One our biggest “to-dos” on the path to net-zero is transportation. Electric vehicles are replacing internal combustion engines; public transit is expected to grow, while private vehicle use is likely to decrease. Buses are a particularly attractive option for expanding public transportation: Unlike railways, they require virtually no new infrastructure. If private vehicle use declines at the same time as bus capacity expands, existing roads will have enough space to accommodate additional buses.

However, to fulfill their role in promoting sustainability, the buses must run on electricity. Today, diesel buses are increasingly being replaced by electric buses. But this process is still in its infancy: In 2023, just under three percent of all buses on European roads were electric. “If the bus fleet remains constant, it will take until at least 2055 for more than 95% of all European buses to be replaced by electric ones,” says Harald Desing from the Technology and Society laboratory at Empa in St. Gallen. “That’s after 2050, the year by which the net-zero target is supposed to be achieved in Europe and Switzerland – and many countries and regions have set themselves even more ambitious goals.”

In a paper recently published in the journal Environmental Research: Infrastructure and Sustainability, Desing therefore examined the potential of a different approach. “If we retrofit existing buses to run on electricity instead of replacing them with new ones, we’ll achieve full electrification of the bus fleet about 15 years earlier – and save on emissions and raw materials in the process,” concludes the researcher.

Simple conversion for lower emissions

As part of the EU research project CircEUlar, Desing has examined in detail the potential of this so-called e-retrofitting for the European bus fleet. His study shows that the conversion 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 with buses is that the process and the required components could be standardized. “In contrast to the wide variety of cars, there are only a few model series of city buses, but they are produced in large numbers,” explains Desing.

The average lifespan of a diesel bus in Europe is about 20 years. After that, the end-of-life vehicles are usually sold to other countries, where they continue to operate for many decades – and continue to produce emissions. “That’s not the most sustainable solution. Climate change doesn’t stop at national borders,” says Desing. Retrofitting prevents the bus from continuing to run on diesel elsewhere – and the conversion itself causes about 20 to 50 percent less environmental impact per bus than the production of a new bus.

To convert a diesel bus into an electric bus, you essentially need to replace the engine and transmission. Batteries are installed in place of the exhaust system and diesel tank. Any auxiliary drives for the air conditioning, braking system, and power steering can be converted to small electric motors relatively easily. “With standardized retrofit kits, a single conversion would take only a few days. The electrification of the fleet could thus take place without significantly impacting day-to-day operations,” the researcher explains. Furthermore, the removed parts consist largely of steel and aluminum and can be recycled.

A faster and more cost-effective path to an e-bus fleet

Another advantage of retrofitting: Fleet operators would not have to wait for their vehicles to reach the end of their 20-year service life or artificially shorten it but could make the switch at any time. This could even extend the buses' service life: “Today, buses are replaced because they no longer meet modern emissions standards, such as those for particulate matter or noise,” explains Desing. “When the powertrain is replaced, the body and interior can often remain in service for much longer.” Bus operators thus save costs in the long term. Alternatively, these savings could also be invested in expanding the bus fleet.

The additional charging infrastructure for electric buses was not the subject of Desing’s study. However, the researcher is confident that this could be implemented relatively easily. “In locations with existing overhead lines, for example, buses can be charged while in motion,” the researcher explains. This would enable even greater cost savings during retrofitting, as smaller batteries would suffice.

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 – though this would first need to be investigated more closely. It would also be conceivable to retrofit trucks, which are on the roads in even greater numbers.