An optimum load management strategy for stand-alone photovoltaic power systems

The subject of load management for stand-alone photovoltaic (SAPV) power systems is addressed. The objective is to minimize the total life-cycle cost of the system while, at the same time, the battery is protected and the load priorities are observed. The first step in this approach involves a general load classification. The idea is to manipulate the controllable loads in order to reduce battery size. For this reason, optimum curves are obtained for the controllable loads. Then an optimum load management strategy is mathematically formulated. Finally, a tracking algorithm has been devised in order to implement the optimum load management scheme. The previously described method yields cost optimum SAPV systems. An illustrative example using data similar to the first village PV power system of Schuchuli, Arizona shows the practical application of the proposed optimum load management strategy.     

Multi-stage dual priority regulator for photovoltaic systems

The design and application are investigated of an array shedding voltage regulator for use in photovoltaic (PV) systems with two separate loads of different priority. The multistage dual-priority regulator charges two separate battery systems for high- and low-priority loads. This controller switches the array modules from the main battery to the auxiliary battery as the main battery nears full charge, thus utilizing the PV panels to the maximum without jeopardizing the reliability of the critical load. Both the main and auxiliary voltage settings are adjustable, allowing the use of different types of batteries. A dual-priority voltage regulator is useful in any stand-alone system with multiple loads. Uses include PV-powered homes, medical clinics, and recreational vehicles     

Enhancing the operation of fuel cell-photovoltaic-battery-supercapacitor renewable system through a hybrid energy management strategy

It is necessary to have an energy management system based on one or more control strategies to sense, monitor, and control the behavior of the hybrid energy sources. In renewable hybrid power systems containing fuel cells and batteries, the hydrogen consumption reduction and battery state of charge (SOC) utilizing are the main objectives. These parameters are essential to get the maximum befits of cost reduction as well as battery and hydrogen storage lifetime increasing. In this paper, a novel hybrid energy management system (HEMS) was designed to achieve these objectives. A renewable hybrid power system combines: PV, PEMFC, SC, and Battery was designed to supply a predetermined load with its needed power. This (REHPS) depends on the PV power as a master source during the daylight. It uses the FC to support as a secondary source in the night or shading time. The battery is helping the FC when the load power is high. The supercapacitor (SC) is working at the load transient or load fast change. The proposed energy management system uses fuzzy logic and frequency decoupling and state machine control strategies working together as a hybrid strategy where the switching over between both strategies done automatically based on predetermined values to obtain the minimum value of hydrogen consumption and the maximum value of SOC at the same time. The proposed HEMS achieves 19.6% Hydrogen consumption saving and 5.4% increase in SOC value compared to the results of the same two strategies when working as a stand-alone. The load is designed to show a surplus power when the PV power is at its maximum value. This surplus power is used to charge the battery. To validate the system, the results were compared with the results of each strategy if working separately. The comparison confirms the achievement of the hybrid energy management system goal.     

Single-Stage Dual Priority Regulator for photovoltaic systems

In this paper the design and application of a voltage regulator for use in Photovoltaic Systems with two separate loads of different priority is presented. The Single-Stage Dual Priority Regulator (SSDPR) charges two separate battery systems for high and low priority loads. The controller always charges the main battery first. It toggles between the two batteries when the main battery is nearly charged, and reduces the charge duty cycle of the main battery as it approaches full charge. The SSDPR utilizes the PV more efficiently by supplying low priority loads without jeopardizing the reliability of the critical load. Both the main and auxiliary voltage settings are adjustable, allowing the use of different types of batteries. The SSDPR is being tested in the University of Lowell Vaccine Refrigerator Laboratory. A dual priority regulator is useful in any stand-alone system with multiple loads. Uses include photovoltaic powered homes, medical clinics, and recreational vehicles.     

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.     

Operation strategy of residential centralized photovoltaic system in remote areas

Photovoltaic (PV) systems have found fairly wide application in remote isolated area. However, each individual PV system usually supplies energy only to one consumer. In such a case we have several consumers that each one of them uses a stand-alone PV system. This situation would expose such stand-alone systems to transient excessive loads larger than the power generated by the PVs, and then the battery is bound to discharge even during the day. For overcoming this problem, we suggest an autonomous centralized PV system, comprising one battery bank and plural subsystems connected to each other. From solar radiation data and load profiles, the performance of the PV centralized system is simulated by using a time step scheme. The advantages of this system are found to be the large charging rate of power, high efficiency, and low cost compared with conventional individual PV systems and hybrid systems. In addition, the economic study shows that the life cycle cost and the price of kilowatt hour generated in the centralized system is lower than that for the individual systems.     

Generation unit sizing and cost analysis for stand-alone wind,photovoltaic, and hybrid wind/PV systems

This paper presents the results of investigations on the application of wind, photovoltaic (PV), and hybrid wind/PV power generating systems for utilization as stand-alone systems. A simple numerical algorithm has been developed for generation unit sizing. It has been used to determine the optimum generation capacity and storage needed for a stand-alone, wind, PV, and hybrid wind/PV system for an experimental site in a remote area in Montana with a typical residential load. Generation and storage units for each system are properly sized in order to meet the annual load and minimize the total annual cost to the customer. In addition, an economic analysis has been performed for the above three scenarios and is used to justify the use of renewable energy versus constructing a line extension from the nearest existing power line to supply the load with conventional power. Annual average hourly values for load, wind speed, and insolation have been used     

DESIGN OF HYBRID-PHOTOVOLTAIC POWER GENERATOR, WITH OPTIMIZATION OF ENERGY MANAGEMENT

A methodology is developed for calculating the correct size of a photovoltaic (PV)-hybrid system and for optimizing its management. The power for the hybrid system comes from PV panels and an engine-generator – that is, a gasoline or diesel engine driving an electrical generator. The combined system is a stand-alone or autonomous system, in the sense that no third energy source is brought in to meet the load. Two parameters were used to characterize the role of the engine-generator: denoted SDM and SAR, they are, respectively, the battery charge threshold at which it is started up, and the storage capacity threshold at which it is stopped, both expressed as a percentage of the nominal battery storage capacity. The methodology developed is applied to designing a PV-hybrid system operating in Corsica, as a case study. Various sizing configurations were simulated, and the optimal configuration that meets the autonomy constraint (no loss of load) was determined, by minimizing of the energy cost. The influence of the battery storage capacity on the solar contribution is also studied. The smallest energy cost per kWh was obtained for a system characterized by an SDM=30% and an SAR=70%. A study on the effects of component lifetimes on the economics of PV-hybrid and PV stand-alone systems has shown that battery size can be reduced by a factor of two in PV-hybrid systems, as compared to PV stand-alone systems.     

Sizing optimization,dynamic modeling and energy management strategies of a stand-alone PV/hydrogen/battery-based hybrid system

This paper presents a sizing method and different control strategies for the suitable energy management of a stand-alone hybrid system based on photovoltaic (PV) solar panels, hydrogen subsystem and battery. The battery and hydrogen subsystem, which is composed of fuel cell (FC), electrolyzer and hydrogen storage tank, act as energy storage and support system. In order to efficiently utilize the energy sources integrated in the hybrid system, an appropriate sizing is necessary. In this paper, a new sizing method based on Simulink Design Optimization (SDO) of MATLAB was used to perform a technical optimization of the hybrid system components. An analysis cost has been also performed, in that the configuration under study has been compared with those integrating only batteries and only hydrogen system. The dynamic model of the designed hybrid system is detailed in this paper. The models, implemented in MATLAB-Simulink environment, have been designed from commercially available components. Three control strategies based on operating modes and combining technical-economic aspects are considered for the energy management of the hybrid system. They have been designed, primarily, to satisfy the load power demand and, secondarily, to maintain a certain level at the hydrogen tank (hydrogen energy reserve), and at the state of charge (SOC) of the battery bank to extend its life, taking into account also technical-economic analysis. Dynamic simulations were performed to evaluate the configuration, sizing and control strategies for the energy management of the hybrid system under study in this work. Simulation results show that the proposed hybrid system with the presented controls is able to provide the energy demanded by the loads, while maintaining a certain energy reserve in the storage sources.     

Optimal energy management system for stand-alone wind turbine/photovoltaic/hydrogen/battery hybrid system with supervisory control based on fuzzy logic

This paper presents a novel hourly energy management system (EMS) for a stand-alone hybrid renewable energy system (HRES). The HRES is composed of a wind turbine (WT) and photovoltaic (PV) solar panels as primary energy sources, and two energy storage systems (ESS), which are a hydrogen subsystem and a battery. The WT and PV panels are made to work at maximum power point, whereas the battery and the hydrogen subsystem, which is composed of fuel cell (FC), electrolyzer and hydrogen storage tank, act as support and storage system. The EMS uses a fuzzy logic control to satisfy the energy demanded by the load and maintain the state-of-charge (SOC) of the battery and the hydrogen tank level between certain target margins, while trying to optimize the utilization cost and lifetime of the ESS. Commercial available components and an expected life of the HRES of 25 years were considered in this study. Simulation results show that the proposed control meets the objectives established for the EMS of the HRES, and achieves a total cost saving of 13% over other simpler EMS based on control states presented in this paper.     

Monitoring and data analysis of a PV system connected to a grid for home applications

Monitoring of a PV system connected to a grid was conducted to collect the system performance and compared to a PV stand-alone system. Daily solar inputs and load outputs for home applications of the two systems were recorded. Balance and surplus of energy in the systems were observed during dry and summer seasons when high solar radiation was recorded. During the raining season, with thunderstorms, solar radiation was low and grid cut-off occasionally occurred. Consequently, energy deficiencies and grid back-up of the systems were observed. It was found that the battery size of the PV system connected to the grid was reduced by a factor of 0.5–0.7 compared to the PV stand-alone counterpart for similar load behaviors. The merits of the PV system connected to the grid during grid cut-off was confirmed; the system proved to be appropriate for tropical countries where unstable electricity supply from the grid can occur during monsoon season.     

Demonstration and assessment of dynamic performance of a hybrid power system based electric passenger cart

Fuel cell (FC) technology is showing excellent promise for many applications ranging from portable devices to vehicular systems. A stand-alone FC may not always satisfy the fast and transient load demands of a vehicular power system. As a result, FC units are usually hybridized with supplementary sources to meet the total power demand of the vehicle. In this paper, the energy demands of a light vehicle (a passenger cart) is developed using a hybrid power supply system involving a photovoltaic (PV) panel, a proton exchange membrane fuel cell (PEMFC) and a battery based energy storage system (ESS). In addition, the details of the physical construction of the modified hybrid cart are given. The most critical feature of an energy management strategy for a multiple-source based hybrid vehicle is the sharing of fast and transient load demands among the available power sources. For this purpose, a 300-s drive cycle is created in this paper to test the effectiveness of the load sharing strategy between FC, battery pack and PV panel. It is found that PEMFC dominates slow and moderate dynamic behaviors of the vehicle, while fast response of the battery group governs the rapid dynamic behaviors. The results also show that the integrating PV panel contributes noticeably to the dynamic behaviors of the system. Furthermore, a control-oriented simulation model for a PEMFC unit is verified with experimental data to test the success of the proposed technique.     

Alternative energy facilities based on site matching and generation unit sizing for remote area power supply

This paper presents the decision support technique and influencing factors in the design of an integrated solar-wind power system for stand-alone applications. Results of investigations on application of alternative energy facility like wind, photovoltaic (PV), and Integration of wind–PV power generating systems for Remote Area Power Supply have been presented. A weather model-based site matching of equipment and a simple numerical algorithm for generation unit sizing have been presented. The program has been used to determine the optimum generation capacity and storage needs for a stand-alone Wind, PV, and integrated wind–PV system for a remote site in India (Sukhalai situated near Suktawa in Hoshangabad district of Madhya Pradesh) that satisfies a typical load. Generation and storage units for each system are properly sized in order to meet the annual load demand for the above three scenarios. Annual average hourly values for load, wind speed, and insolation have been used for analysis. The results are used to justify the use of renewable energy source as a reliable option for remote areas.     

Analysis and implementation of an adaptative PV based battery floating charger

In a system composed of a photovoltaic (PV) cell, a converter and a resistive load, the Maximum Power Point Tracking (MPPT) techniques are applied at the output of the PV panel and not at the level of the load. In this study, the considered load is a battery at different States Of Charge (SOC) that is charged by the PV panel. The power consumed by the battery is related to its SOC. Consequently, an empty battery consumes more current than a charged one. At full state of charge, the battery does not call for more energy and thus it is not rewarding to extract more power from the PV panel.Besides, in a stand-alone photovoltaic system, the size of the PV panel and the battery should be respected. Thus, the PV current at different irradiances should be compatible with the charging current required to charge the battery at different SOC. A critical situation occurs at high irradiance when the PV panel delivers a high current at Maximum Power Point (MPP) that exceeds the tolerated charging current. The current reaches the top limit when the battery is totally empty, caused by the big difference in potential between the converter output and the battery voltages. In this case, the battery starts to gas when attempts are made to charge it faster than it can absorb the energy. On the other hand, in a fully charged battery, the difference in potential between the converter and the battery is zero. In this case, there is no need to track the MPP.In this study, we will focus on the load type and suggest new methods to reach the MPP depending on the load state. In the proposed designs, the components of the stand-alone system are protected even if they are not well sized. In addition, we will focus on the development of the PV array mathematical model. The results achieved with the system, as well as the experimental results of a laboratory prototype, will be given.     

Multi-Stage Dual Priority Regulator for Photovoltaic Systems

This paper investigates the design and application of an array shedding voltage regulator for use in photovoltaic systems with two separate loads of different priority. The Multi-Stage Dual Priority Regulator (MSDPR) charges two separate battery systems for high and low priority loads. This controller switches the array modules from the main battery to the auxiliary battery as the main battery nears full charge, thus utilizing the PV panels to the maximum without jeopardizing the reliability of the critical load.     

Energy dispatch schedule optimization and cost benefit analysis for grid-connected,photovoltaic-battery storage systems

A linear programming (LP) routine was implemented to model optimal energy storage dispatch schedules for peak net load management and demand charge minimization in a grid-connected, combined photovoltaic-battery storage system (PV+ system). The LP leverages PV power output and load forecasts to minimize peak loads subject to elementary dynamical and electrical constraints of the PV+ system. Battery charge/discharge were simulated over a range of two PV+ system parameters (battery storage capacity and peak load reduction target) to obtain energy cost for a time-of-use pricing schedule and the net present value (NPV) of the battery storage system. The financial benefits of our optimized energy dispatch schedule were compared with basic off-peak charging/on-peak discharging and real-time load response dispatch strategies that did not use any forecast information. The NPV of the battery array increased significantly when the battery was operated on the optimized schedule compared to the off-peak/on-peak and real time dispatch schedules. These trends were attributed to increased battery lifetime and reduced demand charges attained under the optimized dispatch strategy. Our results show that Lithium-ion batteries can be a financially viable energy storage solution in demand side, energy cost management applications at an installed cost of about $400–$500 per kW h (approximately 40–50% of 2011 market prices). The financial value of forecasting in energy storage dispatch optimization was calculated as a function of battery capacity ratio.     

Technical and economic assessment of hybrid photovoltaic/wind system with battery storage in Corsica island

The sizing and techno-economical optimization of a stand-alone hybrid photovoltaic/wind system (HPWS) with battery storage is presented in this paper. The main objective of the present study is to find the optimum size of system, able to fulfill the energy requirements of a given load distribution, for three sites located at Corsica island and to analyze the impact of different parameters on the system size. The methodology used provides a useful and simple approach for sizing and analyzing an HPWS. In the proposed stand-alone system, a new concept such as the supply of wind power via a uninterruptible power supply (UPS) is introduced and therefore the energy produced by the wind generator can be sent directly to the load.     

非均衡负载独立光伏发电系统参数设计方法的研究

常规光伏系统设计多以获取最大电能为目标,而对于非均衡负载独立光伏系统,为满足各时间段负载正常供电不得不提高整体光伏发电系统的容量,而导致大量能量浪费。本文剖析了独立光伏发电系统内部各模块间的匹配关系,特别考虑非均衡负载对系统参数设计的影响,在此基础上构思独立光伏系统参数设计新方法,以提高光伏系统的可靠性和经济性的匹配。新算法能同时适应均衡负载和非均衡负载系统的参数设计。本文最后,利用该算法,对广州某一非均衡负载的独立光伏发电系统进行了参数设计,并做了分析和对比。     

Optimal operation of microgrid using hybrid differential evolution and harmony search algorithm

This paper proposes the generation scheduling approach for a microgrid comprised of conventional generators, wind energy generators, solar photovoltaic (PV) systems, battery storage, and electric vehicles. The electrical vehicles (EVs) play two different roles: as load demands during charging, and as storage units to supply energy to remaining load demands in the MG when they are plugged into the microgrid (MG). Wind and solar PV powers are intermittent in nature; hence by including the battery storage and EVs, the MG becomes more stable. Here, the total cost objective is minimized considering the cost of conventional generators, wind generators, solar PV systems and EVs. The proposed optimal scheduling problem is solved using the hybrid differential evolution and harmony search (hybrid DE-HS) algorithm including the wind energy generators and solar PV system along with the battery storage and EVs. Moreover, it requires the least investment.     

Implementation of repowering optimization for an existing photovoltaic‐pumped hydro storage hybrid system: A case study in Sichuan,China

For a remote area or an isolated island, where the grid has not extended, a standalone hybrid energy system can provide cheap and adequate power for local users. However, with the development of society, the load demand will increase and the original system cannot completely meet the load demand. This situation occurs in Xiaojin, Sichuan, China. The existing photovoltaic‐pumped hydro storage (PV‐PHS) hybrid system in this area as the original system cannot completely meet the load requirements at present. The term “repowering” aims to maximize the reliability of power supply and the utilization of the PV‐PHS hybrid energy system that differs from traditional planning optimization to build all components. The repowering strategy is to integrate wind turbines (WTs) and battery into the original system. For the repowering system, a power management strategy is proposed to determine the operating modes of the PHS and battery. Three objectives, which are minimizing percentage of the demand not supplied, levelized cost of energy, and curtailment rate of renewable energy, are considered in the optimization model. Simulation is conducted by single‐objective, biobjective, and triobjective particle swarm optimization (PSO) techniques. For the single‐objective optimization, the comparison of PSO and genetic algorithm (GA) is made. For the double‐objective optimization, multiobjective PSO (MOPSO) is compared with weighted sum approach (WSA), and fuzzy satisfying method is utilized to find the win‐win solution. The results reveal that the repowering strategy can help to achieve maximum reliability of power supply after load demand increases significantly, and the battery plays an important role in such a hybrid system.