From Old to Electric – Retrofit Diesel Buses Instead of Replacing Them

One of the most important challenges on the path to net zero is transport. Electric vehicles are replacing internal combustion engine vehicles; public transport is expected to increase, while private motorized transport should decline. Buses are a particularly attractive option for expanding public transport: unlike rail transport, they require hardly any new infrastructure. If the expansion of bus capacity is accompanied by a reduction in private motorized transport, there will be sufficient space on existing roads for additional buses.

To fulfill their role in terms of sustainability, however, buses must run on electricity. Today, diesel buses are increasingly being replaced by electric buses. However, this process is only just beginning: in 2023, only just under three percent of all buses on European roads were electric. “If the bus fleet remains constant, it will take at least until 2055 before more than 95% of all European buses have been replaced by electric ones,” says Harald Desing from the Technology and Society department at Empa in St. Gallen. “That is after 2050, the year by which the net-zero target in Europe and Switzerland is supposed to be achieved and many countries and regions have set even more ambitious targets.”

In a paper recently published in the journal Environmental Research: Infrastructure and Sustainability, Desing therefore investigated the potential of a different approach. “If we retrofit existing buses for electric operation instead of replacing them with new ones, we can achieve the complete electrification of the bus fleet around 15 years earlier while also saving emissions and raw materials,” says the researcher.

A Simple Conversion for Lower Emissions

As part of the EU research project CircEUlar, Desing examined the potential of this so-called “e- retrofitting” for the European bus fleet in detail. His study shows that the transition would be technically and economically feasible. “There are already companies today that offer e-retrofits for diesel and gasoline vehicles,” says the researcher. The major advantage for buses is that the process and the required components could be standardized. “Unlike the enormous variety of passenger cars, there are only a few model series of city buses, each produced in large numbers,” Desing explains.

The average service life of a diesel bus in Europe is around 20 years. Once retired, these vehicles are usually sold to other countries, where they continue operating for many more decades and continue producing emissions. “That is not the most sustainable solution. Climate change does not stop at national borders,” says Desing. Retrofitting prevents the bus from continuing to operate on diesel elsewhere, and the conversion itself causes around 20 to 50 percent lower environmental impacts per bus than manufacturing a new bus.

Converting a diesel bus into an electric bus essentially requires replacing the engine and transmission. Instead of the exhaust system and diesel tank, batteries are installed. Auxiliary drives for the air conditioning system, the braking system, and power steering can be converted relatively easily to small electric motors. “With standardized retrofit kits, a single conversion would take only a few days. This would make it possible to electrify the fleet without major disruptions to ongoing operations,” the researcher explains. In addition, most of the removed components are made of steel and aluminum and can be recycled.

A Faster and More Cost-Effective Path to an Electric Bus Fleet

Another advantage of retrofitting is that fleet operators would not have to wait for the full 20-year service life of their vehicles or shorten it artificially but could carry out the conversion at any time. In fact, the service life of the buses could even be extended. “Today, buses are replaced because they no longer meet modern emissions standards, for example regarding particulate matter or noise,” explains Desing. “If the drivetrain is replaced, the body and interior can often remain in service for much longer.” This allows bus operators to save costs in the long term. Alternatively, these savings could also be invested in expanding the bus fleet.

The additional charging infrastructure required for electric buses was not part of Desing’s study. However, the researcher is confident that it could be implemented relatively easily. “In locations where overhead power lines already exist, buses could, for example, be charged while driving,” he explains. This would also generate additional cost savings during retrofitting, since a smaller battery would be sufficient.

To further pursue this promising strategy, the technology for e-retrofitting would need to be standardized and scaled up. Although his study focused on the European bus fleet, Harald Desing also sees potential for other countries and regions, although this would first need to be examined in greater detail. Retrofitting trucks, which are present on the roads in even greater numbers, would also be a conceivable option.

Contact:
Anna Ettlin
Kommunikation
Tel. +41 58 765 47 33
redaktion@empa.ch