Most vehicle systems that are in development today rely on battery technology because of its relatively high energy density, its relative maturity, and its familiarity to designers. However, the unique characteristics of the ultracapacitor allow additional dimensions in design to be explored, and open up opportunities for the development of new powertrain and subsystem architectures which can improve performance, efficiency, and cleanliness. Consequently, ultracapacitors are becoming more popular as their advantages are recognised by the automotive industry.
Hybrid Electric Vehicles
The most promising short-term solution is the Hybrid Electric Vehicle (HEV). HEV technology combines the best characteristics of fuel-driven engines, electric motor drives, and energy storage components. An HEV is designed with a combustion engine that functions as the primary power source and an electric power-storage system as the secondary power source. Through this basic design structure, HEVs offer low maintenance, clean operation, and high fuel economy. The secondary power source allows designers to size the combustion engine for cruising-power requirements. It also handles peak power demands for acceleration, where the excellent low-speed torque performance of electric motors is useful. The electrically-powered motor also allows the combustion engine to be stopped when the car is stationary in traffic, and then restarted as it moves forward: this increases fuel efficiency and reducing pollution. In addition, the secondary source is used for capturing regenerative braking energy and employing that energy for further acceleration or for the basic energy needs of supplementary electrical systems. Compared to conventional diesel engines, the reduction of fuel consumption is estimated at greater than 50%; reduction in particulate emissions is greater than 90%; and reduction of nitrogen oxide emissions is 50% when HEV technology is employed.
Ultracapacitors in HEVs
Ultracapacitors significantly improvepower management in hybrid electricvehicles, with a number of significant advantages over batteries. Ultracapacitors offer high-pulse power capability, fast transient response, and high efficiency during discharge and recharging (due to their virtually zero interior resistance), plus full-charge cycling in excess of 500,000 cycles.They will last the lifetime of a car with no maintenance, and offer better performance at extreme temperatures than batteries. For example, a 15V Bootscap ultracapacitor module from Maxwell will support the high-pulse power loading imposed by the high occurrence of engine warm restarts under idle-stop-start control. The module can store approximately 45kJ to meet the idle-start-stop requirement; it can also capture braking energy. This allows a 5kW braking charge to be absorbed for several seconds and then reused as several seconds of engine starting power. Ultracapacitors have already found application in the propulsion system of conventional gasoline and diesel hybrid as well as fuel-cell hybrid vehicles. BMW's hybrid X5 and X3 concept car as well as Volkswagen's fuel-cell-powered Bora are example prototype vehicles, while Honda's IMA and Toyota's ES are production vehicles incorporating ultracapacitors. Originally announced at the Frankfurt motor show in September 2005, BMW's X3 is an interesting example. It uses ultracapacitors with a quoted specific power density of around 15kW/kg, substantially more than the 1.3kW/kg available from a nickel/metal hydride battery. Given the same weight, voltage and power supply, the ultracapacitor offers an efficiency of 98%, compared to 84% from a NiMH battery, operates well at low temperature and is not subject to internal thermal runaway (fire) like a NiMH battery.
Distributed power systems
As well as power trains, the modern car has high demands for electrical power, particularly for the short peaks in demand which are generated by systems such as powered braking and steering. These base-level loads have dramatically increased from 1 or 2kW in the 1980s to over 6kW today, and dynamic loads have gone from 2 to over 18kW in high-end modern automobiles. If a short-term demand causes a voltage drop on the boardnet (the power distribution supporting the logic boards), the control electronics may stop functioning due to low voltage cutoff. With 50 to 100 control modules competing with 50 to 100 electrical actuators and motors in a modern car, this is a major reliability and safety issue. Electronic fuel controls may stop, causing the engine to stall, and lights and sound systems may fluctuate. In the worst case, the car will require towing to a service centre to be reset. Board-net stabilisation is one of the best andeasiest-to-implement applications forultracapacitors. The short-term powerdemands that cause voltage dippingcan be buffered with a 14V powermodule designed with enough energystorage to "ride through" or supplement peak power demands. Thisreplaces the need for a second battery,takes less weight and space, is not amaintenance item, lasts the life of thecar, and performs reliably at -40°C.The cost in high volume is about thesame as a second battery and associated cabling, and the life-cycle cost tothe consumer is lower. Ultracapacitorscan also be used to support higherdemand intermittent power applications, such as electrical power steeringand braking. These applications usually demand 1 to 3kW for a second or two, then a smaller demand for several more seconds. To meet safetyrequirements, the ultracapacitor module has sufficient energy to performthis operation 5 to 10 times withoutrecharging.
