Solar energy management system for stand-alone application : analysis of power management strategies under different solar irradiance profiles

Abstract

Clean power generation from various renewable energy sources has been gaining much attention due to sustainable and environmental reasons. As renewable energy sources and power demand vary, a hybrid power system and proper energy management are required. This study is focused on the development of an energy management system (EMS) in the hybrid renewable energy system consisting of photovoltaic cell (PV), proton exchange membrane fuel cell (PEMFC), and proton exchange membrane electrolysis cell (PEMEC). A vanadium redox flow battery (VRFB) as energy storage for long-term operation is integrated into such a hybrid system. The models of each subsystem are developed based on conservative equations, electrochemistry theory, and power balances used to evaluate the efficiency of the developed EMS. During the system operation, the electric power from PV is supplied to the load, and the excess power is used to charge the battery and then run PEMEC for hydrogen production. When power shortages occur in the system, PEMFC using stored hydrogen or battery is operated. Firstly, the performance of the hybrid energy system using VRFB is investigated and compared with that using a traditional battery (i.e., lead-acid battery). The simulation results show that under the same energy supply and demand situation, the hybrid power system with VRFB outperforms, which leads to a more extended battery operating period. Furthermore, the use of VRFB can decrease the operational time of the fuel cell and electrolysis cell by 18.35% and 14.97%, respectively. Then, the effect of changes in solar energy according to weather conditions on the performance of each unit in the hybrid power system is analyzed in terms of the amount of hydrogen consumed by PEMFC and produced by PEMEC and the amount of dump load power. In the case of high solar irradiance such as in the summer, the hybrid power system is stable due to the balance between hydrogen usage and production, and low dump load power is observed. In the winter and arbitrarily weather, although there is remaining excess power after supplying to the load, it is not sufficient for hydrogen production; taking electricity from the main grid or importing hydrogen from other sources is required to make the system stable for the long-term operation. Finally, the two new energy management strategies of the hybrid power system in which the minimum power of PEMEC operation is differently managed are presented. A suitable energy management strategy for each scenario of the irradiance profiles is identified. In the first strategy, the excess power that is lower than the minimum power for PEMEC operation will distribute to the dump load. This strategy is suitable for the summer scenario due to much excess energy. In the second strategy, it is determined that there is enough power from battery discharging to operate PEMEC at the minimum power point. It is found that this strategy is suitable for the situation with low excess power such as in the winter and arbitrarily weather.