Advent of nanotechnology has accelerated progress in ultracapacitors (also known as supercapacitors, Electrochemical Double Layer Capacitors or EDLC). These capacitors have values in Farads, or even thousands of Farads. Ultracapacitors (U-caps for short) store energy much beyond the capacity of electrostatic or electrolytic capacitors, and somewhere comparable to batteries (Fig. 1). These are finding increasing use in electronics, automobile industry – as well as power supply systems. Each one of these applications makes for a path breaking technology in itself.
Fig. 1: Comparison of energy storage technologies…
Since ultracapacitors store energy in the form of electrical energy, and also deliver it back in the same form with no chemical reaction involved, energy efficiency is very high, around 95 to 98%, as against 70-80% for battery. Lead acid battery on an average has a life of 2 to 4 years, while U-caps last for 15 to 25 years, often longer than the vehicle. Further, most materials that go in the U-cap are eco-friendly, and coupled with long life (and no electrolyte leakage), they are a perfectly eco-friendly option.
Growth of ultracapacitors has mainly been driven by automobile industry, where it finds applications in many areas. They have large energy densities- up to 1/10th of battery or even more, but their strengths lie in high power delivery. Auto sector has gained immensely by these capacitors. Many new vehicles are using them with huge benefits.
A comparison of ultracapacitors with battery (Table 1) shows that ultracapacitors are far superior to batteries in high power delivery, response time, charge / discharge time as also the temperature range. Battery suffers loss of capacity at low temperatures, which leads to starting problems in vehicles in extreme cold weather. U-caps are able to start the vehicle without problem at these temperatures. U-caps are not intended to replace batteries, but assist batteries by taking over high power or high current short time duties. U-cap-battery combination works very well to extend battery life several folds.
Battery size for a vehicle is decided by its ability to start the vehicle in extreme weather conditions – where its capacity goes down substantially. With ultracapacitor taking over starting function, Lead acid battery size can be reduced to almost 1/3rd, with consequent reduction in weight, volume and cost (Table 2). Even the downsized battery has double the life, and replacement cost is reduced because of smaller battery.
New vehicles coming out every year depend more and more on electrical systems, and electrical power demand goes up by 100-150 W every year – from 1kW in 90s, the demand today touches 2kW, and will very well go to 3 kW in near future. Power steering takes about 2kW and typical 14V bus supplies 500 – 1500A for several seconds. Cable harnesses, switches and relays ratings have to be very large.
Applications of U-caps in vehicles include the following:
- Battery backup
- Extending battery life
- Boardnet stabilization
- Increase cranking reliability
- Cold start / jump start
- Accessories like horn, radio, A/C
- Start-stop systems
- Power steering
- As a stand-alone power source
Battery backup
Ultracapacitors take away all transient or short term loads from battery. These include steering, brake, horns, window operation, door closers, and air conditioners etc., which get power from U-caps. Power supply to modern day car electronic systems is sensitive to voltage fluctuations. U-caps help boardnet stabilization to ensure steady supply and reliability of the system by reducing both peak power and average load on batteries. Their low ESR enables fast recovery of regenerative energy capture and storage.
Fig. 2: Ultracapacitor meets short term demand, while battery/engine /fuel cell supplies steady power load…
Boardnet stabilization
LED brake lights and indicators reduce lighting power to 25% of conventional bulbs. However, they need electronic ballasts. Multiple function modules for safety, fuel control, lights and sound need system stabilization. Power and control signals for smart actuators and servos are sent over communication lines from buses. Electromechanical braking reduces cost and weight of brake system and improves performance. All these functions need boardnet stabilization. Separate U-caps are used at different locations as also at centralised control – so as to have a steady battery voltage and reduce its current drain.
Without boardnet stabilization, sensitive components could get unworkable, when voltage drops due to a cranking / overload. Short-term power demand can be compensated by a 15V module weighing under 700 g. During phases without electrical load the U-cap storage can be recharged through the main energy source. A second battery is avoided with advantage in weight, space, maintenance and reliability over a vehicle’s lifetime. Cost is about the same as a second battery and associated cabling, and the life cycle cost is lower.
Fig. 3: Electrical contents in future cars…
A number of operations in modern cars are performed by electrical systems with centralised controls (Fig. 3). Large numbers of actuators around the vehicle need power and energy. Many of these are away from control centre, and short time large currents in these applications means a voltage drop in lines, use of heavy wire harnesses, and also frequent voltage drops across battery terminals, which may lead to control system problems.
Start-stop, regenerative braking application
Fig. 4: Start-stop U-cap modules…
U-caps in Fig. 4 have three terminals, out of which one positive terminal goes to motor starter / engine starter, while second charges the battery. Battery does not have to deliver 1000A burst of power for engine starts. This enables a car to start in extreme cold, and also increases battery life. Battery replacement is drastically reduced or is rendered unnecessary. The U-cap is directly connected to starter, and battery takes care of steady load.
Fig. 5: Integrated starter generator system with ultracapacitor for start-stop…
One way to fuel saving and engine efficiency is to completely take away car starting function from IC Engine (ICE), and use ultracapacitor-motor combination instead. In Fig. 5, a belt-driven motor generator set powered by ultracapacitor starts the car, and ICE starts subsequently, once the car is set in motion. The engine can then be much smaller (since it does not have to start the car), and the motor generator set also doubles for regenerative braking to convert kinetic energy of decelerating vehicle into electricity to be stored in U-cap.
This makes start-stop function very effective, since neither the engine nor the battery is involved in moving the car from a stationary position. Not only are U-caps much lighter than a battery of the same energy capacity, they deliver power more quickly. All the energy to start the engine comes from regenerative braking energy recovery.
This arrangement substantially increases battery life by as much as 400%, reducing battery replacement costs in vehicles. The combination increases engine reliability and even a run-down battery may still be able to charge U-cap enough to get the vehicle moving.
Fig. 6: Electrical system for U-cap engine start application…
In a normal vehicle, with every start of the engine, battery is one step closer to its end-of-life condition. U-caps ensure that the vehicle has no cranking problem even in extreme cold weather. The electrical system architecture in Fig. 6 takes care of start-stop application, and through a separate branch, takes care of GPS, lamps, brakes and other system components.
Start-stop system is gaining momentum as a means to save fuel, and fuel efficiency goes up 4 to 10%.
Considering that average start-stop requirement may be needed 50 to 100 times a day, a battery-only solution just cannot be considered, as thereby the battery life will be drastically reduced.
U-cap can support 300-1000 A needed for engine start in all weather conditions.
Regenerative braking takes most load off the mechanical brakes, increasing their life, and reducing maintenance & replacement.
Maximum exhaust emissions occur during start operations, as engine load is the greatest and maximum power is required.
In extensive tests under the New European Drive Cycle (NEDC) standard, ultracapacitor module completed more than 110,000 stop-start cycles at room temperature, successfully maintaining the battery voltage above 11.8 Volts, meaning it was still working efficiently and fit to go a long way yet. In parallel comparative tests of a battery-only system, also at room temperature, the battery failed after only 44,000 cycles (battery is considered failed when its voltage drops to 10V). U-cap power density is 10-100 times that of a lead acid battery, and they add less than 10 kg to the weight of a car.
Fig. 7: Regenerative system in Mazda I-ELOOP…
Mazda uses regenerative braking in its I-Eloop model, and the system configuration is shown in Fig. 7. A DC-DC converter has a maximum throughput of 50 A at 14.5 V.
Headlamps and other lighting, HVAC, wipers, and the audio system take up about 40 A. Kinetic energy from deceleration is converted to electricity by the variable-voltage alternator and transmitted to U-cap, from which it flows through DC-DC converter to 12-V electrical components. High-resolution crank position sensor with an electronic strategy uses the alternator to stop the engine such that piston of one cylinder is in optimum position at the start of the power stroke.
A precisely timed squirt of fuel and spark creates some downward force, which combines with just a quick boost from the starter motor to restart the engine in under 0.4 sec.
One plus point of U-cap is that energy left in a capacitor can be monitored by reading its voltage. A battery, on the other hand, cannot be judged when it comes to its end of life.
A meter on the panel also shows energy flow to different systems. The alternator has output varying from 12 to 25 V to adjust to capacitor voltage while charging.
Power steering
Many cars today use Electric Power Steering (EPS). Sensors detect the position and torque of the steering column, and control system applies necessary assistive torque via a motor connecting either the steering gear or steering column. Steering-gear response can be tailored to variable-rate and variable-damping suspension systems, optimising ride, handling, and steering. The steering and suspension systems of a car are directly inter-related and are important for safety, as also for comfort level.
An EPS Motor used in power steering systems draws current from battery – only when the steering wheel is being turned. This load is substantial – even up to 1000A, and U-caps supply this current for the short duration of steering. This can result in a fuel efficiency improvement of about 3 to 5%. Overall reduction of fuel consumption can go over 50%, reduction in particulate emissions over 90% and NO2 emissions by 50%. Peak power of 1.2 kW to an EPS for 2 sec (2.4 kW-sec) can be met with 15 V ultracapacitor pack storing less than 6 kJ (6 kW-sec) of energy.
A modern hybrid car with power steering, regenerative braking, door locks, electronic seat adjustments and power brakes have general arrangement as in Fig. 8. Ultracapacitors are arranged in different positions (shown in red) to take care of individual lines or systems.
Fig. 8: Systems in modern car…
Automotive Power Train Solutions
U-caps are placed in parallel with primary source of power (ICE, battery or fuel cell) to handle peak loads and capture braking energy. With ICE, U-caps can replace batteries as secondary energy source.
Toyota is running U-cap based cars in endurance tests since a few years. In 2012, Toyota hybrid TS030 (Fig. 9b) took part in 24-hour endurance test at Tokachi, Japan with petrol engine and ultracapacitor powered system (no batteries). U-caps start 300HP motor in the rear, and once car gets speed, ICE starts and in turn charges U-caps. Power of both U-cap and ICE can be combined to get 830 BHP when necessary. Regenerative braking ensures maximum fuel efficiency. Such hybrids are becoming common in racing cars.
Fig. 9(a): Power train using U-cap…
Jump start/ cold start
In cold climates, cranking of engine of a car or truck or bus is problematic, since battery does not work. U-cap modules are now being fitted in trucks or buses to overcome the problem. Mobile modules are available, which can start trucks one after another in remote cold places, and these modules themselves need to be charged once in a six months or a year.
Fuel cell cars
Fuel cells though have far more energy than a battery; their power delivery is much lower. Hence, it is beneficial to pair it with U-cap and route the electricity to motor and other systems via U-cap modules. Fuel economy of the car is matchless 2 to 3 times that of petrol, with running range quite long. Toyota FCV Concept has a range of 480Km. Morgan Life car has a fuel economy worth 53 Km/ l!
Ultracapacitor powered vehicles
Golf carts, mini buses, fork lifts are already being powered by U-cap power at many places. China is running buses (fully air-conditioned, 41 passengers plus driver) on few routes in Shanghai since past couple of years. These buses are charged at umbrella bus stops located at selected intervals along the route, where the U-caps are charged in the time it takes for the passengers to board and alight.
Fig. 9(b): Toyota Hybrid TS030 racing car (2012)…
Charging takes place via overhead pantographs, which go up and touch charging strips overhead at these stops. The bus has a range of 3.5 miles when charged, and could be extended in winter to 6 miles when A/C is not required. A variant of this bus works on U-cap and Li-ion battery combination for intercity buses with a range of 45 miles, and is charged en route at stops to make them run longer. The charging stations may be in turn be charged by main supply or solar energy.
Way forward
Ultracapacitor energy capabilities are increasing, and we may find more and more applications coming up with more demanding models of vehicles coming up every year. Fuel cell vehicles being introduced in market depend on U-caps, and increasing functions and controls are being managed by U-cap systems. China has planned to introduce metro trains running on U-caps in Guangzou (Guangdong) in 2014, which get charged at every station. They intend to expand this metro network in over 100 cities across China in near future. Batteries and ultracapacitor technologies are both evolving and it will be interesting to watch further developments coming up in near future.
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