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Shahnia F.,Sustainable Power | Chandrasena R.P.S.,Curtin University Australia | Rajakaruna S.,Curtin University Australia | Ghosh A.,Queensland University of Technology
IET Generation, Transmission and Distribution

To minimise the number of load sheddings in a microgrid (MG) during autonomous operation, islanded neighbour MGs can be interconnected if they are on a self-healing network and an extra generation capacity is available in the distributed energy resources (DER) of one of the MGs. In this way, the total load in the system of interconnected MGs can be shared by all the DERs within those MGs. However, for this purpose, carefully designed self-healing and supply restoration control algorithm, protection systems and communication infrastructure are required at the network and MG levels. In this study, first, a hierarchical control structure is discussed for interconnecting the neighbour autonomous MGs where the introduced primary control level is the main focus of this study. Through the developed primary control level, this study demonstrates how the parallel DERs in the system of multiple interconnected autonomous MGs can properly share the load of the system. This controller is designed such that the converter-interfaced DERs operate in a voltage-controlled mode following a decentralised power sharing algorithm based on droop control. DER converters are controlled based on a perphase technique instead of a conventional direct-quadratic transformation technique. In addition, linear quadratic regulatorbased state feedback controllers, which are more stable than conventional proportional integrator controllers, are utilised to prevent instability and weak dynamic performances of the DERs when autonomous MGs are interconnected. The efficacy of the primary control level of the DERs in the system of multiple interconnected autonomous MGs is validated through the PSCAD/EMTDC simulations considering detailed dynamic models of DERs and converters. © The Institution of Engineering and Technology 2014. Source

Hamidi V.,Transmission and Distribution Unit | Li F.,Sustainable Power | Yao L.,AREVA
IEEE Transactions on Power Delivery

Availability of wind energy which differs across locations primarily determines the appropriate location for installing windfarms. However, the grid location of windfarms must always be considered in an attempt to accurately quantify the benefits which can be achieved from windfarms during their lifetime operation to the supply network. The value of wind power is significantly affected by their penetration and concentration, and is further affected by their location within a network. This is because the location and penetration level of wind generation will result in a significant impact on power-flow distribution across the network. The objective of this paper is to measure the impact of grid location of windfarms on economic and operational parameters of a power system in the lifetime of a windfarm project. This paper first develops an assessment tool to quantify the economic and operational impact of wind power in the grid. This is followed by developing different scenarios in which different penetrations of wind power are installed at different locations in the grid, and it shows how the value of wind power is affected by location and network constraints. © 2010 IEEE. Source

Shahnia F.,Sustainable Power | Ghosh A.,Queensland University of Technology | Ledwich G.,Queensland University of Technology | Zare F.,Danfoss A/S
International Journal of Electrical Power and Energy Systems

Voltage unbalance is a major power quality problem in low voltage residential feeders due to the random location and rating of single-phase rooftop photovoltaic cells (PV). In this paper, two different improvement methods based on the application of series (DVR) and parallel (DSTATCOM) custom power devices are investigated to improve the voltage unbalance problem in these feeders. First, based on the load flow analysis carried out in MATLAB, the effectiveness of these two custom power devices is studied vis-à-vis the voltage unbalance reduction in urban and semi-urban/rural feeders containing rooftop PVs. Their effectiveness is studied from the installation location and rating points of view. Later, a Monte Carlo based stochastic analysis is carried out to investigate their efficacy for different uncertainties of load and PV rating and location in the network. After the numerical analyses, a converter topology and control algorithm is proposed for the DSTATCOM and DVR for balancing the network voltage at their point of common coupling. A state feedback control, based on pole-shift technique, is developed to regulate the voltage in the output of the DSTATCOM and DVR converters such that the voltage balancing is achieved in the network. The dynamic feasibility of voltage unbalance and profile improvement in LV feeders, by the proposed structure and control algorithm for the DSTATCOM and DVR, is verified through detailed PSCAD/EMTDC simulations. © 2013 Elsevier Ltd. All rights reserved. Source

Sustainable Power | Date: 2014-02-26

A system for gasifying a carbonaceous feedstock, such as municipal waste, to generate power includes a devolatilization reactor that creates char from the feedstock and a gasifier that creates a product gas from both the char and from volatiles released when devolatilizing the feedstock. The product gas is reacted in a fuel cell to create electrical energy and process heat. The process heat is used to heat the devolatilization reactor and the gasifier. The gasifier comprises a plurality of configurable circuits that can each be tuned to meet the individual needs of the char material being gasified.

Crawled News Article
Site: http://phys.org/physics-news/

"This work is an important starting point for studying the material surfaces and interfaces, where many novel properties can be found. We've developed a new resource that can be used to better understand surface science and find better materials for surface-driven technologies," said Shyue Ping Ong, a nanoengineering professor at UC San Diego and senior author of the study. For example, fuel cell performance is partly influenced by the reaction of molecules such as hydrogen and oxygen on the surfaces of metal catalysts. Also, interfaces between the electrodes and electrolyte in a rechargeable lithium-ion battery host a variety of chemical reactions that can limit the battery's performance. The work in this study is useful for these applications, said Ong, who is also part of a larger effort by the UC San Diego Sustainable Power and Energy Center to design better battery materials. "Researchers can use this database to figure out which elements or materials are more likely to be viable catalysts for processes like ammonia production or making hydrogen gas from water," said Richard Tran, a nanoengineering PhD student in Ong's Materials Virtual Lab and the study's first author. Tran did this work while he was an undergraduate at UC San Diego. The work, published Sept. 13 in the journal Scientific Data, provides the surface energies and equilibrium crystal shapes of more than 100 polymorphs of 72 elements in the periodic table. Surface energy describes the stability of a surface; it is a measure of the excess energy of atoms on the surface relative to those in the bulk material. Knowing surface energies is useful for designing materials that perform their functions primarily on their surfaces, like catalysts and nanoparticles. The surface energies of some elements in their crystal form have been measured experimentally, but this is not a trivial task. It involves melting the crystal, measuring the resulting liquid's surface tension at the melting temperature, then extrapolating that value back to room temperature. This process also requires that the sample have a clean surface, which is challenging because other atoms and molecules (like oxygen and water) can easily adsorb to the surface and modify the surface energy. Surface energies obtained by this method are averaged values that lack the facet-specific resolution that is necessary for design, Ong said. "This is one of the areas where the 'virtual laboratory' can create the most value—by allowing us to precisely control the models and conditions in a way that is extremely difficult to do in experiments." Also, the surface energy is not just a single number for each crystal because it depends on the crystal's orientation. "A crystal is a regular arrangement of atoms. When you cut a crystal in different places and at different angles, you expose different facets with unique arrangements of atoms," explained Ong, who teaches the course NANO106 - Crystallography of Materials at UC San Diego. To carry out this ambitious project, Ong and his team developed highly sophisticated automated workflows to calculate surface energies from first principles. These workflows are built on the popular open-source Python Materials Genomics library and FireWorks workflow codes of the Materials Project, which were co-authored by Ong. "The techniques for calculating surface energies have been known for decades. The major accomplishment is the codification of how to generate surface models and run these complex calculations in a robust and efficient manner," Tran said. The surface model generation software code developed by the team has already been extended by others to study substrates and interfaces. Powerful supercomputers at the San Diego Supercomputer Center and the National Energy Research Scientific Computing Center at the Lawrence Berkeley National Lab were used for the calculations. Ong's team worked with researchers from the Berkeley Lab's Materials Project to develop and construct Crystalium's website. Co-founded and directed by Berkeley Lab scientist Kristin Persson, the Materials Project is a Google-like database of material properties calculated by supercomputers. "The Materials Project was designed to be an open and accessible tool for scientists and engineers to accelerate materials innovation," Persson said. "In five years, it has attracted more than 20,000 users working on everything from batteries to photovoltaics to thermoelectrics, and it's extremely gratifying to see scientists like Ong providing lots of high quality computed data of high interest and making it freely available and easily accessible to the public." The researchers pointed out that their database is the most extensive collection of calculated surface energies for elemental crystalline solids to date. Compared to previous compilations, Crystalium contains surface energies for far more elements, including both metals and non-metals, and for more facets in each crystal. The elements that have been excluded from their calculations are gases and radioactive elements. Notably, Ong and his team have validated their calculated surface energies with those from experiments, and the values are in excellent agreement. Moving forward, the team will work on expanding the scope of the database beyond single elements to multi-element compounds like alloys, which are made of two or more different metals, and binary oxides, which are made of oxygen and one other element. Efforts are also underway to study the effect of common adsorbates, such as hydrogen, on surface energies, which is key to understanding the stability of surfaces in aqueous media. "As we continue to build this database, we hope that the research community will see it as a useful resource for the rational design of target surface or interfacial properties," said Ong, Explore further: Accelerating materials discovery with world's largest database of elastic properties More information: Richard Tran et al, Surface energies of elemental crystals, Scientific Data (2016). DOI: 10.1038/sdata.2016.80 Crystalium can be accessed online at crystalium.materialsvirtuallab.org The surface energies and Wulff shapes are also available in the Materials Project at www.materialsproject.org

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