This study identifies the design conditions for the charging infrastructure at the parking lot in two ways:
The first scenario assumes that there is always a CP available for each vehicle arriving at the parking lot, resulting in a surplus of CPs. In this case, a smart charging system only needs to manage the power distribution among CPs to meet the network’s needs.
In contrast, the second scenario involves a limited number of CPs, which is fewer than the number of vehicles in the parking lot. Here, it is necessary to manage a queue for charging requests. When multiple vehicles arrive in a short period, it becomes important to handle the movement of vehicles between charging stalls and parking spaces. To meet the charging demand effectively, the queue management must be “smart.” This means it should consider the expected parking durations and the amount of energy each vehicle requires. The goal is to ensure that no vehicle leaves without charging while fully charged vehicles remain connected to the CPs in the parking lot.
3.2.1. Surplus of CPs
In this scenario, the availability of CP exceeds the demand, meaning it does not restrict the formulation of the charging problem. However, a constraint in the provision of the service arises from the overall power limit that the charging system can provide.
To verify the maximum charge requirements, we evaluate the power supplied to the load on the most critical day. On this day, the 28 vehicles in the parking lot collectively need a total of 325 kWh for charging. We consider two different charging power levels for the CPs: 3 kW and 6 kW.
Since the average residual time is not zero, we can evaluate a charging strategy that establishes a time interval between the vehicle’s arrival at the parking lot and the start of charging. To assess the impact of this procedure on the power profile, we introduced the following delays:
- (a)
The vehicle waits for a time equal to half of its residual time.
- (b)
The vehicle waits for a time equal to its full residual time.
Under the first condition, the residual time is halved compared to the scenario without waiting. In the second case, the residual time becomes zero. This temporal shift results in slight changes to the maximum power demand. The highest power request occurs in case (a), due to an increased number of overlapping charging events.
For high-power CPs, while the charging load trends remain similar, we observe a higher peak in committed power with a shorter withdrawal time, due to reduced charging duration for each vehicle.
The temporal analysis of charging requests indicates that the maximum number of active CPs at any given time is lower than the number of vehicles present. For example, in a no-delay scenario, out of 27 vehicles parked simultaneously, only 22 CPs are active when using 3 kW CPs, and only 16 are active with 6 kW CPs.
While both CP configurations are sufficient to meet the charging needs of the EV fleet in all three delayed charging approaches, ensuring that no vehicle leaves the parking lot without being charged, the 6 kW option increases the maximum peak power demand by up to 40% compared to the 3 kW case.
The duration of the charging session is influenced by the vehicle’s technical specifications and the power level of the charger. If the dwell time is equal to or shorter than the necessary charging time, charging will commence immediately. However, if the parking duration exceeds the required charging time, charging will begin at a later moment . The goal for the final SOC is 100%, although it could not be fulfilled in some situations.
= charging time.
= end-of-parking time.
= start charging time.
= residual time.
Another goal of charging management is to improve the performance of the parking charging system by reducing power peaks.
- (a)
All the engaged CPs are powered, but with a power equal to half the nominal power of the CP.
- (b)
The CPs are grouped in groups of two and are powered alternately at the nominal power of the CP.
Approach (b), which involves grouping the CPs, allows the same charging power to be applied to two vehicles but at different times. The result is a halved overall power due to the limitation of access to charging.
The graph on the right illustrates mode (a), where power is distributed across all 28 CPs. In contrast, the graph on the left represents mode (b), where a maximum of 14 CPs can be activated simultaneously. Additionally, the figure shows the power trend when vehicles are charged at 3 kW (green line), with charging beginning upon arrival, following the FIFO (first-in-first-out) logic.
The maximum power drops from 70 kW of the uncontrolled charging to 42 kW. The overall charging time is extended by two hours due to the staggered charging start times for some vehicles in the management method. However, the service capacity—meaning the ability to meet the energy demand for charging—remains unchanged; the total energy supplied, represented by the area under the red curve, is equal to that under the green curve and totals 325 kWh.
The previous evaluations were conducted under the assumption that the exit time of the vehicles was known. When there is uncertainty about the parking duration, we can implement a round-robin charging approach, where the charging alternates between the two vehicles at regular intervals.
The results of simulations carried out with charging powers of 3 kW, 2.5 kW, and 2 kW, with time intervals of 30 min for the charging alternation, are presented below. We will compare the findings from this round-robin charging method with those from Approach (b).
Tests conducted using Approach (b) charging consistently yield the best results due to the prior knowledge of the end-of-stay time. For 3 kW CP, the results are also excellent for round-robin charging; in fact, there is only one instance in which not all requested energy is supplied, with the shortfall being less than 1% of the total request and 17% for the specific vehicle involved.
As the maximum charging power decreases, the percentage of unsupplied energy for Approach (b) rises to 4.5% for 2.5 kW and 17.1% for 2 kW. The round-robin charging method results in an additional decrease of about 2% in both cases, with 6.2% and 19.4% of unsupplied energy, respectively.
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Natascia Andrenacci www.mdpi.com
