High Temperature energy storage systems are widely explored due to the increasing need of storage systems in the electric utility industry. With the use of such systems, load leveling can be achieved. The energy storage system of interest here is based on Sodium - β alumina solid electrolyte (BASE) with liquid sodium as anode and molten ZnCl2 as cathode. BASE is an excellent sodium ion conductor with a very high conductivity at 300oC. The discharge reaction: 2Na + ZnCl2 à 2NaCl + Zn OCV (350oC) = 2.4V Unlike sodium sulfur batteries, the volumetric power density of Na - ZnCl2 battery is higher due to the planar cell construction. If a cell fails, it develops a low resistance so that the other cells continue to deliver power in a normal manner. This paper will compare the Na - ZnCl2 batteries: planar design, construction and testing, with other high temperature storage batteries.
The partial free energy Ḡi of a chemical species is altered by pressure according to the relation dḠi=V̄idP, where Vi is the partial molar volume. In a glass sheet under a bending stress, there is a gradient of hydrostatic pressure (as well as shear) through the sheet, tending to drive the mobile Na ions from regions of compression to regions of tension, and equilibrium will be reached when the space-charge buildup balances the driving force of the pressure gradient. The initial current prior to space-charge limitation was measured in a soda-lime—silica glass sheet, and the partial volume of the Na ion in this glass calculated to be 2.2 Å3. This figure is surprisingly high, about ⅗ of the cation volume. A naive calculation attributing the volume change to electrostrictive effects gives reasonable agreement with this result, but further experiments are needed to test this explanation.
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