The ins and outs of vehicle-to-grid charging
Interest in vehicle to grid (V2G) charging is driven by two factors. The first is growing uptake of electric vehicles (EV), which, for many consumers, represent a significant capital asset that, paradoxically, isn’t used very much. The second is the increasingly complex nature of electricity generation, storage, and demand, which is challenging the power companies’ ability to ‘balance’ their grids, that is, match supply and demand efficiently. A well-implemented V2G strategy could address both issues, providing wins for drivers and power companies while reducing the carbon impact of both car and electricity usage.
For consumers, renting the energy storage and delivery capabilities of their car batteries to power companies for a fee could reduce the total cost of ownership of their cars. Fleet operators could gain the same advantages at mass scale. Power generation and distribution companies could use these highly distributed batteries to support local grids during peak loads, help absorb excess generator output, and mitigate the effects of fluctuating renewable generation.
V2G pilot schemes are already underway. At Utrecht in the Netherlands, the top floor of a car park has been given a shade roof made up of 2,160 solar panels, which together can capture between 850,000 and 1,000,000 kWh of energy per year. The car park has bidirectional charging facilities for 250 cars now, with plans for this number to grow to 450 or 500. The hope is that the energy stored in their batteries will be able to power some local office buildings during the morning demand peak and will then be replaced during the rest of the working day by the solar panels. The pilot scheme’s operators calculate that the whole city could run on renewable energy during the day if just 10,000 of the 140,000 cars on its roads joined a V2G scheme.
One of the challenges in implementing bidirectional charging is deciding who carries the costs for the infrastructure needed to convert between the alternating current (AC) that the power companies use to distribute energy across the grid, and the direct current (DC) used to charge and discharge a battery. At low powers, EVs are charged using either AC or DC, but fast charging at hundreds of kilowatts is done using DC via rectification circuitry mounted in the roadside charger rather than onboard the vehicle.
The tension between the preferences of power companies and car owners is magnified when it comes to bidirectional charging. Power companies would like to use AC bidirectionally, so that stored energy could be injected into their grids easily. Car companies worry about the costs of adapting vehicle electronics to meet the rigorous technical and safety standards of the grid. There are concerns, too, about the round-trip efficiency with which AC energy from the grid could be brought onboard a vehicle, rectified to DC, stored in the battery, discharged from the battery, inverted back to AC, and delivered back to the grid.
The installed base of charging connectors presents another challenge. The CHAdeMO charging connector is used for DC charging now, but an AC version is under consideration. The Utrecht project has been developing an AC version of the Type 2 connector, borrowing some protocols from CHAdeMO and working with GE and Renault to build the enhanced connector. Renault is expected to put it in some of its EVs in 2024, while Hyundai may bring it to market in 2023.
If the practicalities of bidirectional charging can be overcome, it promises to offer many advantages. One of these is to help deal with sudden peaks of demand on the grid, which can be expensive to service without a ready source of stored energy. Traditional AC generators have massive armatures and while their rotational inertia helps stabilise the frequency of their output, they are slow to start and stop and therefore bad at servicing sudden changes in load. This can force power companies to buy power on the open market at peak prices. EV batteries could provide a near-instantaneous source of power to fill in during these demand peaks until more traditional forms of generation can be brought online. At the consumer level, car batteries could be used to store renewable energy during the day and then feed it into the household’s AC system at night – some variants of Ford’s F 150 Lightning electric pickup already do this. The pickup also has with AC mains sockets to drive power tools and other domestic devices and can top up other EVs at the same rate as a domestic charger.
However, it may be that the most challenging part of making V2G happen at mass scale is not the technology but finding a business model that attracts both sides of the deal equally. Consumers will have to be shown that they can make money by joining these schemes and reassured that the additional charging and discharging cycles won’t shorten their batteries’ lifetimes. Fleet operators will have to learn to use their insights into their vehicles’ future usage to sell bulk battery capacity slots during the day and buy charging energy off-peak.
By decoupling the time at which energy is generated from when it is used, generation companies and grid suppliers will be able to balance their grids at lower cost, avoid having to buy power at peak prices from other generators, increase the use of renewables in their generation mix, and reduce international energy imports.
The ability to use EV batteries to feed power into the grid, keep a home going during a blackout, drive a power tool on a work site, or top up another EV gives us an opportunity to rethink how we use energy and the role of the car in our lives. However, we need to recognise that V2X strategies involve multiple lossy energy conversion, storage, and discharge stages. If we don’t minimise these losses, through the use of high-efficiency circuit design and modern switching semiconductors, two things may happen. First, the logic of the enabling business models will be undermined as the advantages of V2G strategies are diminished. Second, efforts to use V2G strategies to cut the carbon impact of personal travel will be reduced. Delivering true climate mitigation will take both a systemic understanding of how energy can be generated, stored, and used, and a deep attention to the details of V2X implementations.
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Harvey Wilson
Harvey Wilson is a Systems Engineer Professional (Smart Industry) for Avnet Silica in the EMEA regio...
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The ins and outs of vehicle-to-grid charging | Avnet Silica
