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Long-Duration Energy Storage

  • Because of the intermittent nature of renewable generation, long-duration storage at low costs is needed to decarbonize the electrical grid.
  • Due to the fact that renewables have a near zero marginal cost of energy production, there is enormous opportunity for energy storage systems, which can offer load shifting, peak shaving, and operational flexibility in addition to reducing the need for new transmission lines.
  • While Li-ion batteries are able to provide 4–10 hours of storage, their supply chain risks and relatively high costs make them unsuitable for large-scale long-duration grid storage.
  • Green hydrogen, molten salt systems, metal-air batteries, flow batteries, mechanical/kinetic energy storage, compressed air storage, and gravity based storage show promise but are still in the early stages of development.
  • Load shifting, shifting the supply of electricity later in the day to meet the demand, and peak shaving, eliminating short-term demand spikes during periods of high demand
  • Reduced need for new transmission lines
  • Operational flexibility for grid generation during inclement weather, without resorting to peaker plants
  • Reduced need to overbuild renewable energy capacity, which would otherwise be necessary for an all-renewable grid
  1. Reliable / Resilient
  2. Dispatchable
  3. Scalable
  4. Low Cost
  1. Thermal: This includes sensible (changing temperature) and latent (constant temperature) heat storage as well as thermo-chemical energy storage. These systems all work by storing excess electrical energy as thermal energy and then reversing the process and converting that thermal energy back to electrical energy when it is needed–or simply providing that heat for industrial or residential use. The efficiency and cost may vary depending on the technology, but most of the systems in this category come with good reliability and degradation. However, some of these thermal storage options have lower dispatchability in that they are unable to accommodate high ramp rates and may have limited scalability. One prominent example of thermal technologies is molten salt thermal storage used in conjunction with concentrated solar systems. Sand batteries that store heat for district heating are also a promising development.
  2. Mechanical: The most common types of mechanical storage are flywheel and compressed air energy storage systems. Although both of these technologies hold great promise, we have yet to see the successful deployment of a large number of mechanical storage systems on the US grid. While flywheel technologies have traditionally operated on the timescale of minutes, frictionless flywheels with a very high moment of inertia hold promise for extending the duration and 4-hour commercial systems have been announced. As of 2018 only 0.6% of total electricity storage in the United States used some form of mechanical energy storage.
  3. Electrochemical: Batteries, including Li-ion and flow batteries fall into this category. New battery chemistries have been steadily pushing down the cost of long duration storage. The biggest advantage of this category is scalability and modularity. The architecture and power electronics make it straightforward to connect battery cells electrically in series and in parallel, providing a great deal of flexibility for system integration. The future of batteries in stationary storage will depend on the technology’s ability to scale and capture a large swath of the market, especially when paired with renewable energy. In fact, battery storage projects have been going up alongside many renewable energy installations in recent years. Metal-air and flow batteries are two examples of new technologies for long-duration storage. Li-Ion is the most prominent example of this category; however, it is used mostly for applications requiring only 4–10 hours of energy storage.
  4. Gravity: This category includes pumped hydro and potential mass storage systems. These technologies typically have high efficiency, but the systems are usually custom built and often require special conditions like water and elevation. In the absence of these geographic features, some even incorporate their own cranes and elevators to provide potential energy storage (at a lower density than electrochemical systems).
  5. Chemical: Non-battery chemical storage systems, such as green hydrogen and green ammonia, make up their own category. These systems have the potential for excellent dispatchability if paired with generation technology, such as fuel cells, linear generators, or turbines. Of course, this generation technology is only carbon neutral when using renewable fuels like green hydrogen and green ammonia. However, the cost of producing these green fuels needs to come down dramatically over the next decade in order for them to be commercially viable.



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Siddhartha Banerjee, PhD

Computational physicist | Propulsion scientist | Data science communicator | Author | Inventor | Introvert | Indian Classical Musician