Analytical study and evaluation results of power optimizers for distributed power conditioning in photovoltaic arrays

The use of modular or ‘micro’ maximum power point tracking (MPPT) converters at module level in series association, commercially known as “power optimizers”, allows the individual adaptation of each panel to the load, solving part of the problems related to partial shadows and different tilt and/or orientation angles of the photovoltaic (PV) modules. This is particularly relevant in building integrated PV systems. This paper presents useful behavioural analytical studies of cascade MPPT converters and evaluation test results of a prototype developed under a Spanish national research project. On the one hand, this work focuses on the development of new useful expressions which can be used to identify the behaviour of individual MPPT converters applied to each module and connected in series, in a typical grid‐connected PV system. On the other hand, a novel characterization method of MPPT converters is developed, and experimental results of the prototype are obtained: when individual partial shading is applied, and they are connected in a typical grid connected PV array. Copyright © 2011 John Wiley & Sons, Ltd.     

Individual Module Maximum Power Point Tracking for Thermoelectric Generator Systems

In a thermoelectric generator (TEG) system the DC/DC converter is under the control of a maximum power point tracker which ensures that the TEG system outputs the maximum possible power to the load. However, if the conditions, e.g., temperature, health, etc., of the TEG modules are different, each TEG module will not produce its maximum power. If each TEG module is controlled individually, each TEG module can be operated at its maximum power point and the TEG system output power will therefore be higher. In this work a power converter based on noninverting buck–boost converters capable of handling four TEG modules is presented. It is shown that, when each module in the TEG system is operated under individual maximum power point tracking, the system output power for this specific application can be increased by up to 8.4% relative to the situation when the modules are connected in series and 16.7% relative to the situation when the modules are connected in parallel.     

Analysis and design of phase‐interleaving series‐connected module‐integrated converter for DC‐link ripple reduction of multi‐stage photovoltaic power systems

Recently, installation of photovoltaic power systems such as building‐integrated photovoltaic in urban area has been spotlighted in renewable energy engineering field, even at the expense of the performance degradation from partial shading. The efficiency degradation of maximum power point tracking (MPPT) performance can be compensated by a kind of power‐conditioning system architecture such as module‐integrated converters (MIC), which can handle the optimal‐operation tracking for its own photovoltaic (PV) module. In case of a MIC with series‐connected outputs, it is easy to obtain a high DC‐link voltage for multiple stage PV power conditioning applications. However, switching ripple of the DC‐link voltage also increases as number of the modules increases. In this paper, as a solution for the ripple reduction, interleaved pulse width modulation‐phase synchronizing method is applied to the PV MIC modules. The switching‐ripple analysis of the MPPT power modules were performed and compared between the cases such as phase control or not. For the implementation of the phase control among the modules, Zigbee (XBee Pro, Digi International, Minnetonka, MN, USA) wireless communications transceiver and DSP (TMS320F28335, Texas Instruments, Dallas, TX, USA) series communications interface are utilized. Hardware prototype of the double‐module boost‐type 80‐W MICs has been built to validate the DC‐link voltage ripple reduction. Copyright © 2012 John Wiley & Sons, Ltd.     

“In‐field” cell temperature evaluation in solar modules through time‐dependent open circuit voltage measurements

A method for the evaluation of p–n junction cell temperature in PV modules operating in the maximum power point (MPP) mode has been proposed. The method does not require specialized equipment and (for the concentrator modules) the data on the open circuit (OC) voltage temperature coefficients measured under pulse illumination. It consists of measuring several open circuit voltage magnitudes together with temperature measurements on the external module surface near one of the cells. In this procedure, a fast transition from MPP to OC operational mode is carried out, during which a time‐dependent voltage measurement is carried out with the help of a memory oscilloscope. A “reference” OC voltage magnitude in a “cold” module (a condition, as if the cells are kept at ambient temperature) is obtained by calculations, so that there is no necessity in a fast mechanical shuttering of the module aperture area. In the case of the concentrator modules, the module OC voltage temperature coefficient can be measured, if heat sinking process is artificially modified during outdoor measurements. Copyright © 2015 John Wiley & Sons, Ltd.     

A novel analytical model for determining the maximum power point of thin film photovoltaic module

Achieving the maximum power output from photovoltaic (PV) modules is indispensable for the operation of grid‐connected PV power systems under varied atmospheric conditions. In recent years, the study of PV energy for different applications has attracted more and more attention because solar energy is clean and renewable. We propose an efficient direct‐prediction method to enhance the utilization efficiency of thin film PV modules by tackling the problem of tracking time and overcoming the difficulty of calculation. The proposed method is based on the p–n junction recombination mechanism and can be applied to all kinds of PV modules. Its performance is not influenced by weather conditions such as illumination or temperature. The experimental results show that the proposed method provides high‐accuracy estimation of the maximum power point (MPP) for thin film PV modules with an average error of 1.68% and 1.65% under various irradiation intensities and temperatures, respectively. The experimental results confirm that the proposed method can simply and accurately estimate the MPP for thin film PV modules under various irradiation intensities and temperatures. In future, the proposed method will be used to shed light on the optimization of the MPP tracking control model in PV systems. Copyright © 2012 John Wiley & Sons, Ltd.     

Fabrication of monolithic photovoltaic arrays on crystalline silicon by wafer bonding and deep etching techniques

In this work, a novel technology to fabricate small (∼1 cm²) c‐Si photovoltaic mini‐modules is shown. This technology combines two main bulk micro‐machining techniques: fusion (or adhesive) bonding and anisotropic etching of silicon. Due to the fact that the photovoltaic cells are fabricated in the same wafer, it is mandatory to etch the whole substrate to ensure electrical isolation. Once the individual cells are bulk‐isolated they can be connected in series so as to scale up the output voltage of the mini‐array. A handling wafer is required to provide mechanical stability to the device wafer. Adhesive and fusion bonding are used to join the handling and the device wafer. First electrical results, under standard Air Mass 1·5 (AM 1·5) solar spectrum light (100 mW/cm²), using a 9‐cell series connected mini‐module fabricated by fusion bonding, leads to a total open‐circuit voltage of 4·11 V, a short‐circuit current of 2·45 mA, and a maximum delivered power of 3·8 mW for each mini‐module (1·4 cm²). A 16‐cell series‐connected mini‐module fabricated by adhesive bonding and wire bonding, yields an open‐circuit voltage of 7·45 V, a short‐circuit current of 390 µA, and maximum delivered power of 1·8 mW, with 1·1 cm² of mini‐module area. Copyright © 2005 John Wiley & Sons, Ltd.     

Operation of series‐connected silicon‐based photovoltaic modules under partial shading conditions

Partial shading has been recognized as a major cause of energy losses in photovoltaic (PV) power generators. Partial shading has severe effects on the electrical characteristics of the PV power generator, because it causes multiple maximum power points (MPPs) to the power‐voltage curve. Multiple maxima complicate MPP tracking, and the tracking algorithms are often unable to detect the global maximum. Considerable amount of available electrical energy may be lost, when a local MPP with low power is tracked instead of the global MPP. In this paper, the electrical characteristics of series‐connected silicon‐based PV modules under various partial shading conditions are studied by using a Matlab/Simulink simulation model. The simulation model consists of 18 series‐connected PV modules, corresponding to a single‐phase grid‐connected PV power generator. The validity of the simulation model has been verified by experimental measurements. The voltage and power characteristics of the PV power generator have been investigated under various system shading and shading strength conditions. The results can be utilized to develop new MPP tracking algorithms and in designing, for example, building integrated PV power generators. Copyright © 2011 John Wiley & Sons, Ltd.     

Control of a Single-Phase Cascaded H-Bridge Multilevel Inverter for Grid-Connected Photovoltaic Systems

This paper presents a single-phase cascaded H-bridge converter for a grid-connected photovoltaic (PV) application. The multilevel topology consists of several H-bridge cells connected in series, each one connected to a string of PV modules. The adopted control scheme permits the independent control of each dc-link voltage, enabling, in this way, the tracking of the maximum power point for each string of PV panels. Additionally, low-ripple sinusoidal-current waveforms are generated with almost unity power factor. The topology offers other advantages such as the operation at lower switching frequency or lower current ripple compared to standard two-level topologies. Simulation and experimental results are presented for different operating conditions.     

The results of performance measurements of field‐aged crystalline silicon photovoltaic modules

This paper presents the results of electrical performance measurements of 204 crystalline silicon‐wafer based photovoltaic modules following long‐term continuous outdoor exposure. The modules comprise a set of 53 module types originating from 20 different producers, all of which were originally characterized at the European Solar Test Installation (ESTI), over the period 1982–1986. The modules represent diverse generations of PV technologies, different encapsulation and substrate materials. The modules electrical performance was determined according to the standards IEC 60891 and the IEC 60904 series, electrical insulation tests were performed according to the recent IEC 61215 edition 2. Many manufacturers currently give a double power warranty for their products, typically 90% of the initial maximum power after 10 years and 80% of the original maximum power after 25 years. Applying the same criteria (taking into account modules electrical performance only and assuming 2·5% measurement uncertainty of a testing lab) only 17·6% of modules failed (35 modules out of 204 tested). Remarkably even if we consider the initial warranty period i.e. 10% of P_max after 10 years, more than 65·7% of modules exposed for 20 years exceed this criteria. The definition of life time is a difficult task as there does not yet appear to be a fixed catastrophic failure point in module ageing but more of a gradual degradation. Therefore, if a system continues to produce energy which satisfies the user need it has not yet reached its end of life. If we consider this level arbitrarily to be the 80% of initial power then all indications from the measurements and observations made in this paper are that the useful lifetime of solar modules is not limited to the commonly assumed 20 year. Copyright © 2008 John Wiley & Sons, Ltd.     

基于DSP的光伏最大功率跟踪技术的研究

光伏电池的最大功率点受光照强度、环境温度和负载大小等外界因素的影响而不断变化,因此很有必要对光伏电池的最大功率点进行时时跟踪以提高光伏电池的利用率.以分析光伏电池输出特性为基础,在实验室环境下用直流电源和可变电阻来模拟光伏电池,并选择Boost电路实现DC/DC变换来搭建实验电路;同时该实验采用DSP(TMS320F2812)作为控制器来进行最大功率跟踪算法的控制.实验结果表明,该方法可以有效地完成光伏的最大功率跟踪.     

A novel high-performance utility-interactive photovoltaic inverter system

This paper presents a novel photovoltaic inverter that cannot only synchronize a sinusoidal AC output current with a utility line voltage, but also control the power generation of each photovoltaic module in an array. The proposed inverter system is composed of a half-bridge inverter at the utility interface and a novel generation control circuit which compensates for reductions in the output power of the system that are attributable to variations in the generation conditions of respective photovoltaic modules. The generation control circuit allows each photovoltaic module to operate independently at peak capacity, simply by detecting the output power of the system. Furthermore, the generation control circuit attenuates low-frequency ripple voltage, which is caused by the half-bridge inverter, across the photovoltaic modules. Consequently, the output power of the system is increased due to the increase in average power generated by the photovoltaic modules. The effectiveness of the proposed inverter system is confirmed experimentally and by means of simulation.     

Enhancing the power output of PV modules by considering the view factor to sky effect and rearranging the interconnections of solar cells

The solar diffuse radiation incident on a photovoltaic (PV) module is unevenly distributed along the module's width because its solar‐cells “see” different view‐factor values with respect to their position. This fact causes PV modules to experience undesired power losses brought about by current mismatch. The paper addresses this issue and presents new interconnection strategies for the module's cells. The proposed interconnections are shown to introduce power gain vis‐à‐vis the all‐series connected module. Having established the theoretical framework, a case study is examined, comparing two sites with considerable different amounts of diffuse radiation with the aim of quantifying the power production enhancement with regard to the site's prevailing annual extent of diffuse radiation. It is found, for example, that by converting the all‐series cell interconnection into parallel strips in Desert Rock (NV, USA), each module can be supplemented with a 6.5 [kW_ph] annually on average. The study may have financial implications for the PV industry which strives to increase power generation while maintaining reduced costs. Copyright © 2017 John Wiley & Sons, Ltd.     

Annual degradation rates of recent crystalline silicon photovoltaic modules

Long‐term reliability and durability of recently installed photovoltaic (PV) systems are currently unclear because they have so far only been operated for short periods. Here, we investigated the quality of six types of recent crystalline silicon PV modules to study the viability of PV systems as dispersed power generation systems under operating conditions connected to an electric power grid. Three indicators were used to estimate the annual degradation rates of the various crystalline silicon PV modules: energy yield, performance ratio, and indoor power. Module performance was assessed both with indoor and outdoor measurements using electric measurements taken over a 3‐year period. The trends in the results of the three indicators were almost consistent with each other. Although the performance of the newly installed PV modules decreased by over 2% owing to initial light‐induced degradation immediately after installation, little to no degradation was observed in all the PV modules composed of p‐type solar cells over a 3‐year operation period. However, the PV modules composed of n‐type solar cells clearly displayed performance degradation originating from the reduction of open‐circuit voltage or potential‐induced degradation. The results indicate that a more continuous and detailed outdoor actual investigation is important to study the quality of new, high‐efficiency solar cells, such as heterojunction, interdigitated back contact solar cells, and passivated emitter rear cells, which are set to dominate the PV markets in the future. © 2017 The Authors. Progress in Photovoltaics: Research and Applications published by John Wiley & Sons Ltd.     

基于MATLAB的光伏模块输出特性及MPPT的建模与仿真

基于光伏模块的等效电路模型,结合simulink中的S函数,建立了光伏电池仿真模型。利用该模型,可以模拟实际光伏模块产品在不同太阳辐射强度、环境温度下的I-V和P-V特性。分析了常见的最大功率跟踪(MPPT)方法爬坡法,并建立了仿真模型。     

一种基于Boost电路的光伏最大功率跟踪方法

最大功率跟踪(MPPT)是太阳能光伏发电的重要组成部分,依靠最大功率跟踪可使光伏电池工作在最大功率点(MPP)附近,提高太阳能的利用率.在分析光伏电池的数学模型的基础上,选用Boost电路作为DC/DC变换来搭建仿真模型;针对传统的定步长扰动观测法存在的震荡和误判现象,提出一种改进的扰动观测法,并在Matlab/Simulink环境下进行了仿真.与定步长的扰动观测法的仿真结果进行对比,表明该算法的响应速度更加迅速;在外界环境发生变化时,该算法能够快速做出判断,准确地跟踪到光伏电池的最大功率点.     

光伏组件直流模块的控制策略研究

在非理想工况下,光伏阵列的多峰现象会使得输出功率大幅下降,严重影响整个光伏发电系统的工作效率.为保证每个组件在不同工况下都能运行在各自的最大功率点处,提出一种基于双开关管的Buck-Boost直流斩波电路的光伏组件优化器,能够根据设计的控制策略自动切换工作模式和寻找最大功率点.实验验证了该控制策略的有效 性和高效性.     

基于可编程逻辑电路的相机高精度同步信号研究

介绍了一种相机曝光控制实时误差补偿外同步信号产生方法.系统由一系列硬件和软件组成,硬件包括通用GPS接收设备、信号接收整理模块和外同步信号相机电平匹配化模块;软件包括设计在可编程逻辑电路中的外同步信号频率生成与调整模块及外同步信号误差分析与调整模块、外同步信号生成模块及外同步信号相机逻辑匹配化模块.GPS接收设备接收卫星信号并实时输出时间B码,信号接收整理模块将时间B码整理成可编程逻辑器件可以接收的电平信号,可编程逻辑器件通过一系列处理模块生成目标相机的外同步信号,随后外同步信号相机电平匹配化模块将外同步信号整理成相机匹配的电平信号来控制相机曝光.实践证明所提方法可以在不添加额外器件的情况下实现相机曝光时间的实时精确控制,将连续成像的图像序列在时间上的精度从通常的数十微秒级提高到单微秒级.     

Innovation and fabrication of 5.5 generation image‐patterned translucent photovoltaic module by using laser scribing technology

The building‐integrated photovoltaic (PV) technology is one of the most promising applications for amorphous silicon (a‐Si) thin film solar cells. It is necessary to develop more various building‐integrated PV modules, which will provide architects and industries more options for the PV installation to their buildings or construction bodies. In this paper, a new type of a‐Si PV module, called image‐patterned translucent a‐Si PV module, is developed. Any required image can be displayed on the module by using laser processes. In the present result, a 5.5 generation (1100 × 1400 mm) image‐patterned translucent PV module with 10% transmittance exhibits the stabilized maximum power output (P_max) of 92.5 W, which can be further improved by optimizing the laser parameters. The remarkable features of our module such as the image displaying, natural light transmission, and heat reduction create entirely new applications including windows and logos and provide an option that adds personal style and unique design to the building interiors. Copyright © 2011 John Wiley & Sons, Ltd.     

An enhanced scanning-based MPPT approach for DMPPT systems

In this paper, an improved maximum power point tracking (MPPT) approach being low parameter dependency, simple structure and limited search interval has been presented for distributed MPPT photovoltaic (PV) systems. Basically, this approach is based on scanning of power–voltage (P-V) characteristic curve of PV modules in a limited duty ratio interval which makes tracking operation simple, fast and efficiently available in both uniform irradiance and partial shading conditions (PSCs). By limiting the scanning interval of maximum and minimum values of duty ratio via some analyses related to P-V characteristic for PSCs, global MPPT (GMPPT) is achieved in an efficient way. So as to validate performance of the proposed approach, a single-ended primary inductance converter has been used in both simulation and experimental studies. PV simulator has been used as a PV source to obtain different module characteristics with different number of bypass diodes and PV power levels. Both simulation and experimental results clarify that improved MPPT approach realises GMPPT effectively. Due to the high performance results, this approach can be an alternative technique in module-integrated converters, smart modules and PV power optimisers in which single module is used.     

Toward multiple maximum power point estimation of photovoltaic systems based on semiconductor theory

Environmental conditions, such as temperature, non‐uniform irradiation, and solar shading, deeply affect the characteristics of photovoltaic (PV) modules in PV‐assisted generation systems. Several local maximum power points (MPPs) are found in the power–voltage curve of PV systems constructed by series/parallel‐connected PV modules under partially shaded conditions. The characteristics of PV systems change unpredictably when multiple MPPs occur, so the actual MPP tracking (MPPT) becomes a difficult task. Conventional MPPT methods for the PV systems under partially shaded conditions cannot quickly find the actual MPP such that the optimal utilization of PV systems cannot be achieved. Based on the p–n junction semiconductor theory, we develop a multipoint direct‐estimation (MPDE) method to directly estimate the multiple MPPs of the PV systems under partially shaded conditions and to cope with the mentioned difficulties. Using the proposed MPDE method, the multiple MPPs of the PV systems under partially shaded conditions can be directly determined from their irradiated current–voltage and power–voltage characteristic curves. The performances of the proposed MPDE method are evaluated by examining the characteristics of multiple MPPs of PV systems with respect to different shading strengths and numbers of the shaded PV modules and also tested using the field data. The experimental results demonstrate that the proposed MPDE method can simply and accurately estimate the multiple MPPs of the PV systems under partially shaded conditions. The optimization of MPP control models and the MPPT for PV systems could be achieved promisingly by applying the proposed method. Copyright © 2014 John Wiley & Sons, Ltd.