Research On Optimal Sizing Of Energy Storage Unit For Photovoltaic Micro-Grid

The energy storage unit (ESU) plays an important role in the micro grid. And the costs of the ESU accounted for a great proportion of the total cost of the micro-grid. That determines the necessity of micro grid energy storage optimization method research. In this paper, several common energy storage units have been compared in price, capacity, power and lifetime. The applications in different types of micro grid according to their own characteristics have been described. This paper analyzes the capacity characteristics, the power characteristics and the effect of the charge and discharge on battery lifetime. And the paper focuses on the study of the ESU optimization, in the aim of minimizing the price of the micro grid during the whole lifetime cycle.The research on the calculation of the service life of the battery with the operation of the micro-grid is particularly detailed. Battery replacement costs are included in the micro-grid life cycle costs. The charging and discharging cycles of the battery are described by three parameters:the depth of charge and discharge, the initial state and the end state. Rain-flow counter is used to count the half cycles of the whole year, recording three parameters of each cycle. For getting more accurate battery life time model, life loss of each kind of cycles is calculated using these three parameters. Combined with this method, one parameter, using-degree, is used to descript the battery status to convenient for determining the times of battery replacements and replacement time in the whole life cycle of micro-grid. In the study of hybrid ESU sizing, battery and super-capacitor hybrid ESU with a strategy of power dispatching has been used to reduce the life loss of the battery caused by the power fluctuation in the micro-grid. Genetic algorithm is used to solve the optimization model. The way of selection, crossover and mutation of genetic algorithm are set up according to the characteristics of the model. Some parameters of the algorithm are adjusted several times for the better solution effect. All the details of the algorithm are showed in the paper. In the end, two battery life lost calculation methods have been compared, according to the calculation results. The necessity of applying the battery life model of this paper is analyzed. Optimal sizing configurations have been obtained under several different SOC lower limits and LOLP upper limits. Based on the comparison of the operation data and the cost of several schemes, the influence of these two parameters on the configuration of the micro network is analyzed. And the analysis results also confirmed the accuracy of the optimal sizing model of this paper.