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Vehicle to grid balancing

By Dave Elliott

The vehicle to grid (V2G) debate continues, offering a way to balance variable renewables and also demand peaks, by using the batteries of electric vehicles, linked to the grid when parked at home, to store excess power during low demand periods, ready to export when demand is high and renewables low. It sounds a clever idea but in addition to economic issues (e.g. the extra costs of the home-based power uploading system) it opens up some interesting logistical issues.

In the UK, peak energy demand coincides with coming-home-from-work time (6.30-7.30 pm?), so many cars will not be “nested” and linked in at home yet.  You will get a steady stream of arrivals from work, but most will have low charge, having just been driven. So when they arrive home they will be power hungry and not available for V2G power uploading. Indeed some see the arrival of millions of EVs as being a major issue – creating a new additional evening demand peak. However, some recent US work has suggested this may not be as big a problem as expected:

Certainly those cars that are nested at home all day could be trickle charged during daylight hours from domestic PV, in summer especially, and then be (partly) discharged into the grid at peak demand times. The only problem is how would you like it if you decided to go out later in the evening (or next morning) and found the car almost totally discharged?  There would have to be negotiated/contracted-for limits, and opt-out options.  That’s important also since it has been claimed that regular extra deep cycling of batteries will age them fast – so costing car owners a lot for replacements, given that the batteries are the most expensive element in EVs. A wear and tear element has to be included.  Indeed it might be that it would be best for V2G-enabled cars to have more robust outsized batteries designed for this function. That would add to the cost and would only be worth it for car owners if the cash value of V2G exports was high. That income, and the scale of use of the V2G option, would also be variable around the year, making investment assessments harder, whatever size or type of battery was used. That said, variable income from Feed In Tariffs has not put off home-owners from investing in PV solar, so the same may be true of V2G, and in theory, despite the occasional power drain inconvenience, V2G income should offset the cost of running an EV and the cost of PV.

Nevertheless, looking at the energy system as a whole, some say the V2G idea is at best marginal and opportunistic – if you really want to store power, have proper dedicated stores. While it may seem clever to use all those batteries, which are mostly just sitting in parked cars, perhaps 80% of the time, there is also the fact that energy conversion losses using standard EVs will be high. Charging a fairly efficient battery system from the grid is around 70 to 80% efficient.  Returning that energy from the battery to the grid, which will require converting the DC back to AC with efficiencies of about 90%, yields 63-72% overall system efficiency. Other larger-scale bulk storage options may be better e.g. pumped hydro reservoirs – except you have to build them and make connections to them.  The beauty of V2G is that the batteries already exist (or will soon) and V2G makes use of the same grid links already used by homes. There is also the point that it doesn’t have to be just peak matching: there are often small input and output perturbations on the grid that need to be balanced, and that can be done without decreasing the life of the battery – dealing with small ups and downs is a gentler process. So there may be a less strenuous role for V2G, not just at peaks.

To improve the (time) spread of available vehicles, it is conceivable that charging could be done in the daytime at the workplace e.g. there are some large PV arrays on solar shades at car parks in the US. Things also get a bit easier if we move away from PV to storing wind-derived grid electricity – which comes in at most times of the day in varying amounts (from wind farms), including at night, though some of the logistic issues still apply – you still don’t want your batteries drained without warning. There may also be synergistic timing problems with other power uses. The government’s current plan is for most houses to be heated electrically rather than by gas. However, if vehicle charging is required in the same dwellings as those containing electric heat-pumps, used for evening heating, then as the Energy Technologies Institute has noted “not only will the local sub-system be very likely to be overloaded, but the individual connection to the building may need to be replaced”. And, nationally, with two systems requiring electricity, the evening peak demand will be raised even more.

Will V2G happen? Some analysts are very optimistic. For example, Zpryme’s Smart Grid Insights projected that by 2020 there will be over one million V2G-enabled vehicles on the road in the US with an estimated market value of $26.6 billion and an additional $6.7 billion infrastructure market: There are certainly some interesting projects and programmes going ahead around the US and EU: for examples see However, for the moment in the UK, National Grid says that it doesn’t consider “vehicle to grid (V2G) services as economically viable in the near term due to additional costs that be incurred to make them export capable”.

The viability of the V2G idea also, of course, relies on wide uptake of electric vehicles, which may take time, or not occur on a significant scale. There may also be better ways to use the excess electricity that will be produced at times by variable renewables and indeed by nuclear plants. Not just pumped hydro storage (the main current approach) but also (storable) hydrogen production, with hydrogen, and syngases produced from it, being an option for vehicle use. It is not clear yet which transport options will win out. EVs may dominate for cars, but there could be a major role for green-gas-fuelled trucks and buses, with gas storage generally becoming a significant buffer for the variability of renewables. In which case there may be less of a role for V2G. Time will tell.

Being optimistic, the Centre for Alternative Technology’s 2030 Zero Carbon Britain scenario had 25 GWh annually coming from V2G EV batteries. Useful but not huge. Even the high “Level 3” in DECC’s 2050 UK Pathways model only has 5.3 GW of V2G by 2050. That may change, but for the moment V2G looks marginal.

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