Power Electronics

The Future of Electric Buses

The major growth in electric buses is in free running versions which cost $50,000 to $500,000 and can usually be deployed without infrastructure along the route. The larger electric buses tend to be hybrid to get the range: smaller ones tend to be pure electric. The IDTechEx report, "Electric Vehicles 2010-2020" estimates that there are about 480,000 buses in the world - mostly small ones - with about 135,000 being bought each year as the fleets grow. Although only 12% of these new buses are electric, penetration is increasing and there is a multibillion dollar market awaiting.

Electric buses reduce noise and air and land pollution and do the job better. They are often adopted regardless of payback because local government wishes to be seen to be green. Passengers note improved quietness and hill climbing ability. Arnhem in the Netherlands saw use of local buses increase by 17% when electric versions were introduced.

Electric buses cost up to 50% more per vehicle and need charging infrastructure but they are cost effective. Fuel is cheaper, maintenance is less and life is longer.

Today, the typical hybrid bus is a series or series-parallel hybrid because the benefits of having an electric motor drive the wheels. Lithium-ion batteries are preferred for all but the smallest ones because of size, weight, charge retention and faster charge and discharge. Regenerative braking saves up to 30% of energy because there is so much stop start. Many hybrid electric buses automatically stop the engine when the bus stops for more than a few seconds and automatically start it again when the accelerator is depressed.

Maverick technologies are at the testing and small scale at present such as buses that have no battery but use ultracapacitors (supercapacitors). These only give them a range of two miles or so but last for twenty years and have superior charge and discharge rate to that of batteries. The best ultracapacitors only store 5% of the energy that lithium-ion batteries hold. Cost may be less than for batteries - which often need ultracapacitors anyway to manage fast charge and discharge. The idea is to top them up by infrastructure at stops on suitably intensive routes, using coils in the road, just like your electric toothbrush. Sinautec's ultracapacitor buses in the USA are charged at bus stops every two or three miles.

Inductive charging, though wasteful, is being tried for charging regular battery driven buses as well for increased safety and convenience. In one type the cable connector is inductive and in another the coil is in the road so the driver does not need to get out. Hino, a division of Toyota, is testing wireless electric buses using this electromagnetic inductive charging. Alternative charging systems for electric buses include very rapid charging installed at specific bus stops.

In contrast to today's pure electric buses with 50-100 miles range, FAW Bus and Coach Company with battery maker Tongkun New Energy Technologies plans people-carriers with 186 miles (300 km) range near two cities in Jilin. FAW made over 1000 diesel-electric, parallel hybrid buses for the Beijing Olympics and Shanghai Expo. It now has orders for 70 long range, pure electric buses. The lithium iron phosphate batteries recharge in only twenty minutes. Useful life is 2,000 cycles. They could last 300,000 miles, price is only 500,000 yuan ($73,145) and there are low running costs.

There are about 30 electric bus manufacturers in the world that are taking substantial orders. For large buses, a high proportion use electric drive systems from leaders such as BAE Systems, Siemens and UQM Technologies. BAE Systems' HybriDrive technology powers over 2,500 buses in the UK and North America, transporting more than one million passengers every day. These buses have travelled over 150 million miles, saved 10 million gallons of diesel fuel and prevented more than 100,000 tons of carbon dioxide emissions. The HybriDrive system consists of a generator, an electric motor and a computer controlled energy-storage system. The diesel engine that turns the generator operates independently of the electric drive motor, allowing it to run at nearly constant speed. There is no mechanical transmission, a major maintenance item on conventional buses.

The power train manufacturers buy the most expensive part, the battery pack, from lithium traction battery manufacturers such as Panasonic, LGChem, Altairnano, EnerDel, BYD, ThunderSky and Valence Technologies.

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