Supercapacitors: Making transport more efficient
What are supercapacitors? Simply put, they are very large capacitors that can store very large amounts of energy. The technology bridges the gap between conventional capacitors and rechargeable batteries. Supercapacitors’ energy density is much lower than a battery, but the power density is much higher, that is, supercapacitors can charge and discharge much quicker than batteries can. This means they can be used to power applications that have high peak power requirements or often need a boost of energy which may be more than a battery (or the main power source) can provide.
Today, supercapacitors are widely used in transport applications. A typical way of using them in vehicles is to charge them with waste energy from the braking system and discharge them into the powertrain when high peak power is required, often when setting off. The supercapacitors can charge quickly from even short periods of braking, and can discharge with high power density when required. The overall effect is to stabilise the output power of the main power source. Fuel efficiency can be dramatically improved this way.
Supercapacitors are well suited to hybrid electric vehicles (HEVs) as they can compensate for the weaknesses of battery technology. Aside from their lack of ability to provide energy quickly enough when setting off, batteries are susceptible to hot and cold weather conditions. They also require charge equalisation management to make sure their life is long enough, and at the end of their life, they have to be disposed of, which isn’t good for the environment. Supercapacitors, by contrast, function over a broader temperature range (down to -40°C), don’t require charge management and last as long as the vehicle will (they are also largely recyclable). However, these benefits are balanced by the devices’ size, weight and cost.
Hybrid electric vehicles are particularly efficient in stop-start situations like city traffic, which is especially a feature for hybrid buses. The ultracaps provide the power for acceleration when setting off, meaning the buses can use a hybrid engine that is half the size of the traditional diesel equivalent. Fuel costs are cut by 25-30%, enough to pay for the increased cost of the system.
The powertrain is based on Toyota’s previous-generation car, the TS030, which was powered by a 530bhp V8 petrol engine with an ultracapacitor providing a 300bhp boost to the back wheels when required. The ultracapacitor captures the energy released when the car is braking, charging completely in under three seconds. The TS040 (pictured above) uses the same V8 petrol engine, but with supercapacitors mounted on both the front and back axles to allow temporary four wheel drive.
In the rail arena, there are numerous projects around the world making use of supercapacitors. The Long Island Rail Road uses ultracaps in trackside units that collect and store energy from braking trains. The units, actually big banks of ultracaps housed in shipping containers (right), then provide voltage support for the traction power system when acceleration is required. Energy efficiency is improved, while energy consumption is reduced by 500,000 kW/h per year at each station.
|The Long Island Rail Road uses ultracaps to collect and store energy from braking trains|
In Cerro Negro, Spain, a similar system at the side of the rail tracks captures energy from braking trains and draws on it when trains need to set off again. It’s possible to capture 8-10% of the total energy used by the railway system for reuse, vastly increasing efficiency. The energy collected by the ultracaps is also used to charge batteries which power a charging point for electric vehicles at the railway station.
Meanwhile, in Korea, Metro trains in Seoul, Daejon and Incheon have achieved power savings of more than 20% using ultracapacitor braking energy recuperation technology. The energy storage units, installed in Metro stations, use up to 200 48-V ultracap modules each.
Other, more novel uses of supercapacitors in transport systems include the Emirates Air Line in London, which is a cable car across the river Thames. The Gondola cars have ultracapacitor modules on the roof, which are recharged in a few seconds when the cable cars pass through the stations at either end of the line. The modules then power entertainment, lighting and air conditioning in the cable car while it navigates its 1-km (0.62-mile) trip, up to 90 m (300 feet) above the river.
Avnet Abacus stocks supercapacitors from some of the world’s leading suppliers including Murata, Panasonic, KEMET, Taiyo Yuden and Vishay. If you’re designing a transportation application and need some advice on supercapacitor selection, get in touch with our team of technical specialists in your local language by visiting our Ask an Expert page.