1,720,980 research outputs found
Development and validation of direct contact gas liquid heat exchange for energy storage
Full text linkDecarbonisation of the electrical grid, necessitated by international targets to limit further
global warming, will require a steadily increasing penetration of non-dispatchable
intermittent renewable electricity generation sources. Energy storage has the potential
to substantially increase the grid’s ability to accept greater quantities of renewables
while maintaining stability. Pumped-Heat Energy Storage (PHES) is a form of electrical
energy storage targeted to provide storage on the order of days or weeks, as opposed
to short durations of storage currently available through battery technologies. PHES
systems could be utilised at substations across the country to help the grid endure diurnal
load fluctuations and periods of low wind and solar resource. This system is based
upon the Joule-Brayton cycle, which operates in the reverse direction to store exergy
and the forward direction to generate electricity. An inert gas is the cycle working
fluid, and a liquid is used to transfer heat to and from thermal exergy stores. Exergy is
stored as temperature differences from ambient in balanced hot and cold stores.
PHES development at the University of Edinburgh has iteratively explored different
system architectures, and focused on increasing confidence in components within
these architectures where there is uncertainty with regard to performance. Early work
concentrated on gas-liquid mixing within the cylinder of a compressor/expander machine,
while current work has eliminated such mixing and instead proposes the use of
large scale, direct-contact heat exchangers. Such exchangers suffer from significant
uncertainty for this application owing to the lack of existing experimental correlations
with which to predict their behaviour at the proposed operating pressure and temperature.
As a result, gas liquid surface interactions and heat-transfer between gas and
liquid streams are largely unknown, hindering system development.
Two experimental campaigns were conducted to verify components in both the
early and current system iterations. The first demonstrated a novel in-cylinder gasliquid
mixing device and quantified device behaviour against the no-mix condition.
The second campaign demonstrated operation of a scaled pilot packed-column direct
contact heat exchanger, where gas and liquid comingled to exchange heat. Existing experimental
correlations for high pressure packed column flooding were verified against
experimental results, and the overall heat exchange coefficient was calculated.
Results
were used to validate a finite volume heat transfer model based upon previous correlations.
Successful gas-liquid heat exchange in the temperature and pressure range of
interest was demonstrated, advancing PHES development and informing future iterations
of the system
Study and optimisation of ultra-high temperature thermal insulation structures with application to thermal energy storage
Full text linkToday, high capacity energy storage remains one of the major obstacles to an economical full-scale development of renewable energy technologies, without which hundred-percent decarbonised electricity grid, power and heating integrated networks will not be achievable. Unfortunately, it would not be economically viable to rely on current grid-connected storage technologies to provide enough capacity to compensate for the inherent intermittency of renewables, since most suffer from high upfront costs, low energy densities, low e ciencies, long-term degradation or inconvenient deployment location requirements. Ultra-high temperatures (>1600°C) would unlock greater energy densities and allow heat engine extraction cycles to operate at higher efficiencies, consequently improving the overall round-trip energy effciency - see, for example, an Ultra-High Temperature Thermal Storage (UHTS) [1, 2]. However with higher operating temperatures come additional energy losses which prove increasingly di cult to prevent as radiative transfers become predominant, and efficient ultrahigh temperature thermal insulators are needed.
Recently, high-temperature thermal insulation has been the focus of numerous studies which have led to the development of promising technologies including aerogels, nano fibres and ceramic fibre blankets. Of all these, ceramic brief and nano fibres have the highest maximum operating temperature of up to 1400°C with low-pressure thermal conductivities of order 0.1 W/m/K at such temperatures. However, while very efficient, such technologies would restrict the operating temperatures and storage duration of UHTS type thermal stores, and do not have sufficient structural strength. To remedy this, this doctoral project investigates the potential of insulating structures based on evacuated honeycomb geometries to both improve their thermal performance at ultra-high temperatures and raise their maximum operating temperatures to UHTS requirements whilst providing the required structural support.
First, a theory based on fundamental energy transfer equations and its numerical implementation are built to model combined conductive-radiative energy transfers in three-dimensional complex rectangular multi-media structures with obstacles and internal boundaries of any kind. Two or more media may be present, with multiple interfaces between them, and may be opaque or semi-transparent to thermal radiation so that radiative and conductive transfers may happen through participating (potentially rare ed) gaseous and solid media alike. Experimental results are then presented to validate the model where a vacuum chamber is built, a metal honeycomb heated up to 600°C, and the measured energy flows compared to numerical predictions. In a second stage, the model is applied to the design and optimisation of uniform honeycomb based ultra-high temperature thermal insulation. First, a thermal optimisation is presented during which a parametric analysis of thermal transfers in uniform honeycombs is conducted which correlates optimal geometries and thermal conductivities to a new dimensionless parameter Nrc which is shown to be an appropriate tool for the thermal characterisation of such structures. All correlations are then validated by numerical results, following which numerical optimisation experiments show that honeycomb-based insulators can retain thermal conductivities as low as 0.01 W/m/K at temperatures of 1400°C, thus largely outperforming current technologies. The case of non-uniform honeycombs is subsequently addressed, for which numerical results are analysed, and an analytical condition for optimal performance is derived and numerically validated. Mechanical loads are then introduced into the model and their effects on thermal performance are analysed, which is necessary in a UHTS context where insulators must bear a load whilst preserving optimal thermal performance. Finally, the results of the thermo-mechanical optimisation are applied to the thermal transient modelling of a UHTS type thermal store which relies on optimal honeycomb-based insulators. Its thermal storage efficiency is characterised for short and long term (seasonal) storage operation, highlighting its operating exibility and making it an essential tool for achieving fully decarbonised electricity, heat and gas networks
Triggering redox activity in a thiophene compound: radical stabilization and coordination chemistry
Full text linkThe synthesis, metalation, and redox properties of an acyclic bis(iminothienyl)methene L− are presented. This π-conjugated anion displays pronounced redox activity, undergoing facile one-electron oxidation to the acyclic, metal-free, neutral radical L* on reaction with FeBr2. In contrast, reaction of L− with CuI forms the unique, neutral Cu2I2(L*) complex of a ligand-centered radical, whereas reaction with the stronger oxidant AgBF4 forms the metal-free radical dication L*2+
Ultra-high temperature concentrated solar thermal energy
Full text linkGiven the extremely high surface temperature of the Sun (~5778 K), solar radiation has the theoretical potential, in accordance with the second law of thermodynamics, to heat a receiver on Earth up to ultra-high temperatures (specified in this thesis as >1300 K). However, there is a gap between theory and practice, as contemporary solar thermal energy systems are still limited to temperatures below 900 K due to material and mechanical limitations. Running solar thermal energy at ultra-high temperatures promises greater energy conversion efficiencies for power plants by upgrading their basic cycles to include more advanced power cycles. Furthermore, the provision of solar thermal energy at ultra-high temperatures can unlock a wide range of energy-intensive industrial applications, including hydrogen and cement production, which can contribute to decarbonising sectors which are difficult to electrify.
This thesis proposes a novel concept of an ultra-high temperature solar cavity receiver based on an optically exposed liquid metal heat transfer fluid, which flows down a corrugated back plate. The concept is investigated using a quasi-steady-state analytical energy model, in addition to a radiation-coupled Computational Fluid Dynamics (CFD) solution. The developed analysis methods are tailored to the proposed class of receivers, nonetheless, they can be generalised for broad solar receiver analysis or for analysing similar problems involving volumetric radiation absorption in other thermal applications. The concept is shown implementable at its absorptive cavity configuration with an overall (optical and thermal) receiver efficiency exceeding 70%. The proposed concept is a step towards narrowing the technological mismatch, in terms of temperature and scale, between state-of-the-art thermal energy storage and concentrated solar thermal at ultra-high temperatures.
A characterisation of prospective ultra-high temperature receivers is presented, which involved a review of state-of-the-art solar thermal technologies with the purpose of identifying the existing challenges to operating at ultra-high temperatures. Based on this characterisation, the proposed receiver is designed to address the literature concerns. The proposed receiver concept involved novel engineering features, including the use of refractory containment materials and a transparent ceramic window to seal the aperture. Therefore, the conceptual investigation attempted to address possible concerns that might be introduced by the new features. Finally, the proposed receiver is demonstrated in a concentrated solar power plant application to emphasise, using quantitative terms, the benefits of operating the receiver at ultra-high temperatures for large-scale applications
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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