1,721,281 research outputs found
Multi-ejector R744 booster refrigerating plant and air conditioning system integration – A theoretical evaluation of energy benefits for supermarket applications
Get PDFThe multi-ejector rack is the most promising technology to push the so-called “CO2 equator” further south
and improve the global energy efficiency of R744 supermarket refrigeration systems.
This paper theoretically compares the energy consumption of a CO2 refrigerating plant equipped with a multiejector
unit with that of a R404A direct expansion system (DXS), of a conventional CO2 booster configuration
and of two CO2 solutions using parallel compression. The energy benefits related to the adoption of low
temperature (LT) overfed display cabinets were also assessed. Furthermore, various scenarios involving
different sizes of the supermarket, integration and capacity of the air conditioning (AC) system and efficiency
of the parallel compressors were investigated. The evaluations were carried out by considering different
locations in Southern Europe. The results showed that, as a function of the selected boundary conditions,
energy savings ranging from 15.6% to 27.3% could be accomplished with the multi-ejector concept over
DXS
Development and validation of CO2 cooling systems with expansion work recovery
No full textThe 2023 revised strategy of the International Maritime Organization (IMO) emphasises achieving net-zero greenhouse gas emissions within the shipping sector. Notably, nearly 40% of the energy demand in passenger ships is attributed to cooling and heating systems. Currently, these systems predominantly utilize R134a, a synthetic PFAS (per- and polyfluoroalkyl substances) chemical with a high global warming potential. In 2020, the European Union observed an approximate emission of 75,000 tonnes of PFAS into the environment. Subsequently, in 2023, a joint proposal was initiated by several member states of the EU aimed to prohibit the utilization of PFAS. If ratified, this proposition would lead to the ban of PFAS usage by either 2025 or 2026 [1, 2]. To align with existing and forthcoming environmental regulations, there is a critical need to transition towards zero-emission fuels and adopt natural refrigerants.
Within the realm of natural refrigerants, carbon dioxide (CO2) stands out due to its non-toxic and non-flammable nature, addressing safety concerns onboard passenger ships. CO2 refrigerants have gained widespread acceptance in sectors such as supermarkets, the hotel industry, process industries, and fishing vessels. Given its suitability and environmental advantages, adopting CO2-based cooling and heating systems is anticipated to witness an upward trend in passenger ships, aligning with the evolving regulatory landscape and the IMO’s overarching goal of achieving net-zero emissions.
Operating a CO2 transcritical system in high ambient temperatures presents challenges, particularly expansion losses. Over the past decade, ejectors have been widely employed to recover expansion work, but with certain limitations. The pressure exchanger (PX) is a recent advancement gaining prominence, specifically designed for expansion work recovery.
The PX device is distinct in its capability to recover expansion work from the gas cooler to the receiver pressure and employ it to compress the flash gas. With its four ports and internal rotor, the PX seamlessly expands and compresses without requiring physical separation. A pressure lift is essential for the PX to propel the flash gas into its system, which is subsequently compressed to a pressure slightly lower than the gas cooler’s. This underscores the PX’s need for two low-pressure lift devices to effectively compress the flash gas from the receiver to the gas cooler pressure. The current practice involves employing two small booster compressors to fulfil this essential role in the CO2 transcritical system.
This study investigates a novel integration concept incorporating a pressure exchanger (PX) with two innovative low-lift ejectors. Instead of relying on two booster compressors, the proposed approach leverages two ejectors to achieve the same objective. To enable the ejectors to compress flash gas from the PX compression outlet to the gas cooler pressure, a prerequisite is to maintain a compressor discharge pressure higher than the gas cooler pressure.
A numerical analysis of this innovative PX integration concept is conducted, and its performance is compared with that of standard booster, parallel, and ejector configurations. The investigation findings indicate the viability of the PX integration concept, estimating a potential performance enhancement ranging from 2% to 6% when compared to the ejector configuration.
Following the theoretical investigation, an experimental setup was constructed in the Varmeteknisk NTNU laboratory to validate the conceptual findings. For this endeavour, a retrofit was performed on the SuperSmart CO2 transcritical facility, integrating the pressure exchanger (PX), two newly introduced ejectors, and associated fittings and measuring instruments. A tailored control strategy was developed, and two PID controllers were incorporated into the existing control software to facilitate the operation of the PX mode.
Experiments were conducted to assess the cooling capacity of 70 kW with an evaporation temperature of 0 ℃ and gas cooler outlet temperatures of 33 ℃, 35 ℃, 37 ℃, and 38 ℃. The system maintained steady-state conditions for a duration of 10 minutes while recording data. The experimental outcomes have successfully validated the proof of concept, establishing a solid foundation for further exploration of the pressure exchanger (PX) and applying low-lift ejectors. Discrepancies observed between numerical predictions and experimental results are thoroughly discussed, with potential solutions highlighted for addressing these disparities.
The completion of the PhD thesis signifies the successful achievement of its objectives, encompassing the development of the pressure exchanger (PX) integration concept, numerical investigation, the establishment of an experimental setup, and the subsequent verification of the conceptual framework
Investigation of Hydrocarbon Two-phase Flow for Charge Reduced Heat Exchangers
Get PDFHydrocarbons off er an alternative to the HVAC industry as a natural working fluid with minimal environmental impact. The main challenge with hydrocarbons utilization is flammability which can be mitigated by reduction of charge in systems. Internally enhanced tubes provide a powerful tool for designing a more efficient heat exchanger leading to reductions in both volume and charge. Microfinned tubes are the most commonly utilized internally enhanced tubes that increase the heat transfer coefficient on the refrigerant fluid side in heat exchangers. Thanks to the higher heat transfer coefficients, the internal volume of the heat exchangers can be reduced.
One of the main challenges for using microffinned tubes has been the design of the heat exchangers. This is caused by a lack of reliable predictive methods. In this context, the present thesis presents experimental results for characteristics of two-phase flow of hydrocarbons. Propane (R290), isobutane (R600a), and propylene (R1270) were studied since they are commonly used in HVAC applications.
Three tubes with an outer diameter of 5 mm were tested, one smooth and two microffinned. The two microffinned tubes differed in the number of fins and helix angle, causing a different increase in the available heat exchange area. One test rig was used to obtain both condensation and evaporation characteristics, which was possible due to the design of the rapidly interchangeable test sections. In evaporation tests, the effects of fluid properties, heat flux, mass flux and saturation temperature were studied in addition to the effect of internally enhanced tubes. Condensation tests were focused on fluid properties, mass flux and internal enhancement of tubes.
Additionally, the data obtained for the heat transfer coefficient and pressure drop were compared against predictive methods to find the most reliable correlations. Finally, this data was used for numerical simulation of fin-and-tube heat exchangers in different environmental conditions to compare the charge with other types of heat exchangers
CFD Modelling for Improved Components in CO2 and Ammonia Vapour Compression Systems
No full textRefrigeration and heat-pumping systems are shifting to natural, environmentallyfriendly refrigerants like CO2 to combat global warming. Ejectors are often used in these systems for expansion work recovery to enhance efficiency. However, ejector design is complex due to interdependent parameters and flow complexity, necessitating advanced models and tools. This thesis aims to improve CO2 ejector modeling for robust design optimization and a better understanding of system operation.
To create a fundamental background for this work, key knowledge gaps on this topic are reported in an exhaustive review of CO2 two-phase ejector flow modeling. An overview of different available ejector models is reported and highlights the strengths and weaknesses of the different approaches. Other aspects, such as turbulence, non-equilibrium conditions, experimental data, and model applications are thoroughly reviewed. Different models are implemented into the ANSYS Fluent computational fluid dynamics(CFD) framework, and a comparative study of these models is performed. An experimental test campaign is conducted to validate models implemented in this work. To explore novel ejector concepts and develop improved ejector design methodologies, an algorithm for automated CFD model setup was developed to generate a database of CFD results. The Gaussian Process Regression (GPR) machine learning model is applied for modeling ejector performance trained on this database. Additionally, a numerical investigation of a novel swirl bypass concept for performance improvements of ejectors at off-design conditions is conducted.
Based on the review of current CO2 ejector models, it is found that significant discrepancies between experiments and model prediction are still found. These are typically attributed to non-equilibrium thermodynamics and turbulence modeling. Based on CFD model comparisons, it was found that stable and accurate modeling of CO2 ejectors with a numerically efficient CFD method is challenging and requires further development, especially for low motive-pressure conditions. The novel two-fluid model presented in this work can predict CO2 ejector performance with appropriate parameter selection, but potential challenges for further studies include the complexity of experimental tuning and numerical instabilities. The homogeneous equilibrium CFD model was experimentally validated and reproduced mass flow rates within 2-12% and 3-50% error for the motive and suction flow rates, respectively.
The GPR machine learning model algorithm was applied for modeling ejector performance with various ejector geometries and at various operating conditions with mean errors in entrainment ratio below 0.1 [-]. The algorithm was able to map ejector performance for off-design conditions, explore and optimize ejector designs, and predict local flow structures. The numerical investigation of the swirl bypass ejector indicated that designing such a concept is sensitive to the specific ejector design and operating conditions. A reduction in entrainment ratio of 2-20% is obtained when operating with a swirl bypass inlet. The flow structure inside the ejector with a swirl bypass is also investigated in detail.
In conclusion, it is found that CO2 ejector modeling using CFD is a valuable tool for their design. These models in combination with machine learning have been shown to be applicable for ejector design algorithms and performance mapping. Exploration of the novel ejector concepts using CFD is considered a key way to discover novel ejector improvements. Improved modeling approaches have a direct impact on design tool accuracy, and further experimental studies and numerical developments are valuable to enhance CO2 ejector models in terms of accuracy, speed, and stability
Development of cold thermal energy storage for industrial refrigeration applications
No full textRefrigeration technology is a vital part of modern society, covering multiple applications from comfort cooling and process cooling of servers in data centres, to domestic, commercial, and industrial refrigeration systems. The market for refrigeration systems is continuously expanding, and space cooling is the fastest-growing end user of electricity in buildings today. Furthermore, refrigeration is essential in the food cold chain to preserve fresh and frozen goods and to prevent the important challenge of food loss. Refrigeration technology is crucial in every link of the cold chain, all the way from the processing plants, in transport, in retail and during the final stage at the consumer. Refrigeration systems are large electricity consumers, and some of these systems face high peak refrigeration loads and the associated high electricity consumption. Some examples of applications that experience large variations in the refrigeration load during the day are process cooling for the dairy industry and industrial freezing processes in food processing plants.
Thermal energy storage (TES) technology can be applied to refrigeration systems to decouple the supply of cooling from the refrigeration system and the demand for cooling from the consumer. When TES technology is applied to store thermal energy at sub-ambient temperatures, it is often called cold TES (CTES). The two methods of achieving CTES are sensible heat storage and latent heat storage. In the former method, CTES is achieved due to the change of temperature of a storage medium, such as water. In the latter method, CTES is achieved in the phase transition of a storage medium, often by melting and solidification. A substance capable of storing large quantities of thermal energy in the solid-liquid transition is often denoted as a phase change material (PCM). Common PCM for CTES application are paraffins and various salt-water solutions.
An in-depth review of applications of CTES using PCMs in refrigeration systems was carried out to identify the current research gaps and establish the state-of-the art. It was found that the interest in PCMs for the temperature range relevant to CTES applications has been increasing in the last few years, and commercial PCMs have become available on the market. It was found that research on the implementation of CTES technology in refrigeration has been carried out for multiple applications, including food transport and packaging, commercial refrigeration and various other refrigeration systems. Common to many of these applications is the pressing need to conduct experimental investigations of promising concepts studied theoretically in the past, such as for large-scale CTES systems for industrial cooling and freezing processes using ammonia or CO2 as the refrigerant. Common ways to implement CTES in refrigeration systems in the past have consisted of using ice/water as the latent storage medium, particularly for space cooling and process cooling applications. These CTES systems are connected to the refrigeration plant by an intermediate heat transfer circuit with glycol. This concept reduces the efficiency of the refrigeration plant because the evaporation temperature must be reduced considerably compared to supplying the cooling directly at the consumer temperature. The cooling must be cascaded through the intermediate circuit and then to the consumer, while the charging process of the storage requires even lower supply temperatures. To improve the efficiency of CTES systems for refrigeration plants, it can be beneficial to integrate CTES units directly into the primary refrigerant circuit, effectively avoiding the secondary heat transfer circuit. However, the review revealed a research gap for CTES solutions suitable for industrial scale that can handle the operating pressure of refrigeration systems.
In the present research work, a novel concept for a CTES unit suitable for integration into the primary refrigerant circuit of a CO2 refrigeration system has been developed. A lab-scale demonstration unit and an experimental test facility using CO2 as the refrigerant was constructed. The novel concept is based on a special type of welded heat exchanger (HEX) plate called pillow plate (PP). The PPs are constructed of stainless steel and are stacked together to form a PP-HEX placed into a container. The container is filled with the PCM, immersing the PP-HEX into the storage medium. The PPs have flow channels inside for the refrigerant to exchange heat with the PCM outside the PPs. During the charging process of the CTES unit, the refrigerant evaporates due to heat extraction from the PCM,
which solidifies on the PP surface. During the discharging process, the heat transfer direction is reversed so that the refrigerant condenses while the PCM is melting. The CTES unit is flexible by accepting various types of PCMs and refrigerants at multiple temperature levels.
The novel CTES unit has been tested experimentally by applying two types of storage media. Water/ice was first used as the PCM to provide the proof of concept, show the feasibility of operating the CTES unit in charging and discharging cycles and provide a benchmark for future PCMs. Then, a low-temperature commercial PCM with a melting temperature of - 9.6 C, suitable for the temperature requirement in food processing plants, was experimentally characterised using established methods before being tested in the CTES unit. Various refrigerant parameters and various PP-HEX geometries were tested. The experimental test campaigns on using water/ice and the commercial PCM have shown that the evaporation and condensation temperatures of the refrigerant are the most critical parameters influencing the performance of the charging and discharging cycles of the CTES unit, respectively. It was found that the charging time was mainly affected by the refrigerant evaporation temperature, while the discharging rate and discharged energy over the cycle increased with higher refrigerant condensing temperature. Furthermore, it was found that the distance between the PPs in the PP-HEX (plate pitch) influences the discharging characteristics of the CTES unit. A smaller plate pitch resulted in high discharge rates at the cost of lower thermal storage capacity. Increasing the plate pitch improved the thermal storage capacity, but the discharging cycle length increased. Hence, the average discharge rate was reduced.
In summary, the flexible design of the CTES unit allows the designer to select a discharge characteristic of the CTES unit that matches the refrigeration load curve of the refrigeration plant by changing the plate pitch. The present research work establishes the foundation for further improvement of the concept and considerations for up-scaling and industrial implementation in the future
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Get PDF“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Get PDFWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
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