Lightweight high-voltage energy storage system for electrified semi-trailers.
Alternative drive system using EV trailers
The transport sector is the third largest source of CO2 emissions in Germany, accounting for 18% of total emissions. In the freight transport sector, trucks account for around 72% of total transport performance. That is why the European Parliament and the European Commission decided in February 2019 to set binding targets for CO2 reduction in heavy-duty vehicles. In addition to an improved CO2 balance, discussions about inner-city driving bans for diesel vehicles are also promoting the development and use of trucks with alternative drive systems. Current studies assume that a quarter of all new trucks sold in Germany in 2030 will have alternative drive systems. However, battery-electric vehicles are a difficult concept to implement, especially for long-distance freight transport and the typical driving distances involved, due to their very high mass and the cost of electrochemical energy storage.
Lithium-ion cells as storage devices
Electric traction motors are predominantly used in road vehicles in conjunction with energy storage devices based on lithium-ion secondary cells, as these cells retain their properties over a large number of charging and discharging cycles. The new energy storage system from “evTrailer” uses cylindrical NMC cells in the 18650 form factor with a nominal capacity of 3,000 mAh and a nominal voltage of 3.6 V. These round cells can be produced relatively inexpensively and, thanks to their casing, which is usually made of nickel-plated sheet steel, are comparatively rigid and robust. The NMC cells have a relatively large storage capacity in relation to their mass – in this case 213 Wh/kg – as required for the construction of mass-efficient traction components in electromobility.
Weight and thermally efficient
The energy storage system built from these cells provides a nominal voltage of 605 V and a gross capacity of 100 kWh. Aging and capacity degradation of lithium-ion batteries (LIB) are stimulated by high and low temperatures, high charging and discharging rates, as well as state of charge and calendar effects: The consequences are a decrease in cell capacity and a reduction in performance due to increased internal resistance. For this reason, the battery management system (BMS) of the “evTrailer” energy storage system always takes into account reserves with regard to deep discharge protection, cold start capability, self-discharge, and cell capacity asymmetries.
Special battery connectors and ultra-light housing
The round cells are assembled into 12s20p modules, each with 240 cells. To this end, the research teams developed special current connectors made of electrolytically nickel-plated cold-rolled strip and spot-welded them to ensure larger cross-sections and thus a lower current load than bond wires, resulting in improved electrical safety and durability. The energy storage system was constructed from a total of 42 modules arranged in two rows one above the other with a cooling plate between them. This balances the thermal load for all individual cells and prevents hot spots. Despite a mass of 475 kg for the entire number of cells alone, the total mass of the energy storage system, including the cooling system, BMS, and housing, was limited to just over 600 kg. The lightweight construction concept required for the housing uses advanced sandwich structures and glass fiber-reinforced thermoplastics. This achieves a ratio of 0.8 between cell mass and total weight, which is particularly favorable for high-voltage energy storage systems. Such an ultra-lightweight construction solution is only possible because energy storage systems that are located between the vehicle side members and at least 700 mm above the road surface do not require vehicle crash tests.
CO2 reduction even on long distances
The “evTrailer” concept achieves a significant reduction in CO2 emissions for heavy semi-trailers, even on long distances and with very reasonable techno-economic costs in terms of vehicle technology and charging infrastructure. A kingpin coated with thin-film sensor technology and a dedicated control and regulation system were developed to create a particularly cost-efficient semi-trailer that can be combined with any tractor unit. This means that communication with the tractor unit can be limited to the electronic braking system (EBS) protocol. The installation space available under the front loading floor of the trailer is consistently used for the integration of a larger traction battery, enabling not only brake energy recovery but also short-term traction support and shift and load point shifting of the tractor unit.
This enables fuel savings of over 30 percent in distribution transport and significantly more than 10 percent for uninterrupted journey lengths of up to 400 km. The option of self-sufficient driving in the logistics center is another plus point that contributes to a favorable assessment of the overall economic efficiency of this concept.