Strategic and technical planning of truck charging parks at rest areas

As part of the HoLa project, Fraunhofer IAO developed a simulation-based analysis tool to support the strategic and technical planning of charging and energy infrastructures at motorway rest areas. The focus is on the supply of heavy electric trucks in long-distance transport, using megawatt charging systems (MCS) for intermediate charging during breaks and night charging systems (NCS) with lower power for longer rest periods.

This online tool is intended to provide the user with insights into the analyses for a reference location with 100 truck parking spaces on the A2 motorway in Germany. In particular, the effects of various boundary conditions on the energy system and the quality of service at the site are conveyed. The results are based on simulations over one year in minute resolution.

Further information can be found here. 

Share of electric trucks

The share of electric trucks refers to the heavy goods vehicles arriving at the rest area during the course of the day. This is based on a mobility model based on the utilisation of real rest areas, mobility studies and traffic counts on motorways. For incoming trucks, a distinction is made between whether the driver arrives at the rest area for a break (45 minutes), a rest period (9 or 11 hours) or a weekly rest period (45 hours). The probabilities for each option depend on the time of day and the day of the week. The future share of e-trucks at an individual motorway rest area will depend not only on the general electrification rates in Germany and Europe, but also on the availability of alternative charging facilities in the surrounding area.

Break charging vs. Demand charging

After 4.5 hours of driving, truck drivers typically take a 45-minute break. During this break, the MCS can be used to provide intermediate charging with an electrical output of up to one megawatt (break charging). Alternatively, it is possible to charge only the energy required for the remaining distance to the day's destination (plus a reserve of 100 km) (demand charging). In this case, the charging point is immediately available again, but the statutory break must be taken elsewhere. Particularly in the case of larger batteries that are still quite full, short remaining distances or even higher charging capacities, the charging time for on-demand charging is short and can be carried out very flexibly (similar to refuelling).

Vehicle batteries

Assuming a full overnight charge, the battery size of the electric truck has a significant impact on the minimum charge required for MCS intermediate charging. In contrast to break charging, where the basic charge time for all electric trucks is 45 minutes, this comes into full effect for on-demand charging. The larger the battery, the higher the remaining charge level before intermediate charging and the lower the additional energy required to reach the day's destination. For shorter remaining distances, the charging requirement during the 45-minute break can even be completely omitted. Three battery sizes can be selected in the online tool, as well as a mixed scenario with proportions of 40/40/20% for the small/medium/large battery sizes.

Number of MCS and NCS charging points

The MCS charging points are generally used during the day for break or on-demand charging, while the NCS charging points are mainly used in the evening for overnight charging. On Saturdays in particular, the NCS is also used during the day at the start of the weekly rest period before the Sunday driving ban. The number of charging points available affects the quality of service at the site, as well as the electrical peak loads that must be provided by the local grid connection.

Charging power for MCS and NCS

The simulated MCS charging points can each provide up to 1,000 kW of charging power. The underlying MCS standard is also defined for higher charging powers of up to 3,750 kW, but this is not yet state of the art in the medium term. Up to 100 kW was set for each of the NCS charging points, enabling full charging over the minimum rest period of 9 hours in every scenario. In many cases, a maximum charging power of 50 kW would be sufficient, especially in combination with a full charge during MCS intermediate charging (break charging option), for small battery sizes and/or longer rest periods. For both MCS and NCS, typical charging curves were used for the charging powers actually used by the electric trucks. This results in decreasing charging powers at higher charge levels.

Quality of service at the site

If sufficient charging points are provided, all charging requirements can be met without waiting times at the location. The result shows the maximum number of charging points occupied (parking & charging) or the maximum number of charging points with simultaneous power flow (charging only). If fewer charging points are used, there will be waiting times for some electric trucks. The model includes a waiting time that depends on the battery charge level. If this is exceeded, the vehicle cannot be operated at the location and no charging takes place.

Presence and charging times

Presence time at the charging park is defined as any waiting time before charging plus the actual time spent at the charging point. The latter is made up of the charging time itself and any additional parking time after charging is complete, until the end of the break or rest period. In the case of MCS intermediate charging, the charging point is used either for on-demand charging only (without additional parking time) or for the entire 45-minute break. In any case, the stay at the NCS charging points covers the entire rest period or weekly rest period (9, 11 or 45 hours). Due to the long weekly rest period, the average value is higher than the more frequent individual rest periods of 9 and 11 hours. The average waiting times refer only to the users affected by waiting times and therefore in many scenarios have little impact on the average presence time of all electric trucks at the charging park.

Peak loads and energy turnover

Peak loads are calculated separately for MCS and NCS charging points and the total charging park. The value for the charging park is generally significantly lower than the sum of the two individual values for MCS and NCS. This is due to the different times of day or days of the week when the respective peak loads occur: for MCS in the afternoons during the week, for NCS in the evenings during the week and on Saturday afternoons. The use of a common grid connection is strongly recommended because of this synergy.

The energy turnover depends mainly on the number of vehicles served and the MCS intermediate charging option selected (break charging vs. demand charging). If only the demand is covered during intermediate charging, the energy required for the subsequent night charge will increase if the vehicles are to be ready in the morning with a full battery again.

The online tool presented here can only illustrate some of the functionalities of the underlying simulation tool and the associated interpretation of the results. In addition to the technical aspects, many other strategic questions are addressed: How many vehicles are waiting at the same time and how can waiting areas be organised? What can reservation systems contribute? How can resilient processes be enabled, e.g. in the event of delays and charging infrastructure failures? How can different business models and pricing structures influence user behaviour? What are the feedback effects on the input variables of infrastructure provision and its constraints and limitations?

The planning of the truck charging infrastructure and the expansion of the superordinate power grid require a long lead time and must be driven forward quickly despite all uncertainties regarding the boundary conditions. The tools developed in the HoLa project are intended to support and facilitate this process.

Working paper