Dissertations / Theses on the topic 'Transcritical CO2'

The interest in utilizing the energy in low‐grade heat sources and waste heat is increasing. There is an abundance of such heat sources, but their utilization today is insufficient, mainly due to the limitations of the conventional power cycles in such applications, such as low efficiency, bulky size or moisture at the expansion outlet (e.g. problems for turbine blades). Carbon dioxide (CO2) has been widely investigated for use as a working fluid in refrigeration cycles, because it has no ozonedepleting potential (ODP) and low global warming potential (GWP). It is also inexpensive, non‐explosive, non‐flammable and abundant in nature. At the same time, CO2 has advantages in use as a working fluid in low‐grade heat resource recovery and energy conversion from waste heat, mainly because it can create a better matching to the heat source temperature profile in the supercritical region to reduce the irreversibility during the heating process. Nevertheless, the research in such applications is very limited. This study investigates the potential of using carbon dioxide as a working fluid in power cycles for low‐grade heat source/waste heat recovery. At the beginning of this study, basic CO2 power cycles, namely carbon dioxide transcritical power cycle, carbon dioxide Brayton cycle and carbon dioxide cooling and power combined cycle were simulated and studied to see their potential in different applications (e.g. low‐grade heat source applications, automobile applications and heat and power cogeneration applications). For the applications in automobile industries, low pressure drop on the engine’s exhaust gas side is crucial to not reducing the engine’s performance. Therefore, a heat exchanger with low‐pressure drop on the secondary side (i.e. the gas side) was also designed, simulated and tested with water and engine exhaust gases at the early stage of the study (Appendix 2). The study subsequently focused mainly on carbon dioxide transcritical power cycle, which has a wide range of applications. The performance of the carbon dioxide transcritical power cycle has been simulated and compared with the other most commonly employed power cycles in lowgrade heat source utilizations, i.e. the Organic Rankin Cycle (ORC). Furthermore, the annual performance of the carbon dioxide transcritical power cycle in utilizing the low‐grade heat source (i.e. solar) has also been simulated and analyzed with dynamic simulation in this work. Last but not least, the matching of the temperature profiles in the heat exchangers for CO2 and its influence on the cycle performance have also been discussed. Second law thermodynamic analyses of the carbon dioxide transcritical power systems have been completed. The simulation models have been mainly developed in the software known as Engineering Equation Solver (EES)1 for both cycle analyses and computer‐aided heat exchanger designs. The model has also been connected to TRNSYS for dynamic system annual performance simulations. In addition, Refprop 7.02 is used for calculating the working fluid properties, and the CFD tool (COMSOL) 3 has been employed to investigate the particular phenomena influencing the heat exchanger performance.
QC 20111205

La domanda di energia nel mondo è in continuo aumento e, contemporaneamente, l'aumento dell'inquinamento ambientale spinge verso una diminuzione dell'utilizzo di combustibili fossili. Per questo motivo, oggi è fondamentale sia lo sviluppo di nuove fonti energetiche pulite, sia il recupero di calore di scarto da varie applicazioni che, in passato, veniva perso. Lo scopo di questo lavoro è un'analisi termodinamica di impianti ORC e impianti a CO2 transcritica, entrambi usati per il recupero di calore di scarto industriale a bassa temperatura (=200°C). In particolare, verranno analizzati due diversi layout per ogni tipologia di impianto: il layout semplice e quello recuperato. Le quattro simulazioni son state sviluppate in Matlab, implementando le equazioni di bilancio entalpico per ogni ciclo, in modo tale da poter calcolare le performance termodinamiche e, dunque, poter confrontare i diversi layout, in modo da scegliere quello che assicura le migliori prestazioni. Le quattro simulazioni son state svolte considerando un set di parametri di progetto, in linea con lo stato dell'arte attuale, e per diversi valori della pressione massima del ciclo, in modo tale da trovarne il valore che massimizzi l'efficienza termodinamica. In seguito, son stati confrontati tutti i valori calcolati, così da scegliere l'impianto più adatto al recupero di calore di scarto a bassa temperatura. Successivamente, le simulazioni son state svolte con un diverso valore della temperatura di uscita dei fumi e della temperatura di uscita dell'acqua di raffreddamento, per motivi di impatto ambientale. In seguito, i cicli son stati valutati per diversi valori della temperatura di ingresso dei fumi caldi e per diversi valori del rendimento isoentropico di turbina, così da valutare l'effetto di una variazione di tali parametri sul rendimento termodinamico di ogni ciclo.

Cette étude consiste en l’expérimentation de mélanges de fluides frigorigènes à base de CO2 dans les applications de pompes à chaleur domestiques. L’objectif est l’obtention de performances supérieures à une pompe à chaleur au CO2 double-service (eau chaude sanitaire et chauffage). L’ajout d’autres composés au CO2 déplace le point critique et de façon plus générale modifie les lignes d’équilibre de phases. On peut alors s’attendre à la réduction des pressions de fonctionnement et à une augmentation du rendement énergétique du cycle thermodynamique. Une pompe à chaleur mono-étagée au CO2 est utilisée comme référence, et les conditions de température externes à la boucle thermodynamique sont invariantes. Deux scénarii sont considérés : La production d’eau chaude sanitaire (ECS), où l’eau est chauffée de 10 °C à 65 °C ; la production d’eau de chauffage (CH) où l’au est chauffée de 30 °C à 35 °C. Dans les deux cas, l’eau glycolée en l’entrée de l’évaporateur est régulée à 7 °C. Afin de pouvoir analyser le comportement des cycles thermodynamiques avec mélanges, il est essentiel de connaitre la composition du fluide frigorigène en circulation. Pour ce faire, nous avons mis au point une technique de mesure statistique et non-intrusive de la composition: Des cellules optiques installées sur les tubes de la boucle permettent de recueillir les spectres d’absorption du fluide en circulation, grâce à un spectromètre proche infrarouge à transformée de Fourier. Un étalonnage méticuleux est effectué via l’acquisition de nombreux spectres d’échantillons ayant des compositions connues. Un modèle statistique est alors créé pour lier les concentrations aux données spectrales. Les compositions peuvent ensuite être estimées à partir de nouveau spectres, dont l’acquisition rapide permet l’analyse de la composition du fluide même en fonctionnement dynamique de la pompe à chaleur. Des mélanges de CO2 & propane, et CO2 & R-1234yf ont été testés, montrant des potentiels d’amélioration des performances de la pompe à chaleur pour les applications de chauffage des locaux
The aim of this work is to experiment CO2 based mixtures as working fluids for heat pump applications in buildings, in order to enhance their performances compared to pure CO2 dual services heat pumps. Since adding other chemicals to CO2 moves the critical point and generally equilibrium lines, it is expected that lower operating pressures as well as higher global efficiencies can be reached. A simple stage pure CO2 cycle is used as reference, with fixed external conditions. Two scenarios are considered: water is heated from 10 °C to 65 °C for Domestic Hot Water (DHW) scenario and from 30 °C to 35 °C for Central Heating (CH) scenario. In both cases, water at the evaporator inlet is set at 7 °C to account for such outdoor temperature conditions. In order to understand the dynamic behaviour of thermodynamic cycles with mixtures, it is essential to measure the fluid circulating composition. To this end, we have developed a non intrusive method. Online optical flow cells allow the recording of infrared spectra by means of a Fourier Transform Infra Red spectrometer. A careful calibration is performed by measuring a statistically significant number of spectra for samples of known composition. Then, a statistical model is constructed to relate spectra to compositions. After calibration, compositions are obtained by recording the spectrum in few seconds, thus allowing for a dynamic analysis. Mixtures of CO2 & propane and CO2 & R-1234yf have been tested and showed great potential on enhancing performances of the heat pump for central heating applications

Les fluides naturels employés en réfrigération et en conditionnement d’air possèdent de faibles PRG et sont de ce fait une véritable alternative aux HFC. Cependant, leur généralisation se heurte à des limites provenant de leur caractère toxique (NH3), inflammable (hydrocarbures, NH3) ou de leurs caractéristiques thermodynamiques défavorables (CO2). Leur utilisation accrue nécessite la mise en œuvre de composants spécifiques (échangeurs de chaleur intermédiaire, éjecteur) qui sans qui les performances seraient inférieures à celles obtenues avec les HFC (COPCO2 = 55 % du COPHFC-134a pour des températures de sources de 0 °C et 40 °C). L’utilisation d’un éjecteur comme organe de détente est une solution envisagée pour réduire les irréversibilités. Les éjecteurs diphasiques constituent une alternative intéressante pour les dispositifs de détente classiques utilisés depuis plusieurs décennies. Le principal avantage de l’éjecteur est de récupérer une partie de l’énergie cinétique du processus de détente de la haute pression à la basse pression pour augmenter la pression d’aspiration du compresseur. Ceci entraîne une diminution du travail consommé par ce dernier et, par suite, une augmentation du coefficient de performance du système. Néanmoins, une bonne conception d’un éjecteur diphasique nécessite une analyse détaillée en termes de simulations numériques et travaux expérimentaux. Ainsi, l’objectif de ce travail est d’apporter une contribution expérimentale à l’étude des machines frigorifiques au CO2 transcritique équipées d’éjecteur diphasique. Des efforts importants ont été investis dans la conception d’un éjecteur diphasique avec diverses géométries pour évaluer les principales caractéristiques à savoir le facteur d’entraînement et le rapport de compression. Les essais effectués ont permis de mettre en évidence l’influence des différents paramètres géométriques sur les performances de la machine (différents diamètres au col des tuyères primaires, différents diamètres de mélangeurs, longueurs de mélangeurs, distance entre le plan de sortie de la tuyère primaire et l’entrée du mélangeur, l’angle de divergent des tuyères primaires…) ainsi que les paramètres thermodynamiques (température d’évaporation, température à l’entrée de la tuyère primaire)
Natural refrigerants used in refrigeration and air conditioning have low GWP and are therefore a real alternative to HFCs. However, their generalization comes up against limits due to their toxic (NH3), flammable (hydrocarbons, NH3) or their unfavorable thermodynamic characteristics (CO2). Their increased use requires the implementation of specific components (intermediate heat exchangers, ejector) which without performance would be lower than those obtained with HFCs (COPCO2 = 55% of COPHFC-134a for temperatures source of 0 °C and 40 °C). The use of an ejector as an expansion device is a solution considered to reduce irreversibility. Two-phase ejector has been an interesting alternative for conventional expansion devices for several decades. The main advantage of the ejector is to recover some of the kinetic energy of the process of expansion from high pressure to low pressure to increase the suction pressure of the compressor. This results in a reduction of the work consumed by the latter and, consequently, an increase in the coefficient of performance of the system. Nevertheless, a good design of a two-phase ejector requires a detailed analysis in terms of numerical simulations and experimental work. Thus, the objective of this work is to make an experimental contribution to the study of transcritical CO2refrigeration machines equipped with two-phase ejector. Significant efforts have been invested in the design of a two-phase ejector with various geometries to evaluate the main characteristics namely the entrainment ratio and the compression ratio. The tests carried out made it possible to highlight the influence of the various geometrical parameters on the performances of the machine (different diameters of the throat of the primary nozzle, different mixers diameters and lengths, distance between the exit of the primary nozzle and the inlet of the mixer, the divergence angle of the primary nozzles ...) as well as the thermodynamic parameters (evaporation temperature, temperature at the inlet of the primary nozzle)

L’adozione di refrigeranti a basso impatto ambientale sintetici, HFO, o naturali, CO2 e NH3, in sostituzione dei refrigeranti tradizionali potrebbe ridurre le emissioni dirette in atmosfera. Il settore della refrigerazione commerciale sta progressivamente sostituendo i refrigeranti tradizionali con l’anidride carbonica, R744. La CO2, come refrigerante, presenta delle proprietà termodinamiche che collimano con i fabbisogni commerciali, essendo un fluido non infiammabile, non esplosivo, non tossico e con GWP ed ODP rispettivamente pari ad 1 e a 0. Le problematiche impiantistiche relative all’adozione dell’anidride carbonica sono dovute alle elevate pressioni di lavoro spesso maggiori della pressione critica pari a 73,8 bar infatti, per la realizzazione del ciclo frigorifero, la pressione massima tende ad incrementarsi ad elevate temperature esterne, peggiorando l’efficienza e le condizioni operative. Nonostante gli impianti transcritici abbiano delle prestazioni energetiche maggiori degli impianti tradizionali a basse temperature esterne, la loro efficienza, in termini di COP, si degrada sensibilmente con temperature crescenti. Il miglioramento del COP potrebbe contribuire alla promozione ed all’adozione della CO2 in aree che hanno generalmente condizioni climatiche sfavorevoli, riducendo le emissioni indirette e l’impatto ambientale degli impianti tradizionali. Nella sala motori “C. Caputo” dell’Università degli Studi dell’Aquila, in collaborazione con la società Epta Refrigeration S.p.A., è stata installata, collaudata ed avviata una centrale transcritica a CO2 con una potenza nominale agli evaporatori di 18 kWt ed ai compressori di 17 kWe. La possibilità di sperimentare questo sistema di refrigerazione innovativo consente di monitorare la variazione dei principali parametri termodinamici a diverse temperature esterne, permettendo il calcolo del COP e la ricostruzione modellistica dei cicli frigoriferi transcritici. L’estesa campagna sperimentale effettuata rappresenta la situazione di riferimento su cui implementare modelli teorici di efficientamento energetico, supportati dal software di calcolo NIST-RefProp. I modelli riportano incrementi percentuali del COP fino al 50%, giustificando una validazione sperimentale delle configurazioni proposte.

Biochemistry
Ph.D.
Antigen presentation to T cells results in their activation through T Cell Receptor (TCR) stimulation, resulting in sustained elevation of cytosolic Ca2+ concentration critical for T cell activation. Sustained Ca2+ signals are important for the activation of Nuclear Factor of Activated T cells (NFAT), which is a key regulator of T cell activation through its transcriptional control of genes in multiple process including cytokine production, proliferation and differentiation(Rao, Luo, & Hogan, 1997). Recently it was shown that Stromal Interaction Molecule 1 (STIM1) inhibits plasma membrane Ca2+/ATPase 4 (PMCA4) function during T cell activation resulting in sustained elevation of Ca2+ signals(Ritchie, Samakai, & Soboloff, 2012). This interaction requires upregulation of both STIM1 and PMCA4. In this thesis, I hypothesize that changes in Ca2+ signals arising from transcriptional changes of STIM1 and PMCA are important for the efficient activation of T cells. In the first part of this thesis, I assess the transcriptional regulation of STIM1 and PMCA4. My in vitro studies show that expression of both proteins is regulated by the EGR family members, EGR1 and EGR4. Additionally, transcriptional regulation of PMCA inhibition by EGR1 and EGR4 is required for efficient activation of T cells. Interestingly, whereas significant roles for EGR1, EGR2 and EGR3 in T cell development and function have been established, a role for EGR4 has not, hitherto been elucidated. In the second half of this thesis, using qPCR, I reveal that EGR4 expression is stimulated by TCR engagement in primary double positive, CD4 and CD8 positive murine T cells. Further, EGR4-null mice exhibit shifts in early thymic development, although this does not affect the relative number of double or single positive T cells in the thymus. Interestingly, EGR4-null primary T cells exhibit normal Ca2+ entry, but fail to exhibit activation-induced inhibition of Ca2+ clearance. Although not all subsets of EGR1 and EGR4 null primary T cells exhibited decreased STIM1 expression, significant defects in proliferation, migration and/or cytokine production were observed upon stimulation in all populations, albeit to different extents. These findings reveal a two-faceted role in which EGRs regulate T cell development and function through both Ca2+-dependent and independent methods. I believe that these findings have important implications towards the general understanding of transcriptional control of Ca2+ signaling, as well as having a possible impact in the quest to advance therapies targeting immunological disorders.
Temple University--Theses

博士
國立交通大學
機械工程系所
107
In this thesis, a constrains-free transcritical CO2 heat pump model for medium and large system applications and a transient heat pump for clothes drying applications are developed. On each model, the detailed geometric characteristics of the major component are taken into account and, unlike existing models existing in literature applicable for CO2 system, the developed models do not impose constraints upon simulation, such as fixed operating pressures and constant temperatures. Yet, pressure optimization is also addressed and heat rejection pressure is modulated through a recent sophisticated generalized dimensionless log-linear correlation of the Poisson type. The models are tested against experimental data for a wide range of operating conditions and the results accurately reflected an actual system with a maximum error of 9.6% and 3.9% for the coefficient of performance and heating capacity, respectively. Moreover, the simulation results of both models are discussed and substantiated in the context of experimental results reported in literature. Yet, the generalized correlation employed for pressure optimization is also tested, validated and discussed thorough comparison to experimental data and other correlations available from literature. The correlation can accurately predict heat rejection pressure with an average error of 1.31% and a standard deviation of 4.26 bar, with a valid range of applicability for ambient temperatures from -18 to 50 °C, and within -7 to 15 °C and 10 to 50 °C for the evaporator and gas cooler outlet, respectively. Once the transcritical heat pump model, pressure optimization method and heat pump dryer models are validated, a fully transient heat pump dryer model is developed and simulated using CO2 and then R-134a, for comparison purposes. A standard performance test is carried out and employed later as the reference case when investigating the influence of relevant parameters affecting the overall performance of the system for both refrigerants. Subsequently, comprehensive parametric studies are conducted to identify relevant parameters influencing system performance. In this regard, simulations for both systems (CO2 and R-134a) are conducted for comparison purposes. Additionally, guidelines and control strategies to optimize system performance and highlight the advantages of transcritical CO2 systems are provided.

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.