Operation of power systems - Wind Power Integration: Connection and System Operational Aspects - page 211

materials involved, low efficiency (70-80 per cent) and the limited life and energy
density, secondary batteries based on other designs are being sought for utility-
scale applications. The sodium-sulphur battery consists of molten sulphur at the
positive electrode and molten sodium at the negative electrode separated by a solid
beta-alumina ceramic electrolyte - the electrolyte permits only sodium ions to
traverse, combining with the sulphur to form sodium polysulphide. Operating at
approximately 300 C, they have a high energy density, approaching 650 MJ/m 3 ,
and a cycle efficiency of 85-90 per cent including heat losses, making them
suitable for large-scale storage applications. Over one hundred projects have been
installed worldwide, with most of these in Japan. The latter include 2 8 MW, 60
MWh installations, used for daily peak shaving, with a nominal discharge time of
7.5 hours at rated power (Nourai et al. , 2005).
Alternatively, the sodium-nickel-chloride battery employs nickel/sodium
chloride at the positive electrode and liquid sodium acting as the negative electrode,
with a liquid NaAlCl 4 electrolyte contained within beta alumina. In comparison
with sodium-sulphur, safety characteristics are improved, along with an ability
to withstand limited overcharge/discharge. However, both the energy density
(550 MJ/m 3 ) and power density (300 kW/m 3 ) are reduced. Although currently
aimed at the automotive market, proposals within the 100 kWh-10 MWh range
have been made for load levelling applications. Similarly, lithium ion and lithium
polymer batteries are expected to be expandable from their current prevalence in
the portable electronics market to larger-scale utility needs, which may also include
widespread electric vehicle take-up. The cathode can be formed from a range of
lithiated metal oxides, for example LiCoO 2 and LiMO 2 , while the anode is graphite
carbon, and the electrolyte is formed from lithium salts, such as LiPF6, dissolved in
organic carbonates (Schaber et al. , 2004). The operating temperature is 60 C, with
an energy density of 720 MJ/m 3 and an efficiency over 85 per cent. The main
obstacle to further development is the high cost of special packaging and internal
overcharge protection circuits.
5.5.1.3 Flow batteries
Flow batteries store and release electrical energy through a reversible electro-
chemical reaction between two liquid electrolytes. The liquids are separated by an
ion-exchange membrane, allowing the electrolytes to flow into and out from the
cell through separate manifolds and to be transformed electrochemically within
the cell. In standby mode, the batteries have a response time of the order of
milliseconds to seconds, making them suitable for frequency and voltage support.
Battery capacity depends on the volume of solution, providing economies of scale
with larger installations. This contrasts with secondary batteries where both energy
and power density are affected by both the size and shape of the electrodes - larger
batteries are, therefore, not directly scalable.
There are three types of flow battery that are approaching commercialisation:
vanadium redox, polysulphide bromide and zinc bromide. The vanadium redox
design utilises vanadium compounds for both electrolytes, which eliminates the
possibility of cross contamination and simplifies recycling. The polymer membrane