Numerical Analysis of Suction Mufflers

The aim of this paper is to present a group of numerical experiments of the fluid flow through different suction mufflers geometries to analyse pressure and velocity behaviour in addition with acoustic pressure effect. The numerical results are oriented to compare how suction muffler geometry affects mas flow rate and compressor efficiency performance on one side and pressure pulsations, transmission losses and compressor noise on the other side (1). The CFD&HT results have been obtained by means of TermoFluids code, a new unstructured and parallel object-oriented CFD&HT code for accurate and reliable solving of industrial flows (2). In all studied cases a multi-dimensional explicit finite volume fractional-step based algorithm has been used with symmetry preserving discretization scheme. When turbulence modelling is needed, an extension of the Yoshizawa non-equilibrium fixed-parameter subgrid-scale (3) model to non-structured meshes is used. The pressure equation is solved by means of parallel Fourier Schur decomposition solver, which is an efficient direct solver for loosely coupled PC clusters (4). Different CFD analysis of compressor suction mufflers has been developed (5) mainly based on k-e RANS models. The present paper is oriented on Large Eddy Simulation (LES) models. On the other hand, boundary conditions are coupled with a modular, unstructured and object oriented tool of the numerical simulation of hermetic reciprocating compressors (6)(7). (1) W. Soedel, Mechanics, simulation and design of compressor valves, gas passages and pulsation mufflers, Purdue University, 1992. (2) O. Lehmkuhl, R. Borrell, C.D.Pérez-Segarra, M. Soria, A. Oliva, A new parallel unstructured CFD code for the simulation of turbulent industrial problems on low cost PC cluster, Parallel Computational Fluid Dynamics, Ankara, Turkey, 2007. (3) W. Rodi, DNS and LES of some engineering flows, Fluid Dynamic Research, vol. 38, pp. 145-173, 2006 (4) R. Borrell, O. Lehmkuhl, M. Soria, A. Oliva; Direct Schur method for the solution of Poisson equation with unstructured meshes; Parallel CFD Congress, Antalya, Turkey, 2007 (5) (4)K. Sar?o?lu, A. Özdemir, A. Kaya, E. O?uz, An experimental and numerical analysis of refrigerant flow inside the suction muffler ofhermetic reciprocating compressor, International Compressor Engineering Conference at Purdue, 2012. (6) R. Damle, J. Rigola, C.D. Pérez-Segarra, J. Castro, A. Oliva, “Object oriented simulation of reciprocating compressors: Numerical verification and experimental comparison”, International Journal of Refrigeration, 34 (2011) 1989-1998. (7) J. López, O. Lehmkuhl, J. Rigola, C.D. Pérez-Segarra, “ Use of Low-Mach modelo n a CFD&HT solver for the elements of an object oriented program to numerically simulate hermetic reciprocating compressors”, International Compressor Engineering Conference at Purdue, 2012

A generalized computational model for the simulation of adsorption packed bed reactors – Parametric study of five reactor geometries for cooling applications

Environmental concerns regarding global warming and ozone depletion urge towards sustainable solutions for satisfying the increasing cooling demand. Adsorption cooling technology could form part of the solution since it can be driven by solar energy and industrial or vehicular waste heat, as well as it employs non ozone-destructive refrigerants. However, its low performance hinders its extensive development and commercialization. The design of the adsorption reactor is crucial for its performance improvement, since its inherent cyclic operation imposes a compromise between the Specific Cooling Power and the Coefficient of Performance. A generalized three-dimensional computational model based on unstructured meshes is presented, capable to simulate all potential geometries. Dynamic conjugate simulations of the packed bed and the heat exchanger allow to study the latter’s influence on the reactor performance. A parametric study of five reactor geometries was conducted, demonstrating quantitatively the strong impact of the solid volume fraction, fin length and fin thickness on the performance. Within the studied range, the Specific Cooling Power is maximized for the highest solid volume fraction and for the lowest fin thickness and fin length. The effect of the adsorbed mass spatial distribution on the desorption phase is discussed. A sensitivity analysis exhibits the importance of the heat transfer coefficient between the two domains. Copper and aluminium are compared as heat exchanger materials, revealing that the former performs more effectively, although the difference is appreciable only for longer fin lengths. The presented numerical model can be employed for improving the design of adsorption packed bed reactors.Peer ReviewedPostprint (author's final draft

Control strategy approach based on the operational results of a small capacity direct air -cooled LiBr -Water absorption chiller

The scope of this paper is to give a short overview of the state of the art regarding control strategies, identify the role of different operating conditions, and provide useful suggestions for the design and operation of a solar assisted absorption cooling system, in line to the European regulation as well as its directly related directives. The operation of a solar absorption cooling system under real conditions is subjected to various limitations regarding its ability to satisfy the required cooling demand, as well as to avoid certain internal conditions which would lead to problematic situations and jeopardize the smooth operation of the system - such as solution crystallization and water freezing. Thus, it is very important to define and refine new control operating strategies, from an internal and external perspective. Several control strategies are discussed, altogether with a new fuzzy logic approach, which shall be experimentally validated as future actions, due to its highly promising capability.Peer ReviewedPostprint (published version

Computational modeling of adsorption packed bed reactors and solar-driven adsorption cooling systems

Environmental concerns regarding climate change and ozone depletion urge for a paradigm shift in the cooling production. The cooling demand exhibits an alarmingly increasing trend, thus its satisfaction in a sustainable manner is imperative. Adsorption cooling systems (ACSs) are a potential candidate for a sustainable future of cooling production, since they can utilize solar energy or waste heat, as well as they can employ substances with zero ozone depletion and global warming potential. The objective of this thesis is to contribute to the investigation and improvement of ACSs, through the development of two computational models - which approach ACSs from different perspectives - and their respective utilization for the conduction of related numerical studies. The first research direction focuses on the design of the adsorption reactor, the most vital component of ACSs. Its geometrical configuration is determinant for the system performance. The reactor design is a crucial task since it creates a dichotomy between the two performance indicators - the Specific Cooling Power (SCP) and the Coefficient of Performance (COP). Individual optimizations based on the SCP and the COP would result in completely opposite geometrical configurations. A computational model for the simulation of adsorption packed bed reactors was developed, capable of simulating any potential reactor geometry. A multi-timestep approach is adopted, resulting in a drastic reduction of the computational cost of the simulations. Verification and validation assessments were performed in order to evaluate the reliability of the model. Two major studies were conducted within this research direction. The first aspires to provide a comparison between five reactor geometries, motivated by the lack of comparability across different studies in the literature. Thirteen cases of each geometry are simulated, by varying the fin thickness, fin length and solid volume fraction. The second study pertains to a thorough investigation of a geometry that remained underexplored hitherto - the hexagonal honeycomb adsorption reactor. A parametric study is conducted with respect to the three dimensions that define the geometry, as well as for various operating conditions. The second research direction is dedicated to the investigation of adsorption cooling systems, and in particular, to their integration within a wider thermal system, a solar-cooled building. Such integration is not straight-forward due to thermal inertia effects and the inherent cyclic operation of ACSs, as well as due to the dependence on an intermittent source and an auxiliary unit, with a clear objective to prioritize solar energy. A numerical model was developed using 1-d models for the adsorption reactors and 0-d models for the evaporator and condenser. The model is validated against experimental results found in the literature. The model is coupled to the generic optimization tool GenOpt, thus allowing the conduction of optimization studies. The ACS model is then coupled to solar collectors and thermal storage models, as well as to a building model. The latter was previously developed in the CTTC laboratory. This coupling results in a comprehensive simulation tool for adsorption-based solar-cooled buildings. A case study for a solar-cooled office is considered, with the objective to investigate the potential of satisfying its cooling demand using solar energy. A control strategy is proposed based on variable cycle duration, using optimized values for the instantaneous operating conditions. The variable cycle duration approach allows to satisfy the cooling demand using significantly less solar collectors or less auxiliary energy input. The potential carbon dioxide emissions avoidance is calculated between 28.1-90.7% with respect to four scenarios of electricity-driven systems of different performance and carbon emission intensity.La preocupació mediambiental sobre el canvi climàtic i l'esgotament d'ozó exigeix un canvi de paradigma en la producció de fred. La demanda de refredament mostra una tendència alarmant creixent, així és imperatiu satisfer-la de forma sostenible. Els sistemes de refredament per adsorció (ACS) són un candidat per a un futur sostenible de la producció de fred, ja que poden utilitzar energia solar o calor residual, emprant substàncies amb zero potencial d'esgotament d'ozó i d'escalfament global. L'objectiu d'aquesta tesi és contribuir a la investigació i millora dels ACS, mitjançant el desenvolupament de dos models computacionals - que aborden els ACS des de diferents perspectives - i la seva utilització per a la realització d'estudis numèrics. La primera línia d'investigació se centra en el disseny del reactor d'adsorció, el component més important dels ACS. La seva configuració geomètrica és determinant pel rendiment de sistema. El seu disseny és una tasca crucial, ja que crea una dicotomia entre la potència específica de refrigeració (SCP) i el coeficient de rendiment (COP). Les optimitzacions individuals basades en el SCP i el COP resultarien a configuracions geomètriques completament oposades. S'ha desenvolupat un model computacional per a la simulació de reactors d'adsorció tipus "packed bed", capaç de simular reactors de qualsevol geometria. S'adopta una estratègia multi-timestep, que permet una dràstica reducció del cost computacional de les simulacions. La fiabilitat del model es va avaluar a través de processos de verificació i validació. Dins d'aquesta línia de recerca es van realitzar dos estudis principals. El primer aspira a proporcionar una comparació entre cinc geometries de reactors, motivat per la falta de comparabilitat entre diferents estudis en la literatura. Es simulen tretze casos de cada geometria, variant el gruix de les aletes, la seva longitud i la fracció de volum de sòlid. El segon estudi presenta la investigació d'una geometria sub-explorada previament, el reactor d'adsorció de honeycomb hexagonal. Es realitza un estudi paramètric pel que fa a les tres dimensions que defineixen la geometria, així com per a diverses condicions de funcionament. La segona línia de recerca es dedica a la investigació dels ACS. i en particular, a la seva integració dins d'un sistema tèrmic més ampli, un edifici refredat per energia solar. Aquesta integració no és senzilla a causa de la inèrcia tèrmica i a el funcionament cíclic inherent dels ACS, així com a la dependència d'una font intermitent i d'un sistema auxiliar, amb l'objectiu de prioritzar l'energia solar. S'ha desenvolupat un model numèric utilitzant models 1-d pels reactors i models 0-d per l'evaporador i el condensador. El model es va validar amb resultats experimentals trobats en la literatura. El model es va acoblar amb l'eina d'optimització genèrica GenOpt, permetent així estudis d'optimització. El model ACS es va acoblar amb models de col·lectors solars, emmagatzematge tèrmic i amb un model d'edifici. Aquest últim va ser desenvolupat prèviament al CTTC. Aquest acoblament resulta a una eina de simulació integral per a edificis refredats per energia solar utilitzant adsorció. Es considera un cas d'estudi per a una oficina refredada per energia solar, amb l'objectiu d'investigar el potencial de satisfer la seva demanda de fred utilitzant energia solar. Es proposa una estratègia de control basada en la duració variable del cicle, utilitzant valors optimitzats per a les condicions instantànies. La durada variable d'el cicle permet satisfer la demanda utilitzant una quantitat significativament menor de col·lectors solars o un menor aportació d'energia auxiliar. Les emissions de CO2 evitades es calculen entre 28.1-90.7% respecte a quatre escenaris de sistemes elèctrics de diferent rendiment i intensitat d'emissions de carboni.Postprint (published version

Cyprus Energy Policy : Scenaria for the Achievement of the Targets for Renewable Energy Sources

110 σ.Η Κυπριακή Δημοκρατία, ως κράτος μέλος της Ευρωπαϊκής Ένωσης, δεσμεύεται βάσει της ευρωπαϊκής οδηγίας 2009/28/ΕΚ, να παράγει το 13% της ετήσιας ενεργειακής της κατανάλωσης από ανανεώσιμες πηγές ενέργειας μέχρι το 2020. Η εκπόνηση της παρούσας διπλωματικής εργασίας αποσκοπεί στην κατάστρωση σεναρίων για την επίτευξη του στόχου αυτού. Η διπλωματική εργασία αποτελείται από τρία μέρη. Στο πρώτο μέρος παρουσιάζονται οι κυριότεροι λόγοι που καθιστούν επιτακτική την ανάγκη για στροφή προς τις ανανεώσιμες πηγές ενέργειας (ανεπάρκεια ορυκτών καυσίμων, ενεργειακή ασφάλεια, κλιματικές αλλαγές). Το δεύτερο μέρος ασχολείται με την Ευρωπαϊκή Ενεργειακή Πολιτική για τις ΑΠΕ, εστιάζοντας στην οδηγία 2009/28/ΕΚ. Αναλύονται οι προθέσεις και το αναμενόμενο αποτέλεσμα ορισμένων σημείων της οδηγίας, όπως τα κριτήρια αειφορίας των βιοκαυσίμων, ο επιμερισμός του συνολικού στόχου της ΕΕ, ο "μηχανισμός άνω οριοθέτησης για την πολιτική αεροπορία", η ευελιξία που παρέχεται για τους τομεακούς στόχους, η υιοθέτηση συγκεκριμένο προτύπου για τα Σχέδια Δράσης κ.α.. Το τρίτο μέρος αφορά την Κυπριακή Ενεργειακή Πολιτική για τις ΑΠΕ. Αξιολογείται η οικονομική βιωσιμότητα του Ειδικού Ταμείου ΑΠΕ και στη συνέχεια εξετάζεται κάθε ενεργειακός τομέας μεμονωμένα. Λαμβάνοντας υπόψη το αξιοποιήσιμο δυναμικό, την παρούσα κατάσταση, το σχέδιο χορηγιών και γενικά όλα τα χαρακτηριστικά που διέπουν το ενεργειακό σύστημα, προτείνεται ένα σενάριο, ικανό για την επίτευξη του στόχου. Το σενάριο αυτό, παρουσιάζεται στη μορφή του προτύπου Σχεδίου Δράσης. Αξιολογούνται τα σενάρια που κατέστρωσαν το Κέντρο Ανανεώσιμων Πηγών Ενέργειας (ΚΑΠΕ) και το Ινστιτούτο Fraunhofer, και εν τέλει, συγκρίνονται τα τρία σενάρια μεταξύ τους.The Republic of Cyprus, as a member state of the European Union, is obligated under the European directive 2009/28/EC to produce 13% of its annual energy consumption from renewable energy sources by 2020. This diploma thesis aims to construct scenaria for the achievement of this target. The diploma thesis is consisted of three parts. The first part presents the main concerns that led EU to set mandatory targets to its member states in order to accelerate the implementation of RES in the European energy mix (fossil fuels depletion, energy security, climate change). The second part is dealing with the European Energy Policy for RES, focusing on the directive 2009/28/EC. The expected outcome and the intentions behind certain points of the directive such as the sustainability criteria for the biofuels, the allocation of the overall target of the EU, " the aviation capping mechanism", the flexibility on the sectoral targets, the adoption of a specific template for the Action Plans etc., are examined thoroughly. Cyprus Energy Policy is the third part of this diploma thesis. The financial sustainability of the Special Fund for RES is assessed, and then, every energy sector is examined individually. Taking into account the exploitaple potential, the current situation, the subsidy scheme and all the rest characteristics of the energy system, a scenario is proposed, capable for the achievement of the target. This scenario is presented in the form of the template for the Action Plans. The scenaria constructed by the Centre for Renewable Energy Sources (CRES) and by the Fraunhofer Institute are assessed; and finally the three scenaria are compared.Γιώργος Μ. Παπακόκκινο

A generalized computational model for the simulation of adsorption packed bed reactors – Parametric study of five reactor geometries for cooling applications

Environmental concerns regarding global warming and ozone depletion urge towards sustainable solutions for satisfying the increasing cooling demand. Adsorption cooling technology could form part of the solution since it can be driven by solar energy and industrial or vehicular waste heat, as well as it employs non ozone-destructive refrigerants. However, its low performance hinders its extensive development and commercialization. The design of the adsorption reactor is crucial for its performance improvement, since its inherent cyclic operation imposes a compromise between the Specific Cooling Power and the Coefficient of Performance. A generalized three-dimensional computational model based on unstructured meshes is presented, capable to simulate all potential geometries. Dynamic conjugate simulations of the packed bed and the heat exchanger allow to study the latter’s influence on the reactor performance. A parametric study of five reactor geometries was conducted, demonstrating quantitatively the strong impact of the solid volume fraction, fin length and fin thickness on the performance. Within the studied range, the Specific Cooling Power is maximized for the highest solid volume fraction and for the lowest fin thickness and fin length. The effect of the adsorbed mass spatial distribution on the desorption phase is discussed. A sensitivity analysis exhibits the importance of the heat transfer coefficient between the two domains. Copper and aluminium are compared as heat exchanger materials, revealing that the former performs more effectively, although the difference is appreciable only for longer fin lengths. The presented numerical model can be employed for improving the design of adsorption packed bed reactors.Peer Reviewe