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Multiple exciton generation, the creation of two electron-hole pairs from one high-energy photon, is well established in bulk semiconductors, but assessments of the efficiency of this effect remain controversial in quantum-confined systems like semiconductor nanocrystals. We used a photoelectrochemical system composed of PbS nanocrystals chemically bound to Ti[0.sub.2] single crystals to demonstrate the collection of photocurrents with quantum yields greater than one electron per photon. The strong electronic coupling and favorable energy level alignment between PbS nanocrystals and bulk Ti[O.sub.2] facilitate extraction of multiple excitons more quickly than they recombine, as well as collection of hot electrons from higher quantum dot excited states. Our results have implications for increasing the efficiency of photovoltaic devices by avoiding losses resulting from the thermalization of photogenerated carriers. 10.1126/science.1191462

The work focused on the analysis of the connection between a photovoltaic (PV) plant and an electrolyzer for hydrogen production. On the basis of PV-plant and electrolyzer experimental data, the effectiveness of the models adopted in the simulation program has been verified in order to choose the best model and, eventually, modify some parameters. By running the simulations, the procedure to optimize the PV-plant and the electrolyzer combination has been established. In fact, the simulation results might be considered to size an electrolyzer as small as possible, which is able to exploit up to the maximum power actually produced by the PV-plant during a working year. This criterion allows minimizing the overall plant costs. Furthermore, the possibility, of deleting the maximum power point tracker and the dc/dc converter has been analyzed. On the basis of the obtained results, this opportunity is preferable to avoid the energy losses in the power control system; and it is convenient even from an economic point of view, considering that the electronic device costs are comparable with the PV-plant ones. [DOI: 10.1115/1.4001673]

Distributed maximum power point tracking (DMPPT) is one of the most promising solutions to overcome the drawbacks associated with mismatching phenomena in photovoltaic (PV) applications. DMPPT is based on the adoption of a dc/dc converter dedicated to each PV module. The design of the power stage of such a converter is a challenging task because of the very high efficiency requirements and of the continuous changes of the operating point during the day, depending on the sun irradiation conditions. In this paper the guidelines for the design of dc-dc converters for DMPPT applications are presented and discussed. [DOI: 10.1115/1.4001465]

ABSTRACT As the world faces an impending dearth of fossil fuels, most immediately oil, alternative sources of energy must be found. 174 PW worth of energy falls onto the top [...]

The design and construction of miniature prototype of nonimaging planar concentrator, which is capable of producing much more uniform spatial irradiance and reasonably high concentration ratio, were presented in the previous paper. In this paper, we further explore the optical characteristics of the new concentrator that is specially designed to be incorporated in concentrator photovoltaic systems. For this study, we have carried out a comprehensive analysis via numerical simulation based on all the important design parameters, i.e., array of facet mirrors, f/D ratio, receiver size, and the effect of sun-tracking error, which lead to the overall optical performance of the new concentrator. [DOI: 10.1115/1.4000355] Keywords: nonimaging, solar concentrator, concentrator photovoltaic, uniform illumination, flux distribution, ray-tracing

In this paper, a minimal-order observer-based control method is proposed for small power utility connected photovoltaic (PV) systems. In the proposed method, the PV power is controlled according to load variation to minimize the frequency deviation. Load power and diesel power are estimated successfully by a minimal-order observer. From the estimated load power, load variation index is calculated. A second-order low-pass filter is used to produce PV base power from available maximum PV power. Then, the PV base power is added with the load variation index to generate the command PV power. The proposed method is compared with the method where maximum power point tracking control is used to produce the command PV power. From simulation results, it has been found that the proposed method is effective to reduce the frequency deviation of the utility and also delivers PV power near maximum PV power. Index Terms--Frequency deviation, load variation index, maximum power point tracking (MPPT), minimal-order observer.

The techno-economic optimum sizing of a stand-alone photovoltaic (PV) system is a basic requirement for its proliferation. One of the primary concerns in designing a stand-alone PV system is the determination of an optimum relationship between the PV array and storage battery capacity to supply the required energy at a specified energy load fraction. In this paper, a complete analytical methodology has been presented for that. The solar radiation utilizability concept and the monthly average daily PV array efficiency have been used to estimate the performance of a stand-alone PV system and hence for the designing. The techno-economic optimization of a PV system has been done by using levelized energy cost computation based on the total number of battery replacements (brp's) through battery life-cycle model by using the solar radiation utilizability concept over the life period of the system. It has been found that energy load fractions as well as the number of brp's have a significant impact on the selection of optimum sizing of a stand-alone PV system. Index Terms--Solar energy, solar photovoltaic (PV) system, stand-alone renewable energy system, techno-economics.

This paper describes ramp rate control of a photovoltaic (PV) generator with an electric double-layer capacitor. The capacitor absorbs rapid fluctuations of PV generation, and allows the generator to change its output at a limited ramp rate. The output is mainly determined by a moving average of the PV generation, but the capacitor voltage is also counted to keep the voltage at a specified value, and accordingly, to make the capacitor small. An expression for the capacitor size is derived. The feedback gain of the capacitor voltage is also examined to maintain enough energy storage in the capacitor. Some experiments are executed to demonstrate and validate the proposed method. Index Terms--Capacitor, control, energy storage, photovoltaic (PV) power system, power system dynamics.

As an intermediate solution between Glaser's satellite solar power (SSP) and ground-based photovoltaic (PV) panels, this paper examines the collection of solar energy using a high-altitude aerostatic platform. A procedure to calculate the irradiance in the medium/high troposphere, based on experimental data, is described. The results show that here a PV system could collect about four to six times the energy collected by a typical U.K.-based ground installation, and between one-third and half of the total energy the same system would collect if supported by a geostationary satellite (SSP). The concept of the aerostat for solar power generation is then briefly described together with the equations that link its main engineering parameters/variables. A preliminary sizing of a facility stationed at 6 km altitude and its costing, based on realistic values of the input engineering parameters, is then presented. Index Terms--Energy conversion, photovoltaic (PV) power systems, PV space power systems, power conversion, solar energy, solar power satellites, solar radiation, terrestrial atmosphere.

Severe voltage distortion, due to power system harmonic resonance, has been reported in recent years. This issue becomes more significant in high penetration of a photovoltaic (PV) network. A conventional voltage detection active filter operates as similar conductance for all harmonic frequencies to resolve this problem whether in a fixed conductance command or in an automatic gain adjustment control. However, its filtering capability is impeded by the mismatch between the active filter and the radial line, and the voltage distortion may still be significant. This paper proposes a discrete frequency tuning active filter to suppress power system harmonics. The active filter operates as variable conductance for each individual harmonic frequency. Each harmonic conductance is dynamically adjusted according to the corresponding harmonic voltage distortion of the active filter installation point in response to increase or decrease of nonlinear loads, or variation of resonant frequency in the power system. The mismatching problem between the feeder impedance and the active filter can be avoided effectively. Therefore, harmonic voltage distortion can be maintained at an allowable level throughout the feeder with lower peak current and lower rms current of the active filter, and loads installed at various locations of the power system receive more uniform voltage waveform. Index Terms--Active power filter, harmonic resonance, harmonic voltage damping.

The electrification of autonomous electrical networks is in most cases described by low quality of electricity available at very high production cost. Furthermore, autonomous electrical networks are subject to strict constraints posing serious limitations on the absorption of renewable energy sources (RES)-based electricity generation. To bypass these constraints and also to secure a more sustainable electricity supply status, the concept of combining photovoltaic (PV) power stations and energy storage systems comprises a promising solution for small scaled autonomous electrical networks, increasing the reliability of the local network as well. In this context, the present study is devoted in developing a complete methodology, able to define the size of an autonomous electricity generation system, based on the maximum available solar potential exploitation at minimum electricity generation cost. In addition special emphasis is given in order to select the most cost-efficient energy storage configuration available. According to the calculation results obtained, one may clearly state that an optimum sizing combination of a PV generator along with an appropriate energy storage system may significantly contribute on reducing the electricity generation cost in several island electrical systems, providing also abundant and high quality electricity without the environmental and macroeconomic impacts of the oil-based thermal power stations. [DOI: 10.1115/1.3097271] Keywords: energy storage, autonomous electrical network, photovoltaic generator, electricity production cost, optimum solution

The choice of switching frequency for pulsewidth modulation single-phase inverters, such as those used in grid-connected photovoltaic application, is usually a tradeoff between reducing the total harmonic distortion (THD) and reducing the switching loss. This paper discusses an approach to minimize the switching loss while meeting a given THD requirement using variable switching frequency schemes (switching schemes with the switching frequency varying within a fundamental period). An optimal switching scheme is proposed based on time-domain current ripple analysis and the calculus of variations. The analysis shows that, to meet the same THD requirement, the optimal scheme has a significant saving on switching loss, compared to the fixed switching frequency scheme and the hysteresis control scheme, in addition to other benefits such as reduced peak switching loss and a spread spectrum of the current harmonics. The optimal scheme has been implemented in a prototype and the experimental results have verified the theoretical analysis. Also, a straightforward design method for designing filter inductors for single-phase converters is provided based on the time-domain current ripple analysis. Index Terms--Calculus of variations, photovoltaic (PV), pulsewidth modulation (PWM), single-phase converter, switching loss, variable switching frequency.

Renewable energy sources based on photovoltaic (PV) along with battery-based energy storage necessitate power conditioning to meet load requirements and/or be connected to the electrical grid. The power conditioning is achieved via a dc-dc converter and a dc-ac inverter stages to produce the desired ac source. This is also the case even when the load is of dc type, such as the typical portable electronic devices that require ac adaptors to be powered from the ac mains. The letter presents a hybrid PV-battery-powered dc bus system that eliminates the dc-ac conversion stage, resulting in lower cost and improved overall energy conversion efficiency. It is also shown experimentally that the switching ac adaptors associated with the various commonly used portable electronic devices can be reused with the proposed dc bus system. A novel high-gain hybrid boost-flyback converter is also introduced with several times higher voltage conversion ratio than the conventional boost converter topology. This arrangement results in higher dc bus levels and lower cable conduction losses. Moreover, the voltage stress on the hybrid boost-flyback converter power switch is within half the output voltage. Experimental results taken from a laboratory prototype are presented to confirm the effectiveness of the proposed converter/system. Index Terms--DC bus, dc--dc converter, hybrid boost-flyback converter, photovoltaic (PV) power system.

This paper assesses the protection against electric shock in a photovoltaic generator (PVG), the dc side of a PV installation. Within this context, we discuss the applicability of the protection requirements of the International Electrotechnical Commission 60364, the international standard that provides guidelines for wiring in low-voltage (LV) electrical installations. The unique operational characteristics of a PVG, which differ from those of a conventional ac LV system, made it necessary to revise and adapt these requirements. With a view to discovering the effectiveness of electric shock protection in ungrounded PVGs, we carried out both a theoretical and practical study in a real PVG in order to analyze its electrical behavior. As part of our study, the feasibility of applying an 'active' means of protection was experimentally tested in this same PVG. Index Terms--Electric shock, grounding, insulation, International Electrotechnical Commission (IEC), leakage currents, photovoltaic (PV) power systems, protection, safety.

The cost and efficiency of a photovoltaic (PV)-based grid-connected system depends upon the number of components and stages involved in the power conversion. This has led to the development of several single-stage configurations that can perform voltage transformation, maximum power point tracking (MPPT), inversion, and current shaping--all in one stage. Such configurations would usually require at least a couple of current and voltage sensors and a relatively complex control strategy. With a view to minimize the overall cost and control complexity, this paper presents a novel MPPT scheme with reduced number of sensors. The proposed scheme is applicable to any single-stage, single-phase grid-connected inverter operating in continuous conduction mode (CCM). The operation in CCM is desirable as it drastically reduces the stress on the components. Unlike other MPPT methods, which sense both PV array's output current and voltage, only PV array's output voltage is required to be sensed to implement MPPT. Only one current sensor is used for shaping the buck-boost inductor current as well as for MPPT. The information about power output of the array is obtained indirectly from array's voltage and the inductor current amplitude. Detailed analysis and the flowchart of the algorithm for the proposed scheme are included. Simulation and experimental results are also presented to highlight the usefulness of the scheme. Index Terms--Continuous conduction mode (CCM), current sensor, grid-connected, inverter, maximum power point tracking (MPPT), photovoltaic (PV), single-stage.

A Photovoltaic (PV) system's power output is not constant and fluctuates depending on weather conditions. Fluctuating power causes frequency deviations and reduction in reliability of the isolated power utility or microgrid when large output power from several PV systems is penetrated in the utility. In this paper, to overcome these problems, a simple coordinated control method for leveling the fluctuations of combined power output from multiple PV systems is proposed. The conflicting objective of output power leveling and acquisition power increase is achieved by means of the proposed method. Here, output power command is generated in two steps: central and local. Fuzzy reasoning is used to generate the central leveling output power command considering insolation, variance of insolation, and absolute average of frequency deviation. In local step, a simple coordination is maintained between central power command and local power commands by producing a common tuning factor. Power converters are used to achieve the same output power as local command power employing PI control law for each of the PV generation systems. The proposed method is compared with the method where a modified maximum power point tracking control is used for smoothing the short-term change in each of the PV system's output. Simulation results show that the proposed method is effective for leveling output power fluctuations and feasible to reduce the frequency deviations of the isolated power utility to maintain reliability. Index Terms--Coordinated control, frequency deviation, fuzzy reasoning, insolation, photovoltaic (PV) output power fluctuations.

A transformer provides galvanic isolation and grounding of the photovoltaic (PV) array in a PV-fed grid-connected inverter. Inclusion of the transformer, however, may increase the cost and/or bulk of the system. To overcome this drawback, a single-phase, single-stage [no extra converter for voltage boost or maximum power point tracking (MPPT)], doubly grounded, transformer-less PV interface, based on the buck-boost principle, is presented. The configuration is compact and uses lesser components. Only one (undivided) PV source and one buck-boost inductor are used and shared between the two half cycles, which prevents asymmetrical operation and parameter mismatch problems. Total harmonic distortion and dc component of the current supplied to the grid is low, compared to existing topologies and conform to standards like IEEE 1547. A brief review of the existing, transformer-less, grid-connected inverter topologies is also included. It is demonstrated that, as compared to the split PV source topology, the proposed configuration is more effective in MPPT and array utilization. Design and analysis of the inverter in discontinuous conduction mode is carried out. Simulation and experimental results are presented. Index Terms--Double grounding, IEC 1547, IEC 61727, maximum power point tracking (MPPT), NEC 690, partially shaded conditions, photovoltaic.

This paper describes the connected photovoltaic (PV) power generation system's grid overvoltage protection function and summarizes the occurrence of the output power loss due to the grid voltage rise. Power injection from the PV system will raise the voltage at the power distribution line. A power conditioning subsystem (PCS) needs to regulate its output if the voltage becomes higher than the upper limit in order to avoid the overvoltage at the power grid. Thus, a PV system cannot generate electricity under the high grid voltage. There are 553 residential PV systems installed in Ota, Japan, for the demonstration research project of clustered PV systems. Measurement data of these 2.1-MW grid-connected PV systems are used for the analysis. Only the limited number of PV systems experienced a significant amount of output energy loss due to the high grid voltage in a particular day, whereas the other system's outputs also raise the grid voltage. The causes of this maldistribution of the output energy loss are the difference of the line impedance, the difference of the starting voltage of the PCS's grid overvoltage protection function, and the imbalance of the load in single-phase three-wire power distribution systems. The present control of the PCS successfully avoids the overvoltage on the grid but cannot share the loss. Index Terms--Grid disturbances, loss analysis, overvoltage protection, photovoltaic power systems.

This paper presents a two-control loop design considering the nonlinear time-varying characteristics of a single-phase grid-connected photovoltaic (PV) full-bridge central inverter. The control scheme design is based on the energy-balance modeling of the PV system and enables the design of a voltage loop linear discrete controller ensuring the stability of the system for the whole range of PV array operating conditions. A set of experimental results carried out on a laboratory prototype is provided to validate the proposed approach. Index Terms--Photovoltaic power systems, power control, power electronics.

A substantial increase of photovoltaic (PV) power generators installations has taken place in recent years, due to the increasing efficiency of solar cells as well as the improvements of manufacturing technology of solar panels. These generators are both grid-connected and stand-alone applications. We present an overview of the essential research results. The paper concentrates on the operation and modeling of stand-alone power systems with PV power generators. Systems with PV array-inverter assemblies, operating in the slave-and-master modes, are discussed, and the simulation results obtained using a renewable energy power system modular simulator are presented. These results demonstrate that simulation is an essential step in the system development process and that PV power generators constitute a valuable energy source. They have the ability to balance the energy and supply good power quality. It is demonstrated that when PV array-inverters are operating in the master mode in stand-alone applications, they well perform the task of controlling the voltage and frequency of the power system. The mechanism of switching the master function between the diesel generator and the PV array-inverter assembly in a stand-alone power system is also proposed and analyzed. Finally, some experimental results on a practical system are compared to the simulation results and confirm the usefulness of the proposed approach to the development of renewable energy systems with PV power generators. Index Terms--Autonomous power systems, master/slave inverter operation, photovoltaic (PV) systems, power system modeling and simulation.

This paper presents a transformerless three-phase inverter designed for the integration into a special type of a photovoltaic (PV) module, which is capable of providing an output voltage of several hundred volts. The chosen topology, a current-source inverter, features a single-stage power conversion system that directly feeds into the grid. The principle operation and control is described, and a modified modulation strategy is proposed to attenuate common-mode currents. A robust and highly efficient laboratory prototype of a 250-W module integrated converter has been implemented and tested. Its compact and flat design allows the direct attachment to the PV module. Index Terms--Alternating current (AC) module, current-source inverter (CSI), photovoltaic (PV) power systems, pulsewidth-modulated (PWM) inverters.

Power processing systems will be a key factor of future photovoltaic (PV) applications. They will play a central role in transferring, to the load and/or to the grid, the electric power produced by the high-efficiency PV cells of the next generation. In order to come up the expectations related to the use of solar energy for producing electrical energy, such systems must ensure high efficiency, modularity, and, particularly, high reliability. The goal of this paper is to provide an overview of the open problems related to PV power processing systems and to focus the attention of researchers and industries on present and future challenges in this field. Index Terms--Inverters, maximum power point tracking (MPPT), photovoltaic (PV) systems, power electronics, power processing, solar energy.

In evaluating the energy-generation potential of a photovoltaic (PV) energy system, the system is usually assumed to work without interruptions over its entire life. PV energy systems are fairly reliable, but as any complex system, they may fail. In PV systems, the inverter is responsible for the majority of failures, and most inverter failures are blamed on the aluminum electrolytic capacitors typically used in the dc bus. This paper investigates the effects of common failure modes on the reliability of PV inverters and suggests a model framework for decomposing the inverter into subsystems for more detailed study. The challenges of statistical analysis based on small data sets are discussed, and simulations are performed to illustrate the proposed model using a simple decomposition into subsystems of the inverter used in the 342-kW PV system at the Georgia Tech Aquatic Center. Index Terms--Photovoltaic (PV) power systems, power-system reliability, power-system simulation, reliability modeling, simulation.

Islanding is one important concern for grid connected distributed resources due to personnel and equipment safety. Inverter-resident active islanding detection methods (IDMs) inject disturbances into the supply system and detect islanding based on the responses of locally measured parameters. Frequency drifting IDMs are very effective in systems with a single inverter but it is believed that their effectiveness might be reduced in multi-inverter systems due to mutual interferences. This paper discusses the effect of inverters with passive IDM on the islanding detection capabilities of inverters with active frequency drifting IDMs and also interferences between the latter. Non-detection zones (NDZs) of multi-inverter systems in a load parameter space are used as analytical tool. Simulation and experimental results are provided to validate the NDZs of the multi-inverter systems with active frequency drifting IDMs. Index Terms--Distributed generation (DG), inverters, islanding detection, non-detection zones, photovoltaic systems.

This paper presents a computational fluid dynamics (CFD) study of a building-integrated photovoltaic thermal (BIPV/T) system, which generates both electricity and thermal energy. The heat transfer in the BIPV/T system cavity is studied with a two-dimensional CFD model. The realizable k-[epsilon] model is used to simulate the turbulent flow and convective heat transfer in the cavity, including buoyancy effect and long-wave radiation between boundary surfaces is also modeled. A particle image velocimetry (PIV) system is employed to study, the fluid flow in the BIPV/T cavity and provide partial validation for the CFD model. Average and local convective heat transfer coefficients are generated with the CFD model using measured temperature profile as boundary condition. Cavity temperature profiles are calculated and compared to the experimental data for different conditions and good agreement is obtained. Correlations of convective heat transfer coefficients are generated for the cavity surfaces; these coefficients are necessary for the design and analysis of BIPV/T systems with lumped parameter models. Local heat transfer coefficients, such as those presented, are necessary for prediction of temperature distributions in BIPV panels. [DOI: 10.1115/1.2770750]

In a first and completely new approach, a vacuum plasma-spray coating technique was used to deposit selective emitting rare-earth oxide films of ytterbia ([Yb.sub.2][O.sub.3]) on porous silicon-infiltrated silicon carbide foams (Si-SiC). The plasma-spray coating technique offers a new and promising way to produce selective emitting coatings on different refractory substrates with complex geometries. The adhesion and thermal shock stability were tested until a film thickness of 130 [micro]m was achieved; the selective emittance of the oxide coating has been found to be dependent on the film thickness. The material combination Si-SiC and [Yb.sub.2][O.sub.3], however, needs some major improvement regarding hightemperature stability and high thermal cycling loads. In a different approach, the advantage of low emitting [Al.sub.2][O.sub.3] fibers and good thermal matching was combined with [Yb.sub.2][O.sub.3] slurry coating of flexible alumina ([Al.sub.2][O.sub.3]) fiber bundles, formed into a cylindrical shape. The thin fiber structure tried to imitate the famous incandescent mantle emitters of Auer von Welsbach, but with a more rugged structure. Even though the fibers of the woven emitter were thin, the low thermal conductivity of [Al.sub.2][O.sub.3] led to a distinct reduction of the surface temperature and emittance, and a shielding effect of the radiation emanating from the hot inner walls by the cooler outer grid structure was inevitable. Optical filters consisting of a water film and of transparent conducting oxides (TCO) have been developed and tested to protect the photocells against overheating and to reflect nonconvertible off-band radiation back to the emitter. The water film led to a significant reduction of the cell temperature and increased cell performance, whereas with the TCO filters only a reduction of the cell temperature was observed. [DOI: 10.1115/1.2770749] Keywords: fuel-fired thermophotovoltaics, ceramics, rare-earth oxides, emission, ytterbia, silicon carbide, coating

The many different techniques for maximum power point tracking of photovoltaic (PV) arrays are discussed. The techniques are taken from the literature dating back to the earliest methods. It is shown that at least 19 distinct methods have been introduced in the literature, with many variations on implementation. This paper should serve as a convenient reference for future work in PV power generation. Index Terms--Maximum power point tracking (MPPT), photovoltaic (PV).

This paper describes the construction and the performance of a distributed power generating system of photovoltaics (PV) using solar energy estimation. Instead of conventional batteries a new storage device, called energy capacitor system (ECaSS), is used for its various advantages. A procedure is described to estimate the PV output power by calculating solar radiation. To calculate the daily insolation, Hotters equation and Liu-Jordan's equation have been used with some modification. The accuracy of the procedure is verified by comparing the calculated solar radiation with the practically measured one. To apply the procedure even in cloudy/rainy weather, the characteristic of the PV output in different weather conditions has been studied using 1-day-ahead weather forecasts. With an aim to set the optimum amount of buy power, a simulation program has been developed. The system is being operated in optimal economic mode in different weather conditions. The financial benefit and load-leveling capacity of the system have been studied and presented here. It was found that the system provides an excellent economic benefit for nonflat price of electricity and load-leveling facility with a good overall efficiency. Index Terms--Distributed generation, electric double layer capacitor (EDLC), energy capacitor system (ECaSS), load leveling, photovoltaic (PV).

The objective of this paper is to propose a novel multi-input inverter for the grid-connected hybrid photovoltaic (PV)/wind power system in order to simplify the power system and reduce the cost. The proposed multi-input inverter consists of a buck/buck-boost fused multi-input dc-dc converter and a full-bridge dc-ac inverter. The output power characteristics of the PV array and the wind turbine are introduced. The perturbation and observation method is used to accomplish the maximum power point tracking algorithm for input sources. The operational principle of the proposed multi-input inverter is explained. The control circuit is realized by using a digital signal processor and auxiliary analog circuits. For practical applications, functions of soft-start and circuit protection are implemented. Experimental results have shown the performance of the proposed multi-input inverter with desired features. Index Terms--Inverter, photovoltaic (PV), wind energy.

Cost reductions methods in designing photovoltaics and effects of external factors including energy policies on these methods are analyzed.

This paper presents a novel approach for the connection of renewable energy sources to the utility grid. Due to the increasing power capability of the available generation systems, a three-level three-phase neutral-point-clamped voltage-source inverter is selected as the heart of the interfacing system. A multivariable control law is used for the regulator because of the intrinsic multivariable structure of the system. A current source (playing the role of a generic renewable energy source) is connected to the grid using a three-level inverter in order to verify the good performance of the proposed approach. Large-and small-signal d--q state-space averaged models of the system are obtained and used to calculate the multivariable controller based on the linear quadratic regulator technique. This controller simultaneously regulates the dc-link voltage (to operate at the maximum power point of the renewable energy source), the mains power factor (the power is delivered to the grid at unity power factor), and the dc-link neutral-point voltage balance. With the model and regulator presented, a specific switching strategy to control the dc-link neutral-point voltage is not required. The proposed controller can be used for any application, since its nature makes possible the control of any system variable. The good performance of the presented interfacing solution in both steady-state and transient operation is verified through simulation and experimentation using a 1-kW neutral-point-clamped voltage-source-inverter prototype, where a PC-embedded digital signal processor board is used for the controller implementation. Index Terms--Distributed power generation, grid interface, multilevel conversion, multivariable control, photovoltaic power system, three-level inverter, wind power system.

The improved maximum power point tracking (MPPT) control method for small-scale dual-module photovoltaic (PV) systems is presented in this paper. With this method, the voltage and current information of each module are shared and utilized for the detection of the maximum-power point (MPP) without measuring power. This approach can be implemented in a simple structure, especially due to the elimination of memory and multiplication devices. The proposed method is verified by a hardware prototype of grid-connected dual-module PV systems with the proposed analog-implemented MPPT controller. In addition, practical issues of the proposed scheme are considered. Index Terms--Dual module, grid-connected inverter, maximum power point tracking (MPPT), photovoltaic (PV) systems.

Photovoltaic power systems are usually integrated with some specific control algorithms to deliver the maximum possible power. Several maximum power point tracking (MPPT) methods that force the operating point to oscillate have been presented in the past few decades. In the MPPT system, the ideal operation is to determine the maximum power point (MPP) of the photovoltaic (PV) array directly rather than to track it by using the active operation of trial and error, which causes undesirable oscillation around the MPP. Since the output features of a PV cell vary with environment changes in irradiance and temperature from time to time, real-time operation is required to trace the variations of local MPPs in PV power systems. The method of real-time estimation proposed in this paper uses polynomials to demonstrate the power-voltage relationship of PV panels and implements the recursive least-squares method and Newton-Raphson method to identify the voltage of the optimal operating point. The effectiveness of the proposed methods is successfully demonstrated by computer simulations and experimental evaluations of two major types of PV panels, namely: 1) crystalline silicon and 2) copper-indium-diselenide thin film. Index Terms--Curve fitting, least-squares method (LS), Newton-Raphson method (NRM), parameter estimation, photovoltaic (PV) power systems, polynomials, real-time systems.

In moderate climates, short fluctuations in solar irradiance and their impact on the distribution grid will become an important issue with regard to the future large-scale application of embedded photovoltaic systems. Several related studies from the past are recalled. The approach that is presented here applies a localized spectral analysis to the solar irradiance and derived quantities in order to determine the power content of fluctuations, depending on their characteristic persistence. Pseudorandom time series of solar irradiance, based on measured values of the instantaneous clearness index, are applied as input data. Power-flow calculations are carried out in order to assess the impact of fluctuating solar irradiance on the grid voltage. The 'fluctuation power index' is defined as a measure for the mean-square value of fluctuations of a specific persistence. A typical scenario is simulated, and the results are interpreted. Index Terms--Dispersed storage and generation, Monte Carlo methods, random number generation, signal analysis, solar energy, solar radiation, wavelet transforms.

Islanding detection is a mandatory feature for gridconnected inverters. The effectiveness of passive islanding detection methods (IDMs) is usually demonstrated by means of nondetection zones (NDZs) represented in a power mismatch space ([DELTA]P versus [DELTA]Q). Active frequency drifting IDMs have been shown to provide improved performance but their theoretical NDZ cannot be described in the [DELTA]P versus [DELTA]Q space for a general RLC load. This paper shows that a load parameter space based on the values of the quality factor and resonant frequency of the local load ([Q.sub.f] versus [f.sub.o]) can be used in these cases. It employs a single curve to represent the NDZ of frequency drifting IDMs for any RLC loads. Equations that represent NDZs of three common active IDMs in the Q f versus [f.sub.o] load parameter space are derived and it is shown that the slip mode frequency shift and the Sandia frequency shift IDMs can be designed to guarantee islanding detection for equivalent RLC loads with a quality factor smaller than a design value. The accuracy of the NDZs is verified with simulation and experimental results. Index Terms--Dispersed storage and generation, islanding, inverters, photovoltaic power systems, protection, safety.

A new multiunit optimization algorithm is formulated to plan participation by photovoltaic sources with significant penetration in utility generation. The focus is on the minimization of the overall levelized cost of energy, with constraints on demand, capacity as decided by penetration, efficiency levels, and various cost components. Collector areas, unit ratings, and required investment evolve as part of the plan. A case study involving a utility service area, inclusive of 12 potential installation sites, is used to illustrate features of the algorithm. Index Terms--Energy resources, photovoltaic power systems, power generation, power generation planning, solar energy.

This paper presents the experimental results and discusses the track of the maximum power point on the current-voltage curve of a PV module due to changes of the illumination level and temperature. A time decrease of the voltage and simultaneous temperature increase during the initial stage of irradiation has been observed. Some practical implementation aspects of a maximum powerpoint tracking unit, which match the current and voltage characteristics of the load to the PV module's maximum power point automatically, are also discussed. A linear decrease of the maximum output power [P.sub.m] with temperature increase has been observed and the temperature coefficient was derivate. Temperature coefficients for [V.sub.OC], [I.sub.SC], [V.sub.mpp], [I.sub.mpp], and [[eta].sub.PV] have been determined for the photovoltaic module. Also the radiation-rate coefficient at constant temperature has been calculated. [DOI: 10.1115/1.2147586] Keywords: solar cells, photovoltaic module, temperature coefficients, maximum power point

Photovoltaic (PV) systems, grid-connected or stand-alone, use the power conditioning unit (PCU) to optimize the energy transfer from the PV generator to the user load by using the maximum power point tracker, and also to invert, regulate, and wave shape the power between the components of the system. To study and optimize this system, different PCU models are applied. In this work different empirical models of the PCU have been set up. The purpose of this investigation is to verify the possibility of the PCU application to this system and study the different mathematical energy efficiency models. All these models have been simulated and compared with a typical real system to show which one is better. The second part of the paper includes the optimal size of the PCU with the PV system and also an application comparison between two possible locations for varied geographical latitudes, in Kassel/Germany (middle Europe) and Cairo/Egypt (North Africa). [DOI: 10.1115/1.2148978] Keywords: power conditioning unit, modeling, sizing, photovoltaic system

Sizing represents an important part of photovoltaic system design. This paper describes a sizing procedure based on the observed time series of solar radiation. Using a simple geometrical construction, the sizing curve is determined as a super-position of contributions from individual climatic cycles of low daily solar radiation. Unlike the traditional methods based on loss-of-load probability, the reliability of supply enters in this method through the length of the time series of data used in the analysis. The method thus resembles techniques used in other branches of engineering where extreme values are considered as functions of certain recurrence intervals. Keywords: Photovoltaic systems; Sizing; Solar radiation

Inverter sizing strategies for grid-connected photovoltaic (PV) systems often do not take into account site-dependent peculiarities of ambient temperature, inverter operating temperature and solar irradiation distribution characteristics. The operating temperature affects PV modules and inverters in different ways and PV systems will hardly ever have a DC output equal to or above their STC-rated nominal power. Inverters are usually sized with a nominal AC output power some 30% (sometimes even more) below the PV array nominal power. In this paper, we show that this practice might lead to considerable energy losses, especially in the case of PV technologies with high temperature coefficients of power operating at sites with cold climates and of PV technologies with low temperature coefficients of power operating at sites with warm climates and an energy distribution of sunlight shifted to higher irradiation levels. In energy markets where PV kW h's are paid premium tariffs, like in Germany, energy yield optimization might result in a favorable payback of the extra capital invested in a larger inverter. This paper discusses how the time resolution of solar radiation data influences the correct sizing of PV plants. We demonstrate that using instant (10 s) irradiation values instead of average hourly irradiation values leads to considerable differences in optimum inverter sizing. When calculating inverter yearly efficiency values using both, hourly averages and 1-min averages, we can show that with increased time resolution of solar irradiation data there are higher calculated losses due to inverter undersizing. This reveals that hourly averages hide important irradiation peaks that need to be considered. We performed these calculations for data sets from pyranometer readings from Freiburg (48[degrees]N, Germany) and Florianopolis (27[degrees]S, Brazil) to further show the peculiarities of the site-dependent distribution of irradiation levels and its effects on inverter sizing. Keywords: Inverter sizing; Grid-connected PV systems

A compact stand-a/one PV power system was recently designed and built to run an air sampler for environmental monitoring at the Nevada Test Site. This paper presents an overview of the system design and analysis of some of the recorded daily cycles of various power flows during the summer period. The system long-term performance during both high and low solar resource periods is simulated with the computer code PVFORM using historical weather data. [DOI: 10.1115/1.1562951]

Solar FIT module This is the green power generator of the system. There are many flavours of solar modules on the market now. Most are multi-crystalline silicon solar cells, with [...]

Photovoltaic (PV) generators exhibit nonlinear v-i characteristics and maximum power (MP) points that vary with solar insolation. An intermediate converter can therefore increase efficiency by matching the PV system to the load and by operating the solar cell arrays (SCAs) at their maximum power point. An MP point tracking algorithm is developed using only SCA voltage information thus leading to current sensorless tracking control. The inadequacy of a boost converter for array voltage based MP point control is experimentally verified and an improved converter system is proposed. The proposed converter system results in low ripple content, which improves the array performance and hence a lower value of capacitance is sufficient on the solar array side. Simplified mathematical expressions for a PV source are derived. A signal flow graph is employed for modeling the converter system. Current sensorless peak power tracking effectiveness is demonstrated through simulation results. Experimental results are presented to validate the proposed method.