Sodium-sulfur (NaS) redox flow battery
Energy storage for the future
Renewable energy generation is expected to play a growing role in the Dutch economy. It is therefore desirable to develop novel electricity storage systems that are flexible, cheap, and capable of delivering a lot of power and storing a lot of energy. With those goals in mind, the potential for a sodium-sulfur (NaS) redox flow battery has been investigated. The NaS redox flow concept combines the advantages of two other concepts. One of those is the NaS battery: a mature, highly efficient battery technology that uses very cheap chemicals.
Originally developed in Japan, several commercial applications of this concept are currently in use, providing up to 50 MW/350 MWh. The battery relies on liquid sodium and sulfur to achieve this power level and typically operates at a temperature of 350ºC. The high temperature used in the design combined with the low internal resistance results in an 87% DC-to-DC efficiency and a long battery life (more than 5 years).
Another very important technological development is the redox flow cell. This utilizes two separate streams of aqueous electrochemically reactive species, which are pumped into a reaction chamber. A reaction takes place in the chamber, generating power and energy. With a configuration of this kind, power and energy can be controlled independently. In larger systems, this leads to a lower cost per energy equivalent. The principle is currently applied in the all-vanadium redox flow cell.
Project
This project aimed to investigate the feasibility of an NaS redox flow battery. Such a battery would combine the best of two technologies: the high energy density of NaS chemistry and efficient membranes, and the lower relative cost of a redox flow system. An NaS redox flow system would have several advantages over both the classical redox flow setup and the NaS battery. However, a flow cell of this kind has never been developed, probably because the technical challenges were considered too daunting.
The NaS battery uses molten, metallic sodium and molten elemental sulfur. Upon discharge, a sodium atom (Na) acts as the reductor, releasing an electron (e). The sodium cation (Na+) then migrates through a special type of ceramic membrane that is highly efficient. This results in a very high efficiency (87% DC-to-DC) and a long cell life. The whole cell must be kept at a temperature of approximately 350ºC, in order to achieve high ionic conductivity in the membrane. Whereas organic membranes can support current densities up to about 0.2 A/cm2, the ceramic membrane operates at 1 A/cm2.
Project coordinator
> KEMA, the Netherlands
Project details
> SenterNovem EOS-NEO
> Duration: September 2008 – July 2009