Although demonstrated in a number of pilot projects (for example, a 3 MW battery storage plant was installed in Berlin for frequency control in emergencies and a 35 MW battery system is used to smooth the output of a wind farm in Japan) the large-scale use of battery storage remains expensive and the key area where the use of secondary batteries is likely to have impact is in electric vehicles. The popular lead-acid cell, although reasonable in price, has a low energy density (15Wh/kg). Nickel-cadmium cells are better (40Wh/kg) but more
Figure 1.14 Hydrogen-oxygen fuel cell
expensive. Still under intensive development and demonstration is the sodium- sulphur battery (200Wh/kg), which has a solid electrolyte and liquid electrodes and operates at a temperature of 300 °C. Modern electric vehicles use Lithium ion batteries (100-200 Wh/kg) but these remain expensive. Other combinations of materials are under active development in attempts to increase output and storage per unit weight and cost.