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This paper in refrigeration analyses the effects of internal and external irreversibilities on the steady-state of a double irreversible absorption refrigeration system powered by a waste wood boiler. At the optimal steady-state of the maximum coefficient of performance, expressions of relaxation times of a double irreversible absorption refrigerator are derived. The influence of internal irreversibility parameters, heat leakage coefficient and heat transfer coefficients on relaxation times are discussed. Phase portraits for the trajectories are presented. The results of our analysis show that the eigen-values of the generator-absorber assembly are real with opposite signs while those of the evaporator-condenser are real and both negative. From these results, it follows that internal and external irreversibilities parameters have a draconian effect on the steady-state of the generator-absorber assembly which leads to the disturbance of the absorption system and consequently to its instability. The result can provide some theoretical guidelines for the designs of real absorption refrigerators.

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.

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.

This paper compares the local stability of an irreversible compression refrigerator working in the maximum ecological function (E) and in the maximum ecological coefficient of performance (ECOP). The goal was to analyze the dynamic of the steady-state of the refrigerator when it undergoes a small perturbation. By using the linearization technique and the stability analysis, the expressions of the relaxation times of the irreversible refrigerator are derived at two optimal steady-states of the maximum E and ECOP functions. It is found that the relaxation times are function of the heat capacity, overall heat transfer surface area, heat reservoirs temperature ratio, degree of internal irreversibility, heat transfer coefficients, and heat leakage rate. The influences of heat leakage rate, internal irreversibilities and heat resistances on the relaxation times are discussed. The phase portraits for the trajectories are presented. Finally, a comparative study was done on these two thermo-ecological functions and it has been found that the local stability of the optimal steady-state of the maximum ECOP function is better than that of the maximum E function.

Finite-time thermodynamics optimization analysis based on the coefficient of performance and the ecological coefficient of performance criteria has been carried out. This was done analytically and numerically for a double-effect parallel flow absorption refrigerator with losses of heat resistance, heat leakage and internal irreversibility. The maximum of the coefficient of performance and the corresponding optimal conditions have been derived analytically. The optimum performance parameters, which maximize the coefficient of performance objective function have been investigated. The effects of irreversibility parameters on the general and optimal performances on the basis of COP and ECOP functions have been discussed. The results obtained may provide the basis for designing real double-effect parallel flow absorption refrigerators.

A general cycle model including several major irreversibilities existing usually in real absorption refrigeration systems is established and used to investigate the new thermo-ecological optimal performance of a four-heat-source refrigerator affected by the irreversibilities of finite-rate heat transfer between the working fluid and the external heat reservoirs, heat leak losses of the cooled space, and internal dissipation of the working fluid. The ecological coefficient of performance (ECOP) of the cycle system is used as objective functions for optimization. The ECOP objective function is defined as the ratio of the cooling load to the loss rate of availability (or entropy generation rate). The maximum of the new thermo-ecological performance criterion and the corresponding optimal conditions have been derived analytically. The effects of irreversibility parameters on the general and optimal performances are discussed detailed. The practical operating region of the four-heat-source refrigerator is discussed. The obtained results may provide a general theoretical tool for the ecological design of four-heat-source absorption refrigerators.

Irreversibility is a phenomenon that affects much the performance of absorption refrigeration systems. Internal irreversibility is produced by the dissipation of the working fluid. This paper studies and analyzes the effect of two internal irreversibilities parameters (the internal irreversibility parameter for generator–absorber assembly and the internal irreversibility parameter for condenser–evaporator assembly) on the optimal ecological performances of an irreversible three-heat-source absorption refrigerator based on the ECOP criterion. The results obtained show that the internal irreversibility parameter for condenser–evaporator assembly affects more the ecological performance of the system than the internal irreversibility parameter for generator–absorber assembly. This is of importance to the optimal design and performance improvement of absorption refrigeration cycles.

The new thermo-ecological performance optimization of an absorption heat pump operating between three temperature levels with the losses of heat resistance, internal irreversibility and leakage is analyzed by taking the ecological coefficient of performance (ECOP) as an objective function. The new thermo-ecological criterion takes into account the first and second law of thermodynamics and is defined as the heating rate per unit loss rate of availability. The ecological coefficient of performance has been expressed and maximized in terms of the temperatures of the working fluid in the main components of the system. The corresponding optimal temperatures and other optimum performance parameters have been derived analytically, and the effects of the internal irreversibility, the heat leakage coefficient and the source temperature ratio on the global and optimal performances are discussed. The obtained results may provide a general theoretical tool for the ecological design of absorption heat pumps.

This paper presents an optimization study of an irreversible refrigeration absorption system based on a new thermo-ecological criterion. The considered objective function is the ecological coefficient of performance (ECOP). It takes into account the first and second law of thermodynamics and is defined as the cooling load per unit loss rate of availability. The ecological coefficient of performance has been expressed and maximized according to the temperatures of the working fluid in the main components of the system. The corresponding optimal temperatures and other optimum performance parameters have been derived analytically, and the effects of the internal irreversibility, the heat leakage coefficient and the source temperature ratio on the global and optimal performances are discussed. The results show that the maximum coefficient of performance (COP) and ecological coefficient of performance (ECOP) occurs for the same operating conditions.