1,721,070 research outputs found
Levels of infant mortality in a context perspective: a focus on Africa
Full text linkThe world under-five mortality rate declined by 56 per cent, from 93 deaths per 1,000 live births
in1990 to 41 in 2016. However, Africa remains the region with the highest global burden of child mortality.
The study intends to describe the transition to lower infant mortality of most African countries in the last
decades. Applying a cluster analysis on DHS data representing the levels of infant mortality and the values of
some covariates regarding health, woman’s status and economic situation, we found four scenarios ranging
from a lowest-low to a medium development. Related to these differences, we found a reduction of infant
mortality from 96.8to 42.2, a decline of fertility from 6.0 to 4.2 birth per woman, an increase of the woman’s
median age at first marriage from 17 to 20 years, and an increase of the percentage of women with a diploma
from 13 to 43%
Implementation of reactive and resistive load matching for optimal energy harvesting from piezoelectric generators
No full textThis paper discusses the implementation of an equivalent resistive-reactive load for the optimum energy harvesting from vibration sources. In photovoltaic applications, the converter impedance that ensures the maximum power is real and the Maximum Power Point Tracker (MPPT) uses a single variable optimization algorithm (resistive load matching). The piezoelectric generator, instead, is characterized by a high capacitive impedance. Thus, the optimal power generation is obtained by a resistive and reactive load impedance (resistive and reactive load matching). Taking into account that the load matching inductor should be changed depending on the excitation frequency and it is usually impractically large if implemented by a passive element, the adaptive resistive-inductive (RL) impedance is here implemented using a bidirectional dc-dc converters and their parameters are tuned using a two-dimentional MPPT using the downhill simplex method. Simulation and experimental results obtained on a prototype, where the digital control has been implemented on a microcontroller platform, confirm the proposed analysis and the performance achievable by the proposed method. The resistive and reactive load matching strongly increases the power generation especially for excitation frequencies different from the resonant and antiresonant frequencies of the piezoelectric generator
Li-Ion Battery-Supercapacitor Hybrid Storage System for a Long Lifetime, Photovoltaic-Based Wireless Sensor Network
No full textThis paper proposes a power management architecture that utilizes both supercapacitor cells and a lithium battery as energy storages for a photovoltaic (PV)-based wireless sensor network. The supercapacitor guarantees a longer lifetime in terms of charge cycles and has a large range of operating temperatures, but has the drawback of having low energy density and high cost. The lithium battery has higher energy density but requires an accurate charge profile to increase its lifetime, feature that cannot be easily obtained supplying the wireless node with a fluctuating source as the PV one. Combining the two storages is possible to obtain good compromise in terms of energy density. A statistic analysis is used for sizing the storages and experimental results with a 5-W PV energy source are reported
ZVS isolated active clamp Sepic converter for high power LED applications
No full textThis paper analyses a ZVS isolated active clamp Sepic converter for high power LED applications. Due to the recent advancement in the light emitting diode technology, high brightness, high efficient LEDs becomes achievable in residential, industry and commercial applications to replace the incandescent bulbs, halogen bulbs, and even compact fluorescent light bulbs. Generally in these devices, the lumen is proportional to the current, so the converter has to control the LED string current, and for high power applications (greater than 100W), is preferable to have a galvanic isolation between the bus and the output; among different isolated topologies and taking into account the large input voltage variation in the application this paper is targeting, a ZVS active clamp Sepic converter has been adopted. Due to its circuit configuration, it can step up or down the input voltage, allowing a universal use, with lamps with different voltages and powers. A 300W, 5A, 48V input voltage prototype has been developed, and a peak efficiency of 91% has been reached without synchronous rectification
Supercapacitor-based hybrid storage systems for energy harvesting in wireless sensor networks
No full textThis paper investigates a power management architecture based on a hybrid accumulator system that utilizes the supercapacitor cell as storage element for energy harvesting applications. The supercapacitor guarantees a longer lifetime in terms of charge cycles, it presents itself as a ¿green¿ technology compared to batteries and it has a wide range of operating temperature. The drawbacks of this type of solution are the low energy density and the leakage current that reduce the performance for very low power applications. In this paper a photovoltaic scavenger based on the supercapacitors is investigated and it can work only with supercapacitors, or together with the lithium battery cell in order to obtain a good compromise in terms of energy density and lifetime. A dedicated power management strategy is also proposed. Experimental results with a 5 W photovoltaic energy source are reported
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
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