46,501 research outputs found
Assessment of the performance of commonly used DFT functionals vs. MP2 in the study of IL-Water, IL-Ethanol and IL-(H2O)3 clusters
We present a comparative study of the accuracy of different DFT approaches vs. MP2 for evaluating ionic liquids (ILs) + cosolvent. Namely, we are interested in [XBmim] + cosolvent (X being Cl−, BF4−, PF6−, and CH3SO3− anions and cosolvent being water or ethanol) and [XBmim] + (H2O)3 clusters. In this study the B3LYP, B3LYP-D3, M06, M06-2X and M06-HF functionals with Pople and Dunning basis sets are considered. We find that the influence of the basis sets is a factor to take into consideration. As already seen for weakly bonded systems when the basis set quality is low the uncorrected counterpoise (unCP) or averaging counterpoise (averCP) energies must be used due to cancellation errors. Besides, the inclusion of extra diffuse functions and polarization is also required specially in the case of ILs interacting with water clusters. The B3LYP functional does not reproduce either the structure or the interaction energies for ILs + H2O and ILs + EtOH aggregates, the energetic discrepancies being more significant than the structural ones. Among the dispersive corrected functionals, M06-2X results resemble to a great extent the reference data when the unCP interaction energies are considered for both water and ethanol. In turn, M06 and B3LYP-D3 functionals are the best option for ILs containing polar and non-polar anions, respectively, whether the averCP interactions energies are taking into consideration. From the structural point of view, B3LYP and M06 functionals describe more open structures whereas B3LYP-D3, M06-2X and M06-HF structures resemble quite well MP2 results. When the number of water molecules increases the H bonding motif gains importance and the effect depends on the underlying functional. Only M06-2X and M06-HF behaviour is similar to that observed for one water molecule. This is important because to describe ILs-cosolvent solutions is not only necessary to take into account the ILs-cosolvent interactions but also the cosolvent-cosolvent ones in the ensemble of the system.Junta de Andalucía FQM282Ministerio de Ciencia e Innovación CTQ2011-2593
The Intermediate Band Solar Cell: Progress Toward the Realization of an Attractive Concept
The intermediate band (IB) solar cell has been proposed to increase the current of solar cells while at the same time preserving the output voltage in order to produce an efficiency that ideally is above the limit established by Shockley and Queisser in 1961. The concept is described and the present realizations and acquired understanding are explained. Quantum dots are used to make the cells but the efficiencies that have been achieved so far are not yet satisfactory. Possible ways to overcome the issues involved are depicted. Alternatively, and against early predictions, IB alloys have been prepared and cells that undoubtedly display the IB behavior have been fabricated, although their efficiency is still low. Full development of this concept is not trivial but it is expected that once the development of IB solar cells is fully mastered, IB solar cells should be able to operate in tandem in concentrators with very high efficiencies or as thin cells at low cost with efficiencies above the present ones
Can Impurities be Beneficial to Photovoltaics?
The state of the art of the intermediate band solar cells is presented with emphasis on the use of impurities or alloys to form bulk intermediate band materials. Quantum dot intermediate band solar cells start to present already attractive efficiencies but many difficulties jeopardize the immediate achievement of record efficiency cells. To complement this research it is worthwhile examining bulk materials presenting an IB. Four or perhaps more materials have already proven to have it and several paths for the research of more are today open but no solar cell has yet been published based on them. This topic has already attracted many researches and abundant funds for their development worldwide
"Comment on "Thermodynamics limits of quantum photovoltaic cell efficiency" [Appl. Phys. Lett. 91, 223507 (2007)]"
This article claims the photovoltaic cell efficiency calculated using the Luque and Marti model
Reconstructing cosmic acceleration from f(R) modified gravity
A variant of the accelerating cosmology reconstruction program is developed
for gravity. Reconstruction schemes in terms of e-foldings and by using
an auxiliary scalar field are developed. An example of a model with a transient
phantom behavior without real matter is explicitly discussed in both schemes.
Detailed comparison of the two schemes of reconstruction is presented. It seems
to support physical non-equivalence of the two frames.Comment: 3 pages. Contribution to the Proceedings of the Spanish Relativity
Meeting (ERE) 2011, Madrid, Spai
Experimental advances in the next generation of solar cells
We consider next generation solar cells concepts those that have the potential to exceed the limiting efficiency calculated by Shockley and Queisser for single gap solar cells (40.7 %) and still have not been commercialized. Among these concepts, this paper deals with the multiple exciton generation (or impact ionization or multiple carrier generation) solar cell, the intermediate band solar cell and the hot carrier solar cell. These concepts were proposed theoretically more than ten years ago. In the last years, the number of experiments supporting the theories behind and paving the way towards their practical implementation has leaped forward. This work reviews these experimental advance
Formas de implementação das funções de proteção de motores de indução de média tensão
Este trabalho tem como objetivo apresentar e
comparar as diferentes formas de implementação das
funções de proteção de motores de média tensão.
Primeiramente, visando entender a finalidade de cada
uma das principais funções de proteção, o artigo apresenta
uma descrição geral das principais falhas que afetam
tais motores e como protegê-los.
Finalmente, o artigo faz o estudo comparativo das funções
de proteção disponíveis nos relés, 7SK80, SEL-710
e GE-339, fabricados, respectivamente, pela Siemens,
SEL e GE.Ingeniería Industria
Resource location based on precomputed partial random walks in dynamic networks
The problem of finding a resource residing in a network node (the
\emph{resource location problem}) is a challenge in complex networks due to
aspects as network size, unknown network topology, and network dynamics. The
problem is especially difficult if no requirements on the resource placement
strategy or the network structure are to be imposed, assuming of course that
keeping centralized resource information is not feasible or appropriate. Under
these conditions, random algorithms are useful to search the network. A
possible strategy for static networks, proposed in previous work, uses short
random walks precomputed at each network node as partial walks to construct
longer random walks with associated resource information. In this work, we
adapt the previous mechanisms to dynamic networks, where resource instances may
appear in, and disappear from, network nodes, and the nodes themselves may
leave and join the network, resembling realistic scenarios. We analyze the
resulting resource location mechanisms, providing expressions that accurately
predict average search lengths, which are validated using simulation
experiments. Reduction of average search lengths compared to simple random walk
searches are found to be very large, even in the face of high network
volatility. We also study the cost of the mechanisms, focusing on the overhead
implied by the periodic recomputation of partial walks to refresh the
information on resources, concluding that the proposed mechanisms behave
efficiently and robustly in dynamic networks.Comment: 39 pages, 25 figure
Is the Pedestrian going to Cross? Answering by 2D Pose Estimation
Our recent work suggests that, thanks to nowadays powerful CNNs, image-based
2D pose estimation is a promising cue for determining pedestrian intentions
such as crossing the road in the path of the ego-vehicle, stopping before
entering the road, and starting to walk or bending towards the road. This
statement is based on the results obtained on non-naturalistic sequences
(Daimler dataset), i.e. in sequences choreographed specifically for performing
the study. Fortunately, a new publicly available dataset (JAAD) has appeared
recently to allow developing methods for detecting pedestrian intentions in
naturalistic driving conditions; more specifically, for addressing the relevant
question is the pedestrian going to cross? Accordingly, in this paper we use
JAAD to assess the usefulness of 2D pose estimation for answering such a
question. We combine CNN-based pedestrian detection, tracking and pose
estimation to predict the crossing action from monocular images. Overall, the
proposed pipeline provides new state-of-the-art results.Comment: This is a paper presented in IEEE Intelligent Vehicles Symposium
(IEEE IV 2018
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