Vehicle-to-grid (V2G) technology provides power from electric vehicles (EVs) to the grid in response to peak load demands. Depending on the demand an EV could charge or discharge power; therefore, EVs can be seen as moving storage devices.

V2G technology is evolving steadily, and developments are taking place at many levels; especially on the power electronic side of the design where power-efficient components are paramount for a successful V2G implementation. A V2G system (Figure 1) includes power converters which are vital to the system because the interconnection between the grid and the batteries of EVs requires not only bidirectional AC-DC (or DC-AC) converters but also bidirectional DC-DC converters where step-down and step-up conversions are needed. There are already products in the market capable of providing bidirectional DC-DC conversions, and they can be used in V2G applications.

For instance, TDK Lambda has developed the EZA series, and the latest product of the series is the EZA11K (Figure 2) which is capable of handling bidirectional power conversion at a maximum rated output capacity of 11kW with maximum efficiency of 95 percent. The current, voltage and direction of change modes can be controlled and monitored by external devices via RS485 serial communications.

Recently, Diamond Electric Mfg Co Ltd developed a business-card-sized, thin isolated bidirectional DC-DC converter (IBDC) that can substantially downsize and reduce the weight of dc-dc converters essential for rechargeable batteries. The GaN-based device (Figure 3) can meet safety standards for large-capacity batteries for EVs with higher voltages, and it can switch up to 2 MHz.

V2G communications are very important as well; it is essential to have real-time communication to create a connection between electric vehicles and the operator of the grid to efficiently manage the power of the grid. There are several communication protocols that could be used for V2G
systems; here are a couple of options:

• ISO/IEC 15118: It is an international standard defining a vehicle-to-grid (V2G) communication interface for the bi-directional charging/discharging of electric vehicles. It uses the IEEE P1901.2 HomePlug Green PHY (HPGP) broadband PLC specification as the best protocol to ensure
robust communication and a high data rate. Operating at frequencies between 2 MHz and 30 MHz, HPGP enables the system to distinguish valid data on a connected pilotline against noise from other nearby sources.

• IEC 61850: It is an international standard defining communication protocols for intelligent electronic devices at electrical substations that can help manage energy flow between renewable electricity resources and consumers. Another vital element of a V2G system is the lithium-ion battery of EVs. A lot of research and efforts are taking place in this area. One study from the University of Warwick tells that V2G may not degrade EV battery life, but another study from the University of Hawaii suggests that additional cycling to discharge EV batteries to the power grid can be detrimental to battery performance shortening the life of the batteries to five years. The researchers on these two studies got together to analyze their opposite results and concluded that:

Vehicle-to-grid (V2G) technology provides power from electric vehicles (EVs) to the grid in response to peak load demands. Depending on the demand an EV could charge or discharge power; therefore, EVs can be seen as moving storage devices.

“The simplistic approach adopted by current V2G pilot studies, namely that an EV is discharged and charged without consideration of battery degradation, on-line, is not economically viable because of the impact additional V2G cycling has on battery life.

However, a smart control algorithm with the objective of maximizing battery longevity can reverse this. In such an approach, the control algorithm only allows access to the battery’s stored energy if there are no adverse effects on battery longevity; therefore, the worst case is that a battery degrades as if there was no V2G. This approach relies upon the development of accurate battery prognostic models and further advances in understanding the causes, mechanisms, and impacts of battery degradation.”

In other words, V2G could be successful if the degradation of lithium-ion batteries is taken into account (Figure 4) and not just only the typical factors: capacity throughput (CTP), temperature (T), State of Charge (SoC), swing in State of Charge (ΔSoC).

Degradation modes are often associated with a phase of accelerated, nonlinear fading that seems unpredictable relying on capacity and power fade alone. V2G applications still need more research on subjects like this before major efforts are put in place for mass implementation.

Current pilot projects 

V2G is still in its infancy but is has the potential of transforming the way we see the grid. Take a look at few of the projects:

• V2G project by WMG at University of Warwick: The project started last April, and it will run for three years where researchers at WMG—a research and education group and an academic department of the University of Warwick—led by vehicle electrification and energy storage expert Dr James Marco, will build a technoeconomic model of how V2G will be viable within the UK. A key innovation will be the inclusion of new models of battery degradation within the analysis that will underpin new methods to optimize the vehicle’s battery system.

• SEEV4-City Project: The main goal of SEEV4-City is to develop the concept “vehicle4Energy services” into sustainable (commercially and socially viable) business models to integrate EVs and renewable energy in a sustainable urban mobility and energy plan (SUMEP). The project is in progress and it contains seven operational, long-term pilots in six cities in five European countries (Newcastle, Loughborough and Leicester, Kortrijk, Amsterdam, Hamburg, and Oslo). The project aims to increase energy autonomy, ultra-low emissions mileage, and avoid extra investments to make existing electrical grids compatible with an increase in electromobility and local energy production.

• City-Zen Project: The objective is to demonstrate a V2G concept in Amsterdam and to determine the effect of V2G on the electricity grid by using multiple EVs and bi-directional chargers. Another objective of the project is to test how V2G technology and services can influence and support the grid stability. Also, new services, delivered by market parties such as aggregators,  will be tested in the demonstration.

The future of V2G

Even though there are real possibilities of V2G becoming the next disruptive technology in the power sector, not everybody believes that is the case, which is slowing down adoption. The technologies to make it a reality are here; they are still improving with many projects and research efforts at all levels (from economic and social studies to much more complex technical studies). Manufacturers and owners of EVs are still questioning the stress on the batteries, which is one of the most expensive parts of the EV. Mass implementation of V2G is achievable, but will require more time to make the economics of vehicle ownership more attractive.