Forecasting photovoltaic array power production subject to mismatch losses

The development of photovoltaic (PV) energy throughout the world this last decade has brought to light the presence of module mismatch losses in most PV applications. Such power losses, mainly occasioned by partial shading of arrays and differences in PV modules, can be reduced by changing module interconnections of a solar array. This paper presents a novel method to forecast existing PV array production in diverse environmental conditions. In this approach, field measurement data is used to identify module parameters once and for all. The proposed method simulates PV arrays with adaptable module interconnection schemes in order to reduce mismatch losses. The model has been validated by experimental results taken on a 2.2 kW_p plant, with three different interconnection schemes, which show reliable power production forecast precision in both partially shaded and normal operating conditions. Field measurements show interest in using alternative plant configurations in PV systems for decreasing module mismatch losses.     

A cell-to-module-to-array detailed model for photovoltaic panels

This paper presents a modified current–voltage relationship for the single-diode model. The single-diode model has been derived from the well-known equivalent circuit for a single photovoltaic (PV) cell. A cell is defined as the semiconductor device that converts sunlight into electricity. A PV module refers to a number of cells connected in series and in a PV array, modules are connected in series and in parallel. The modification presented in this paper accounts for both parallel and series connections in an array. Derivation of the modified current–voltage relationships begins with a single solar cell and is expanded to a PV module and finally an array. Development of the modified current–voltage relationship was based on a five-parameter model, which requires data typically available from the manufacturer. The model accurately predicts voltage–current (V–I) curves, power–voltage (P–V) curves, maximum power point values, short-circuit current and open-circuit voltage across a range of irradiation levels and cell temperatures. The versatility of the model lies in its accurate prediction of the aforementioned criteria for panels of different types, including monocrystalline and polycrystalline silicon. The model is flexible in the sense that it can be applied to PV arrays of any size, as well as in simulation programs such as EMTDC/PSCAD and MatLab/Simulink. Accuracy of the model was validated through a series of experiments performed outdoors for different configurations of a PV array.     

Performance enhancement of partially shaded solar PV array using novel shade dispersion technique

Solar photo voltaic array (SPVA) generates a smaller amount of power than the standard rating of the panel due to the partial shading effect. Since the modules of the arrays receive different solar irradiations, the P-V characteristics of photovoltaic (PV) arrays contain multiple peaks or local peaks. This paper presents an innovative method (magic square) in order to increase the generated power by configuring the modules of a shaded photovoltaic array. In this approach, the physical location of the modules in the total cross tied (TCT) connected in the solar PV array is rearranged based on the magic square arrangement pattern. This connection is done without altering any electrical configurations of the modules in the PV array. This method can distribute the shading effect over the entire PVarray, without concentrating on any row of modules and can achieve global peaks. For different types of shading patterns, the output power of the solar PV array with the proposed magic square configuration is compared with the traditional configurations and the performance is calculated. This paper presents a new reconfiguration technique for solar PV arrays, which increases the PV power under different shading conditions. The proposed technique facilitates the distribution of the effect of shading over the entire array, thereby, reducing the mismatch losses caused by partial shading. The theoretical calculations are tested through simulations in Matlab/ Simulink to validate the results. A comparison of power loss for different types of topologies under different types of shading patterns for a 4×4 array is also explained.     

Power loss in photovoltaic arrays due to mismatch in cell characteristics

Variations in the current-voltage characteristics of photovoltaic cells can lead to significant power loss “due to mismatch” when the cells are connected together in a network. This study explores how this mismatch loss depends on variations in max-power current and max-power voltage from cell to cell. An analysis of a series string is first performed. Losses in a parallel string are also determined. Estimates of mismatch losses in more complex arrays are then obtained. In addition to generally excellent comparison with several numerical studies, results show that, for a series string, there exists a critical magnitude of deviation in cell max-power current beyond which the power loss due to mismatch is sensitive to both the number of cells placed in series and the shape of the probability density function defining variations in max-power current. This critical level also depends on the cell fill-factor.     

双面光伏组件发电性能影响因素理论与实验研究

基于视角系数模型,理论计算与实验分析双面光伏组件的背面辐照度分布,根据组件背面的辐照度模型和太阳电池等效电路的单二极管模型,采用I-V曲线叠加法计算其微失配损耗;并提出简化反射光谱模型准确计算反射太阳光谱,以光谱失配因子量化反射光谱对双面组件背面发电性能的影响,结合组件背面辐射视角系数模型,建立双面组件背面功率输出的计...     

Low light conditions modelling for building integrated photovoltaic (BIPV) systems

The low irradiance efficiency of photovoltaic modules is important to the optimization of BIPV systems. When photovoltaic modules are integrated into a building, architectural design considerations compete with maximizing photovoltaic energy production. As a result, BIPV arrays are often not facing south and are frequently mounted vertically. Under these conditions, a greater portion of the total sunlight striking the array is diffuse or at high angles of incidence. In northern latitudes a significant amount of the total yearly energy is produced at low light levels.A grid-connected array of BIPV modules integrated into the BCIT Technology Centre building in Burnaby, B.C. was used for assessing the accuracy of an energy performance model developed for BIPV systems. The BIPV system uses AC modules and a computerized data acquisition system for monitoring the performance of modules and inverters. The performance model was developed from analysis of the open circuit voltage, maximum power point voltage and maximum power point current of the individual modules comprising the BIPV array.The algorithm for calculating power output of the photovoltaic array is derived from the ideal diode equation using the single diode model of a photovoltaic cell. An empirically derived parameter modifies the equation. Once the parameters for different module technologies are established, it is possible to compare their annual performance in a BIPV system.     

串并联太阳能电池弧形光伏组件的发电性能研究

将弧形光伏组件安装在建筑和汽车上获取电能,已受到人们越来越多的关注。为获得更高的输出功率,有必要研究由互连太阳能电池组成的、电流不匹配的弧形光伏组件的特性。研究重点关注由串并联太阳能电池组成的弧形光伏组件的发电性能,设计了不同曲率的非平面微型光伏模块,并通过测量获取光伏模块的参数。与平面光伏模块相比,弧形光伏模块的发电量较小。此外,利用二极管模型分析了光伏模块的特性,说明并联比串联功率高的原因。最后研究了实际应用中太阳能电池的互连问题。结果表明,在理想模型下并联能获取更多电能,但大尺寸的光伏模块会产生更大电流,可能会在实际运行中产生额外损耗。因此,在实际应用中设计弧形光伏组件时也应考虑太阳能电池的互连。     

阴影遮蔽条件下光伏发电效率的增大方法

受阴影遮蔽影响,光伏阵列输出功率下降,功率曲线呈现多峰值,使传统的最大功率点(MPPT)算法失效陷入局部极值点。基于光伏电池的等效电路模型,在正常和有阴影遮蔽情况下对光伏组件串并联输出特性进行仿真,分析功率曲线上局部极值点的产生原因及变化规律。针对阴影遮蔽对光伏发电效率的影响,提出建立微型光伏发电系统,采用光伏组件间解耦的方法,跟踪每个光伏组件的最大功率点,使光伏发电效率达到最优。试验结果验证了该方法的可行性,为今后光伏电站的建设提供了指导。     

高原高寒地区光伏组件背板冷却对输出功率影响的实验研究

针对光伏组件表面温度影响光伏电站出力的技术难题,以2行4列的光伏阵列为例,在拉萨搭建光伏阵列输出功率实验测试平台,采用液冷方式在背板铺设冷却循环管道构建背板冷却系统,并基于冷却系统的连续运行（实验Ⅰ）、先停后启（实验Ⅱ）、先启后停（实验Ⅲ）3种运行工况,分别进行光伏组件有无背板冷却的对比实验,探究高原高寒地区组件表面温度对光伏阵列输出功率的影响规律。研究表明：采用背板冷却技术降低光伏组件的表面温度,可有效提升光伏阵列的输出功率,在实验Ⅰ、Ⅱ、Ⅲ中光伏阵列输出功率分别提升了1.4%、1.3%、1.0%;光伏组件采用背板冷却技术时,冷却介质循环泵耗功高于光伏阵列提升的输出功率,但在高原高寒地区可回收利用冷却介质吸热量,加热生活用水,可使采用背板冷却的光伏阵列综合效益提高。     

Performance analysis of a grid-connected photovoltaic system

Grid-connected photovoltaic systems are required to introduce photovoltaic solar energy into urban areas. To analyze these systems, a 2.0 kW_p power system has been installed at the University of Málaga, Spain. The array power output was estimated by using measured I–V curves for the installed modules with minimization of mismatch losses. The supplied grid energy and main performances are described. The effects on system yield of threshold-inverter and coupling losses of the inverter to the grid have been studied. During 1997, the system supplied 2678 kWh to the grid, i.e. the mean daily output, was 7.4 kWh. The annual performance ratio was 64.5% and the optimal value 67.9%.     

光伏组件在非标准测试条件下的能量分布

该研究基于非标准测试条件,结合五参数电学模型、热阻模型和损失模型,提出一种评估光伏组件能量分布的耦合模型.通过模拟数据和实验数据的对比,验证该模型在模拟光伏组件发电量和太阳电池温度方面的准确性,并研究非标准测试条件下光伏组件的运行状态和能量分布情况.结果表明,在某个晴天下,对于一个实际运行的光伏组件,21.9％的入射太...     

Partial shadowing of photovoltaic arrays with different system configurations: literature review and field test results

Partial shadowing has been identified as a main cause for reducing energy yield of grid-connected photovoltaic systems. The impact of the applied system configuration on the energy yield of partially shadowed arrays has been widely discussed. Nevertheless, there is still much confusion especially regarding the optimal grade of modularity for such systems. A 5-kWp photovoltaic system was installed at K.U. Leuven. The system consists of three independent subsystems: central inverter, string inverter, and a number of AC modules. Throughout the year, parts of the photovoltaic array are shadowed by vegetation and other surrounding obstacles. The dimensions of shadowing obstacles were recorded and the expectable shadowing losses were estimated by applying different approaches. Based on the results of almost 2 years of analytical monitoring, the photovoltaic system is assessed with regard to shadowing losses and their dependence on the chosen system configuration. The results indicate that with obstacles of irregular shape being close to the photovoltaic array, simulation estimates the shadowing losses rather imprecise. At array positions mainly suffering from a reduction of the visible horizon by obstacles far away from the photovoltaic array, a simulation returns good results. Significant differences regarding shadow tolerance of different inverter types or overproportional losses with long module strings could not be confirmed for the system under examination. The negative impact of partial shadowing on the array performance should not be underestimated, but it affects modular systems as well as central inverter systems.     

An investigation of mismatch losses in solar photovoltaic cell networks

Solar photovoltaic (PV) arrays in field conditions deliver lower power than the array rating. In this paper, the sensitivity of solar cell parameters in the variation of available power from the array is investigated. The parameters characteristic of aging and fresh cells used in prototype field systems have been used for computation of reduction in the available power. It is found that in series string the fractional power loss would increase from 2% to 12% with aging of solar cells. However, this fractional power loss may be reduced to 0.4–2.4% by an appropriate series-paralleling.     

Monte Carlo techniques to analyse the electrical mismatch losses in large-scale photovoltaic generators

In large-scale photovoltaic generators, the arrangement of modules with different electrical characteristics could involve a considerable mismatch between the single components resulting in a power loss. This means the actual power is less than the sum of the maximum output powers of the individual PV modules, operating at same irradiance-temperature conditions. To reduce the mismatch losses and to calculate it under operating conditions, a statistical approach based on Monte Carlo simulation techniques, has been developed and validated. The simulation model shows that it is possible to meet the required mismatch level, with a random arrangement, starting from a modules population characterized in terms of short circuit current, I_sc and open circuit voltage V_oc, by a probability density function with a imposed variance. The method has been successfully applied for a 100 kWp standard unit photovoltaic generator, the computational results have shown good agreement with the experimental data.     

Reliability of photovoltaic modules I. Theoretical considerations

The effect of interconnection and bypass diodes on the reliability of photovoltaic modules and arrays have been evaluated. Analysis of different cell or panel interconnections for the determination of the optimum item configuration and circuit design strategies to achieve reliable modules and arrays has been carried out. A computer program for finding the optimum configuration which produce maximum module and array reliability is presented. The roll of bypass diodes for module and array reliability improvements have been studied theoretically and experimentally. It is found that module and array reliability depends on the item's (cells or panels) probability of failure, arrangement of the items, and failure mode. Discussion and suggestions for optimum design are presented.     

Calculating the shading reduction coefficient of photovoltaic system efficiency using the anisotropic sky scattering model

The front-row shading reduction coefficient is a key parameter used to calculate the system efficiency of a photovoltaic(PV) power station. Based on the Hay anisotropic sky scattering model, the variation rule of solar radiation intensity on the surface of the PV array during the shaded period is simulated, combined with the voltage–current characteristics of the PV modules, and the shadow occlusion operating mode of the PV array is modeled. A method for calculating the loss coefficient of front...     

Effect of concentration on the performance of flat plate photovoltaic modules

The effect of low concentration ratios on the performance of passively cooled conventional photovoltaic modules has been investigated. Peak power outputs of up to 140 W per square metre of module area have been obtained with single crystal modules of high cell packing factor using a 2.2X plane mirror concentrator. Both cell temperature and series resistance losses are found to be important in limiting module efficiency. Performance simulations indicate that the use of a 4X concentrator with polar axis tracking will increase annual peak output by a factor of 3.2 over that of a fixed flat plate module.     

Experimental study on the effect of dust deposition on solar photovoltaic panels in desert environment

The accumulation of dust particles deteriorates the performance of solar cells and results in appreciable losses in the generated power due to the sun irradiance scattering effects on the surface of the solar panel. This study investigates the impact of dust accumulation on photovoltaic solar modules in Baghdad city in Iraq. For this purpose an experiment has been conducted to quantify losses caused by the accumulation of dust on the surface of three identical photovoltaic solar modules. The modules have been installed with direct exposure to weather conditions, in a well controlled experimental setup. Subsequently, measurements of dust accumulation on modules have been taken on daily, weekly and monthly basis. The dust density and size distribution of aerosol particles and fibers have been also investigated and measured by a highly sensitive aerosols measuring system. The dusted module and another similar clean module have been then exposed to constant radiation and constant temperature using a solar simulator as light source. The deposition of the dust on the surface of the photovoltaic solar modules showed a reduction in both the short circuit current (I_sc) and the output power compared to the same parameters of the clean module. The average degradation rate of the efficiencies of the solar modules exposed to dust are; 6.24%, 11.8% and 18.74% calculated for exposure periods of one day, one week and one month. The experimental results are well compared with the calculations obtained by a theoretical model recently developed by the authors.     

Positioning of PV panels for reduction in line losses and mismatch losses in PV array

Partial shading decreases the power output of PV arrays due to mismatch losses. These losses are dependent on the shading pattern and the relative positions of shaded modules in the array. Various static and dynamic reconfiguration techniques have earlier been proposed to mitigate these losses. In an earlier proposed static reconfiguration technique, the power generation is enhanced by altering the physical location of the PV panels using a random Sudoku configuration without modifying the TCT (Total-Cross-Tied) based electrical connections. However, this arrangement faces drawbacks due to ineffective dispersion of shade and significant increase in wiring required. In this work, an optimal Sudoku arrangement to overcome these drawbacks is formulated. Further analysis indicate that the global peak of the optimal Sudoku based PV array occurs as the right most peak in the curve for most shading conditions, thus evidently obviating the need for complex MPPT (Maximum-Power-Point-Tracking) algorithms. The proposed configuration is compared with various other existing reconfiguration schemes in terms of power output and the comparison is presented. In addition, a general formulation is proposed to expand this pattern to any generic array. A strategy is also proposed to make such an interconnection practicable for very large size PV arrays.     

Flexible polymer photovoltaic modules with incorporated organic bypass diodes to address module shading effects

We present experimental results on large-area low-cost processed flexible organic photovoltaic (OPV) modules incorporating organic bypass diodes to eliminate the negative effects of shading on the module power output. A fully organic-based structure (organic solar module combined with an organic bypass diode) is essential to allow monolithic interconnection of the bypass diode during the solar module production within the same printing steps. The origin of shading losses in organic photovoltaic modules is analyzed in detail, and guidelines for the design and architecture of flexible OPV modules are derived. Inorganic and organic diodes were tested on their functionality as bypass diodes, and a set of diode specifications to minimize shading losses is summarized. Organic bypass diodes were found to efficiently reduce the adverse shading effects in OPV modules.