Supercapacitors as Sidekicks
Owners and operators of all types of vehicles – from small vehicles such as cars and light trucks to large diesel-engine commercial vehicles and locomotives – have an interest in technologies that can help save running costs and reduce environmental impact.
Owners and operators of all types of vehicles – from small vehicles such as cars and light trucks to large diesel-engine commercial vehicles and locomotives – have an interest in technologies that can help save running costs and reduce environmental impact.
Supercapacitors can be used to improve energy management in a number of vehicle electrical systems, such as starting and stop-start for conventional combustion engines, as well as energy storage and power boosting in hybrid and full-electric drivetrains. They can be used effectively alongside a lead-acid or lithium-ion battery, and could even replace a significant portion of lithium-ion battery capacity in hybrids and EVs of the future.
Key advantages of supercapacitors, compared to batteries, include high power density, fast charging, superior low-temperature performance, high duty cycle, and longer service life. They can also be dispatched safely in an uncharged state, whereas batteries can be subject to strict regulations regarding shipping, such as the International Air Transport Association (IATA) or International Maritime Organization (IMO) regulations on transporting lithium batteries. Such regulations are based on United Nations manuals for transporting dangerous goods. In addition, supercapacitor cell voltage is independent of the device chemistry, which gives designers more freedom to determine the cell voltage according to the needs of the application. On the other hand, higher self discharge and lower energy density means supercapacitors are likely to be used in combination with batteries, rather than as a replacement.
Supercapacitor Structure and Properties
Supercapacitors are otherwise referred to as double-layer capacitors. The internal construction comprises two carbon electrodes on either side of a cellulose separator impregnated with an electrolyte. When a charging voltage is applied, ions in the electrolyte migrate towards the electrodes of opposing polarity, resulting in the formation of two separate charged layers as shown in figure 1. This action is similar to that seen in a battery, but is the result of an electrostatic – rather than chemical – effect. As such, the reaction is reversible, which allows the supercapacitor to have a very long cycle life. In addition, the cell voltage is independent of the chemistry used. Although energy density is lower than that of a battery, at less than 15Wh/Kg, supercapacitors offer very high power density of around 4,000 W/Kg.
Figure 1. Supercapacitor operating principle
Supercapacitors can be produced with capacitance values of thousands of Farads. The value of capacitance is governed by the equation:
C= ε_0 KA/d
Where A is the surface are of carbon electrode, and d is the thickness of the inner Helmholtz layer closest to the electrode, as shown in figure 2.
Figure 2. Attraction of ions to supercapacitor electrodes
Modules and Voltage Balancing
To provide the energy needed by the target application, supercapacitors are grouped into modules as shown in figure 3. The module incorporates voltage balancing as well as safety features such as a pressure relief valve and circuitry to discharge the capacitors.
Figure 3. Internal construction of supercapacitor module
A supercapacitor module typically comprises multiple low-voltage capacitors connected in series, creating a stack. When system voltage is applied at the terminals, the voltage distribution across the cells is initially a function of the individual capacitances. After the stack has been held at voltage for a period of time, the voltage distribution becomes determined by individual cell leakage current. Cells with higher leakage currents have lower voltage, leaving others exposed to higher voltage leading to shorter cell life and premature module failure.
In fact, voltage and temperature are the primary factors determining cell lifetime. The capacitor’s end of life can be determined according to capacitance loss, which may be a proportion such as 20%, or an increase in Equivalent Series Resistance (ESR) ESR by a margin such as 100%. The rate of capacitance loss is known to slow over time. On the other hand, operating conditions such as discharge rate and depth of discharge have no effect on the time to end of life. This contrasts with the lifetime of a battery, which can be significantly shortened by rapid or deep discharging.
Voltage balancing can be used to maintain consistent voltages across individual cells in a stack. This can be done with a passive circuit, using resistors to minimize the effects of capacitor leakage currents. Alternatively, active balancing using a charge pump circuit to bleed off excess charge can provide a more energy-efficient alternative.
KEMET S301 series supercapacitors are weld-able axial screw-termination cells that are ideal for use in high-power modules for transportation applications as well as Uninterruptible Power Supplies (UPS), power correction and backup systems. The cells have a rated voltage of 2.7V, and are available with capacitance values up to 3000F and capable of supplying 2200A peak current (one-second duration). In addition the KEMET Supercapacitor Development Kit helps engineers build modules containing up to six S301 60mm-diameter cells, as shown in figure 4. The kit comprises balancing cards containing two-stage active balancing circuitry, in addition to bus bars, screws, washers and wires to aid assembly.
Figure 4. Supercapacitors in a six-cell module with two-stage active balancing
Supercapacitor Applications
By providing high energy-delivery capability, supercapacitor modules can be used to assist batteries in engine-start applications. These can range from passenger cars with start-stop technology up to very high power applications such as starting systems for diesel locomotives.
Start-stop operation is known to place excessive demands on conventional lead-acid batteries typically used for starting passenger vehicles. Battery voltage can fall by 50% or more during cranking, and frequent restarting during town driving can result in deep discharge and so dramatically shorten the battery’s service life.
The supercapacitor, on the other hand, has very high cycle life and fast recharging capability, and so can assist the lead-acid battery to crank the engine during stop-start operation when driving. Supercapacitors are also highly effective for storing energy from a recovery system such as regenerative braking. This not only helps improve battery lifetime and general vehicle usability in start-stop applications, but can also be used to improve economy and electric driving range of hybrid cars and EVs.
The largest diesel engines, as used in equipment such as mining trucks and locomotives, can be units from 1000 hp to over 3000 hp, which place heavy demands on a conventional starter battery. In a locomotive, the battery is first used to power up hydraulic pumps that prepare the engine for starting, and in cold weather a pre-heat phase must be completed to allow smooth start-up without damaging engine components.
The pre-start procedures can take up to 60 seconds, with the result that the battery is usually significantly discharged even before engine cranking begins. In normal use the battery also operates other on-board equipment such as compressors and the brake system, even if the engine is not running. When the time comes to restart the engine the battery may not be able to supply the engine cranking current, which can be 2000 Amps or more. Oversizing the battery for reliable starting to avoid breakdowns adds weight and cost. Alternatively, leaving the engine running when the locomotive is idle wastes fuel and also generates emissions that may be unacceptable in some areas, such as passenger terminals.
The large lead-acid or lead-iron batteries used in locomotive applications also suffer inherent disadvantages due to their size and construction. Batteries of sufficient Amp-hour rating to supply the required peak current tend to have high ESR, which can reduce the starting-mode voltage by as much as 50% compared to the nominal value. These types of batteries are also typically unsealed, vented types that have high maintenance requirements.
A supercapacitor can be used to assist the battery to ensure more reliable starting. If connected in parallel with the battery, the supercapacitor is pre-charged before use and both sources deliver energy when the starting circuit is activated.
In a system designed to deliver peak current of 2000A to start a 3000 hp engine, a supercapacitor power module has been shown to deliver over 1600A, thereby reducing demand on the battery to around 400A. This has several advantages. A smaller, lighter and lower-cost battery can be specified, and engine start can be assured even if the battery is not fully charged. In addition, deep discharging of the battery is avoided, which helps prolong service life. A further advantage is that power delivery is not affected by cold or hot weather, unlike the lead-acid or lead-iron battery.
Boosting Main-Battery Capability
Supercapacitors are used to supply high short-term peak power demands in various other applications, such as industrial fork-lift trucks. In a full-electric truck the peak power demand when raising or lowering a load can be as much as ten times the average power needed to move the vehicle. If battery power alone is used the battery needs to be sized to satisfy this peak power demand. The demands of a full working shift can require one or more battery changes to allow the truck to operate continuously. Operating costs can be reduced if the number of batteries needed per truck is lower. However the overall size of the power pack is restricted by limitations on truck size, since the physical layout of warehouses tends to be governed by the dimensions of a standard pallet.
To prolong the operating envelope of the battery, supercapacitors can be used to supply extra power at times of peak demand. A supercapacitor module can be integrated with a simple connection across the battery, or may be used in conjunction with regenerative or current-injection subsystems to further optimise performance and battery sizing.
The supercapacitor helps to prolong the run-time of the truck before the battery must be changed, thereby enabling operators to save cost by downsizing their fleet of batteries. In addition, preventing the truck battery from becoming deeply discharged helps extend the service life. Moreover, the superior low-temperature performance of the supercapacitor provides a significant advantage where trucks are used in cold-storage areas or outdoors in particularly cold climates.
Conclusion
Compared to current battery technologies, which have high energy density and a low self-discharge rate, supercapacitors are able to supply very high power for short periods while sustaining a high duty cycle and long cycle life. A supercapacitor module is the ideal companion to a battery to help optimise engine starting, range, performance, reliability and ownership costs in transportation applications ranging from hybrid and electric cars to large industrial vehicles and locomotives.
