Wind turbine speed control of a contactless piezoelectric wind energy harvester

ABSTRACT

Wind turbine control is an important task to make the electricity generation secure in terms of energy demand and machine safety. It also yields to control the desired power level and optimized energy because of the assignment of turbine speed. The contactless piezoelectric wind energy harvester (CPWEH) used in this study has three piezoelectric layers located around the shaft with 120 degrees apart and they are buckled by the magnetic force without any physical contact. The superiority of this device is to generate energy for low wind speeds such as 1.5 m/s. However, for high speeds, high total harmonic distortions (THDs) govern the waveforms, thus controlling the turbine speed becomes necessary for optimizing the output power. Encouraged by this, a small low inertia dc generator is coupled with the wind turbine, and the generator terminals are connected to a resistor through a power switch to generate a braking torque that opposes to wind speed direction. By controlling the switch properly, turbine speed is ensured to remain within a certain band, which accordingly prevents the turbine from rotating very fast at damaging wind speeds. Several experiments are performed on the developed CPWEH with/without the presented control scheme which prove the existence of promising performance of our proposal.     

Reusable water oxidation catalyst with dual active center for enhanced water oxidation: Iridium oxide nanoparticles immobilized on monodisperse-porous Mn5O8 microspheres

A reusable catalyst with dual active center for chemical water oxidation is synthesized for the first time by immobilization of iridium oxide nanoparticles (IrO₂ NPs) on monodisperse-porous manganese oxide microspheres acting both catalytic active center and support. Individual catalytic activity of manganese oxide microspheres is explained by multiple oxidation states of manganese which are capable of forming oxidative oxygen species. Monodisperse-porous microspheres in the form of Mn₅O₈, MnO₂ and Mn₂O₃ are used for synthesis of different catalysts and the highest activity in water oxidation is observed with the catalyst synthesized using Mn₅O₈ microspheres. The catalytic activity is correlated with the total Mn(II) and Mn(III) percentage of manganese oxide type selected for composite catalyst. The oxygen evolution up to 244 μmol is achieved in 30 min with the catalyst synthesized using Mn₅O₈ microspheres. Maximum TON and TOF numbers are obtained as 298 and 557 h^?1 with excellent reusability.     

The effect of cement alkali content on ASR susceptibility of mortars incorporating admixtures

In this study, the effects of three types of plasticizing chemical admixtures (modified lignosulfonate, sulfonated naphthalene formaldehyde and polycarboxylate based) on deleterious expansion due to alkali–silica reaction (ASR) have been investigated. Two different types of cements with low (0.53 Na₂O eq.) and high (0.98 Na₂O eq.) alkali contents, a non-reactive crushed limestone as fine aggregate and a reactive river sand were used within the scope of the experimental program. ASR tests were conducted according to accelerated mortar bar method (ASTM C 1260). Additionally the flow value, dry unit weight, capillary water absorption and compressive strength tests were performed. Test results indicated that mortars prepared with inert fine aggregate caused no significant expansion, regardless of cement type, admixture type and dosage. However, for mixes containing reactive sand, admixtures increased or decreased the expansion values (compared to plain mortars) depending on the alkali content of cement used. The magnitude of change of expansion also depended on the type and amount of admixture incorporation which have a dominant effect on stability and compactability of mortars. The high-alkali cement usually revealed the ASR expansion augmentation behaviour of admixtures. In contrast, low alkali cement decreased the expansion values compared to the control specimens.     

Investigation of the soil amplification factor in the Adapazari region

Turkey is located on the highly active Eurasian plate. A very active strike slip fault, the North Anatolian Fault Zone (NAFZ), crosses Turkey from east to west; earthquakes occurred on this fault on August 17, 1999 (Kocaeli) and November 12, 1999 (Düzce). Regional geology and subsoil conditions can significantly change the characteristics of ground motion. For this reason, determining soil amplification during an earthquake, especially for soft soils, is a very important topic for researchers. In this study, one dimensional ground response analyses were performed for selected Adapazari sites using the August 17, 1999 Kocaeli earthquake strong ground motion record with SHAKE2000 software. Soil characteristics and depth to engineering bedrock at the selected sites are different and the observed level of structural damage at these sites during the Kocaeli earthquake was also different. Calculated soil surface response spectrums at these sites were compared with the recommended design spectra of the Turkish Earthquake Code and the Eurocode 8. According to one dimensional ground response analyses, the calculated response spectra of the selected sites exceed the recommended design spectra of the Turkish Earthquake Code and the Eurocode 8. Calculations show that higher amplification ratios occur at higher periods due to soil behaviour. Results of this study indicate that local geological conditions may amplify ground motion at some periods and, due to this amplification, the calculated response spectra may exceed the recommended design spectra. Therefore, it is clear that local site conditions must be considered for earthquake-resistant engineering designs on soft alluvial soil deposits.     

An attitude scale for smart board use in education: Validity and reliability studies

The main purpose of this study was to develop an instrument to measure the attitudes of elementary students towards smart board (SB) use in education. A sequential exploratory mixed method was used. So the study started with a qualitative approach to establish the content and face validity of the scale, followed by a quantitative approach to test the construct validity and reliability of the scale items. Ten students, one teacher trainer specialized in SB use, three teachers experienced in SB use, and a language expert participated in the qualitative part. Data from 203 elementary students from two local schools in Malatya, Turkey, were used to test the psychometric properties of ‘smart board attitude scale’ (SBAS). The exploratory factor analysis (FA) yielded a two-factor model with 10 items. The goodness of fit indices produced by confirmatory FA confirmed the fit of the model to the data. Reliability of SBAS was also proved through multi-analyses.     

Fracture load and microcrack comparison of crowns manufactured from tooth‐shaped and traditional blocks

This study intended to analyze microcracks and fractographic markings on the surface of all ceramic crowns after milling and compare the fracture loads. 90 crowns were manufactured from two feldspathic (Priticrown‐Pr and Vita Mark II‐Vi) and a lithium disilicate (EmaxCAD‐Em) blocks (n = 30). Two groups (n = 15) were prepared for each ceramic. In the first group, crowns were analyzed twice via the fluorescent penetrant method for microcrack detection, after the manufacturing process and thermal cycles. The load to fracture test was applied at a crosshead speed of 0.5 mm/min until catastrophic failure. Second group crowns were directly cemented onto the Co‐Cr dies following the manufacturing process and loaded to fracture. Fractographic markings were analyzed through scanning electron microscope. Spearman correlation analysis, Kruskal–Wallis H test, Mann–Whitney U test, and Wilcoxon Signed Rank test were applied (α = .05). Fracture loads of Em crowns were higher than other groups (p < .05), with and without the aging procedure. Except for second group Pr (r = ?.532), no significant relationship was found between microcrack numbers and fracture loads (p > .05). Thermal cycling did not affect microcrack numbers and fracture loads (p > .05). Tooth‐shaped multilayered Pr blocks did not provide an advantage in terms of microcrack and fracture loads.     

Compatibility of a polycarboxylate-based superplasticiser with different set-controlling admixtures

The compatibility of specific combinations of chemical admixtures for cement pastes and mortars have been investigated. The consistency, setting time and compressive strength of cement pastes and mortars incorporating a polycarboxylate-based plasticizing admixture (PCA) with four different set-controlling admixtures (SCA); calcium formate (CF), tri-ethanolamine (TEA), mixture of calcium nitrite and nitrate salts (CNN blend) and sodium aluminate (SA) have been determined within the scope of this study. Fifty-two mortar and paste specimens with different amounts of PCA and SCA combinations have been prepared. Water/cement ratios of cement paste and mortar mixtures were 0.24 and 0.50, respectively. The paste mixtures have been prepared to determine the setting time data and the mortar mixtures have been used to determine the consistency and compressive strength values. Test results indicated that the PCA employed in this study, retards the setting time, improves consistency and reduces the early strength development of all paste and mortar mixtures at dosages greater than 0.4% by mass of cement. However, the use of different set-controlling admixtures in combination with PCA caused drastic changes in the performances of the mixtures depending on the type and amount of SCAs. It was concluded that, in order to reduce the setting time and increase the early strength of PCA incorporated mortars without causing consistency loss, the combination of PCA, and SCA should be optimized according to their types. A new 3D box-plot method has been used to determine the optimum admixture combinations for the desired properties. Maximum flow, shortest setting time and highest early strength criterion were the targets of the optimization. The most effective combination to obtain desired set acceleration and high early strength values was 1.8% PCA with 1.6% nitrite and nitrate salts based SCA by mass of cement. If consistency lost is in secondary importance, 1.8% PCA based plasticiser with 1% sodium aluminate based SCA may be an alternative combination. The proposed method can be used to optimize the targeted properties for a specific combination of cement, PCA and SCA.