Surprise: the fleet of electric buses adopted in Oslo to drastically reduce emissions have been paralyzed in recent days due to the severe cold (between minus 7 and minus 14) that hit Scandinavia. Technicians and professionals know well that the output of a battery is proportional to the temperature, and in winter users of electric cars and buses have to deal with a reduced autonomy compared to the summer. And since this heat drop is also compounded by the problem of managing the temperature of the passenger compartment (with many passengers in the case of urban buses) which can constitute 30% of consumption, the experts have underlined that a corresponding saving in the charge of the batteries used to heat vehicles it can be obtained with diesel burners.
Anna Stefanopoulou, professor of mechanical engineering
The cold paralyzed part of the new fleet of Ruter, the company that manages electric buses in the Norwegian capital, adding this criticality to what had been happening since the beginning of December with more than 140 bus departures compromised every day. Ruter itself admitted that it had some “problems with the shorter range of buses in cold weather” but also that it had “resolved this problem by changing the charging shifts and fixing the relevant charging infrastructure”. However, the problem remains of the compatibility of electric traction systems – be they cars, buses or trucks – with the cold”. Every mobility system, whether it is a battery or living beings like humans, does not work well in the cold” explained Anna Stefanopoulou, professor of mechanical engineering at the University of Michigan and specialized in the chemistry of electrical systems, recalling that ” even internal combustion engines have difficulty starting at low temperatures.” “The lower the temperature, the slower the reactions of the batteries needed to generate energy. And at -12 degrees centigrade a parked electric vehicle can lose up to 30-40% of its autonomy before it even starts to move.”
But electric vehicles – underlined Stefanopoulou – “are more efficient than petrol or diesel engines, which means that having low losses they cannot exploit the heat generated to heat the interior of the vehicle”. “The buses must remain connected to the network even in the start-up phase, until a few moments before starting to move – she said – and remain connected during the night. If these modes are adopted the battery can operate at maximum capacity. So transport authorities must adjust their schedules and equip buses with diesel heaters for the cold months to compensate for the 30% loss of range if they want to maintain their usual routes.
But installing diesel burners that heat the air means going against the zero-emissions goal that the city of Oslo aims to achieve already this year for all its public transport. “The solution is not black or white – he concludes – because completely decarbonising is not easy even if we are working hard on it. I think one should be aware of the improvements and at the same time understand the shortcomings. This will help us avoid creating negative publicity against the major effort we are currently making towards electrification.”
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