Electric Vehicles for a Flexible Energy System: Challenges and Opportunities

Abstract

As the adoption of Electric Vehicles (EVs) accelerates, driven by increasing urbanization and the push for sustainable infrastructure, the need for innovative solutions to support this growth has become more pressing. Vehicle-to-Grid (V2G) technology presents a promising solution by enabling EVs to engage in bidirectional interactions with the electrical grid. Through V2G, EVs can supply energy back to the grid during peak demand periods and draw power during off-peak times, offering a valuable tool for enhancing grid stability, improving energy management, and supporting environmental sustainability. Despite its potential, the large-scale implementation of V2G faces significant challenges, particularly from a technological and regulatory standpoint. The success of V2G requires coordinated efforts among various stakeholders, including vehicle manufacturers, infrastructure providers, grid operators, and policymakers. In addition to the technical barriers, such as battery degradation due to frequent charging cycles and the need for advanced bidirectional charging systems, regulatory frameworks must evolve to accommodate this new energy paradigm. This review aims to provide a comprehensive analysis of V2G technology, focusing on different perspectives—such as those of users, vehicles, infrastructures, and the electricity grid. This study will also explore ex ante, ex post, and ongoing assessment studies, alongside the experiences of pioneer cities in implementing V2G.

1. Introduction

In response to the growing urgency of addressing climate change and reducing dependence on fossil fuels, the transition to sustainable energy systems has gained significant momentum in recent years. A key element in this evolution is the increasing adoption of Electric Vehicles (EVs), which not only offer a cleaner alternative to traditional internal combustion engine vehicles, but also hold the potential to enhance the flexibility and stability of modern power grids. Since Renewable Energy Sources (RESs) such as solar and wind are unstable and intermittent during generation and, therefore, difficult to apply continuously and stably, in the evolving energy landscape, decentralized energy storage systems have become essential to improve the resilience, flexibility, and sustainability of new energy networks [1].

Decentralized storage solutions, including home batteries and community energy storage, mitigate this intermittency by storing surplus energy during periods of high production and releasing it during peak demand. This approach not only stabilizes the grid, but also reduces reliance on centralized power plants, thereby enhancing energy security and minimizing the risk of widespread outages. Moreover, decentralized storage empowers local communities and individuals to manage their energy consumption more effectively, potentially leading to cost savings and a reduced environmental footprint. The adoption of such systems is essential for achieving decarbonization goals and transitioning towards a more resilient and sustainable energy infrastructure [1,2].

It is certain that the exponential growth in the number of EVs will correspond proportionally to an increasing difficulty in the capacity of the electrical network to cope with such a significant distributed load, but EVs can also be a resource if seen as a powerful distributed storage system [3]. A pivotal technology in this context is Vehicle-to-Grid (V2G), which enables EVs to return energy to the grid during times of peak demand and recharge when energy supply exceeds consumption [4]. As cities worldwide aim for smarter and more sustainable futures, V2G technology is emerging as a cornerstone for resilient urban energy systems. By allowing EVs to interact dynamically with the grid, V2G enables smart grids—those that merge traditional energy networks with cutting-edge digital technologies—to respond flexibly to real-time energy demands. The bidirectional energy exchange that V2G facilitates not only helps balance supply and demand, but also plays a critical role in stabilizing voltage and frequency levels. Furthermore, V2G enhances the integration of RESs by offering essential storage and balancing capabilities, which are crucial for managing the intermittent nature of solar and wind energy [5]. To support this transition, regulatory frameworks are adapting. For instance, the European Union’s Clean Energy Package, specifically Directive 2019/944, advocates for consumer empowerment by encouraging EV owners to actively participate in the electricity market. This directive promotes the use of Decentralized Energy Resources (DERs), such as V2G, and implements Demand Response (DR) mechanisms that allow consumers to modify their energy usage in response to market fluctuations. Similarly, the UK’s “Smart Systems and Flexibility Plan” outlines strategies to promote smart grid development and position EVs as key providers of grid services [6]. V2G technology has been widely recognized for its potential to improve grid reliability and resilience, as highlighted in numerous studies. However, despite the promising developments, several technical and economic hurdles continue to impede its widespread adoption. One major technical issue concerns the impact of frequent charge–discharge cycles on the degradation of EV batteries, which could significantly shorten their lifespan. Moreover, the large-scale deployment of V2G requires substantial investment in infrastructures, such as bidirectional chargers and smart meters. These costs are a significant burden on both governments and private stakeholders. Another key consideration in the adoption of V2G is the importance of demand forecasting. In any V2G ex ante assessment, understanding the potential energy that V2G services can offer is crucial [7]. Since individual EV owners cannot participate in the power market on their own, comprehensive plans and regulatory guidelines must be established to coordinate all V2G participants. Economic incentives and supportive regulatory frameworks are essential to driving the adoption of V2G technology [8,9]. Countries such as Japan and the United States are already making strides in creating markets for ancillary services like frequency regulation and voltage support, which EVs can provide. These regulatory efforts are supported by innovative business models that offer financial rewards to encourage EV owners to participate in V2G programs [10]. In summary, while V2G technology holds immense potential to enhance grid flexibility and support the transition to renewable energy, significant challenges remain. Issues such as battery degradation, the high costs of infrastructure, and the need for robust regulatory frameworks—particularly about demand forecasting—must be addressed.

Even though the road to fully exploit the potential of V2G technology is still long and winding, there is substantial literature on the subject. In [11], the authors start from the perspective of having to exploit significant flows of investment and financing, together with the commitment of the private sector, identifying a series of business markets and policy implications. In [12], privacy-preserving approaches for the V2G network that achieve various privacy-preserving objectives are analyzed and summarized. The impacts of V2G integration to support power grid security are instead treated by [13] who examined the specific effects of V2G integration on grid security and stability, focusing on aspects such as frequency, voltage, phase angle, and harmonics. A survey on enhancing grid flexibility through bidirectional interactive electric vehicle operations is proposed by [5] who highlight the benefits of convenient charging, affordable rates, and environmental sustainability. V2G connected technologies and charging strategies are deepened by [14] who analyze different systems using various types of power flow and highlight the need for further studies on capacity prediction modeling, behavior in steady-state, transient, and dynamic processes. Other articles provide analysis of current progress, challenges and prospects in the technology of on-board chargers for EVs, devices that are fundamental and whose characteristics are enabling for the implementation of V2G services [15,16,17].

However, most of the articles in the literature address the issue by analyzing its strengths and weaknesses from the perspective of one or two actors involved. This is the gap that this work intends to fill by offering an overview as exhaustive as possible of the different players involved. In particular, the potential that a distributed storage system, such as that of EV batteries, can offer to new distributed energy systems that will increasingly depend on RES-based production is outlined, with the advantages and disadvantages that this entails.

The structure is as follows: Section 2 offers a comprehensive review of the existing literature on V2G, focusing on research efforts related to vehicles, infrastructures, users, the electricity grid, and aggregators. Section 3 examines key pilot projects worldwide, highlighting how they have tackled the challenges of V2G implementation. Lastly, Section 4 presents the conclusions and discusses potential future developments in the field of V2G.

2. Different Perspectives of V2G

The opportunities arising from V2G, including increased flexibility and operational efficiency of electricity grids, are achieved by exploiting EVs, even aggregated together, as DERs, enabling them to function as integrated components of the energy distribution system [18,19]. A comprehensive cost–benefit analysis of V2G technology highlights divergent motivations and challenges from the perspectives of EV owners and grid operators, underscoring both opportunities and critical barriers for broader implementation. However, for V2G to be successfully implemented, it is essential to address a range of significant challenges, which arise from multiple perspectives: the vehicles themselves, the aggregator, the supporting infrastructure, the electricity grid, and the behavior and involvement of users (Figure 1). Another relevant aspect is the potential cybersecurity risks and possible mitigation strategies.

2.1. Electric Vehicles

EVs are built on a variety of drivetrain architectures, including Battery Electric Vehicles (BEVs), Plug-in Hybrid Electric Vehicles (PHEVs), and Fuel Cell Electric Vehicles (FCEVs) [20,21,22]. In 2023, almost 14 million new electric cars were registered worldwide, increasing the total number of EVs on the road to 40 million [23] (Figure 2).

However, when it comes to V2G applications, BEVs stand out as the most effective option [24]. FCEVs, which generate electricity using a fuel cell powered by hydrogen, represent a unique approach to sustainable transportation. However, they face specific challenges, such as managing fuel cell degradation and maintaining optimal operational temperatures. Recent advancements in fuel cell technology include adaptive energy management strategies based on the Rule-DDPG method to mitigate degradation effects, as well as studies on proton exchange membrane (PEM) fuel cells that propose automated temperature calibration models to improve efficiency and extend fuel cell lifespan [25,26].

Unlike PHEVs, which are limited by smaller batteries and an internal combustion engine, BEVs rely entirely on electricity, making them more suitable for providing power to the grid without generating emissions [27]. Their large-capacity batteries, designed specifically for storing and delivering power, make a full battery vehicle ideal for V2G systems, where they can act as mobile energy storage units [28,29]. The evolution of the sector pushes economic expectations for the sector and the global V2G technology market size was valued at USD 3.78 billion in 2023, and it is expected to be worth around USD 45.09 billion by 2033, recording a CAGR of 28.13% over the forecast period from 2024 to 2033. The global V2G technology market is expected to be driven by the emergence of EVs across the globe (Figure 3) [30].

As the demand for BEVs continues to grow, there is a parallel push to standardize vehicle-side charging systems, both AC and DC, that can support V2G operations (

Table 1. Vehicles enabled for V2G service.

Vehicle Model	Battery Capacity (kWh)	Charging Standard	V2G Compatibility	Details
Nissan LEAF	40–62	CHAdeMO	Fully compatible with V2G	One of the first vehicles to support V2G. Widely used in projects like Powerloop in the UK to return energy to the grid [31,32,33,34].
Mitsubishi Outlander PHEV	13.8	CHAdeMO	Supports V2G for grid stabilization in Japan and Europe	Popular hybrid SUV used for grid stabilization in Japan and Europe, particularly beneficial for energy peaks [35,36,37].
Volkswagen ID.3, ID.4 and ID.4 GTX	45–77	CCS	V2G-ready	Volkswagen has optimized ID.4 for V2G to support smart grids, particularly in Europe where grid services are expanding [38,39,40].
Renault ZOE	52	CCS	V2G-enabled	Used in French pilot projects for grid stabilization and renewable energy integration [41,42,43,44].
Honda e	35.5	CCS	Fully compatible with V2G	Participates in European V2G pilot projects aimed at renewable energy and grid management support [45,46,47,48].
Nissan e-NV200	24–40	CHAdeMO	V2G-compatible	Commercial electric van, ideal for businesses, capable of stabilizing grids and reducing energy costs [49,50,51,52].
Cupra Born	58–77	CCS	V2G-capable	SEAT’s electric model with V2G technology for grid interaction and energy balancing in urban areas [53,54,55].
Volvo EX90	111	CCS	Will support V2G	Scheduled for release in 2024, the EX90 will support V2G, enhancing urban decarbonization efforts [55,56,57,58].
Kia EV9	99.8	CCS	V2G-capable	Kia’s first large SUV designed to fully integrate V2G, helping with grid load management and energy optimization [59,60,61].

).

In terms of technology, two main components are key for enabling V2G in EVs: the batteries themselves and the bidirectional chargers. Ensuring the longevity and efficiency of V2G systems depends largely on overcoming these hurdles [15]. One of the main concerns is battery aging, which occurs in two forms: calendar aging, where the battery degrades over time even when not in use, and cyclic aging, which happens with charging and discharging cycles [62,63]. Additionally, developing bidirectional chargers that enable efficient energy flow between the vehicle and the grid remains a critical hurdle [64,65].

2.1.1. Battery Degradation

In the context of V2G technology, the performance and longevity of Lithium-Ion Batteries (LIBs) are crucial, as they serve as the energy storage systems that facilitate bidirectional energy flow between EVs and the grid. Understanding the internal structure of LIBs and the mechanisms that contribute to their degradation is essential to maximize their effectiveness in V2G applications. Besides these issues, accurately and timely monitoring the health state of LIBs is equally important. Micro-health parameters reflect the performance of active materials and electrolytes within the battery, with changes in these parameters indicating the battery’s internal health state [66]. LIBs are made up of four essential components: the cathode, the anode, an electrolyte, and a separator that isolates the two electrodes. The anode typically consists of materials like graphite or lithium titanate (LTO), while the cathode is made from compounds such as lithium iron phosphate (LFP) or nickel cobalt manganese (NCM). The electrolyte facilitates the movement of lithium ions between the electrodes through the separator, which prevents electron flow but allows lithium ions to pass. However, over time, the battery’s performance can degrade due to two primary aging processes: calendar aging and cyclic aging [62,63]. Calendar aging occurs as the battery degrades over time, even when it is not actively used, as mentioned before. Environmental factors such as high storage temperatures and humidity accelerate this type of degradation by corroding internal components and leading to electrolyte decomposition [67,68]. This results in a natural loss of charge, known as self-discharge, and a gradual reduction in capacity. On the other hand, cyclic aging results from the repeated charge–discharge cycles that the battery undergoes during regular use. V2G operations, which require frequent cycling to supply energy to the grid and then recharge during off-peak periods, can accelerate cyclic aging. Factors such as high charge/discharge rates (C-rate) and high Depth of Discharge (DoD) place stress on the electrodes, accelerating the resistance breakdown of active materials and increasing the risk of forming lithium dendrites [69]. These dendrites, especially when formed during high-current charging or at low temperatures, can cause internal short circuits and compromise the battery’s safety and lifespan. From the perspective of V2G, the bidirectional energy flow puts additional strain on the battery’s internal components [70]. The frequent cycling associated with V2G systems can accelerate the growth of the Solid Electrolyte Interphase (SEI) layer on the anode, a process that consumes lithium ions and contributes to capacity loss [71,72,73]. Furthermore, this repetitive charging and discharging increases the internal resistance of the battery, reducing its overall efficiency and making it less effective in providing the grid with energy. At the same time, the V2G system’s demand for fast response times and high-energy throughput can aggravate the mechanical stresses on the electrodes, leading to particle fractures and further degradation [74,75].

The cathode is also vulnerable to degradation in V2G applications. As lithium ions move in and out of the cathode during charge–discharge cycles, the material can undergo structural changes that reduce its ability to store energy. In addition, the formation of a Cathode–Electrolyte Interphase (CEI), which is similar to the SEI layer on the anode, restricts the movement of lithium ions and contributes to capacity fade [76]. Over time, both the anode and cathode experience a gradual loss of active material, diminishing their ability to store and release energy.

The frequency and depth of charge–discharge cycles directly affect battery degradation. High DoD, where batteries are cycled between a wide SOC range (e.g., 90% to 20%), tends to accelerate aging due to increased mechanical and chemical stress on battery materials. Research on LIBs with NCM cathodes has shown that deeper discharges intensify degradation mechanisms like lithium plating and electrode material loss, which reduce capacity over time. To mitigate this, some studies suggest limiting the discharge range (e.g., 90% to 65%) in V2G applications, which can potentially lower the degradation rate while still contributing to grid support [77]. Calendar aging—degradation that occurs even when the battery is not actively cycled—is also influenced by the SOC at which batteries are stored or frequently held. High SOC levels (above 80%) increase the rate of degradation due to heightened internal resistance and side reactions within the battery. In V2G settings, strategies that maintain batteries at intermediate SOC levels when they are not in active use can significantly reduce calendar aging. For example, maintaining SOC around 50–60% during idle times has been shown to improve battery longevity in scenarios that involve frequent but shallow discharges, such as DR services [64]. Advanced control algorithms, such as aging-aware optimization, have been proposed to mitigate battery wear in V2G applications. These algorithms use real-time data to adjust charging and discharging patterns based on current battery health, grid demand, and forecasted energy needs. By scheduling discharges during peak demand and keeping the battery within moderate SOC ranges, these algorithms can enhance battery life and maintain V2G profitability by minimizing degradation costs [64].

In summary, while V2G technology presents significant opportunities for enhancing grid flexibility and energy management, it also introduces additional challenges for battery longevity. The increased cycling and higher stress on the battery’s internal components in V2G operations lead to accelerated aging, particularly in the form of SEI and CEI growth, electrode degradation, and lithium dendrite formation [78]. Understanding these degradation mechanisms [79] is essential to improving battery management systems (BMSs) that can optimize the charging and discharging cycles, thereby extending battery life in V2G applications [80,81,82]. By addressing these challenges, V2G systems can maximize the potential of EVs as distributed energy resources while maintaining the long-term viability of the batteries that power them. Battery degradation has direct economic implications for the viability of V2G systems, as increased degradation rates reduce the resale value of EVs and raise replacement costs. The financial returns from participating in V2G must therefore outweigh the costs associated with accelerated battery aging. Research shows that well-managed V2G programs, that limit deep discharges and incorporate predictive maintenance, can enhance the economic feasibility of V2G. However, without such management, the cost of degradation could deter EV owners from participating in V2G, undermining the technology’s potential as a scalable energy solution.

2.1.2. Bidirectional Charger

Bidirectional chargers are a key component of V2G technology, allowing EVs to both receive power from and supply energy back to the grid. These chargers enable EVs to act as dynamic energy storage units, providing grid support during peak demand periods and storing excess energy during off-peak times. Despite their potential, implementation challenges persist. The high cost and complexity of bidirectional power converters are major barriers, as they must efficiently manage power flow in both directions while minimizing energy loss, requiring advanced control systems and designs [83,84,85]. An efficient bidirectional battery charger for V2G applications, utilizing an open-end winding multilevel converter, enhances energy flow optimization, reduces costs, and seamlessly integrates V2G functions; however, it still encounters challenges related to control complexity and power management [86]. Additionally, standardization remains a critical issue. Wireless power transfer (WPT) poses specific interoperability challenges in bidirectional charging systems. As WPT technology advances, ensuring seamless operation between devices from various manufacturers is increasingly important. Poor interoperability in WPT can lead to low charging efficiency or even charging failures, causing substantial power loss. Establishing clear interoperability standards is essential to meet performance requirements and ensure effective energy exchange across different WPT systems [87]. Different EV manufacturers use varied charging standards, like CCS or CHAdeMO, leading to compatibility issues that hinder V2G system interoperability and scalability across regions. Efforts to standardize charging protocols and harmonize regulatory frameworks are vital for the growth of V2G technology [19,88]. On the positive side, bidirectional chargers offer considerable opportunities for improving energy management and grid resilience. They can help to reduce peak loads and to increase energy arbitrage [88] on the grid by allowing EVs to supply energy during high-demand periods, thereby reducing the strain on conventional power plants. In addition, they provide financial incentives for EV owners, who can earn revenue by selling stored energy back to the grid when prices are high [19,89]. Recent studies on bidirectional converters for V2G applications focus on improving efficiency and addressing key challenges. For instance, research into single-phase bidirectional converters explores designs that provide high efficiency through interleaved buck-boost stages, ensuring wide battery voltage ranges and low ripple currents during energy transfer between EVs and the grid [90]. Innovations in three-phase bidirectional chargers integrate AC-DC and DC-DC conversion to optimize power flow, allowing for flexible grid integration and efficient energy exchange, while minimizing transmission losses [84]. The main technical challenges associated with bidirectional charging systems in V2G systems are therefore multiple and involve power electronics, grid synchronization, battery management, infrastructure, and cybersecurity. The high switching frequencies needed for efficient power conversion can lead to electromagnetic interference (EMI) issues, impacting both the grid and surrounding electronic devices. The design and durability of these converters are critical, as they must operate reliably over extended periods without excessive power losses, which would affect both system efficiency and grid stability. For V2G to function seamlessly, bidirectional chargers must synchronize precisely with grid voltage, frequency, and phase. This synchronization ensures that EV power can be injected into the grid without causing fluctuations in grid parameters. Mismatches can lead to voltage spikes, harmonics, and frequency imbalances, threatening grid stability. Advanced grid-tied inverters are required to maintain seamless synchronization, and these often rely on real-time communication with grid operators to adjust to the dynamic conditions of the power system [14,64]. Reliable, real-time communication between EVs, charging stations, and grid operators is essential to enable load balancing, DR, and optimized power flow. Implementing this requires low-latency communication networks capable of supporting high data throughput. Protocols like OCPP (Open Charge Point Protocol) and IEEE 2030.5 are becoming more common, but achieving interoperability across different hardware and software platforms is challenging. Furthermore, delays or disruptions in communication can cause miscoordination, resulting in energy loss or even grid instability [64].

In conclusion, as the demand for renewable energy grows, bidirectional chargers are essential for grid stability. Future developments should focus on reducing costs, improving efficiency, and ensuring compatibility across different vehicle models and grid systems. Advances in power electronics, energy management, and supportive policies will be crucial for scaling up V2G technology.

2.2. Infrastructures

Charging infrastructures serve as the critical link between EVs and the power grid, enabling the implementation of V2G services (Figure 4) [91,92,93].

AC-based infrastructures are the more prevalent option, largely due to their lower cost [94], but they demand higher technological capabilities from vehicle systems, resulting in reduced flexibility for V2G operations. Conversely, DC charging infrastructures are designed for greater efficiency and faster charging, particularly beneficial for V2G applications, though they require more significant financial investment in charging stations [95]. In AC charging systems, the energy provided to the EV is in AC. However, since EV batteries store energy as direct current (DC), the vehicle’s onboard inverter is required to convert this AC into DC for storage. When the vehicle participates in V2G services and returns energy to the grid, the onboard inverter must perform a second conversion, transforming the stored energy as DC back into AC, as the power grid operates on AC. To handle these demanding tasks, vehicles equipped for V2G using AC infrastructure need advanced inverters capable of managing these dual conversions efficiently. However, the need for such sophisticated inverters not only raises the technological complexity but also contributes to higher vehicle costs, making this a critical consideration in the widespread adoption of V2G technology from a vehicle point of view. DC charging, on the other hand, bypasses these conversion issues by providing energy in DC, directly compatible with the battery’s storage. Although DC charging infrastructure is more expensive and complex, incorporating advanced energy converters for fast charging and bidirectional energy flow, it reduces the technological burden on the vehicle and simplifies the V2G process on the vehicle side. DC chargers can deliver much higher power, typically ranging from 50 kW to over 350 kW, making them ideal for rapid charging, for example, along highways or in high-density urban areas. In summary, AC charging tends to be slower and is limited by the inverter’s onboard capacity, usually between 7 and 22 kW. While DC infrastructure demands higher investment and is more complex, it provides significantly faster charging times and requires less technological complexity within the vehicle itself. Advances in power electronics have optimized chargers, reducing size and cost, yet widespread V2G adoption also depends on developing support systems for efficient coordination between the grid and EVs. Effective compensation models are also integral; these models should account for battery degradation and the value of grid services provided by EVs. Some research explores financial models where compensation is tied directly to the energy fed back during high-demand periods, improving the economic feasibility of V2G for EV owners [96].

Table 2. Summary of AC and DC infrastructures in V2G.

Feature	Alternating Current (AC)	Direct Current (DC)
Functionality	Conversion from AC to DC takes place inside the vehicle via the onboard inverter.	The conversion from AC to DC is handled directly within the charging station.
Cost and Infrastructure	Charging stations are less expensive, but vehicles need more advanced onboard inverters.	Charging stations are more costly, as they include sophisticated energy conversion systems.
Charging Speed	Slower charging rates, constrained by the vehicle’s onboard inverter capacity (7–22 kW).	Significantly faster charging, capable of delivering power up to 350 kW or more.
Onboard Vehicle Complexity	The vehicle’s inverter handles the energy conversion process.	The vehicle requires simpler inverter technology, as conversions occur at the station level.

provides a comparison of the key differences between AC and DC charging infrastructures. Meanwhile,

Table 3. Key standard in V2G charging infrastructures.

Standard	Description	Key Functions	References
ISO 15118-20	A communication protocol facilitating interaction between EVs and charging points, enabling V2G and seamless plug and charge features.	▪ Advanced bidirectional communication ▪ Wireless charging compatibility ▪ Enhanced security ▪ Plug and charge ▪ Interoperability	[97]
IEC 61851-1	Specifies the general requirements for conductive charging systems for EVs.	▪ Charging modes ▪ Electrical safety ▪ Communication and control ▪ Environmental requirements ▪ Electromagnetic Compatibility (EMC)	[98]
IEC 61850	A communication framework designed for smart grid control and management.	▪ Data modeling ▪ Abstract Communication Services Interface (ACSI) ▪ Protocol mapping ▪ Fast event transfer ▪ Substation Configuration Language (SCL) ▪ Scalability and flexibility	[99]
IEC 63110	A developing standard focusing on the management of EV charging infrastructures with emphasis on V2G capabilities.	▪ IEC 63110-1: basic definitions, use cases, and architectures ▪ Energy transfer management ▪ Charging resource management ▪ Authentication, authorization, and payment ▪ E-mobility services ▪ Cybersecurity ▪ IEC 63110-2: technical protocol specifications and requirements ▪ Technical protocol specifications ▪ Functional requirements ▪ Interoperability ▪ Security measures ▪ Scalability ▪ IEC 63110-3: requirements for conformance tests ▪ Conformance testing procedures ▪ Test case specifications ▪ Certification guidelines ▪ Interoperability assurance ▪ Continuous compliance monitoring	[100,101,102]
CHAdeMO 3.0/ChaoJi	A DC fast-charging standard designed for high-power, bidirectional charging, enabling compatibility across Asian markets through ChaoJi integration.	▪ High power capacity ▪ Compact and lightweight connector ▪ Backward compatibility ▪ Enhanced safety ▪ Global interoperability	[103,104]
Combined Charging System (CCS)	A widely adopted fast-charging standard in Europe and North America that combines AC and DC charging in a single solution.	▪ AC and DC charging ▪ DC fast-charging ▪ European standardization ▪ Compatibility ▪ Advanced safety	[105]
OCPP 2.0.1	An open-source communication protocol between charging stations and central systems, enabling interoperability and data management for bidirectional power flow.	▪ Real-time monitoring ▪ Demand response signals ▪ Flexible power distribution ▪ Battery status reporting	[106]
NACS (North American Charging Standard)	Adopted by automakers for simplicity and reliability, NACS builds on CCS2 principles, focusing on streamlined EV and charging infrastructure in North America.	▪ Bidirectional support ▪ Simplified connector design ▪ V2G readiness ▪ Reliable high-efficiency charging	[107]

outlines the main standards relevant to V2G charging systems, emphasizing their specific roles and functions. Recent advancements in V2G-compatible charging standards focus on enabling seamless, bidirectional energy transfer between EVs and the grid, supporting both home energy needs and broader grid stability. The global development of these standards addresses key interoperability, safety, and efficiency aspects essential for scaling V2G technology.

2.3. Users

V2G technology brings multiple benefits to end users, although it is accompanied by a series of challenges that need to be addressed [108]. A significant advantage for users is the potential to lower their electricity bills by utilizing energy arbitrage. By charging their EVs during off-peak hours, when electricity is cheaper, and discharging stored energy back to the grid during peak times, users can benefit financially. This practice allows the vehicle to function as an energy storage unit, storing energy at low costs and selling it back to the grid at higher prices [5,95,109]. In addition to reducing their energy costs, users can also generate revenue by participating in grid services, such as energy trading or frequency regulation, which helps balance the grid during periods of fluctuating demand [110]. According to [111], when considering both operational and investment costs alongside the associated benefits, V2G can represent a highly attractive financial opportunity for certain EV owners. This is especially true in situations where regulatory hurdles are addressed, interoperability between vehicles and systems is ensured, and vehicle usage is low, making the vehicle more suitable for providing grid services. When energy demand fluctuates, EVs can supply stored energy back to the grid, allowing users to earn profits by helping stabilize grid operations during periods of high or low demand [7,95,112].

V2G systems also enable EVs to participate in grid stabilization services like frequency regulation, which balances short-term fluctuations in the power grid [73,113]. Although the revenue from these services may be modest, it adds another layer of financial incentive, making V2G systems more appealing to users who seek to capitalize on their vehicle’s energy storage capacity [114]. They also play a direct role in decarbonizing the energy grid by facilitating the integration of RESs [115,116]. By participating in V2G, EV owners support the management of intermittent renewable energy generation, particularly from sources like solar and wind [117]. This not only helps stabilize the grid, but also allows users to feel actively involved in the transition to cleaner energy. Many users see environmental impact, especially the reduction in emissions, as one of the most meaningful benefits of V2G technology [118]. In addition to the environmental and financial benefits, V2G provides the additional security of backup power in case of emergencies. During power outages or blackouts, EVs equipped with bidirectional charging capabilities can power essential home devices or even sustain an entire household for short periods. This function is particularly valuable in regions prone to natural disasters or frequent power interruptions, offering a layer of resilience and independence for users [119].

Despite the clear benefits, concerns about the long-term impact of V2G on battery health persist among users. Frequent charge–discharge cycles required for V2G services can, as mentioned, accelerate battery degradation, leading to fears about reduced vehicle performance and increased maintenance costs over time [115,118]. Although smart charging strategies can mitigate some of these effects, the uncertainty surrounding battery longevity remains a barrier for widespread V2G adoption [120]. From the EV owner’s perspective, battery degradation remains a crucial factor. Studies model degradation based on depth and frequency of battery cycling, revealing that battery wear leads to capacity loss and heightened resistance, which, in turn, impacts long-term usability and profitability [121]. High DoD values correlate with accelerated aging, making it essential to employ tailored discharge strategies. Often modeled using degradation curves, such as Peukert’s Law, these strategies balance battery utilization against lifespan preservation. To make V2G participation viable for EV owners, earnings from peak-period grid services must compensate for these degradation costs. Additionally, compatibility issues limit access to V2G services for many users. Currently, only certain EV models, such as those with the CHAdeMO standard, and specific bidirectional chargers are V2G-capable. This lack of widespread compatibility prevents many EV owners from participating in V2G until these systems are standardized across more vehicle and charger models. Installing the necessary infrastructure, such as bidirectional chargers, can also be costly, adding another hurdle to adoption [122]. The upfront cost of setting up a V2G-compatible system, especially when installing dedicated DC charging infrastructure at home or work, can also be prohibitively high. Although long-term savings on energy costs and potential revenue generation can offset these costs, the initial financial investment remains a significant barrier for many potential users. Government incentives or subsidies can help alleviate some of these costs, but the financial feasibility of V2G often depends on local policies [123]. Finally, V2G technology requires users to adopt new habits when it comes to charging their vehicles. This involves either planning charging schedules more carefully or relying on smart systems to manage energy use based on grid conditions and fluctuating electricity prices. For many users, this increased involvement in energy management may be seen as an inconvenience, but it is a necessary shift for maximizing the benefits of V2G [123].

In summary, the main economic incentives for EV owners to participate in V2G services, as highlighted in recent studies, include the following:

▪

Direct Financial Compensation: V2G programs often provide monetary rewards for EV owners who discharge energy back to the grid, particularly during peak demand periods. This compensation can come in various forms, such as per-kWh payments, fixed monthly income, or demand response payments for supporting grid stability during peak times [124].

▪

Reduction in EV Ownership Costs: By participating in V2G, EV owners can offset ownership costs, including battery maintenance and replacement. This economic benefit helps make EV ownership more financially viable, especially for those using their vehicles in V2G-compatible parking lots with bidirectional chargers, where they can also benefit from specific cost-sharing schemes for infrastructure use [125]

▪

Time-of-Use (ToU) Price Arbitrage: V2G allows EV owners to take advantage of fluctuating electricity rates by charging their vehicles during low-cost, off-peak hours and discharging during high-cost, peak periods. This not only reduces energy costs but also enables EV owners to earn from energy sold back to the grid at premium peak rates [126]

▪

Battery Longevity and Customized Participation: Many V2G programs address battery degradation concerns by offering guarantees on minimum charge levels and discharging limits, which reassure owners about battery health. Flexible contract options and “fit-and-forget” strategies, where vehicles can autonomously participate in V2G without user intervention, also encourage participation [127]

▪

Environmental and Social Responsibility: For owners motivated by environmental impact, V2G offers an opportunity to support renewable energy and grid decarbonization. This aligns with the interests of idealistic or environmentally conscious users who see V2G as contributing to a low-carbon future [128]

2.4. Aggregators

The role of vehicle aggregates in the V2G ecosystem is extensively explored in recent and foundational research, which highlights their critical contribution to the scaling, efficiency, and viability of V2G services. Aggregates refer to a collection of EVs that are managed as a single coordinated resource, allowing for more efficient and impactful participation in grid services. The importance of vehicle aggregates in the context of V2G can be understood from several perspectives such as scalability and enhanced grid services, improved grid stability and flexibility, economic viability and market participation, optimization and resource management, distributed and decentralized energy resources, and integration with smart grid technologies [129,130,131]. Aggregators act as intermediaries between individual EVs and the energy grid, pooling multiple EVs to form a collective Virtual Power Plant (VPP). This allows even small, distributed energy resources to participate in energy markets and deliver grid services. Aggregators coordinate the charging and discharging of these EVs in response to grid signals, balancing demand and supply dynamically. By managing a network of EVs, aggregators can optimize charging schedules to match the grid’s needs, either by absorbing excess energy during low-demand periods or dispatching stored power back into the grid during peak times [132]. Aggregators also handle the complexity of communication and billing, enabling V2G to integrate seamlessly with grid management systems and helping overcome barriers to entry for individual EV owners. By using advanced algorithms, they optimize when and how EVs participate in grid services, maximizing both the financial returns for EV owners and the operational efficiency of the grid. The importance of aggregation strategies and distributed control architectures in optimizing the collective use of EVs are emphasized in [133]. By coordinating large numbers of EVs as a unified resource, aggregates can provide more efficient grid services, especially in scenarios involving renewable energy variability and fluctuating grid demands. The ability to manage a large fleet of vehicles allows for significant contributions such as frequency regulation, voltage support, and peak load management. The relationship between the size of the aggregate and its effectiveness is treated in [134]. Their research shows that as the number of vehicles in the aggregate increases, the ability to provide grid services such as DR and energy balancing improves. However, they also highlight the challenges associated with managing larger aggregates, such as increased complexity in communication and coordination between EVs and the grid. Their findings suggest that distributed control algorithms may help mitigate the difficulties of scaling aggregates to large numbers. An important contribution on the use of dynamic aggregation models for EVs is provided in [135]. The authors focus the study on the variability of vehicle availability and energy storage capacity. Their research highlights that dynamic aggregation, as opposed to static models, allows for more flexible and efficient responses to real-time grid conditions. By incorporating stochastic elements into their models, Zhao et al. show that dynamic aggregation enhances the predictability of available resources, thus reducing the risk of under-delivering during critical grid services. In [116], authors extend the literature on the use of artificial intelligence (AI) in managing EV aggregates. Their study highlights how AI and machine learning algorithms can optimize the charging and discharging schedules of large EV fleets in real time, based on predictive models of vehicle availability and energy demand. They demonstrate that these AI-driven management techniques significantly improve the reliability and economic performance of vehicle aggregates, especially in grids with high penetration of renewable energy sources. The integration of machine learning and demand response mechanisms in managing large EV aggregates has been explored also by [136]. They propose a predictive model based on historical data of EV usage patterns and grid demand fluctuations. Their results show that machine learning can significantly enhance the accuracy of load forecasting for EV aggregates, thereby improving their ability to provide fast-response grid services like frequency control and peak shaving. The use of artificial intelligence is also supported by [112,137,138]. In another significant contribution, the use of blockchain technology to manage decentralized EV aggregates in V2G systems is explored [139]. Their study demonstrates how blockchain can be utilized to create secure, transparent, and efficient management of energy exchanges within EV fleets. The authors argue that blockchain’s decentralized nature complements the distributed control systems often used in large EV aggregates, facilitating trust and coordination between various stakeholders without the need for centralized oversight. The impact of spatial distribution on the performance of EV aggregates in the provision of ancillary network services is explored in depth in [140]. Their study shows that the geographic dispersion of EVs affects the timing and quantity of energy that can be injected into the grid. They propose optimization algorithms that account for the location of charging stations and the distribution of EVs to enhance the aggregate’s ability to meet demand response and frequency regulation needs, particularly in urban versus rural settings. Recent research emphasizes that for V2G to benefit grid operators economically, advanced dispatch strategies must be employed [95]. Active and reactive power dispatch (APD and RPD), for example, are advanced approaches that allow grid operators to manage EV charging and discharging patterns in real time, optimizing grid stability and minimizing power losses [141]. These methods have been shown to reduce grid losses, but they require sophisticated infrastructure and data integration capabilities that add to the upfront and ongoing costs of V2G deployment. The role of aggregator business models and their impact on the performance of EV aggregates is discussed by [142]. Their study compares different aggregator schemes, from fully centralized to hybrid models, where EV owners retain partial control over their energy resources. They conclude that hybrid models offer a balance between efficiency and user engagement, enabling higher participation rates from EV owners while still maintaining robust grid service delivery.

The economic aspect of EV aggregators is instead evaluated in [143], where the authors focus particularly on participation in real-time electricity markets. They develop a bidding strategy that optimizes the financial returns of EV aggregates while ensuring compliance with grid requirements. Their research reveals that advanced forecasting models, combined with real-time data on grid conditions, can significantly improve the profitability of EV aggregates when participating in energy and frequency regulation markets.

2.5. Electric Grid

The bidirectional interaction of V2G introduces a new layer of flexibility and resilience to grid operations, particularly as energy systems increasingly incorporate intermittent RESs [5,144]. From the grid’s viewpoint, V2G provides several essential services that improve overall grid stability and efficiency.

Demand-Side Management (DSM) in V2G applications leverages bidirectional energy flow between EVs and the grid to dynamically adjust charging and discharging schedules based on real-time grid demands by shifting load from peak to off-peak periods. Studies indicate that V2G can reduce peak load by 3–20%, depending on factors like EV penetration levels, charging modes, and efficient aggregator management, which significantly bolsters grid reliability and resilience [145]. Through predictive algorithms and advanced load-shifting models, aggregators analyze past consumption patterns, grid conditions, and real-time forecasts to optimize charging and discharging schedules, which enhances system efficiency. These adjustments also support renewable energy integration, allowing EVs to absorb excess energy generated during low-demand periods, such as midday solar production, and redistribute it as needed, reducing reliance on fossil-fuel plants and supporting a more sustainable grid structure. Advanced control systems integrating AI and machine learning (ML) play a critical role in DSM for V2G, enabling accurate demand predictions and optimizing EV behavior accordingly. AI-driven forecasting models anticipate demand fluctuations, aligning EV discharges with peak grid needs to avoid expensive peaking plants. Aggregators manage the coordination of energy during peak hours, ensuring that discharge schedules are aligned with dynamic pricing schemes that reward EV owners [130]. As regulatory and technological advancements support V2G, these strategies will become integral to modern, sustainable electric grid operations. A key aspect is frequency regulation, where EVs contribute to maintaining grid frequency within the desired operational range by either charging or discharging in response to real-time signals. The research by [96] explores how aggregated EVs can supply ancillary services like frequency regulation, showing that their rapid response times make them particularly suitable for this role. Frequency regulation is crucial for balancing supply and demand, especially as renewable energy sources, such as solar and wind, introduce greater variability into the power system [113,146]. Through inertia emulation, V2G compensates for the inertia loss caused by high penetration of non-synchronous renewables, like wind and solar, responding quickly to load changes. This approach enables EVs to participate in primary and secondary frequency control, managing charging and discharging to stabilize the grid and reduce fluctuations, thereby supporting effective intermittency management [147,148]. Another important service provided by V2G is voltage support. As EVs discharge energy back into the grid, they help stabilize voltage levels, particularly in local distribution networks. Studies such as those by [149] indicate that aggregated EVs can assist in mitigating voltage fluctuations by acting as distributed energy storage systems, ensuring that power quality is maintained at both the local and grid-wide levels. The study proposed by [150] discusses the application of V2G control techniques, comparing the results for the voltage reading at the PCC and at the medium to low voltage transformers upstream of the EVs’ outlet buses. V2G also plays a role in peak shaving, which involves reducing the grid’s load during periods of high demand. Research by [151] highlights the role of EVs in DSM, showing that by shifting load from peak to off-peak periods, EVs can significantly reduce the need for additional grid infrastructure. In [152], existing research on the DSM operation of EVs, which has witnessed significant interest in the energy management domain in the last few years, has been reviewed extensively. The impacts of penetrating the EVs and V2G system on the demand profile, especially during the peak demand are explored by [145] through different scenarios that are defined to assess the performance of the EVs and V2G system in shaving the peak demand and filling the valley demand. Aggregated EVs can store excess energy generated during periods of high renewable output and then discharge it during times of lower generation. This flexibility makes V2G an important component of the future grid, which must accommodate variable energy sources while maintaining reliable energy delivery. In [153], the authors discuss how V2G services can complement the integration of renewables, enabling a smoother transition towards sustainable energy systems. The study proposed by [154] provides, instead, theoretical and model support for the future development of V2G and policy formulation, underscoring the comparative advantages of V2G in enhancing green energy utilization efficiency. The capability of EV fleets to function as a unified storage system for excess power facilitates greater integration of RESs into current grid infrastructures and future smart grids via bidirectional communication, are also explored in [155]. The integration of V2G into the electrical grid also improves grid resilience. In the event of grid disturbances, such as outages or instability, EVs can serve as mobile backup generators, providing emergency power to critical infrastructure or local areas. Studies such as those by [156] explore the potential for EVs to support grid recovery efforts after power interruptions, enhancing overall grid security and resilience in the face of increasing climate-related disruptions.

However, implementing V2G services at scale presents challenges from the grid’s perspective. Managing the bidirectional flow of energy between millions of EVs and the grid requires sophisticated control systems and communication protocols. In [157], the authors analyse the technical complexities of integrating large fleets of EVs into grid operations, emphasizing the need for real-time communication and coordination between grid operators and EV aggregators. These systems must ensure that the energy supplied by EVs is both reliable and delivered in a way that supports grid requirements without negatively affecting the EVs’ primary function as transportation. As [158] underlines, the non-linearity of the load resulting from the characteristics of the EVs causes various energy quality problems to the network that must be appropriately managed. Several studies emphasize the potential of V2G to participate in ancillary service markets, showing that the collective power of aggregated EVs can become a competitive asset for grid operators. The financial incentives for grid operators to adopt V2G solutions lie in the cost savings from avoided infrastructure expansion and reduced reliance on traditional power plants during peak demand [159,160,161]. In conclusion, the provision of V2G services from the perspective of the electrical grid offers a wide range of benefits, from improving grid stability and flexibility, to enhancing the integration of renewable energy sources. However, its widespread implementation requires addressing technical, economic, and regulatory challenges to ensure that EVs can reliably and effectively contribute to grid operations without disrupting their primary role as vehicles.

2.6. Potential Cybersecurity Risks Associated with V2G and Smart Grid Integration

Integrating V2G technology into smart grids introduces several cybersecurity risks, given the complex interactions among EVs, charging infrastructure, and grid management systems. Addressing these vulnerabilities is essential for maintaining grid reliability and securing sensitive information within V2G networks. Data interception risks can be particularly relevant in V2G systems, where sensitive information, such as user data and energy consumption patterns, is exchanged between EVs and grid operators. Such data breaches can lead to unauthorized access and compromise user privacy, necessitating robust encryption measures to secure communication channels [162]. Man-in-the-Middle (MitM) attacks could allow attackers to intercept and alter communications between EVs, charging stations, and grid operators. This manipulation would allow unauthorized control over charging processes and billing, potentially leading to financial losses and grid instability. Implementing mutual authentication protocols, such as those outlined in the ISO 15118 standard, helps mitigate these risks by ensuring all communications are authenticated before access is granted [163]. Denial of Service (DoS) and Distributed Denial of Service (DDoS) attacks could also be substantial threats to V2G infrastructure. These attacks overwhelm the network with excessive traffic, rendering V2G services inaccessible to legitimate users and disrupting energy supply–demand balance, potentially leading to grid instability. Intrusion detection systems (IDSs) and traffic monitoring tools are essential for mitigating these attacks and maintaining system responsiveness during peak demand [164]. Malware attacks introduced through firmware vulnerabilities or infected software could severely impact V2G systems by intercepting communications, manipulating charging schedules, or initiating unauthorized transactions. Regular firmware updates and secure development practices help protect against such threats, preserving operational integrity and data security [163]. Physical tampering is an additional risk, as direct access to EVs or charging infrastructure enables attackers to alter hardware, intercept data, or disrupt power exchange operations between the grid and vehicles.

Mitigation strategies for these risks include robust encryption protocols, such as TLS, combined with mutual authentication, like ISO 15118’s standards, to ensure secure communication between EVs and the grid, keeping data accessible only to authorized entities [162]. Intrusion Detection Systems (IDSs) and real-time monitoring enable continuous detection of unusual network activity indicative of threats like MitM or DoS attacks. Quick detection and response to suspicious activity are essential for maintaining system integrity [164]. Regular firmware updates address known vulnerabilities and ensure secure operations, while endpoint security measures protect against malware, preserving data and operational integrity in V2G systems [163]. Physical security measures, such as locks and surveillance, are essential for safeguarding V2G network components, while Role-Based Access Control (RBAC) ensures only authorized personnel access critical systems and data, reducing insider threat risks [162].

3. Pilot Projects and Regulatory Framework

As highlighted in the literature, the implementation of V2G systems poses several significant challenges, as evidenced by various V2G projects currently being developed around the world.

Table 4. Overview of the main projects at a global level.

Country	Project	Description	Benefits	References
United Kingdom	Powerloop	Led by Octopus Energy, utilizing Nissan Leaf EVs to return energy to the grid.	Reduction in peak demand, integration of renewable energy, cost savings for EV owners	[32]
	Electric Nation Vehicle to Grid (EN-V2G)	National Grid’s V2G project using residential EVs to support the grid.	Frequency regulation, cost savings for users, increased grid flexibility	[165]
	Project Sciurus	Residential V2G project involving Nissan Leaf owners supplying energy to the grid.	Frequency regulation, reduced energy costs for participants	[34]
	Vehicle-to-Building User Interface Learning Device	Optimize energy use by utilizing EV batteries for buildings	Cost savings, grid flexibility, renewable energy use, and emission reductions	[166]
	V2X-Flex	V2X-Flex develops bidirectional charging to optimize energy usage and flexibility,	Cost reduction, energy flexibility, renewable integration, enhanced grid efficiency	[167]
Japan	Nissan LEAF V2G	Nissan LEAFs using bidirectional charging to supply energy during peaks or emergencies.	Emergency backup power, improved grid flexibility	[33]
Denmark	NUVVE Project	The first commercial V2G project, allowing EVs to sell electricity back to the grid.	Profit generation for EV owners, carbon emission reduction	[168]
Netherlands	City-zen V2G	Amsterdam-based V2G pilot with solar energy integration and smart charging.	Renewable energy storage, peak shaving	[169]
USA	University of Delaware V2G	Academic-led V2G project with NUVVE to optimize grid services.	Frequency regulation, energy storage for grid stability	[170]
France	Renault ZOE V2G	Pilot program enabling Renault ZOE owners to feed energy back into the grid during high demand.	Grid stabilization, lower energy bills for participants	[44]
Australia	ACT Government V2G	Government EV fleets testing V2G technology for grid stability.	Reduction in emissions, grid support during demand peaks	[171]
Italy	Enel X V2G Projects	Initiatives allowing EVs to contribute energy back to the grid during demand peaks.	Peak load reduction, greater renewable energy utilization	[172]
Germany	BDL (Bidirectional Charging Management)	BMW-led project focused on V2G charging in residential and commercial spaces.	Improved grid flexibility, energy storage optimization	[173]
Swiss	SunnYparc	SunnYparc integrates EV charging and V2G into a smart microgrid.	Grid stability, renewable energy use, flexible pricing, and blackout prevention	[174]
Finland	LEMENE Project	Large-scale smart energy project integrating V2G for grid stabilization.	Microgrid optimization, peak demand management	[175]

provides an overview of the most prominent V2G initiatives across different countries, showcasing achievements such as reductions in peak energy demand, enhanced integration of renewable energy, and cost savings for both energy providers and EV owners. However, despite these promising outcomes, these initial projects are still grappling with several technological and infrastructural obstacles. Scaling V2G to a broader level remains complex, requiring progress in areas such as battery degradation analysis, infrastructure development, standardization, and overcoming regulatory hurdles.

In analyzing evolving regulatory frameworks for V2G adoption across regions, insights from recent publications reveal how policies influence the technology’s integration into energy systems underlining the need for global alignment on technical standards, particularly ISO 15118-20 and CHAdeMO, to improve interoperability and reduce market fragmentation. Regulatory frameworks that mandate bidirectional charging capabilities in public and fleet vehicles could accelerate adoption and help address challenges, such as double taxation, on stored energy. The European market leads in terms of regulatory maturity, but other regions could benefit from harmonizing their policies to support both local and cross-border V2G applications.

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European Union: According to [167], the EU has positioned itself as a leader in V2G regulation, with comprehensive frameworks aimed at accelerating the technology’s adoption. Directives under the Clean Energy for All Europeans and Fit for 55 packages set ambitious targets for charging infrastructure, incorporating bidirectional capabilities as part of future-ready energy systems. The Alternative Fuels Infrastructure Regulation (AFIR), effective from 2024, requires member states to assess V2G in their resource planning, supporting V2G-capable installations. EU standards like ISO 15118-20 and the revised Renewable Energy Directive (RED III) also encourage V2G by offering incentives for EVs that store and redistribute renewable energy. Despite these advancements, challenges remain in achieving uniform adoption across member states due to different national priorities and infrastructure capabilities.

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United States: The U.S. regulatory environment for V2G is fragmented, with state-level initiatives leading the way. California is notable for its ambitious Vehicle-Grid Integration (VGI) Roadmap, which outlines goals for using EVs as flexible grid resources. The National Electric Vehicle Infrastructure (NEVI) program, part of the 2021 Bipartisan Infrastructure Law, supports V2G infrastructure, though more emphasis is currently placed on unidirectional charging. Policies around grid interoperability are still emerging, with few federal mandates for V2G technology, though private and state-funded projects show promise in regions like California, where utilities actively support V2G pilot programs [132,167]

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Japan and Republic of Korea: Japan’s CHAdeMO standard remains foundational for V2G, enabling bidirectional charging since its inception. Japan’s regulatory focus includes V2G’s role in energy resilience, particularly in disaster-prone regions. Republic of Korea’s Green New Deal promotes V2G as part of its energy transformation goals, incentivizing both EV owners and manufacturers to invest in compatible technology. Subsidies and partnerships among automakers, such as Hyundai and Kia, support V2G deployment, showcasing how regulatory encouragement and infrastructure alignment are critical in driving V2G growth in Asia.

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Australia: Australia’s V2G framework is influenced by its need for decentralized energy solutions in remote areas. The country’s regulatory approach is progressive, with state-level incentives, particularly in New South Wales, to deploy V2G-capable infrastructure within local microgrids. Regulatory efforts focus on grid flexibility and resilience, with incentives directed toward households and businesses that incorporate V2G to supplement renewable energy sources like solar. However, a lack of cohesive national policy results in inconsistent adoption, with some states lagging behind in V2G uptake due to limited funding and infrastructure support.

4. Conclusions

The exponential growth in the number of EVs is anticipated to be matched by a proportional increase in the challenges faced by the electricity grid in managing such a substantial distributed load. V2G plays a key role in this scenario, integrating RESs and fostering more sustainable and resilient energy systems, particularly within smart cities. While the benefits—such as enhanced grid resilience and lower energy costs—are substantial, large-scale implementation faces several obstacles. These include the impact of frequent charging and discharging on battery life, the high cost of infrastructure like bidirectional chargers, and the lack of standardization across different charging systems and vehicle models. This paper reviews major studies and pilot projects worldwide to highlight best practices and strategies for addressing these challenges.

From the literature analysis, it emerges that the advantages of V2G technology are notable:

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Enhanced grid resilience: V2G improves grid stability, especially during peak demand by allowing EVs to feed energy back to the grid.

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It supports the transformation of the energy system from centralized to decentralized: the problem of managing the growing demand for electricity by EVs from a critical issue becomes an opportunity deriving from the presence of distributed storage.

However, large-scale implementation faces significant obstacles:

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Battery degradation: Frequent charging and discharging accelerate battery aging, impacting EV efficiency and lifespan. In particular, declined as calendar aging (gradual loss of active material over time); cyclic aging (driven by the growth of the SEI layer, especially in high-temperature or DoD conditions, leading to reduced battery capacity); and lithium plating (occurs during high-current charging or low temperatures, limiting the battery’s energy storage and release capabilities).

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High cost of infrastructure: Bidirectional chargers, crucial for V2G, are expensive and lack standardization, adding to the overall cost.

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Compatibility issues: Many EV models are not compatible with bidirectional charging systems, restricting broader V2G adoption.

Key components are required to optimize V2G functionality and recent technological advancements promise improvements:

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Advanced Battery Management Systems (BMSs): Essential for managing charge–discharge cycles, minimizing battery degradation, and optimizing performance.

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Bidirectional converters: Facilitate energy flow between the vehicle and the grid but are complex and costly due to advanced electronics needed for managing power and voltage variations.

To ensure successful V2G integration, standardization across different EV models and charging networks is essential:

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Standardizing charging infrastructure is helping to lower costs and improve the accessibility of bidirectional converters.

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Standards for connectors and communication protocols are crucial for seamless V2G integration, as interoperability across systems is necessary for effective V2G deployment.

Successful pilot projects, such as Powerloop in the UK and the Nissan LEAF initiative in Japan, show encouraging results, but further technological advances and investment are needed to fully unlock the potential of V2G. The results also highlight the need for economic incentives, supportive regulations and adequate infrastructure to enable widespread adoption of V2G.

The policy landscape will play a critical role in V2G adoption. Incentive mechanisms, including infrastructure subsidies and grid demand-based compensation structures, will increase the likelihood that EV owners will engage in V2G. Policies that can support long-term infrastructure development, such as tax credits or subsidies for bidirectional chargers, will also provide financial relief to grid operators.

In conclusion, while V2G represents a valuable opportunity to promote sustainable urban development by leveraging EVs as dynamic energy resources, several technical, financial, and regulatory barriers still need to be overcome. The authors also intend to include methodologies for identifying hotspots and estimating demand in future work, with a focus on predicting user participation as a critical component for the development of V2G services.