An energy storage system (ESS) is a technology or arrangement that enables the storage of excess energy produced during times of low demand or high generation for use at a later time when demand is higher or generation is low. It involves capturing and storing energy in various forms and releasing it when needed, providing flexibility and stability to electrical grids, renewable energy systems, and other power systems.

The primary purpose of an energy storage system is to balance energy supply and demand, address intermittency issues of renewable energy sources, enhance grid stability, and optimize energy usage. By storing surplus energy during periods of low demand or high generation, an ESS can help meet peak energy demands, reduce reliance on fossil fuel power plants, and support a more efficient and sustainable energy infrastructure.

Energy storage systems can utilize different technologies and storage mediums, depending on factors such as the desired storage capacity, duration, efficiency, and application requirements. Some common types of energy storage technologies include:

1. Batteries: Battery energy storage systems (BESS) store electrical energy in rechargeable batteries, such as lithium-ion, lead-acid, or flow batteries. They are widely used in applications ranging from small-scale residential systems to utility-scale installations.
2. Pumped Hydro Storage: Pumped hydro storage systems use excess electricity to pump water from a lower reservoir to an upper reservoir during low-demand periods. When electricity demand increases, water is released from the upper reservoir, passing through turbines to generate electricity.
3. Flywheels: Flywheel energy storage systems store energy by spinning a rotor at high speeds. When needed, the rotor’s kinetic energy is converted back into electrical energy using a generator. Flywheel systems provide quick response times and are suitable for applications requiring high power output for short durations.
4. Thermal Energy Storage: Thermal energy storage systems store and release heat or cold energy using materials with high heat capacity, such as water, ice, or phase change materials. They can be utilized for applications like district heating/cooling, solar thermal energy storage, or industrial processes.
5. Compressed Air Energy Storage (CAES): CAES systems store energy by compressing air and storing it in underground caverns or tanks. When electricity demand rises, the compressed air is released and expanded through turbines to generate electricity.
6. Hydrogen Storage: Hydrogen can be produced through electrolysis during periods of excess electricity and stored for later use in fuel cells or combustion engines to generate electricity.

Energy storage systems offer several benefits, including load leveling, grid stability, peak shaving, backup power supply, integration of renewable energy sources, and the potential for energy cost savings. They are vital for achieving a more resilient, flexible, and sustainable energy system that can efficiently handle fluctuations in energy supply and demand.