Wind energy storage technologies - Wind Power Generation and Wind Turbine Design - page 677

large-scale projects. Although just to note, a 40 MW NiCd storage facility was
constructed in Alaska; comprising of 13,760 cells at a cost of $35M [2]. The cold
temperatures experienced were the primary driving force behind the use NiCd
as a storage medium. NiCd will probably remain more expensive than LA bat-
teries, but they do provide better power delivery. However, due to the toxicity of
cadmium, standards and regulations for NiCd batteries will continue to rise.

4.4.3 NaS battery
NaS batteries have three times the energy density of LA, a longer life span, and
lower maintenance. These batteries are made up of a cylindrical electrochemi-
cal cell that contains a molten-sodium negative electrode and a molten-sulphur
positive electrode. The electrolyte used is solid

β -alumina. During discharging,

sodium ions pass through the -alumina electrolyte where they react at the posi-

tive electrode with the sulphur to form sodium polysulphide (see Fig. 8). During
charging, the reaction is reversed so that the sodium polysulphide decomposes,
and the sodium ions are converted to sodium at the positive electrode. In order to
keep the sodium and sulphur molten in the battery, and to obtain adequate con-
ductivity in the electrolyte, they are housed in a thermally insulated enclosure that
must keep it above 270°C, usually at 320-340°C.
A typical NaS module is 50 kW at 360 kWh or 50 kW at 430 kWh. The aver-
age round-trip energy effi ciency of a NaS battery is 86% [2] to 89% [3]. The
cycle life is much better than for LA or NiCd batteries. At 100% DoD, the NaS
batteries can last approximately 2500 cycles. As with other batteries, this
increases as the DoD decreases; at 90% DoD the unit can cycle 4500 times and
at 20% DoD 40,000 times [3].

β

Figure 8: Sodium-sulphur battery [ 3 ].