Excess electricity problem in off-grid hybrid renewable energy systems: A comprehensive review from challenges to prevalent solutions

Excess electricity, surplus power, or dumped energy refers to the unused portion of energy in hybrid renewable energy systems (HRESs), which can significantly impact the stability, affordability, and reliability of the energy system. Surplus power is often generated due to the intermittent nature of renewable energy resources when battery is fully charged or the generator's minimum output exceeds the load. While it can be transferred to the grid utility in grid-connected HRESs, off-grid systems face a significant challenge with high amounts of excess power. Therefore, surplus electricity is a crucial factor that affects the development of stand-alone HRESs. This review study aims to identify and classify prevalent and practical methods for reducing excess electricity in stand-alone HRESs based on the performance concepts in excess power reduction. Accordingly, four categories of excess electricity direct use, storage of excess electricity, indirect use of excess electricity, and decrease of excess electricity production are introduced. Finally, deferrable load, power to heat, storage banks, power to hydrogen, power to gas cycles, multiple generators, and loss of power supply were detected as the most prevalent methods. These methods were discussed based on their impacts on energy cost, renewable fraction, and excess electricity reduction potential in HRESs.