With more restrictions regarding the use of refrigerants with high GWP-values, refrigeration systems that uses R744 (Carbon dioxide) as a refrigerant increases. Supermarkets are energy intense facilities where the refrigeration system utilize a significant part of the total utilized energy. Heat recovery have been proven beneficial and reduces the total energy utilization for the building.  This master thesis evaluates an existing R744 refrigeration system with heat recovery, for the purpose to find an optimal system configuration and control strategy that minimizes the operational costs. Today the refrigeration system uses a water-cooled condenser/gas cooler that during the period from February to November 2021 consumed 8 194 m3 of cooling water, that resulted in high water costs for the supermarket. With the development of a model that takes historical data as input can different system configurations and control strategies be tested and compered to the historical analysis of the existing refrigeration system. Four system configurations with different heat recovery solutions were evaluated.  The result of this study shows that the operational cost can be reduced by 31% (55 400 SEK) with a control strategy that adapts the discharge pressure to the facility’s heating demand. With maximized heat recovery the condenser/gas cooler load decreases and therefore the cooling water flow decreases too. With an additional heat exchanger in the supermarkets ventilation exhaust duct can the water consumption further be reduced, compared with similar system without an additional heat exchanger. The additional heat exchanger reduced the cooling water consumption by 1 400 m3 for the investigated period and reduced the total operational cost by 9 800 SEK. However, it is recommended to first apply the control strategy that adapts the discharge pressure to the heating demand, and from there evaluate the benefits of a heat exchanger in the exhaust duct. / Med ett allt strängare regelverk angående användandet av köldmedier med höga GWP-värden, ökar antalet kylanläggningar som använder R744 (Koldioxid) som köldmedium. Livsmedelsbutiker är energikrävande där kylsystemen är en stor del av den totala energimängd som nyttjas. Studier visar att värmeåtervinning för dessa system kan med fördel användas för att minska energianvändningen i fastigheten.  Detta masterexamensarbete utvärderar en befintlig R744-kylanläggning med värmeåtervinning, i syfte att finna en optimal systemkonfiguration och styr-strategi som minimerar driftkostnaderna. Idag används en vattenkyld kondensor/gaskylare som under perioden februari till november 2021 förbrukade 8 194 m3 kylvatten, vilket resulterade i höga vattenkostnader för butiken. Genom att utveckla en modell som använder historisk mätdata som indata kan olika systemlösningar och styr-strategier testas och jämföras med analysen av det befintliga kylsystemet. Fyra systemkonfigurationer med olika grad av värmeåtervinning utvärderades. Studiens resultat visar att driftkostnaderna kan reduceras genom att anpassa kylsystemens högtryck till fastighetens värmebehov. Genom att maximera värmeåtervinningen minskar nyttjandet av kondensorn/gaskylaren och därmed flödet av kylvatten. Med en extra värmeväxlare i butikens frånluftskanal kan vattenförbrukningen reduceras ytterligare jämfört med liknande system utan denna värmeväxlare. Dock rekommenderas att initialt nyttja en styrstrategi som anpassar högtrycket efter värmebehov, för att sedan utvärdera fördelarna med en värmeväxlare i ventilationens frånluftskanal.

When released into the atmosphere, traditional refrigerants contribute to climate change several orders of magnitude more than a corresponding amount of carbon dioxide. For that reason, an increasing amount of interest has been paid to transcritical vapor compression systems in recent years, which use carbon dioxide as a refrigerant. Vapor compression systems also impact the environment through their consumption of energy. This can be greatly increased by faulty operation. Automated techniques for detecting and diagnosing faults have been widely tested for subcritical systems, but have not been applied to transcritical systems. These methods can involve either dynamic analysis of the vapor compression cycle or a variety of algorithms based on steady state behavior. In this thesis, the viability of dynamic fault detection is tested in relation to that of static fault detection for a transcritical refrigeration system. Step tests are used to determine that transient behavior does not give additional useful information. The same tests are performed on a subcritical air-conditioner showing little value in dynamic fault detection. A static component based method of fault detection which has been applied to subcritical systems is also tested for all pairings of four faults: over/undercharge, evaporator fouling, gas cooler fouling, and compressor valve leakage. This technique allows for low cost measurement and independent detection of individual faults even when multiple faults are present. Results of this method are promising and allow distinction between faulty and fault-free behavior.

In this thesis, one of the current control algorithms for the R744 cycle, which tries tooptimize the performance of the system by two SISO control loops, is compared to acost-effective system with just one actuator. The operation of a key component of thissystem, a two stage orifice expansion valve is examined in a range of typical climateconditions. One alternative control loop for this system, which has been proposed byBehr group, is also scrutinized.The simulation results affirm the preference of using two control-loops instead of oneloop, but refute advantages of the Behr alternate control approach against one-loopcontrol. As far as the economic considerations of the A/C unit are concerned, usinga two-stage orifice expansion valve is desired by the automotive industry, thus basedon the experiment results, an improved logic for control of this system is proposed.In the second part, it is investigated whether the one-actuator control approach isapplicable to a system consisting of two parallel evaporators to allow passengers tocontrol different climate zones. The simulation results show that in the case of usinga two-stage orifice valve for the front evaporator and a fixed expansion valve forthe rear one, a proper distribution of the cooling power between the front and rearcompartment is possible for a broad range of climate conditions.

Este estudo avalia o desempenho de um sistema em cascata em operação subcrítica utilizando o par R134a/R744, como uma opção a sistemas convencionais em supermercados que normalmente utilizam R404A ou R22. O aparato experimental consiste em um compressor alternativo de velocidade variável para R744 e uma válvula de expansão eletrônica que promove a evaporação direta do CO2 dentro de uma câmara fria (2,3m x 2,6m x 2,5m) para manter a temperatura interna do ar estável. O ciclo de alta temperatura consiste em um compressor alternativo para R134a, uma válvula de expansão eletrônica, e um condensador a ar. Um trocador de calor de placas, que é ao mesmo tempo, o condensador para o R744 e o evaporador para R134a completa a configuração. Durante os testes experimentais, dois parâmetros foram manipulados: o grau de superaquecimento do R744, 515 K, e a frequência de operação do compressor R744, 45-65 Hz. Os valores da capacidade de refrigeração, para R134a/R744 variaram entre 4,30 ± 0,01 e 5,57 ± 0,02 kW, demonstrando a aplicabilidade desse sistema cascata em condições de carga térmica variável. Essa flexibilidade operacional também se estendeu aos valores estabelecidos para a temperatura do ar dentro da câmara fria, sendo o menor valor de -17,7 °C e o mais alto -0,8 °C. A fim de contribuir para a melhoria dos processos de refrigeração, principalmente no que se refere à eficiência energética, foi feito um drop-in no ciclo de alta temperatura, cuja carga de R134a foi substituída por R438A. A comparação foi feita de forma que os sistemas proporcionem capacidades de refrigeração e valores de temperatura do ar dentro da câmara fria semelhantes. Como resultado, o consumo de compressor com R438A foi maior em todos os testes. Os valores de COP para o sistema R134a/R744 variaram de 1,81 ± 0,01 a 2,09 ± 0,01, enquanto no sistema R438A/R744, de 1,41 ± 0,01 a 1,66 ± 0,01. O impacto total equivalente de aquecimento global para o sistema R438A/R744 foi maior em relação ao sistema original, devido ao maior potencial de aquecimento global do fluido R438A e ao maior consumo de potência elétrica do compressor do ciclo de alta temperatura. Logo, o refrigerante proposto no drop-in do sistema de alta temperatura de R134a por R438A provou-se uma alternativa menos eficiente, para as aplicações específicas. / This study evaluates the performance of a cascade system in subcritical operation using the pair R134a/R744, as an option to conventional systems in supermarkets, which usually uses R404A, or R22. The experimental apparatus consists of a variable speed reciprocating compressor for R744 and an electronic expansion valve that promotes direct evaporation of the CO2 inside a cold room (2,3m x 2,6m x 2,5m) to maintain the internal air temperature stable. The high-temperature cycle consists of a reciprocating compressor for R134a, an electronic expansion valve, and an air-cooled condenser. A plate heat exchanger, which is at the same time, the condenser for the R744 and evaporator to R134a completes the setup. During the experimental tests, two parameters were manipulated: The superheating degree of the R744, 5-15 K, and the R744 compressor operation frequency, 45-65 Hz. The cooling capacity values for R134a/R744 ranged between 4,30 ± 0,01 and 5,57 ± 0,02 kW, demonstrating an application to the cascade system under variable thermal load conditions. This operational flexibility also extends to the air temperature values established inside the cold room, being the lower value of -17,7 ° C and higher -0,8 ° C. In order to contribute to the improvement of the cooling processes, mainly about the energy efficiency, a drop-in has been made at the high- temperature cycle, which R134a refrigerant charge has been replaced by R438A. The comparison was done regard both systems providing similar cooling capacities and air temperature values inside the cold room. As a result, the consumption of the compressor operating with R438A was higher in all tests. The COP values for the R134a/R744 system ranged from 1,81 ± 0,01 to 2,09 ± 0,01, while with the R438A/R744 system, from 1,41 ± 0,01 to 1,66 ± 0,01. The total equivalent warming impact for the R438A/R744 system was higher compared to the original system, as a result of the higher GWP for the R438A and higher electrical power consumptions. It has been proven the proposed drop-in, at the high temperature cycle from R134a by R438A, is a less efficient choice for such specific applications. / Dissertação (Mestrado)

No description available.

Une thermofrigopompe (TFP) est une machine frigorifique qui produit du froid et de la chaleur utiles. L’objectif de ces travaux est de développer le concept de thermofrigopompe pour la production de froid et le dessalement. Le dessalement est réalisé en utilisant la chaleur rejetée au condenseur de la TFP. La technique de dessalement retenue après étude bibliographique est l’AGMD (air gap membrane distillation) pour ses températures de fonctionnement compatibles avec la température de condensation des machines frigorifiques classiques. Ce procédé de distillation a été caractérisé grâce à une installation pilote pour diverses conditions de températures, de débits, d’épaisseurs d’air gap et de compositions de solutions. Une étude de longue durée associée à une observation au MEB a permis évaluer le niveau colmatage des membranes. Un modèle numérique a ensuite été développé à partir des premiers résultats expérimentaux. Des simulations ont permis de dégager des tendances de comportement d’une machine couplée TFPMD. Enfin, un prototype a été construit à partir d’un petit réfrigérateur et d’une cellule d’AGMD fabriquée par impression 3D. Les mesures expérimentales ont permis de valider le concept de TFPMD et d’obtenir de premiers résultats de performance prometteurs. La valorisation de l’énergie thermique perdue par les équipements frigorifiques pour effectuer du dessalement semble donc une solution intéressante au manque d'eau douce qui peut survenir dans de nombreuses régions de la planète. / A heat pump or a refrigerating device produces simultaneously cooling and heating energies. The objective of this research is to develop the concept of heat pump for simultaneous cooling and desalination. Desalination is carried out by recovering the heat rejected by the condenser of the machine. The desalination technique, chosen thanks to the literature review, is AGMD (air gap membrane distillation) because of the compatibility of operating temperatures with the condensing temperature of standard heat pumps. AGMD was characterized using a pilot for different conditions of temperature, flow rate, air gap thickness and solution compositions. A long term study associated to scanning electron microscope images enabled to evaluate the fouling level of the membrane. A numerical model was then developed using the first experimental results. Simulations have revealed patterns of behaviour for a coupled heat pump and AGMD machine. Finally, a prototype was built with a small refrigerator and an AGMD cell manufactured by 3D printing. Experimental measurements were used to validate the concept of heat pump for simultaneous cooling and desalination and to obtain promising performance results. The valorization of the heat lost by refrigeration equipment for desalination seems therefore an interesting solution to overcome the lack of fresh water that can occur in many regions of the planet.

Ce travail s’inscrit dans le cadre de l’utilisation du dioxyde de carbone comme réfrigérant dans les systèmes de climatisation automobile en remplacement du HFC-134a. L’objectif est d’assurer le bon dimensionnement de l’évaporateur. Cela nécessite une étude du comportement de la vaporisation du CO2 dans les mini-canaux et la caractérisation expérimentale de l’échange de chaleur et de masse côté air assuré par des ailettes à persiennes. La vaporisation du CO2 est dominée par un régime d’ébullition nucléée offrant des coefficients d’échange très élevés et par un assèchement (disparition du film liquide en contact de la paroi) précoce. À partir de données expérimentales issues de la littérature, une méthode prédictive du coefficient d’échange de chaleur en fonction du titre en vapeur a été développée. Cette méthode combine des modèles de transfert de chaleur d’ébullition nucléée, d’évaporation convective et de post-assèchement. Du côté air de l’évaporateur, un travail expérimental a été mené pour étudier l’impact de l’humidité absolue sur la performance thermohydraulique des ailettes à persiennes. Il ressort qu’à partir d’un nombre de Reynolds relativement faible et en mode de déshumidification, l’augmentation de l’humidité absolue de l’air dégrade le coefficient d’échange de chaleur sensible alors que l’efficacité d’ailette reste quasiment inchangée. Enfin, tous ces résultats nous ont permis de développer un modèle de simulation du fonctionnement des évaporateurs à mini-canaux basé sur une discrétisation fine de l’échangeur ; c’est un outil précieux pour le dimensionnement et l’optimisation de tels échangeurs de chaleur. / This work deals with the use of carbon dioxide as refrigerant for automotive air conditioning systems in replacement of HFC-134a. The objective is to provide the right design of evaporators. For that, it is necessary to study the vaporization of CO2 inside minichannels and to characterize the heat and mass transfer on the air-side. CO2 vaporization is characterized by a predominant nucleate boiling, leading to high heat transfer coefficients, and an early dryout (disappearance of liquid film in contact with the wall). From experimental data found in the literature, a predictive method of heat transfer coefficient according to the vapor quality has been developed. This method combines heat transfer models of nucleate boiling, convective evaporation and post-dryout. On the air-side of the evaporator, an experimental work has been performed to study the impact of absolute humidity on the thermal-hydraulic performance of louvered fins. We noted that from a relatively low value of Reynolds number, in dehumidifying conditions, the increase in absolute humidity degrades the sensible heat transfer coefficient, while the fin efficiency remains almost unchanged. Finally, all these results allowed us to develop a numerical model of minichannels evaporator based on a fine discretization of the heat exchanger; it is a valuable tool for the design and optimization of such heat exchangers.