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V2G / V2H, what’s the difference?

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Xavier Moreau's picture
Consultant AlterGrids

Xavier MOREAU is an freelance consultant with a background in customer experience and energy technologies. He develops for corporations and start-ups, deep insights of complex changes in energy...

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  • Mar 1, 2021 11:33 am GMT
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In discussions and articles about electric vehicles (EVs) that could both charge and discharge their battery, I often see the assumption that if a vehicle can back-feed electricity into the grid it can also be used as a back-up power source. Well, this is far from automatic.

 

In Vehicle-to-grid (V2G), vehicles exchange energy from their traction battery with the grid in two directions, charging or discharging. The purpose is to stabilize the grid, dampen fluctuations due to variable renewable energy such as solar, resolve bottlenecks or make arbitrage on electricity prices.

This bidirectional exchange with the grid can be through the home’s wiring in which parking or garage the car is parked and plugged. It requires a bidirectional charger, with power electronics that can transform Alternative Current (AC) from the grid into Direct current (DC) for the battery and vice-versa. It can be either on-board the car, or off-board, for example in a wallbox.

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Vehicle-to-Home (V2H), while its definition is still debated, is usually understood as the ability in the event of a black-out, when the grid is down, to continue to power your home from the car battery. For resilience, the car can have been charged from the grid before the event, or be recharged at another location (for example at work), or at a public charging point. If the house has rooftop photovoltaics (PV), its battery is used to smooth the output, storing excess solar energy when the electricity needs are low and releasing it into the home when there is not enough PV electricity to meet all the loads.

(Vehicle-to-Building (V2B) is similar, but with a building and several vehicles in its garage or parking lot. Vehicle-to-Load (V2L) is also similar, but aims to power from a socket in the vehicle, your computer, camping gear or power tools. Both (and other variants) have slight differences that will not be covered in this article.)

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While in V2G the car works with the grid, in V2H the car is at the core of a microgrid limited to the home, its appliances and possibly rooftop PV. In a grid, for the systems to work in sync, the tempo is given by the frequency of current that is set by the large power plants supplying it. In a microgrid, you need an orchestrator to set this tempo, and in V2H that’s the charger. It is impossible for electric assets to exchange flows of alternative currents if they are not in sync.

In V2G, all assets align to the frequency of the grid. This is called grid following.

InV2H, the charger has to set the frequency, this is called grid forming because its power electronics need to shape the waveform of the current all other appliances will align on.

If the grid goes down, all connected assets with power electronics that followed its frequency just stop working. This explains why in case of blackouts, even homes that have rooftop PV cannot continue to power their appliances. Unless they can switch to V2H, where the charger with the car generates a frequency to sync on. But this requires a charger with different power electronics.

Pieter Kuiper, CC BY-SA 2.5 https://creativecommons.org/licenses/by-sa/2.5>, via Wikimedia Commons

But with grid forming comes other responsibilities. In V2H, the home becomes an islanded microgrid where the car is the main voltage source. If it remains connected to the grid, engineers or line technicians working on grid repairs and expecting it to be powered down face the risk of electric shock. Trying to maintain voltage, the car would feed a fault current that will jump to intensities the electronics and wiring cannot withstand.

A device is thus needed to detect the grid is down and disconnect the home before switching the car to V2H mode and before it supplies voltage. It is called an anti-islanding device. The switching can be automated for a seamless transfer, thanks to a device called an automated transfer switch (ATS).

But once disconnected the earthing scheme, that connects the electric power system with the ground for safety, changes. In V2G, the Distribution Network Operator (DNO) of the grid makes the link between the neutral and the Earth. In V2H, the Neutral is not grounded, it is said to be floating. Protection devices must be adapted to detect any current leakage even in this scheme, for example with an isolation monitoring device.

However big the car and its battery are, they might well not have enough capacity to power the whole home. So when switching from the grid to V2H, non-critical loads must be shed.

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In summary, while V2G and V2H stem both from the concept of bidirectional charging and discharging of an EVs battery, they serve different purpose and require different power electronics and controls. So next time you read a car or charger is bidirectional, do not automatically assume this means it can do both V2G and V2H.

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Matt Chester's picture
Matt Chester on Mar 1, 2021

Really interesting, thanks for sharing! When it comes to the cost to the car-owner of bidirectional charging, I've had some skepticism about how much V2G would be accepted and what sort of compensation they'd need to maybe sacrifice lifetime of their battery, but I could so the direct personal benefits from V2H changing that mental calculus

Bob Meinetz's picture
Bob Meinetz on Mar 1, 2021

A 40 kWh vehicle battery would be capable of powering a 500-watt kitchen oven for 4 minutes, 4 seconds - not even enough time to preheat it.

Some perspective is in order.

Matt Chester's picture
Matt Chester on Mar 1, 2021

Perhaps when faced with the blackout and using V2H, households would then prioritize what is needed for safety-- basically how they tend to use emergency portable generators today. Keep the fridge running so food doesn't spoil, ensure you have necessary lights and you can keep your phones charged, maybe use electric cooktops to boil some water, etc. 

Roger Arnold's picture
Roger Arnold on Mar 1, 2021

Uh, Bob, how late at night or early in the morning was it when you posted that? 40 kWh would power a 500 watt oven for 80 hours. That's a long way from 4 minutes and 4 seconds. 

Bob Meinetz's picture
Bob Meinetz on Mar 1, 2021

Late in the morning, but short on coffee. Thanks.

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