Ali Fellah

Cooling loads analysis of an endoreversible solar absorption refrigerator cycle

This paper deals with the analysis of the Cooling Loads (CL) of a solar absorption refrigerator. For this purpose, a new Process Conceptual Design (PCD) approach based on equivalent endoreversible model and hierarchical decomposition is developed. The influences of many cooling load aspects on the model performances are investigated. A sensitive study is made: performance coefficient (i.e. its inverse) vs. different cooling load aspects. New relationships between technical and economical criteria and optimal parameters stand out. An endoreversible new bound for the Inverse Specific Cooling Load (ISCL) is achieved. For every case, functional and practical domains are defined. Academic interest domains are also notified.

Conceptual and functional study of a solar absorption refrigeration cycle

The paper presents an optimised conceptual design approach. It deals with the analysis of the solar absorption refrigerators performances. Hierarchical decomposition and endoreversibility principles of technical and energetic systems are used. Optimisation is carried out according to the Lagrange multipliers method. The general optimal performances of the cycle are coupled into a derived set of equations. The functional and conceptual characteristics are studied and the effects of the external sources temperatures on both the performances and the functional and conceptual characteristics are highlighted. This approach could be useful all time when we have to handle with a great number of variables.

Hierarchical Decomposition Thermodynamic Approach for the Study of Solar Absorption Refrigerator Performance

A thermodynamic approach based on the hierarchical decomposition which is usually used in mechanical structure engineering is proposed. The methodology is applied to an absorption refrigeration cycle. Thus, a thermodynamic analysis of the performances on solar absorption refrigerators is presented. Under the hypothesis of an endoreversible model, the effects of the generator, the solar concentrator and the solar converter temperatures, on the coefficient of performance (COP), are presented and discussed. In fact, the coefficient of performance variations, according to the ratio of the heat transfer areas of the high temperature part (the thermal engine 2) Ah and the heat transfer areas of the low temperature part (the thermal receptor) Ar variations, are studied in this paper. For low values of the heat-transfer areas of the high temperature part and relatively important values of heat-transfer areas of the low temperature part as for example Ah equal to 30% of Ar, the coefficient of performance is relatively important (approximately equal to 65%). For an equal-area distribution corresponding to an area ratio Ah/Ar of 50%, the COP is approximately equal to 35%. The originality of this deduction is that it allows a conceptual study of the solar absorption cycle.

Chapter 2.11 – Exergetic and Energetic Performance Evaluation of a Flat Plate Solar Collector in Dynamic Behavior

Abstract This work aimed to evaluate the energy and the exergy performance of a flat plate solar collector under transient conditions. A theoretical model based on the first and second laws of thermodynamics was developed to predict the thermal behavior of the system. An exergy analysis was performed to determine the location, type, and magnitude of exergy destroyed and losses over the collector. The mathematical model obtained was solved numerically using the MATLAB computational program and Stat Ease Expert Design software. A detailed parametric study was conducted to assess the effect of various parameters. The results showed that the values for exergy efficiency were low compared with energetic ones. Furthermore, the main cause of exergy destruction was the absorption of radiation by the absorber plate. The mass flow rate, inlet water temperature, and absorber emissivity are critical parameters for a solar collector. However, the number of tubes and the diameter of the pipes have negligible effects on the collector’s performance. Thus, more accurate results and useful applications of the exergy method in transient behaviors were obtained to design solar collectors.

Thermodynamic Analysis of the Irreversibilities in Solar Absorption Refrigerators

A thermodynamic analysis of the irreversibility on solar absorption refrigerators is presented. Under the hierarchical decomposition and the hypothesis of an endoreversible model, many functional and practical domains are defined. The effect of external heat source temperature on the entropy rate and on the inverse specific cooling load (ISCL) multiplied by the total area of the refrigerator A/Qe are studied. This may help a constructor to well dimension the solar machine under an optimal technico-economical criterion A/Qe and with reasonable irreversibility on the refrigerator. The solar concentrator temperature effect on the total exchanged area, on the technico-economical ratio A/Qe, and on the internal entropy rate are illustrated and discussed. The originality of these results is that they allow a conceptual study of a solar absorption refrigeration cycle.

Simulation of the absorption phase of an intermittent absorption solar refrigeration system

The use of solar energy in sunny countries in rural areas is an effective way to overcome the lack and the difficulty to supply into electrical energy. It is therefore important and necessary to exploit this natural and clean resource. Among the methods of thermal conversion of solar energy, solar refrigeration is the most suitable application for the storage of foodstuffs and pharmaceutical products. This refrigeration may be ensured through compression and absorption cycles in continuous or in intermittent operation. It is up to solar absorption cycles operating intermittently that we are interested. This cycle consists of two phases: generation and absorption phase. The phase of generation was modeled for the installation which was designed and realized in the research unit. The modeling of the absorption phase of a refrigeration cycle operating intermittently, is the subject of this study, it was determined the mass and energy balances for each element. The resolution of the system of equations obtained is based on computing and mathematical tool. Exploitation of the results of the simulation has been performed in a first time without considering thermal losses and then introducing these losses, in different compartments of the installation, in a second step. The interpretation of different gaits aims to describe quantitatively the evolution of the characteristic variables of functional components for the absorption phase in particular temperatures, flow rates and powers exchanged.

A dynamic model for study and optimization of an irreversible solar refrigerator

A general dynamic model for the study and optimization of reversed cycle thermal machines with three heat reservoirs is presented. The model is based on the First and Second Laws of Thermodynamics, heat transfer equations at finite thermal source and sink capacities, and entropy generation terms in order to consider the internal and external irreversibilities of the cycle. The proposed model is applied to an irreversible absorption machines for which several constraints are imposed. Some results generated by the model when applied to refrigeration machines are presented. They point out interesting results regarding the limits of the variation range for the model variables under different operating conditions and also for different variation laws for the internal entropy generation term. As a result, the model is expected to be a useful tool for simulation, design, and optimization of solar collector based energy systems.

Performance evaluation of a solar absorption refrigerator with dynamic simulation

This contribution deals with the theoretical study in dynamic mode of an absorption refrigerator endoreversible model. The system is a cold generating station driven by solar energy. The main elements of the cycle are a refrigerated space, an absorption refrigerator and a solar collector form. A simplified mathematical model, which combines fundamental and empirical correlations, and principles of classical thermodynamics and heat transfer, is developed. The proposed endoreversible model is then utilized to simulate numerically the system transient and steady state response under different operating and design conditions. In order to obtain a fixed temperature in the cold room in a minimal lapse of time, the determination of the command necessary heat flow is based on the energy balances. Compared to the collector efficiency, the collector temperature presents major influence on the conceptual and functional characteristics.

On the Thermal Transformer Performances

Different approaches are considered to select optimum criteria for technical process analysis. The maximization of the efficiency and the minimization of the total cost enclosing capital and running costs are the main purposes e.g. Munoz and Von Spakovsky (2003). Physical and thermodynamic criteria and technical and economic considerations have to be joined while analyzing energetic conversion processes (Berlitz et al. 1999; Chen 1995). Thus, deducing economic findings is the common objective of all intentions. Furthermore, the use of the interdisciplinary modeling methods has recently constituted the most important orientation of the technical system studies and process analyses. The simplification of both mathematical description and hypothesis definition of the interaction effects due to internal irreversibilities lead to the development of interesting simple but universal models. Internal irreversibilities due to heat transfer, throttling, mixing and internal dissipation of the working fluid, which are responsible for entropy generation are always present in a real heat driven refrigerator (Chen et Schouten, 1998). However, many works do not satisfy all the futures because the distribution of heat transfer properties between the components is taken as inputs and no as a result to be deduced from the optimization procedure. In the other hand, the interaction effects in the internal processes do not favor separated studies. Therefore, it will be necessary to consider discreet parts of the whole system as they were independent. Then, the characteristics will be treated according to mathematical and physical couplings. To perform these approaches, the theory of finite time thermodynamic is mainly used. Seeing that the heat transfer processes are defined according to temperature finite difference method and the inner and outer reversibilities should be taken into consideration (Fellah et al., 2010). According to the study’s finality, the decomposition of an overall system into subsystems may constitute a helpful tool, for which the physical and mathematical couplings would permit the efficient application of the investigation methods than a whole problem with a unique task. This could reduce the size of the mathematical problem. In fact, many attempts have been made to reduce the size of the problem, using the decomposition method on stage or/and on block models (Berlitz et al. 1999; Feidt and Lang 2002; Chen 1995; Chen and Wu 1996; Fellah, 2008 and Fellah et al., 2010). The results obtained for various thermodynamic cycle analyses using FTT are closer to real device performance than those obtained using classical thermodynamics. During the last two decades, many optimization studies for refrigerators based on endoreversible and irreversible models have been performed by considering various objective functions. Wijeysundera, 1997;