What Are Vehicle-to-Grid Services?
Electric vehicles, once connected to the electricity grid, offer a wide range of functionalities. If equipped with the right hardware and software, an electric vehicle (EV) is capable of both recharging its battery and feeding energy back into the grid. This feature is referred to as vehicle-to-grid (V2G) and is an important facilitator for what are called ‘ancillary services’.
Vehicle-to-grid technology is fast attracting interest within the e-mobility industry. With that context in mind, this article will walk you through each of these topics:
What Are Ancillary Services?
You can even view the actual frequency in the European electric grid live at https://www.mainsfrequency.com/.
According to ENTSO-E, the European Network of Transmission System Operators for Electricity, these ancillary services include:
- Black start capability: the ability to restart a grid following a blackout
- Frequency response: which maintains system frequency with automatic and very fast responses
- Fast reserve: which can provide additional energy when needed
- Provision of reactive power and various other services
Traditionally, providing ancillary services has been the sole responsibility of utilities. Such utilities operate huge, centralized generators like nuclear power plants, fossil-fueled power plants, hydropower plants, and gas turbines. However, the integration of decentralized wind- and solar-based power plants and the development of smart grid technologies have changed the landscape of devices that provide ancillary services.
Electric Vehicles Can Stabilize the Grid
This is why it makes so much sense to use EVs which are connected to the grid via home chargers at night to harness this renewable energy.
Think about the solar panels installed on your roof at home. On a particularly sunny day, they can produce a lot more energy than all your household appliances combined could consume. A power converter installed at your home will then feed this excessive renewable energy into the grid.
However, several countries already significantly lowered financial compensation for homeowners to provide energy to the grid. It would be better to temporarily store this free solar energy in your EV for later use in the evening. After all, the battery capacity of modern electric cars is already enough to supply a household with electricity for several days. This concept is referred to as vehicle-to-home (V2H).
The ABCs of Reactive Power
Whether (some of) these ancillary services will be mandatory or based on a proper compensation for the EV owner is currently under intense debate and will ultimately vary from region to region. This article doesn’t cover possible compensation schemes or potential business models for vehicle-to-grid services. Instead, we’ll approach the topic from a technical viewpoint and explore how the EV can be enabled to provide such services via the standardized charging communication protocol in ISO 15118.
Grid Codes and Ancillary Services
When it comes to feeding energy back to the grid, there are certain technical regulations that any generating device (aka producer) connected to a public electric network needs to comply with. These regulations guarantee the stable and secure operation of the electrical grid. They are known as ‘grid codes’ and specify voltage regulation, power factor limits, reactive power supply (see the video linked above for an explanation of these terms), and response to short circuits or frequency changes on the grid. There are shared aspects included in the national grid codes across countries. However, there is no perfect match or clear international consensus on naming or features of a grid code. Thus, it is quite hard to design a mass-market product that complies with a set of highly complex and inconsistent rules.
Why Vehicle-to-Grid Is Easier for DC Charging Than for AC Charging
Although ISO 15118 is entitled ”Road vehicles – Vehicle to grid communication interface”, the vehicle-to-grid feature has at first been described only as a use case in ISO 15118-1. ISO 15118-2, which was published in 2014, does not define any messages that would allow a bidirectional power transfer. Such a feature would enable the EV to feed energy back to the grid. Depending on the financial compensation, this could be a lucrative service for the driver to opt in to – given that the EV’s battery will be charged by the time the driver needs to leave the charging station.
In one of our blog posts we informed readers that the next generation of ISO 15118 features, defined in ISO 15118-20, include wireless and bidirectional charging. Another blog post explains how ISO 15118-20 currently enables the EV to act as a distributed energy resource (DER) and feed energy back to the grid in a grid-friendly way.
To do this, the EV needs a power converter to convert the direct current (DC) provided by the EV’s battery into the alternating current (AC) provided on the grid. In DC charging mode, the power converter is located “off-board” in the charging station and is always a stationary device. Therefore, the location-dependent grid codes can be programmed into the controller of the charging station that manages the power flow to and from the grid.
This makes bidirectional energy transfer in DC charging mode an easy task to achieve in ISO 15118. Why? Because no additional grid-related information (like the necessary amount of active and reactive power) needs to be exchanged between the EV and the charging station.
In AC discharging, however, the power converter that manages the power flow is located “on-board”, i.e. inside the EV. This means that the charging station needs to provide the EV with information on how to feed energy back to the grid. Technical requirements need to be defined that clearly specify e.g. the amount of active power and reactive power needed for a reverse power flow to guarantee a stable operation of the grid.
In general, there are two possible approaches to guarantee that the EV will comply to the local grid codes of the country in which it is currently charging/discharging:
- The parameters of a certain set of national grid codes (e.g. inside Europe) are stored in the EV’s communication controller (EVCC), associated with a unique grid code identifier. The charging station’s communication controller (SECC) would then only need to communicate the grid code’s specific identifier to the EVCC to ensure a safe and secure operation of bidirectional power transfer.
- The SECC needs to explicitly communicate a set of parameters to the EVCC to ensure that the reverse power flow from the EV to the grid does not violate local grid code constraints.
While the first option allows for a rather easy adaptation of the ISO 15118 communication protocol, it requires EV manufacturers to make sure a multitude of grid code parameters are stored on their communication controllers. Most EV manufacturers are not fond of this option, which is one of the reasons why the Draft International Standard (DIS) of ISO 15118-20 focuses more on the second option.
How ISO 15118 Enables Vehicle-to-Grid
By now, you’ve learned why and how vehicle-to-grid (V2G) services can help to keep the electrical grid in a secure and safe operating state. The importance of V2G will dramatically increase with the ever-growing number of renewable energy sources being installed across the globe and the fast-growing number of EVs that enter the market. The standardization community around ISO 15118 has intensively discussed the necessary technical requirements for enabling EVs around the world to provide V2G services.
Eager to find out more? Then have a look at this blog post to learn how the newest draft of ISO 15118-20 enables V2G services.