Ultracapacitors, the New Thinking in the Automotive World

By Adrian Schneuwly, Bobby Maher, Jürgen Auer
Maxwell Technologies

Introduction

Due to the increasing power demand in future vehicles for comfort improvement, as well as ongoing pressures for more environmentally friendly means of transportation, automotive manufacturers are developing alternatives to existing fossil fuel-driven vehicles.

Perhaps the most promising near-term alternative to fuel-cell vehicles, which will not be ready for volume production for at least a decade, is hybrid electric vehicle (HEV) technology. While progress has been made in control, engine and motor design, they have not been successful with regard to the electric power storage systems. This is due primarily to the fact that batteries are used to provide the power peaks in most of the currently developed hybrid electric vehicles. But the deficiencies of battery storage systems are multiple and they create many design challenges for automotive engineers.

Batteries have a bad low temperature performance, a very limited lifetime under extreme conditions which results in repeated replacement throughout the life of the vehicle, and they are not designed to satisfy the most important requirements of hybrid electric vehicles power source; to provide bursts of power in the seconds range for events such as acceleration, braking and cold starting.

Recently, a very promising technology has been introduced with the potential to improve energy storage in automotive applications: double-layer capacitors. These devices, also known as ultracapacitors or supercapacitors, represent a new generation of electrochemical components with very high capacitance for energy storage.

Today, ultracapacitors are a viable component for production-intent designs. Ultracapacitors are available from major production firms in the United States, Asia, and Europe. They are available in a variety of sizes, in a variety of configurations. Ultracapacitor prices are within the cost targets of many automotive systems, and are approaching $0.01 per farad in automotive production volumes by 2004 in millions.

When appropriately designed with a systems approach, they offer excellent performance, wide temperature range, long life, and flexible management. When used in combination with other energy storage solutions (e.g. lead-acid batteries, fuel engines, fuel cells), the complete system can meet performance and cost goals unachievable with a single energy storage device.

Ultracapacitors from Maxwell Technologies are available under the trade name of BOOSTCAP®.

Ultracapacitor Technology

Energy Storage Technology Comparison

In terms of energy density and access time to the stored energy, double-layer capacitors are placed between large aluminum electrolytic capacitors and smaller rechargeable batteries [1]. The Ragone diagram, presented in Figure 1, shows the domain occupied by double-layer capacitors in the power and energy densities space. It can be seen that the BOOSTCAPs from Maxwell Technologies cover the highest part in power and energy density of double-layer capacitors.

Figure 1: Ragone diagram, comparison of different energy storage and conversion devices

Peak power applications, as they occur in the transportation domain, need passive components to store the electrical energy that are as small as possible in volume and weight. The choice of the storage device type depends particularly on the speed of the storage process, or in other words on the power required by the application.
While the slower storage processes may be performed with batteries and the faster ones with capacitors, the ideal storage devices to supply bursts of power in the seconds range are double-layer capacitors.

Double-layer Technology

Ultracapacitors consist of two activated carbon electrodes, which are immersed into an electrolyte. The two electrodes are separated by a membrane that allows the mobility of the charged ions but prevents electronic contact. The organic electrolyte supplies and conducts the ions from an electrode to the other if an electrical charge is applied to the electrodes. In the charged state, anions and cations are located close to the electrodes so that they balance the excess charge in the activated carbon. Thus across the boundary between carbon and electrolyte two charged layers of opposed polarity are formed. This layer, discovered in 1879 by Helmholtz, is called an electrochemical double-layer.

Ultracapacitors rely on an electrostatic effect, which is purely physical and therefore highly reversible. Charge and discharge occurs upon movement of ions within the electrolyte. This energy storage process is in contrast to all battery technologies, as they are based on chemical reactions. Consequently, there are some fundamental property differences between double-layer capacitor and battery technologies; namely long shelf life, an extended useful life and a high cycle life. These advantages result in an almost maintenance-free storage device.

Cell Construction

Double-layer capacitors are assembled by winding or stacking in parallel electrodes, current collectors and separator foils. For the stacking processes separate electrode and collector foils are assembled in the device. In this case, it is important to have a very good mechanical contact between the electrode and the current collector. By applying a controlled high pressure on the stack, low internal resistance can be obtained. The disadvantage of the stacking technology is a low productivity and therefore higher production costs. Stacked devices allow for a prismatic design.

Manufacturers assembling electrodes deposited directly on a current collector are usually using winding processes. Advantages of the winding technology are a very reliable process, high productivity and therefore low costs. Maxwell Technologies has many years experience in the winding technology and today produces the fastest and most reliable winding machines worldwide.

Figure 2: Maxwell Technologies winding machine and assembly line

Thanks to this technology, high capacitance BOOSTCAPs® are today produced in a cylindrical shape with variable sizes and dimensions. Due to a precise control of all winding parameters such as the foil length and the foil tension a very low dispersion of the devices performances is achieved.

BOOSTCAP® Features

Available Products

Today, Maxwell Technologies manufactures BOOSTCAPs® based on both, stacking and winding technology. PC products are stacked devices using carbon cloth electrodes and BCAP devices are based on winding technology and use carbon powder electrodes. Ultracapacitors in 4, 10, 100, 450, 900, 1800 and 2600 Farad sizes are available, which can be combined into power modules for higher voltage.

The top of the range when it comes to power and energy density is represented by the BCAP0010 capacitor rated at 2600 F. With an internal resistance of typically 0.25 mOhm at 100 Hz this capacitor is suitable for power applications of any kind [2].

Figure 3: PC 10 rated at 10 F and an integrated system built up with BCAP0010

Future Performance Trends

The double-layer capacitor performance development goals are a longer lifetime, an increase of the rated voltage, an improvement of the operating temperature range and an increase of the energy and power densities. The rated voltage will be increased up to 3V within the next years. An increase in capacitance by 50% as well as a reduction of the ESR by a factor of two is also possible. The energy and power density for BOOSTCAP double-layer capacitors will therefore be increased significantly. To easily access automotive applications a temperature range from –35 to 105°C will be advantageous.

What is expected from the research and development is the increase of the electrolyte decomposition voltage and ionic conductance, the increase of the electrode accessible surface, chemical and mechanical stability as well as electronic conductance and the separator electronic insulation level and ionic conductance. A primary focus is the development of new electrolytes based on the combination of novel organic solvents and improved conduction salts, permitting not only a higher rated voltage and a higher conductivity but also a larger operating temperature range.

Automotive Applications

BOOSTCAP ultracapacitors are ideal wherever high bursts of power are needed. Existing and new applications include automotive engineering, rail traction, telecommunication, uninterruptible power supplies, renewable energy resources, industrial electronics and medical engineering.

Numerous automotive firms are well into the production design cycle for ultracapacitor-based power trains and subsystems, recognizing the advantages and availability of the ultracapacitor to meet their business and technical requirements. Ultracapacitors are available, cost effective, and perform well in automotive systems, and today are considered a peer to other options for production energy storage system requirements.

BOOSTCAPs Used in EV, HEV or HEFC Vehicles

In the last few years, new propulsion hybrid drive trains have been intensively studied. An interesting concept is a fully electric hybrid drive train that consists of a primary constant power source such as a fuel cell or a battery and a secondary peak power source, e.g. double-layer capacitors [3]. The primary power source handles continuous load requirements, such as cruising and basic electric needs. The secondary power source is sized for short-duration load leveling, absorbing the kinetic energy from braking and releasing it later to accelerate the vehicle, resulting in energy savings of up to 25 % [4] and increased mileage of the vehicle. Because these short-duration events are experienced many thousands of times throughout the life of a vehicle, they are very well suited for the long life of ultracapacitors. Their cycle lives are much longer than those of batteries, so it may never be necessary to replace the energy storage medium. Therefore the life-of-system costs are reduced, and adverse environmental effects are diminished.

In collaboration with VW and other partners, a hybrid electric fuel cell (HEFC) vehicle was built up with an ultracapacitor energy storage device [5, 6]. The BOOSTCAPÒ bank, shown in Figure 4, is capable of providing a constant power of 50 kW during 15 seconds of discharge from full to half rated voltage. This is equivalent to an energy content of 210 Wh at 50 kW.

Figure 4: The fully assembled BOOSTCAP system in its storage boxes

14/42 V Electrical Subsystem and Integrated Starter Generators

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 systems by electrical systems such as electric power steering, electromagnetic valve control, electric water pump, electromechanical braking, electric air conditioning, electric door opening, catalyst preheating etc. as well as the introduction of new drive train functions like start-stop and recuperative braking. The storage of braking energy 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 [7]. Conventional lead-acid batteries cannot furnish the energy in the seconds range because of the slow chemical processes. Double-layer capacitors work quite differently; they are designed to store the energy generated within a very short time and release the energy with high efficiency, even in cold weather. Future 42V electrical subsystems will be able to furnish the power demands in the range of 8 to 20 kW. Thanks to their long life and high cycle life BOOSTCAPs are ideal for the variable power loading required of new subsystems.

Regenerative Braking

In vehicles with internal combustion engines, up to 25% of the total energy used is uselessly converted to heat during braking. This effect is even more critical in urban traffic. Therefore an obvious solution is to introduce an energy storage system that allows storage of the braking energy which can then be applied to acceleration. Such systems allow an important reduction of the fuel consumption in urban traffic where stop-and-go is very common. Regenerative braking is thus absolutely imperative to extend the range of EVs. However this method of energy saving can also be usefully applied to vehicles with internal combustion engines.

Buses are being closely watched as pioneering vehicles of environmentally friendly transportation. Until fuel cells go into volume production, combustion engine-electric drives represent the most interesting drive systems to reduce the emission levels of buses. Typically it is possible to combine a diesel engine with an electrical power train. Distinctive features of this diesel-electric drive are low fuel consumption, advantages for low-floor chassis, more flexibility for the bodywork and quiet running during starting and part load operation. Here BOOSTCAPs have proven to be an ideal energy storage medium for regenerative braking. Conventional lead-acid batteries cannot store the braking energy in the seconds range because of the slow chemical processes underlying their operation. Ultracapacitors work quite differently; even energy that is generated within fractions of a second can be stored and released with high efficiency.

Reliable Starting With BOOSTCAPs

Ultracapacitors cannot replace the car battery, but they do extend its application range significantly. They ensure reliable starting when this must be done frequently or in case of low temperatures, where they improve the vehicle’s cold starting properties by increasing the starter torque and stabilizing the automotive power system voltage. Even if the battery output is low, the peak power needed for starting can be supplied by double-layer capacitors connected in parallel to the battery. Therefore a smaller battery can be used or a faster starting can be applied over a shorter time. The latter effect reduces environmental pollution during the starting process.

Transportation Applications

On a larger scale, double-layer capacitors are well suited to many transportation applications. The endless cycles of acceleration, followed by braking, of mass transit train, subway, and metro systems are ideal for ultracapacitor technology.
In conclusion, ultracapacitors could play a large part in revolutionizing the entire transportation industry, an industry which needs power technologies that respond to changing consumer demands for environmentally sensitive, high performance and low-cost products.

Several projects are actually running in the field of transportation applications. For example, tram supply without catenary [11] and voltage drop compensation for weak distribution network [9]. In industrial electronics, double-layer capacitors can be used in uninterruptible power supplies, elevators, pallet trucks etc. BOOSTCAPsÒ can be combined into large modules with integrated balancing that span outages in all power categories. In power electronics they are particularly suitable for backup in operation independent of the line voltage.

Realistic Cost Estimates

During the next years, the costs of ultracapacitors will decrease significantly due to the following reasons:

• First, an additional cost reduction can be expected as production quantities increase. High production volumes will permit purchase of the capacitor materials at much higher volumes and therefore lower prices. The electrode is the key material in a double-layer capacitor and the price strongly depends on the quantity ordered.
• Second, an important cost reduction is obtained through the automation of the whole production process. Here the winding technology used for the production of double-layer capacitors offers a big potential for cost reduction.
• Third, the nominal voltage of ultracapacitors, currently rated at 2.5 V, will be increased to 3 V within the next five years. In this way, the costs of a module built up from several devices will be reduced by approximately 25%.

Thanks to these improvements, the future cost per energy content of BOOSTCAPÒs will come down to $0.01 per farad. In addition, module costs will decrease due to higher production volumes and new low-cost voltage sharing technologies. Ultracapacitors will then be used in many applications that are currently still powered by accumulators or batteries.

Conclusion

Double-layer capacitors are ideal wherever high bursts of power are needed. They are an optimum storage medium for absorbing and releasing large amounts of energy within the seconds range. These inherent properties make BOOSTCAPÒs an ideal solution for automotive applications, where they can save energy from braking and release it for acceleration, thus reducing fuel consumption and environmental pollution while increasing efficiency. In addition they can contribute to the improvement of car comfort thanks to their ability to furnish the high power demands needed for the substitution of mechanical systems by electrical ones.

References

1. A. Burke and M. Miller, “Characteristics of advanced carbon-based ultracapacitors”, The 10th international seminar on double-layer capacitors, Deerfield Beach 2000
2. V. Hermann, A. Schneuwly, R. Gallay, “ High performance double-layer capacitor for power electronic applications ”, Proceeding PCIM2001, Nürnberg, 2001
3. E. Faggioli, P. Rena, V. Danel, X. Andrieu, R. Mallant, H. Kahlen, “Supercapacitors for the energy management of electric vehicles”, J. of power sources, 84, 261, 1999
4. Ph. Desprez et al., “Supercondensateurs: un tampon de puissance pour sources d’énergie”, Colloque Piles à combustible et Interfaces pour les transports, Belfort 2000
5. P. Dietrich et al., “Supercapacitors for peak-power application with fuel cell system”, Proceedings of the 2nd BOOSTCAP meeting, Fribourg, Switzerland, March 29, 2001
6. R. Kötz et al., “Supercapacitor for peak-power demand in full-cell-driven cars”, ECS Proceedings, PV 2001-21, The Electrochemical Society, Inc., Pennington, NJ
7. R. Schöttle, G. Threin, “Electrical power supply systems: Present and future”, VDI Berichte, Nr. 1547, 2000
8. G. Pereira et al., “Transport urbain sans catenaire et nouvelles techniques de stockage”, Colloque Piles à combustible et Interfaces pour les transports, Belfort 2000
9. A. Rufer, "Key developments for supercapacitive energy storage: power electronic converters, systems and control", 2nd BOOSTCAP meeting, Fribourg, 2001