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Most of the absorbent-refrigerant pairings are evaluated considering single-effect vapour absorption refrigeration (VAR) cycle. The coefficient of performance (COP) based modelling of VAR enables its quick performance evaluation. The development of efficient and environmentally benign fluid pairing is required to replace conventional refrigerants. Operating parameters-based COP models are scarce for single-effect VAR systems, and fewer correlations are available for the COP estimation. The paper compares five performance-based models reported in the literature for operating absorption cooling systems with various refrigerant and absorbent pairing. The comprehensive mean absolute percentage error (MAPE) analysis was performed for five reported correlations for more than 1601 data points of different fluid pairings. Results revealed that RMSD and MAPE values seem significantly higher for the reported correlations apart from the recently developed COP correlation accounted for fluid parameters in prediction. This indicates that the earlier reported correlations only accounted for the specific fluid pairing and could not incorporate different fluid pairing, which has been considered a recently reported correlation that resulted in the significantly improved prediction ability for COP. The finding from this study highlighted that the newly registered COP prediction correlation could be beneficial for developing new single-effect VAR cycles as it accounted for both the operating parameters and fluid parameters.

This paper investigates the dynamic behavior of dual vapor compression Refrigeration (VCR) chillers, considering nine ecofriendly refrigerants such as R134a, R32, R1234yf, R1234ze(E), R717, R600, R...

District Cooling Plants (DCP) is the sustainable way of energy consumption, refrigeration, and supply of chilled water to end-users via chillers and distribution networks. Thermodynamic assessment of the vapor absorption refrigeration (VAR) in the open literature has been performed for cooling capacity well below the actual requirement (less than 300 kW) of DCP. Therefore, this work aims to investigate energy and exergy analysis of large-scale single effect VAR cycles operated on NH3-water and water-LiBr systems for a cooling capacity of 4000 kW and assess several operating parameters’ influence on cycle performance. Different parameters such as evaporator operating pressure (water-LiBR = 0.010–0.020 atm, NH3-water = 5.39–6.20 atm), chilled water outlet temperature (288–297 K), absorber temperature (300–305 K), condenser temperature (310–315 K), cooling output (2900–4000 kW) and heat input (4300–5500 kW) to the generator are varied during the parametric analysis. The exergy destruction analysis of the cycle’s components is also included in the study. Overall, the COP (0.89) and Exergy efficiency (89%) of water-LiBr were found to be higher than that of an equivalent NH3-water system which is 0.697 and 81%, respectively. The maximum exergy destruction was found at the absorber unit (24%), followed by the generator (23%). Furthermore, the NH3-water and water-LiBr simulations were optimized for different parametric conditions, and optimal operating conditions were identified for large-capacity district cooling systems. Therefore, the findings will provide the roadmap for designing and optimizing large-scale DCP operated on VAR cycles.

A district cooling system (DCS), that generally operates with vapor compression refrigeration (VCR) chillers, is superior to conventional air conditioning as it helps to reduce energy consumption and to protect the environment by reducing carbon dioxide emissions. The advantages of the district cooling plant can be further enhanced by integrating the VCR system with a vapor absorption refrigeration system (VAR). In this context, this paper presents the comparative assessment of a VCR chiller and two configurations of an integrated VCR and VAR chiller, consisting of the parallel and the cascade configuration. Two refrigerant-absorbent mixtures in the VAR system are considered, namely the conventional NH3/H2O and the novel acetaldehyde/N,N-dimethylformamide (Acet/DMF). The energy, exergy, environmental and economic analysis of the systems have been performed considering the coefficient of performance (COP), the exergy efficiency (ηexergy), the total equivalent warming impact (TEWI) and the cost rate (Ċtot) as performance indicators. It is observed that the connection of the evaporator of the VAR with the condenser of the VCR in the cascade configuration increases the COP of the VCR unit by 292% compared to the stand-alone VCR system. Overall, the COP of the parallel and the cascade refrigeration system with the novel Acet/DMF is better than that of NH3/H2O. This performance can further be improved by enhancing the exergy efficiency. Overall, the thermodynamic, environmental and cost benefits of the novel Acet/DMF with the cascade arrangement of the VCR and the VAR system are superior to those of NH3/H2O.

Σκοπός της διατριβής είναι η μελέτη της αντίδρασης αναμόρφωσης μεθανίου με ατμό σε αντιδραστήρα μεμβράνης παλλαδίου για την παραγωγή υδρογόνου. Εξετάστηκε τόσο πειραματικά όσο και υπολογιστικά την αντίδραση και παρουσιάστηκαν αποτελέσματα σχετικά με την επαλήθευση του μαθηματικού μοντέλου και την επίδραση διαφόρων παραμέτρων στην απόδοση του συστήματος όπως αυτή ορίζετε από την μετατροπή μεθανίου. Επιπλέον, πραγματοποιήθηκε και παρουσιάστηκε ο βέλτιστος σχεδιασμός τόσο της λειτουργικής μονάδας του αντιδραστήρα μεμβράνης όσο και τριών εναλλακτικών διαγραμμάτων ροής για τη διεργασία. Στη συνέχεια, σχεδιάστηκε ένας βέλτιστος προβλεπτικός ελεγκτής και εφαρμόστηκε για τον έλεγχο της διεργασίας. Τέλος, αναπτύχθηκε ένα πλαίσιο ολιστικού βέλτιστου σχεδιασμού και ελέγχου διεργασιών και εφαρμόστηκε για το σχεδιασμό των εναλλακτικών διαγραμμάτων ροής. Η μεθοδολογία περιλαμβάνει την επίλυση ενός μαθηματικού μοντέλου σε μόνιμη κατάσταση, για τον υπολογισμό του πάγιου και λειτουργικού κόστους της διεργασίας καθώς και την επίλυση ενός δυναμικού μαθηματικού μοντέλου μαζί με την επίλυση του αλγορίθμου του ελεγκτή, για την διόρθωση του λειτουργικού κόστους της διεργασίας και για τον υπολογισμό την επίδρασης ενός προκαθορισμένου σεναρίου διαταραχής στο σύστημα. Η ελαχιστοποίηση του υπολογιστικού κόστους πραγματοποιείται με τον υπολογισμό της μέγιστης μεταβλητότητας του συστήματος ως προς τις προκαθορισμένες πολλαπλές ταυτόχρονες διαταραχές. Επίσης, η ποσοτικοποίηση της δυναμικής συμπεριφοράς πραγματοποιείτε είτε μέσω απόκλισης από την επιθυμητή τροχιά είτε μέσω της οικονομικής επιβάρυνσης της λειτουργίας εκτός προδιαγραφών. Τα αποτελέσματα αποδεικνύουν την ικανότητα της προτεινόμενης μεθοδολογίας στο να προσδιορίζει σχεδιαστικά σημεία που παρουσιάζουν ανώτερη συμπεριφορά ως προς τα κριτήρια σχεδιασμού. Η μεθοδολογία είναι μοναδική στο ότι συνδυάζει τον βέλτιστο σχεδιασμό και έλεγχο ολοκληρωμένων, εντατικοποιημένων διεργασιών υπό την επήρεια πολλαπλών ταυτόχρονων διαταραχών με αλγόριθμους στοχαστικής βελτιστοποίησης εξασφαλίζοντας την εύρεση της ολικά μέγιστης λύσης. Τέλος, το διάγραμμα ροής που χρησιμοποιεί αντιδραστήρα και διαχωριστή σε σειρά παρουσιάζει ανώτερη δυναμική συμπεριφορά σε σχέση με το διάγραμμα ροής με τον αντιδραστήρα μεμβράνης, λόγω επιπλέον χειραγωγούμενων μεταβλητών. Ωστόσο, το διάγραμμα ροής που είναι εξοπλισμένο με τον αντιδραστήρα μεμβράνης είναι πιο ελκυστική λύση λαμβάνοντας υπόψη τη συνολική οικονομική επίδοση του συστήματος.

Vapor absorption refrigeration (VAR) is a sustainable alternative to the conventional vapor compression refrigeration (VCR) cycle, owing to its lower non-renewable energy requirements and potentially for exploitation of renewable energy sources. Traditionally, the coefficient of performance (COP) of the conventional single effect VAR cycle is considerably lower than VCR cycles. This provides room for improvement which can be attained through double effect VAR cycles that provide relatively higher performance. The COP of the dual effect VAR cycle is enhanced due to the waste/rejected heat energy utilization from the condenser or the absorber into a secondary generator. Models that correlate the COP of the double effect VAR cycle with operating parameters are not available in the open literature, with Iyer’s correlation being the only exception. This work applies this COP correlation using literature data for double effect VAR that operate with a variety of refrigerant and absorbent pairs. A comprehensive Mean Absolute Percentage Error (MAPE) analysis is performed for more than 2028 data points of various fluid pairs. Results reveal that MAPE (86.6–839%) values appear to be quite high for the reported correlation. Furthermore, the model is optimized using the proposed data set, considerably reducing the MAPE up to 36.03%. The results also indicate that due to the lack of fluid-specific parameters, the application of this correlation may not support the development of new double effect VAR cycles. Therefore, it is crucial to establish a performance-based correlation that considers both operational parameters and fluid parameters to assess the performance of new and efficient dual effect VAR cycles.

The presented work includes a detailed review of organic working fluid mixtures and corresponding Absorption Refrigeration (ABR) cycles available in published literature. Such processes are important as they enable exploitation of cleaner and renewable energy sources for cooling generation. Research efforts may be benefited by a systematically organized account of previous works in organic working fluids, which have received considerably less attention than conventional inorganic options. The reviewed developments are separated into working fluids used in single effect cycles and alternative configurations such as double effect, half effect and so forth. Details are reported regarding the operating conditions tested, the criteria used for working fluid selection and the ones selected as desirable options either experimentally or in simulation studies. Research on thermodynamics of organic working fluids suitable for ABR processes is also reported with respect to measured properties, experimental conditions and types of thermodynamic models. The characteristics of different process flowsheets are also analyzed, while commercial scale ABR applications are reported to motivate research in industrial/commercial scale systems. It is observed that there are few types of chemical groups repeated in working fluid investigations, with halogenated refrigerants and ether- and amide-based absorbents prevailing compared to other substances. Most experimental works pertain to single effect systems, with model-based approaches mainly used due to introduction of increasingly complex process modifications. The latter have been assessed with considerably fewer different fluid options compared to single effect systems. Thermodynamic investigations mainly combine experiments with parameter estimation for model development. Most works derive data and employ the NRTL activity coefficient model, often combined with cubic equations of state.

This work focuses on the development and implementation of an integrated process design and control framework for a membrane-based hydrogen production system based on low temperature methane steam reforming. Several alternative flowsheet configurations consisted of either integrated membrane reactor modules or successive reactor and membrane separation modules are designed and assessed by considering economic and controller dynamic performance criteria simultaneously. The design problem is expressed as a non-linear dynamic optimization problem incorporating a nonlinear dynamic model for the process system and a linear model predictive controller aiming to maintain the process targets despite the effect of disturbances. The large dimensionality of the disturbance space is effectively addressed by focusing on disturbances along the direction that causes the maximum process variability revealed by the analysis of local sensitivity information for the process system. Design results from a multi-objective optimization study, where only the annualized equipment and operational costs are minimized, are used as reference case in order to evaluate the proposed design framework. Optimization results demonstrate the controller&rsquo, s ability to track the imposed setpoint changes and alleviate the effects of multiple simultaneous disturbances. Also, significant economic improvements are observed by the implementation of the integrated design and control framework compared to the traditional design methodology, where process and controller design are performed sequentially.

The present study aims to design and investigate the performance of a multivariable PI controller for the control of single, double and triple effect absorption refrigeration (ABR) processes. Dynamic modeling of ABR has been previously reported in literature for single effect systems using LiBr/H2O and NH3/H2O and for double effect systems using LiBr/H2O, whereas there is a lack of such models for triple effect systems. With few exceptions, existing implementations employ reduced-order models, mainly focused on heat exchangers, whereas the rectification process is generally omitted. For the first time, comprehensive dynamic models that include the rectifier and a multivariable control scheme are presented for a) a double effect, and b) a triple effect ABR using NH3/H2O. The main control challenge is to efficiently meet cooling loads while maintaining the performance level expressed as sustainable energy usage. The control strategy consists of multiple temperature, flowrate, level and capacity control loops, in order for the system to withstand the effect of anticipated disturbances (ambient conditions variation and part load operation). The presented case study confirms the controller ability to achieve the desired dynamic behavior both in the case of performance optimization and part load operation.

The systematic and rigorous model-based optimization of the configuration and operating conditions of a methane membrane steam reforming reactor for hydrogen production is performed. A permeable membrane with Pd–Ru deposited on a ceramic dense support is used to selectively remove the produced hydrogen from the reaction zone. The shifted chemical equilibrium towards hydrogen production enables the achievement of high methane conversion at relatively low reactor temperature levels. Steam reforming takes place over a Ni–Pt/CeZnLa ceramic foam-supported catalyst that ensures better thermal distribution, at an operating temperature of 773 K and a pressure of 106 Pa. A nonlinear, two-dimensional, and pseudo-homogeneous mathematical model of the membrane fixed-bed reactor is developed and subsequently validated using experimental data. For model validation purposes, two sets of experiments have been performed at the experimental reactor installed at CPERI/CERTH. The first set of experiments aims to investigate membrane permeability in order to estimate the parameters involved in the applied Sieverts law. The second set of experiments explores the performance of the membrane reactor at different steam to carbon ratios and total inlet volumetric flowrates. The derived mathematical model, consisted of mass, energy, and momentum balances that consider both axial and radial gradients of temperature and concentration, is then utilized within a model-based optimization framework that calculates the optimal operating conditions for the highly interactive reactor system. The optimal steam to carbon ratio and sweep gas flow rate that minimize the overall methane utilization (i.e., reformed methane and equivalent methane for heating purposes) are calculated for a range of hydrogen production rates. Τhe optimal reactor design configuration described by the length of the catalyst zone is also obtained for a given pure hydrogen production rate.

One area of vital importance in the field of absorption refrigeration (ABR) is the search of alternative combinations of refrigerant/absorbent pairs that can enhance its performance relatively to the widely used H2O-NH3 or H2O-LiBr. In this work, three binary (H2O-LiBr, H2O-LiCl, H2O-LiI), three ternary (H2O-LiBr + LiI, H2O-LiBr + C2H6O2, H2O-LiBr + LiCl) and four quaternary (H2O-LiBr + LiCl + ZnCl2, H2O-LiBr + ZnCl2+CaBr2, H2O-LiBr + ZnBr2+LiCl, H2O-LiBr + LiI + C2H6O2) mixtures of refrigerant/absorbent are tested to investigate heat transfer and pressure drop characteristics across the solution heat exchanger used in ABR systems. Subsequently, seven different single effect absorption cooling flowsheet configurations are explored to assess the potential for exploitation of the generated results at the cycle level. The effects of the concentrations of the absorbent in the mixture of refrigerant/absorbent on heat transfer and pressure drop characteristics are addressed. As a result, H2O-LiI, H2O-LiBr + LiI and H2O-LiBr + ZnCl2+CaBr2 shows higher heat transfer coefficient and lower pressure drop among the tested binary, ternary and quaternary mixtures respectively. Furthermore, it is concluded that with increase in mass fraction of the absorbent in the mixture, the heat transfer coefficient is decreased. A process configuration with part of the refrigerant mixed with the strong solution through an ejector, prior to the solution heat exchanger, is found to exhibit highest coefficient of performance.

Pure hydrogen production is of great interest as it is an energy carrier which can be used in PEM fuel cells for power production. Methane Steam Reforming (MSR) is commonly used for hydrogen production although the produced hydrogen is not free of other components. Membrane Reactors (MR) enable a pure hydrogen product stream and allows the reaction to take place at significantly lower temperatures (lower than 550 °C) than in conventional reactors (greater than 800 °C) with comparable methane conversion. This is achieved by hydrogen removal through a permselective Pd–Ag based membrane that cause a favorable shift in chemical equilibrium towards hydrogen production. In the present study, a two-dimensional, nonlinear, and pseudo-homogeneous mathematical model of a catalytic fixed-bed membrane reactor for methane steam reforming over a nickel-based foam supported catalyst is presented. Simulated results referring to the distribution of species, methane conversion, temperature and hydrogen flowrate along the reactor for different radial positions are obtained and analyzed. The performance of structured catalyst and catalyst supported on foam configurations under the same operating conditions is also studied. Experimental results for the membrane facilitate the identification of suitable operating conditions.