Energy management for a hybrid renewable micro-grid system

Abstract

This study presents an energy management system (EMS) for a hybrid renewable micro-grid system. The increasing demand for energy has led to the accelerated depletion of fossil fuel reserves, outpacing their natural replenishment. It is anticipated that the available fossil fuel resources can only suffice for approximately the next 40 years, given the limitations in extracting these resources. Moreover, the widespread reliance on fossil fuels and internal combustion engines (ICE) has contributed substantially to environmental pollution and raised concerns about anthropogenic (human-induced) climate change. To address these issues, it is imperative to transit to a production system that is not based on fossil fuels as the primary energy source, hence the adoption of renewable micro-grids. The utilization of renewable micro-grids has experienced significant advancements and growth in the past decade, driven by technological maturity, the expected scarcity of fossil fuels and government policies emphasizing environmental protection. While hybrid renewable micro-grid systems are now widely available in the market, renewable micro-grids are still considered as a promising alternative to traditional power grids. On-site hybrid micro-grids offer advantages such as minimizing disruptions, reducing costs, optimizing system component sizes to minimize operating costs, as well as ensuring access to energy sources which are affordable, reliable, and sustainable. The incorporation of an energy storage system, especially in renewable micro-grid systems, provides unconditional benefits for electric power reliability, including smoothing power fluctuations, supplying initial power during the transition from grid-connected mode to island operating mode, and offering the ability to ride through dynamic fluctuations in intermittent energy sources. Battery energy storage systems are predominantly utilized in renewable micro-grid systems. However, both renewable hybrid micro-grids and battery storage systems have shown certain limitations, not related to rising fossil fuel prices or environmental pollution but rather within the EMS. Some of these limitations include the intermittence of renewable energy sources, the efficiency dependence of the equipment on the operating point, the relationship between battery charging/discharging rates and operating conditions, overcurrent, overvoltage, state of charge (SOC), unbalanced conditions, all of which may impact battery lifetime and the reliability of hybrid micro-grids. The main research problem focused on how to develop a real-time energy management system able to control the power flow within the hybrid renewable micro-grid system and the variable AC load as well as the utility grid. Secondly, how to control the charge and discharge of the battery. The developed EMS algorithm including the micro-grid system was performed utilizing MATLAB/Simulink software, and Typhoon HIL’s real-time digital simulator was used to test, validate and optimize their performance. This research aimed to develop a strategy to manage the energy flow within the hybrid renewable micro-grid system, which optimizes the renewable energy resources utilization and augments their incorporation into the power system. Hence, this research had as its principal objective to develop an energy management strategy capable to control the flow of energy in the hybrid micro-grid system and the main grid. The second objective of this research has been based on the control of the battery charging/discharging. To achieve these objectives, different sequences have been considered, which included the design of the components model of the hybrid micro-grid utilized in the simulation model, the development of the hybrid micro-grid simulation, the development of the hybrid micro-grid control scheme and finally, the development of the hybrid micro-grid system EMS algorithm. To demonstrate the developed EMS control robustness and effectiveness, different cases have been utilized in the simulations, which showed its ability to handle the load in isolated as well as grid connected modes. The developed EMS control also ensures an appropriate battery operation within the hybrid micro-grid system. The main advantage of this algorithm is that it controls the flow of energy into the hybrid micro-grid and the variable AC load including the main grid; it additionally maintains a suitable battery charge/discharge, according to their conditions of operation as well as allows to keep its SOC within acceptable bounds. Additionally, the real-time experimental results obtained using the Typhoon HIL software demonstrated good performance in comparison to those achieved utilizing the Stateflow logical language in the MATLAB/Simulink software. This developed EMS algorithm was tested according to the different scenarios and shown to be extremely effective. Both results were similar and were based on the renewable energy sources production, utility grid, diesel generator, variable AC load demand and battery SOC. In discharge mode, the battery responded instantly, and during high production, it was perfectly recharged. The hybrid micro-grid and utility grid were able to share power effectively under the given conditions, which ensured the smooth operation of both sources. Finally, a higher charging power was used to charge the battery. The results indicate the feasibility of maximizing charging time using higher power, leading to improved energy density utilization and possibly increasing the longevity of the batteries.