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.
The term “ancillary services” refers to a variety of operations provided by the electricity grid that go beyond the generation and transmission of electricity. These services are required to support the continuous flow of electricity so that supply will continually meet demand, this way guaranteeing stability and security in the electricity grid.At each point in time, the amount of electricity produced by power plants connected to an electric grid must exactly match the amount of energy taken from the grid by consumers (households, factories, buildings, EVs, etc.). Otherwise the grid’s nominal frequency (e.g. 50 Hz in Europe or 60 Hz in the U.S.) will exceed a specific range and we risk running into a blackout. Picture the electric grid as a balance that needs to be in a constant equilibrium between load and generation, as shown in this illustration by PJM (click the image to view the animation).
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:
Traditionally, providing ancillary services has been the sole responsibility of utilities. Such utilities operate huge, centralised generators like nuclear power plants, fossil-fueled power plants, hydropower plants, and gas turbines. However, the integration of decentralised wind- and solar-based power plants and the development of smart grid technologies have changed the landscape of devices that provide ancillary services.
Did you know that EVs have the potential to provide ancillary services to the grid? It’s true; they can be used to do so in several ways. One way is to regulate the overall load on the grid by either increasing or decreasing the charging power as needed. In that context, the terms "self regulation", “peak shaving” or “load shifting” may sound familiar to you. This basically means that a consumer (e.g. an EV) reduces its power consumption for a short period of time to avoid a load peak on the grid. Later, when the overall demand on the grid is lower, the EV will continue to recharge its battery. This way, tiny variations of each EV's charging power, combined with millions of other devices connected to the grid, help to smoothen the constantly occurring small grid disturbances. EVs can also be used to consume excessive renewable energy that would otherwise have to be wasted. Take wind turbines, for example, which tend to produce a lot of energy in the winter season or at night when the wind blows stronger. It does happen from time to time that the amount of energy wind turbines produce exceeds the overall demand on the grid, as this article illustrates. Often times, these wind farm operators would try to sell the excessive energy to neighbouring countries, which might be connected to the same transnational power grid (e.g. ENTSO-E). If that's not an option and even the installed stationary batteries can’t consume any more of that energy, then there is only one way out: the grid operator has to shut down these wind turbines to maintain the nominal frequency and avoid destabilising 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.
Assume the EV can provide the information about the amount of energy it needs to recharge its battery and the anticipated departure time set by the driver to a charging station. A smart charging device built into such a charging station can then use a scheduling algorithm to exploit the driver’s flexibility and shift the EV’s charging process to more grid-friendly time slots. What’s the benefit of this practice? Simple: the charging station can charge the EV in times of high availability of renewable energy and/or overall low demand on the grid while still guaranteeing that the EV will be fully charged when the driver wants to leave. Alternatively, EVs’ can also be enabled to discharge electricity, meaning: feeding energy from the internal battery back into the grid. If you can control the discharging process of many EVs, each of them connected to a charging station, then you can aggregate them to a pool of EVs that acts like a virtual power plant. These virtual power plants can then serve as energy reserves, known as ‘spinning reserves’ or ‘operating reserves’. Spinning reserves are energy producers that are connected to the electrical grid but are currently not producing any energy. They can quickly provide energy to the grid when needed, this way contributing to the overall stability of the power grid.
Another service that EVs are capable of offering is the provisioning of reactive power. What is reactive power and how closely does it relate to the terms active power, apparent power, and power factor? To find out, watch this short video (which includes this entertaining example using a glass of beer). For our German readers, there is also a very informative article on the Q(U) regulation published by Vorarlbergnetz. That article will provide you with more insights into why providing reactive power is essential for balancing the nominal voltage in a low voltage grid.
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 standardised charging communication protocol in ISO 1118.
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.
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 news articles we informed readers that the next generation of ISO 15118 features, defined in ISO 15118-20, include wireless and bidirectional charging. Another news article 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:
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.
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 article to learn how the newest draft of ISO 15118-20 enables V2G services.
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