1. Introduction
The development of road transport in the near future will be based on the widespread use of electric vehicles (EVs). They are currently equipped with an internal battery that must be periodically charged by a plug-in connection at home or at a charging station. One of the most critical issues in EVs is the battery, which has high cost, high weight, and long recharge time. All these drawbacks today are the main impediment to the spread of EVs. In the future, a wide diffusion could be fostered by new technologies such as wireless power transfer (WPT) that will certainly allow overcoming the above-mentioned obstacles. The working principles of this technology are well described in [
1,
2,
3]. In order to increase safety, comfort, and range, the idea is to recharge the battery using a wireless connection based on inductive coupling instead of using a plug-in connection. At present, the resonant inductive coupling technology is almost commercially ready for stationary EVs by using a fixed transmitter coil embedded in a parking area and a receiver coil mounted on board the EV [
4]. Several studies were presented to increase the system performance and to verify the compliance with the safety limits [
5,
6]. Moreover, advanced techniques to reduce the magnetic field in the environment were investigated [
7]. In perspective, the idea is to create an electric infrastructure (i.e., electrified roads) where EVs can wirelessly recharge while moving along the roads and, hence, increasing their range significantly [
8,
9,
10]. This type of WPT technology is known as dynamic wireless power transfer (DWPT). The effort required for the implementation of the DWPT will be very large and will involve many players such as car manufacturers, electricity boards, traffic management, etc. Road electrification is mainly based on two different architectures for the transmitter coils: a series of short-track pads or a long-track coil. In both cases, the transmitter coils will be embedded in the road pavement and powered by an AC or DC bus connected to the main network, and every EV must be equipped with a receiver coil in order to be recharged. Many experimental demonstrations carried out so far on DWPT adopt short-track architecture. By this solution, the transmitter coils will be switched on, one at a time, only when an EV passes over them. A critical electrical aspect of the short-track DWPT is provided by the strong variability of power absorbed by the EVs and delivered by the AC main grid. Indeed, the transmitter and receiver coils are rarely perfectly aligned due to the EV movement. Therefore, the coupling factor between the coils is strongly variable, being dependent on the position of the onboard receiver coil relative to the fixed position of the transmitter coil. Since the coupling factor is time variant, there are some reciprocal positions/instants where it is at its maximum (perfect alignment of the coils), but there are also other positions/instants where it is close to zero (large misalignment of the coils or deactivated transmitter coil). This results in significant power variation with spikes and holes. Investigations of the short pad architecture can be found in [
8] where a demonstration system is presented and tested while showing good performance. Additional insight on several aspects such as efficiency, EMF emission, etc., are presented in [
9,
10]. In all the available literature, the problem of the power discontinuity is evident, and the oscillation of power is still an open problem.
The aim of this work is the stabilization of the transferred power, which is very important for both the AC power grid and the EV battery charging on board the EV. Some studies have been published in technical literature to solve these problems [
11,
12]; however, they are mainly based on the use of multiple overlapped transmitter coils. This solution when adopted for the transmitter permits reducing power variation and spikes but with a significant increase in road infrastructure cost. Another solution was proposed in [
13] where a complex vehicle assembly (VA) composed of three overlapped coils was presented while retaining an unchanged traditional architecture of the transmitting short pad coil. Here, we propose a new solution to reduce the cost of road infrastructure and, at the same time, to guarantee an almost stable power consumption of an EV in motion. The proposed solution is based on a new architecture of the VA coils in which the EV is equipped with two independent receiver coils mounted one in the front and the other in the rear of the vehicle underneath. The two coils operate one at a time. The receiver coil to be used is selected to maximize inductive coupling with the transmitter coil depending on mutual positions. On the ground assembly (GA), the necessary number of transmitter coils and electronics is reduced as much as possible by increasing the separation between adjacent transmitter coils along the electrified road, thus reducing the cost of the road infrastructure. The first section deals with the state of art of the DWPT system and the simulation tools able to analyze these systems. Then, the proposed architecture based on two pick-up coils is presented and described. Finally, an application of the proposed solution is compared with the traditional approach (i.e., single coil receiver). The first advantage of the proposed approach is the reduction in the number of GA coils required, with a significant reduction in the cost of the infrastructure. In addition, charging power and efficiency are nearly constant with a dual benefit for the AC grid and for the vehicle where the battery is charged with a constant current, reducing battery stress and improving its lifespan.
4. Applications
The proposed system is applied to realize a DWPT system working at the resonant frequency of 85 kHz with a nominal power of 7.7 kW for each VA in the electrified road section. The considered system is composed of multiple transmitting GA coils and two receiving VA coils installed under the vehicle body. The two VA coils have square shapes with exterior side lengths of
lVA = 50 cm. A square ferrite layer with side
wfe = 50 cm and thickness
tfe = 8 mm [
19] is interposed between the copper VA coil and the metallic body of the EV to mitigate the eddy currents in the conductive chassis. The GA coils have rectangular shapes with a width of
wGA = 85 cm to ensure an adequate tolerance with respect to possible lateral misalignment of the coupled coils. The length of the GA coil
lGA, the separation
SGA between adjacent GA coils, and the separation
SVA between the two VA coils are assumed to be the design variables that need to be optimized. To keep costs low, no conductive shields or magnetic materials are used in GA coils.
Magnetic field analysis is based on the solution of the MQS equations by COMSOL [
18]. In order to simplify the configuration under study, the metallic body of the vehicle has been modelled by only its underbody, i.e., an aluminum plane with a dimension of
lv = 350 cm and
wv = 165 cm. The multi-turn copper coils are suitably modelled in the computational domain by considering the ferrite layers of the two VA coils. The road pavement and any other parts of the EV are neglected in the magnetic field calculation. The simplified configuration adopted for the magnetic field analysis is shown in
Figure 6.
The proposed design flow is illustrated in
Figure 7. As a first step, the size of the VA coil is fixed depending on car specifications and weight constraints. Then, a field analysis is carried out in order to find the best tradeoff between the GA coil length and electrical efficiency. A longer GA coil allows the use of fewer coils in the electrified road section but with an increase in magnetic flux leakage with a consequent reduction in efficiency. Finally, the optimum size of the GA coils is calculated to ensure nearly constant output power and efficiency when the vehicle is in motion.
First, the self-inductances
LGA and
LVA are numerically calculated by (5)–(6) assuming a fixed number of turns for GA and VA coils set at
NGA = 8 and
NVA = 16, respectively, but also by assuming the length
lGA of the GA coil variable. The values calculated for
LGA and
LVA for different GA coil lengths are reported in
Table 1. Then, the mutual inductance
M =
M(
x) is calculated by (9) for several GA coil lengths
lGA and for several positions
x of the VA coil center in order to simulate a moving vehicle. The behavior of the mutual inductance
M =
M(
x) obtained by interpolation is shown in
Figure 8. The coupling factor
k =
k(
x) between GA and VA coil, calculated by (10), is shown in
Figure 9. As observed, the variation of k is slightly higher than M due to the increase in GA coil self-inductance for larger GA coils. The position starts from the perfectly aligned coils (
x = 0) up to a distance from the coil centers along the road direction equal to
x = 1 m. The efficiency behavior η = η(
x), calculated when considering SS compensation and the load modeled as a simple resistor
RL = 10 Ω, is shown in
Figure 10. The resistor
RL is dimensioned to reproduce a load power of 7.7 kW at a voltage level of
V2 = 280 V.
Considering a minimum target efficiency of η
min = 92%, the maximum GA coil length is
lGA = 150 cm; thus, this size is selected for the considered application. With this configuration, the efficiency η is good for coil separations (i.e., misalignment between GA and VA coil centers) up to
xmax = 65 cm, as shown in
Figure 10. Considering also the negative misalignment (thus, all the coils), we can find the optimum distance
SVA between two receiving VA coils, which is, therefore, fixed to
SVA = 2
xmax = 130 cm. Likewise, the separation
SGA between adjacent GA coils is calculated in order to ensure that when the second VA coil exceeds the maximum admissible misalignment, the first VA coil is activated with a misalignment
x ≤
xmax. The separation
SGA can be calculated as follows.
For the considered case, the optimum distance is found to be at
SGA = 260 cm. A sketch of the configuration is reported in
Figure 11.
In order to understand the proposed configuration well, the coupling factor of the complete system is simulated by considering a road with several coils and an electric vehicle in motion. The coupling factor is calculated by considering the two proposed VA coils and a pair of GA coils. As observed from the results shown in
Figure 12, the calculation of the optimal separation distance between both the GA and VA coils ensures a good coupling factor for all vehicle positions.
Then, the performance of the complete system is calculated. The switching transition between the transmitter and receiver coil is assumed to be instantaneous. This approximation is valid since the transient of the electronic switch is very fast and of the order of nanoseconds; thus, it is negligible compared to the time variation of the coupling factor/mutual inductance that depends on the speed of the vehicle.
The system performances in terms of transferred power, efficiency, and coil currents are evaluated by considering a road length of 8 m covered by about four GA coils. In this numerical test, the input power is kept fixed at
P1 = 7.7 kW. The obtained results are compared with a traditional system composed by a single VA coil in terms of efficiency
η, coil currents
IVA and
IGA, and output power
P2 vs. the position of the EV along the electrified road, as shown in
Figure 13. The results demonstrated as the efficiency of the system and the transferred power are very high for all positions of the vehicle when assuming the solution based on two independent VA coils. The results obtained are very promising as they clearly demonstrate that the traditional configuration using a single VA coil exhibits large variations in current and power during the EV movement resulting in many power holes and over currents in the transmitter coil. On the contrary, by the proposed solution based on two independent VA coils, the power delivered to the EV is almost constant. We calculated the variation γ of the real power
P2 as follows:
where
Pmax is the peak value, and
Pav is the average value of
P2. We found γ = 2.8% for the proposed configuration. This excellent value is very useful for applications as it allows the battery of the electric vehicle to be optimally recharged and also reduces transients and electromagnetic interference. Note that the simultaneous use of both VA coils would not result in an improvement in system performance because only one pair of GA and VA coils at a time has a good coupling factor while the other VA coil does not have a good coupling factor. With the proposed coils design and arrangement, when one of the VA coils is operating in the working region of length
SVA = 130 cm, the other VA has a low coupling factor because it is out of the working region. As observed in
Figure 8 and
Figure 9, out of the working region, the coupling factor and the efficiency drops very fast; thus, it is not convenient to use a second VA coil in this condition. The results obtained show how, with the proposed solution, it is possible to significantly mitigate power instability when the vehicle is in motion, solving one of the main problems of DWPT systems. The logic that can be used for the switching process of the transmitting coils is not reported for the sake of brevity; however, it is the same logic adopted for the traditional dynamic configuration that can be based on both sensors (optic, infrared, etc.) or on the assessment of coil impedance [
20]. A very efficient solution for the activation of the primary coil is described in [
21] where the real time calculation of input impedance is adopted for activating the GA coil only when a VA coil is detected. Similar techniques can be used for the selection of the receiving coil. In this case the decision on which coil should be activated is based on the real time calculation of efficiency. The calculation of the efficiency is made by the control unit and the communication channel between onboard electronics and ground units [
21].
The flowchart of the charging coil selection is reported in
Figure 14. As a starting condition, VA
1 is selected as receiving coil (considering forward drive direction, it will be the first coil that receives energy). VA
1 is used until the efficiency drops below a fixed value of efficiency (η
min). When this occurs, the receiving unit is switched to VA
2, and it is maintained until the efficiency drops again under η
min; then, the receiving unit is switched again to VA
1. This procedure is repeated and permits obtaining the power profile reported in previous figures.