Power management strategies for off-grid hybrid power systems

Abstract

At present, there are still a large number of people living in isolated areas, particularly in developing countries, who have no immediate access to the main electricity grid. Most of the energy demands of these remote communities are met by diesel-operated power systems, which are relatively affordable and available. With the ever increasing awareness of climate change, many local authorities have taken initiatives to reduce the carbon footprint of certain energy sectors. In some rural applications, diesel generator power systems are augmented by single or multiple renewable energy supply units to form an off-grid hybrid power system.Generally, the majority of off-grid hybrid power systems include a massive battery bank to store excess renewable energy to supply the user load demand during the period when renewable energy is deficient. In the charging and discharging processes, energy losses may occur due to the inefficiency of the charger and the battery cells. Also, inclusion of an energy storage element into a hybrid power system incurs additional investment costs and involves recycling issues. Therefore, it is necessary to minimise the size of storage, whenever possible, and operate the system under an appropriate power management strategy to ensure efficient system operation.The chosen power management strategy impacts long-term performance of a system as well as components’ longevity. The research presented in this thesis describes the development of an advanced power management concept for the operation of a photovoltaic-variable speed diesel generator hybrid power system.A general introduction regarding the research background to hybrid power system applications and fundamentals of solar energy is presented. A component sizing and control program is developed to facilitate hybrid power system design. Then, various off-grid power system configurations are further discussed with emphasis on the system performances and economic aspects.A prediction technique, namely the Hourly-based Prediction Model for solar irradiance and load demand forecasts is discussed. Forecast algorithms for the hourly solar irradiance and load demand predictions are presented. The proposed prediction models are implemented in the power management strategy for the off-grid photovoltaic-variable speed diesel generator hybrid power system. Assessments of the prediction models through comprehensive analyses of statistical measures are presented.HOMER simulation software has been adopted for time series generation and economic analyses for several off-grid power system configurations. Also, the HOMER simulation results for electrical aspects are used as a benchmark to evaluate the component models developed in this thesis. Due to the fact that HOMER offers limited choices of power management strategy and users do not have access to modify the control algorithms, it is impossible to determine the performance of a system under advanced power management strategy. Therefore, analytical performance models of system components have been developed using the MATLAB/Simulink software to allow the implementation of the proposed power management strategy.The concepts and flow charts of the predictive power management strategy are described. This power management strategy consists of predictive and adaptive dispatch. The time step of the predictive dispatch is fixed to one hour while the time step of the adaptive dispatch is one minute. Operation of the additional generator capacity of the hybrid power system is based on the predicted net load. The adaptive dispatch supports the predictive dispatch to handle fluctuations of net load that occur in between prediction intervals.Simulation results of the performance of hybrid power systems using different types of diesel generator and power management strategies are presented. Particular emphasis is on the comparisons of the system performances using non-predictive and predictive power management strategies. These simulations allow quantitative assessment of the system performances in terms of electrical output, fuel consumption and carbon dioxide emission.Last but not least, the entire research is summarised and concluded with suggestions for future research. In short, the photovoltaic-variable speed diesel generator hybrid power system topology and the proposed power management strategy offer an alternative to the off-grid hybrid power system design, with the aims of overcoming the complex technical issues associated with energy storage and of contributing to market extension of hybrid power systems, particularly in remote locations where financial and technical issues are the major concerns.