How supercapacitors can help fleets implement no-idling

Wednesday, May 6, 2015 - 09:51
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The key to London’s no-idling law?

In a response to Steve Towe’s previous Expert Blog regarding recent idling legislation introduced in the capital, Carl Postiglione from Saft gives his view on the potential for supercapacitors to cut out idling time in London bus fleets.

Recent developments in London have focused attention on air quality and the impact of idling. This May, Westminster council is introducing fines for motorists who idle their vehicles and air pollution is a top issue for Londoners.

And while the capital has introduced ultra low emission buses in central London, where challenging legislation has been set, it’s difficult to make the financial case for such vehicles for suburban areas as a hybrid bus costs around twice the price of a conventional bus and requires battery replacement every six years.

One solution is to introduce a fleet-wide no-idling policy. A typical bus sits at idle for 30 percent of the time and consumes 3.2 litres of fuel per hour. Over the course of a 5-day week, 50 weeks per year and with diesel costing in excess of £1.10 per litre, that adds up fast across a fleet.

Restarting a bus uses the amount of fuel consumed in 7 seconds of idling, therefore as long as the bus stop lasts longer than 7 seconds a no-idle strategy can save fuel and emissions. By reducing idling from 3 hours per day to half an hour can reduce carbon emissions by 80 percent from 22.5 kg to 3.8 kg

By switching off, fleet operators have the potential to save around £2,600 annually per bus but one thing holding them back is that normal lead-acid bus batteries are not designed to start the engine upwards of 10 times a day. Even in optimal conditions bus batteries tend to last less than three years.

The number of starts is directly related to the maximum number of cycles a battery can handle. The harder the battery works the lower the Depth of Discharge, leading to fewer cycles and a shorter life.

Going to a no-idle policy means that a different technology is needed and that is where nickel supercapacitors have a role to play. A supercap can be installed with a smaller sized battery which supports hotel loads like lighting and can start a bus one million times during its life.

Saft has more than 2,000 capacitors installed in many different transit agencies, where they have a proven history of providing reliable cranking power. A supercapacitor can cost around one percent of the bus’ overall price. Together with an automated no-idling system and battery powered air conditioning and heating, a total investment of five percent results in a payback of less than five years, together with the benefit of cleaner air.

So with a supercap, an aggressive no-idle policy is well within the reach of fleet operators beyond central London.

Carl Postiglione is Product Manager at Saft, a company involved in the design, development and the manufacturing of batteries for use in transport, industry and defense.

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