Computer simulation of stand-alone photovoltaic systems with battery storage

Abstract

This report describes a computer program which has been developed to simulate accurately the performance of stand alone photovoltaic systems with battery storage on an hourly basis for one simulated year. The program incorporates models of the POA irradiance, the photovoltaic cell · temperature and the battery temperature to simulate the environmental conditions of the system. These require hourly weather data as input. Typical meteorological years, which constitute a suitable form of input weather data, have been generated for those weather stations in Southern Africa which contain sufficient data. The energy flows within the system are simulated using models of the following parameters: photovoltaic module current, regulator efficiency and voltage, battery current and voltage, inverter efficiency, load shed voltage and load current. These models incorporate versatility in the level of modelling complexity (determined typically by the availability of the data used to characterise the components). The various models are encapsulated in modular units to facilitate alteration and updating at a later stage. The program is designed to simulate photovoltaic systems without maximum power point trackers, necessitating the use of interactive curve solving to compute the system operating point at any time. A robust and comprehensive algorithm has been implemented to execute this function. Improved battery modelling has been effected using data and experience acquired from a parallel research project. The program facilitates, with the judicious selection of input weather data, the economical sizing of systems in that it incorporates loss of power probability analysis and offers a high level of modelling precision. The simulation performance of the program compared favourably with that of PVFORM. The system performance estimated by PVFORM was marginally better, which is expected because PVFORM assumes that the system operates with a maximum power point tracker. In the development of the program there has been a focus on creating an effective user interface. This is designed to simplify and speed up program operation, and to present output in a form which is useful and illustrative.