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Summarization: An extensive utilization of wireless sensor networks has evolved during the last years for monitoring various environmental and artificial processes. When operating in remote locations, the nodes of wireless sensor networks are typically power supplied by an energy production and management system, comprising low-power renewable energy sources, a power electronic converter, and a battery-based energy storage unit. In this paper, a methodology is proposed for optimally designing the energy production and processing system of a wireless sensor network node simultaneously at both the renewable power-supply system level and the power converter circuit level, through a unified design process. The impact of the objective function type on the power-supply design is also investigated in this paper. Design optimization and experimental results are presented, which demonstrate that the optimized power-supply structures derived by applying the proposed optimization technique exhibit lower cost of generated energy compared to partially optimized or totally nonoptimized structures and by that reduce the cost of the overall wireless sensor network node. Presented on: IEEE Transactions on Industrial Informatics

This work described the operational capabilities of a platform developed to assist decision makers to determine the optimal location and configuration of desalination plants to be powered by renewable energy sources. The platform, called DESiRES, operates under an open source geographical information system interface. Interoperability with geostatictical and optimization design algorithm modules was achieved via the location based information provided by the geographical module. The current stage of DESiRES platform development (i.e., study areas, data used, etc.) was presented along with the basic platform functionalities, showing a great potential for design and assessment of desalination in the Mediterranean region.

Nowadays, the management of water is of paramount importance for modern societies due to the high water-availability requirements. The application of water management schemes requires the installation of water level data-acquisition systems in multiple, geographically isolated large-scale storage tanks of water distribution networks. Existing techniques for liquid level sensing have either been applied over a relatively small measurement range, or require special scientific equipment of high cost, or they are not convenient for transportation, installation and long-term maintenance in multiple large-scale water storage tanks of water distribution networks in cities, communities, etc. In this paper, a review of prior art on liquid level sensing is initially presented. Then, the operational characteristics and performance of a novel capacitive-type water level measurement system are investigated through simulations and experimental tests conducted in two water storage tanks of a city-scale water distribution network. It is demonstrated that the proposed capacitive water level measurement system achieves equivalent performance with that of a commercially-available ultrasound water-level sensing device and simultaneously exhibits a much lower manufacturing cost.

ThermoElectric Generators (TEGs) are capable to harvest the ambient thermal energy for power-supplying sensors, actuators, biomedical devices etc. in the μW up to several hundreds of Watts range. In this paper, a Maximum Power Point Tracking (MPPT) method for TEG elements is proposed, which is based on controlling a power converter such that it operates on a pre-programmed locus of operating points close to the MPPs of the power–voltage curves of the TEG power source. Compared to the past-proposed MPPT methods for TEGs, the technique presented in this paper has the advantage of operational and design simplicity. Thus, its implementation using off-the-shelf microelectronic components with low-power consumption characteristics is enabled, without being required to employ specialized integrated circuits or signal processing units of high development cost. Experimental results are presented, which demonstrate that for MPP power levels of the TEG source in the range of 1–17 mW, the average deviation of the power produced by the proposed system from the MPP power of the TEG source is 1.87%.

Nowadays, wind energy conversion systems (WECSs) are widely employed in stand-alone systems for providing power to isolated loads, as well as in distributed generation systems, microgrids, and smart grids. In all of these applications, appropriate energy management processes must be performed for maximizing the energy produced by the wind turbines and transfer the generated energy to the consumer with high efficiency and required power quality. In this chapter, an overview of modern energy management techniques for WECS is presented, in terms of both the hardware architecture employed and the control methods that are applied.

—A substantial growth of the installed photovoltaic systems capacity has occurred around the world during the last decade, thus enhancing the availability of electric energy in an environmentally friendly way. The maximum power point tracking technique enables maximization of the energy production of photovoltaic sources during stochastically varying solar irradiation and ambient temperature conditions. Thus, the overall efficiency of the photovoltaic energy production system is increased. Numerous techniques have been presented during the last decade for implementing the maximum power point tracking process in a photovoltaic system. This article provides an overview of the operating principles of these techniques, which are suited for either uniform or non-uniform solar irradiation conditions. The operational characteristics and implementation requirements of these maximum power point tracking methods are also analyzed to demonstrate their performance features.

Transformerless PV inverters are known to generate undesirable ground leakage currents due to the lack of galvanic isolation between the PV array and the electric grid. This paper initially discusses the different types of transformerless PV inverter topologies that have been proposed to overcome this problem. It then proposes a new topology, which extends the commercially successful HERIC inverter with the addition of a mid-DC-link clamping configuration. Such configurations are known to cause a highly desirable reduction of the ground leakage currents, which is demonstrated through simulation results from MATLAB-Simulink and PLECS. The proposed topology is finally compared to other transformerless topologies with respect to other aspects, such as efficiency and component count.

Countries with considerable PhotoVoltaic (PV) installations are facing a challenge of overloading their power grid during peak-power production hours if the power infrastructure remains the same. To address this, regulations have been imposed on PV systems, where more active power control should be flexibly performed. As an advanced control strategy, the Absolute Active Power Control (AAPC) can effectively solve the overloading issues by limiting the maximum possible PV power to a certain level (i.e., the power limitation), and also benefit the inverter reliability due to the reduction in the thermal loading of the power devices. However, its feasibility is challenged by the associated energy losses. An increase of the inverter lifetime and a reduction of the energy yield can alter the cost of energy, demanding an optimization of the power limitation. Therefore, aiming at minimizing the Levelized Cost Of Energy (LCOE), the power limit is optimized for the AAPC strategy in this paper. The optimization method is demonstrated on a 3-kW single-phase PV system considering a real-field mission profile (i.e., solar irradiance and ambient temperature). The optimization results have revealed that superior performance in terms of LCOE and energy production can be obtained by enabling the AAPC strategy, compared to the conventional PV inverter operating only in the maximum power point tracking mode. In the presented case study, the minimum of the LCOE is achieved for the PV system when the power limit is optimized to a certain level of the designed maximum feed-in power (i.e., 3-kW). In addition, the LCOE-based analysis method can be used in the design of PV inverters considering long-term mission profiles.

A substantial growth of the installed photovoltaic (PV) systems capacity has occurred around the world during the last decade, thus enhancing the availability of electric energy in an environmentally friendly way. The maximum power point tracking (MPPT) technique enables to maximize the energy production of PV sources, despite the stochastically varying solar irradiation and ambient temperature conditions. Thereby, the overall efficiency of the PV energy production system is increased. Numerous techniques have been presented during the last decades for implementing the MPPT process in a PV system. This chapter provides an overview of the operating principles of these techniques, which are suited for either uniform or nonuniform solar irradiation conditions. The operational characteristics and implementation requirements of these MPPT methods are alsoanalyzed in order to demonstrate their performance features.

The flexible photovoltaic (PV) modules have the advantage of easily fitting on curved surfaces, but in that case their power-voltage characteristic exhibits local maximum power points (MPPs) where the PV module power production is suboptimal. In this paper, the effect of geometrical installation parameters of flexible PV modules, such as the bending angle, tilt angle, and orientation, on the shape of the power-voltage characteristic is experimentally investigated. Then, a new method of tracking the global MPP of flexible PV modules is proposed. An experimental, comparative study is also presented, which demonstrates that compared to the past-proposed MPP tracking (MPPT) techniques, the system proposed in this paper is capable to detect the global MPP of a flexible PV module with less search steps. Thus, the power loss during the global MPPT process is minimized and the energy production of the flexible PV module is maximized.

The digital implementations of Sinusoidal Pulse Width Modulation (SPWM) generators have dominated over their counterparts based on analog circuits. In this paper, an FPGA-based SPWM generator is presented, which is capable to operate at switching frequencies up to 1 MHz (requiring FPGA operation at 100-160 MHz), thus it is capable to support the high switching frequency requirements of modern single-phase dc/ac power converters. The proposed design occupies a small fraction of a medium-sized FPGA and, thus, can be incorporated in larger designs. Additionally, it has a flexible architecture that can be tuned to a variety of single-phase dc/ac inverter applications. The post-layout simulation and experimental results confirm that compared to the past-proposed SPWM generation designs, the SPWM generator presented in this paper exhibits much faster switching frequency, lower power consumption, and higher accuracy of generating the desired SPWM waveform.

The design optimization of H5, H6, neutral point clamped, active-neutral point clamped, and conergy-NPC transformerless photovoltaic (PV) inverters is presented in this paper. The components reliability in terms of the corresponding malfunctions, affecting the PV inverter maintenance cost during the operational lifetime period of the PV installation, is also considered in the optimization process. According to the results of the proposed design method, different optimal values of the PV inverter design variables are derived for each PV inverter topology and installation site. The H5, H6, neutral point clamped, active-neutral point clamped and conergy-NPC PV inverters designed using the proposed optimization process feature lower levelized cost of generated electricity and lifetime cost, longer mean time between failures and inject more PV-generated energy into the electric grid than their nonoptimized counterparts, thus maximizing the total economic benefit obtained during the operational time of the PV system.

Large-scale photovoltaic (PV) plants enable the reduction of the PV plant cost per watt of nominal power that is installed. In this paper, a new method is presented for the calculation of the optimal configuration of large PV plants, such that the levelized cost of the generated electricity (LCOE) is minimized. The proposed design optimization process is performed by considering the impact of the number of components, as well as their type and arrangement within the installation field, on the tradeoff between the lifetime cost and the corresponding energy production of the PV plant. The high-accuracy feature of the energy production calculations that are performed by the proposed design tool has been validated using experimental operational data of an existing PV plant. The design results demonstrate that using the proposed optimization method allows a reduction of the cost of the energy that is generated by the large-scale PV plant, thus enabling the maximization of the economic benefit that is obtained during the operational lifetime period of the PV system.

Summarization: This paper presents a new methodology for optimal design of transformerless photovoltaic (PV) inverters targeting a cost-effective deployment of grid-connected PV systems. The optimal switching frequency as well as the optimal values and types of the PV inverter components is calculated such that the PV inverter LCOE generated during the PV system lifetime period is minimized. The LCOE is also calculated considering the failure rates of the components, which affect the reliability performance and lifetime maintenance cost of the PV inverter. A design example is presented, demonstrating that compared to the nonoptimized PV inverter structures, the PV inverters designed using the proposed optimization methodology exhibit lower total manufacturing and lifetime maintenance cost and inject more energy into the electric-grid and by that minimizing LCOE. Παρουσιάστηκε στο: 27th Annual IEEE Applied Power Electronics Conference and Exposition

Summarization: The power-voltage characteristic of photovoltaic (PV) arrays operating under partial-shading conditions exhibits multiple local maximum power points (MPPs). In this paper, a new method to track the global MPP is presented, which is based on controlling a dc/dc converter connected at the PV array output, such that it behaves as a constant input-power load. The proposed method has the advantage that it can be applied in either stand-alone or grid-connected PV systems comprising PV arrays with unknown electrical characteristics and does not require knowledge about the PV modules configuration within the PV array. The experimental results verify that the proposed global MPP method guarantees convergence to the global MPP under any partial-shading conditions. Compared with past-proposed methods, the global MPP tracking process is accomplished after far fewer PV array power perturbation steps. Presented on: IEEE Journal of Photovoltaics

Several countries with considerable PhotoVoltaic (PV) installations are facing a challenge of overloading the power infrastructure during peak-power production hours. Regulations have been imposed on the PV systems, where more active power control should be flexibly performed. As an advanced control strategy, the Absolute Active Power Control (AAPC) can effectively solve the overloading issues by limiting the maximum possible PV power to a certain level (i.e., the power limitation), and also benefit the inverter reliability. However, its feasibility is challenged by the energy loss. An increase of the inverter lifetime and a reduction of the energy yield can alter the cost of energy, demanding an optimization of the power limitation. Therefore, aiming at minimizing the Levelized Cost of Energy (LCOE), the power limit is optimized for the AAPC strategy in this paper. The optimization method is demonstrated on a 3-kW single-phase PV system considering a real-field mission profile (i.e., solar irradiance and ambient temperature). The optimization results have revealed that superior performance in terms of LCOE and energy production can be obtained by enabling the AAPC strategy, compared to the conventional PV inverter operating only in the maximum power point tracking mode. In the presented case study, the minimum of LCOE is achieved for the system when the power limit is optimized to a certain level of the designed maximum feed-in power (i.e., 3 kW). In addition, the proposed LCOE-based analysis method can be used in the design of PV inverters considering mission profiles.

The market of PV systems is rapidly expanding during the last years and the transformerless DC/AC inverters are key components in this evolution. Despite its design and control simplicity, a transformerless full-bridge DC/AC inverter may exhibit a high leakage ground current, due to capacitive coupling between the solar panels and ground, which prohibits its employment in single-phase PV systems. Thus, multiple alternative transformerless PV inverter topologies have been developed in the past, which, however, comprise additional power devices and/or passive components. In this paper, a new modulation method is proposed for optimally controlling the power switches of transformerless single-phase full-bridge PV inverters, such that their leakage ground current is minimized. The design results demonstrate that by using the proposed technique, the leakage ground current of transformerless full-bridge inverters is reduced below the maximum limit imposed by the VDE 0126-1-1 standard, thus enabling their application in single-phase PV installations, instead of more complex PV inverter structures.

Wireless Sensor Networks (WSNs) are widely used during the last years in various environmental and industrial applications and they are frequently installed in geographically isolated areas, where access to electricity is not available. In such cases, the WSN nodes are power-supplied by Renewable Energy Sources (RES), accompanied by energy-storage units. In this paper, a design optimization method is presented for deriving the optimal configuration of the RES-based energy production system of a WSN node, such that its total lifetime cost is minimized, while simultaneously it is guaranteed that the data-acquisition equipment of the WSN node is uninterruptedly power-supplied during the entire year. The design results verify that by applying the proposed optimization technique, RES-based power-supply structures with a lower lifetime cost are derived, compared to the non-optimally designed configurations. Experimental results are also presented, demonstrating the successful operation of a RES-based power-supply system, which has been optimally designed using the proposed method.

Summarization: In this paper, a detailed study and implementation of a reliable and efficient regenerative braking system is presented. It is applied on a prototype electric vehicle, that uses a hydrogen fuel cell as its only power source. Supercapacitors are used to store the energy that is generated during braking, transistors for switching the alternative circuits and an embedded computer controller program undertakes the synchronization of the system tasks. A finite state machine was designed to create a simple but robust technique to control the transistor switches according to the system's sensory inputs. The system is powered by its own supercapacitors, and thus it may be used in a plug-and-play manner. On-road test drives proved the system's reliability and efficiency. Presented on

The transformerless dc/ac inverters are critical components in the rapidly growing market of grid-connected photovoltaic (PV) applications. They are synthesized by combining available solutions in terms of the power-section topology, power-semiconductors manufacturing technology, and structure of the output filter. In addition, the silicon carbide (SiC)-based power semiconductors are capable to operate reliably at high operating temperature and switching frequency levels combined with a very high efficiency. In this paper, a new design technique is presented for optimizing the switching frequency and structure of the output filter (either LCL- or LLCL-type) in transformerless H5 and conergy-neutral point clamped (Conergy-NPC) PV inverters, which employ SiC-type power devices. The design results demonstrate that the optimized SiC-based H5 and Conergy-NPC transformerless PV inverters are more effective in terms of energy production than their nonoptimized and silicon (Si)-based counterparts. In addition, by reducing the market price of SiC-type power semiconductors to the level of Si-based power devices, enables the development of optimized SiC-based PV inverters with a lower cost of energy than the corresponding PV inverters based on Si technology, thus maximizing the economic profitability of the PV system.

The implementation of water management techniques in various environmental applications requires the measurement of the level of water stored in artificial storage tanks (e.g. in cities, communities etc.) or natural reservoirs. In this paper, the design of a water-level sensor is presented, which is based on the Time-Domain Reflectometry (TDR) technique. The proposed TDR sensor comprises a sensing probe constructed using low-cost and widely-available multilayer and stainless steel tubes. A simple signal-conditioning circuit has been designed for interfacing the acquired measurements to a digital data-acquisition device through an I2C communication bus. Thus, the manufacturing cost and power consumption of the proposed sensor are relatively low, enabling its incorporation in Wireless Sensor Networks which are power-supplied by Renewable Energy Sources. The operation of the proposed sensor has been tested in a city-scale storage tank of drinking water. The experimental results verify that the proposed sensor achieves equivalent accuracy performance with a commercially available ultrasound water-level sensor, which, however, is of higher cost.

Reliability is one of the most important parameters in aircrafts. In this paper, a method for the calculation of reliability (i.e. number of failures / 106 hours) of a three-phase full-bridge inverter, which is employed in the dynamic waste heat recovery system of an aircraft, is presented. The main factors for the reliability analysis that have to be considered is the topology of the inverter, the ambient temperature conditions, the power switches (type and modulation technique), and the harmonic filter. The power inverter reliability has been calculated using a software program developed under the Matlab platform, which was used to calculate the failure rate of each device of the inverter, such as the power switches, the DC-bus capacitor and the filter inductor, given the operating switching frequency value. The results indicate that at higher switching frequency levels the inverter of the DWHR system exhibits a high failure rate, thus resulting in a lower Mean Time Between Failures (MTBF).

Water-level sensors are indispensable for monitoring the level of water in storage tanks, which are used in drinking water distribution networks. In this paper, a long-range capacitive-type water-level sensor is presented. The proposed sensor is constructed using widely-available multilayer tubes, which are used for building drinking water systems. Thus, both the manufacturing cost of the sensor and the cost of the associated electronic circuits, which are used for interfacing the sensor to a digital data-acquisition unit, are low. The performance of the proposed sensor has been evaluated in a water storage tank of a city-scale water distribution network. The experimental results indicate that the accuracy of the proposed experimental prototype sensor is equivalent to that of a commercially available ultrasound water level sensor, while, additionally, its manufacturing cost is significantly lower.

Summarization: A methodology for optimal sizing of stand-alone PV/WG systems is presented. The purpose of the proposed methodology is to suggest, among a list of commercially available system devices, the optimal number and type of units ensuring that the 20-year round total system cost is minimized subject to the constraint that the load energy requirements are completely covered, resulting in zero load rejection. The 20-year round total system cost is equal to the sum of the respective components capital and maintenance costs. The cost (objective) function minimization is implemented using genetic algorithms, which, compared to conventional optimization methods such as dynamic programming and gradient techniques, have the ability to attain the global optimum with relative computational simplicity. The proposed method has been applied for the design of a power generation system which supplies a residential household. The simulation results verify that hybrid PV/WG systems feature lower system cost compared to the cases where either exclusively WG or exclusively PV sources are used. Παρουσιάστηκε στο: Solar Energy

Pulse width modulation (PWM) has been widely used in power converter control. Most high power level converters operate at switching frequencies up to 500 kHz, while operating frequencies in excess of 1 MHz at high power levels can be achieved using the planar transformer technology. The contribution of this paper is the development of a high-frequency PWM generator architecture for power converter control using FPGA and CPLD ICs. The resulting PWM frequency depends on the target FPGA or CPLD device speed grade and the duty cycle resolution requirements. The post-layout timing simulation results are presented, showing that PWM frequencies up to 3.985 MHz can be produced with a duty cycle resolution of 1.56%. Additionally, experimental results are also presented for low cost functional verification of the proposed architecture.

A wind-generator (WG) maximum-power-point-tracking (MPPT) system is presented, consisting of a high-efficiency buck-type dc/dc converter and a microcontroller-based control unit running the MPPT function. The advantages of the proposed MPPT method are that no knowledge of the WG optimal power characteristic or measurement of the wind speed is required and the WG operates at a variable speed. Thus, the system features higher reliability, lower complexity and cost, and less mechanical stress of the WG. Experimental results of the proposed system indicate near-optimal WG output power, increased by 11%-50% compared to a WG directly connected via a rectifier to the battery bank. Thus, better exploitation of the available wind energy is achieved, especially under low wind speeds.

The installation of photovoltaic (PV) plants has been expanding rapidly across the world during the last years. In this paper, a methodology for the design optimization of PV plants is presented, which, in contrast to the conventional PV plant design approaches, is suitable to be executed using high time-resolution (i.e., 1-min-average) values of the meteorological input data. Due to the nonlinear operation of the devices comprising a PV plant, this allows for the accurate estimation of the PV plant performance during its operational lifetime period. A parallel processing-based implementation of genetic algorithms has been employed, which, compared to the serial execution, provides the ability to accomplish the proposed optimal design procedure in a considerably shorter time interval. The design optimization results confirm that the proposed method successfully accounts for both the meteorological conditions and the operational characteristics of the PV plant components and incorporates their impact on the PV plant energy production and cost in the design process. Thus, the proposed optimization method allows for optimum design of PV systems, which will provide maximum economic profit during their lifetime period.

Δημοσίευση σε επιστημονικό περιοδικό Summarization: A real-time system for protecting and monitoring a DC/AC converter has been designed and constructed. The proposed system consists of (a) a hardware protection unit for fast reaction, load protection and inverter fail-safe operation and (b) a microcontroller unit for calculating critical parameters of the inverter operation. The control unit malfunctions have not been investigated in this study. The proposed hardware architecture and sensors form a low-cost and reliable control unit. The experimental results show that the proposed system ensures the inverter protection and fail-safe features. The proposed unit can be used to increase the reliability of any power inverter in AC motor drives, renewable energy systems, etc. or can be incorporated in any UPS system. Presented on: Microelectronics Journal

The transformerless DC/AC inverter topologies are employed in Photovoltaic systems in order to improve the power conversion efficiency, power density and cost. The Active-Neutral Point Clamped (Active-NPC) transformerless inverters have the advantage of achieving better thermal balance among their power semiconductors. In this paper, a new modulation technique is proposed for optimally controlling the power switches employed in transformerless Active-NPC inverters. The design results demonstrate that compared to the existing PWM strategies, using the proposed method results in lower total power losses and significantly better distribution of the power losses among the semiconductors of the Active-NPC inverter.

Maximum power point tracking (MPPT) is used in photovoltaic (PV) systems to maximize the photovoltaic array output power, irrespective of the temperature and irradiation conditions and of the load electrical characteristics. A new MPPT system has been developed, consisting of a buck-type DC/DC converter, which is controlled by a microcontroller-based unit. The main difference between the method used in the proposed MPPT system and other techniques used in the past is that the PV array output power is used to directly control the DC/DC converter, thus reducing the complexity of the system. The resulting system has high-efficiency, lower-cost and can be easily modified to handle more energy sources (e.g., wind-generators). The experimental results show that the use of the proposed MPPT control increases the PV output power by as much as 15% compared to the case where the DC/DC converter duty cycle is set such that the PV array produces the maximum power at 1 kW/m/sup 2/ and 25/spl deg/C.

Dense monitoring of environmental parameters (e.g. air/soil humidity, ambient temperature) is critical in precision agriculture, urban area monitoring and environmental modeling applications. In this paper, the design of a novel wireless sensor network (WSN) is proposed, consisting of low-power and low-cost sensor nodes, deployed in a bistatic architecture (i.e. carrier emitter in a different location than the receiver) and achieving long-range backscatter communication. The tags modulate sensor information using analog frequency modulation (FM) and frequency division multiple access (FDMA) at the subcarrier frequency, even though a single carrier is assumed. In sharp contrast to prior art, the developed backscatter sensor network performs environmental monitoring over a relatively wide area. A proof-of-concept prototype WSN application has been developed for capacitive relative humidity (RH) sensing, with 1.5 mW per tag, 0.9 RMSE and range on the order of 50 m.

The electrical signals generated by plants provide information about their physiology. A low-cost and low-power-consumption (≈ 240 μW) wireless sensor node is proposed, which measures the electrical signal generated across selected points on a plant and wirelessly transmits it to a data-collection device (reader) through a backscattering-based communication link. The proposed sensor node is appropriate for large-scale Wireless Sensor Networks (WSNs) agricultural applications, which are capable of collecting information about the health status of multiple plants over a broad area, with low cost and low power-consumption. Experimental results are presented for avocado and orange trees, demonstrating the successful operation of the proposed sensor node.

Single-phase transformerless Photovoltaic (PV) inverters are synthesized by combining available solutions in terms of the power section topology, power semiconductors manufacturing technology and structure of the output filter. A design method is presented in this paper for optimizing the power semiconductors type (Si- or SiC-based), switching frequency and output filter (LCL- or LLCL-type) employed in H5 and Conergy-NPC PV inverters, considering the simultaneous impact of the factors affecting the PV energy processing performance and PV inverter cost. According to the design results, the optimized SiC-based PV inverters will inject more energy into the electric grid, compared to the Si-based structures and enable the reduction of the output filter size, weight and cost. Employing an LLCL-type output filter and simultaneously reducing the cost of SiC power semiconductors to the level of their Si-based counterparts will enhance the economic superiority of the resulting optimized H5 and Conergy-NPC PV inverters with respect to the cost of the electricity they generate. Single-phase transformerless Photovoltaic (PV) inverters are synthesized by combining available solutions in terms of the power section topology, power semiconductors manufacturing technology and structure of the output filter. A design method is presented in this paper for optimizing the power semiconductors type (Si- or SiC-based), switching frequency and output filter (LCL- or LLCL-type) employed in H5 and Conergy-NPC PV inverters, considering the simultaneous impact of the factors affecting the PV energy processing performance and PV inverter cost. According to the design results, the optimized SiC-based PV inverters will inject more energy into the electric grid, compared to the Si-based structures and enable the reduction of the output filter size, weight and cost. Employing an LLCL-type output filter and simultaneously reducing the cost of SiC power semiconductors to the level of their Si-based counterparts will enhance the economic superiority of the resulting optimized H5 and Conergy-NPC PV inverters with respect to the cost of the electricity they generate.

Targeting at a cost-effective deployment of grid-connected PhotoVoltaic (PV) systems, this paper presents a new methodology for the optimal design of transformerless PV inverters, which are based on the Neutral Point Clamped (NPC) and the Active-Neutral Point Clamped (ANPC) topologies. The design optimization results demonstrate that a different set of optimal values of the PV inverter switching frequency and output filter components are derived for the NPC and ANPC topologies, respectively, as well as for each of the PV inverter installation sites under study. The NPC and ANPC PV inverter structures, which are derived using the proposed design optimization methodology exhibit lower Levelized Cost Of generated Electricity (LCOE) and manufacturing cost and they are simultaneously capable to inject more energy into the electric grid than the corresponding non-optimized PV inverters. Thus, the proposed optimal design methodology enables to maximize the economic benefit obtained during the lifetime period of the installed PV system.

Summarization: System latency (time delay) and its visible consequences are fundamental Virtual Environment (VE) deficiencies that can hamper user perception and performance. This paper presents an immersive simulation system which improves upon current latency measurement and minimization techniques. Hardware used for latency measurements and minimization is assembled based on low-cost and portable equipment, most of them commonly found in an academic facility without reduction in accuracy of measurements. A custom-made mechanism of measuring and minimizing end-to-end head tracking latency in an immersive VE is assembled. The mechanism is based on an oscilloscope comparing two signals. One is generated due to the head-tracker movement by a shaft encoder attached on a servo motor moving the tracker. The other signal is generated by the visual consequences of this movement in the VE using a photodiode attached to the computer monitor. Visualization and application-level control of latency in the VE was implemented using the XVR platform. Minimization processes resulted in almost 50% reduction of initial measured latency. The description of the mechanism by which VE latency is measured and minimized will be essential to guide system countermeasures such as predictive compensation. Παρουσιάστηκε στο: 10th International Conference on Virtual Reality Continuum and Its Applications in Industry

Summarization: The DC/AC inverters are the key elements in grid-connected PV energy production systems. In this paper, new design optimization techniques focused on transformerless (very high efficiency) PV inverters are proposed. They have been developed based on an analysis of the deficiencies of the current, state-of-the-art PV inverters design technology, which limits the amount of PV energy supplied into the electric grid. The influences of the electric grid regulations and standards and the PV array operational characteristics on the design of grid-connected PV inverters have also been considered. The simulation results verify that the proposed optimization techniques enable the maximization of the PV energy injected into the electric grid by the optimized PV installation. Παρουσιάστηκε στο: 26th Annual IEEE Applied Power Electronics Conference and Exposition

Summarization: Nowadays Photovoltaic (PV) plants have multi MW sizes, the biggest plants reaching tens of MW of capacity. Such large-scale PV plants are made up of several thousands of PV panels, each panel being in the range of 150-350W. This means that the design of a Large PV power plant is a big challenge and configuring such a plant should be implemented taking into consideration not only the cost of the installation, but also the Annual Energy Production, the Performance Ratio and the Levelized Cost Of Energy. In this paper, an algorithm is presented including the most important models of the PV system, that optimizes the structure of the plant based on predefined targets, such as the minimum Levelized Cost Of Energy. Παρουσιάστηκε στο: International Symposium on Industrial Electronics

Summarization: This paper presents an experimental methodology exploring the effect of tracking latency on object recognition after exposure to an immersive VE, in terms of both scene context and associated awareness states. System latency (time delay) and its visible consequences are fundamental Virtual Environment (VE) deficiencies that can hamper spatial awareness and memory. The immersive simulation consisted of a radiosity-rendered space divided in three zones including a kitchen/dining area, an office area and a lounge area. The space was populated by objects consistent as well as inconsistent with each zone's context. The simulation was displayed on a stereo head-tracked Head Mounted Display. Participants across two conditions of varying latency (system minimum latency vs added latency condition) were exposed to the VE and completed an object-based memory recognition task. Participants also reported one of three states of awareness following each recognition response which reflected either the recollection of contextual detail, the sense of familiarity unaccompanied by contextual information or even informed guesses. Preliminary results from initial pilot studies reveal better memory performance of objects in the low latency condition. A disproportionately large proportion of guess responses for consistent objects viewed with high latency is also observed and correspondingly a disproportionately low proportion of remember responses for consistent objects in the same latency condition. Παρουσιάστηκε στο: 10th International Conference on Virtual Reality Continuum and Its Applications in Industry

An absolute sea-level monitoring and altimeter calibration permanent facility has been established on the isle of Gavdos, 50 km south of the island of Crete, Greece. This calibration/validation facility has been chosen because Gavdos is under a crossing point of the ground-tracks of Jason-1 satellite, and adjacent to an Envisat pass. Satellite altimeter missions are evaluated at that site using external measurements from tide gauges, GPS, a DORIS beacon, meteorological sensors, wave-height sensors, airborne campaigns for gravity and sea-surface topography, water-vapour radiometry, solar atmospheric spectrometry, GPS buoys, altimeter transponder, Satellite Laser Ranging, etc. The mean sea level and the earth's tectonic deformation field in the region have also been determined accurately. Comparison over the cycles 70 to 77 of the Jason-1 satellite indicate that its absolute mean bias for the sea-surface heights is 13 mm ± 20 mm. The GAVDOS project has started in December 2001 and has been in the context of an international calibration/validation effort of the Jason-1 Science Working Team.

Summarization: The location of the GAVDOS facility is under a crossing point of the original ground-tracks of TOPEX/Poseidon and the present ones for Jason-1, and adjacent to an ENVISAT pass, about 50 km south of Crete, Greece. Ground observations and altimetry comparisons over cycles 70 to 90, indicate that a preliminary estimate of the absolute measurement bias for the Jason-1 altimeter is 144.7 ± 15 mm. Comparison of Jason microwave radiometer data from cycles 37 and 62, with locally collected water vapor radiometer and solar spectrometer observations indicate a 1–2 mm agreement. Presented on: Marine Geodesy

In this study, a methodology for the design optimisation and the economic analysis of photovoltaic grid-connected systems (PVGCSs) is presented. The purpose of the proposed methodology is to suggest, among a list of commercially available system devices, the optimal number and type of system devices and the optimal values of the photovoltaic (PV) module installation details, such that the total net economic benefit achieved during the system operational lifetime period is maximised. The decision variables included in the optimisation process are the optimal number and type of the PV modules and the DC/AC converters, the PV modules optimal tilt angle, the optimal arrangement of the PV modules within the available installation area and the optimal distribution of the PV modules among the DC/AC converters. The economic viability of the resulting PVGCS configuration is explored according to the net present value, the discounted payback period and the internal rate of return methods. The proposed method has been applied for the optimal design of a PVGCS interconnected to the electric network of an island with significant solar irradiation potential and the corresponding optimal sizing and economic analysis results are presented.

Summarization: A new method for the design of a bidirectional inverter based on the sinusoidal pulse-width modulation principle and the use of a low-cost and lightweight ferrite-core transformer is presented. The inverter is designed for either ohmic or inductive loads. In the case of inductive loads, the reactive power is transferred back to the DC input power source using a new active rectifier design. The inverter is controlled by two minimum-time feedback loops, providing relatively low output voltage distortion (less than 2% for DC input higher than 24 V) and good load regulation (better than 2%), while the inverter efficiency remains relatively constant (from 80 to 85%) over a wide output power range (75 to 200 W) and DC input voltage range (23 to 28 V). Theoretical results are experimentally verified using a laboratory prototype Presented on: IEE Proceedings Electric Power Applications

Summarization: A battery-charging system for stand-alone photovoltaic (PV) applications is presented. Advantages of the proposed method are: better exploitation of the available PV energy by means of a maximum power point tracking (MPPT) technique employed in the control algorithm, increased battery lifetime due to higher level state of charge operation, and the charging control process does not depend on accurate battery current measurements, reducing the effect of the current sensor sensitivity on the battery final state of charge. Also, since it is based on the battery current regulation principle, it can be effectively used with large battery strings. The experimental results verify that, using the proposed method, a better exploitation of the available PV energy, compared to a commercial battery charger based on the on/off principle, is achieved, and simultaneously a 100% battery state of charge is reached in shorter time. Presented on: IEE Proceedings Electric Power Applications

Summarization: Grid-connected photovoltaic (PV) systems are currently developed by merging a PV array and a DC/AC inverter which are designed separately, without considering the impact of the PV array operational characteristics on the power losses of the DC/AC inverter. In this paper, a co-design technique is presented, where the optimal design parameters of the PV array and DC/AC inverter in a grid-connected PV system are calculated concurrently through a unified design process. The proposed technique enables to optimally match the PV array configuration and the DC/AC inverter structure. A study has been performed, where the PV systems synthesized by applying the proposed co-design technique are compared with the PV system configurations comprising PV arrays and DC/AC inverters that have been designed separately, through distinct optimization processes based on various alternative optimization objectives. The design results for two installation sites, with different meteorological conditions during a year, demonstrated that only the proposed co-design optimization technique is capable of ensuring the maximization of the annual energy production of the overall grid-connected PV system. Παρουσιάστηκε στο: IEEE Transactions on Energy Conversion

Summarization: A Monte-Carlo based, probabilistic power flow method is used to evaluate different power system interconnection options. Suitable indices are proposed to assess the potential profit of each interconnection scenario and the required interconnection rating. Separate investment perspectives are identified and the respective optimal solutions are compared. Indicative results are presented regarding a possible submarine interconnection of the Greek island of Crete with the mainland grid. Παρουσιάστηκε στο: 10th IASTED European Congress on Power and Energy Systems

In this paper, a methodology for the optimal sizing of a Battery Energy Storage System (BESS) targeting to supply a predefined and constant (guaranteed) amount of power to an isolated electric grid during the peak load-demand hours is presented. The BESS battery bank is charged using that part of the energy produced by wind parks interconnected with the power system, which is rejected by the electric network due to grid stability limitations. Thus, using the proposed BESS configuration results in increment of the wind energy penetration in the isolated power system and reduction of the air pollution caused by the thermal power stations. The economic viability of the optimally sized BESS configuration is investigated by calculating the investment discounted payback period and internal rate of return. The proposed method has been applied for the optimal design of a BESS supporting the isolated electric network of a remote island with significant wind potential. According to the economic analysis results, the installation of the proposed BESS is economically beneficial for both the BESS investor and the local electric network operator.

Summarization: The development of a server database system for monitoring and operational evaluation of remote Renewable Energy Sources (RES) plants is presented. Meteorological and operational parameters of multiple RES systems are measured and transmitted in real-time to a database (DB) server. An integrated data management system, comprised of programs running on the DB server, displays the received data on screen, stores them on local disk and inserts them in the DB in real-time. Remote clients access the DB using the TCP/IP protocol in order to create charts, calculate statistical and operational parameters regarding each RES plant and perform DB administration actions. The proposed system can be used for the exploration of the available RES potential during the design of RES systems, the development of statistical models describing the spatial variability of RES resources and the remote monitoring and control of RES plants. Παρουσιάστηκε στο: Renewable Energy

Summarization: Environmental variables monitoring with wireless sensor networks (WSNs) is invaluable for precision agriculture applications. However, the effectiveness of existing low-power, conventional (e.g., ZigBee-type) radios in large-scale deployments is limited by power consumption, cost, and complexity constraints, while the existing WSN solutions employing nonconventional, scatter-radio principles have been restricted to communication ranges of up to a few meters. In this paper, the development of a novel analog scatter-radio WSN is presented, that employs semipassive sensor/tags in bistatic topology (i.e., carrier emitter placed in a different location from the reader), consuming

Summarization: The power supply system design of a marine data buoy system is a challenge. Due to cost reasons, it is difficult for marine buoys to obtain electricity directly from the power grid. At present, marine buoys use solar energy, wind energy or wave energy to supply power to sensors. Among them, the employment of wave energy and solar energy has broad application prospects due to the complementary power production of these sources. This paper presents a new wave energy conversion (WEC) device for a marine buoy, which is composed of a floating body and a power generation device. In addition, a three-dimensional finite element simulation model of the buoy movement is established, and the simulation is carried out using hydrodynamic software. On the basis of simulation, experiments are carried out on a 50 kg core power generation unit to estimate the power generation performance of the proposed WEC device. Παρουσιάστηκε στο: 47th Annual Conference of the IEEE Industrial Electronics Society

Summarization: This paper presents the analysis and design procedure of a low voltage, reconfigurable, on-chip DC/DC - DC/AC power conversion system for PV applications. The whole work is carried out in the context of a novel smart PV system development, which is based on integrated PV cell inverters. Various power converter topologies are investigated in order the most appropriate for the DC/DC and DC/AC power stages to be selected. The synchronous boost DC/DC converter and the H-bridge inverter are the preferred solution, in terms of complexity, efficiency and cost. The specifications of the power conversion system are given, along with the theoretical analysis for the synchronous boost converter and H-bridge inverter operation. Moreover, the chip design process is presented, highlighting the design constraints and limitations obtained. Finally, in order the functionality of the above concept to be validated, PSpice simulations are carried out, indicating the high efficiency and low complexity of the proposed power conversion system. Παρουσιάστηκε στο: 5th Panhellenic Conference on Electronics and Telecommunications