With the rapid growth in the adoption of electric vehicles globally, particularly in regions like China where the electric vehicle market is expanding at an unprecedented rate, there is an increasing need for advanced charging and discharging interfaces. The integration of electric vehicles into the grid, known as vehicle-to-grid (V2G) technology, has highlighted the importance of bidirectional power flow capabilities. Among various solutions, bidirectional wireless power transfer (BWPT) systems have gained significant attention due to their convenience, flexibility, and strong interactive potential. This review comprehensively examines the key technologies, research progress, and future trends in BWPT systems for electric vehicles, with a focus on system architectures, compensation networks, control strategies, and communication methods. The emphasis is on enhancing efficiency, interoperability, and scalability to support the widespread adoption of electric vehicles, especially in the context of China’s evolving EV infrastructure.
The proliferation of electric vehicles, including the rapid expansion of China EV markets, has driven innovations in wireless charging technologies. BWPT systems enable energy to flow in both directions—from the grid to the vehicle during charging and from the vehicle to the grid during discharging—facilitating V2G applications. This not only supports grid stability by providing distributed energy storage but also promotes the use of renewable energy sources. A typical BWPT system consists of several key components: grid-side AC-DC converters, high-frequency inverters/rectifiers, compensation networks, and wireless communication modules. The system operates by converting grid AC power to high-frequency AC for wireless transmission through magnetic coupling, and vice versa for reverse power flow. The efficiency and performance of these systems depend on factors such as power topology, compensation design, and control algorithms, which are critical for meeting the demands of modern electric vehicle applications.
In recent years, research on BWPT systems has advanced significantly, with numerous studies focusing on optimizing power conversion stages. The power topology can be broadly classified into two types: two-stage AC-DC-AC conversion and single-stage AC-AC conversion. Two-stage systems are more common due to their simplicity and high power factor, but they involve additional components like DC-link capacitors, which can increase size and cost. Single-stage systems, such as those using matrix converters, offer higher power density but pose challenges in control complexity and harmonic distortion. For instance, the power transfer in a two-stage system can be modeled using equations that account for inverter output and resonant behavior. A general formula for the power transferred in a resonant inductive system is given by:
$$P = \frac{V_1 V_2}{2 \pi f M} \sin(\delta)$$
where \(P\) is the power, \(V_1\) and \(V_2\) are the voltages on the primary and secondary sides, \(f\) is the frequency, \(M\) is the mutual inductance, and \(\delta\) is the phase shift angle. This highlights the importance of frequency and phase control in regulating power flow. The following table summarizes the key characteristics of different power topologies used in BWPT systems for electric vehicles:
| Topology Type | Key Features | Efficiency Range | Complexity | Suitability for Electric Vehicles |
|---|---|---|---|---|
| Two-Stage AC-DC-AC | Uses DC-link capacitor, stable operation | 90-95% | Moderate | High, due to reliability |
| Single-Stage AC-AC | Compact, no DC-link, direct conversion | 85-90% | High | Moderate, for specific applications |
| Matrix Converter-Based | Bidirectional, high flexibility | 80-88% | Very High | Emerging, with control challenges |
Compensation networks are crucial in BWPT systems to enhance power transfer efficiency and stability by compensating for reactive power in the magnetic coupling. Common topologies include series-series (SS), double-sided LCC, and LCC-S configurations, each offering distinct advantages in terms of output characteristics and interoperability. For electric vehicles, the choice of compensation network affects the system’s ability to maintain constant current or voltage output under varying conditions, such as misalignment or load changes. The impedance of the compensation network can be expressed as:
$$Z_{eq} = R + j\left(\omega L – \frac{1}{\omega C}\right)$$
where \(Z_{eq}\) is the equivalent impedance, \(R\) is the resistance, \(L\) is the inductance, \(C\) is the capacitance, and \(\omega\) is the angular frequency. This impedance plays a key role in determining the power factor and efficiency. The table below compares popular compensation topologies for BWPT systems in electric vehicle applications:
| Compensation Topology | Output Characteristic | Efficiency | Interoperability | Robustness to Misalignment |
|---|---|---|---|---|
| SS | Constant Current | High | Moderate | Low |
| Double-Sided LCC | Constant Current | High | High | |
| LCC-S | Constant Voltage | High | Moderate | Moderate |
Interoperability is a critical aspect for BWPT systems, ensuring that different electric vehicle models and charging infrastructure can work together seamlessly. This involves magnetic, electrical, and communication compatibility. For instance, in China EV ecosystems, standardization efforts focus on defining impedance ranges and communication protocols to achieve interoperability. The port impedance, which reflects the interaction between the vehicle and grid sides, can be modeled as:
$$Z_{port} = \frac{V}{I} = \frac{j\omega M^2}{Z_{load} + j\omega L}$$
where \(Z_{port}\) is the port impedance, \(V\) and \(I\) are voltage and current, and \(Z_{load}\) is the load impedance. By optimizing this impedance, systems can achieve better power transfer and efficiency across various operating conditions.
Control and communication strategies are essential for regulating power flow, optimizing efficiency, and maintaining synchronization in BWPT systems. Common control methods include frequency control, phase-shift control, and pulse-density modulation, which adjust parameters like switching frequency and phase angles to manage power direction and magnitude. For example, in a phase-shift control scheme, the power can be controlled by varying the phase difference \(\delta\) between the primary and secondary inverters, as shown in the equation:
$$P = \frac{V_p V_s}{2 \pi f M} \sin(\delta)$$
where \(V_p\) and \(V_s\) are the primary and secondary voltages. Efficiency optimization often involves tracking the maximum efficiency point by adjusting control variables, such as in algorithms that minimize losses in the resonant tank and power converters. Additionally, wireless communication for phase synchronization is vital to ensure coherent operation between the grid and vehicle sides without physical connections. Techniques like current-based synchronization or harmonic analysis help achieve this, with recent advances focusing on reducing latency and improving reliability for electric vehicle applications.
Looking ahead, the development of BWPT systems for electric vehicles is poised to focus on multi-power level interoperability, wide-range efficiency, and advanced control techniques. In the context of China EV growth, research directions include the design of reconfigurable compensation networks, wide-bandgap semiconductor-based converters for higher efficiency, and AI-driven control algorithms for real-time optimization. For instance, future systems may incorporate adaptive impedance matching and machine learning to predict and respond to grid demands, enhancing the role of electric vehicles in smart grids. The integration of these technologies will support the sustainable expansion of electric vehicle networks, contributing to reduced carbon emissions and improved energy security.
In conclusion, bidirectional wireless power transfer systems represent a transformative technology for electric vehicles, enabling efficient energy exchange between vehicles and the grid. This review has outlined the fundamental components, including power topologies, compensation networks, and control strategies, that underpin BWPT systems. As the adoption of electric vehicles, particularly in markets like China EV, continues to rise, further innovations in BWPT will be crucial for achieving seamless V2G integration. Future research should prioritize scalability, cost-effectiveness, and standardization to unlock the full potential of this technology for a sustainable transportation ecosystem.