15
Wireless Charging for Electric Vehicles in the Smart Cities: Technology Review and Impact

Alicia Triviño‐Cabrera1 and José A. Aguado2

1 Associate Professor at the University of Málaga, Spain

2 Full Professor at the University of Málaga, Spain

15.1 Introduction

Electric vehicles constitute an important asset for the smart grid. In this new context, their functions are not just limited to the users' transportation, but they can also participate in the smart grid operations actively (Shareef et al., 2016). The expected inclusion of a relevant number of EVs in the near future will alter the power network performance in such a way that they will affect the load flattening, on the fast frequency control and on easing the integration of renewable energy sources.

Its influence depends on the number of EVs, the time their charges are scheduled, and their ability to deliver energy to the grid acting as mobile network storage systems. This last function is considered in the V2G (vehicle‐to‐grid) scenarios where the power transfer is also feasible from the vehicle to the grid. In this sense, the vehicles operate as sources when they are parked and the V2G function is enabled. The EV owners obtain some economical revenues for their participation in this kind of service (Shareef et al., 2016) while the power network also gets important benefits. The V2G procedures, when done in a coordinated way, are expected to improve power network performance and its reliability. As summarized in Shareef et al. (2016), the implementation of V2G mitigates voltage fluctuation in distribution networks, it contributes to improve the power quality, and it also helps to realize a frequency control in a decentralized power supply. In addition, considering a smart home, the EV may potentially serve as an emergency supply for home or business in case of power outages (Kalwar et al., 2015).

The scheduling and the V2G operations need to rely on the battery and on the charger features. Thus, it is important to know how the vehicles charge in order to design the procedures related to both activities. We can classify the way the EV is charged/actively discharged in two broad categories: conductive and wireless charge. In the conductive charge, the EV is connected through a wire to the grid charger. This physical connection implies some safety issues, especially when rain and snow are present (Su et al., 2012). Moreover, it is necessary that the user participate in this action.

Users' intervention is eluded in the wireless chargers as the vehicle can be charged without any physical connection. Additionally, this capability adds the autonomy to the charge process so the charge can be achieved in more situations than those available with the conductive one. For instance, the EV can be charged via wireless while stopped for a short time or even while it is in motion. The range anxiety, which is a drawback conventionally associated to EVs, is alleviated with these new mechanisms (range anxiety is the fear perceived by drivers when considering that their EVs will not be able to fulfil their destinations due to the reduced battery capacities). It is said that wireless charge will allow for ubiquitous charge.

But this convenience is not the only advantage of wireless charge; it also provides electrical isolation, which makes the charge safer from an electrical point of view. In addition, since a lower number of components are manipulated by humans, the robustness of the system is increased (Mi et al., 2016).

On the other hand, the design of a wireless charge should be carefully accomplished as new issues need to be considered. Firstly, as it is supported by an electromagnetic field, it is necessary to meet the standard guidelines concerning the maximum electromagnetic emissions allowed. In this sense, the ICINRP (International Commission on Non‐Ionizing Radiation Protection) guidelines should be followed, and the design of the coils, the electronics and additional modules such as shielding and ferromagnetic components need to be designed altogether for this purpose. Additionally, the interferences produced to other electronics components need to be reduced in order to prevent malfunctions on them (Song et al., 2016).

These are not the only issues to be considered. As a conventional electronic device, it has to deliver a restricted amount of reactive power to the grid and the harmonics injected to the grid must be also reduced. Both conditions are measured by two well‐known electrical parameters: the power factor and the total harmonics distortion. The use of nonlinear components make a wireless charge especially productive in terms of harmonics. Moreover, as the resonant condition is not always guaranteed, the impedance offered by the wireless charge is not purely resistive, which decreases the power factor. There are some contributions in the literature to deal with these two parameters in a wireless charger for EVs, as we will study in the present chapter. We will also address how the electronics in a wireless charger is adapted to enhance power flow bidirectionality so that the EV can feed electricity back to the grid.

Electromagnetic emissions, interferences, and the electrical integration issues can be treated in every wireless charger independently. However, when considering the impact of the electric vehicle into the grid and its role as an active asset, we need to take into account a group of vehicles. In fact, to obtain evident improvements on the power network performance, we need to control the charge of a complete group of vehicles and coordinate this process following a market‐based algorithm. This is the role of the aggregators.

The remainder of the chapter is structured as follows. Section 15.2 reviews the technologies to enhance wireless power transfer in an EV. It also deals with the different ways that this technology can be used. Section 15.3 focuses on the relevant electronics aspects when integrating an EV wireless charger to the grid. Section 15.4 analyzes how the coordinated process of charge/discharge a group of vehicles, that is, the scheduling, is adapted to the particularities of basing on wireless power transfer. Finally, Section 15.5 draws the main conclusions of this work and details some future guidelines that need to be addressed to make this technology progress.

15.2 Review of the Wireless Charging Methods

Wireless power transfer can be supported by multiple technologies. The suitability of this technology for its application to charge EVs depends on some physical constraints (e.g., the distance between the power emitter and the receiver), but it is also affected by the way it is going to be used (i.e., while the vehicle is moving or static). These two issues are analyzed in the following subsections.

15.2.1 Technologies Supporting Wireless Power Transfer for EVs

Wireless charging of an electric vehicle is mainly supported by an electric or magnetic field, which induces voltage in the receiver (installed in the car) when the transmitter (the power source) is electrically excited on some specific conditions.

The first approximation to wireless charging an EV could be the handheld resonant magnetic field charger proposed in Song et al. (2016). To operate, the user needs to insert the charger into the vehicle just in a similar way to fueling it. There is a coil in the handheld component that is on resonant condition with the coil in the vehicle. Since it is supported by inductive wireless power transfer, the efficiency of the power transfer greatly degrades when both terminals are not in physical contact (Musavi & Eberle, 2014). As can be observed, this approach still demands the user's intervention to put close a transmitter coil to a receiver coil, but it adds some electrical isolation in comparison with a conductive charge. This restriction is avoided in the so‐called wireless power transfer systems for EV, where there is a significant gap between the transmitter coil and the receiver. This distance typically ranges from 100 to 300 mm.

The distance between the power transmitter and the receiver along with the frequency of the electromagnetic field involved in the wireless power transfer determines the technology to be used. Specifically, these two parameters decide whether the wireless power transfer is going to occur in a near‐filed area, a mid‐range area, or a long‐distance area. Taking into account the conditions of a wireless charge for EV, near‐field techniques are commonly employed. In this category, wireless power transfers applied to electric vehicles are divided into two groups: inductively coupled transfer and capacitive transfer.

In the inductively coupled wireless power transfer technology, a coil (named primary or transmitter coil) is installed in the pavement and another one (named secondary or receiver coil) is placed in the vehicle. The primary coil is excited with an alternating current, which generates a magnetic field around it according to Ampère's law. When this magnetic field traverses the secondary coil, it creates an induced voltage on this component as explained by Faraday's law. This induced voltage is used to charge the battery as the secondary coil is connected through electronic converters to the battery.

Both coils are adapted with reactive structures so that the whole system is on resonance conditions at a specific designed frequency. In this way, the battery gets the maximum real power from the grid.

Depending on the components of the reactive structures, the compensation topologies can be classified into single‐resonant and multi‐resonant (Villa et al., 2012). Single‐resonant structures add a capacitor to each coil whereas multi‐resonant topologies use multiple reactive components in the transmitter and/or the receiver coil.

Due to their robustness, single‐resonant compensation topologies are frequently used. In this category, there are four compensation topologies: series‐series (SS), series‐parallel (SP), parallel‐series (PS), and parallel‐parallel (PP). The first word stands for the connection between the primary capacitor and the transmitter coil while the second word refers to the type of connection between the secondary capacitor and the receiver coil.

As observed in Villa et al. (2012), inductively coupled wireless power transfer is highly sensitive to misalignments, that is, to displacements of the primary and the secondary coils from their original designed positions. Misalignments can occur in the horizontal and vertical axis and in an angular position. The electrical consequence of misalignment is that the mutual inductance between the two coils differs from the one used in the design process. As a result, the system does not operate under resonance phenomena. To cope with misalignments, Villa et al. (2012) propose a multi‐resonant compensation topology capable of offering similar power transfer efficiency when some misalignments occur. It is based on a series‐parallel compensation network placed in the primary coil whereas the secondary coil is tuned with a series capacitor. Some other approaches opt for keeping single‐resonant compensation networks but incorporate a control mechanism that adapts the system according to the perceived misalignment. In this sense, there are two main strategies to follow: adjust the operating frequency so that the system is on resonant conditions with the real coupling factor or adjust the angle phase of the power converters to maintain the output power.

Inductively coupled wireless power transfer technology is the most mature technology in the market by now. Nevertheless, there are alternative ways to wireless recharge the battery. Firstly, we can rely on highly coupled resonant wireless power transfer, also known as strongly coupled power transfer (Wei et al., 2014). It is perceived as the natural evolution of inductively coupled wireless power transfer, and the main difference is the use of strongly coupled coils. The primary and the secondary coil are built with high quality factors. Operating in the MHz frequency band, they can work under misalignment conditions. However, the current prototypes for EV offer a low efficiency.

Finally, the electric induction/coupling has been recently proposed to transfer power to an EV (You & Yi, 2016). Known as capacitive power transfer (Musavi & Eberle, 2014), it is based on a pair of coupling capacitors. Initially, its applications were limited to low‐power transfer over short transfer distances. Chargers for tooth‐brushes or cellular mobile phones were the initial candidates to benefit from this technology. However, recent progress on this type of wireless power transfer is making this technology also appropriate for the powers involved in an EV charge (Dai & Ludois, 2015). There are other non‐mature proposals for wireless power transfer in EV. We can highlight microwave power transfer, which has also been applied to EVs but is still showing a reduced output power (Kalwar et al., 2015).

15.2.2 Operation Modes for Wireless Power Transfer in EVs

So far, we have detailed how the wireless power transfer from the charger to the vehicle is achieved. These technologies are also adapted to the way the chargers are used. In this sense, we can differentiate the following charging operations:

  1. – Static, when the charge takes place in a specific position and the vehicle is expected to be turned off while a full charge is done. This is the case of home chargers or those installed in parking lots. The chargers can be enhanced with a control system that advises where to place the vehicle in order to avoid the coil misalignments. Algorithms and specific equipment to detect the presence of objects or animals between the power emitter and the receiver are also incorporated to ensure the correct operation of the charger.
  2. – Stationary, when the vehicle is stopped but the motor is still working, and this situation holds over a short period of time that is not enough to get a full charge. This will be of utility when offering wireless charge during the stops imposed by the traffic lights or the taxi/bus/tram stops. Under these circumstances, the transmitter and the receiver could be greatly misaligned and it is not feasible to adjust their positions as that may interfere with other vehicles.
  3. – Dynamic, which refers to the charge that is carried out when the vehicle is moving, that is, when it is conventionally circulating on a prepared road. This system promotes the roadway powered electric vehicles (RPEV; Mi et al., 2016) and it is foreseen to be applied in specific lanes for private use or for public transport such as buses or trams. In other words, it will enhance the ubiquitous charge. By now, it has already been tested in some cities in South Korea (promoted by the OLEV project) and in Spain (through the Victoria project). In order to mitigate the electromagnetic emissions and the unused power, the primary charger is divided into segments, tracks, or pads. The control techniques are responsible to detect the vehicle when it is over the primary coil and to activate the pad once the vehicle has been automatically authorized. Dynamic charging is clearly affected by misalignments, which must be handled by the control techniques. It is necessary to accomplish a study about where to place the transmitters. A work about this concern is presented in Jang, et al. (2016).

Although we have differentiated the static and the stationary charge, works in the literature name these methods indistinctly. The use of stationary and dynamic chargers make the charge available in more situations so that the battery can be charged more frequently. As a result, the battery of the EVs can be downsized, which leads to cheaper battery and lighter components. It is necessary to adopt the agreements to make the three types of charge interoperable so that a vehicle is able to recharge its battery in all the situations.

Table 15.1 summarizes the review of wireless chargers for EV done in this section.

Table 15.1 Summary of the Wireless Power Transfer Technologies

Wireless power transfer for electric vehicles
Supporting technology Charge timing
  1. – Inductive
  2. – Inductively coupled / Magnetic‐Resonant
  3. – Highly resonant coupled
  4. – Capacitive
  1. – Static
  2. – Stationary
  3. – Dynamic, i.e., while in motion

15.3 Electrical Effect of Charging Technologies on the Grid

By now, ICPT technology is the most mature technique applied to wireless charge of EVs. In this technology, a wireless charger for EVs is an electro‐mechanical system composed of multiple blocks as illustrated by Figure 15.1.

Figure 15.1 Scheme of a wireless charger for EV.

The primary side is the one illustrated outside the car, which is built in the pavement. It counts on a power electronics block, which includes a rectifier, a power factor corrector (sometimes ignored in the literature), an optional DC/DC converter, and an inverter. Compensation topologies are incorporated in both the primary and the secondary side to make the coils operate on resonant conditions (Villa et al., 2012). The secondary side stands up for the electronics placed inside the car. It also requires the power electronics converter and the battery to be charged.

It is important to note that power converters are a requirement for wireless chargers in order to make the operation at high frequency feasible. These power converters usually operate in the 20–90 kHz range.

As a conventional electronic device connected to the grid, there are electrical consequences that must be taken into account when integrating a wireless charger into the grid. Concerning these integration issues, the designs proposed in the literature mainly focus on correcting the power factor and on controlling the harmonics introduced into the grid. In addition, it is relevant to analyze how the charger is designed to offer bidirectional wireless power transfer. These three aspects are studied next.

15.3.1 Harmonics Control in EV Wireless Chargers

Harmonics are mainly caused by the nonlinear components of these systems, which are localized in the power converters, and the way these components are controlled.

The mode that harmonics affect the integration of the wireless chargers is diverse. Firstly, harmonics may be responsible for the generation of undesirable magnetic near‐field noise, which could affect adjacent sensitive electronic devices belonging to the wireless charger (i.e., sensors or control systems). This behavior is known as EMI (electromagnetic interference). Harmonics also decrease the efficiency of the system as losses due the harmonic currents occur too. On the other hand, the grid admits a maximum of current harmonics for those devices connected to it. Thus, harmonics provoked by the wireless chargers should be minimized.

A first step to avoid EMI consists of using shielding techniques and ferromagnetic materials to restrict the electromagnetic field in non‐desirable zones (e.g., where other sensible electronic equipment may be) while guiding the electromagnetic field in the direction where the power transfer should be maximized (Kim et al., 2013). This strategy is followed in Song, et al. (2016) for a handheld wireless charger. However, the efficiency of the system decreases because of the losses occurring in the shielding. In addition, the design of the wireless power transfer system becomes quite more complex.

As an alternative, we can include lumped components. Song et al. (2014) propose the reduction of the magnetic fields by incorporating decoupled coils in both the transmitter and the secondary. By this inclusion, the impedance perceived by the harmonics is increased, especially for high frequencies. As a consequence, the harmonic currents and the current distortion are reduced.

A different strategy for reducing the harmonics currents derives in the control of the inverter. In this sense, the work in Kim et al. (2011) proposes the use of spread spectrum clock technology. In this way, the number of harmonics involved in the current of the coils increases, but their total power is reduced. Two mechanisms based on spread spectrum are also proposed in Inoue et al. (2016) to reduce harmonics in inductive chargers. According to the proposal, the implementation of the spread spectrum randomly varies the operational frequency at which the transistors in the inverter switch. The selection of the operational frequency is done using a uniform discrete probability distribution or a biased discrete probability distribution. In this last case, the probability of a frequency depends on the impedance offered by the coils and the capacitors. The main disadvantage of these approaches lies on the difficulty to implement the controller to activate the inverter transistors so that the spread spectrum is achieved.

15.3.2 Power Factor Control in EV Wireless Chargers

Reactive power should be controlled in the wireless chargers. To do so, a power factor corrector (PFC) may be incorporated in the primary side of the system. There are two main options when implementing it: a two‐stage or a single‐stage implementation. The first one, named two‐stage PFC, consists of making it an independent block and placing it between the primary rectifier and the primary inverter. In this case, the usual implementation in wireless chargers is a boost DC/DC converter (González‐Santini et al., 2016). This is a valid option when the output power does not exceed 3.5 kW. Otherwise, the ripple current in the boost capacitor makes itself so relevant that dissipation and efficiency problems appear. Nevertheless, the use of this type of scheme has been applied to a 7 kW prototype, but it requires a DC/DC converter previous to the primary inverter (Deng et al., 2014). Following a two‐stage approach, the work in Shi et al. (2016) designs the PFC based on a SEPIC structure. This DC‐DC (Deng et al., 2014) converter is known to force its switch to support higher voltages than boost converters for the same power outputs. Although this stress is a clear drawback, SEPIC allows for a complete regulation of the output voltage; that is, input voltage can be augmented or diminished. In order to cope with this stress feature, Shi et al. (2016) propose the use of two interleaved SEPICs as PFC. Thus, the control is more complex, and the application is still restricted to a 500‐W prototype.

Including a PFC also minimizes the harmonic distortion, which has been reported in Kitamoto et al. (2016) and in Kitano et al. (2015). The authors in these works have checked this relationship by including a simple and independent PFC in their designed wireless charger. In particular, their design relies on a single‐ended inverter operating with a PWM (pulse width modulation) control.

Alternatively, it is also possible to integrate the PFC in the primary rectifier leading to a single‐stage PFC. In contrast to these previous works, Hsieh et al. (2016) opt for an integrated PFC based on a bridgeless structure (Siu & Ho, 2016). A single‐stage PFC is also employed in Chinthavali et al. (2015).

A different approach for building a PFC is presented in González‐Santini et al. (2016). Based on a Z‐source network, they simultaneously control the output power and the power factor delivered by the front stage without the need of additional switches. In fact, a Z‐source network is built based exclusively on reactive components with no control system applied to them. This makes the proposed PFC more reliable. Its control is indirectly related to the inverter and its duty cycles.

On the other hand, Keeling et al. (2010) address the power factor corrector from a different point of view. These researchers propose the design of a LCL compensation topology to control the reactive power delivered back on the utility. The study is theoretical, and there is no evidence about how this strategy behaves in a real prototype where the designed values are different from the values of the real components due to tolerances.

15.3.3 Implementation of Bidirectionality in EV Wireless Chargers

There is another concern on integrating wireless chargers into the grid: bidirectionality. Obviously, implementing bidirectionality has a clear effect on the grid as the vehicle is able to act as an energy storage system capable of transferring power back to the grid when needed. Next, we will review how the bidirectional chargers are implemented in the most outstanding proposals.

A symmetrical scheme is employed in Lee & Han (2015) so that the primary AC/DC and the secondary DC/AC converters are exactly the same. The symmetry is also maintained in the compensation structures and a series‐series topology is followed. By these conditions, the control is eased. These structures operate at the same resonant frequency, which is kept constant, and the power output is regulated through the duty cycles of the above‐mentioned power converters. Particularly, there are two current control systems based on a self‐resonant PWM. The one in the primary controls the power flow from the grid to the battery whereas the second system is in charge of setting the duty cycle of the secondary DC/AC converter. The primary controller needs some measurements concerning the battery, so a wireless communication module is installed in the charger. A Bluetooth‐based communication module is the usual implementation for this kind of information transfer.

As can be observed, the previous work relies on the modification of the duty cycles to control the wireless charger. The next proposal uses this parameter, but it also adjusts the operational frequency (Bojarski et al., 2014). In order to minimize the losses due to the nonlinear properties of the coils, the phase between the voltage provided by the primary inverter and the voltage of the secondary AC/DC converter is adjusted. Specifically, Bojarski et al. (2014) developed an analytical expression to maximize the system efficiency according to this phase. A previous publication applied this approach to CLCL compensation topology (Nguyen et al., 2014).

In Madawala & Thrimawithana (2010), the wireless charger relies on a parallel‐parallel compensation topology as the authors argue this is the most convenient reactive structure to maintain the primary current constant independently of the number of loads. The authors derive a mathematical framework to demonstrate that the sense of the power flow can be easily controlled by the phase angle between the primary voltage (the one provided by the primary inverter) and the secondary voltage (the one set by the AC/DC converter). A matrix‐based approach is followed in Thrimawithana & Madawala (2010).

15.3.4 Discussion

The implementation of a PFC, the harmonics reduction, and the bidirectionality are essential to obtain efficient V2G wireless chargers. Paying attention to the research work, we can conclude that their adequate design greatly relies on the proposal and use of appropriate control algorithms. PFC in EV wireless chargers and bidirectionality are more mature techniques than the control of harmonics.

15.4 Scheduling Considering Charging Technologies

The potential benefits of massive EVs deployments are widely recognized (Agency, 2016) (Agency, 2016). Particularly, EVs allow for a transportation sector with low carbon emissions and, due to their use patterns, they can also operate as flexible electric loads providing ancillary services to distribution grids and enabling the integration of renewable energy (Dallinger & Wietschel, 2012)(Dallinger & Wietschel, 2012). Despite these potential benefits, the EV industry is still facing several important challenges for massive EVs deployments. The first one is the limited EV driving range, which varies between 100 and 180 km for light‐duty EVs. The second is related to the long charging time of EVs; full charge can take from 30 minutes for fast chargers up to 8 h for domestic chargers. The third challenge concerns the integration of EVs in the power system since it requires the development of charging infrastructure conveniently designed to comply with some integration requirements as explained in the previous sections. Moreover, uncontrolled simultaneous charging of a large number of EVs can result in voltage excursions or distribution line congestion increasing the need for capacity reinforcements both in the distribution grid and in the electricity system related to buildings and charging sites. These enforcements are expensive and will probably have small utilization factors. Thus, it is important to control when and for how long the EVs are charged. This is what scheduling is about. Scheduling defines charging coordination mechanisms, which must cope with a high degree of uncertainty as users' behavior is not predefined. Aggregators will be responsible for a fleet of vehicles and control their charge/discharge operations following a market‐driven algorithm. For that, the aggregators and the electrical network must exchange information while the electrical network must manage updated information about the status of its assets. Communication techniques specifically designed for smart grids environments are proposed in Khan et al. (2016, 2017).

Many studies suggest that adopting proper scheduling and planning strategies are necessary to mitigate the aforementioned limitations. Charging cost and the convenience level of the users are among the most relevant criteria for evaluating the adequacy of charging and scheduling algorithms. While there are several studies that address user convenience level and optimizing charging cost (Tang et al., 2014; Shao et al., 2016) separately, only a few studies consider both factors as the underlying merit factors (Tusha et al., 2014; Malhotra et al., 2016). Tang et al., (2014) try to minimize the charging cost for the parking station owner whereas a similar research in Shao et al. (2016) minimizes grid generation cost.

Scheduling and planning strategies can be classified into two main categories:

  1. – Charging infrastructure network design strategies: EV owners prefer charging activity to be subordinated to the mobility goal and not vice versa. As a consequence, users should plug in their EVs within walking distance from their destination. Apart from domestic chargers at households and work places, numerous public charging stations will be necessary for massive EV penetration. Since massive simultaneous charging of EVs may lead to problems in distribution grids and in the electric infrastructure at charging sites, a crucial question is to determine locations and sizes of charging stations (He et al., 2013) (He et al., 2013) where up to 70% of the incremental investment costs can be avoided with optimal planning strategies. These strategies usually consider a planning expansion problem where size and location are determined subject to technical and operational constraints. Some authors have also proposed to complement the charging infrastructure with separate battery storages, potentially in combination with local generation such as solar photovoltaic panels (PV).
  2. – EVs smart (or controlled) charging and routing strategies. One of the simplest charging strategies is the dumb charging strategy, where the vehicle owner plug in and charge once she arrives home during the evening. This behavior may add some extra load during peak demand periods. However, assuming that vehicle‐to‐grid (V2G) technology is available and, under the a profit maximization strategy, the EVs are to be charged when the electricity price is low and discharge back to the grid at high prices subject to mobility constraints. The idea is that since the energy price pattern often follows similar patterns as consumers demand, this strategy helps to flatten the load curve. Usually, this type of problem can be formulated as follows: given a fleet of EVs, a set of tours to be processed by vehicles, and a charging infrastructure, the problem aims to optimize the assignment of vehicles to tours and minimize the charging cost of EVs while considering several operational constraints mainly related to chargers, the electricity grid, and EVs driving range. As for the routing problem, with limited cruising range, long recharge times, and energy recuperation ability of battery‐powered EVs, the problem is now to find energy efficient routes, rather than just fast or short routes. Several works that address this or similar problems can be found in López et al. (2013, 2015).

The particular case of wireless technology makes the charging and scheduling problem even more complex. Although recognized as a promising technology, it currently faces some technological barriers. This technology is available but not fully mature, and it is usually more expensive than its conductive counterpart. In terms of electrical losses, it achieves lower efficiency, particularly for the dynamic version. In order to become a relevant actor within the EV industry, it will require a relatively high critical mass of electric vehicles since installation and maintenance for the long term is still an issue that has not been well addressed.

As of today, there have been only a few scientific contributions addressing the charging and scheduling of wireless EV (Theodoropoulos et al., 2016).

From a mathematical point of view, the inclusion of a static wireless charge is nearly equivalent to a conductive one. It would only differ on the efficiency, which will be lower for a wireless charger, and it would be also associated to a low level of uncertainty due to potential misalignments. In contrast, it is a requirement to consider the particularities of the stationary and dynamic charging when including them in the scheduling algorithm. These approaches force the charge to occur over short times and frequently. Thus, both conditions must be modelled in the scheduler. Aggregators should coordinate to exchange relevant information about the incoming members and cope with the frequent changes in their fleet. More recently, the potential of demand side management during dynamic wireless charging of EVs has been studied in Theodoropoulos et al. (2016).

15.5 Conclusions and Future Guidelines

This chapter has reviewed the wireless power transfer applied to the charge of electric vehicles. When considering the operation of a wireless charge independently, we have studied it as a stand‐alone electromechanical device. In particular, we have analyzed its effects on the grid in terms of harmonics and how the power factor can be adjusted so that reactive power is not delivered back to the power network. As EVs are foreseen as an important asset for future smart grids, we have also studied how they can be adapted to ensure a bidirectional wireless power transfer. Concerning these electric aspects, we can conclude that there is not a definitive solution that copes with these three functionalities. Control algorithms are foreseen as vital, as they play an important role in the implementation of these three features.

Additionally, we have studied the scheduling algorithms that control the timing of the charging process in a group of EVs. Wireless charging imposes some new challenges to the definition of scheduling algorithms. From a mathematical point of view, the static wireless charge of EVs is a mere extension of the classical conductive‐based scheduling algorithms. The only difference to take into account is that the efficiency of the wireless chargers is usually lower than the conductive ones. However, wireless charge may be performed in other operation modes as in a stationary way (done in short times while the car is temporarily stopped) or dynamically (while the car is moving). Scheduling algorithms should characterize the new dynamic loads that these behaviors impose. As of today, there are still few works dealing with this issue.

After the study done in this chapter, we can conclude that there are still some open issues that need to be solved to improve the integration of wireless charging technology into the grid. We outline the following ones:

  1. – There is no standard for the operational principles of wireless chargers. The SAE (Society of Automotive Engineers) is currently working on finalizing a standard, for which there is a strong demand, so that wireless charger operations can be performed in all types of wireless chargers in the future market and for all operation modes.
  2. – It will be desirable to have interoperable conductive and wireless charge so that the user could use the one that is available, independently of the technology on which it is built.
  3. We have identified three types of charge according to the time and conditions of charge. It is necessary to adopt the agreements to make the three types of charge interoperable so that a vehicle is able to recharge its battery in all the situations.
  4. – There is a demand to design specific aggregators to support wireless charge of EVs. In this sense, the development of these agents for static wireless charge is quite straightforward, but some new aspects need to be carefully considered for stationary and dynamic chargers. Among these aspects, aggregators should be defined for a restricted area. Aggregators should exchange data relative to vehicles when the vehicles under their control move to a different area.

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