Optimization and multivariable control of refrigeration systems

Los ciclos de refrigeración por compresión de vapor constituyen el método más extendido a nivel mundial para la generación de frío. Estos sistemas se utilizan en áreas tan diversas como regulación de la temperatura en estancias habitadas, almacenamiento y transporte de alimentos y múltiples procesos industriales. Dado el considerable impacto causado por el consumo energético de estos sistemas en los balances económicos y medioambientales de los países desarrollados y en vías de desarrollo, y teniendo en cuenta la escasez creciente de fuentes de energía fósiles y el desarrollo todavía lento de las diferentes tecnologías de energía renovable, la operación óptima en términos de eficiencia energética de los sistemas de refrigeración por compresión de vapor existentes se presenta como un problema clave que abordar. Esta Tesis aborda la operación óptima de los ciclos de refrigeración desde el punto de vista de la eficiencia energética. Aunque el trabajo se centra principalmente en sistemas de una etapa de compresión y un recinto a refrigerar, se analizan también otras configuraciones con varias etapas y varios recintos. Existen varios factores clave para alcanzar la operación óptima de un sistema de refrigeración en el campo del Control Automático: el modelado, la optimización y el control propiamente dicho. En primer lugar, se estudia ampliamente el modelado estático y dinámico de los sistemas de refrigeración. En cuanto al segundo, se desarrolla un modelo dinámico simplificado y orientado al control de un ciclo de una etapa de compresión y un recinto a refrigerar. El objetivo es que pueda ser incorporado en estrategias de control basado en modelo, donde se requieren tanto una baja carga computacional como una descripción suficientemente precisa de la dinámica dominante del sistema, de acuerdo con los objetivos de control. En segundo lugar, se analiza la operación óptima en régimen permanente de un ciclo de una etapa de compresión y un recinto a refrigerar. Dada una cierta demanda de frío, el objetivo de la fase de optimización es calcular el ciclo en régimen permanente que alcanza la máxima eficiencia energética posible asegurando la satisfacción de la demanda de frío y a la vez respetando las restricciones de operación. Una vez calculado, se pretende que este ciclo óptimo constituya la referencia a seguir por parte del controlador. Finalmente, se estudia asimismo el problema de control. En la literatura sobre sistemas de refrigeración se encuentran principalmente dos esquemas: el control convencional y el control centrado en la eficiencia energética. En el primer esquema, además de la referencia impuesta por la demanda de frío, se impone un valor bajo pero constante como referencia para el grado de sobrecalentamiento del refrigerante a la salida del evaporador, to achieve the cycle defined by the optimization stage by manipulating the available control actions. Therefore, the controllability of the one-stage, one-load-demand cycle is analysed using linear theory and a nonlinear pointwise analysis based on the phase portrait method. Given the conclusions of the controllability analysis, a suboptimal hierarchical control strategy is proposed to achieve the highest possible efficiency while satisfying the cooling load. Most contributions of this Thesis are of theoretical nature. Notwithstanding, the application of the proposed control strategy to a multi-compression-stage, multi-loaddemand experimental plant is intended. Then, steady-state identification of the plant is performed from experimental data, whereas validation of the models considering different plant configurations is also carried out.Premio Extraordinario de Doctorado U

Modelling and data validation for the energy analysis of absorption refrigeration systems

Data validation and reconciliation techniques have been extensively used in the process industry to improve the data accuracy. These techniques exploit the redundancy in the measurements in order to obtain a set of adjusted measurements that satisfy the plant model. Nevertheless, not many applications deal with closed cycles with complex connectivity and recycle loops, as in absorption refrigeration cycles. This thesis proposes a methodology for the steady-state data validation of absorption refrigeration systems. This methodology includes the identification of steady-state, resolution of the data reconciliation and parameter estimation problems and the detection and elimination of gross errors. The methodology developed through this thesis will be useful for generating a set of coherent measurements and operation parameters of an absorption chiller for downstream applications: performance calculation, development of empirical models, optimisation, etc. The methodology is demonstrated using experimental data of different types of absorption refrigeration systems with different levels of redundancy.Los procedimientos de validación y reconciliación de datos se han utilizado en la industria de procesos para mejorar la precisión de los datos. Estos procedimientos aprovechan la redundancia enlas mediciones para obtener un conjunto de datos ajustados que satisfacen el modelo de la planta. Sin embargo, no hay muchas aplicaciones que traten con ciclos cerrados, y configuraciones complejas, como los ciclos de refrigeración por absorción. Esta tesis propone una metodología para la validación de datos en estado estacionario de enfriadoras de absorción. Estametodología incluye la identificación del estado estacionario, la resolución de los problemas de reconciliación de datos y estimación de parámetrosy la detección de errores sistemáticos. Esta metodología será útil para generar un conjunto de medidas coherentes para aplicaciones como: cálculo de prestaciones, desarrollo de modelos empíricos, optimización, etc. La metodología es demostrada utilizando datos experimentales de diferentes enfriadoras de absorción, con diferentes niveles de redundancia

Experimental comparison between R152a and R134a working in a refrigeration facility equipped with a hermetic compressor

[EN] The EU Regulation 517/2014 has recently been approved in a further attempt to curb the effects of GlobalWarming. As a consequence, the refrigeration sector is moving towards refrigerants with a low GlobalWarming Potential (GWP100) in accordance with the limit fixed by these regulations (150). In this regard, the old refrigerant R152a attracts renewed interest due to its low GWP (138) and its similarity to R134a. The present work shows the results of using R152a in a vapour compression plant equipped with a hermetic compressor and an IHX designed for R134a. The refrigerant was replaced by a conventional drop-in process in order to carry out an energy comparison. The results have revealed an improvement in the COP with R152a up to 13% despite a reduction in the cooling capacity of about 10%. During the test campaign, R134a hermetic compressors have been shown to be capable of operating with R152a.The authors acknowledge Jaume I University of Spain, who financed partially the present study through the research project P1.B2013-10.Cabello, R.; Sanchez, D.; Llopis Doménech, R.; Armendáriz Araúzo, LM.; Torrella Alcaraz, E. (2015). Experimental comparison between R152a and R134a working in a refrigeration facility equipped with a hermetic compressor. International Journal of Refrigeration. 60:92-105. https://doi.org/10.1016/j.ijrefrig.2015.06.021S921056

A hybrid modeling approach for steady-state optimal operation of vapor compression refrigeration cycles

This paper presents a steady-state hybrid modeling approach for vapor compression refrigeration cycles which is intended to achieve an optimal system operation from an energy consumption point of view. The model development is based on a static characterization of the main components of the cycle using a hybrid approach, and their integration in a new optimization block. This block allows to determine completely the system stationary state by means of a non-linear optimization procedure subjected to several constraints such as mechanical limitations, component interactions, environmental conditions and cooling load demand. The proposed method has been tested in an experimental pilot plant with good results. Model validation for each identified hybrid model is carried out from a set of experimental data of 82 stationary operating points, with prediction errors below ±10%. The model is also globally validated by comparing experimental and simulated data, with a global mean relative absolute error less than 5%. The basic control structure consists of three decentralized control loops where the controller variables are the secondary fluid temperature at the evaporator inlet, the superheat, and the condenser pressure. While the secondary temperature is assumed as an imposed requirement, the optimal set-points of the other two control loops are searched offline using the proposed refrigerant cycle model. This set-point optimality is defined according to the coefficient of performance for minimizing the total electrical power consumption of the system at steady-state. This energy saving has been confirmed experimentally. The proposed method can be easily adapted for different sets of controlled variables in case of modification of the basic control structure. Furthermore, other energy efficiency metrics can be handily adopted. Considering the tradeoff between the accuracy and computational cost of the hybrid models, the proposed procedure is expected to be used in real-time applications

Dynamic models for the analysis of vapour-compression refrigerating machines

Vapour-compression refrigeration systems represent one of the most relevant fields in terms of energy consumption. Among the research works aimed at reducing their environmental impact, those related to control systems are appealing, since they usually involve lower costs than interventions at system level and directly affects the energy consumption. In this work, a library of models able to reproduce the dynamics of the main components of a vapour-compression refrigeration machine has been developed in Matlab/SIMULINK to supply the building blocks to investigate the effects of different control strategies on the energy performance and on the control quality of the system. To obtain models with low computational cost, the switched moving-boundary approach was adopted for the mathematical model of the heat exchangers. Firstly, the models were numerically verified, using the Matlab toolbox Thermosys as a benchmark. A good agreement between the predictions of the two models was highlighted in the simulation of an air-to-air refrigerating machine; moreover the in-house models showed a better repeatability in the prediction of important quantities like refrigerant superheating, subcooling and charge. The dynamic models were also experimentally validated, comparing their predictions with the experimental data acquired on a water-to-water refrigerating machine equipped with brazed-plate heat exchangers. Results showed a very good agreement between numerical and experimental data, also in terms of electric power absorption and COP, thus proving the model usefulness in transient energy analysis. The effects of the suction accumulator and liquid receiver on the dynamics of the machine were also investigated. A novel lumped-parameter model combining the dynamics of the condenser and of the receiver was developed, allowing the coexistence of a non-zero subcooling at the condenser outlet and a partial filling condition of the receiver. An improvement in the prediction of the refrigerant pressure and temperature at the compressor outlet was highlighted

Modelling of small capacity absorption chillers driven by solar thermal energy or waste heat

Aquesta recerca es centra en el desenvolupament de models en règim estacionari de màquines d’absorció de petita potència, els quals estan basats en dades altament fiables obtingudes en un banc d’assajos d’última tecnologia. Aquests models podran ser utilitzats en aplicacions de simulació, o bé per a desenvolupar estratègies de control de supervisió dels sistemes d’aire condicionat amb màquines d’absorció. Per tant, l’objectiu principal d’aquesta investigació és desenvolupar i descriure una metodologia comprensible i que englobi el procés sencer: tant els assajos, com la modelització, com també el desenvolupament d’una estratègia de control per a les màquines d’absorció de petita potència. Basant-se en la informació obtinguda de forma experimental en el banc d’assajos, s’han desenvolupat cinc models, cadascun amb una base teòrica diferent. Els resultats mostren que és possible obtenir models empírics summament precisos utilitzant únicament com a paràmetres d’entrada les variables dels circuits externs d’aigua. Aquest treball finalitza amb la proposta de dues estratègies òptimes de control i el seu ús per al control on-line de sistemes basats en refredadores tèrmiques d’absorció.This research deals with the development of the simple, yet accurate steady-state models of small capacity absorption machines which are based on highly reliable data obtained in the state-of-the-art test bench. These models can further be used in simulation tools or to develop supervisory control strategies for air-conditioning systems with absorption machines. Therefore, the main aim of this research is to develop and to describe a comprehensive methodology which encloses entire process which consists of testing, modelling and control strategy development of small capacity absorption machines. Five different models are developed based on the experimental data obtained in the test bench. The results show that it is possible to develop highly accurate empirical models by using only the variables of external water circuits as input parameters. Finally, two optimal control strategies are developed to demonstrate how these models can be used for on-line control of absorption systems

Modélisation dynamique et commande optimale d'un système de réfrigération à base d'éjecteur

Recently, the ejector-based refrigeration system (ERS) has been widely used in the cooling industry as an appropriate alternative to the compressor-based cooling systems. However, the advantages of ERS such as the reliable operation and low operation and maintenance costs are overshadowed by its low efficiency and design complexity. In this context, this thesis presents the efforts to develop a control model enabling the ERS to operate in its optimal operational conditions. The extensive experimental studies of ERS revealed that at a fixed condenser inlet condition, there exists an optimal primary stream mass flow rate (generating pressure) that simultaneously maximizes the compression ratio (Cr) and exergy efficiency and minimizes the evaporating pressure. Then, the steady state models of the heat exchangers were developed and used to investigate the influence of the increase in generating pressure on the coefficient of performance (COP) of the system and it showed that increasing the generating pressure reduces the COP, linearly. In order to predict the choking regime of the ejector and explain the reasons of observed physical phenomenon, the 1D model of a fixed geometry ejector installed within an R245fa ERS was developed. The developed model demonstrated that the ejector operates in the subcritical mode when the generating pressure is below the Cr optimum point, while it operates in critical mode at or above the optimum generating pressure. Next, a dynamic model of the ERS was built to evaluate the ERS transient response to an increase in the primary stream mass flow rate. Since the ERS dynamics is mainly dominated by the thermal dynamics of the heat exchangers, the dynamic models of the heat exchangers were developed using the moving boundary approach and connected to the developed models of the ejector and steady state models of the pump and expansion valve to build a single dynamic model of the system. The built dynamic model of an ERS was used to estimate the time response of the system in the absence of accurate experimental data of the system’s dynamics. Finally, a control model was designed to drive an ERS towards its optimal operation condition. A self-optimizing, model-free control strategy known as Extremum seeking control (ESC) was adopted to minimize evaporating pressure in a fixed condenser thermal fluid inlet condition. The innovative ESC model named batch phasor ESC (BPESC) was proposed based on estimating the gradient by evaluating the phasor of the output, in batch time. The simulation results indicated that the designed BPESC model can seek and find the optimum evaporating pressure with good performance in terms of predicting the steady state optimal values and the convergence rates.Récemment, le système de réfrigération à éjecteur (SRE) a été largement utilisé dans l'industrie du refroidissement en tant que solution de remplacement appropriée aux systèmes de refroidissement à compresseur. Cependant, les avantages du SRE, tels que le fonctionnement fiable et les faibles couts d'exploitation et de maintenance, sont éclipsés par son faible rendement et sa complexité de conception. Dans ce contexte, ce projet de recherche de doctorat a détaillé les efforts déployés pour développer une stratégie de commande permettant au système de fonctionner dans ses conditions opérationnelles optimales. Les études expérimentales approfondies du SRE ont révélé que, dans une condition d'entrée de condensateur constante, il existe un débit massique optimal du flux primaire (générant une pression) qui maximise simultanément le taux de compression (Cr) et l'efficacité exergétique, et minimise la pression d’évaporation. Ensuite, les modèles à l’état d’équilibre des échangeurs de chaleur ont été développés et utilisés pour étudier l’influence de l’augmentation de la pression générée sur le coefficient de performance (COP) du système et il en ressort que l'augmentation de la pression génératrice réduit le COP de manière linéaire. Afin de prédire le régime d'étouffement de l'éjecteur et d'expliquer les raisons du phénomène physique observé, le modèle 1D d'un éjecteur à géométrie fixe installé dans un système SRE R245fa a été développé. Le modèle développé a démontré que l'éjecteur fonctionne en mode sous-critique lorsque la pression génératrice est inférieure au point optimal de Cr, alors qu'il fonctionne en mode critique à une pression égale ou supérieure à la pression génératrice optimale. Ensuite, un modèle dynamique du SRE a été développé pour étudier la réponse transitoire du SRE lors d’une augmentation du débit massique du flux primaire. Puisque la dynamique du SRE est principalement dominée par la dynamique thermique des échangeurs de chaleur, les modèles dynamiques des échangeurs de chaleur ont été développés à l'aide de l'approche des limites mobiles et connectés aux modèles développés de l'éjecteur et des modèles à l'état stationnaire de la pompe et de la vanne un seul modèle dynamique du système. En l’absence de données expérimentales précises sur la dynamique d’un système SRE, le modèle dynamique développé du SRE a été simulé numériquement pour étudier sa réponse temporelle. Enfin, une stratégie de commande extrêmale (ESC) a été élaboré pour régler automatiquement le SRE à ses conditions de fonctionnement optimales, c’est-à-dire pour trouver la vitesse de la pompe qui minimise la pression dans des conditions d'entrée de condenseur fixes. Afin de proposer une ESC implémentable en temps discret sur une installation réelle sujette à un bruit de mesure important et un traitement hors-ligne par trame, une nouvelle commande extrémale basée sur une approche par phaseur avec une procédure de traitement de signal par trame (BPESC) a été développée et simulée avec le modèle numérique. Les résultats de la simulation ont indiqué que le modèle BPESC peut trouver la vitesse optimale de la pompe avec de bonnes performances en termes de précision et de vitesse de convergence

Variable speed liquid chiller drop-in modeling for predicting energy performance of R1234yf as low-GWP refrigerant

This paper presents a model for a variable-speed liquid chiller integrating a compressor model based on Buckingham π-theorem to accurately predict the system performance when R134a is replaced with R1234yf, using a wide range of data obtained from an experimental setup. Relevant variables such as temperature, pressure, mass and volumetric flow rates, compressor power consumption and rotation speed were measured at several positions along the refrigeration and secondary circuits and were used to validate the developed model. Model results show that cooling capacity and power consumption predicted values are in good agreement with experimental data, within ±5%, being slightly higher for the deviation obtained for R134a than for R1234yf. Moreover, model results indicate that R1234yf has a reduction of coefficient of performance (COP) compared with R134a (between 2 and 11.3%), and that R1234yf COP reduction is diminished at intermediate volumetric flow rate and higher inlet temperature for the evaporator secondary fluid, respectively. On the other hand, an environmental analysis based on TEWI (total equivalent warming impact) method showed that direct emissions are almost negligible for R1234yf. However, there are no environmental benefits in terms of indirect greenhouse gas emissions using R1234yf without system modifications (as for instance the addition of internal heat exchanger or R1234yf new design components), which are required to reduce the liquid chiller climate change contribution using it as low GWP alternative in comparison with the typically used R134a refrigerant

Steady State Modeling of Advanced Vapor Compression Systems

The use of heating, ventilation, air conditioning, and refrigeration (HVAC&R) systems is always increasing. This is because the HVACR systems are necessary for food production and ability to inhabit buildings that otherwise would be inhabitable. Thus, there is continued research focused on improving the efficiency and reducing the negative environmental impact of these systems. The basic vapor compression cycle (i.e., evaporator, condenser, expansion device and compressor), which is still the main underlying HVAC&R technology worldwide, has already reached its limits and researchers are investigating more creative and complex cycles to improve capacity and efficiency. This motivates the development of an enhanced general vapor compression system steady state solver. Steady state simulations require less time than transient simulations, and are used in system design optimization and cost minimization for given performance. This paper presents a comprehensive vapor compression system steady state solver which has several novel features compared to the existing solvers. Firstly, this proposed solver is capable of simulating large number of different designs of vapor compression systems. This includes arbitrary system configurations, multiple air and refrigerant paths, and user defined refrigerants. The solver uses a component-based solution scheme in which the component models are treated as black box objects. This allows a system engineer to quickly assemble and simulate a system where in the component models and performance data comes from disparate sources. This allows different vapor compression systems design engineers, and manufacturers to use the solver without the need to expose any possible confidential component data. The solver is validated using a vapor injection heat pump system with a flash tank and the preliminary modeling results match the experimental results within 10% accuracy. This heat pump system model is also tuned in order to improve the validation accuracy. A parametric case study for a variable refrigerant flow (VRF) system is presented as well to demonstrate the applicability to larger systems

CO2 Capture by the Integrated VSA/Cryogenics method including Pipeline Transportation

This thesis proposes a hybrid technology of vacuum swing adsorption and cryogenic liquefaction to capture CO2 at high recovery with an energy penalty comparable with the traditional amine absorption process. After the capture, this thesis also proposes a pipeline transportation system which utilizes the ground/ambient temperature to retain its liquid phase. Significant amount of laboratory experiment and software simulation have been conducted and conclusions and recommendation have been provided for future research work