A Comprehensive Analysis: Integrating Renewable Energy Sources With Wire/Wireless EV Charging Systems for Green Mobility

Integrating renewable energy resources into the existing generation capacity can handle the additional load resulting from the EV charging system, even when the majority of the EVs are charged during off-peak hours. This paper comprehensively analyzes EV charging methods with a particular focus on both grid-based direct charging and the utilization of renewable energy sources. However, the most practical approach for EV charging is through large-scale grid-based renewable energy stations. This study outlines the significant challenges facing the application of EV chargers. Overcoming these challenges is vital for the widespread adoption of new technologies such as Wireless power transfer charging systems in the EV sector. Wireless charging technology still faces several issues, such as energy loss during transmission, alignment between the coils, and quick and safe power delivery. One of the significant challenges facing wireless chargers is the need to optimize efficiency while ensuring convenient and reliable charging as outlined in this study. Different wireless charger configurations are presented for stationary EV charging systems by incorporating photovoltaic systems into charging stations and supervisory bases. The application of series-series wireless charger system as an emerging technology for EV chargers is analyzed and simulated as a recommended solution for EV wireless chargers. The system overall efficiency is approximately 98%. The charging system can operate reliably by mitigating the effects of load and grid disturbances.

SECTION I. Introduction

The global community has reached a crucial juncture in its shift toward a society with reduced carbon emissions. Climate change aims to mitigate the release of harmful gas emissions [1]. In view of these conditions, many nations are actively implementing renewable energy measures [2]. Owing to the ever-increasing use of fossil fuels, car users are interested in vehicles that are powered by alternative energy sources [3], [4], [5]. Electrified transportation has emerged with new technologies and a wide variety of friendly charging technologies [6]. The PV system’s capacity to harness energy presents a significant technological advancement that offers the potential to eliminate wiring and the need for battery maintenance across various critical applications. This breakthrough enables the deployment of self-powered devices, streamlining maintenance requirements [7], [8]. Globally, the integration of photovoltaic power generation into smart grids has experienced a significant increase, with an annual growth rate surpassing 40% over the past two decades [9]. Electrifying conventional vehicles (CVs) offers a highly promising solution for mitigating pollution levels [10], [11]. Additionally, electric vehicles (EVs) provide significant benefits in terms of energy conservation, emission reduction, and grid security. With the growing adoption of EVs and the demand for wireless charging systems, wireless power transfer (WPT) has emerged as a highly suitable solution for EV battery charging. The development of WPT systems for EVs has become a feasible option for both stationary and mobile transportation needs. This makes EVs more efficient and cost competitive. Wireless electric vehicle chargers (WEVCs) have the potential to eliminate the need for traditional plug-in charging methods, thereby addressing the challenges associated with charging EVs. Moreover, the concept of WEVCS, such as Vehicle-to-Grid (V2G) and the Inductive Wire/Wireless Charger System (IWWCS), are reviewed in relation to other existing technologies [12], [13].

In WPT, capacitive or inductive methods are commonly employed. Capacitive power transfer (CPT) offers advantages such as cost-effectiveness and compact size at lower power levels. CPT enables greater transmission distances than does inductive power transfer (IPT) but with lower efficiency. Inductive methods, including resonance systems, are favored for their ease of implementation, efficiency at close distances, and widespread adoption in high-power applications. IPT ensures safety and practicality in commercial and industrial settings [14]. In EV charger applications that utilize IPT, power is wirelessly transferred from an underground transmitting coil to a receiver coil beneath the vehicle. IPT technologies employ specially designed coils, namely, power transmitters and receivers. The authors of [15] explored the operation principles, various topologies and coil designs that are employed in WPT systems. Typically, there is no requirement to physically connect EVs to the wireless charging systems integrated with them. These coils resonate through the inclusion of parallel/series connecting capacitors. This resonance strengthens the electromagnetic field between the loosely coupled coils, enabling efficient high-power transfer [16]. Many precautions are necessary to enhance power transfer between the transmitter and receiver coils, thereby increasing the transmitted power and efficiency. In an IPT system, the coupled coils are often compensated to address the inductance issues and to maximize the transferred power between the primary and secondary sides. Optimal compensation selection is crucial for maximizing the power transfer efficiency and system performance. The four most widely used compensation methods in WPT can be categorized into parallel-parallel (PP), parallel-series (PS), series-parallel (SP) and series-series (SS), and these topologies are discussed in [17].

These compensations emphasize the importance of evaluating different compensation topologies based on the basis of system’s transfer characteristics. The SS topology is adopted to realize the WPT protocol over the other topologies [18]. The proposed SS design outperforms the other topologies because: (1) There is a minimized eddy current in the vehicle frame. (2) There is a low electric stress in the semiconductor components. (3) A low operational frequency, typically approximately 10 kHz, is utilized to prevent electromagnetic interference with the on-board electronics of the vehicle. The relationship between the system efficiency and coil parameters, such as the gap and the coupling with an improved efficiency of approximately 96%, was discussed in [19] and [20]. This paper discusses studies related to EV charging topologies, and their impacts on the electrical network. Additionally, reviewing some typical studies managing the challenges facing EV charging systems and how these studies are analyzed to mitigate these challenges are reviewed. Integrating PV systems into EV charging systems represents an urgent solution to meet the increased demand on the electrical network due to EV chargers. An analysis and comparison of the basic WPT compensation typologies that are employed for the EV charging system are thoroughly analyzed in this paper. The major contributions of this study are: (1) An analysis of some typical studies and reviews related to EV charging methods is presented in Table 1. (2) An analysis of some typical studies addressing the challenges facing EV charging systems and their proposed analysis are detailed in Table 2. (3) Summary of the status of EV charging technologies and their corresponding criteria, encompassing various types of EVs, charging modes and their levels. (4) Determination of charging scenario impacts on the grid. (5) Comparison of commercial secondary battery models and types. (6) A comprehensive review of converter topologies, including both on-board and off-board AC-DC converters, which are used for charging systems. (7) Designs of EV charging stations based on renewable energy sources. (8) Analysis of the SS compensation configuration, relation between transferred power, efficiency and system parameters.

TABLE 1 Reviews of Typical Studies Related to EVs Charging Topologies, Impacts, and Integrating PV Systems

TABLE 2 Review of Typical Studies Handling the Challenges Facing EVs Charging System and Their Proposed Analysis

SECTION II. Photovoltaics: System Design and Modeling

A. Solar PV Generation

Typically, solar forecasting generates predictions for both solar irradiance and PV output power. The characteristics of PV panels are crucial for accurate modeling and forecasting of the generated solar energy. This section clarifies several significant aspects of solar forecasting, such as relevant variables and the prediction horizon. Standardized performance evaluation metrics are introduced for the development of solar energy predictors. The maximum power output of the PV solar panel is presented by (1) as detailed in [51]: $$\begin{equation*} P_{R}=\eta SI\left [{{ 1-0.05(t_{0}-25)}}\right ] \tag {1}\end{equation*}$$ where $\eta$ represents the conversion efficiency (%) of the solar cell panel, S represents the array area (m²), I represents the solar irradiance (kW/m²), and $t_{o}$ represents the ambient temperature (^oC). Fig. 1 represents the architecture of the PV system connected to the boost circuit with its MPPT algorithm and load. The PV system exhibits varying performance under different conditions, specifically when subjected to nonuniform solar irradiance, as observed in Fig. 2a. Fig. 2b shows the PV behavior corresponding to the effect of the panel temperature on both the PV current and the output power respectively. Fig. 3 displays the PV output power, current, and output voltage of the PV array under various solar irradiance level conditions. The proposed system encompasses not only all control blocks for MPPT but also effectively addresses the impact of the parasitic elements. Therefore, incorporating the parasitic resistance ($r_{L}$ ) of the inductor and input capacitor $C_{pv}$ not only lowers the design cost but also enhances the tracking efficiency. The system input capacitor, with a range from nF to $\mu$ F, effectively reduces the ripple of the module output voltage to under 10% for switching frequencies in the range of $\approx ~10$ kHz as detailed in [5] and [52]. Utilization of the duty cycle d in modeling DC/DC converter enhances the convergence speed of the MPPT algorithm. Additionally, modeling of boost converter including inductor current $I_{L}$ and output voltage $V_{o}$ can be expressed as follows: $$\begin{align*} C_{pv}\dfrac {dV_{pv}}{dt}& = (I_{pv}-I_{L}) \tag {2a}\\[1ex] L\dfrac {di_{L}}{dt} & = \Big (V_{pv}-r_{L}i_{L}- (1-d)V_{o}\Big) \tag {2b}\\[1ex] C_{o}\dfrac {dV_{o}}{dt} & = (1-d)(i_{L})-i_{o} \tag {2c}\end{align*}$$

FIGURE 1.

PV array connected to a boost circuit with its MPPT control method and load [53].

FIGURE 2.

(a) PV output power under daily solar irradiance: uniform and nonuniform, (b) PV output current and power versus PV voltage under different temperatures [53].

FIGURE 3.

PV module $P-V$ and $I-V$ characteristics [53].

B. Modeling of Photovoltaic Panels

The PV cell model, as depicted in Fig. 4, can be represented as follows: $$\begin{equation*} I = I_{pv}^{cell} - \underbrace {I_{rs}^{cell} \left [{{ exp \left ({{ \frac {qV_{pv}}{akT} }}\right) -1 }}\right ]}_{I_{d}} \tag {3}\end{equation*}$$

FIGURE 4.

Equivalent model of a PV cell [53].

However, to accurately simulate the real-life performance of PV arrays, which comprise multiple interconnected cells, additional parameters need to be considered [53], [54]. Owing to its accuracy and simplicity, this practical model is extensively utilized. Accordingly, the model presented by (4) can be used to define the behavior of the PV array. $$\begin{align*} \begin{cases} \displaystyle I = I_{pv}-I_{rs}\left [{{ exp \left ({{ \frac {V_{pv} +I r_{se}^{pv}}{aV_{th}}}}\right) -1 }}\right ] - \frac {V_{pv}+I r_{se}^{pv}}{r_{p}^{pv}} \\ \displaystyle I_{pv} = \big (I_{pv}^{n}+k_{sc} \Delta T \big) GG_{n}^{-1} \\ \displaystyle I_{rs} = \frac {I_{sc}^{n}T_{n}^{3}T^{-3}}{exp\big (V_{oc}^{n}a^{-1}V_{th}^{-1} \big)-1} exp \left [{{ \frac {qW_{g}(T-T_{n})}{akTT_{n}} }}\right ] \end{cases} \tag {4}\end{align*}$$

The parameters of the PV panel, according to the equivalent circuit shown in Fig. 4 and the models given by (3) and (4), are listed in Table 3.

TABLE 3 PV Module Parameters

C. Principle of the MPPT-Based Technique

To increase the efficiency of PV systems, it is typically necessary to monitor and adjust the MP of a PV array. The MPPT enables harvesting of the maximum output power $P_{max}$ under specific temperature and irradiance conditions to determine the optimal voltage $V_{m}$ and current $I_{m}$ at which the PV system operates most efficiently. Focusing on PV power maximization, a cross-correlation (CC) algorithm is implemented to obtain the MP from the PV arrays as introduced in [53]. The CC plays a crucial role in signal processing to gauge the likeness between two signals or the degree of self-resemblance when one signal is shifted in relation to another [55], [56].

SECTION III. Charging EV From the Grid Based on Renewable Energy

There is a significant amount of additional electricity that is generated, reaching a few TWh/year, which is based on renewable sources. This additional generation could replace all the energy in traditional resources used for personal transportation by implementing a solar roof/parking system. With the growing focus on conserving conventional energy sources, integrating renewable energy has emerged as a viable solution in the current scenario while minimizing the overload to the grid. Charging stations powered by solar energy have emerged as a sustainable solution for the growing demand in EV charging infrastructure. These charging stations can be integrated into existing infrastructure, such as parking or highways.

A. Integration of an EV Charging System in the Planning of a Power System

The EV charger poses significant challenges to power system planning, operation, and to the electricity market because of the increased load owing to the large-scale EV charging stations. Even at a low-level of EV ownership, issues persist in the local power grid because of the irregular and nonequilibrium distribution of EV chargers. Numerous issues arise from the unscheduled (uncontrollable) charging of EVs, including overloading, power losses, voltage sags, component aging, and power system reliability. Consequently, it is imperative to conduct ongoing and future research to address and alleviate the aforementioned issues. Ensuring the reliability of the power supply and minimizing the risk of power loss and voltage drop can be achieved through careful planning of power grids, including the integration of charging facilities. By optimizing the design of charging facilities, the reliability of the power supply can be enhanced, leading to improved efficiency in the utilization of EVs. Therefore, it is crucial to consider various factors such as charging type, traffic congestion, and the economic issues of power grid development during the planning phase [21]. Considering the EV distribution, the annual operation of charging stations with maximized revenue can be achieved by considering the structure of the power grid, and the availability of public transportation networks [22]. To address the problems of optimizing the location and sizing of EV charging stations, a comprehensive objective function that includes various elements such as staff salaries, initial construction expenses, equipment costs, and operational expenditures was developed in [57] and [58]. This approach involves examining the power, transportation systems and utilizing a model for determining the optimal location of a centralized charging station to address the notion of “concentrated charging and unified delivery”. As peak loads usually occur during evening periods, voltage sags at the distribution system are expected. Since EV chargers have many impacts on the power system which affect the supply quality at the receiving end, an assessment was proposed in [23] and [59] to evaluate the impact of EV chargers on the power supply quality and security. Charging EVs at peak load hours increases the voltage distortion and harmonics. The power system faces limitations due to the power transfer capacities and other considerations under the growth in charging load. The additional load could impact the life cycle of the main equipment of the power system. With a 10% penetration rate of EVs for the grid, distribution power transformers are likely to become highly overloaded [21]. In the study presented in [60], the effects of the charging load on the distribution network and energy consumption were quantified across various penetration rates [60]. Under conditions of low power line loading, the charging load of EVs can improve the operational efficiency and economic viability of the line. By using smart chargers under high penetration of EVs, large-scale EV chargers have little impact on the grid without markedly influencing on the reliability of the power supply. The new additional power capacity is directly related to the charging mode of EVs and is minimized when V2G is considered. The effective growth of the EV sector relies on the presence of four essential networks: power, transportation, charging, and information [21]. The network planning architecture depends on the analysis of the charging characteristics with network coupling including the EV integrated charging system, as shown in Fig. 5.

FIGURE 5.

Planning EV integration across multiple interconnected networks [21], [59], [60].

B. EV Charger Level Configuration

EV owners typically experience charging anxiety because their daily driving range is typically less than 100 kilometers. The on/off-board EV charger illustrated in Fig. 6 uses the IEC standard which is categorized into three levels: Level 1, Level 2, and Level 3, as elaborated in [61] and [62]. The EV battery pack can receive DC power from the off-board charger as designed by level 3, which is known as the DC fast charger and Tesla supercharger. Opting for an off-board charger configuration is generally a preferable option, as it mitigates charging anxiety by providing higher kW transfer and reducing the weight in this type of EV. Conversely, on-board charging adds weight to the vehicle but provides a lower kilowatt (kW) transfer rate. Several constraints, such as weight, space, and cost pose limitations regarding the ability to transfer high power when single-phase on-board chargers are utilized [63]. Consequently, the on-board charger leads to a longer charging time than does the off-board charging configuration, which is a persistent issue for the car owners. As the charging level increases, the charging process accelerates since high power is transferred to the EV. The acceptance of the power levels by each EV is determined through communication between the vehicle and the charger. In Table 4, the charging levels of an EV refer to the speed and power at which the vehicle’s battery can be charged under the charger standards given in Table 5.

TABLE 4 EV Charging Levels [24], [64], [65]

TABLE 5 EV Chargers’ Standards [24], [64], [65]

FIGURE 6.

EV charging methods.

C. International Technical Standards for WPT

The ISO (International Organization for Standardization) and IEC (International Electrotechnical Commission) have developed an international standardization system, with national bodies as members participating in the development of international standards through technical committees. These committees collaborate on standards in fields of mutual interest, with involvement from other international organizations as well. Regarding WPT for EVs, the ITU, IEC, ISO and SAE developed a series of standards to enable standardized communication between EVs and charging infrastructure. This communication is necessary to optimize energy resources, production, and efficient billing systems. In addition to these standards, the ITU and IEC Technical Committee is actively working on developing a variety of WPT-related standards. These international standards help to ensure the safety, interoperability, and compatibility of WPT systems, paving the way for their widespread adoption and application across different industries and domains, as detailed in Table 6. In summary, the development of these standards for WPT charging communication, and the ongoing evolution of these standards represent the key information needed to enable new capabilities for the EV WPT charger.

TABLE 6 International Technical Standards for WPT Used for EVs (Source: IEEE WPT)

SECTION IV. EV Charging Modes

The charging power of EVs is determined by several factors such as power equipment capacity, location, charging time, expense, and impact on the utility network as detailed in Table 7. The implementation of charging infrastructure and EV supply equipment is a challenging task. Numerous issues require resolution: such as the standardization of charging infrastructure, demand policies, and regulatory processes. The availability of appropriate charging infrastructure significantly decreases the necessity and expenses of on-board energy storage system (ESS). The primary elements of supply equipment include cables, charging for residential or public use, power outlets (plugs, connectors), and safety devices. These components are classified into two configurations: One of these configurations is customized as a cord set, whereas the other type is a wall-mounted or pedestal box. These configurations differ depending on the country, and other factors such as grid standards, voltage and frequency [79], [80]. Table. 7 outlines the power levels for EV chargers.

TABLE 7 EVs Charging Power Levels [24], [64], [65], [81]

Choosing an external charger can serve as an efficient strategy for mitigating charging anxiety, as these chargers typically offer higher kW transfer rates and minimize the burden on the vehicle in terms of weight. Importantly, the EV itself determines the amount of power it can accept, ensuring that there is no risk of plugging into a charger that supplies more power than the EV can handle. The availability of charging levels may vary depending on the EV model and regional standards. The EVs utilize three main charging methods: Battery swap station (BSS), wireless charging, and conductive charging [82] as shown in Fig. 7.

FIGURE 7.

Battery charger methods [25], [82], [83].

1) Battery Swap Station (BSS) or Battery Exchange

The battery swapping method, or battery exchange, involves paying a monthly rental fee to the owner of the BSS for battery usage. One major advantage of this approach is that drivers can rapidly replace a depleted battery without leaving the vehicle. Moreover, the battery kept at the station can be repurposed for second-life use and can participate in the V2G initiative. Therefore, the station could possess a specific battery model, whereas the vehicles might adhere to different battery standards. The utilization of this method demands numerous costly batteries and a significant storage space, which could necessitate the acquisition of costly real estate in a busy location. Extending battery life is facilitated by the slow charging rate utilized by the BSS. The battery swapping station can be integrated into the charging/discharging system as a basic unit commitment [27]. Despite the advantages of BSSs, the rental fees charged by BSS owners for EV batteries can make this charging method more expensive than traditional fueling. The integration of renewable energy such as solar and wind energy with a BSS is a relatively straightforward process and can reduce the energy cost.

2) Wireless Power Transfer (WPT)

The use of WPT technology in EVs has garnered interest because of its ability to enable safe and convenient recharging of the EV. This technology relies on electromagnetic induction and employs a pair of coils: a primary coil positioned on the ground and a secondary coil installed within the EV. Moreover, it does not necessitate the use of a typical connector (although a standard coupling technology is required), and it has the ability to charge the EV while it is in motion. Additionally, the WPT may encounter eddy current loss in the EV body if the transmitter is not deactivated. For effective energy transfer between the transmitter and the receiver coils, real-time information communication is necessary, which may be subject to latency issues.

3) Conductive Charging (CC)

EVs need to be physically connected to a charging inlet to use conductive charging, which offers various options (such as Level 1, Level 2, and Level 3) and boasts high charging efficiency owing to its direct connection [25]. A public charging station typically uses charging Level 2 and Level 3. Compared with the first level (i.e., Level 1). These higher levels influence the distribution system, such as system reliability, voltage deviation, transfer/power loss and the lifespan of transformers [84]. A complex infrastructure and access to the grid are needed, and a standardized connector/charging level is necessary. By utilizing the vehicle’s battery, conductive charging offer a V2G capability while also mitigating grid loss, providing active power, enabling reactive power compensation, maintaining voltage levels, and preventing grid overloading [85]. Intensive communication between the vehicle and grid is necessary for V2G technology to function. Additionally, the frequent charging and discharging involved in V2G operations can shorten the lifespan of the battery. The charging station types: BSS, WPT, and CC, are also outlined in [26]. In applications that demand rapid charging, such as buses and trucks with high battery capacities, two charging modes are employed:

1. Depot Charging: Depending on the specific needs, overnight or depot charging can support slow and fast charging capabilities. Typically, this type is installed at the terminus of a line and utilized for charging during nighttime hours. As a result, slow charging is often preferred because of its minimal impact on the grid [83].

2. Pantograph Charging: This type of charging infrastructure is utilized for applications that require rapid charging and more significant power and have high battery capacity, such as buses and trucks. For this charging type, the investment required for the bus battery is reduced, resulting in lower overall bus investment costs. However, the cost of the pantograph charging infrastructure increases.

The pantograph charger is classified into two distinct categories:

• Top-down Pantograph: The charging system is installed on the roof of the bus; thus, it is referred to as an off-board top-down pantograph. This approach delivers high DC-power. This charging type has already been successfully implemented in Singapore, Germany, and the USA.

• Bottom-up Pantograph: This type is known as an on-board bottom-up pantograph which is suitable for buses already integrated with charging equipment.

SECTION V. Scale of Charging System

The chosen site for this study is Mansoura-Egypt. The annual average solar irradiance for a fixed flat solar panel in Mansoura is estimated to be 1924 kW/m² by using the solar irradiance measurement atlas [86]. The chosen solar panel provides $230~W$ , where $P_{m}$ and $\eta _{m}$ are $230~W_{p}$ and 17.3%, respectively. According to the specified solar resources, the average power of the panel is 172 W/m², or 30 kWh/m²/month for a panel area of 1.33 m² under normal operating conditions. If each car requires 20 kWh/day =109 kms/day for the medium EV-Tesla Model 3 charger or 25 kWh/day =109 kms/day for the large EV-Tesla Model S charger, a PV system of approximately 550 panels could provide power to charge over 50 cars to 70% SoC per day [87]. The proposed solar photovoltaic system will generate an average of 0.3 GWh of energy per year from an average sunlight duration of 6 hours/day. To determine the number of solar panels required to charge an EV with a power consumption of 20 kWh/day on the basis of the average daily sunlight hours at the selected location, the following steps are applied:

1. Step 1: Calculation of the total energy required by the EV per day: 20 kWh/day.

2. Since the specific details of the solar panels chosen in this paper are listed in Table 3, a standard panel size of 1.33 m² is used. Additionally, an average solar irradiance of 5.2 kWh/m²/day is estimated for Mansoura via measurement. The solar panel output power = efficiency*panel size*solar irradiance =0.173* 1.33 m²*5.2 kWh/m²/day = 1.196 kWh/day.

3. Step 3: Calculation of the number of solar panels needed, where the number of solar panels = total energy required by the EV per day/energy generated by a single solar panel per day. Therefore, the number of solar panels = 20 kWh/day/1.196 kWh/day $\approx ~16.7$ . Since no fraction of a solar panel is used, this value is rounded to the nearest whole number. Therefore, approximately 17 solar panels are needed to charge an EV with a power consumption of 20 kWh per day. These calculations are estimations, and actual requirements may vary based on factors such as panel efficiency, sunlight availability, and other environmental conditions.

The scale of the system was evaluated with two general concepts. The first possible concept is to create a system that can charge all EVs in a residential area, where car owners need to park their vehicles and connect them to a central location. The second option would be to design the system to be situated on the owner’s property, enabling it to supply power to one or multiple vehicles parked in the home garage.

A. Onsite Energy Storage for an Off-Grid Charger

For the off-grid charging stations, the batteries are charged from renewable energy sources such as the PV panels. A charge regulator is used to manage energy transfer and to safeguard the station’s batteries by preventing overcharging. Fig. 8 illustrates the structure of a multi EV charger.

FIGURE 8.

Structure of a multi EV bidirectional charger.

The controller is activated at regular intervals from a specific point, with the purpose of lowering the average current of the batteries within the permissible threshold voltages. Using large station batteries allows more energy to be stored. The energy storage mechanism is possibly the most critical and intricate design choice.

B. Off-Grid EV Charging System

This section discusses the feasibility of a standalone charging system. No improvements to the grid infrastructure are required for the off-grid system to operate effectively. This type of charging system has various advantages and disadvantages in comparison to on-grid large-scale renewable energy plants. It is self-contained, isolated, and independent. However, owing to the smaller size of the system compared with a grid-based solution, the cost of charging a single car is likely greater, primarily because of the absence of economies of scale. The larger the size of the PV system, is, the lower the cost per installed watt of PV power. The physical dimensions of off-grid solar-based solutions might pose a challenge, as the average car owner might not have access to ample sunny land areas. However, solar power has the potential to generate enough energy to charge a vehicle on a daily basis. It is estimated that an average car consumes 16 kWh/day or 480 kWh/month [88], [89].

SECTION VI. Impacts of Uncoordinated EV Charging

The power system could face significant challenges if EV charging is not properly coordinated with the widespread adoption of EVs. These challenges become increasingly severe with increasing levels of EV penetration [90]. In this section, the main operational challenges and hurdles that power grid operators could encounter with EVs are thoroughly examined. These include issues such as power quality, grid congestion, peak demand, and the need to meet generation adequacy requirements. The effect of EVs on the power grid can be evaluated by considering various factors, including the degree of EV penetration. The impacts of EVs on the power grid include charging strategies, characteristics of EV batteries, charging patterns, charging durations, and patterns for rapid EV charging [91]. The effects of uncoordinated EV charging on the generation and distribution system are illustrated in Fig. 9.

FIGURE 9.

Impact of uncoordinated EV charging.

A. Impacts of EV Charging Stations on Grid

This study confirms the validity of integrating solar power into the EV industry. Incorporating EVs into the grid in an impractical manner can have a negative effect on the overall power quality of the grid. The control mechanism will utilize information regarding the quantity of available power, the number of active EVs, and the number of charging stations to determine the demand and allocate power with the highest level of quality. Regulating the charging station, prevent overload conditions. The power distribution grid is considerably impacted by the uncoordinated charging of EVs, as the charging process for EVs demands a significant amount of electrical power/energy (ranging 1.4-25 kW under charging Level 1 to 3). The charging demand mentioned above can result in extremely undesirable spikes in electricity consumption. Research has indicated that network congestion can occur on a local network at approximately 50% EV penetration, even when low-capacity chargers (ranging from 1.4 kW to 7.4 kW) are used. In the case of uncoordinated domestic charging, the presence of 10% EVs can result in a peak demand increase of 17.9%, whereas 20% EV penetration can cause a peak demand increase of 35.8%. If many EVs are charged during peak hours, approximately 4% of the distribution transformers could be overloaded when the EV market penetration is approximately 5% [91], [92]. The overloading of transformers is further exacerbated by the charging of EVs. When 40-70% of customers own EVs, uncoordinated charging may result in 32% of distribution transformers in the power system being replaced [28].

B. Effects on Power Quality

The impact of the EV charging infrastructure on energy losses, particularly due to the charge-discharge cycles of batteries, is a significant concern. Randomly uncoordinated EV charging results in various power quality challenges. When evaluating power quality, various factors are considered including harmonics, poor power factors, voltage sags, voltage violations, and flicker. These issues lead to substandard and poor power quality resulting in a reduced lifespan of equipment (e.g. distribution transformers) and reduced grid reliability [93], [94], [95]. Accordingly, the THD level has increased to 11.4%, surpassing the permissible VTHD limit of 8% [96]. According to Da Silva et al., the introduction of EVs at a 60% penetration level, along with different charging strategies, led to a 15% increase in distribution network investment costs and a 40% rise in energy losses during off-peak hours [97]. To address this, the distribution system operator aims to minimize power loss during charging for economic reasons, while also mitigating transformer and feeder overloads.

Poor planning of EV charging infrastructure could result in a significant increase in substation load demand if EVs become widely adopted, necessitating the expansion of the existing distribution grid [98]. In addition, integrating renewable energy into uncoordinated EV charging could exacerbate issues related to generation adequacy and economic viability. These findings indicate that the deployment of EVs on large-scale faces challenges related to generation adequacy. Consequently, additional power generation resources may be required for the grid, or coordinated charging schemes need to be implemented. According to Fig. 10, the system configuration includes the load that represents the battery to be charged. The PV system supplies power to the charger or transmits it to the grid-side by converting it into AC electrical energy. In situations where highly turbulent conditions can cause an uncontrolled increase in the output voltage. A charge controller is used to restrict the current flow from the generator. This is done to safeguard the other components of the system from potential damage. Power can transmit from the DC power center to either the vehicle battery or through the inverter to the grid. In situations where solar power is unavailable, the DC power controller regulates the transfer of power from the grid to the vehicle.

FIGURE 10.

Architecture of an on-grid, solar based EV charging system.

SECTION VII. WPT Structure and Modeling

The fundamental operating principle of WPT is based on electromagnetic induction. To maximize the transferred power for WPT, the primary and secondary coils utilize the concept of resonance. Thus, the transmitting and receiving coils must resonate at an identical frequency [99], [100]. In the magnetic resonance circuit, the state of oscillation between the inductor and capacitor can be achieved with sufficient excitation. The applications of these technologies are related to EVs with WPT. The circuit of a WPT EV charging system is shown in Fig. 11. In [101], [102], and [103], the WPT operating frequency was approximately ~ MHz, whereas the recently developed WPT chargers can operate under reduced frequency (e.g., <100 kHz) with an air-gap range of one tenth of a centimeter. Most high-power electronics devices cannot work perfectly at ~ MHz, (i.e., IGBTs, and MOSFETs). A reduction in the switching frequency avoids electromagnetic interference with other on-board electronic devices. This section presents different aspects of WPT technology including challenges, design, and integration into EVs.

FIGURE 11.

Application of the WPT parking EV charger.

A. Challenges of WPT

Table 8 presents the analysis and comparison of various compensation typologies that are used for wireless charging technologies. This comparative analysis focuses on the factors and variables influencing WPT technology, addressing the attributes, characteristics, prospects, opportunities, and challenges facing WPT technologies. Improving the efficiency of power transfer, coupling separation, alignment tolerance, interference, safety and winding resistance represent the major challenges for WPT technology [104], [105]. Compared with wired power transmission, conventional WPT designs typically experience significantly lower transfer efficiency because many factors as discussed below.

1. Transfer efficiency: The system efficiency depends on many factors such as the switching frequency, load, coupling factor, separation distance and winding resistance. To achieve optimal efficient designs, and a stable and reliable system, it is essential to carefully choose these parameters. While the efficiency may exceed 90% under the intended conditions, a slight variation leads to a reduction in efficiency. However, when the value of the coils is increased, the coil resistance increases. Minimizing the coil resistance reduces coil losses. The use of multistranded windings, such as Litzwires, can reduce the impact of coil resistance.

2. Coupling separation and alignment: The coupling between the primary and secondary coils depends on their separation. The coupling coefficient decreases with increasing in separation which leads to a reduction in the mutual inductance between them. Increasing the self-inductance of the coils can mitigate the effect of the coupling coefficient, increase the mutual inductance between the coils, and may increase the losses and take up more space. In typical designs, the standard distance for such applications is approximately 15 cm. For power transfer between two coils, a minimum level of mutual inductance is needed. Viable designs must offer high efficiency over a range of operations and the same level of performance even under variation in the designed parameters or designed for a specific efficiency. Even if the efficiency is greater than 90%, a misalignment of just a few millimeters can cause a significant drop in efficiency for conventional systems. For the optimized system, the parameter variation will be less insignificant.

3. Interference and Safety: The typical approach is to position the transmitter and receiver coil pads near the circuitry or vehicle chassis in conventional designs. Increasing the switching frequency results in a greater electromagnetic field [115]. If a proper shielding arrangement is not installed in place, the electromagnetic waves being transmitted may impact the electronic circuits within the vehicle. In more serious instances, this interference may even lead to electric shocks in humans. To prevent this, ferrite sheets and aluminum foils are commonly used for shielding and protection purposes. However, if the shielding materials are not chosen carefully, significant core and copper losses may occur. By selecting the appropriate ferrite sheet, it is possible to shield against electromagnetic interference while minimizing any associated losses. Since there is no medium guiding WPT, the scattering of electromagnetic waves may occur and potentially impact individuals in close proximity. Magnetic resonance can help mitigate this issue by providing a directional component. To improve the quality factor, the resonant frequency can be increased when dealing with higher coupling separations [116]. When the radiation of a certain frequency exceeds certain limits, it may have an impact on objects in close proximity, including humans. To prevent this issue, designers must strictly follow safety regulations such as ANSI/IEEE C95.1-2019 [117].

TABLE 8 Analysis and Comparison of Wireless Charging Technologies

B. Compensation of the Resonant WPT System

Various wireless charger configurations have been implemented to overcome the limitations concerning power transfer of the WPT. These limitations are related to the tolerance for misalignment, the coupling coefficient, the transfer efficiency and the distance. The coupling coefficient in a wireless transformer is significantly lower than 0.3, whereas it exceeds 0.9 in wired transformers [29], [118]. As a result of weak coupling, there is a high amount of leakage inductance in the wireless transformer. To mitigate the influence of this leakage inductance, compensator circuits are used for WPT. A compensator circuit consists of a configuration of capacitors and inductors that are positioned in relation to each other. In WPT, four types of compensators are employed: series-series (SS), series-parallel (SP), parallel-series (PS), and parallel-parallel (PP), as detailed in [107]. Fig. 12 depicts the fundamental compensator configurations. In compensator design, the primary/secondary capacitor design varies according to the type of compensator and the use of the basic resonant frequency equation. As detailed in [107] and [119], among the four fundamental compensators, the SS compensator is the only one that is independent of the coupling coefficient. This implies that the SS compensator retains its resonance operation throughout any coupling while preserving the transfer efficiency [118], [120]. Table 9 presents a comparison of basic compensation typologies for wireless chargers for EVs including various variables such as coupling dependency, separation distance, impedance, power/efficiency transfer, advantages and disadvantages. For applications requiring charging at varying distances, the SS compensation is the optimal selection. Therefore, the SS configuration can be utilized in the design of a versatile wireless charger for flexible distance applications. The SS-compensated WPT topology is specifically designed to enable long-range WPT for EVs, despite potential alignment issues. Owing to the significant reduction in the input ripple current achievable with this design, it is possible to use a low-current battery at the source end, which in turn helps to lower resistive power loss. Power conversion in the conventional SS-compensated WPT is achieved through a voltage-fed converter. However, the drawback associated with SS compensation is that alterations in the coupling can result in a significant fluctuation in its output voltage. Combining this topology with others may lead to reverse current flow, negatively affecting the transfer efficiency and power source lifespan [121].

TABLE 9 Comparison of the Basic Compensation Typologies for WPT EV Chargers. [105], [106], [119], [122], [123], [124]

FIGURE 12.

Compensation topologies: SS, SP, PS, and PP.

C. Typical Design of the WPT Model: Inductive or Capacitive

The selection between capacitive and inductive WPT systems is based on the advantages of each type. This section discusses the different aspects of each type and its application for the EV charging system.

1) Capacitive WPT

Capacitive wireless power transfer (CWPT) technology uses direct electric fields to transfer the power between two sides. Instead of relying on coils or magnets, the CWPT employs coupling capacitors to transmit power from the transmitter to the receiver. CWPT helps minimize the requirement for electromagnetic field shielding by employing sophisticated geometric and mechanical structures for coupling capacitors [125]. Fig. 13 shows a typical schematic diagram of the series resonant circuit-based CPWT. The absence of magnetic ferrite cores in CWPT allows it to operate at higher frequencies, to be smaller size and less costly. To minimize the impedance between the coupling capacitors and extra inductors are included in series under the resonant configuration. The addition of these inductors facilitates soft switching within the circuitry [126]. At the receiver end of the CWPT, the coupling capacitors allow high frequency power transmission to pass through. Owing to the affordability and uncomplicated nature of CWPT technology, it is highly advantageous for low-power applications such as portable electronic devices [127]. Unlike the inductive power transfer (IPT), CWPT can function effectively at both low current and high voltage. The size of the coupling capacitor and the distance between the two plates affect the level of transferred power. Until now, the use of CWPT in EVs has been restricted because of the need for high power levels and large air-gaps. The smaller the air-gap, is, the stronger the field between the two capacitor plates, leading to superior performance. The authors of [128] suggested the use of a vehicle’s bumper bar as a receiver plate to decrease the air-gap. Reference [129] proposed designs with high capacitance couplings and reduced air-gaps for the rotary mechanism as a potential solution to this issue. They successfully demonstrated a laboratory prototype with a power transfer greater than approximately 1 kW, with an efficiency of approximately 83% at an operating frequency of 540 kHz [30]. CWPT systems have some disadvantages, including the challenge of achieving high-power transfer density with high efficiency, and maintaining effective power transfer when there are changes in the relative position of the capacitive plates. A power factor correction circuit is used to apply the main AC voltage to an H-bridge converter.

FIGURE 13.

Schematic diagram of CWPT.

2) IPT

Nikola Tesla developed the conventional IPT in 1914 as a means of WPT [130]. IPT is a nascent and flexible technology that has the potential to be used in WPT applications. The technology of IPT has undergone testing and application in diverse fields, with the capacity to transfer power wirelessly from transmitter to receiver, ranging from milliwatts to kilowatts and is suitable for stationary and dynamic EV charging. In 1996, General Motors (GM) developed the Chevrolet S10 EV charged by magne-charge IPT (J1773) Level 2 (6.6 kW) slow and Level 3 (50 kW) fast charges [30]. Despite its potential, the application of IPT is limited by its intricate control method and suboptimal efficiency. An IPT based WPT utilizes mutual inductance and coupling between the coils to supply power to the load. To increase the efficiency and power transfer of IPT based WPT, magnetic resonance tuning and matching techniques are employed. However, the need for ferrite cores for magnetic flux guidance and shielding to minimize losses leads to costly and large designs. Operating at low frequencies results in large coil size and low power transfer density. To minimize losses in the magnetic cores, the operating frequencies are in the range of <100 kHz. This charger has the capability to charge batteries within a voltage range of 200 to 400 V. Fig. 14 depicts the fundamental block diagram of the traditional IPT, which forms the basis of EV charging systems. According to [30], a coaxial winding transformer with a power capacity of 10 kVA provided notable benefits in a universal IPT system. These advantages include the ability to easily adjust the power range of the inductive coupling design.

FIGURE 14.

Implementing the V2G/H SS-inductive WPT system.

D. Integration of V2G Technology

The integration of V2G is an innovative use of EVs to relieve their impacts on the power grid. It represents a more advanced version of Vehicle-to-Home (V2H) technology. With V2G/H integration, the excess energy of the EV battery can be transferred to the home electrical network (HEN) as an energy backup. To enable V2G capability, modifications to the HEN are necessary to charge or discharge the EV battery during off-peak hours. This feature can be applied if the EV is parked in close proximity to the HEN. To address the limitation posed by space constraints, the entire grid is made capable of accommodating EV power transfer in V2G. Compared with the traditional methods, the smart grid enables EV fabricators to load sharing and demand control features during the charging process. During periods of grid overload, if the grid is unable to handle the necessary additional power, a frequency disturbance/drop appears in the grid, resulting in a decline in grid power quality. The combined surplus charge of multiple EVs helps to regulate the frequency to its nominal value and can meet the power demand. The V2G concept has emerged as a new development in EV technology. Fig. 14 shows a block diagram of the V2G charging system with a dual converter. The wireless charger’s primary component is integrated with the grid or renewable energy sources.

Through the monitoring of power source parameters and the EV battery on the secondary side, the controller can effectively manage power and maintain optimum power quality. Since many EVs are parked during the day when ample sunlight is available, the first stage involves harnessing solar energy. Power exchange is achieved between the grid and EVs through a bidirectional inverter whether connected via a wired or wireless charger to regulate the flow of power. To analyze power quality, the microgrid is linked to multiple wireless communication components at different grid locations. A stand-alone system, which operates independently, will manage, analyze, and process real-time power quality. An enhanced battery management system (BMS) and vehicle longevity can be expected, along with improved power quality. Before supplying power to the grid, the BMS controller verifies whether the battery has adequate power. The BMS initially checks the battery’s state of charge (SoC) before feeding the grid. The power flows from the grid to the vehicle battery if Soc falls below the predetermined level. The battery charging process remains uninterrupted by reactive compensation via bidirectional converter. To optimize the system’s performance, it is recommended that EV users have access to information such as the charging station location, battery Soc status, specifications, etc. Access to these data may be interrupted as a result of issues with wireless communication or cyberattacks. Hence, the user can still obtain the best energy price even in the absence of information about the charging stations. To achieve V2G integration, certain standards must be met [131].

E. Electrochemical Energy Storage Technologies

Batteries and electrochemical double layer capacitors, which are commonly referred to as ultracapacitors, are both options for energy storage [31], [132]. Compared with Ultracapacitors, batteries are a well-established technology with greater energy density. Currently, power batteries are the mainstream choice for EV applications. Fig. 15 illustrates a detailed classification of the energy storage system. These sources offer commendable power and energy densities while producing no emissions [133], [134]. EVs can incorporate a hybrid energy storage system by combining batteries and ultra- or supercapacitors. This configuration offers the advantages of a high energy density, enabling extended driving ranges, as well as high specific power, facilitating immediate energy exchange during acceleration and braking [135]. Lead-acid, nickel-based, and particularly lithium-ion batteries (LIBs) are the main contributors among various battery types [32]. Throughout the evolution of battery technology, LIBs have emerged as a noteworthy breakthrough owing to their exceptional performance metrics, notably their high energy density, extended lifespan, and superior safety features. LIBs have emerged as a notable leap forward in battery technology throughout their developmental timeline, primarily owing to their remarkable key performance indicators (KPIs), particularly in terms of superior energy density, extended lifespan, and enhanced safety. Additionally, high-temperature batteries typically operate within the temperature range of $270~\sim ~400^{o}$ C, providing significant advantages such as high energy [136]. While metal/air batteries offer a higher specific energy, they tend to have a shorter cycle life, meaning that they can undergo fewer charge and discharge cycles before their performance decreases. Metal/air batteries are classified according to the specific metal employed as the anode, including lithium or zinc. The potential of other materials, such as zinc-ion, magnesium-ion, and aluminum-ion batteries, for use in EVs needs to be further explored and developed. As an exception, sodium-ion battery (SIBs) are much more developed than other metal ion technologies. Furthermore, flow batteries resemble fuel cells more closely than traditional batteries. Fig. 16 represents a comparison of several commonly used commercial secondary/rechargeable battery types as a function of their volumetric-energy density. Rechargeable batteries come in various types, but lead acid batteries are often selected for energy storage because they are a widely used, established, and cost-effective technologies with a proven track record as indicated in Table 10 and detail in [137], [138], and [139].

TABLE 10 Comparison of Commercial Secondary Battery Types [137], [138], [139]

FIGURE 15.

Classification of electrochemical energy storage sources [107], [135], [140], [141].

FIGURE 16.

Comparison of commercial secondary battery types.

This comparison is useful for the following battery types:

1. Lithium-ion (Li-ion) Batteries: Advantages: Light- weight, low self-discharge rate, long cycle life, and high energy density wide range of applications (laptops, smartphones, EVs). Disadvantages: Sensitive to high temperatures, risk of fire if mishandled or damaged, and high cost.

2. Nickel-metal hydride (Ni-MH) Batteries: Advantages: Higher energy density compared with NiCd batteries, the absence of toxic metals, and the absence of a memory effect. Disadvantages: Lower cycle life compared with Li-ion batteries, self-discharge rate higher than Li-ion batteries, and a lower energy density compared to Li-ion batteries.

3. Nickel-cadmium (Ni-Cd) Batteries: Advantages: Good cycle life, high discharge current capabilities, low internal resistance, and a wide operating temperature range. Disadvantages: Contains toxic cadmium, exhibits a relatively low energy density, exhibits memory effects (reduced capacity if not fully discharged before recharge), and it is environmentally unfriendly.

4. Lead-acid Batteries: Advantages: Relatively low cost, high discharge current capabilities, wide operating temperature range, and it is a well-established technology. Disadvantages: Heavyweight, low energy density, limited cycle life, requires maintenance (topping up electrolytes and cleaning terminals), and it is environmentally unfriendly (contains lead and sulfuric acid).

Importantly, battery performance can vary depending on specific manufacturers, advancements in technology, and individual product designs. A comprehensive examination of each storage technology is outside the scope of this paper.

F. Battery Management System

To achieve efficient DC/DC conversion with dependable isolation, a high frequency transformer is utilized. Typically, traditional charging systems consist of adapters and plugs for connection. An EV charger generally comprises a single AC-DC converter, which may include a power factor correction (PFC) option and a unidirectional or bidirectional DC/DC converter for the power flow as shown in Fig. 17a and Fig. 17b, respectively [29]. The battery management system (BMS) and power converters can supply power to the entire system of the EV by obtaining energy from the battery pack. Installing a BMS in each EV can provide protection for the battery pack, ensuring that it is shielded from physical harm, performance decline, and thermal runaway. The BMS enables periodic battery testing to obtain detailed information about the battery. The integration of various advanced functions is a key aspect of a BMS, with battery modeling and state estimation becoming essential and vital components. Accordingly, a block diagram of the BMS is depicted in Fig. 18. The system is composed of a measuring unit, battery status estimation, thermal protection, fault diagnosis, computer for data collection, and a user interface. The sensing block obtains digital signals by measuring battery parameters from various locations. These parameters are then utilized to estimate the battery states. The battery state estimation, specifically for the SoC, state of health (SoH), and state of temperature (SoT), can be directly impacted by the accuracy of the model [32]. The capacity estimation block uses appropriate algorithm(s) to generate the charge/discharge current constraints. Battery thermal management involves controlling the temperature by turning on or off the fan or heater as needed to maintain an optimal range. A cell equalizer imposes multidimensional constraints to prevent irregularities in overcharging and overdischarging. Ensuring battery safety is the responsibility of the fault diagnosis block [142]. The CAN block manages the flow of information for sending and receiving data. Tests can be conducted at specific temperatures, and after data are collected from these packs, a battery model can be created. As a result, battery modeling becomes a crucial requirement for battery management systems (BMSs).

FIGURE 17.

Typical charging topologies for EVs. (a) unidirectional charger. (b) bidirectional charger [26], [29].

FIGURE 18.

Functional diagram of the BMS for EVs [32], [143], [144], [145].

G. Battery Equivalent Circuit Model

This section provides an elaboration of three modeling techniques used to represent EV batteries, namely: (i) electrochemical, (ii) equivalent circuit, and (iii) data-driven models. Physical approaches can be used to represent a battery in an electrochemical model, which provides good consistency with the battery’s external characteristics. These electrochemical models rely on nonlinear electrochemical reactions, which represent the structures of the batteries [146]. Furthermore, optimization methods may incur significant memory and computational costs. A recent study focused on the development of a thermal-electrochemical model by combining thermal and electrochemical equations. A new electrochemical model with reduced complexity was developed to achieve high accuracy while maintaining low computational costs. However, the model’s dependence on many variables makes it more complex and less applicable, particularly when factors such as battery temperature and aging are considered. Owing to its simplicity, a single-particle model demonstrates a good adherence to the actual parameters. The single-particle model with electrolyte dynamics has become one of the most well-established models as indicated in Fig. 19, but its accuracy is relatively low [147], [148]. To increase the applicability of the model, online parameter identification was employed. Models of this type provide the most precise data on batteries, but they face two primary challenges:

1. Many non-analytical equations cannot be solved through analytical methods and instead require global optimization techniques.

2. There is a strong interdependence between the control model and boundary conditions.

FIGURE 19.

Evolution map of battery modeling methods [32], [147].

To increase the applicability of the model, some researchers are exploring equivalent circuit models (ECMs) that simulate battery behavior via fundamental electrical components. The process of determining the resistance involves how to construct an equivalent and suitable circuit for loading the resonant circuit during AC analysis. The battery is modeled by a resistance $R_{L}$ = $V_{bat}$ /$I_{bat}$ , where $V_{bat}$ and $I_{bat}$ are the battery voltage and current, respectively. For the series-series compensation arrangement, the effective or equivalent AC side resistance can be expressed as $R_{e} = R_{ac}$ = 8/$\pi ^{2}$ . $R_{L}$ . Consequently, it becomes possible to transform a battery load into a purely resistive load. It is proportional to the ratio of the battery voltage to the charging current. For this case, the equivalent AC resistance is given by: $$\begin{equation*} R_{e}=R_{ac}= \left ({{\dfrac {8}{\pi ^{2}}}}\right).\dfrac {V_{bat}}{I_{bat}}=\left ({{\dfrac {8}{\pi ^{2}}}}\right).R_{L} \tag {5}\end{equation*}$$

The value of $R_{ac}$ can vary depending on the battery connection style, parallel or series compensation, and with or without DC/DC converter. The battery pack resistance $R_{L,bat}$ is obtained by (6). This resistance represents the equivalent resistance of the battery as reflected via a full wave rectifier [138]. $$\begin{align*} R_{ac}=\begin{cases} \displaystyle \left ({{\dfrac {8}{\pi ^{2}}}}\right)R_{L}, & \text {series secondary compensation}. \\ \displaystyle \left ({{\dfrac {\pi ^{2}}{8}}}\right)R_{L}, & \text {parallel secondary compensation}. \end{cases} \tag {6}\end{align*}$$

This model is valid only for steady-state analysis according to the constant current/voltage (CC/CV) charging profile of the battery as shown in Fig. 20. Based on the I-V curve of the battery load, the equivalent resistor continues to increase from the beginning of the charging procedure, as stated in [149], [150], and [151]. For EV wireless charging, the conventional setup involves linking the battery to the compensation circuit via a diode-bridge rectifier [138].

FIGURE 20.

Charging profile of an EV battery with its equivalent resistance based on charging profile [151].

H. Lithium-Ion Battery Modeling

Li-ion batteries have rapidly penetrated the market of high-performance EV storage systems, because of their outstanding features. Compared with other battery types, specific power, energy densities, higher efficiency, lesser self-discharge rates, longer lifetimes, and reasonable costs are the most powerful characteristics. Their components can be recycled, thereby ensuring their eco-friendliness [152], [153]. The voltage of battery packs for commercial EVs (EVs) are different depending on their model, typically falling within the range of 300 V to 600 V for energy specifications exceeding 15 kWh [44], [64], [154]. Fig. 21 depicts the electrical equivalent circuit of the Li-ion batteries. This model consists of an ideal voltage source $V_{oc}$ and an internal resistance $R_{s}$ where $I_{b}$ is the load current and $V_{t}$ is the battery terminal voltage. To represent the internal diffusion process and electrochemical reactions occurring within the Li-ion battery, a parallel network consisting of $R_{p}$ and $C_{p}$ is incorporated alongside the internal resistance. The model of the battery is detailed in [146]. The battery model which represents the electrical characteristics is given by (7). The SoC as a function of the maximum battery capacity Q is given by (8). $$\begin{align*} V_{t}& =V_{oc}-V_{p}-I_{b}R_{s} \tag {7}\\ SoC& =100\left ({{1-\dfrac {1}{Q}\int _{a}^{b}I(t)\,dt}}\right) \tag {8}\end{align*}$$

FIGURE 21.

Equivalent circuit model of a lithium-ion battery [144], [152].

SECTION VIII. Analysis of PV-Based SS-Wireless Charging

The majority of parking spaces are situated in areas with abundant solar energy exposure. By strategically installing solar panels on roofs, these spaces can serve dual purposes, such as both parking slots and charging stations [155], [156]. Fig. 22 shows a block diagram of the EV charger powered by renewable energy. The wireless charger requires the use of a high-frequency inverter because the renewable energy source is the DC form, whereas AC power is needed for the transmitter-side coil. When incorporating the renewable energy source into the conventional compensator topology, it is crucial to consider account the potential reverse current flow [33]. The inverter’s high frequency switching on the transmitter side causes a reverse current flow in the primary circuit, which has the potential to harm the fragile renewable source. To minimize the power loss in the charger caused by the reverse blocking diode, a current source inverter (CSI) can be employed to mitigate the reverse current [157]. However, the inductor interaction in the current-source inverter (CSI) can cause distortion in the magnetic resonance. To mitigate this issue, it is necessary to adopt an appropriate inductance tuning approach, which will be further elaborated in the subsequent section.

FIGURE 22.

Architecture of renewable energy-based EV charger.

A. Series-Series Compensated IPTS Equivalent Circuits

A comprehensive investigation of SS and SP compensation topologies to optimize power transfer efficiency, where SS is suitable for lower loads and SP is suitable for higher loads, is presented in [106]. Based on an earlier analysis, the SS topology has high reliability and efficiency. The SS compensation type appears to be the topology with the greatest potential for realizing wireless battery charging in EVs with high power, the lowest H-bridge current and high voltage [158]. This is one of the major advantages of SS topologies over the other topologies mentioned in [34]. The design of the proposed SS wireless charger is detailed in [35]. The SS charging system comprises an on-board receiver and an off-board transmitter, with the aim of generating high voltage AC power at a high frequency via the inverter. On the receiver side, the receiver coil ($L_{2}$ ) utilizes electromagnetic induction to capture AC power from the transmitter coil ($L_{1}$ ). To increase the available power, capacitors are used to compensate for the secondary inductor, so that it can resonate at or close to the operating frequency. As the operating frequency increases, all AC capacitors may experience stress and will need to be derated. Additionally, a bridge rectifier is responsible for converting the AC into DC power ($P_{L}$ ) for the battery pack. As shown in Fig. 23, two capacitors are utilized and connected in series with their corresponding coils on both sides of the system to achieve compensation. The battery is charged via the rectifier bridge by the output voltage of the secondary coil. The EV battery pack is denoted by $V_{b}$ and $R_{b}$ , which represents the internal resistance of the battery. This section investigates the correlations between circuit parameters; coils and compensating capacitors, the conduction angle, circuit variables, current and voltage, transferred power and transfer efficiency.

FIGURE 23.

Equivalent coupling circuit for series-series topology.

In the equivalent circuit depicted in Fig. 23, to ensure resonant operation, the capacitive reactance $X_{C2}$ and the inductive reactance of the secondary circuit are identical. Under resonance conditions, $C_{2}$ is calculated as follows: $$\begin{align*} X_{L2}& =X_{C2} \tag {9}\\ j\omega _{o}L_{2}-j\frac {1}{\omega _{o}C_{2}}& =0 \tag {10}\\ C_{2}& =\frac {1}{\omega _{o}^{2}L_{2}} \tag {11}\end{align*}$$ where $\omega _{o}$ : resonant angular frequency, $L_{2}$ : secondary side coil self-inductance, $C_{2}$ : secondary side capacitance.

The primary compensation is calculated in a similar manner at the terminals of the primary side as follows: $$\begin{align*} C_{1}& =\frac {1}{\omega _{o}^{2}L_{1}} \tag {12}\\ f_{1}& =\frac {1}{2\pi \sqrt {L_{1}C_{1}}} \tag {13}\end{align*}$$ The secondary capacitance can be calculated as a function of the primary inductance at the resonant frequency as following: $$\begin{equation*} C_{2}=C_{1}\frac {L_{1}}{L_{2}} \tag {14}\end{equation*}$$

The value of $L_{2}$ is fixed; therefore, the value of $C_{2}$ principally depends on the resonant frequency.

As depicted in Fig. 23, the complex power exchanged from $L_{1}$ to $L_{2}$ and vice versa as detailed in [35] and [159] is given by: $$\begin{align*} \begin{cases} S_{12}=-V_{12} i_{2}^{*}=-j\omega L_{m} i_{1} i_{2}^{*} \\ =\omega L_{m} I_{1} I_{2} \sin \varphi _{12}-j\omega L_{m} I_{1} I_{2} cos(\varphi _{12}) \\ S_{21}=-V_{21} i_{1}^{*}=-j\omega L_{m} i_{2} i_{1}^{*} \\ =-\omega L_{m} I_{1} I_{2} \sin \varphi _{12}-j\omega L_{m} I_{1} I_{2} cos(\varphi _{12}) \end{cases} \tag {15}\end{align*}$$

The transferred active power from the transmitter to the receiver can be expressed as follows: $$\begin{equation*} P_{12} = \omega L_{m} I_{1} I_{2} sin \varphi _{12} \tag {16}\end{equation*}$$

A method for design and optimization has been proposed with the aim of improving the overall efficiency while ensuring effective control of the output voltage. Design curves are used to demonstrate a procedure for designing an SS-IPT system that balances the efficiency improvement and current rating of switches while achieving the desired voltage transfer ratio.

B. Maximum Power Transfer

In numerous WPT projects, the impedance matching technique is typically employed to adhere to the maximum power transfer theorem, which requires that the source and load be impedance-matched. To attain maximum energy efficiency, it is possible to utilize a low-impedance power source. This is because a smaller source resistance results in lower losses and greater power transfer to the load, leading to maximum energy efficiency. This method is frequently employed in power converters, particularly when high efficiency is needed. In mid-range WPT, resonators are typically air-cored, resulting in no magnetic core loss. Assuming nonradiative power transfer and neglecting the equivalent series resistance of capacitors, the system will experience only two types of losses: 1) conduction losses resulting from the coil’s AC resistance, and 2) power loss from the source resistance [36]. The primary objective is to attain optimal energy efficiency, although any unwanted stray loss within the system would result in a reduction in efficiency. To achieve this objective, a power source with very low resistance $R_{in}$ is employed, which is intentionally unmatched with the equivalent load. Additionally, copper tubing or Litz wire is used to decrease the AC coil resistance during high-frequency operation. According to (16), the maximum value of the transformer efficiency as a function of $\sin \varphi _{12}$ =1 is achieved when the angle $\varphi _{12} = \pi /2$ . The transferred power is also maximized to the receiver, as shown in Fig. 24. The phase between the current $I_{1}$ and $I_{2}$ is approximately $90^{o}$ . The quality factors of the two coils, $Q_{1}$ and $Q_{2}$ , are expressed as follows: $$\begin{align*} \begin{cases} \displaystyle Q_{1}=(\omega L_{1})/R_{1} \\ \displaystyle Q_{2}=(\omega L_{2})/R_{2} \end{cases} \tag {17}\end{align*}$$

FIGURE 24.

Efficiency and transferred power versus $\varphi _{12}$ when deviating from $90^{o}$ ($\eta _{max}$ to approximately 98%).

The system efficiency is analyzed by using the high-quality factor (Q) of the resonant circuits and the resonant frequency. As detailed in [35], where $L_{m}=k\sqrt {L_{1}L_{1}}$ , k is the coupling coefficient ($k \lt \lt 1$ ). The transferred efficiency $\eta$ can be derived as a function of $Q_{1}$ and $Q_{2}$ as follows: $$\begin{align*} \eta =\dfrac {|I_{2} |^{2} R_{b}}{|I_{1} |^{2} R_{1}+|I_{2} |^{2} (R_{2}+R_{b})}=\dfrac {R_{b}}{\dfrac {(R_{b}+R_{2})^{2}}{k^{2}Q_{1}Q_{2}R_{2}}+(R_{2}+R_{b})} \tag {18}\end{align*}$$

Let $\alpha = R_{b}$ /$R_{2}$ ; therefore, $\eta$ can be reformulated in terms of k, $Q_{1}$ and $Q_{2}$ as detailed in [35] and [37] by: $$\begin{equation*} \eta =\dfrac {\alpha }{1+\alpha +\dfrac {(1+\alpha)^{2}}{k^{2}Q_{1}Q_{2}}} \tag {19}\end{equation*}$$

Therefore, the maximum efficiency is: $$\begin{equation*} \eta _{max} =\dfrac {k^{2}Q_{1}Q_{2}}{(1+\sqrt {1+k^{2}Q_{1}Q_{2}})^{2}} \tag {20}\end{equation*}$$

The maximum efficiency is achieved at $\alpha _{\eta _{max}}=(1+k^{2}Q_{1}Q_{2})^{1/2}$ . The power transfer efficiency approaches 98% as shown in Fig. 24 and as discussed in [160]. The coupling coefficient k, as indicated in [37], can be expressed as a function of the geometry of the coils by: $$\begin{equation*} k =\dfrac {1}{\left [{{1+2^{2/3}\left ({{\dfrac {D}{\sqrt {r_{1}r_{2}}}}}\right)^{2}}}\right ]^{3/2}} \tag {21}\end{equation*}$$ where, D is the separation between the transmitter coil (with loop radius $r_{1}$ ) and the receiver coil (with loop radius $r_{2}$ ). The higher the coupling coefficient k, is, the higher the power transfer efficiency.

SECTION IX. SS Charger Performance Confirmation

A model based on Simulink/MATLAB was developed to validate the proposed analysis. A simulation of the system was created by using the MATLAB/SIMULINK tool for modeling, simulating, and analyzing the system dynamics and performance. The system parameters are given in Table 11.

TABLE 11 System Parameters

By ensuring the efficacy of the SS charger, it is possible to analyze the impact of parameters such as the switching frequency, coupling coefficient and the coil inductance on the output power and efficiency of the system. Fig. 25 shows the transfer efficiency with respect to the switching frequency and the coupling coefficient. The performance of the SS-WPT system is confirmed by varying the coupling from $k =0.1$ up to $k =1$ . Typically, the maximum efficiency is approximately 98%.

FIGURE 25.

Transfer efficiency versus the coupling coefficient and switching frequency.

At the resonant frequency, the system impedance corresponds to the minimum value, consequently, maximizing the power transfer between the two coils as shown in Fig. 26, where $f_{s}$ is the switching frequency. Considering the 10% tolerance of the mutual inductance $L_{m}$ , $L_{1}$ and $L_{2}$ values, the resonant frequency calculated by (13) and (14) can vary in the range of 14 kHz $\leq f_{0} \leq 16$ kHz.

FIGURE 26.

Transferred power characteristics versus frequency.

The wireless charging system is influenced by the coil inductances. Fig. 27 shows the relationship among the transmitter, and receiver coil inductances and the transfer efficiency. However, excessively large inductances of the two coils may result in increased line consumption and economic costs. Therefore, the inductance of the transmitting coil can be larger, and the inductance of the receiving coil can be smaller. Considering that the EV wireless charging system requires an output power of 3 kW, it is advisable to have slightly larger inductance values for the two coils. The previous analysis confirms the ability of the SS-WPT charger to meet the energy demand as needed.

FIGURE 27.

Efficiency versus the transmitter, and receiver inductances.

SECTION X. Conclusion

This paper comprehensively analyzed the integration of PV systems into wired/wireless power transfer in electric vehicle charging systems, enabling the construction of smart solar-powered parking systems for public establishments. The parking systems are considered energy hubs for their respective owners, including PV generation, V2V, V2G, G2V and on grid systems. The study will be used in the future as a basis for designing the solar parking. The study primarily concentrates on series-series WPT by using inductive coupling, considering the coil design parameters. This compensation topology reduces the reflected load resistance, leading to increased current drawn from the available source with high power transfer capability. The transfer efficiency of the system is directly proportional to the switching frequency of the WPT system. Minimizing the internal impedance maximizes the output power and efficiency. Increasing the coupling coefficient enhances the power transfer capability, particularly for inductors with a low frequency. The system can achieve a transfer efficiency of up to 98%. Theoretical analysis and simulation results were conducted to analyze the proposed design.