The energy shortage and environmental pollution are attracting more and more attention with the social development. Hence it is important to develop new energy and renewable energy that are bio-energy, wind energy, solar energy, small waterpower, tide energy, terrestrial heat energy, and hydrogen energy and so on. Hydropower has a central role to play in the grid of the future that will often be more based upon variable output renewable energy generation – especially wind and solar. Hydropower can act as a frequency regulator and energy store, and deliver a range of other ‘ancilliary’ benefits. Hydropower is one of the most flexible sources of electricity production. Power systems with considerable amounts of flexible hydropower potentially offer easier integration of variable generation, e.g., wind and solar. However, there exist operational constraints to ensure the mid-/long-term security of supply while keeping river flows and reservoirs levels within permitted limits. In order to properly assess the effectively available hydropower flexibility and its value for storage, a detailed assessment of hydropower is essential. Hydropower, generated mainly from hydroelectric dams, is a clean, renewable, non-emitting source of energy that provides low-cost electricity and helps reduce carbon emissions.