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English   The electrically powered Mercedes-Benz Citaro *
08.03.2018 von admin

Thermal management: new approaches for heating and climate control

The battery capacity on its own, however, provides little indication of the actual performance capability and, above all, the range of an all-electrically powered city bus - the true measure is that of energy consumption. In the case of a city bus, this is impacted significantly by climatic conditions, through the need to cool and, above all, heat the interior.

At an outside temperature of minus ten degrees Celsius, the energy consumption of a city bus doubles by comparison with journeys where no heating is required - thereby halving the range. The cause: the extreme efficiency of an electric motor means that, compared with a combustion engine, the amount of usable waste heat is negligible. The heating system must therefore be fed from the vehicle's own energy supply. An added factor is the sheer volume of the interior space, particularly when up to three double-width doors are opening regularly and letting outside air in every 400 metres or so, assuming normal bus-stop intervals.

The engineers have therefore put a considerable amount of thought into the issue of thermal management. It is one of the outstanding features of the Citaro and has been honed and refined in every detail: compared with the current Citaro with combustion engine, the energy requirement for heating, ventilation and climate control has fallen by about 40 percent. Achieving this exceptional level of energy efficiency has been a complex process, but it provides the basis for the Citaro's very viable operational range, even under unfavourable conditions.

Batteries at the ideal temperature: maximum performance and service life

Here too, thermal management is important. Mercedes-Benz, for example, cools the batteries to ensure that they remain at the ideal temperature, thereby ensuring maximum performance capability and service life. This cooling is undertaken by a separate battery cooling device mounted on the roof. At extreme outside temperatures, the standard passenger-compartment climate control system is used to boost the cooling of the batteries.

Further flexibility is possible by exploiting the discharge depth of the batteries. This can be extended, although at the expense of range and service life.

Heating: even the passengers help with heating

The passenger compartment of the Citaro is heated in an energy-efficient manner by a heat pump. An even distribution of temperature is ensured by use of the familiar side-wall fan heaters. The conventional heater at the front is boosted by the addition of a double heat exchanger. For use in extreme weather conditions, or to extend the vehicle's range, a fuel-powered auxiliary heater can also be used as an option.

A series of examples makes clear the care and detailed attention that have gone into the thermal management system. All components that give off heat are linked together, so keeping the amount of energy required for their cooling while in operation to a minimum. Since the human body likewise gives off heat, the heating on a bus carrying a full complement of passengers can be turned down earlier. Furthermore, Mercedes-Benz varies the output of the heating and climate control systems according to the number of passengers on board: the intake of fresh air in the bus is matched to the current number of passengers. The capacity utilisation of the bus is measured via its axle load sensors.

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