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Alternative power sources for buses – now and in the future

Franz Redwitz talks about electromobility in public transport

Tighter exhaust standards, stricter legislation and increased sense of responsibility for the environment are fueling the development of alternative means of powering. Franz Redwitz, Head of Product Marketing Bus & Coach at MAN Truck & Bus, spoke in the leadup to Busworld 2013 about the development of alternative powering sources for buses, pointing to the fact that electromobility does not automatically mean ecological friendliness.

Mr Redwitz, what kind of a role does electromobility play for MAN Truck & Bus?

Electromobility is obviously high up on the agenda for MAN, as throughout the vehicle industry. But you have to take a differentiated view of things. Purely electrically powered buses are at present neither economical nor ready to go into series. There are a number of reasons for this: For one thing, operating electric vehicles efficiently calls for large investment in infrastructure. Then, also important, you have operational safety, cost and service life of the batteries. The batteries available at the moment are very expensive, plus they need to be replaced every five to six years.

Not forgetting the extremely high weight of these batteries. Even the next battery generation won't match diesel or gas in terms of energy density. I'll illustrate it by a comparison: The energy density of a modern electrical battery is maximally about 150 watt hours per kilogram. A liter of diesel on the other hand gives you 11,800 watt hours per kilogram. So you see, batteries have a long way to go in energy density if you want to drive a city bus fully electrically for a whole day without any interruptions. The alternative is either very high vehicle weight, or a special and pretty complex infrastructure for recharging in between.

But the biggest challenge posed by electromobility is probably safety. Batteries consist of single cells that have to be combined into packages. The result needs to be absolutely safe because you can't have an electric bus that's in any way less safe than a diesel or gas powered bus. Batteries must be installed and safeguarded in vehicles to prevent them exploding or releasing harmful substances into the environment in case of a defect or accident, for instance. That means another increase in vehicle weight.

There's a bigger demand emerging for emission-free zones in cities. Isn't complete electrification an especially environment-friendly alternative for public transport?

There are different aspects to that kind of argument. The bare facts tell us that an electric vehicle isn't automatically efficient or environment-friendly. In Europe energy is generated from different sources and then mixed. If energy won from wind power is mixed with current from coal-fired power stations, for example, that also increases carbon dioxide emissions. Which means that electric vehicles aren't the optimum way of reducing greenhouse gases.

A recently published study from DENA – the German Energy Agency – found that biogas and electric power have an identically small carbon dioxide footprint. Taking the complete energy chain that's about five grammes per kilometer. But that only applies for two factors: The biogas must be produced entirely from manure, biogenic waste material or waste water, the electricity from purely regenerative sources. The study assumes this. The emissions from all other types of fuel are 15 to 30 times higher. This isn't especially surprising for conventional modes of powering. But it's often not realized that supposedly clean fuels like ethanol or biodiesel cause substantial carbon dioxide emissions.

Is going hybrid an alternative?

Yes, no doubt. In the MAN Lion´s City Hybrid we already have series produced and economical, future technologies up and running. In this way we're constantly gaining experience in what's decisive to make electromobility efficient: in energy management onboard a city bus. With its serial hybrid drive this low-floor bus saves up to 30 percent on diesel and CO2. While conventional buses convert their braking energy into heat – by service brake and retarder – the MAN Lion's City Hybrid saves this braking energy in ultracaps and transforms it into power to drive its two electric motors.

Another advantage of hybrid buses is the reduced noise. When pulling away from a stop, the bus moves fully electrically. The diesel engine doesn't cut in until after a few hundred meters. That avoids bothersome noise where a conventional bus causes most. The noise level in the interior is also extremely low.

Basically then, we're already offering a ready electric bus that can manage without an internal combustion engine if, as the next step, you replace the diesel engine by batteries or another energy source. A series hybrid vehicle also presents the advantage of recharging during operation. In that way we combine the best of two worlds. Experience with the MAN Lion´s City Hybrid is very satisfactory: To date there are more than 200 MAN hybrid buses working successfully in European cities.

What's the role of natural gas and biogas as alternative power sources?

In public transport you see an increasing number of natural-gas-powered buses as an investment for the future, as an economical, environment-friendly alternative to conventional diesel. Compared to other fossil fuels, natural gas shows especially high CO2 efficiency. Natural gas also combusts very cleanly, so engines emit low levels of air pollutants such as carbon monoxide, hydrocarbons, nitrogen oxides and particulate matter. Using biogas, buses can even be operated CO2 neutral.

The price of natural gas is often lower than that of diesel fuel. Operators of natural-gas-powered buses also frequently benefit from state incentives. Given their very soft fuel combustion, CNG buses are especially quiet on the roads.

When do you think we'll see fully electric buses in public transport?

Demands for purely electric operation of buses will increase. So there's no getting around electrification, and longterm it will become a sine qua non for all European bus producers. As soon as economical operation of fully electric buses is possible – and there's appreciable demand for them – MAN will be able to deliver.

In the Lion´s City Hybrid and a wide-ranging portfolio of CNG buses, MAN offers CO2-efficient options as alternatives for the future of public city transport.

The MAN Group is one of Europe's leading industrial players in transport engineering with annual revenue of some € 15.8 billion (2012). MAN is a supplier of trucks, buses and coaches, diesel engines, turbo machinery and special transmissions and employs 54,300 persons worldwide. MAN's business areas are all leaders in their markets.

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