Barbosa, F., "Ultracapacitor Transit Bus - Urban Electric Mobility into an Opportunity Charging Concept," SAE Technical Paper 2016-36-0176, 2016.
Environmental concerns and limited fossil fuels reserves have fostered an increased interest in alternative propulsion systems. In this scenario, electric traction, with its inherent zero local emissions, high efficiency and improved operational performance (acceleration and hill climbing potential), emerges as a desired option for public transport systems. Transit buses, the prevailing transport system in cities, and, hence, strong contributors to traffic environmental impact on urban areas, can reduce considerably their environment burden with the use of electric traction. This means less local pollutants, specially particulate matter - PM and nitrogen oxides - NOx, currently the “Achilles heel” of diesel engines, as well as CO2 greenhouse emissions - GHG. The so called autonomous electric bus - e-bus, i.e. that one independent of electric grid, fed while running solely by an energy storage device - ESD (batteries, ultracapacitor, flywheel) and charged at bus stops into the so called “opportunity charge” concept emerges as the best option for short range/medium demand corridors in downtown areas, as enables a low vehicle space and weight impairment for ESD assembly, in benefit of passenger capacity and operational performance. The so called ultracapacitors or supercapacitors are energy storage devices based mainly on the highly reversible electrostatic double layer capacitance, formed at the interface between an electrode and an electrolyte solution. Their charge and discharge process, in contrast to battery technologies (based on chemical reactions), occurs upon the movement of ions. With no chemical bonds being made or broken, very fast charge and discharge processes are possible, even at low temperatures, with lifecycle of over 1 million cycles with minimal degradation, albeit at a lower energy density than batteries. These features favor ultracapacitor use in high power burst charge (charge at bus stops and regenerative brake) and discharge (acceleration and uphill climbing) regimes, typical of “stop and go” urban cycles. In this context, ultracapacitor buses appears as a promising technology to be used in short range central bus routes, in which e-bus runs solely with ultracapacitor ESS (without batteries) and can travel along its route with few seconds’ ultrafast charging (up to 500 kw) events at predetermined bus stops or a single, few minutes’ charge each time upon returning to its terminal (depending on the duty cycle and air conditioning use). This work is supposed to present an overview of ultracapacitor basics technology and an evaluation of the applicability of this groundbreaking cutting edge technology into central short range transit bus market, followed by a technical evaluation of some prototypes ultracapacitor bus experiences currently in test in Europe and China.