1,721,001 research outputs found
Conceptual Design of a Self-Sufficient Agricultural Greenhouse
No full textEvaporative cooling systems are the currently used technology for cooling greenhouses
in arid regions where typically the greenhouse is ventilated. However, they consume a
large quantity of water to cool the air. Theoretical energy assessment investigations
and on-site operation observations have demonstrated the water saving potential of
closed greenhouses; however, the literature lacks the application of this concept in arid
regions, as well as studies related to closed greenhouse cooling and dehumidification
methods. This thesis proposes a closed greenhouse system that is temperature regulated
via an earth-air heat exchanger (EAHE) and dehumidified via a solar powered water
harvest/dehumidifier unit (PV-WH), whereby the greenhouse becomes self-sufficient in
terms of water and energy needs. Thereby, year-round crop cultivation is possible. The
results showed that EAHE are able to effectively provide cooling in summer and heating
in winter, managing to keep the greenhouse temperature within 20-30°C. At an early
planting stage (a bare-soil greenhouse), the cooling provided by the combined operation
of EAHE and fogging in a closed greenhouse was comparable to a fan-pad system in a
ventilated greenhouse in terms of temperature regulation. However, water consumption
was lower by tenfold, and it maintained a stable relative humidity (RH) of 70%. When a
fully-grown crop was included to the closed greenhouse, a drop in the air temperature
was observed and accompanied by a rise in RH. The high RH limited the fogging system
operation, and dehumidification was necessary for all seasons. Moreover, it was noticed
that the cooling provided by the EAHE in the summer season reduced the crop evapotranspiration which translates to an annual irrigation water saving of 73 kg/year/m².
The findings revealed that the dehumidified water could be harvested by the proposed
PV-WH system powered by PV-panel area 1.6 times the greenhouse floor area. The harvested water recovered 80-90% of the irrigation water, which would have been lost to
the environment if using dehumidification through active ventilation. The developed system provides a sustainable and economically feasible cultivation solution for arid regions, which is fundamental for securing agri-food for a growing population.
Keywords: Greenhouse cooling; Arid region; Water scarcity; Closed greenhouse;
Evaporative Cooling; Water Harvesting ; Solar energy ; Geothermal ; Earth-air heat
Exchanger (EAHE) ; Agriculture ; Horticultur
Numerical Investigation of Humidity Changes by Pressure Reduction Within a Rotary Vane Expander
No full textThe Rotary Vane Expander (RVE) is a positive displacement device capable of
achieving spontaneous condensation through large pressure reductions via volumetric
expansion. By exploiting this phenomenon, humidity reductions can be achieved.
The objective of this research was to develop a model that could reasonably predict
the behaviour of an RVE with a humid air working fluid in order to gain insight into
the effect of humidity on RVE performance and how condensation forms.
A numerically solved, one-dimensional mathematical model of the RVE was
developed. It considers the RVE geometry, port characteristics, internal leakages,
wall heat transfer, and humidity. The one-dimensional model approaches the phase
change problem from a psychrometric perspective.
In addition, a CFD model was also developed, with a greater focus on modelling
the condensation process. The CFD model possesses a simplified geometry and only
considers wall heat transfer. Unlike the one-dimensional model, the CFD model
approaches the phase change problem using the Lee model.
The one-dimensional model was validated against experimental data from the
literature. Good agreement was obtained for pressure evolution predictions, whilst
a more moderate agreement was obtained for exhaust temperature predictions. The
one-dimensional model suggests that humidity does not significantly impact RVE
performance. The CFD model comes to the opposite conclusion, finding that humidity
has a much greater impact. However, the impact it predicts is so large that it
raises doubts over the validity of the results. Regardless, it does suggest that the onedimensional
model may be underestimating the impact of humidity and that further
investigation is needed.
It was determined that modelling spontaneous condensation from an equilibrium
thermodynamics perspective was flawed due to the possibility of supercooled
vapour. Modelling phase change using equilibrium thermodynamics suggests a
100% humidity reduction upon encountering subzero temperatures. It is clear that
the phase change problem is a non-trivial issue and that future research will need to
use more sophisticated models
Refrigerant Selection and Compressor Performance Analysis for High Temperature Heat Pumps
No full textFull Text is available to authenticated members of The University of Auckland only.It is prevalent that a push towards a more sustainable energy future is needed and is
in action. Heat pumps (HPs) are a known technology that are able to offer an energyefficient
alternative to other heat sources, such as furnaces and electric heaters. However,
conventional HPs are only able to deliver heat up to 80°C. High temperature heat pumps
(HTHPs) are an effective technology capable of providing heat above 80°C and could
reduce the energy consumption of many residential and industrial applications. HTHPs
however, have several barriers before they are widely used in industry, which includes but
are not limited to the lack of available working fluids and suitable equipment.
This project looked into finding a promising working fluid for a HTHP and the complications
that it entailed. The chosen HTHP that was investigated was to provide a heat
delivery of 100°C from common ambient conditions; hence the condensing and evaporating
temperatures were 120 and 5°C respectively. A broad study was first completed to test
the suitability of working fluids that may or may not have been used in a refrigeration
or heat pump system before. These fluids were then screened upon their thermophysical
properties and their performance. It was found that ethanol had the highest coefficient of
performance (COP) of 2.73. Ethanol also had a few intrinsic issues, from being flammable,
having a high compression ratio, in addition to the system having to operate under vacuum
pressures.
A multi-stage compressor consisting of the cross-vane mechanism was analysed to address
these issues. The mechanism allows the introduction of intercooling between the two
compression stages as well as the isolation of vacuum pressures to the atmosphere. A
mathematical model was formed to analyse the geometry, kinematics, thermodynamics,
mass flow and power of the chambers. The model showed a mechanical efficiency of
49% with the current geometry and operating conditions. It was found that the mechanical
efficiency was significantly influenced by the geometry and the operating conditions, where
the highest mechanical efficiency was found to be 70% with the change in the end face
clearances
Investigation of a Four-Intersecting-Vane Rotary Expander in a Vapour Compression Refrigeration System
No full textRefrigeration, air conditioning, and heat pumps have contributed significantly to the growth of various industrial sectors including food, thermal control of manufacturing process, building and medicine. The vapour compression refrigeration (VCR) system is still the most commonly used system due to its simplicity. However, a significant energy loss occurs during the expansion process in the throttling device. Hence, there have been proposals to modify the expansion process to one that is isentropic to recover the expansion work. This can be achieved by using alternative technologies: an ejector or an expander. In general, ejectors are simpler to build than expanders however they require a more extensive modification to the cycle. By contrast, expanders are easier to install and are especially suitable for retrofitting. This research aims to improve the energy efficiency and performance of a subcritical VCR system by using an expander. In order to achieve this objective, a unique rotary vane mechanism having four vanes made up of two intersecting bars was adopted and developed as the expander in this study and it is the first reported example of such an expander in the literature. The geometry of the stator is not circular but is instead designed so that there is always contact between the vanes and the inner wall of the stator. This differs from most traditional rotary vane machines which use centrifugal forces and/or springs to maintain contact between the vanes and the stator. This configuration reduces the vane tip forces, although the internal leakages might increase because of the additional leakage paths.
The first set of experiments was carried out on an open system using compressed air as the working fluid. The analysis of the experimental data allowed a better understanding of the expander’s performance in static and dynamic conditions. The main parameters studied were volumetric efficiency, isentropic efficiency and output power. Dynamic tests were carried out to study the effects of different grades of lubrication on the performance of the expander at various operating conditions. The maximum volumetric efficiency was 31.2% for grade 46 oil. The highest isentropic efficiency was 31.2% for grade 46 oil and it was 45.6% for grade 22 oil. Static leakage tests were carried out to investigate the leakage characteristics. There was no leakage from the prototype’s housing. The average contributions of internal leakages through the radial clearance, vane tips, rotor slot and end face gaps were 37.2%, 33.3%, 16.2% and 14.1%, respectively. It was observed that the internal leakage paths were generally independent of each other. Mathematical and numerical models of the expander were developed and validated with the experimental data. The analytical model associates a geometrical description of the prototype, thermodynamic processes, heat transfer, forces and frictions, and radial clearance leakage. The theoretical model showed the effects of internal leakages and frictions on the performance of the expander. Three-dimensional numerical simulations were carried out using computational fluid dynamics (CFD). The effects of vane tip clearance gap sizes and expander rotational speeds on the performance of the expander were studied. Finally, a static CFD model was developed to simulate the individual leakage paths in the expander, the interaction between various operating parameters and the internal leakages, and the effects on the total efficiency.
A refurbished heat pump system with R22 as the working fluid was investigated. The relationships between the expander’s rotational speed, condensing load and degree of subcooling with the output work and expander efficiency were studied. Additionally, the thermodynamic behaviours of the system fitted with the expander or with a throttle valve were compared and discussed. The maximum expander isentropic efficiency of the prototype was 41.9% at the optimum rotational speed of 750 rpm and the Coefficient of Performance (COP) was improved by 4.16% compared to that of the system with an expansion valve. This heat pump test rig had significant pressure losses which were due to the components in the circuit such as mass flow meter, sub cooler and bends in the pipeline. This resulted in the low performance of the prototype and the system. Also, the test rig did does not have the flexibility to test how different expander cycle configurations affect the performance.
To overcome the limitation on the previous R22 heat pump system, a customised R134a vapour compression test rig was constructed to further investigate the expander, its effects on a VCR cycle and how different expander arrangements affect its performance. Locating the expander immediately after the condenser gave the best performance. The expander prototype improved the COP by 6.4% compared to the system with a throttling valve. The maximum expansion isentropic efficiency was 34.9% at 500 rpm.
In summary, this work aimed to improve the energy efficiency and performance of a subcritical VCR system using a four-intersecting-vane expander. The results show that the concept can effectively improve the COP of a system without affecting the thermodynamic behaviour significantly, suggesting its suitability for retrofitting to existing systems. Various factors may affect the general performance of the expander, including the operating conditions, lubrication system and expander arrangement in the cycle. Further optimisation of the expander’s performance is possible and this should be investigated in the future
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe 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
Full text link“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
Full text linkWe 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
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
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