Long-Duration Energy Storage
The technologies and players driving long-duration energy storage growth
The International Energy Agency (IEA) announced that it expects grid-scale storage to be the fastest-growing energy technology. Due to a variety of market forces and tech innovation, it could reach 30-40 GW in 2025 in global capacity.
What is long-duration energy storage?
Long-duration energy storage (LDES) systems store energy to balance energy supply and demand over extended periods, ranging from hours to weeks, for variable renewable sources like wind and solar. They store excess energy generated during periods of high production and low demand and release it during times of low production or high demand.
Types of LDES
There are four main types of LDES systems, mechanical, thermal, electrochemical, and chemical.
Mechanical
Mechanical LDES systems include pumped hydro storage, gravity-based energy storage, compressed air energy storage, liquid air energy storage, and liquid CO2 energy storage.
Pumped hydro storage is the most used mechanical LDES system. It accounts for 95% of global energy storage capacity. It works by using excess electricity to power turbines to pump water from a low reservoir to an upper reservoir. When electricity is needed, the water flows from the upper reservoir into the lower reservoir through turbines connected to generators to produce electricity. Gravity-based energy storage works similarly but uses heavy solid objects like steel and concrete instead of water.
Compressed air energy storage stores energy as compressed air in pressure-regulated structures. When energy is needed, the compressed air is released to drive a turbine and generate electricity. Liquid air energy storage works similarly but uses electricity to cool and liquify air and stores it in cryogenic storage tanks at low pressure. When energy is needed, the liquid air is heated and expands to drive a turbine and generate electricity.
Liquid CO2 energy storage stores CO2 at high pressure and ambient temperature in a closed loop. When energy is needed, the liquid CO2 is released and expands to drive a turbine and generate electricity.
Challenges include operational inefficiencies, high construction costs, and geographic requirements, like mountains or elevated areas and water for pumped hydro.
Stage: Near to early commercialization stages.
Thermal
Thermal LDES systems use excess electricity to power a heating device that heats up thermal storage materials like molten salt, rocks, and phase-change materials. These materials absorb large amounts of heat during phase changes. When electricity is needed, the stored heat is transferred to a heat exchanger that drives a turbine or other device to generate electricity. Systems can also directly release heat for uses like industrial and district heating.
Challenges include heat loss over long periods of time, less efficient conversion from heat to electricity relative to other LDES, and large material and infrastructure requirements.
Stage: Near to early commercialization stage.
Electrochemical
Electrochemical LDES systems are made up mostly of batteries, including flow, iron-air, lithium-ion, sodium-based, and solid-state batteries. Flow batteries use liquid electrolytes stored in external tanks that store energy and release it during oxidation-reduction reactions. Iron-air batteries use iron as the anode and oxygen from the air as the cathode. During discharge, the iron reacts with oxygen to form iron oxide, which is rust, and releases electricity. When the battery is charged, the iron oxide is converted back into iron and stores energy.
Although lithium-ion batteries dominate short-duration energy storage, as mentioned in my previous post, they are not economically optimal to use for long-duration stationary storage due to their $/kWh cost. LDES are complements to short-duration energy storage systems.
Solid-state batteries use solid electrolytes made of materials like ceramics, glass, or polymers.
Challenges include high initial and maintenance costs, low energy density, high space requirements, limited cycle life, temperature sensitivity, and supply chain and materials sourcing.
Stage: R&D to early commercialization stages.
Chemical
Chemical LDES systems use chemicals like hydrogen, methanol, biomass liquid fuels, redox flow battery electrolytes, and synthetic fuels.
Electrolysis is the process of using electricity to split water (H2O) into hydrogen (H2) and oxygen (O2). Excess electricity from renewable energy sources powers the process, and the hydrogen is stored as a compressed gas in tanks, as a liquid at very low temperatures, or as a solid. When electricity is needed, fuel cells, as mentioned in my green hydrogen post, combine oxygen (from the air) and hydrogen to produce electricity, with water as the only byproduct.
A challenge with fuel cells is that their round-trip efficiency, which is the amount of energy used to create hydrogen that is converted back into electricity, typically ranges from 40-60%.
Stage: Near commercialization for certain use cases.
Key Companies
The following seven companies are tackling the main challenges and innovating in the LDES space.
Antora Energy
Antora makes solid carbon modular thermal batteries that capture and store excess renewable energy as heat at temperatures up to 2,400°C for days and deliver heat for industrial processes. Antora’s next product will deliver both heat and electricity using thermophotovoltaic cells. In February, Antora raised a $150M Series B round, led by Decarbonization Partners, a partnership between BlackRock and Temasek.
Form Energy
Form Energy makes iron-air batteries that store energy for up to 100 hours. Form Energy has a partnership with Georgia Power utility for a 15MW project and with Xcel Energy for a 10MW project to store solar power and replace energy from a retiring coal plant. In October, Form Energy raised a $405M Series F round.
Malta
Malta converts electricity from the grid to thermal energy using a heat pump. The heat is stored in molten salt, and the cold is stored in a chilled liquid antifreeze coolant. When energy is needed, the temperature difference between the heat and cold drives a heat engine and converts the stored thermal energy back into electricity.
Rondo Energy
Rondo Energy makes heat batteries that use renewable electricity to heat iron wires, which warm bricks up to 1,500°C. The bricks provide a 24/7 supply of hot air or steam to industrial facilities to replace fossil-fuel-powered boilers, furnaces, and kilns.
Since June 2023, Rondo Energy has raised over $140M and developed 4 partnerships to deploy over 3 GWhs of heat battery across 5 different industries, food & beverage, cement, fuel, chemicals, and textiles.
Energy Vault
Energy Vault designs 300-400 ft tall energy storage systems where heavy blocks are raised to upper floors and stored there. When energy is needed, the blocks are lowered, which converts the gravitational potential energy into electricity through a turbine. Energy Vault is partnering with Enel Green Power to build an 18MW large scale gravity storage facility in Texas.
Fourth Power
FourthPower makes modular thermal batteries that store renewable energy as heat and generate electricity on demand. A ceramic pump pumps molten tin carrying heat from the grid to large graphite blocks. When energy is needed, thermophotovoltaic cells convert the thermal energy back into electricity.
RedoxBlox
Redoxblox uses electricity from the grid to heat its modular metal oxide containers to 1,000–1,500°C. At this temperature, an endothermic reaction occurs, during which oxygen separates from the metal oxide. Heat is then absorbed by the chemical bonds in the metal oxide. When energy is needed, air is blown through the metal oxide, and an exothermic reaction occurs, during which oxygen binds with the metal oxide and heat is released. The thermal energy can be discharged as baseload grid power or industrial heat.
Future Growth
The LDES space faces several market challenges. First, the LDES manufacturers are mostly startups and new public companies without large scale production capacity. Second, markets currently favor shorter duration energy storage, especially lithium-ion batteries. Third, LDES systems are only needed if renewable energy sources account for a large share of the energy mix. Fourth, regulatory issues including inconsistent regulation, permitting delays, and lack of standardized rules slow the deployment of LDES projects. Policy incentives like tax credits, grants, or guaranteed purchase agreements could lower costs, promote innovation, reduce risk for investors, and integrate LDES systems into the grid.
There are four main drivers for the demand of LDES systems. First, the global rise in renewable energy sources like wind and solar. Second, Chinese battery manufacturers brought the price down for batteries for EVs and personal devices (by 97% since 1991) and with the slowing of EV sales in some regions, they will focus more on grid-scale batteries. Third, the immense power needs of AI companies have driven tech giants to invest in renewable energy. Fourth, the innovations of the LDES companies, including the ones mentioned above.
The market for LDES is projected to reach $1.3T and 1.5-2.5 TW capacity by 2040, which means around 10% of global electricity will be stored in LDES. This will be a critical suite of technologies for renewable energy and decarbonizing industry, and a huge opportunity for LDES manufacturers and their customers and investors.