Vehicle-to-grid (V2G) technology is often touted as one key to a more sustainable energy infrastructure. Like other areas where technology strongly impinges on economic and social interactions, however, the reality is complex and nuanced. Battery chargers can be a particularly daunting technical problem when implementing V2G. The charger must be bidirectional and offer high efficiency. As discussed below, the high voltages and currents needed for high-power EV chargers can present significant design challenges.
This FAQ begins with an overview of the complexities of V2G implementations including V2G system architectures, the expectations and needs of various stakeholders from vehicle owners to government entities, and looks at the technical requirements and challenges related to bidirectional power converters including a couple of design examples and closes with a glance at consumer-related adoption challenges to widespread use of V2G.
One of the driving factors behind the development of V2G technology is the fact that renewable energy sources (RES) like wind and solar are neither dispatchable nor reliable. The growing penetration of RES into the utility grid brings challenges related to grid stability that V2G may help to address. Electric vehicles (EVs) that include V2G technology are sometimes referred to as griddable EVs (GEVs). GEVs need more than bidirectional power conversion to be effective. GEVs need to have a real-time communications connection with the grid through their telematic system, not just through the battery charger to support vehicle and grid integration (VGI).
A VGI infrastructure is complex and includes charging stations, communication protocols, security, a networked grid with both conventional generation and RES, and control algorithms. VGI implementation can also vary during different times of the day (Figure 1). From an economic perspective, the provision of ancillary services like frequency and/or voltage regulation, peak shaving, load leveling, congestion mitigation, spinning reserves, and reductions in intermittence and curtailment that can be provided with VGI could be more valuable than simple energy storage. But they are also more complex to implement.
Satisfying the sometimes-conflicting expectations of a wide range of stakeholders can also be difficult. Examples of stakeholders and their expectations include:
- EV owners expect VGI to reduce the total cost of ownership (TCO) of the vehicle. It can provide home energy storage for regular use or as a backup power source, in addition to a revenue stream from the power put onto the grid, but it can also reduce the lifetime of the very expensive EV battery.
- Grid operators see VGI as a solution to fluctuations in energy availability and grid performance resulting from increased penetration of RES. VGI can also reduce the need for updating some grid infrastructure, but it comes with increased security challenges.
- EV fleet operators such as bus systems or fleets of taxis can realize new business opportunities like grid balancing and energy storage services to generate incremental revenue streams but with the challenge of reduced battery lifetimes.
- Businesses and office building owners can potentially use VGI of parked EVs to implement load leveling and peak shaving to reduce their electricity costs.
- Government entities view VGI as a component that can help create a circular economy, support energy security, improve sustainability, and provide a range of environmental and economic benefits.
Bidirectional power flow
Bidirectional power flow is foundational to VGI. A common topology for bidirectional power conversion includes a three-phase bidirectional power factor correction (PFC) section that’s not isolated and an isolated bidirectional DC/DC converter section (Figure 2). That topology is used to implement an EV battery charger that directly connects the battery to the grid. The charger is usually external to the EV, but three-phase onboard charger (OBC) designs can also support VGI. That contrasts with lower-power single-phase AC EV charging designs where AC power is fed to a battery charger inside the EV. The lower power designs are not generally seen as the best candidates for supporting VGI.
The bidirectional converters used for VGI will have to handle the high voltages and currents inherent in EV charger designs. The DC bus voltage between the PFC and DC/DC sections can be as high as 750 V, and the output of the DC/DC converter needs to match the battery voltage which can be 800 to 1,000 V in new EVs.
One basic choice when designing a bidirectional EV charger is whether to use silicon (Si) MOSFETs, Si IGBTs, or silicon carbide (SiC) MOSFETs for the power switches. That initial choice affects all aspects of the design and can have a major impact on the efficiency and cost of the charger. Si MOSFETs are low-cost and can be used in compact high-frequency designs, but at the required voltage levels, they have high on-resistance which limits efficiency, and high output capacitance contributes to high switching losses, further reducing efficiency. IGBTs can deliver higher efficiencies at the required voltage and current levels, and they can support cost-effective designs. However, they suffer from lower switching frequencies resulting in lower power density solutions.
SiC MOSFETs present a third alternative that can support high efficiencies and high-power densities. Advantages of SiC MOSFETs include lower on-resistances, higher withstand voltages, lower switching losses, and better thermal characteristics compared with Si devices. Additionally, the price premium associated with SiC MOSFETs has declined significantly, and in many designs, cost parity can be achieved with Si devices. Reference designs using SiC MOSFETs are available for bidirectional DC/DC converters and AC/DC power supplies suitable for EV charging.
One of the numerous reference designs that are available is for an 11 kW capacitor-inductor-inductor-capacitor (CLLC) resonant bidirectional DC/DC converter for use in OBCs and off-board EV chargers. CLLC resonant converters are getting increased attention for their superiority in soft switching, wide output range, and symmetrical bidirectional operation. When implemented with SiC MOSFETs, they can deliver high-efficiency and cost-effective solutions. This reference design uses 1,200 V SiC MOSFETs and operates with an input voltage of 700 V and can provide output voltages from 550 to 800 V with a peak efficiency of 97.2% with Schottky diodes for output rectification and a switching frequency up to 250 kHz. The board, including the enclosure, measures 360 mm x 160 mm x 65 mm, resulting in a power density of about 3 W/cm³ (Figure 3).
A second exemplary reference design using SIC MOSFETs is a 4 kW AC/DC power supply with a 3-phase 400 Vac input and a 750 Vdc output that can be used to implement bidirectional EV charging. It has an efficiency of 97% and a power factor of 0.99.
The use of 1,200 V SiC MOSFETs for the power switches improves efficiency. Implementing this design using IGBT would require devices rated for 1,000 V. The SiC MOSFETs have low conduction losses combined with lower switching losses compared to the IGBTs. In addition, the Si MOSFETs maintain their efficiency even at higher switching frequencies which makes it possible to use smaller inductors and reduce the size of the power supply (Figure 4).
While the technical challenges to VGI are significant, the challenges don’t stop there. Simply getting consumers to embrace and use the technology is expected to be difficult. Changes in consumer behavior often need to be incentivized and the benefits amplified and reinforced.
EV batteries are expensive, and battery degradation will be a concern for commercial and consumer users. The use case adopted for VGI will be an important determinator of the degree of battery degradation. For example, energy-intensive services like peak shaving may need to be limited to under 20 times per year. Additionally, limiting battery use to the middle range of the battery capacity minimizes the reduction in operating life. These and other approaches to minimizing battery degradation also minimize the benefits of VGI technology.
Finally, cybersecurity concerns with VGI technology are significant. The operation needs to be seamless to encourage the use of VGI systems, but the simplicity of use can result in lower levels of security. Maximizing the benefits of VGI requires a large diversity of users, but that diversity further increases cybersecurity challenges.
V2G technology holds the promise of improving sustainability. For that promise to be realized, numerous technological, business, and societal challenges must be addressed. Infrastructure development will be extensive and must address the needs and concerns of a wide range of stakeholders. The technology must also provide high efficiency and strong cyber security.
11 kW SiC bi-directional DC/DC converter board for EV Charging and ESS applications, Infineon
A Three-level Dual-active-bridge Converter with Blocking Capacitors for Bidirectional Electric Vehicle Charger, IEEE Access
Designing with Silicon Carbide in Bidirectional On-Board Chargers, Wolfspeed
LLC DC-DC Converter Performances Improvement for Bidirectional Electric Vehicle Charger Application, MDPI World Electric Vehicle Journal
PFC circuit for 3-phase 400V AC input, Toshiba
Utilization of Electric Vehicles for Vehicle-to-Grid Services, MDPI energies