First-principles study on electronic and structural properties of Cu(In/Ga)Se alloys for solar cells

Thin-film photovoltaic solar cells based on the Cu(In1−xGax)Se2 (CIGS) alloys have attracted more and more attention due to their large optical absorption coefficient, long term stability, low cost, and high efficiency. Modern theoretical studies of this material with first-principles calculations can provide accurate description of the electronic structure and yield results in close agreement with experimental values, but takes a large amount of calculation time. In this work, we use first-principles calculations based on the computationally affordable meta- generalized gradient approximation of the density-functional theory to investigate electronic and structural properties of the CIGS alloys. We report on the simulation of the lattice parameters and band gaps, as a function of chemical composition. The obtained results were found to be in a good agreement with the available experimental data.

Power Electronic Converters in Low-Voltage Direct Current Distribution – Analysis and Implementation

Over the recent years, smart grids have received great public attention. Many proposed functionalities rely on power electronics, which play a key role in the smart grid, together with the communication network. However, “smartness” is not the driver that alone motivates the research towards distribution networks based on power electronics; the network vulnerability to natural hazards has resulted in tightening requirements for the supply security, set both by electricity end-users and authorities. Because of the favorable price development and advancements in the field, direct current (DC) distribution has become an attractive alternative for distribution networks. In this doctoral dissertation, power electronic converters for a low-voltage DC (LVDC) distribution system are investigated. These include the rectifier located at the beginning of the LVDC network and the customer-end inverter (CEI) on the customer premises. Rectifier topologies are introduced, and according to the LVDC system requirements, topologies are chosen for the analysis. Similarly, suitable CEI topologies are addressed and selected for study. Application of power electronics into electricity distribution poses some new challenges. Because the electricity end-user is supplied with the CEI, it is responsible for the end-user voltage quality, but it also has to be able to supply adequate current in all operating conditions, including a short-circuit, to ensure the electrical safety. Supplying short-circuit current with power electronics requires additional measures, and therefore, the short-circuit behavior is described and methods to overcome the high-current supply to the fault are proposed. Power electronic converters also produce common-mode (CM) and radio-frequency (RF) electromagnetic interferences (EMI), which are not present in AC distribution. Hence, their magnitudes are investigated. To enable comprehensive research on the LVDC distribution field, a research site was built into a public low-voltage distribution network. The implementation was a joint task by the LVDC research team of Lappeenranta University of Technology and a power company Suur-Savon S¨ahk¨o Oy. Now, the measurements could be conducted in an actual environment. This is important especially for the EMI studies. The main results of the work concern the short-circuit operation of the CEI and the EMI issues. The applicability of the power electronic converters to electricity distribution is demonstrated, and suggestions for future research are proposed.

Static waves in corporate space : characterizing oscillating trading patterns in New York stock exchange

Various researches in the field of econophysics has shown that fluid flow have analogous phenomena in financial market behavior, the typical parallelism being delivered between energy in fluids and information on markets. However, the geometry of the manifold on which market dynamics act out their dynamics (corporate space) is not yet known. In this thesis, utilizing a Seven year time series of prices of stocks used to compute S&P500 index on the New York Stock Exchange, we have created local chart to the corporate space with the goal of finding standing waves and other soliton like patterns in the behavior of stock price deviations from the S&P500 index. By first calculating the correlation matrix of normalized stock price deviations from the S&P500 index, we have performed a local singular value decomposition over a set of four different time windows as guides to the nature of patterns that may emerge. I turns out that in almost all cases, each singular vector is essentially determined by relatively small set of companies with big positive or negative weights on that singular vector. Over particular time windows, sometimes these weights are strongly correlated with at least one industrial sector and certain sectors are more prone to fast dynamics whereas others have longer standing waves.

Pairs trading profitability in the Finnish stock market: A comparison between three methods

Pairs trading is an algorithmic trading strategy that is based on the historical co-movement of two separate assets and trades are executed on the basis of degree of relative mispricing. The purpose of this study is to explore one new and alternative copula-based method for pairs trading. The objective is to find out whether the copula method generates more trading opportunities and higher profits than the more traditional distance and cointegration methods applied extensively in previous empirical studies. Methods are compared by selecting top five pairs from stocks of the large and medium-sized companies in the Finnish stock market. The research period includes years 2006-2015. All the methods are proven to be profitable and the Finnish stock market suitable for pairs trading. However, copula method doesn’t generate more trading opportunities or higher profits than the other methods. It seems that the limitations of the more traditional methods are not too restrictive for this particular sample data.