Bus2Grid: the next step in decarbonising transport?

In summer 2018, Bus2Grid, the world’s largest vehicle-to-grid (V2G) project got underway and after two years of hard work, a North London bus garage has been transformed into a ‘virtual power station’, discharging electricity from buses when not in use. Led by SSE Enterprise in partnership with BYD, UK Power Networks and the University of Leeds, Northumberland Park bus depot is now the biggest electric bus facility in Europe and houses nearly 100 new zero-emission electric buses. With the implementation of Bus2Grid, the site will be capable of returning over 1MW of power to the grid.

Each V2G bus has a battery capable of holding up to 382kWh, charged at 80kW on an AC basis and with two electrical motors, each up to 150kW. V2G is the name used for a group of combined activities that enable a vehicle’s battery to discharge back into an energy system rather than the more conventional approach of just allowing the vehicle to charge. V2G technology enables energy stored in an electric vehicle’s battery to be fed back into the electricity network through a communications module that informs the battery management system to reverse the direction of energy flow.

By recharging when demand is low and putting energy into the grid when it is high, V2G technology can help manage peaks and troughs, balance the network and make it more efficient. Benefits of Bus2Grid Put simply, if the entire London bus fleet of around 9,000 vehicles were to be converted with the V2G technology being used in the Bus2Grid project, it could theoretically replace the peak capacity of a small CCGT power station for around nine hours.

Central to the challenge of decarbonising our transport and achieving climate change targets is how we can optimise the existing flexibility within the energy system. Developing a charging infrastructure that operates in two directions, like this one, so that vehicle batteries can give back as well as take from the grid is an important part of this. Furthermore, it enables support to the electricity system in periods of stress, as well as allowing vehicles to tap into additional revenue streams.

Electrifying transport will have huge benefits for air quality in cities and for meeting the UK’s climate change commitments. Electric vehicles such as buses can also support the energy system, but this means creating new ways of working between energy utilities, grid managers, and transport providers – this project is creating new business models to make this happen. There’s also the commercial value and social benefits of this project to the energy and passenger transportation systems by developing services for the national grid, regional distribution network operators, bus operators and transport authorities.

Chargers at the Northumberland Park site. Image: SSE. The technical challenges

Collaboration is central to this project. It works directly in partnership with a number of companies to enable the original vehicle manufacturer, the vehicle owner and the electricity network to interact. To do this we have had to work together, each bringing our joint sector expertise to solve a range of challenges.

One of the biggest challenges that this project faced was around dispatch. This project is based on an Alternating Current (AC) based charging system. Under an AC-based V2G process, the on-board charger controls the discharge of the battery, however in order to do this, it requires a signal to tell it so.

At first, it was believed that we would be able to communicate sufficient information for the discharge capability with type-2 sockets. However, we quickly established that we could not do this, so we moved over to a control system based on the bus. This approach now allows us to communicate via the Cloud to the bus to enable safe discharge.

Origami energy routers located at the bus garage are used to manage site dispatch. A range of energy services are going to be tested, including some wholesale energy market transactions, some local Distribution System Operator (DSO) services and some of the National Grid Electricity System Operator (ESO) services, including frequency response. Frequency response is particularly challenging.

In frequency sensitive mode, the Origami energy router detects site frequency. If there is a deviation, a signal is then sent to the Origami cloud, then to the BYD cloud and then onto the bus for it to respond to the signal. Bus2Grid will be assessing both static frequency response and the more challenging dynamic frequency response characteristics of the buses.

Of course, it’s important to make sure that V2G activity does not adversely affect the vehicles, and a key focus has been the possible impact on battery life. At an early stage it became apparent that battery degradation effects would be too small to measure effectively within the project timescale, so a modelling approach was adopted. BYD’s battery engineers have assessed the effects of expected levels of V2G activity and have reported that the impact of this use is small enough that standard warranty is unaffected.

Meeting regulations In 2019, the new G99 regulation came into effect, requiring that generators have a voltage control system that can inject or absorb reactive power into the grid to control voltage. This meant that it was essential for us to confirm the certification standards when the discharge facility – the bus – was added to the site.

The challenge we faced here is that the G99 regulation related to stationary generators, but because this project is an AC-based system, the “generator” in the form of the battery and on-board inverter is on the bus. It drives out every day, delivers passengers to where they need to go before returning to the depot in the evening. As a result, G99 requires us to test and confirm the voltage, harmonics and other characteristics for these aggregated buses.

The future When it comes to electrification of buses, a fleet of bus batteries harnesses large amounts of electricity. They are also habitual, with regular and predictable routes, driving patterns and timings.

That means we can easily predict and plan for how we can use any spare electrical capacity they can offer and use them as energy storage devices by providing services to the electricity network. Furthermore, V2G and smart charging of buses can enable greater levels of electrification in certain more constrained areas of the country. With ambitious net zero targets in the UK, it’s critical that we continue to develop the country’s EV infrastructure if it is to encourage the widespread adoption of these vehicles – and buses have a big part to play in this roll out.

Exploiting the potential of electric buses to act as ‘mobile power stations’ will be critical to the efficient running of the grid once electric vehicles of all types eventually become commonplace.

While V2G represents a huge challenge to the industry, it is something that the energy and automotive markets are currently navigating through together.

There is no reason why projects like this cannot be easily replicated in bus, truck and fleet depots up and down the country – so long as the industry continues to collaborate.

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