Supercapacitors for Electric Mobility

Electric Vehicles
World wide there is growing concern for Environmental pollution caused due to Gasoline Fuel Vehicles, there are concerted efforts worldwide to promote use of Electric Vehicles. India like other developed nations intends to go 100% Electric by the year 2030. Electric vehicles curb the use of hydrocarbons and ultimately decrease greenhouse gases (GHG). In addition to being profitable in terms of the environment and the economy, the growth in the use of electric vehicles promotes the emergence of leading-edge technologies like driver-less vehicles. A high-performance on-board electric energy storage at affordable price is must for development of electric vehicles. The most common energy storage/sources in electric vehicles are fuel cells, electro-chemical batteries and Supercapacitors. It has been observed that installing only one type of energy storage/source is often insufficient, so use a hybrid energy storage is recommended, consisting of more than one type, to complement the each single type drawbacks.

Advantage of Supercapacitor assisted Power-pack
Fuel Cells, Batteries and Supercapacitors are distinguished by their energy and power density. Fuel Cells are capable of providing more Energy density as compare to Batteries, but very low on power density. Batteries are known to have greater energy capacity than supercapacitors, But Supercapacitors have Power density more than 20 times to that of Batteries. The requirement of Electric Vehicle is to have best of Energy density and Power density. Supercapacitors can fill up gap for Fuel Cells and Electro-Chemical Batteries, and can be used in tandem to combine the benefits of both technologies.

Apart from compensating for Power Densities, use of Supercapacitor based Power-pack provides solutions to solve some key problems encountered in electric vehicles as:

- regenerative braking, while the main source of energy is unidirectional in case of fuel cell or availability of fast charge is heavily limited like in lead-acid battery.

- providing large power pulse, while the main source is designed to average power like energy storage based on molten-salt battery,

- significant deterioration of energy storage performance in harsh exploitation conditions like in Lithium Chemistry Batteries, which are sensitive to the operating conditions. Typically lithium battery provides the energy density of around 80-110Wh/kg and power density of 500W/kg in pulse, which may satisfy the dynamic requirements of an electric vehicle, but decent performance of Lithium batteries relates solely to temperatures close to nominal 25 deg C. Low temperatures and high currents (above 1 deg C-rate) cause a drastic decrease in capacity and increases internal resistance which in turn causes reduction in range of the vehicle, deterioration of the efficiency and reduces power. High temperatures and currents also significantly reduce the battery life time.

Advantage of using Supercapacitor with Lithium Battery
Unlike batteries, Supercapacitors can undergo millions of cycles without significantly impacting their performance or lifespan. Combination of Lithium Batteries and Supercapacitors improves the efficiency and reliability of the source. This hybrid source can provide desired maximal power independent of battery ageing effect, in various temperature conditions. Also energy recovered during regenerative braking can be harvested in same Supercapacitors. Regenerative braking takes most of the load off mechanical brakes, reducing brake maintenance and replacement expenses, which makes system more efficient. Presence of Supercapacitor also reduces the maximum battery current and number of executed cycles, slowing down battery ageing process and extending usable life of Lithium battery pack.

Guidelines for possible Topology
There are many possible topologies for complementing Battery and Supercapacitor. The basic and simplest configuration would be to connected in parallel with each other and the load. Power distributions between them is determined only by their internal resistance. This is known as Passive hybrid, and is suitable for use with Lead Acid Chemistry, but is amost ineffective in Lithium Chemistry for source of propulsion.

It is strongly recomended that in order to optimize complementing of Supercapacitors and Lithium batteries power electronic converter is required to manage power flow between them and main DC bus of drive converters. This is known as Active Hybrid, Supercapacitor hould be connected to the battery through bidirectional DC-DC converter which allows proper division of power load between two storage as well as control of energy flow between storages, in order to maintain the proper voltage on Supercapacitors, and also cover average power demand from main source (battery) and at the same time to cover all power fluctuation related to deceleration and acceleration from Supercapacitors. The Supercapacitor source should be recharged during regenerative braking or from batteries at periods of low power demand.


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