Energy Storage News

NGK Insulators Sodium Sulfur Batteries for Large Scale Grid Energy Storage

Sodium sulfur (NAS) batteries are the only commericially mature, large scale energy storage technology which can be sited at arbitrary locations. Pumped hydro storage is also a mature form of energy storage, but such storage facilities can only be sited where there a suitable geographical location with high and low storage reservoirs.

Only one company, NGK Insulators manufactures NAS batteries. Here is some information about the batteries taken from their web site:

NAS battery consists of sulfur at positive electrode, sodium at negative electrode as active materials, and Beta alumina of sodium ion conductive ceramic which separates both electrodes. This hermetically sealed battery is operated under the condition that the active materials at both electrodes are liquid and its electrolyte is solid.

If a load is connected to terminals, electric power is discharged through the load. 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. The electric power is generated by such current flow.

With the progress of the discharge, sodium polysulfide is formed in positive electrode; on the contrary, sodium in negative electrode will decrease by connsumption. 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.

Typical System:
Coniderably more detail about NGK's batteries can be found in the paper Overview of the Sodium-Sulfur(NAS) Battery for the IEEE Stationary Battery Committee published in 1995 by Akihiro Bito of NGK Insulators.Some extracts of this paper are given below:

15-year service life, high cycle life
(2500 cycles at 100% DOD – 4500 at 90% – 6500 at 65%)

Insensitive to ambient temperature
No emissions, noise or vibrations
Remote/automated operation, minimal maintenance

Development History:

US & Europe (late '60s):
– Ford and BBC (now ABB) pioneered EV applications

Japan & Europe ('84-'98):
– TEPCO/Hitachi/NGK advanced stationary applications
– NGK/ABB established worldwide licensing agreement

NGK: Supplier resource with extensive ceramics expertise
– NAS technological key: High strength/purity, dimensionally stable ceramic solid electrolyte

Installation History:

June 1995 – First 500 kW NAS system at TEPCO’s Kawasaki substation NAS batteries from NGK’s pilot manufacturing plant Followed by 57 pre-commercial demonstration projects over 7 years

July 2004 – Largest NAS load-leveling (LL) application 9.6 MW/58 MWh for Hitachi’s auto systems factory

NAS "PS-G50" Module (320 T5 Cells)
Energy Density: 151kWh/m3 (=151Wh/liter)
Rated Capacity & Power: 360kWh @ 50kW (60kW max)

(Note: PS stands for Peak Shaving while PQ stand for Power Quality)

NAS "PQ-50" Module for PQ and Combined PQ & PS Applications:

Pulse Factor, % Rated PS Power
500% x 30sec (250kW)
450% x5min (225kW)
400% x15min (200kW)
330% x30min (165kW)
260% x1hr (130kW)
190% x2hr (95kW)
150% x3hr (75kW)
130% x4hr (65kW)
110% x6hr (55kW)
100% x7.2hr (50kW)

The paper above references a Hitachi auto systems factory 9.6MW/58MWh storage facility as the largest NAS battery installation in the world, but since the paper was written two even larger installations have occurred, a 12MW/73MWh installation at the Honda Automobile R&D center, and a 34MW/204MWh installation at the Rokkasho Wind Farm in Japan.

Furthermore within the last year or so NGK insulators has announce two large orders of NAS batteries, 150MW to France's largest electric power firm EDF, and 300MW worth to the United Arab Emirates.

The sale price for the UAE order was reported as 65 billion Yen. We can use this number to estimate the cost of NAS battery storage. NGK normally quotes 6 hours worth of storage for 4500 cycles as typical performance. However, if you read the details given in the above paper it appears that at six hours of discharge the battery can deliver power 10% above the nominal power rating. Therefore the total kWh that will be delivered over the 4500 cycles is:

Total kWh = 330,000kW×6hours×4500 = 8.91 billion kWh

65 billion yen is US $0.72 billion, so that the cost of electricity coming out of the battery is:

cost 0.72/8.91 = US $0.08/kWh + interest

Of course you also have to count the cost of the electricity going into the battery. In AC mode the round trip storage efficiency is 75% so that the base electricity cost must be multiplied by 4/3. So, for example, if wind generated electricity costs US $0.06kWh then the cost of electricity that is delivered from an NAS battery is:

Cost = (4/3)×0.06 + 0.08 + interest = US $0.16 + interest

This is certainly not cheap electicity by the standards we have become used to in the OECD countries, but it is not clear that the cost is high enough to cause industrial civilzation to crumble.

Of course if you do not store all of your energy in the battery then the cost is the weighted average of this cost with the cost of that part of the generation that does not require storage.

I do not explicitly incude an assumption about the cost of interest since my belief is that current system of economic thinking in which it is assumed that if you do not invest resources in valuable infrastructure you can simply invest them in toy factories and you will become richer at a rate of 6 to 10% a year forever will be demonstrated to be complete and utter nonsense.

One good thing about NAS batteries is that they are made of comparatively common materials: Sodium (23,000,000ppb in crustal rocks), Sulfur (928,000ppb in sea water), and aluminium (82,000,000 in crustal rocks), so that a substantial ramp up in production volumes will not drive prices through the roof.

Of course one may wonder why, if NAS batteries are a good product, no one but NGK Insulators is manufacturing them. One reason is that electrochemical energy storage at utility scale is a very difficult market to enter which requires great patience and a long term view of economic possibilities. Batteries are expensive and their full value is not realized until after years of use. Unless considerable operating experience exists proving the usefulness an durability of such batteries, utilities will be very reluctant to invest in them. NGK appears to be the first and only company to have gotten over the hump with respect to the commecialization of grid scale storage batteries. How large a market they will find in the long run remains to be seen.

November 8, 2009

Energy Storage News

rogerkb [at] energystoragenews [dot] com