Long Duration Energy Storage (“LDES”) is a group of technologies that are more economical storage mediums than lithium-ion over longer durations. Duration is a measure of how many hours a storage technology can discharge energy at full capacity. Energy is generally discharged at full capacity because discharging below this level lowers efficiency (increasing overall cost) and may lead to variations in the level of output, which is detrimental to the stability of an electricity grid.
The predominant Short Duration Energy Storage technology is lithium-ion. However, lithium-ion energy storage is economically limited to durations of up to 4–8 hours. This means that once the sun sets (ie solar PV power generation), li-ion energy storage can only provide energy up to right after midnight. Providing power through the night with the current state of li-ion technology (i.e. for use in data centers, cold storage, 24/7 manufacturing, or transport infrastructure) exponentially increases the cost of energy per kWh.
LDES is a catch-all term for technologies that are more economically efficient at discharging energy over durations greater than 8 hours. LDES complements lithium-ion energy storage, which is projected to dominate the stationary storage market for at least this decade. Intro to utility-scale energy storage in previous article.
The decrease in Levelized Cost of Energy of renewables has made storage more attractive as a complementary product in an electricity grid. At the same time the intermittency of wind and solar generation has increased the need for the grid-stabilizing feature of storage.
The most bankable LDES: Pumped Storage Hydropower (“PSH”)
PSH is a system that utilizes two large reservoirs at differing elevations. During charging, a water pump forces water from the low elevation reservoir to the high elevation reservoir. During discharge, gravity forces water through a hydroelectric turbine to generate electricity.
This technology has been around for over a century. It is well understood by banks and investors, has low technology risk, and is found in both emerging and developed countries. Globally, there is currently 8,429 GWh of PSH capacity. 40% of this capacity is in the US, 15% is located in India, 11% is found in China, and 9% in Norway.[i]
PSH is currently the lowest-cost and most financeable LDES technology. Additionally, projects can be built with a much larger energy capacity than most LDES. However, PSH is constrained by geographic siting, the need for construction expertise, and environmental-social issues. While developed countries such as the US and Norway have hit the limit on the PSH capacity they can install in their countries, there are still numerous sites that can be developed in emerging countries. As PSH requires the construction of a hydroelectric dam, project proponents require specialized water infrastructure experience at the corporate/institutional level to successfully implement a project. Lastly, PSH requires a long consultation period, stakeholder engagement and permitting due to the impact a project’s development has on the surrounding land and population.
PSH’s constraints (together with the competition between the EVs and storage sectors for the lithium-ion supply chain) have increased demand for Novel LDES technologies in the past couple years. Commissioned Novel LDES projects are mostly located in developed countries (the oldest grid-scale project in the BNEF database was commissioned in Japan in 2005 [iii]), but technology providers are increasingly looking at expansion into emerging markets.
The three sets of Novel LDES that are in the demo project and commercial project phases today are: Mechanical, Electro-chemical and Thermal.
A. Mechanical LDES
Similar to PSH, mechanical LDES systems store potential or kinetic energy in systems.
1. Compressed Air “CAES” (Duration of 4–24hrs): Air is pumped into an underground cavern. During discharge, air is released back up into a facility where it is heated. The expanding gas turns a turbine that generates electricity.
2. Liquid Air “LAES” (Duration of 8–24hrs): Air is cooled until liquified, then stored in a tank. During discharge, the liquid is gasified, through exposure to ambient air or heat. The expansion of the gas turns a turbine that generates electricity.
3. Gravity (Duration of 5–24hrs): Objects are raised to a height by cranes or elevators and locked into place. During discharge, the objects are released causing the crane to turn a motor that generates electricity.
4. Flywheel (Duration of 10–24hrs): A low-friction rotating wheel is sped up as it is charged by electricity. During discharge, the mass gives off electricity, which also slows the rotation down.
B. Electrochemical LDES
Electrochemical batteries store electricity in chemical solutions. From external tanks, the chemicals are pushed through a stack of cells, where the charge and discharge processes take place facilitated by a selective membrane.
5. Zinc Bromine (Duration of 10hrs): A hybrid flow battery where a zinc bromide solution is circulated through two independent loops. Charging and discharging is done via the pumping of the solution into a central stack separated by a membrane. The process either gives off or absorbs electrons.
6. Vanadium Redox “VRFB” (Duration of 8–12hrs): A flow battery where a liquid vanadium metal electrolyte with different charges is stored in two separate tanks. Charging and discharging is done via pumping vanadium into a central stack separated by a membrane. This process either gives off or absorbs electrons.
7. Zinc Air (Duration of 10hrs): A flow battery powered by oxidizing zinc. During charging, electrons are run through zinc to remove the oxygen. During discharge, the zinc is exposed to air, which oxidizes the zinc and gives off electrons.
8. Iron Flow (Duration of 12–24hrs): Similar to VRFB, iron electrolyte with different charges is stored in two separate tanks. Charging and discharging is done via pumping electrolyte with different charges into a central stack separated by a membrane.
C. Thermal LDES
Thermal systems store electricity or heat. During charging, heat is transferred to a fluid, which is used to power a heat engine and discharge the electricity back to the system. Thermal energy storage can be classified into sensible heat (increasing the temperature of a solid or liquid) and latent heat (changing the phase of matter). These technologies use different mediums to store heat.
9. Liquid Metal (Duration of 6–20hrs): Molten salt or liquid metal is heated and kept in an insulated environment. During discharge, a heat engine converts the heat into electricity. See intro to Concentrated Solar Plants integrated with latent heat storage in previous post
10. Solid State (Duration of 6–20hrs): Rocks, concrete or minerals are heated and kept insulated. During discharge, water is pumped onto the rocks to produce steam which turns turbines that generate electricity.
Each of these technologies has its own investment drivers:
· Project returns (Installed cost, modularity, revenue streams, and cycle/project life)
· Technical attributes impacting opex (Efficiency and serviceable adjacent industries)
· Addressable market size (Use cases, supply chain and geographic suitability)
· Stage of maturity in certain markets (Presence of demo or commercial projects)
However, it is clear that a future energy grid will require a mosaic of storage technologies to get closer to a decarbonized electricity grid. LDES can reduce the overall cost of electricity to end-users by negating curtailment of solar and wind, and providing stability to a grid. Complementing Li-ion’s short duration and Green Hydrogen’s seasonal duration, LDES will continue to increase in importance with higher penetration of variable generation (wind & solar).
[i] International Energy Agency. “How rapidly will the global electricity storage market grow by 2026?” (December 1, 2021): https://www.iea.org/articles/how-rapidly-will-the-global-electricity-storage-market-grow-by-2026
[ii] Environmental Management Bureau of the Philippines. “Public Hearing on 120MW Aya Pumped Storage Project.” (February 4, 2021): http://eia.emb.gov.ph/wp-content/uploads/2021/02/FGHPC-Aya-PSP-PH-Documentation.pdf
[iii] BloombergNEF energy storage database
Disclaimer: This post reflects my personal views and not those of the International Finance Corporation, World Bank, or any other member of the World Bank Group