There is increasing interest world-wide in the use of so-called “hybrid vehicles” for improved fuel economy, lower emissions and sustainable energy use. Hybrid vehicles are difficult to define but can be broadly described as vehicles that use a combination of technologies for power generation and energy storage. In particular, parallel and series hybrids are widely discussed and consist of an auxiliary power unit (APU) mechanically connected to the driven wheels or with no direct mechanical connection respectively.
Series hybrids offer significant advantages over parallel hybrids, such as mechanical simplicity, design flexibility and modularity allowing easier incorporation of technological advances (Gosden, 1996). Hybrid electric vehicles (HEV) are now under development in many countries (Lovins, 1997).
Energy storage is a critical component of any hybrid vehicle. The options for energy storage include batteries, supercapacitors, flywheels and hydraulic devices. Batteries can provide high energy density (eg Li-ion up to 100Wh/kg, Pb-acid 25 Wh/kg) but only relatively low power density (eg 300 W/kg for commercial Pb-acid). Flywheels are still under development and require some significant technological advances before their widespread use can be envisaged. Supercapacitors are the only technology available today that can provide high power capability, ie over 1kW/kg, with long life at reasonable cost. Supercapacitors also have other features that make them attractive for use in hybrid electric vehicles such as their ability to be used for regenerative braking (energy efficiency), maintenance-free operation, high electrical efficiency and low toxicity with easy disposal.