While lithium-ion batteries have been able to address the need for multi-hour energy storage needs, there is still a need to develop technologies that can affordably store energy for days to weeks.

The need: The Department of Energy estimates that 4-hour lithium-ion batteries can support grids with up to 50% to 60% of variable renewable energy, but that long-duration energy storage (LDES) with 10+ hours of storage are needed much above that.

There are various families of storage technologies being developed:

• Electrochemical: Stores energy by charging a battery with electricity and then releasing it later through chemical reactions.
• Hydrogen: Uses electricity to split water into hydrogen, which can be stored and later burned or used in a fuel cell to generate power.
• Mechanical: Stores energy by moving or lifting physical objects—like spinning flywheels or pumping water uphill—and later retrieves it by reversing the motion.
• Thermal: Stores excess electricity as heat in materials like molten salt or bricks and then converts it back into electricity when needed.
• Thermo-mechanical: Converts electricity into heat and pressure (like compressed air or supercritical carbon dioxide), then uses that to drive a turbine and make electricity later.
• Thermo-chemical: Uses electricity to drive a chemical reaction that stores energy, which can be reversed later to release heat or electricity.

Funding is needed to support their development. To date, Eos Energy, Hydrostor, Form Energy and Plug Power have led the charge in capital raised.

+Bonus article: The Emerging Long-Duration Storage Technologies

+Bonus article: Form Energy and the 100-hr Battery