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August 25, 2020
Renewable energy is the fastest-growing energy source globally. According to the Center for Climate and Energy Solutions, renewable energy production increased 100 percent in the United States from 2000 to 2018, and renewables currently account for 17 percent of U.S. net electricity generation. As renewables have grown, so has interest in energy storage technologies. As Jason Burwen, Vice President of Policy at the Energy Storage Association (ESA), explained during a 2019 EESI briefing, energy storage technologies are critical to renewable energy development because they stabilize the energy grid. When there is higher demand for energy, or when intermittent renewables (such as solar and wind) are not generating power, the energy retained by storage systems can be used to meet demand. The latest U.S. Energy Storage Monitor report from ESA and Wood Mackenzie Power & Renewables suggests that the amount of energy storage capacity deployed in the United States is predicted to rise from 523 MW deployed in 2019 to 1,186 MW deployed in 2020. Further, the market value for energy storage is set to increase from $720 million today to $5.1 billion in 2024.
The International Energy Agency's (IEA) ETP Clean Energy Technology Guide, released on July 2, provides development and deployment plans, key initiatives, assessments on importance for net-zero emissions, and Technology Readiness Level (TRL) ratings (on a scale of 1-11) for over 400 technologies that are integral to reaching a net-zero emissions world. The guide describes 38 energy storage technologies, five of which overlap with energy storage technologies EESI has highlighted because of their capacity to store at least 20 MW, as of 2019. Here, we dive into the current status of those five technologies as described by the IEA Guide, listed from highest to lowest Technology Readiness Level.
TRL
Description of rating
Example technologies*
1
Initial idea: basic principles have been defined
Li-Air electric vehicle batteries (TRL 1-2)
2
Application formulation: concept and application of solution have been formulated
Multivalent ions electric vehicle batteries
3
Concept needs validation: solution needs to be prototyped and applied
Chemical reaction thermochemical heat storage
4
Early prototype: prototype proven in test conditions
Active latent heat storage
Li-S electric vehicle batteries
5
Large prototype: components proven in conditions to be deployed
Solid state + Li-metal electric vehicle batteries
6
Full prototype at scale: prototype proven at scale in conditions to be developed
Building integrated phase change materials
7
Pre-commercial demonstration: prototype working in expected conditions
High-temperature-latent heat storage (TRL 5-7)
8
First of a kind commercial: commercial demonstration, full-scale deployment in final conditions
Compressed air energy storage (CAES)
9
Commercial operation in relevant environment: solution is commercially available, needs evolutionary improvements to stay competitive
Flywheel
Lithium-ion batteries (Li-ion batteries)
Redox flow batteries
10
Integration needed at scale: solution is commercial and competitive but needs further integration efforts
Salt cavern storage
11
Proof of stability reached: predictable growth.
Pumped hydro storage (PHS)
*Bolded technologies are described below. See the IEA Clean Energy Technology Guide for further details on all technologies.
IEA Guide TRL: 11/11
IEA Importance of PHS for net-zero emissions: Moderate
In pumped hydro storage, electrical energy is converted into potential energy (stored energy) when water is pumped from a lower reservoir to a higher reservoir. To transform the stored energy into electricity again, the water falls from the higher reservoir to the lower reservoir to drive a turbine. PHS accounts for 95 percent of U.S. utility-scale energy storage and nearly 96 percent of global storage capacity. The Dominion Bath County Pumped Storage Station in Virginia powers 750,000 homes and is the largest PHS plant in the world.
Still, EIA's guide lists some technological improvements that are currently underway, including re-purposing and retrofitting PHS plants to accommodate different renewables and expanding PHS plants to seawater and underground locations.
IEA Guide TRL: 9/11
IEA Importance of flywheels for net-zero emissions: High
In flywheel energy storage, electric motors power flywheels to spin at high speeds, turning electric power into kinetic rotational energy that can be stored. In the discharging process, the motors go into reverse and the mechanical energy is turned into electrical energy once again, gradually slowing the flywheel's spinning. The Stephentown Spindle in Stephentown, NY, which was created with a $43 million loan from the Department of Energy in 2010, was the first commercial flywheel program in the United States.
Flywheels have rapid response times, long life cycles, low maintenance costs, and high energy efficiencies of about 90-95 percent, but energy outputs are small. The Guide does not list any key initiatives for flywheel technology.
IEA Importance of Li-ion batteries for net-zero emissions: Very High
Li-ion batteries are already widely used for battery storage in the power and transportation sectors around the globe. According to the MIT Energy Initiative, Li-ion batteries represent more than 90 percent of the global and domestic battery storage markets. Li-ion battery prices have dropped by 80 percent over the past five years, leading to further deployment. EESI’s fact sheet states that Li-ion batteries are becoming increasingly common in developing countries for the purpose of rural electrification. As a part of its 25-year plan to become more sustainable, the University of Massachusetts Boston has partnered with Enel X to add a lithium-ion energy storage system to its campus. This project will reportedly reduce the university’s energy spending by more than $1.5 million during the lifetime of the contract.
In the future, developers hope to increase the competitiveness of lithium-ion batteries in energy storage by increasing their flexibility and capacity.
IEA Guide TRL: 8/11
IEA Importance of CAES for net-zero emissions: Moderate
Compressed air energy storage converts thermal and mechanical energy into electrical energy.
Air that has been compressed and stored in underground caverns or above-ground vessels is released in a turbine where it expands and generates electricity. Certain CAES technologies also capture the heat that is released when compressed air expands (compressing air makes it warmer, whereas allowing it to expand makes it cooler). Indeed, CAES technology can either be diabatic, where generated heat is dissipated as waste, or adiabatic, where heat is stored and used for heating the air. The MacIntosh Power Plant in MacIntosh, Alabama, is the only utility-scale CAES plant in the United States. It contains one 110 MW Simple Cycle, Gas-Fired, Compressed Air Energy Storage (CAES) Turbine.
The first adiabatic plant in the world, the Adiabatic Compressed-Air Energy Storage Project for Electricity Supply (ADELE) demonstration plant built by RWE power in Germany, saw its progress stall in 2017 due to “uncertain business conditions,” so there is not yet full proof of concept for this system. There are many technological improvements that still need to be made for CAES technology to be a widely viable option for energy storage.
IEA Importance of Redox flow batteries for net-zero emissions: High
Redox flow batteries, or simply flow batteries, are composed of tanks of electrolytes separated by an ion-selective membrane. An alternative to Li-ion batteries, flow batteries are far less popular and represent less than 5 percent of the battery market.
While redox flow batteries have a long lifespan (up to 30 years) and unlimited energy capacity (as long as electrolytes are added to the system), they have low energy density and are not commercially mature. U.S.-based companies such as Lockheed Martin Corporation and U.S. Vanadium are trying to advance redox flow battery technology.
To learn more about energy storage technologies, read EESI’s Energy Storage Fact Sheet and IEA’s ETP Clean Energy Technology Guide.
Author: Maeve Arthur
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