a bubble-pump-driven LiCl-H2O absorption refrigeration system that can be powered by solar thermal energy or waste heat energy is discussed and analysed in this study. The new concept of enhanced exergy analysis is integrated in the thermodynamic analyses, which quantifies the available exergy destruction of each component for overall system performance improvement. The analyses uncovered that 80% of the total exergy loss is due to each component's own internal irreversibilities, whereas the remaining is through the interaction of the irreversibilities of other components in the system. The analyses revealed that though the highest exergy losses (46%) are in the absorber, priority for improvement should be given to the generator. Furthermore, the exergy losses of the condenser, the evaporator, and the solution heat exchanger are mostly unavoidable and can be reduced by improving the other components of the system.

Experimental thermodynamic first and second law analysis of a variable output 1–4.5 kWe, ICE-driven, natural-gas fueled micro-CHP generator

This work experimentally assesses the thermodynamic performance of the only single kWe, variable-output, continuous duty, internal combustion engine driven, residential micro-combined heat and power (mCHP) generator (Marathon Engine Systems ecopower) available in the United States. The system underwent steady state testing over its full operating range, and first (energy) and second (exergy) law analyses were conducted on the collected data. At rated speed, first law results reveal the ecopower operated at an electrical efficiency of 24.4 ± 0.7% and a utilization factor of 94.5 ± 12.6%. At the same speed, the ecopower’s second law electrical efficiency was 24.3 ± 0.8% and total second law efficiency (including exergy in both the recovered heat and electrical streams) was 33.7 ± 1.9%. This total second law efficiency was higher than that of common residential heating devices, including electric and gas furnaces and boilers, electric air-source heat pumps, and gas-engine driven heat pumps. Further, the ecopower outperformed most of these devices at all part-load conditions as well, indicating it would be an appropriate choice for a dispatchable generator to provide ancillary grid-support services in a future with increased variable renewable generator penetration. System-level and internal combustion engine irreversibilities are also presented to identify areas of inefficiency. The most prominent irreversibilities were (in decreasing magnitude) irreversible heat transfer, combustion irreversibility, frictional and pumping losses, followed by generator and power electronic losses.

Exergy-based ecological optimization for a four-temperature-level absorption heat pump with heat resistance, heat leakage and internal irreversibility

Based on irreversible four-temperature-level (FTL) cycle models, an ecological criterion function of absorption heat pump (AHP) plants is proposed. This function can attain a compromise in inter restricted relations between exergy output and exergy loss of AHP plants. Relations among ecological function, coefficient of performance (COP), heating load, exergy output and exergy loss, and various loss factors are derived. Simplified relations are derived when various irreversibility factors can be ignored. The exergy-based ecological characteristics are analyzed through numerical examples. The results can provide some new suggestions about parameter selection for practical AHP plants.

Performance optimization of single and two-stage micro/nano-scaled heat pumps with internal and external irreversibilities

Based on the thermosize effects of a confined ideal Maxwellian gas, we established a model of micro/nano-scaled heat pump including two isothermal and two isobaric processes, where the Joule-like heating effect was analyzed in detail by using the nonequilibrium thermodynamic theory. Further, in order to improve the performance of a single thermosize device, a model of two-stage micro/nano-scaled heat pump with multiple micro/nano components is for the first time put forward. The general expressions of heating load and coefficient of performance for the single and the two-stage heat pump are derived. The influences of the internal and external irreversibilities of the system, i.e., Fourier heat flow and the Joule-like heating and the finite rate heat transfer with the reservoirs, on the performance characteristic of thermosize devices are studied in detail. Moreover, we also discuss the effect of the allocation the thermosize components on the optimal performance of the two-stage micro/nano-scaled heat pump cycle. It is found that in the presence of the internal and external irreversibilities, the two kinds of the micro/nano-scaled heat pump can achieve specific heating performance. In particular, when the total number M of the thermosize components is fixed, both the maximum of the optimal coefficient of performance Φopt,max and the maximum of the heating load Πopt,max corresponding to the optimal coefficient of performance appear near n=M/2 with n being the number of the thermosize elements of the top stage. The results obtained here show several main irreversibility source of the micro/nano-scaled heat pump and have potential applications in the efficient utilization and conversion of energy in the microchannel system using gas.