Integrating intermittent sources of energy requires additional flexibility resources and results in a new momentum for electricity-storage solutions
Power systems are challenging to operate since supply and demand must be precisely balanced at all times. Power demand is a constant state of flux; although it generally follows predictable patterns, it is impossible to forecast with precision. As a result, power systems have always had to be flexible. At present, flexibility comes primarily from the generation side: system operators adjust the output of generators upwards or downwards in response to predefined time frames and ramping rates. By storing primary energy sources, such as coal and gas, or water in hydro dams, system operators have avoided the need to store electricity.
Wind and solar photovoltaic systems make demand-supply matching more difficult since they increase the need for flexibility within the system, but do not themselves contribute significantly to flexibility. The increased need for flexibility is reflected in the residual load variations (demand minus intermittent output). The minimal participation in flexibility pool resources is mirrored by the low capacity credit of wind and solar that are granted by system operators to measure the amount of power that they can reliably be expected to produce at peak of demand.
Flexibility management can be optimized by perfecting models for forecasting output from wind and solar plants, fine-tuning market regulations and refining the design of power systems. But additional flexibility will be needed in the form of demand-side participation, better connections between markets, greater flexibility in base-load power supply or electricity storage.
Electricity storage is a three-step process that consists of withdrawing electricity from the grid, storing it and returning it at a later stage. It consists of two dimensions: the power capacity of the charging and discharging phases, which defines the ability of the storage system to withdraw or inject electricity instantaneously from or into the grid; and the energy capacity of the storing phase, which measures how much energy can be stored and for how long. As a consequence, electricity storage has very different uses, depending on the combination of the power rating and discharge time of a device, its location within the grid and its response time.
The primary purpose of electricity storage consists of ensuring power quality and reliability of supply, whether it is to provide operating reserves, uninterrupted power-supply solutions to end-users, or initial power to restart the grid after a blackout. A secondary purpose of electricity storage is driven more by energy requirements. This involves leveling the load – storing power in times of excess supply and discharging it in times of deficit. Leveling enables the deferral of grid investment on a congestion node and optimal utilization of low-operating-cost power plants, and presents opportunities for price arbitrage. The increased penetration of variable renewables is making these applications more critical. It is also creating a new application, known as intermittent balancing, to firm their output or avoid curtailment. For these reasons, variable renewables have resulted in renewed interest in electricity storage.