Energies2015, 8(9), 9655-9669; doi:10.3390/en8099655 (registering DOI) - published 4 September 2015 Show/Hide Abstract
Abstract: Magnetic components are important parts of the phase shifted full bridge (PSFB) converter. During the dead-time of switches located in the same leg, the converter can achieve zero-voltage-switching (ZVS) by using the energies stored in magnetic components to discharge or charge the output capacitances of switches. Dead-time is usually calculated under a given set of pre-defined load condition which results in that the available energies are insufficient and ZVS capability is lost at light loads. In this paper, the PSFB converter is controlled by variable dead-time method and thus full advantage can be taken of the energies stored in magnetic components. Considering that dead-time has a great effect on ZVS, the relationship between available energies and magnetic component values is formulated by analyzing the equivalent circuits during dead-time intervals. Magnetic component values are chosen based on such relationship. The proposed choice procedure can make the available energies greater than the required energies for ZVS operation over a wide range of load conditions. Moreover, the burst mode control is adopted in order to reduce the standby power loss. Experimental results coincide with the theoretical analysis. The proposed method is a simple and practical solution to extend the ZVS range.
Energies2015, 8(9), 9640-9654; doi:10.3390/en8099640 (registering DOI) - published 3 September 2015 Show/Hide Abstract
Abstract: Agricultural residues, such as corn stover, wheat straw, and nut shells show promise as feedstocks for lignocellulosic biorefinery due to their relatively high polysaccharide content and low or no nutritional value for human consumption. Apricot pit shells (APS) were studied in this work to assess their potential for use in a biorefinery. Hot water extraction (HWE; 160 °C, 2 h), proposed to remove easily accessible hemicelluloses, was performed to evaluate the susceptibility of APS to this mild pretreatment process. The chemical composition of APS before and after HWE (EAPS) was analyzed by standard methods and 1H-NMR. A low yield of the remaining HW-extracted APS (~59%) indicated that APS are highly susceptible to this pretreatment method. 1H-NMR analysis of EAPS revealed that ~77% of xylan present in raw APS was removed along with ~24% of lignin. The energy of combustion of APS was measured before and after HWE showing a slight increase due to HWE (1.61% increase). Near infrared radiation spectroscopy (NIRS), proposed as a quick non-invasive method of biomass analysis, was performed. NIRS corroborated results of traditional analysis and 1H-NMR. Determination of antioxidizing activity (AOA) of APS extracts was also undertaken. AOA of organic APS extracts were shown to be more than 20 times higher than that of a synthetic antioxidizing agent.
Energies2015, 8(9), 9620-9639; doi:10.3390/en8099620 (registering DOI) - published 3 September 2015 Show/Hide Abstract
Abstract: German electricity giants have recently taken high-level decisions to remove selected fossil fuel operations from their company portfolio. This new corporate strategy could be seen as a direct response to the growing global influence of the fossil fuel divestment campaign. In this paper we ask whether the divestment movement currently exerts significant influence on decision-making at the top four German energy giants—E.On, RWE, Vattenfall and EnBW. We find that this is not yet the case. After describing the trajectory of the global fossil fuel divestment campaign, we outline four alternative influences on corporate strategy that, currently, are having a greater impact than the divestment movement on Germany’s power sector. In time, however, clear political decisions and strong civil support may increase the significance of climate change concerns in the strategic management of the German electricity giants.
Energies2015, 8(9), 9594-9619; doi:10.3390/en8099594 (registering DOI) - published 3 September 2015 Show/Hide Abstract
Abstract: A benchmarking exercise was organized within the framework of the European Action Weather Intelligence for Renewable Energies (“WIRE”) with the purpose of evaluating the performance of state of the art models for short-term renewable energy forecasting. The exercise consisted in forecasting the power output of two wind farms and two photovoltaic power plants, in order to compare the merits of forecasts based on different modeling approaches and input data. It was thus possible to obtain a better knowledge of the state of the art in both wind and solar power forecasting, with an overview and comparison of the principal and the novel approaches that are used today in the field, and to assess the evolution of forecast performance with respect to previous benchmarking exercises. The outcome of this exercise consisted then in proposing new challenges in the renewable power forecasting field and identifying the main areas for improving accuracy in the future.
Energies2015, 8(9), 9584-9593; doi:10.3390/en8099584 - published 2 September 2015 Show/Hide Abstract
Abstract: In this paper, the hydrothermal method was adopted to synthesize nanostructure Co-doped pyrite (FeS2). The structural properties and morphology of the synthesized materials were characterized using X-ray diffraction (XRD) and scanning electron microscopy (SEM), respectively. Co in the crystal lattice of FeS2 could change the growth rate of different crystal planes of the crystal particles, which resulted in various polyhedrons with clear faces and sharp outlines. In addition, the electrochemical performance of the doping pyrite in Li/FeS2 batteries was evaluated using the galvanostatic discharge test, cyclic voltammetry and electrochemical impedance spectroscopy. The results showed that the discharge capacity of the doped material (801.8 mAh·g−1) with a doping ratio of 7% was significantly higher than that of the original FeS2 (574.6 mAh·g−1) because of the enhanced conductivity. Therefore, the doping method is potentially effective for improving the electrochemical performance of FeS2.
Energies2015, 8(9), 9565-9583; doi:10.3390/en8099565 - published 2 September 2015 Show/Hide Abstract
Abstract: Up to the present day, wood has been used to supply the needs for cooking in rural Africa. Due to the ongoing deforestation, households need to change to other energy sources. To cover this need, a large amount of people are using residues from agriculture (straw, manure) instead. However, both straw and manure also have a function in agriculture for soil improvement. Using all the straw and manure will seriously affect the food production. In this paper we first determine the amount of energy that households need for cooking (about 7 GJ per year). Then we estimate the amount of residues that can be obtained from the agricultural system and the amount of energy for cooking that can be derived from this amount when different conversion techniques are used. The amount of residues needed is strongly affected by the technology used. The traditional three stone fires require at least two times as much resource than the more advanced technologies. Up to 4 ha of land or 15 cows are needed to provide enough straw and manure to cook on the traditional three stone fires. When more efficient techniques are used (briquetting, biogas) this can be reduced to 2 ha and six cows. Due to large variation in resource availability between households, about 80% of the households own less than 2 ha and 70% holds less than four cows. This means that even when modern, energy efficient techniques are used the largest share of the population is not able to generate enough energy for cooking from their own land and/or cattle. Most rural households in Sub-Saharan Africa may share similar resource holding characteristics for which the results from the current findings on Ethiopia can be relevant.