Is Sodium Sulfur (NaS) Battery a viable Grid Energy Storage Solution?

By David Anthony and Tao Zheng

On September 21st, 2011, sodium-sulfur (NAS) batteries installed at Mitsubishi Materials Corp’s Tsukuba Plant, Japan, caught on fire. It took firefighters more than 8 hours to control the blaze, and two weeks to extinguish the fire. NGK Insulators Ltd., the company that manufactured the energy storage system, said the fire authorities are still investigating the cause of the fire. NGK has suspended production of its NAS cells, and advised customers around the world refrain from using their batteries until it tracks down the cause of the fire and finds a solution. NGK began shipping NAS batteries in 2002, and has installed 305MW capacity in a total of 174 locations across six countries, including US and Europe. The largest NAS installation is a 34MW, 245MWh unit for wind stabilization in Rokkasho, Japan, as shown in Figure 1. From its press release, NGK expects to incur an extraordinary loss of around 60 billion Yen, approximately $750 million, for the fiscal year ending March 31st, 2012, based on the sum total of the cost of investigation, upgrades and other safety measures, as well as loss from valuation of assets.

Figure 1. NGK’s 34MW NaS Battery at Rokkasho, Japan.

A NaS battery is a molten-metal battery with molten sulfur as the positive electrode and molten sodium as the negative. The electrodes are separated by a solid ceramic, sodium alumina, served as the electrolyte. During the discharge, sodium ions converted from sodium in a negative electrode pass through solid electrolyte then reach to sulfur in positive electrode. The electrons finally flow to outside circuits, and the electric power is generated by such current flow. With the progress of the discharge, sodium polysulfide is formed in positive electrode. During the charge, the electric power supplied from outside form sodium in negative electrode and sulfur in positive electrode by following the reverse process of the discharge. Because of this, the energy is stored in the battery.
NaS battery has potential for two major applications in grid energy storage, including energy arbitrage and intermittence stabilization. Energy arbitrage is to use NaS battery to reduce power station fluctuation by load leveling and peak shaving. The battery is charged when electricity is abundant, and discharged into the grid when electricity is more valuable. As shown in Figure 2, NaS battery can be used to stabilize the intermittency from wind and solar renewable energy generation. The variable output of wind and solar generation causes voltage and frequency fluctuations on power network. NaS battery can smooth the output from these resources to meet electricity demand pattern.

Figure 2. Wind and Solar Energy Intermittency Stabilization by NaS Battery. (Courtesy of NGK Insulators, Ltd.)

NaS battery has advantages of high energy density, high efficiency of charge/discharge (89%) and long cycle life, and is fabricated from inexpensive materials. However, the primary disadvantage of NaS battery is its high operating temperature of 300 to 350 °C, and the requirement for thermal management to maintain the ceramic separator and cell seal integrity, which otherwise crack at lower temperature. In addition, the highly corrosive nature of the sodium polysulfides, presents another challenge for ceramic insulator protection. The cracked insulator can cause fire when sodium in contact with moisture.
The fire mentioned earlier is not the first fire happened to NGK’s NAS batteries. The two previous fires occurred in 2010 and 2005, respectively. NaS battery has inherent safety issues, due to its high operating temperature and highly active materials used. Pure sodium spontaneously burns or explodes in contact with water. Special sealing technologies must be used to protect NaS cells from moisture. The fire also becomes a major challenge for firefighters, since water cannot be used to extinguish the battery fire. Therefore, although NaS battery has many advantages, we do not recommend it as a grid energy storage solution. Beside the safety concerns, we also consider per-cycle cost when evaluating large-scale energy storage technologies, as illustrated by data from Electricity Storage Association (ESA) in Figure 3.

Figure 3. Per-cycle Cost Comparison of Grid Energy Storage Technologies. (Courtesy of Electricity Storage Association)

Per-cycle cost can be used to evaluate the cost of storing energy in a frequent charge/discharge application, such as load leveling. The per-cycle chart above shows the capital cost of energy output, taking into account the impact of cycle life and efficiency. NaS battery performs better than lithium iron and lead acid batteries, but not as good as some flow batteries and compressed air energy storage (CAES) technologies. Pumped hydro technology is even 10 times cheaper in per-cycle cost than NaS battery. Conventional pumped hydro uses two vertically-separated water reservoirs. During off peak hours water is pumped from the lower reservoir to the upper reservoir. When required, the water flow is reversed to generate electricity. Pumped hydro technology has advantages of high capacity, low cost, and safety as grid energy storage solution. The example of pumped hydro technology has been demonstrated by Gravity Power, a California Company developing a solution to provide clean, fast-responding peaking power and renewable energy dispatchability in one system. Full disclosure: David Anthony, a co-author of this blog, is the lead investor in Gravity Power. Furthermore there is no venture capitalist who is not self-serving.

Regardless, the winner for grid energy storage technology competition will come from the technology with high energy capacity, low cost, and public safety assurance.

David Anthony is the Managing Director of 21Ventures, LLC, a VC management firm that has provided seed, growth, and bridge capital to over 40 technology ventures across the globe, mainly in the cleantech arena.. David received his MBA from The Tuck School of Business at Dartmouth College in 1989 and a BA in economics from George Washington University in 1982.david blogs at His email is

Tao Zheng is a material scientist in advanced materials and cleantech industry. He held 20+ patents and patent applications, and published many peer-reviewed papers in scientific journals. Tao Zheng received his B.S. degree in polymer materials sciences from Tsinghua University in China, and a Ph.D. degree in chemical engineering from University of Cincinnati. He obtained his MBA degree with distinction in finance and strategy from New York University, Stern School of Business, where he was designated as “Stern Scholar” and received “Harold Price Entrepreneurship Award”. Tao can be reached at zhengta(at)gmail(dot)com

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