Non-intentionally doped GaSb epilayers were grown by molecular beam epitaxy (MBE) on highly mismatched semi-insulating GaAs substrate (001) with 2 offcut towards (110). The effects of substrate temperature and the Sb/Ga flux ratio on the crystalline quality, surface morphology and electrical properties were investigated by Nomarski optical microscopy, X-ray diffraction (XRD) and Hall measurements, respectively. Besides, differential Hall was used to investigate the hole concentration behaviour along the GaSb epilayer. It is found that the crystal quality, electrical properties and surface morphology are markedly dependent on the growth temperature and the group V/III flux ratio. Under the optimized parameters, we demonstrate a low hole concentration at very low growth temperature. Unfortunately, the layers grown at low temperature are characterized by wide FWHM and low Hall mobility.
This paper presents the results of a numerical analysis of nitride-based edge-emitting lasers with an InGaN/GaN active region designed for continuous wave room temperature emission of green and blue light. The main goal was to investigate whether the indium thin oxide (ITO) layer can serve as an effective optical confinement improving operation of these devices. Simulations were performed with the aid of a self-consistent thermal-electrical-optical model. Results obtained for green- and blue-emitting lasers were compared. The ITO layer in the p-type cladding was found to effectively help confine the laser mode in the active regions of the devices and to decrease the threshold current density.
Shale gas mining is mainly viewed as an industrial and economic issue. But we can also look at it from the scientific perspective. Why should we?
This paper presents an improved approach for locating and identifying faults for UHV overhead Transmission line by using GA-ANFIS. The proposed method uses one end data to identify the fault location. The ANFIS can be viewed either as a Fuzzy system, neural network or fuzzy neural network FNN. The integration with neural technology enhances fuzzy logic system on learning capabilities are proposed to analyze the UHV system under different fault conditions. The performance variation of two controllers in finding fault location is analyzed. This paper analyses various faults under different conditions in an UHV using Matlab/simulink. The proposed method is evaluated under different fault conditions such as fault inception angle, fault resistance and fault distance. Simulation results confirm that the proposed method can be used as an efficient for accurate fault location on the transmission line.
In this paper, dynamic response improvement of the grid connected hybrid system comprising of the wind power generation system (WPGS) and the photovoltaic (PV) are investigated under some critical circumstances. In order to maximize the output of solar arrays, a maximum power point tracking (MPPT) technique is presented. In this paper, an intelligent control technique using the artificial neural network (ANN) and the genetic algorithm (GA) are proposed to control the MPPT for a PV system under varying irradiation and temperature conditions. The ANN-GA control method is compared with the perturb and observe (P&O), the incremental conductance (IC) and the fuzzy logic methods. In other words, the data is optimized by GA and then, these optimum values are used in ANN. The results are indicated the ANN-GA is better and more reliable method in comparison with the conventional algorithms. The allocation of a pitch angle strategy based on the fuzzy logic controller (FLC) and comparison with conventional PI controller in high rated wind speed areas are carried out. Moreover, the pitch angle based on FLC with the wind speed and active power as the inputs can have faster response that lead to smoother power curves, improving the dynamic performance of the wind turbine and prevent the mechanical fatigues of the generator.
This paper concerns measurements and calculations of low frequency noise for semiconductor layers with four-probe electrodes. The measurements setup for the voltage noise cross-correlation method is described. The gain calculations for local resistance noise are performed to evaluate the contribution to total noise from different areas of the layer. It was shown, through numerical calculations and noise measurements, that in four-point probe specimens, with separated current and voltage terminals, the non-resistance noise of the contact and the resistance noise of the layer can be identified. The four-point probe method is used to find the low frequency resistance noise of the GaSb layer with a different doping type. For n-type and p-type GaSb layers with low carrier concentrations, the measured noise is dominated by the non-resistance noise contributions from contacts. Low frequency resistance noise was identified in high-doped GaSb layers (both types). At room temperature, such resistance noise in an n-type GaSb layer is significantly larger than for p-type GaSb with comparable doping concentration.
We investigated the germination requirements of Androsace villosa L. (Hairy Androsace), which spreads on limestone or granite screes or ledges of rocky or turfy slopes and hilltops of the alpine zone. With seeds collected from Uludag Mt. (Bursa, Turkey, 2200-2300 m a.s.l.), germination was studied in fresh seeds, seeds subjected to short-time moist chilling (15 d, +4°C), to GA3 (100, 150 and 250 ppm), and to chilling plus GA3. The hormone and moist chilling treatments were carried out in continuous darkness (20°C) and under a 12 h photoperiod at 20/10°C. Seeds maintained in darkness gave higher germination percentages than seeds maintained under a photoperiod. Germination rates rose to 90-97% with 100-250 ppm GA3 and short-time moist chilling in continuous darkness (20°C). Seeds germinated rapidly under a combination of GA3 and short-time moist chilling in continuous darkness, generally giving the lowest mean germination times (4.4-5.0 d) among the treatments.
As is well known, gas consumption and its prices depends on many factors including local factors, geopolitics, the development of the gas transport infrastructure (including liquefied natural gas), distribution and extraction costs – for example unconventional deposits (e.g. shale gas). The global gas market depends primarily on the economic relations between large gas producers and importers e.g. US-China, Middle East/US – Russia etc. (Olayele 2015). In individual countries, the price is also dependent on concluded contracts and delivery directions. Also it should be mentioned that the gas consumption depends on weather conditions, type of day of the year (holiday, business day, month) and economic situation (Kosowski et. al 2010). What impact has the appearance of the COVID-19 epidemic had on the European natural gas market?
The analyzed research problem concerned, in particular, two areas: gas consumption and its prices, in selected European countries in the aspect of the spread of the COVID-19 epidemic with reference to historical data from 2016–2019. Seven European countries belonging to the European Network of Transmission System Operators for Gas (ENTSOG) were selected, for which the highest inland consumption of natural gas by country was observed in the last year. The countries are presented in order or consumption: Germany, the United Kingdom, Italy, France, the Netherlands, Spain, Poland. The data has been downloaded from transmission system operators (TSOs) for each of these countries.
Furthermore the article showed information about the dates of governments restrictions (lockdown), LNG contract volumes, injection/withdrawal volumes (storage).
Describing the gas boiler fuel consumption as a time series gives the opportunity to use tools appropriate for the processing of such data to analyze this phenomenon. One of them are ARIMA models. The article proposes this type of model to be used for predicting monthly gas consumption in a boiler room working for heating and hot water preparation. The boiler supplies heat to a group of residential buildings. Based on the collected data, three specific models were selected for which the forecast accuracy was assessed. Calculations and analyses were carried out in the R environment using “forecast” and “ggplot2” packages. A good quality of the obtained forecasts has been demonstrated, confirming the usefulness of the proposed analytical tools. The article summary also indicates for what purposes the forecasts obtained in this way can be used. They can be useful for diagnosing the correct operation of a heat source. Registering fuel consumption at a level significantly deviating from the forecast should be a signal to immediately diagnose the boiler room and the heat supply system and to explain the reason for this difference. In this way, it is possible to detect irregularities in the operation of the heat supply system before they are detected by traditional methods. The gas consumption forecast is also useful for optimizing the financial management of the property manager responsible for the operation of the boiler room. On this basis, operating fees or financial operations with the use of periodic surplus capital may be planned.
The paper analyzed the natural gas sector in Ukraine for the period 2000 to 2018. This sector was affected by external factors, such as the crisis which began in late 2008/2009, as well as internal factors, including the situation in Ukraine after 2013 (the Annexation of Crimea). A comparative analysis was also conducted of the natural gas sector in European Union countries and Ukraine – compared the specificity of natural gas consumption in 2018. The analysis (I) examined the demand for natural gas in Ukraine between 2000 and 2018; (II) described changes in sources to cover Ukraine’s gas needs with a particular emphasis on its own production; (III) pointed to the fundamental changes that have occurred in the natural gas supply routes to the Ukrainian sector in recent years; (IV) stressed the growing role of own production in balancing Ukraine’s gas needs; (V) described the role of Ukraine as a transit country for Russian gas to be delivered to EU countries (in recent years, the volume of natural gas transmitted via the Ukrainian transmission system has been around 90 bcm annually); and (VI) looked at the structure of natural gas consumption in the Ukrainian gas sector and how it has changed in recent years. Unlike EU countries, the growing role of own production in balancing Ukraine’s natural gas needs was emphasized, which is consistent with the strategy of the Ukrainian government. Also, attention was drawn to the threats that may significantly reduce the role of Ukraine as an important transit country. The paper also puts forward the most important parameters concerning the underground natural gas storage facilities in Ukraine which is one of the largest in Europe.
Energy security is one of the most frequently analysed phenomena in the energy markets. Great variety of scientifc efforts should have indicated clear definition of the phenomenon. However, those studies highlighted more than 80 different definitions of what energy security really is. Due to the fact, that energy security is analyzed by different scientific disciplines, studies have provided a comperehensive reflection on the phenomenon.
The main objective of this paper is of the theoretical nature and focuses on showing energy security externalities. Author delivers an integrative review focusing on existing literature referring to the analyzed phenomena. Energy security is though studied only from the perspective of economics therefore interdisciplinary studies are out of the study scope. The reason for such scientific procedure stem from a belief that each discipline approach is different in terms of concepts, research methods and though results that are obtained. Therefore without undermining high value of interdisciplinary approaches to energy security, author decided to concentrate solely on economic perspective, which in energy security studies seems to be underestimated. Such approach in author’s belief helps achieve theoretical clarity of the below given analysis.
Presented paper is of the theoretical nature and focuses on showing energy security externalities. Critical literature review shows the literature mainstream in which energy security externalities are
Underground gas storage facilities play an important part in the maintenance of balance between the constantly imported raw material and variable gas demand in the discussed part of Europe. They also allow for more the efficient operation of businesses which exploit this raw material in this part of Europe and operators of power lines.
The following issues will be discussed in the article: types, capacity, location and variability of the filling level of underground gas storage facilities in Poland, the Czech Republic, Hungary, and Slovakia; similarities and differences in the policy of natural gas storage between individual Visegrad Group countries; the influence of these differences on the situation in the gas market; the influence of the planned further reconstruction of the natural gas storage facilities system on the energy security of individual countries which belong to the Visegrad Group.
Concern for UGSF is one of the conditions of expansion of transmission pipelines to the north and south, increase of LNG import within the Visegrad Group, or the creation of a gas hub in Poland – initiatives aimed at, among others, securing the continuity of supplies to domestic users.
However, the current and planned investments indirectly indicate that in the policies of the governments of the Visegrad Group countries, UGSF are supposed to soon play a much smaller role than many researchers would expect. An intensive expansion of UGSF is very unlikely. The scale of the state’s effect on the role of storage facilities in supplying gas to users depends on the level of the state’s control over the companies managing UGSF.
Harmonic minimisation in hybrid cascaded multilevel inverter involves complex nonlinear transcendental equation with multiple solutions. Hybrid cascaded multilevel can be implemented using reduced switch count when compared to traditional cascaded multilevel inverter topology. In this paper Biogeographical Based Optimisation (BBO) technique is applied to Hybrid multilevel inverter to determine the optimum switching angles with weighted total harmonic distortion (WTHD) as the objective function. Optimisation based on WTHD combines the advantage of both OMTHD (Optimal Minimisation of Total Harmonic Distortion) and SHE (Selective Harmonic Elimination) PWM. WTHD optimisation has the benefit of eliminating the specific lower order harmonics as in SHEPWM and minimisation of THD as in OMTHD. The simulation and experimental results for a 7 level multilevel inverter were presented. The results indicate that WTHD optimization provides both elimination of lower order harmonics and minimisation of Total Harmonic Distortion when compared to conventional OMTHD and SHE PWM. Experimental prototype of a seven level hybrid cascaded multilevel inverter is implemented to verify the simulation results.