Optimum matching of ohmic loads to the photovoltaic array

Optimum matching of loads to the photovoltaic (PV) generator is most desirable for more accurate sizing, higher system performance and maximum utilization of the costly solar array generator. The quality of load matching depends on the PV array characteristics, the load characteristics, and the insolation profile. A matching factor is defined as the ratio of the load energy to the array maximum energy over a one day period. Optimum matching is achieved by determining the optimal array parameters with respect to the load parameters. Optimization is done using direct-search techniques. Results show that the theoretical optimum matching factor for an ohmic load is 94.34%. For an electrolytic load the matching factor could reach 99.83%. A maximum power tracker can be eliminated if optimum matching is achieved.     

基于滑模变功率跟踪的PV-VSG控制技术

针对含储能装置的传统光伏虚拟同步发电机（PV-VSG）不仅投资成本高且未考虑光伏阵列输出特性的问题,提出一种基于滑模变功率点跟踪（SM-VPT）的PV-VSG控制策略。该方法在滑模控制的基础上引入直流母线电压偏差控制,调整光伏阵列的功率跟踪轨迹,实现光伏出力自适应匹配负载需求,即当光伏容量充足时,只提供与负载相匹配的功率;光伏功率不足时,可实现传统的MPPT控制以减少电力短缺,同时防止直流电压骤降,保证系统稳定运行。该方法使PV-VSG能够按需向负载供电,无需增加额外的储能设备,可实现光伏发电系统直接以VSG形式接入并网,仿真结果验证了所提控制方法的有效性。     

The operation of loads powered by separate sources or by a commonsource of solar cells

A procedure is introduced for analyzing the interaction between different loads connected to a common solar cell (SC) source, and the performances of these loads when powered by separate and common sources are compared. It is shown that for identical loads (the same type and size), either optimal or nonoptimal, powered by identical SC arrays, and for loads of the same type but different sizes that are powered by proportional sizes of SC arrays, the operation of the loads is the same when powered either by separate sources or by a common SC source. The arrays in the separate source systems are of the same sizes, and in the common source system the arrays are of double size. The loads in both setups are of the same type. Two types of loads are considered in this study: two different ohmic loads; two different water electrolyzer loads. A comparison of the operation of the above two load types is performed for systems that incorporate a maximum-power point-tracker (MPPT), for both the separate and common SC sources. The criterion for comparing the performances is the energy utilization efficiency. It is shown that the performance of one load affects the performance of another load in a common source system. In systems not including MPPTs, the total performance of the loads in a common source system is improved as compared to the total performance of the loads when they are powered separately by individual sources. In systems including MPPTs, the total performance of the loads is the same when powered either by a common source or by single sources. MPPTs improve the performance of loads, the amount of improvement depending on the mismatch of the loads to the solar cells     

Theoretical and experimental analysis of the behaviour of a photovoltaic pumping system

The aim of the paper is to present the influence of the solar radiation variation on the performances of a stand alone photovoltaic pumping system which consists of photovoltaic generator, dc-dc converter, dc-ac inverter, an immersed group motor-pump and a storage tank that serves a similar purpose to battery storage. Hence a theoretical analysis (modelling and control) of the system is needed. Attention has been paid to the command of the power converters using MPPT and variable laws. The MPPT control allows the extraction of the maximal output power delivered by the PV generator. The inverter ensures the PWM control of the asynchronous motor and a sine wave form of output signals. From the obtained simulation results, we will show that the decrease of the solar radiation degrades performances (the global efficiency and the flow rate) of the PV pumping system. The analysis is validated by simulation and experimental results.     

带蓄电池的光伏系统中MPPT充电效果理论分析

通过对太阳电池组件平面辐照、太阳电池组件特性以及蓄电池负载数学模型计算,分别对北京和广州地区两种典型气候条件下应用MPPT与直接耦合方式的输出情况进行比较和研究,发现在广州地区MPPT的应用意义不大,而在北京地区冬季则能够明显增加太阳电池组件的输出。在带蓄电池的光伏系统中影响MPPT控制器发挥效能的因素被分析和研究,要综合当地全年气温变化、负载状况、经济性以及可靠性等多方面考虑MPPT的应用。     

Lead acid batteries in solar refrigeration systems

Batteries play a vital role in solar photovoltaic refrigeration systems. Despite breakthroughs in operational characteristics of various components of such systems, lead acid batteries continue to be the only viable electrical energy storage devices as of date. The purpose of this paper is to report the results of characterization of a lead acid battery system as a component of a small capacity uninterrupted refrigeration system. The charging characteristics on solar energy, mains and a generator set powered by an internal combustion engine are presented. It is shown that through a judicious choice of capacity and load matching, the maximum power point tracker can be eliminated.     

Starting and Steady-State Characteristics of DC Motors Powered by Solar Cell Generators

The performance of dc motors (series, separately-excited, and shunt motors) powered by a solar cell generator and loaded by two different types of loads, one a constant load and one a ventilator load, were analyzed with respect to the transient (starting) and steady state operation. Direct current motors are employed in photovoltaic water pumping systems; therefore, the understanding of the system operation and the matching of the system components (solar cells, dc motor type, and load type) are important factors of the system design. Since the solar cell generator in a nonlinear and time-dependent power supply with an output that varies with the insolation (hourly and daily), the performance characteristics of the dc motor are different when supplied by a solar cell generator than when supplied by a conventional constant voltage source. The transient solution was obtained by using an available computer program - SUPER SCEPTRE. The separately - excited (or permanent magnet) motor with a ventilator load was found to be the most suitable for the solar cell generator. The series motor is quite acceptable, but the shunt motor gives poor performance. In all cases the ventilator load is more compatible with the solar cell generator than with the constant load.     

Optimal operation of DC smart house system by controllable loads based on smart grid topology

From the perspective of global warming mitigation and depletion of energy resources, renewable energy such as wind generation (WG) and photovoltaic generation (PV) are getting attention in distribution systems. Additionally, all-electric apartment houses or residence such as DC smart houses are increasing. However, due to the fluctuating power from renewable energy sources and loads, supply-demand balancing of power system becomes problematic. Smart grid is a solution to this problem. This paper presents a methodology for optimal operation of a smart grid to minimize the interconnection point power flow fluctuation. To achieve the proposed optimal operation, we use distributed controllable loads such as battery and heat pump. By minimizing the interconnection point power flow fluctuation, it is possible to reduce the electric power consumption and the cost of electricity. This system consists of photovoltaic generator, heat pump, battery, solar collector, and load. To verify the effectiveness of the proposed system, results are used in simulation presented.     

Combined operation of DC isolated distribution and PV systems for supplying unbalanced AC loads

This paper describes a DC isolated network which is fed by distributed generation (DG) from photovoltaic (PV) renewable sources to supply unbalanced AC loads. The battery energy storage bank has been connected to the DC network via DC/DC converter called storage converter to control the network voltage and optimize the operation of the PV generation units. The PV units are connected to the DC network via its own DC/DC converter called PV converter to ensure the required power flow. The unbalanced AC loads are connected to the DC network via its own DC/AC converter called load converter without transformer. This paper proposes a novel control strategy for storage converter which has a DC voltage droop regulator. Also a novel control system based on Clarke and Park rotating frame has been proposed for load converters. In this paper, the proposed operation method is demonstrated by simulation of power transfer between PV units, unbalanced AC loads and battery units. The simulation results based on PSCAD/EMTDC software show that DC isolated distribution system including PV units can provide the balanced voltages to supply unbalanced AC loads.     

Analytical model for predicting the performance of photovoltaic array coupled with a wind turbine in a stand-alone renewable energy system based on hydrogen

We present the results of an analysis of the performance of a photovoltaic array that complement the power output of a wind turbine generator in a stand-alone renewable energy system based on hydrogen production for long-term energy storage. The procedure for estimating hourly solar radiation, for a clear sunny day, from the daily average solar insolation is also given. The photovoltaic array power output and its effective contribution to the load as well as to the energy storage have been determined by using the solar radiation usability concept. The excess and deficit of electrical energy produced from the renewable energy sources, with respect to the load, govern the effective energy management of the system and dictate the operation of an electrolyser and a fuel cell generator. This performance analysis is necessary to determine the effective contribution from the photovoltaic array and the wind turbine generator and their contribution to the load as well as for energy storage.     

Reliability of rechargeable batteries in a photovoltaic power supply system

We investigated the reliability of a rechargeable battery acting as the energy storage component in a photovoltaic power supply system. A model system was constructed for this that includes the solar resource, the photovoltaic power supply system, the rechargeable battery and a load. The solar resource and the system load are modelled as stochastic processes. The photovoltaic system and the rechargeable battery are modelled deterministically, and an artificial neural network is incorporated into the model of the rechargeable battery to simulate damage that occurs during deep discharge cycles. The equations governing system behaviour are solved simultaneously in the Monte Carlo framework, and a first passage problem is solved to assess system reliability.     

Single-stage sine-wave inverter for an autonomous operation of solar photovoltaic energy conversion system

This paper proposes a high performance single-stage inverter topology for the autonomous operation of a solar photovoltaic system. The proposed configuration which can boost the low voltage of photovoltaic (PV) array, can also convert the solar dc power into high quality ac power for driving autonomous loads without any filter. An MPPT circuit with parallel connection is implemented so that the part of the energy generated is processed by the dc–dc converter to supply dc loads. The line current total harmonic distortion (THD) obtained using this configuration is quite reasonable. The proposed topology has several desirable features such as low cost and compact size as number of switches used, are limited to four as against six switches used in classical two-stage inverters. In this paper analysis, simulation and experimental results are presented.     

Maximum photovoltaic power tracking for the PV array using the fractional-order incremental conductance method

This paper proposes maximum photovoltaic power tracking (MPPT) for the photovoltaic (PV) array using the fractional-order incremental conductance method (FOICM). Since the PV array has low conversion efficiency, and the output power of PV array depends on the operation environments, such as various solar radiation, environment temperature, and weather conditions. Maximum charging power can be increased to a battery using a MPPT algorithm. The energy conversion of the absorbed solar light and cell temperature is directly transferred to the semiconductor, but electricity conduction has anomalous diffusion phenomena in inhomogeneous material. FOICM can provide a dynamic mathematical model to describe non-linear characteristics. The fractional-order incremental change as dynamic variable is used to adjust the PV array voltage toward the maximum power point. For a small-scale PV conversion system, the proposed method is validated by simulation with different operation environments. Compared with traditional methods, experimental results demonstrate the short tracking time and the practicality in MPPT of PV array.     

Improved fuzzy logic control-based energy management strategy for hybrid power system of FC/PV/battery/SC on tourist ship

An improved fuzzy-based energy management strategy (EMS) is proposed for a tourist ship used hybrid power system with multiple power sources consisting of fuel cell(FC)/photovoltaic cell(PV)/battery(BAT)/super-capacitor(SC). The power demand from propeller and user terminal is afforded by the power sources connecting to power converters. To obtain more superior performance of the power system, the maximum power point tracking (MPPT) algorithm is employed to optimize the PV. Meanwhile, the improved fuzzy logic control based on dynamic programming (DP) associated with wavelet analysis and PI control are employed to achieve the output power optimal distribution and online control. In particular, the MPPT algorithm can improve the utilization of solar energy, and the SC can well absorb the high frequency power and reduce the fluctuation of the battery and FC that exhibits the potential of their lifetime extension. The FC outputs the high and stable power satisfying the ship's power demand even under the extreme work conditions. The developed model is able to illustrate well in the operation process of the hybrid power system governed by the proposed EMS. In addition, compared with the rule-based strategy, the improved fuzzy-based EMS can reduce 14.39% hydrogen consumption and keep the consistency of battery SOC.     

Analysis and enhancement of PV efficiency with incremental conductance MPPT technique under non-linear loading conditions

This paper presents experimental evaluations for variation in the efficiency of energy extracted from a photovoltaic (PV) module (under non-linear loading) incorporated with an incremental conductance(IC) maximum power point tracking (MPPT) algorithm. The focus is on the evaluation of the PV panel under non-linear loading conditions using the experimental installation of a 100W_p photovoltaic array connected to a DC–DC converter and a KVA inverter feeding a non-linear load. Under the conditions of non-linear loading, both the simulation and experiment show that the MPPT technique fails to attain maximum power point due to the presence of ripples in the current leading eventually to a reduction in efficiency. In this paper, panel current is taken as a function of load impedance in the MPPT algorithm to eradicate power variation, as load impedance varies with supply voltage under non-linear conditions. The system is simulated for different non-linear loads using MATLAB-Simulink. A TMDSSOLAREXPKIT was used for MPPT control. In case 2, the inverter is connected to a single phase grid. When a voltage swell occurs in the grid, PV power drops. This power loss is reduced using the proposed MPPT method. The results of simulations and experimental measurements and cost efficiency calculations are presented.     

Matching of a DC motor to a photovoltaic generator using a step-upconverter with a current-locked loop

A photovoltaic (PV) generator is a nonlinear device having insolation-dependent volt-ampere characteristics. Because of its relatively high cost, the system designer is interested in optimum matching of the motor and its mechanical load to the PV generator so that maximum power is obtained during the entire operating period. However, since the maximum-power point varies with solar insolation, it is difficult to achieve an optimum matching that is valid for all insolation levels. In this paper it is shown that for maximum power, the generator current must be directly proportional to insolation. This remarkable property is utilized to achieve insolation-independent optimum matching. A shunt DC motor driving a centrifugal water pump is supplied from a PV generator via a step-up converter whose duty ratio is controlled using a current-locked feedback loop     

THD reduction with reactive power compensation for fuzzy logic DVR based solar PV grid connected system

Dynamic voltage restorer (DVR) is used to protect sensitive loads from voltage disturbances of the distribution generation (DG) system. In this paper, a new control approach for the 200 kW solar photovoltaic grid connected system with perturb and observe maximum power point tracking (MPPT) technique is implemented. Power quality improvement with comparison is conducted during fault with proportional integral (PI) and artificial intelligence-based fuzzy logic controlled DVR. MPPT tracks the actual variable DC link voltage while deriving the maximum power from a photovoltaic array and maintains DC link voltage constant by changing modulation index of the converter. Simulation results during fault show that the fuzzy logic based DVR scheme demonstrates simultaneous exchange of active and reactive power with less total harmonic distortion (THD) present in voltage source converter (VSC) current and grid current with fast tracking of optimum operating point at unity power factor. Standards (IEEE-519/1547), stipulates that the current with THD greater than 5% cannot be injected into the grid by any distributed generation source. Simulation results and validations of MPPT technique and operation of fuzzy logic controlled DVR demonstrate the effectiveness of the proposed control schemes.     

Optimum matching of direct-coupled electromechanical loads to aphotovoltaic generator

The objective of the paper is to present a general mathematical formulation for matching electromechanical loads connected to a photovoltaic array. An optimization method is then used to solve the matching problem with the objective of maximizing the gross mechanical energy. The analysis is extended to obtain the sizing of the array and the battery in the direct-coupled system. Results show that optimum matching can be achieved by carefully selecting the PV array rated parameters with respect to the load parameters. The rated power of the array is twice that of the load. The field constant that maximizes the gross mechanical energy can be obtained given the parameters of the load and the array temperature. The separately excited motor offers higher matching performance compared to the series motor. The rotodynamic load offers better matching compared to the viscous friction load     

An analytical method to determine the optimal size of a photovoltaic plant

In this paper, a simplified method for the optimal sizing of a photovoltaic system is presented. The results have been obtained for Italian meteorological data, but the methodology can be applied to any geographical area. The system studied is composed of a photovoltaic array, power tracker, battery storage, inverter and load. Computer simulation was used to obtain the performance of this system for many values of field area, battery storage value, solar flux and load by keeping constant the efficiencies. A simple fit was used to achieve a formula relating the system variables to the performance. Finally, the formulae for the optimal values of the field area and the battery storage value are shown.     

Operation analysis of a photovoltaic lighting system with battery and heater

An operation analysis is presented for a photovoltaic system consisting of the photovoltaic generator, the battery, the light, and the heater. An optimal resistive load for lighting is determined according to the characteristics of the battery. The working domain of the battery is marked out to display the range of the battery charge supplied by the photovoltaic generator. Before sunrise and after sunset, the battery supplies power to the light with an optimal resistive load. When the sun is above the horizon but sunset or not sunrise for the tilled photovoltaic module, the system is at rest. If the beam radiation is really incident on the surface of the module and induces a characteristic inside the working domain, all power will charge the battery. Once the characteristic is outside of the working domain, the resistive load is chosen to balance the currents of photovoltaic generator, battery charge, and heater. It is the only situation of concurrence. If the open-circuit voltage of the photovoltaic array under insolation is less than the zero-current voltage of the battery at the fractional state of charge, or if the battery is at full charge, the battery is disconnected from the photovoltaic system and the power output of the photovoltaic array to the heater is found by the method of maximum power point tracking.