In the automotive domain the supercapacitor applications are classified in three categories:
An overview of non-exhaustive application examples is presented in the following paragraphs.
Combustion engine starting
Enhanced starting of automobile engines is another attractive application for double-layer capacitors. Today the energy required to crank an internal combustion engine, is stored in batteries, either Pb or Ni. Because of their high internal resistance, which limits the initial peak current, they have to be oversized. The fast battery discharging and the cold environmental temperature affect heavily its properties.
A starting demonstration has been done with a 10 V module consisting of a 4 series connection of supercapacitors with a diesel engine assembled on a test bench. The principal advantage of the supercapacitor compared to the battery in this application, is that it can support several tens of thousands of charge/discharge cycles with very high currents. The batteries are limited to only a few hundreds of cycles with currents much weaker. In addition, in the case of the starting of a thermal engine, the battery is extremely solicited, which obliges the manufacturers to install expensive high power batteries. In this application, a battery of low power is sufficient because it is only used at the time of the first supercapacitors charge (with weak current). When the engine is launched, the alternator of the vehicle charges the supercapacitors. It thus results a reduction of the size of the battery and a greater longevity of operation of this one. The automotive starters generally consist of a DC machine with series excitation and of a coil making it possible to put in contact the gearwheel of the rotor with the crown of the driving wheel.
The characteristics of the engine used are the experiment is as follows: Peugeot 604 turbo diesel engine of 80 HP, resistive torque to the starting of 32 daN.m, minimal speed of launching of 130 rpm. In this configuration, the 675 F module can start the engine without problem more than 9 times. Figure 1 shows the evolution of the current and the voltage of the supercapacitors pack for each starting.
The voltage drop after each starting varies between 0.2 and 0.75 V. The supercapacitors are an interesting solution for the applications requesting high power peaks during short times.
The case of the starter of Internal Combustion Engine (ICE) is an other perfect example. Figure 2 shows the experimental results obtained when a standard ICE vehicle is started by using a supercapacitors module.
In the beginning the supercapacitors module is charged at a voltage of 11V. When the first ICE starts, the current request is in order of 220A, after, the current of the vehicle alternator is used to charge the supercapacitors module until 13V. The ICE vehicle is started three times.
42 V electrical power net
The development of innovative automotive systems is determined by the demand for comfort improvement, cut in fuel consumption, reduction of environmental pollution and increase in efficiency. A result is the substitution of mechanical by electrical systems such as power steering, electromagnetic valve control, electric water pump, electromechanical braking, electric air conditioning, catalyst preheating etc. as well as the introduction of new drive train functions like start-stop and recuperative braking. These functions show a power demand in the range of 8 to 20 kW [i]. Future 42 V electrical subsystems will be greatly enhanced by using double-layer capacitors. Their long life and high cycle life are ideal for the variable power loading required of new subsystems. Localized load leveling of pulse loads will reduce the need to run high-current wires for long distances in the vehicle.
Integrated starter generators
The short high-current requirement of engine starting, especially in cold weather, is an excellent application for supercapacitors. Start-stop motoring is very familiar in urban traffic. Today the kinetic energy during braking is converted to heat and expelled into the environment. In view of increasing energy costs as well as the need to reduce environmental pollution, it is imperative to store the braking energy for re-use during acceleration. This method of energy saving can also be usefully applied for vehicles with internal combustion engines, especially for the improved alternators used as braking generators, so-called integrated starter generators. Such crank shaft starter generators will need a power of up to 10 kW in near future. Conventional lead-acid batteries cannot furnish the energy in the seconds range because of the slow chemical processes. The supercapacitors are predestined to store the energy generated within a very short time and release the energy with high efficiency thanks to the very low internal resistance. The integrated starter generators will allow consumption reductions, mainly in urban traffic where start-stop function is very important, of up to 25%. In near future, the integrated starter generators, placed in the drive train between the engine and the gearbox, will furnish enough energy to power so called small hybrids. Such vehicles will recuperate the braking energy during a period of 30 s and re-use it during acceleration of 10 s. Due to the integrated starter generators the faster engine starting will be done in about 300 ms.
Mild and strong hybrid vehicles
These environmentally friendly drives are based on the combination of an internal combustion engine with an electric power train. The double-layer capacitors absorb the kinetic energy from braking and release it later to accelerate the vehicle. In addition, they cover the energy requirements of auxiliary electrical power equipment. The duration and magnitude of typical acceleration and braking events determines the size of the double-layer capacitor bank. The double-layer capacitors can be also a device to improve the lifetime of a storage system as they present a high number of charge/discharge cycles, withstand wide temperature ranges, require little maintenance, and be placed more optimally for vehicle ergonomics.
Fuel cell vehicles
In the future the combustion engine mechanical energy obtained from the fuel combustion could be replaced electrical engine supplied by electricity produced by a fuel cell. The promise of fuel cell technology has had a recent resurgence due to new advancements not in fuel cells, but in the double-layer capacitors. Indeed, high power energy storage is required in all types of fuel cell applications and double-layer capacitors are ideally suited to provide it. These improvements open up opportunities for the development of new power train and subsystem architectures utilizing both double-layer capacitors and fuel cells which can improve performance, efficiency, and cleanliness in electric and hybrid vehicle technology [ii],[iii],[iv].
In collaboration with the Paul Scherrer Institute, the Volkswagen group and other partners, a fuel cell vehicle has been built up with BOOSTCAPs [v].The fuel cell, which acts as a primary power source, is sized for the continuous load requirement. The supercapacitor bank, which acts as the secondary power source, is sized for peak load leveling events such as fuel starting, acceleration and braking. These short duration events are experienced many thousands of times throughout the life of the vehicle and require relatively little energy but substantial power.
California several In hundred of hybrid buses are already on duty. In 2002 General Electric has produced a hybrid diesel-electric bus able to deliver 110kW for 10 s with a dc link voltage of 240 V. In 2003 ISE has started the production of a series of hybrid gasoline-electric buses (720 V, 200 kW peak). 27 buses in 2004, 100 buses in 2005 and >150 buses in 2006 have been produced. In Europe Vossloh Kiepe has developed a solution of gasoline-electric bus (720 V, 140 kW) which will be introduced in the Milan city bus network in 2008. Scania has presented its 198 kW ethanol hybrid bus in Helsinki in 2007. The energy storage has been made with 4 supercapacitors modules of 125 V. The available energy is greater than 400 Wh. This technology offers at least 25% fuel saving and cutting the CO2 emissions by up to 90%.
The buses are an interesting application for the supercapacitor because of the elevated number of stop the vehicle has to do in commercial operation.
[i] Schöttle R, Threin G. Electrical power supply systems: Present and future, VDI Berichte 2000; Nr. 1547.
[ii] Fuglevand W. Avista Laboratories. Fuel cell power system, method of distributing power, and method of operating a fuel cell power system. WO patent/2002/095851.
[iii] Raiser S. General Motors Corporation. Hybrid fuel cell system with battery capacitor energy storage system. WOpatent/2006/065364.
[iv] Pearson M. Ballard Power Systems. Power supply and ultracapacitor based battery simulator. US patent 2004/228055.
[v] Kötz R, Bärtschi M, BuchiF, Gallay R, Dietrich Ph. HY.POWER A Fuel Cell Car Boosted with Supercapacitors. Proc. 12th International Seminar On Double Layer Capacitors, 2002, Deerfield Beach, USA.