Optimisation of real-time control for hybrid diesel–PV–battery systems

Hybrid diesel–PV–battery systems are one of the most cost effective options for off-grid power generation. A methodology for the optimal operation of such systems for an off-grid application is proposed in this paper. The methodology is based on the minimisation of an energy cost function. Based on this function, an optimal operating point for the diesel generator is identified, taking into account the characteristics of the diesel generator, battery bank and converter as well as the costs of fuel and battery usage. The operation of the diesel generator at this optimum operating point results in an overall energy cost reduction for the hybrid diesel–battery system. Simulation analysis shows that the proposed control strategy can achieve up to 4% reduction in the levelised cost of energy. This is mostly due to the savings made from the efficient usage of diesel generator and battery.     

Optimum sizing of photovoltaic battery systems incorporating uncertainty through design space approach

Photovoltaic-battery system is an option for decentralized power generation for isolated locations receiving abundant sunshine. A methodology for the optimum sizing of photovoltaic-battery system for remote electrification incorporating the uncertainty associated with solar insolation is proposed in this paper. The proposed methodology is based on the design space approach involving a time series simulation of the entire system. The design space approach was originally proposed for sizing of the system with deterministic resource and demand. In the present paper, chance constrained programming approach has been utilized for incorporating the resource uncertainty in the system sizing and the concept of design space is extended to incorporate resource uncertainty. The set of all feasible design configurations is represented by a sizing curve. The sizing curve for a given confidence level, connects the combinations of the photovoltaic array ratings and the corresponding minimum battery capacities capable of meeting the specified load, plotted on an array rating vs. battery capacity diagram.The methodology is validated using a sequential Monte Carlo simulation approach with illustrative examples. It is shown that for the case of constant coefficient of variation of solar insolation, the set of sizing curves for different confidence levels may be represented by a generalized curve. Selection of optimum system configuration for different reliability levels based on the minimum cost of energy is also presented. The effect of ambient temperature on sizing a stand-alone photovoltaic-battery system is also illustrated through a representative example.     

Computer-aided design of PV/wind hybrid system

A complete set of match calculation methods for optimum sizing of PV/wind hybrid system is presented. In this method, the more accurate and practical mathematic models for characterizing PV module, wind generator and battery are adopted; combining with hourly measured meteorologic data and load data, the performance of a PV/wind hybrid system is determined on a hourly basis; by fixing the capacity of wind generators, the whole year’s LPSP (loss of power supply probability) values of PV/wind hybrid systems with different capacity of PV array and battery bank are calculated, then the trade-off curve between battery bank and PV array capacity is drawn for the given LPSP value; the optimum configuration which can meet the energy demand with the minimum cost can be found by drawing a tangent to the trade-off curve with the slope representing the relationship between cost of PV module and that of the battery. According to this match calculation method, a set of match calculation programs for optimum sizing of PV/wind hybrid systems have been developed. Applying these match calculation programs to an assumed PV/wind hybrid system to be installed at Waglan island of Hong Kong, the optimum configuration and its hourly, daily, monthly and yearly performances are given.     

Modelling of hybrid energy system—Part II: Combined dispatch strategies and solution algorithm

Computer simulation is an increasingly popular tool for determining the most suitable hybrid energy system type, design and control for an isolated community or a cluster of villages. This paper presents the development of the optimum control algorithm based on combined dispatch strategies, to achieve the optimal cost of battery incorporated hybrid energy system for electricity generation, during a period of time by solving the mathematical model, which was developed in Part I of this tri-series paper.The main purpose of the control system proposed here is to reduce, as much as possible, the participation of the diesel generator in the electricity generation process, taking the maximum advantage of the renewable energy resources available.The overall load dispatch scenario is controlled by the availability of renewable power, total system load demand, diesel generator operational constraints and the proper management of the battery bank. The incorporation of a battery bank makes the control operation more practical and relatively easier.     

Application of design space methodology for optimum sizing of wind–battery systems

A methodology for optimum sizing of different components (i.e., rotor diameter, electrical generator rating, and battery capacity) of a standalone wind–battery system is proposed in this paper. On the basis of time series simulation of the system performance along with different design constraints, the entire set of feasible design options, also known as the design space, has been identified on a rotor diameter vs. rated power diagram. The design space of a standalone wind–battery system identifies the entire envelope within which a feasible system may be designed. The optimum configuration of the standalone system is identified on the basis of minimum cost of energy (US$/kWh). It is observed that the cost of energy is sensitive to the magnitude of average demand and the wind regime. Sensitivity of the capital cost on the minimum cost of energy is also studied.     

Techno‐economic optimization of hybrid power system using genetic algorithm

This paper presents an optimum sizing methodology to optimize the hybrid energy system (HES) configuration based on genetic algorithm. The proposed optimization model has been applied to evaluate the techno‐economic prospective of the HES to meet the load demand of a remote village in the northern part of Saudi Arabia. The optimum configuration is not achieved only by selecting the combination with the lowest cost but also by finding a suitable renewable energy fraction that satisfies load demand requirements with zero rejected loads. Moreover, the economic, technical and environmental characteristics of nine different HES configurations were investigated and weighed against their performance. The simulation results indicated that the optimum wind turbine (WT) selection is not affected only by the WT speed parameters or by the WT rated power but also by the desired renewable energy fraction. It was found that the rated speed of the WT has a significant effect on optimum WT selection, whereas the WT rated power has no consistent effect on optimal WT selection. Moreover, the results clearly indicated that the HES consisting of photovoltaics (PV), WT, battery bank (Batt) and diesel generator (DG) has superiority over all the nine systems studied here in terms of economical and environmental performance. The PV/Batt/DG hybrid system is only feasible when wind resource is very limited and solar energy density is high. On the other hand, the WT/Batt/DG hybrid system is only feasible at high wind speed and low solar energy density. It was also found that the inclusion of batteries reduced the required DG and hence reduced fuel consumption and operating and maintenance cost. Copyright © 2014 John Wiley & Sons, Ltd.     

Optimum sizing of wind-battery systems incorporating resource uncertainty

The inherent uncertainty of the wind is a major impediment for successful implementation of wind based power generation technology. A methodology has been proposed in this paper to incorporate wind speed uncertainty in sizing wind-battery system for isolated applications. The uncertainty associated with the wind speed is incorporated using chance constraint programming approach. For a pre-specified reliability requirement, a deterministic equivalent energy balance equation may be derived from the chance constraint that allows time series simulation of the entire system. This results in a generation of the entire set of feasible design options, satisfying different system level constraints, on a battery capacity vs. generator rating diagram, also known as the design space. The proposed methodology highlights the trade-offs between the wind turbine rating, rotor diameter and the battery size for a given reliability of power supply. The optimum configuration is chosen on the basis of the minimum cost of energy (US$/kWh). It is shown with the help of illustrative examples that the proposed methodology is generic and flexible to incorporate alternate sub-component models.     

Design of isolated renewable hybrid power systems

Isolated electrical power generating units can be used as an economically viable alternative to electrify remote villages where grid extension is not feasible. One of the options for building isolated power systems is by hybridizing renewable power sources like wind, solar, micro-hydro, etc. along with appropriate energy storage. A method to optimally size and to evaluate the cost of energy produced by a renewable hybrid system is proposed in this paper. The proposed method, which is based on the design space approach, can be used to determine the conditions for which hybridization of the system is cost effective. The simple and novel methodology, proposed in this paper, is based on the principles of process integration. It finds the minimum battery capacity when the availability and ratings of various renewable resources as well as load demand are known. The battery sizing methodology is used to determine the sizing curve and thereby the feasible design space for the entire system. Chance constrained programming approach is used to account for the stochastic nature of the renewable energy resources and to arrive at the design space. The optimal system configuration in the entire design space is selected based on the lowest cost of energy, subject to a specified reliability criterion. The effects of variation of the specified system reliability and the coefficient of correlation between renewable sources on the design space, as well as the optimum configuration are also studied in this paper. The proposed method is demonstrated by designing an isolated power system for an Indian village utilizing wind-solar photovoltaic-battery system.     

风光互补发电系统的优化设计(I) CAD设计方法

给出了一整套利用CAD进行风光互补发电系统优化设计的方法。为了精确确定系统每小时的运行状态,采用了更精确地表征组件特性及评估实际获得的风光资源的数学模型。为了寻找出以最小设备投资成本满足用户用电要求的系统配置,首先在风力发电机容量固定不变的前提下,计算了与该容量风力发电机匹配的不同容量的PV方阵和蓄电池所组成的风/光/蓄组合的全年功率供给亏欠率LPSP,根据总的设备投资成本最小化的原则筛选出一组与该容量风力发电机对应的满足用户给定系统供电可靠性即LPSP值的风/光/蓄组合;然后通过改变风力发电机的容量,优选出多个与不同容量风力发电机对应的既能满足用户用电要求同时总的设备购置成本又是最低的风/光/蓄组合,比较它们的成本最终唯一确定出以最小投资成本满足用户用电要求的优化的系统配置。     

Sizing a hybrid wind-diesel stand-alone system on the basis of minimum long-term electricity production cost

Hybrid wind-diesel systems are an interesting solution for the electrification of isolated consumers. The proposed system, including a properly sized battery, leads to a significant reduction of the fuel consumption, in comparison with a diesel-only installation, also protecting the diesel generator from excessive wear. On the other hand, a properly designed wind-diesel installation remarkably reduces the required battery capacity, in relation to a wind-only based stand-alone system, especially in medium-low wind potential areas. In this context, a complete sizing model, based on a long-term energy production cost analysis is developed, able to predict the optimum configuration of a hybrid wind-diesel stand-alone system on the basis of minimum long-term cost. According to the application results obtained for representative wind potential cases, the proposed hybrid system guarantees one year’s long energy autonomy of a typical remote consumer, presenting a significant cost advantage in relation either to a diesel-only or to a wind-based stand-alone system.     

Design and analysis of an aging‐aware energy management system for islanded grids using mixed‐integer quadratic programming

The rapid increase of renewable energy sources made coordinated control of the distributed and intermittent generation units a more demanded task. Matching demand and supply is particularly challenging in islanded microgrids. In this study, we have demonstrated a mixed‐integer quadratic programming (MIQP) method to achieve efficient use of sources within an islanded microgrid. A unique objective function involving fuel consumption of diesel generator, degradation in a lithium‐ion battery energy storage system, carbon emissions, load shifting, and curtailment of the renewable sources is constructed, and an optimal operating point is pursued using the MIQP approach. A systematic and extensive methodology for building the objective function is given in a sequential and explicit manner with an emphasis on a novel model‐based battery aging formulation. Performance of the designed system and a sensitivity analysis of resulting battery dispatch, diesel generator usage, and storage aging against a range of optimization parameters are presented by considering real‐world specifications of the Semakau Island, an island in the vicinity of Singapore.     

Optimal sizing study of hybrid wind/PV/diesel power generation unit

In this paper, a methodology of sizing optimization of a stand-alone hybrid wind/PV/diesel energy system is presented. This approach makes use of a deterministic algorithm to suggest, among a list of commercially available system devices, the optimal number and type of units ensuring that the total cost of the system is minimized while guaranteeing the availability of the energy. The collection of 6 months of data of wind speed, solar radiation and ambient temperature recorded for every hour of the day were used. The mathematical modeling of the main elements of the hybrid wind/PV/diesel system is exposed showing the more relevant sizing variables. A deterministic algorithm is used to minimize the total cost of the system while guaranteeing the satisfaction of the load demand. A comparison between the total cost of the hybrid wind/PV/diesel energy system with batteries and the hybrid wind/PV/diesel energy system without batteries is presented.The reached results demonstrate the practical utility of the used sizing methodology and show the influence of the battery storage on the total cost of the hybrid system.     

Predictive control of an integrated PV-diesel water and power supply system using an artificial neural network

This paper discusses the development of a predictive artificial neural network (ANN)-based prototype controller for the optimum operation of an integrated hybrid renewable energy-based water and power supply system (IRWPSS). The integrated system, which has been assembled, consists of photovoltaic modules, diesel generator, battery bank for energy storage and a reverse osmosis desalination unit. The electrical load consists of typical households and the desalination plant. The proposed Artificial Neural Networking controller is designed to be implemented to take decision on diesel generators ON/OFF status and maintain a minimum loading level on the generator under light load and high solar radiation levels and maintain high efficiency of the generators and switch off diesel generator when not required based on predictive information. The key objectives are to reduce fuel dependency, engine wear and tear due to incomplete combustion and cut down on greenhouse gas emissions. The statistical analysis of the results indicates that the R² value for the testing set of 186 cases tested was 0.979. This indicates that ANN-based model developed in this work can predict the power usage and generator status at any point of time with high accuracy.     

Optimum operation strategy and economic analysis of a photovoltaic-diesel-battery-mains hybrid uninterruptible power supply

This paper presents the optimum operation strategy and economic analysis of a photovoltaic-diesel-battery-mains hybrid uninterruptible power supply (UPS). The system involves a photovoltaic, battery and bi-directional inverter that is connected in parallel to the grid. A diesel generator is required when the grid is not available for a longer time. The optimum operation strategy of the system is proposed for the diesel-connected mode (when the grid fails for several hours), while the economic analysis is evaluated for the grid-connected mode. The optimum strategy determines the ‘set point’ value for starting and stopping the diesel generator, resulting in a lower system operation cost within its lifetime. The optimum value is obtained by comparing the cost of the diesel fuel consumption and the battery wear. The economic analysis includes the system operation as UPS and demand side management. The system will reduce the power flow from the mains by increasing the power from the inverter to the load when the tariff is high. However, when the grid tariff is low, the power from the mains is used to charge the battery and to meet the load simultaneously.     

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.     

Economic optimisation of a wind power plant for isolated locations

This paper presents a model of a wind power plant for isolated locations composed of a vertical axis wind turbine connected to a self-excited induction generator operating at constant voltage and frequency; a back-up diesel generator and a battery system are moreover included in the system. Constant voltage and frequency are obtained only by controlling the generator appropriately. The control system is supposed to be optimised so that the system operates at the highest efficiency. In order to improve the total efficiency even further, a gear-box to vary the gear transmission ratio between the turinbe and the generator has been considered. A “Monte Carlo” type simulation has been used to analyse the operation of that system over a one year period. The model is based on a probability density function of the wind speed derived by statistical data concerning a given location and on the probabilistic curve of the load required by an isolated location. The cost per kWh for different dimensions of the main components has been evaluated and the optimum configuration has been identified.     

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.     

An integrated model for performance simulation of hybrid wind–diesel systems

Stand-alone hybrid systems have turned into one of the most promising ways to handle the electrification requirements of numerous isolated consumers worldwide. The proposed wind–diesel–battery hybrid system consists of a micro-wind converter, a small diesel-electric generator—basically operating as a back up energy production system—and a lead-acid battery bank that stores the wind energy surplus during high wind speed periods. In this context the present work is focused on presenting a detailed mathematical model describing the operational behavior of the basic hybrid system components, along with the representative calculation results based on the developed mathematical model. Accordingly, an integrated numerical algorithm is built to estimate the energy autonomy configuration of the hybrid system under investigation. Using the proposed numerical algorithm, the optimum configuration selection procedure is verified by carrying out an appropriate sensitivity analysis. The proposed methodology may equally well be applied to any other remote consumer and wind potential type, in order to estimate the optimum wind–diesel hybrid system configuration that guarantees long-term energy autonomy.     

Methodology for optimally sizing the combination of a battery bankand PV array in a wind/PV hybrid system

In this paper, a methodology for calculation of the optimum size of a battery bank and the PV array for a standalone hybrid wind/PV power system is developed. Long term data of wind speed and irradiance recorded for every hour of the day for 30 years were used. These data were used to calculate the average power generated by a wind turbine and a PV module for every hour of a typical day in a month. A load of a typical house in Massachusetts, USA, was used as a load demand of the hybrid system. For a given load and a desired loss of power supply probability, an optimum number of batteries and PV modules was calculated based on the minimum cost of the power system     

Optimal sizing of battery storage for hybrid (wind+diesel) power systems

Hourly mean wind-speed data for the period 1986–1997 [except the years 1989 (some data is missing) and 1991 (Gulf War)] recorded at the solar radiation and meteorological monitoring station, Dhahran (26°C 32′ N, 50° 13′ E), Saudi Arabia, have been analyzed to investigate the optimum size of battery storage capacity for hybrid (wind+diesel) energy conversion systems at Dhahran. The monthly average wind speeds for Dhahran range from 4.12 to 6.42 m/s. As a case study, the hybrid system considered in the present analysis consists of two 10 kW Wind Energy Conversion Systems (WECS), together with a battery storage system and a diesel back-up. The yearly and monthly average energy generated from the above hybrid system have been presented. More importantly, the study explores the impact of variation of battery storage capacity on hybrid power generation. The results exhibit a trade-off between size of the storage capacity and diesel power to be generated to cope with specific annual load distribution [41,500], and for given energy generation from WECS. The energy to be generated from the back-up diesel generator and the number of operational hours of the diesel system to meet a specific annual electrical energy demand have also been presented. The diesel back-up system is operated at times when the power generated from WECS fails to satisfy the load and when the battery storage is depleted. The present study shows that for economic considerations, for optimum use of battery storage and for optimum operation of diesel system, storage capacity equivalent to one to three days of maximum monthly average daily demand needs to be used. It has been found that the diesel energy to be generated without any storage is considerably high; however, use of one day of battery storage reduces diesel energy generation by about 35%; also the number of hours of operation of the diesel system are reduced by about 52%.