Sodium Sulfate: Future New Grid Energy-Storage Technology?

Lithium-ion batteries (LIB) have maintained market dominance for the past several years as the primary energy-storage technology. As one data point notes: At the beginning of 2019, the United States had about 870 MW (megawatts) of large-scale battery projects in operation, and more than 90 percent of those projects were LIB systems. Most energy analysts believe LIB will capture most all energy-storage growth in stationery/transportation markets over the next 10 years.

The appeal of LIB storage is its continuous price drop, flexible installation (including modular stacking), fast response, and short construction time. Additional performance benefits include its high-energy density (or the amount of energy per unit volume), long life cycle, and integration with renewable energy sources in grid-level energy-storage systems. In recent years, LIB projects have been successfully deployed with remarkable improvements in performance.

Despite its ubiquitous use, LIB has limitations. LIB faces supply-chain problems with lithium, cobalt, and nickel, because of limited geological reserves, high extraction costs, and human rights violations. As one example, Tesla announced that it will be making its electric vehicle cathodes without cobalt, since it has been mined using child workers and irreversibly damaged the environment of the Democratic Republic of Congo.

LIB cannot deliver full power for more than four hours, and scaling beyond that four-hour limit cannot be done cost-effectively. Grid-scale, long-duration energy-storage projects are becoming more and more mainstream, so what energy-storage technology should be featured if LIB is not the one to choose? Enter the sodium-sulfur battery technology as one option. Leyline is following the developments of this technology from the sidelines as a key option for long-duration energy storage.

How to Think about Long-Duration Storage Systematically

Long-duration energy storage supports the need for high levels of variable renewable electricity by storing surplus energy and releasing it later. The "long-duration" timeframe occurs at the six-hour mark or longer, the outer edge of LIB or flow batteries, but squarely within the capability of sodium-sulfate technology.

The longer-duration energy-storage market is just beginning to take off in the United States, most notably in California. While current LIB can thrive at shorter durations and its value exceeds costs, the cost increases exponentially with duration. Long-duration storage may struggle at shorter durations, but the cost curve is at the point where there are larger jumps in value caused by deferral of capital-intensive baseload plants. And that is where long-duration storage, and by extension, sodium sulfate, may thrive.

Sodium-Sulfur Battery

The global sodium-sulfur battery market size is valued at $78 million in 2020 (the installed base amount), and will reach $289 million by 2025, and slightly over $1 billion by 2030, all based on an annual growth rate of 30 percent every year. Most of the growth is in Asia, because of the presence of regional manufacturers in Japan and South Korea, and the industries demanding energy storage in India, Malaysia, Thailand, and Singapore.

Picture of NGK's Visualization of Sodium-Sulfur Grid Scale Storage Project

A few of the large projects using sodium-sulfur batteries are also in Asia. One was constructed in Mongolia in spring 2021 with batteries made by a Japanese company called NGK Insulators (NGK). NGK will supply a 600kW/3,600kWh NAS battery energy-storage system to Mongolia's western Zavkhan Province.

In 2015, the Chugoku Electric Power Company installed a hybrid battery system as part of a demonstration project in the Oki islands in Japan. This project used a 2-MW/0.7-MWh (megawatt hours) lithium-ion battery in combination with a 4.2-MW/25.2-MWh sodium-sulfur battery to address renewable energy output fluctuations on the island. The hybrid system used the lithium-ion system to address short-term fluctuations in renewable energy output and the sodium-sulfur system to address longer-term changes in output.

The largest energy-storage device in the world is in Abu Dhabi, and it uses sodium sulfur. It is a 108 MW system of 10 separate batteries connected.

There has been one sodium-sulfur demonstration project in the United States with Xcel Energy. The technology was selected, because it has high-storage capacity and the ability to handle numerous charge-recharge cycles from an intermittent wind source. The battery system is the size of two semi-trailers stacked one on top of the other and stores about 7.2 MWh of electricity. A second sodium-sulfur battery system is installed to improve reliability in Presidio, Texas.

In all, NGK batteries have had the greatest success in deployment. They are being used by locations primarily in Japan, the Middle East, and Europe for renewable energy stabilization; transmission/distribution network deficiencies; in microgrids; and for ancillary services.

Increased Deployment and Financing?

Sodium-sulfur batteries can be deployed to support the electric grid or for stand-alone renewable power applications. They have unique benefits compared with LIB: its long duration capability, on the scale of several hours, its high cycle life, fast-charging capability, and easy supply chain availability. Companies have demonstrated sodium-sulfur batteries with impressive results, but it remains to be seen if its deployment will increase in the United States as the cost curve associated with long-duration energy storage improves.

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