1,706 research outputs found
MatchIt: Nonparametric Preprocessing for Parametric Causal Inference
No full textMatchIt implements the suggestions of Ho, Imai, King, and Stuart (2007) for improving parametric statistical models by preprocessing data with nonparametric matching methods. MatchIt implements a wide range of sophisticated matching methods, making it possible to greatly reduce the dependence of causal inferences on hard-to-justify, but commonly made, statistical modeling assumptions. The software also easily fits into existing research practices since, after preprocessing data with MatchIt, researchers can use whatever parametric model they would have used without MatchIt, but produce inferences with substantially more robustness and less sensitivity to modeling assumptions. MatchIt is an R program, and also works seamlessly with Zelig.
Merging GW with DMFT and non-local correlations beyond
No full textWe review recent developments in electronic structure calculations that go beyond state-of-the-art methods such as density functional theory (DFT) and dynamical mean field theory (DMFT). Specifically, we discuss the following methods: GW as implemented in the Vienna ab initio simulation package (VASP) with the self energy on the imaginary frequency axis, GW+DMFT, and ab initio dynamical vertex approximation (DΓA). The latter includes the physics of GW, DMFT and non-local correlations beyond, and allows for calculating (quantum) critical exponents. We present results obtained by the three methods with a focus on the benchmark material SrVO3.© The Author(s) 201
Solid-state Raman laser generating
No full textA new Raman oscillator is described based on a diode end-pumped Nd:YAG laser and an intra-cavity rubidium titanyl phosphate (RTP) crystal that serves as the active Q-switch. This compact and robust laser generates <1 ns, kilohertz pulses at 1096 nm with a 1064 nm pump and has potential for ?1480 nm eye-safe operation using a 1340 nm pump. Based on rate equation modelling a Raman gain of ?1.7–2.2 cm/GW at 1064 nm is estimated for RTP
A WEAP-MODFLOW surface water-groundwater model for the irrigated Miyandoab plain, Urmia lake basin, Iran: Multi-objective calibration and quantification of historical drought impacts
No full textThis study develops and applies the first coupled surface water-groundwater (SW-GW) flow model for the irrigated Miyandoab plain located in the Urmia basin, in the northwest of Iran. The model is implemented using a dynamic coupling between MODFLOW and WEAP and consists of spatially distributed monthly water balances for the aquifer, root-zone, rivers, canals, and reservoirs. Multi-objective calibration of the model using river discharge and GW level data yields accurate simulation of historical conditions, and results in better constrained parameters compared to using either data source alone. Model simulations show that crop water demand cannot be met during droughts due to limited GW pumping capacity, and that increased GW pumping has a relatively strong impact on GW levels due to the small specific yield of the aquifer. The SW-GW model provides a unique tool for exploring management options that sustain agricultural production and downstream flow to the shrinking Urmia Lake.Accepted author manuscriptWater Resource
Model GW study of the late transition metal monoxides
No full textThe model GW method [F. Gygi and A. Baldereschi, Phys. Rev. Lett. 62, 2160 (1989)] is an efficient simplification to the standard GW approximation which uses model dielectric function to describe the long range Coulomb interactions in semiconductors. In this work, the model GW method is used to calculate the quasiparticle band structures of MnO, FeO, CoO, and NiO. All four late transition metal monoxides are predicted to be insulators. The band gaps, magnetic moments, and quasiparticle spectra are in good agreement with the experiments, except for the satellite structures which are missing in the density of states because the model GW self-energy is static. The high accuracy of model GW is due to the usage of the accurate dielectric constants in the construction of the model dielectric functions which ensures the correct asymptotic behavior of the long range Coulomb interactions. Besides, we find that the transition metal 4s states are irrelevant to the formation of the band gaps, which supports the local approaches and the experimental interpretations of the band gaps by photoemission and electron energy loss spectroscopy, while contradicts the recent calculations by hybrid functionals, exact exchange, and one shot GW approximations. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4758986]Physics, Atomic, Molecular & ChemicalSCI(E)EIPubMed3ARTICLE1515411013
Comparing post-combustion CO2 capture operation at retrofitted coal-fired power plants in the Texas and Great Britain electric grids
No full textStuart Cohen is with UT Austin, Hannah Chalmers is with University of Edinburgh, Michael Webber is with UT Austin, and Carey King is with UT AustinThis work analyses the carbon dioxide (CO2) capture system operation within the Electric Reliability
Council of Texas (ERCOT) and Great Britain (GB) electric grids using a previously developed
first-order hourly electricity dispatch and pricing model. The grids are compared in their 2006
configuration with the addition of coal-based CO2 capture retrofits and emissions penalties from 0 to
100 US dollars per metric ton of CO2 (USD/tCO2). CO2 capture flexibility is investigated by
comparing inflexible CO2 capture systems to flexible ones that can choose between full- and zero-load
CO2 capture depending on which operating mode has lower costs or higher profits. Comparing these
two grids is interesting because they have similar installed capacity and peak demand, and both are
isolated electricity systems with competitive wholesale electricity markets. However, differences in
capacity mix, demand patterns, and fuel markets produce diverging behaviours of CO2 capture at
coal-fired power plants. Coal-fired facilities are primarily base load in ERCOT for a large range of CO2
prices but are comparably later in the dispatch order in GB and consequently often supply intermediate
load. As a result, the ability to capture CO2 is more important for ensuring dispatch of coal-fired
facilities in GB than in ERCOT when CO2 prices are high. In GB, higher overall coal prices mean that
CO2 prices must be slightly higher than in ERCOT before the emissions savings of CO2 capture offset
capture energy costs. However, once CO2 capture is economical, operating CO2 capture on half the
coal fleet in each grid achieves greater emissions reductions in GB because the total coal-based
capacity is 6 GW greater than in ERCOT. The market characteristics studied suggest greater
opportunity for flexible CO2 capture to improve operating profits in ERCOT, but profit improvements
can be offset by a flexibility cost penalty.Mechanical Engineerin
Studying the effect of strain induced birefringence on the sensitivity of FBG sensors for Guided wave measurements
No full textTo perform active structural health monitoring (SHM), guided waves (GW) have received great interest as they can inspect large areas with a few sensors and are sensitive to barely-visible structural damages. Fiber Bragg grating (FBG) sensors offer several advantages such as small size, low weight and ability to be embedded but their use has been limited for GW sensing due to their limited sensitivity while using spectrometers. FBG sensors in the edge-filtering configuration have overcome this issue with reasonable sensitivity and there is a renewed interest in their use. It is well known that when subjected to a transverse strain, the circular cross-section of the fiber deforms into an elliptical shape generating the birefringence phenomenon. This deformation, influences the coupling mode of the light inside the FBG and hence, modifies the resulting reflectivity spectrum. This paper investigates how controlled changes in the reflectivity spectrum can be introduced using different transverse loads. The effect of the modified spectrum on the sensitivity of the FBG for GW measurements is then studied. The study also investigates the effect of the transverse strain on the coupling of the GW from the structure into the fiber.Green Open Access added to TU Delft Institutional Repository 'You share, we take care!' - Taverne project https://www.openaccess.nl/en/you-share-we-take-care Otherwise as indicated in the copyright section: the publisher is the copyright holder of this work and the author uses the Dutch legislation to make this work public.Structural Integrity & Composite
EMT Real-Time Simulation Model of a 2 GW Offshore Renewable Energy Hub Integrating Electrolysers
No full textDue to their weak nature, such as low inertia, offshore energy hubs are prone to unprecedented fast dynamic phenomena. This can lead to undesired instability problems. Recent literature, with main focus on onshore systems, suggests that electrolysers could be an attractive option to support wind generators in the mitigation of balancing problems. This paper presents an Electromagnetic Transient (EMT) model for real-time simulation based study of the dynamics of active power and voltage responses of offshore hubs due to wind speed fluctuations. The purpose of this study was to ascertain the ability of an electrolyser to support an offshore energy hub under different scenarios and with different locations of the electrolyser. Two locations of Proton Exchange Membrane (PEM) electrolysers were considered: centralised (at the AC common bus of the hub) or distributed (at the DC link of the wind turbines). Numerical simulations conducted in RSCAD® on a 2 GW offshore hub with 4 × 500 MW wind power plants and 330 or 600 MW PEM electrolysers show that electrolysers can effectively support the mitigation of sudden wind speed variations, irrespective of the location. The distributed location of electrolysers can be beneficial to prevent large spillage of wind power generation during the isolation of faults within the hub.Intelligent Electrical Power Grid
Electronic properties of lanthanide oxides from the GW perspective
No full textA first-principles understanding of the electronic properties of f-electron systems is currently regarded as a great challenge in condensed-matter physics because of the difficulty in treating both localized and itinerant states on the same footing by the current theoretical approaches, most notably density-functional theory (DFT) in the local-density or generalized gradient approximation (LDA/GGA). Lanthanide sesquioxides (Ln(2)O(3)) are typical f-electron systems for which the highly localized f states play an important role in determining their chemical and physical properties. In this paper, we present a systematic investigation of the performance of many-body perturbation theory in the GW approach for the electronic structure of the whole Ln(2)O(3) series. To overcome the major failure of LDA/GGA, the traditional starting point for GW, for f-electron systems, we base our GW calculations on Hubbard U corrected LDA calculations (LDA + U). The influence of the crystal structure, the magnetic ordering, and the existence of metastable states on the electronic band structures are studied at both the LDA + U and the GW level. The evolution of the band structure with increasing number of f electrons is shown to be the origin for the characteristic structure of the band gap across the lanthanide sesquioxide series. A comparison is then made to dynamical mean-field theory (DMFT) combined with LDA or hybrid functionals to elucidate the pros and cons of these different approaches.Physics, Condensed MatterSCI(E)22ARTICLE12null8
Generic EMT Model for Real-Time Simulation of Large Disturbances in 2 GW Offshore HVAC-HVDC Renewable Energy Hubs
No full textThis paper proposes a Electro-Magnetic Transient (EMT) model of a 2 GW offshore network with the parallel operation of two Modular Multi-level Converter (MMC)—High Voltage Direct Current (HVDC) transmission links connecting four Offshore Wind Farms (OWFs) to two onshore systems, which represent a large scale power system. Additionally, to mitigate the challenges corresponding to voltage and frequency stability issues in large scale offshore networks, a Direct Voltage Control (DVC) strategy is implemented for the Type-4 Wind Generators (WGs), which represent the OWFs in this work. The electrical power system is developed in the power system simulation software RSCAD™, that is suitable for performing EMT based simulations. The EMT model of 2 GW offshore network with DVC in Type-4 WGs is successfully designed and it is well-coordinated between the control structures in MMCs and WGs.Intelligent Electrical Power Grid
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