The job of an electric grid operator is, succinctly put, to keep supply and demand in constant balance, as even minor imbalances between the two can damage equipment and cause outages.
This balance is a highly complex undertaking that involves coordinating hundreds of generation units with the demands of millions of individual customers. Historically, this challenge was mitigated by predictability: the generation (supply) side had power plants that could be turned up or down as needed, while the load (demand) side had customers who generally had the same devices in their houses and used them in the same ways. Grid operators knew what was coming and could adjust production to accommodate it.
Given recent changes in energy supply and demand, energy storage is of increasing interest to ensure reliable and sustainable provision. This article explains the current challenges to power supply and demand and then provide an overview of energy storage technologies. Following a summary of the modeling challenges associated with energy storage and recent advances in overcoming those challenges, it discusses systems and technologies needed to maintain a clean and reliable electric grid.
Current Challenges to Power Supply and Demand
Climate change and technological innovations that have made renewable generation financially competitive and increasingly accessible have fundamentally changed the nature of supply and demand. A rapidly increasing share of electricity comes from variable sources, distributed energy resources and electrical vehicles mean that generation can come from just about anywhere on the grid, and customer demands may vary widely.
Fortunately, technical innovations have also delivered new forms of electrical energy storage that can keep generation and load in balance. To maintain that balance, grid operators call on flexible ancillary services to reconcile differences between electric supply and demand. But the services vary in both the size of the differences that they remedy and the duration over which they are employed.
Flexible resources such as energy storage can help to integrate variable sources of generation such as wind and solar. Moment-to-moment variability in the output of renewable resources requires frequency regulation to absorb peaks and fill in valleys to maintain generation and load balance. Longer-term variability in output (e.g., due to a cloudy day) requires activation of spinning reserves to replace the lost production. And finally, the variable output of distributed energy resources such as rooftop solar can vary the demands of individual customers, requiring energy imbalance resources to correct differences between forecast and actual demand.
Understanding Current Energy Storage Technologies
Energy storage devices are unique among grid assets because they can both withdraw energy from the grid during periods of excess generation and inject energy during periods of insufficient generation. These capabilities make storage an ideal source of both ancillary services and the grid flexibility necessary to incorporate variable energy resources such as wind and solar. However, determining how to optimally deploy energy storage is a challenge under traditional electric grid planning practices, and the rapidly changing grid is creating demand for new long-duration energy storage (LDES) technologies that have not yet been commercially proven.
Energy storage is distinct from other electric grid assets in three important ways:
- Flexibility: Because energy storage technologies can act as either a load (when charging) or a generator (when discharging), they can provide a range of grid-balancing services.
- Scalability: Most energy storage technologies are modular, which allows them to be scaled down to a small device that supports the demands of a single customer or scaled up to a large project that supports the demands of thousands of customers.
- Duration: Unlike a power plant that can provide electricity as long as it is connected to its fuel source, energy storage technologies are energy-limited: they store their fuel in a tank and must recharge when that tank is empty.