Utility scale lithium ion Battery Energy Storage System (BESS) installation at Ft. Carson, Colorado, USA. Source: National Renewable Energy Laboratory
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Some renewable energy sources like wind and solar are, by nature, intermittent (the wind is not always blowing and the sun is not always shining). With increasing installation of renewable energy resources, energy storage can fill these gaps. There are times during a 24-hour period when the total supply of electricity can exceed total demand. For instance, there can be excess solar energy generated during a sunny afternoon or excess wind energy generated in the early hours of a day when demand is low. This energy can be stored to meet energy demand during times of no generation or intermittency. 

There are many types of energy storage – mechanical, gravitational, electro-chemical, chemical, electromagnetic, biological or thermal. An example of gravitational storage is a pumped hydro storage system (PHS). PHS systems currently constitute upwards of 96% of global storage capacity. Electro-chemical storage (i.e. battery storage) is the second most common form of energy storage. Mechanical storage systems consist of flywheels, where energy storage is achieved through inertia of mass. The DOE Global Energy Storage Database breaks down the installed capacity of storage by country and technology.  

Projected global energy storage deployments through 2040. Source: BNEF

Energy storage prices have fallen significantly (Lazard, 2019). Bloomberg NEF estimates that global energy storage will grow exponentially in the coming decades to exceed 900 GW by 2040. Like all other emerging technologies in the energy transition, storage requires a robust policy framework to overcome a number of barriers. Fiscal policies, mandates, and research funding can contribute to the acceleration of energy storage deployment. Several U.S. states have passed energy storage mandates with Massachusetts being a great example, targeting 1000 MWh of energy storage by 2025. The Federal Energy Regulatory Commission (FERC) passed Order 841 allowing market access to energy storage.