Electricity price forecasting accounting for renewable energies: optimal combined forecasts

Electricity price forecasting is an interesting problem for all the agents involved in electricity market operation. For instance, every profit maximisation strategy is based on the computation of accurate one-day-ahead forecasts, which is why electricity price forecasting has been a growing field of research in recent years. In addition, the increasing concern about environmental issues has led to a high penetration of renewable energies, particularly wind. In some European countries such as Spain, Germany and Denmark, renewable energy is having a deep impact on the local power markets. In this paper, we propose an optimal model from the perspective of forecasting accuracy, and it consists of a combination of several univariate and multivariate time series methods that account for the amount of energy produced with clean energies, particularly wind and hydro, which are the most relevant renewable energy sources in the Iberian Market. This market is used to illustrate the proposed methodology, as it is one of those markets in which wind power production is more relevant in terms of its percentage of the total demand, but of course our method can be applied to any other liberalised power market. As far as our contribution is concerned, first, the methodology proposed by García-Martos et al (2007 and 2012) is generalised twofold: we allow the incorporation of wind power production and hydro reservoirs, and we do not impose the restriction of using the same model for 24?h. A computational experiment and a Design of Experiments (DOE) are performed for this purpose. Then, for those hours in which there are two or more models without statistically significant differences in terms of their forecasting accuracy, a combination of forecasts is proposed by weighting the best models (according to the DOE) and minimising the Mean Absolute Percentage Error (MAPE). The MAPE is the most popular accuracy metric for comparing electricity price forecasting models. We construct the combination of forecasts by solving several nonlinear optimisation problems that allow computation of the optimal weights for building the combination of forecasts. The results are obtained by a large computational experiment that entails calculating out-of-sample forecasts for every hour in every day in the period from January 2007 to December 2009. In addition, to reinforce the value of our methodology, we compare our results with those that appear in recent published works in the field. This comparison shows the superiority of our methodology in terms of forecasting accuracy.     

Microchemical investigation on Renaissance coins minted at Gubbio (Central Italy)

The bulk and surface chemical composition of Renaissance coins minted at Gubbio (Central Italy) from 1508 to 1516 and from 1521 to 1538 by Francesco Maria della Rovere is investigated by means of the combined use of different analytical techniques such as scanning electron microscopy (SEM), energy dispersive spectrometry (EDS), and optical microscopy (OM). The aim of the work is to determine the bulk chemical composition of these commonly used coins at Gubbio, to ascertain their surface nature and if they were coated by a thin film of silver or other white metals similar to silver.The results indicate that the coins were produced by coating a copper core with a thin film of silver and antimony, and also with lead whose thickness is of a few microns which is now scarcely present because the original silvered surface was almost entirely removed by degradation phenomena. Furthermore, the SEM+EDS results show that the surface content of silver and antimony cannot be attributed to long-term selective corrosion phenomena leaving the coin slightly silver or antimony enriched. Therefore, the presence of silver or apparently silver-like metals i.e. antimony and lead, could be considered as a deliberate surface finishing of the coins obtained via inverse segregation or intentional selective corrosion based on pickling solutions or a combination of them. From a historical point of view the presence of a Ag or Sb film on the surface of the coins discloses the occurrence of a period of economic difficulties. PACS 68.55.Jk; 68.35.Dv; 68.37.Hk; 68.55.Nq; 81.05.Bx     

Hausdorff clustering of financial time series

A clustering procedure is introduced based on the Hausdorff distance as a similarity measure between clusters of elements. The method is applied to the financial time series of the Dow Jones industrial average (DJIA) index to find companies that share a similar behavior. Comparisons are made with other linkage algorithms.     

Influence of shear stress and size on viability of endothelial cells exposed to gold nanoparticles

Screening nanoparticle toxicity directly on cell culture can be a fast and cheap technique. Nevertheless, to obtain results in accordance with those observed in live animals, the conditions in which cells are cultivated should resemble the one encountered in live systems. Microfluidic devices offer the possibility to satisfy this requirement, in particular with endothelial cell lines, because they are capable to reproduce the flowing media and shear stress experienced by these cell lines in vivo. In this work, we exploit a microfluidic device to observe how human umbilical vein endothelial cells (HUVEC) viability changes when subject to a continuous flow of culture medium, in which spherical citrate-stabilized gold nanoparticles of different sizes and at varying doses are investigated. For comparison, the same experiments are also run in multiwells where the cells do not experience the shear stress induced by the flowing medium. We discuss the results considering the influence of mode of exposure and nanoparticle size (24 and 13 nm). We observed that gold nanoparticles show a lower toxicity under flow conditions with respect to static and the HUVEC viability decreases as the nanoparticle surface area per unit volume increases, regardless of size.     

Integrated approach to the characterization and conservation of artefacts of the Brazilian colonial period

Several common-use artifacts (coins, faience, cult objects, etc.) coming from two excavations sites near Rio de Janeiro, two sugar farms Historico do Rochedo and Cruzeiro, have been the subject of this investigation, which aimed to integrate physico-chemical characterizations with the identification of degradation mechanisms, and the proposal of innovative conservation methods, such as PECVD deposition of SiO₂-like films for protection of metallic artifacts. For the microchemical, micromorphological and microstructural analysis, optical microscopy-OM, X-ray diffraction XRD, X-ray fluorescence-XRF, and scanning electron microscopy equipped with electron microprobe-SEM+EDS have been employed.A cleaning step followed by a consolidation step has been carried out on glassy artefacts, while on metallic artefacts the mechanical cleaning has been followed by a stabilization treatment when necessary.For long-term protection the deposition by PECVD of SiO₂-like films has been performed on a series of copper-based alloys in a home made reactor fed with an tetraethoxysilane-oxygen-argon mixture. The coatings show a good barrier effect against the diffusion of water and oxygen from the environment to the metal surface, that increases with increasing the oxygen to monomer ratio in the feeding gas mixture and the input power density. Moreover, the coating adhesion increases if the deposition process is performed after a hydrogen plasma treatment. PACS 68.55.Jk; 68.35.Dv; 68.37.Hk; 68.55.Nq; 81.05.Bx     

Energy levels calculation for Er3+: LiYF4

The complete configuration 4f¹¹ of Er³⁺ was diagonalized with a Hamiltonian containing electrostatic, two-body and three-body configuration interaction, spin-orbit and crystal field parameters of LiYP₄. A set of 18 parameters was adjusted to fit the experimental energy levels as given by Petrov and Tkachuk.