A Review on the Driving Forces, Challenges, and Applications of AC/DC Hybrid Smart Microgrids

The majority of Medium Voltage (MV) and Low Voltage (LV) power systems are based on and operate using Alternating Current (AC) infrastructures. Yet, modern energy market needs, which promote more decentralized concepts with a high Renewable Energy Sources (RES) penetration rate and storage integration, bring Direct Current (DC) to the forefront. In this sense, AC/DC hybrid smart microgrids constitute a newly-introduced research field with a variety of potential applications that combine the benefits of both AC and DC systems. The purpose of this chapter is to review the advantages and disadvantages of AC/DC hybrid grids and analyze potential applications that would benefit from such infrastructures. Also, the most significant efforts and requirements for the constitution of a solid regulatory framework for AC/DC hybrid grids are presented, to pave the way towards their wider adoption by the market

Advanced trauma life support course for medical students. A new era?

IntroductionTrauma represents a major public health issue and is one of the leading causes of death and disability worldwide. A systematic approach toward dealing with trauma patients was facilitated through the ATLS program, which has become a milestone in trauma care. Our new ATLS course for medical students was set in motion in 2015. Our aim was to make medical students familiar with trauma patients interactively, through a program like ATLS, and here we present the results of this endeavor.MethodsA two-day ATLS-Medical Student (MS) course was offered from November 2015 to July 2018, and analysis was performed retrospectively on the data gathered over a three-month period through online questionnaires. Before graduating, 261 newly qualified medical doctors were interviewed and evaluated as part of the ATLS course.ResultsAfter the course, the vast majority of medical students (251 MSs; 96.16%) felt more capable of managing severely injured patients and 58% of students felt that the medical services they offered were better due to the ATLS training. Regarding the educational fee for the course, 56.7% of the students reported that they felt the fee of 100 euros was fair.DiscussionThe interactive format of the course, which differs from more traditional methods of teaching, has been endorsed by medical students. Though they lack clinical experience, that does not prohibit them from acquiring more specialized or specific knowledge, enabling them to excel. Most of the students improved their skillset either in theoretical knowledge, practical skills, or even in the emotional component of the course, i.e., dealing with treating a severely injured patient. It was decided that the program would be re-evaluated and extended to all Greek Medical Schools.ConclusionThe advantage of providing doctors with trauma training at the beginning of their careers is evident. For that reason, it was decided that the program would be re-evaluated and extended to all Greek Medical Schools

Traffic Accidents in Crete (1996-2006): the Role of the Emergency Coordination Center

The general decline in traffic accidents throughout Europe is not the case for Crete, a favorite holiday destination. The extent of problem and reflections on the significant impact of the interplay of organizational, educational, & technological interventions by the Emergency Coordination Center of Crete (ECC-Crete) are presented. 10-year data from 1996-2006 have been analyzed revealing demographic, topological, and qualitative issues of traffic accidents in Grete. Primary source of data is 315000 emergency calls answered by ECC-Crete. Over this 10 year period, ECC-Crete gradually employed advanced medical technologies and electronic protocol-based handling in all phases of an emergency episode contributing to its timely and effective management. GIS/GPS technology and telemetry for biosignals in ambulances, up-to-date triage protocols combined with incidence analysis provide vital information for continuous process improvement. In 2000-2006, process improvement due to technological and organizational changes has led to increased efficiency. The mean reduction was similar to 75% in dispatch time, similar to 50% in the time at accident scene for metropolitan areas, and similar to 75% in time at the emergency ward, mainly due to medical interventions on site

State of the Art of Low and Medium Voltage Direct Current (DC) Microgrids

Direct current (DC) microgrids (MG) constitute a research field that has gained great attention over the past few years, challenging the well-established dominance of their alternating current (AC) counterparts in Low Voltage (LV) (up to 1.5 kV) as well as Medium Voltage (MV) applications (up to 50 kV). The main reasons behind this change are: (i) the ascending amalgamation of Renewable Energy Sources (RES) and Battery Energy Storage Systems (BESS), which predominantly supply DC power to the energy mix that meets electrical power demand and (ii) the ascending use of electronic loads and other DC-powered devices by the end-users. In this sense, DC distribution provides a more efficient interface between the majority of Distributed Energy Resources (DER) and part of the total load of a MG. The early adopters of DC MGs include mostly buildings with high RES production, ships, data centers, electric vehicle (EV) charging stations and traction systems. However, the lack of expertise and the insufficient standards’ framework inhibit their wider spread. This review paper presents the state of the art of LV and MV DC MGs in terms of advantages/disadvantages over their AC counterparts, their interface with the AC main grid, topologies, control, applications, ancillary services and standardization issues. Overall, the aim of this review is to highlight the possibilities provided by DC MG architectures as well as the necessity for a solid/inclusive regulatory framework, which is their main weakness

Walled-off pancreatic necrosis

Walled-off pancreatic necrosis (WOPN), formerly known as pancreatic abscess is a late complication of acute pancreatitis. It can be lethal, even though it is rare. This critical review provides an overview of the continually expanding knowledge about WOPN, by review of current data from references identified in Medline and PubMed, to September 2009, using key words, such as WOPN, infected pseudocyst, severe pancreatitis, pancreatic abscess, acute necrotizing pancreatitis (ANP), pancreas, inflammation and alcoholism. WOPN comprises a later and local complication of ANP, occurring more than 4 wk after the initial attack, usually following development of pseudocysts and other pancreatic fluid collections. The mortality rate associated with WOPN is generally less than that of infected pancreatic necrosis. Surgical intervention had been the mainstay of treatment for infected peripancreatic fluid collection and abscesses for decades. Increasingly, percutaneous catheter drainage and endoscopic retrograde cholangiopancreatography have been used, and encouraging results have recently been reported in the medical literature, rendering these techniques invaluable in the treatment of WOPN. Applying the recommended therapeutic strategy, which comprises early treatment with antibiotics combined with restricted surgical intervention, fewer patients with ANP undergo surgery and interventions are ideally performed later in the course of the disease, when necrosis has become well demarcated

Flexible polyhedral Lyapunov functions for the robust constrained stabilization of bilinear boost DC-DC converters

Δημοσιεύσεις μελών--ΣΤΕΦ--Τμήμα Αυτοματισμού, 2015Recent research efforts, relying on constrained stabilization principles, have been concentrated on the design of robust and efficient state-feedback control laws for switched-mode DC-DC boost converters, characterized by accurate nonlinear dynamics incorporation, nonconservative handling of hard state and control constraints, and robustness to supply voltage variations and load changes. This paper focuses on the efficient construction of flexible polyhedral Lyapunov functions, which can facilitate control design procedures which address all aforementioned issues via the generation of contractive polytopes (safety domains). New non-conservative conditions for the averaged-model bilinear converter dynamics and a corresponding ray-gridding algorithm are proposed that allow near-maximal approximations of the real stability domains with negligible computational cost. A significant improvement in the size of the contractive domains is obtained by resorting to more flexible partial contractivity conditions. This novelty allows the extension of the constrained stabilization framework to robust constrained tracking, whereby a wide operating region may be covered. The proposed ideas can be applied for the specification of affine state-feedback control laws and are numerically evaluated on a boost converter example using the exact switched model of the converter

Automation and Operation Strategies in a Stand-Alone Power System that Uses Solar and Wind Energy in Conjunction with Hydrogen Long-Term Storage

Abstract: The description and analysis of a stand-alone power system that exploits solar and wind energy is presented in this study. More specifically, the integrated system consists of a 10kW photovoltaic (PV) array and three wind generators rated at 1kW each. A lead-acid accumulator with nominal capacity of 3,000Ah/48V is used to absorb the short energy fluctuations and to regulate the operation and integrity of the system. Furthermore, the long-term needs are fulfilled via a hydrogen based system. Hydrogen is produced by a 4.2kW PEM electrolyzer and stored in cylinders under pressure for subsequent use in a 4kW PEM fuel cell. A diesel engine can also be used to provide power in cases of subsystems failure. In the present study the infrastructure of the control system is presented along with the algorithm for the automatic operation which is used to implement the power management strategy. Moreover, experimental results from the operation of each subsystem will be given and a discussion on them will take place. Finally, two operation strategies will be applied through an one-year simulation study, in order to identify their possible implementation in the real system that currently operates under a system operator either remotely or on site

Solid State Transformers: A Critical Review of Projects with Relevant Prototypes and Demonstrators

The ongoing energy transition has changed the architecture of electricity networks in ways that conventional power transformers are not able to cope with the new required functionalities. For this purpose, the solid state transformer (SST), which comprises state of the art power electronics with galvanic isolation to interconnect two separate alternating current (AC) or direct current (DC) power grids, is considered to be the dominant solution. The purpose of this paper is to provide a practical, application-oriented review of the SST. In this context, the main functionalities and possible applications of the SST are presented, including smart grids (SGs), data centres, railways, offshore wind farms, etc. Furthermore, the main developed SST prototypes are analysed with special focus on the related projects, demonstrators, stakeholders and rated values, e.g., voltage, switching frequency and power. The analysis is concluded with the future trends and challenges regarding the wider implementation of SST technology in the electrical grid

Foldings and grazings of tori in current controlled interleaved boost converters

Δημοσιεύσεις μελών--ΣΤΕΦ--Τμήμα Αυτοματισμού, 2014Interleaved boost converters (IBCs) are used when energy conversion is required at high current levels. Such converter systems may undergo various nonlinear phenomena which can affect their performance adversely. In this paper, we study an IBC and demonstrate the first instability through a Neimark–Sacker bifurcation, resulting in a torus. An analysis based on the calculation of the monodromy matrix reveals that the torus has a rather strange form as the complex Floquet multipliers that became unstable have a real value close to 1. We show that further variation in a parameter can result in novel nonlinear phenomena where the torus itself folds and grazes a switching manifold, resulting in a ‘wobbling’ of the closed loop that represents the torus in discrete time. Numerical and analytical results validate our work. Copyright © 2013 John Wiley & Sons, Ltd

Solution-processed hydrogen molybdenum bronzes as highly conductive anode interlayers in efficient organic photovoltaics

Highly efficient and stable organic photovoltaic (OPV) cells are demonstrated by incorporating solution-processed hydrogen molybdenum bronzes as anode interlayers. The bronzes are synthesized using a sol-gel method with the critical step being the partial oxide reduction/hydrogenation using an alcohol-based solvent. Their composition, stoichiometry, and electronic properties strongly correlate with the annealing process to which the films are subjected after spin coating. Hydrogen molybdenum bronzes with moderate degree of reduction are found to be highly advantageous when used as anode interlayers in OPVs, as they maintain a high work function similar to the fully stoichiometric metal oxide, whereas they exhibit a high density of occupied gap states, which are beneficial for charge transport. Enhanced short-circuit current, open-circuit voltage and, fill factor, relative to reference devices incorporating either PEDOT-PSS or a solution processed stoichiometric molybdenum oxide, are obtained for a variety of bulk heterojunction mixtures based on different polymeric donors and fullerene acceptors. In particular, high power conversion efficiencies are obtained in devices that employed the s-HxMoO2.75 as the hole extraction layer. The incorporation of solution-processed hydrogen molybdenum bronzes as anode interlayers in organic photovoltaic cells is presented. High power conversion efficiencies are observed in devices based on polymeric donors and fullerene acceptors that include a bronze with a moderate degree of reduction, namely the s-HxMoO2.75, as the anode interlayer.</p