Electric vehicles as storage – a psychologist’s view

“LONDON (UK) 10th May 2016: Automotive industry leader Nissan and multinational power company Enel, today confirmed plans to launch a major vehicle-to-grid (V2G) trial – the first ever carried out in the UK. The trial will work by installing and connecting one hundred V2G units at locations agreed by private and fleet owners of the Nissan LEAF and e-NV200 electric van. By giving Nissan electric vehicle owners the ability to plug their vehicles into the V2G system, owners will have the flexibility and power to sell stored energy from their vehicle battery back to the National Grid.”

V2G is a novel concept which clearly excites both journalists and academics (always a tricky combination if neither have experience of using the technology in question in their everyday lives).  In contrast, Nissan’s parallel set of trials (redeploying old EV batteries as home storage units) offers a  more practical and environmentally useful initiative but it doesn’t excite the journalists and is even sometimes mis-reported as being vehicle-to-grid (V2G) because the latter is viewed as sexier.

But, as a behavioural psychologist and EV owner, I have always been doubtful as to how V2G would work practically in the home.

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Thierry Twizy

We have owned a small electric vehicle (a Renault Twizy) for 4 years and use it every day for all our local journeys,  rarely using our other car. We try to arrange to charge our Twizy when the sun is shining so it can get as much of the power it can from our 4kW solar panels rather than from the grid. Even so, because the Twizy battery (6kWh) consumes 2 kW for much of its charging cycle, it is rare for it to manage to recharge purely from the output of our 4kW solar panels given the background loads in the house at the same time.

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PV exporting to battery & grid – 4pm in May

Our situation has now changed. A month ago, we had a Wattstor 6 kWh energy storage system installed by Gwent Energy CIC because we were keen to use as much of the solar PV we generate for our own domestic consumption rather than exporting it to the grid. We work from home and, in May through to October, most of the energy our home requires during the day (when the light levels are high) is covered by the solar output with plenty to spare which we then export. But as soon as the sun sets, we are back to importing from the grid regardless of how much solar the day has generated. That is no longer the case. With our new system, our excess solar is now stored by the Wattstor battery enabling the house to use that solar energy during the evening and night rather than importing from the grid.

In May (close to optimum daylight length), the Wattstor system has reduced our grid import to less than 1 kWh a day. However, we are now experiencing a direct conflict between charging the Wattstor battery and the Twizy battery (both of which require 6kWh to fill). They are proving serious competitors for the same energy and this generates extensive discussion (even the occasional dispute) between the household members as we discuss timings and priorities for the disparate power requirements and timings.

If we charge the Twizy during the day,  then we are unlikely to manage to fill the Wattstor battery ready for the evening (unless the sun is blazing for the entire day). If we wait until evening (as sometimes our travel pattern requires), then the Twizy will completely empty the Wattstor battery leaving none for the house. After 4 weeks, we are still discussing which battery is best to fill when and we are lucky in that much of our travel needs are flexible enough to give us some choice. For others who use their EV to commute to work, this would not be the case. It’s also unlikely that any household with a Nissan Leaf could ever generate enough from a domestic Solar PV installation (unless it was twice the average domestic size) to meet its 7kW charge rate.  This suggests that the V2G schemes are not motivated by renewable goals.

Like most EV drivers (even those with much greater range than the Twizy), we prefer to keep our EV battery close to full so we are ready for any journey however unpredictable in length or timing (which local journeys tend to be).  Using an EV battery to power the house at any point or to export electricity to the grid at peak periods would be difficult to optimise via any algorithm (domestic lives and energy uses are too unpredictable). Operating it manually would require daily planning and negotiation among household members about anticipated trips or heavy household consumption (e.g. cooking in the electric oven or operating a tumble dryer).

In my mind therefore, supplying  a household’s disparate energy needs (or making a small return from selling energy to the grid) does not combine easily with supplying a household’s transport energy needs. The two (as we are discovering in our own small domestic experiment) are definitely at odds with one another and working out how to satisfy both (other then maybe than in a fiercely regulated and predictable household) is  too complicated for people to bother. It might sound technically attractive but it’s not a natural behavioural fit. We are used to having a shared energy source (be it grid or domestic battery) which can meet any of the variety of needs which crop up. We don’t need to choose whether or not to operate the washing machine now in case we need spare energy for cooking supper later.

(This leaves aside the separate issue of EV owners who fill up their EV’s from public charge points as they travel who might subsequently sell that electricity back to the grid or use it to power their homes. That’s a separate can of worms and one for an economist rather than a psychologist).

So, I shall watch the upcoming V2G trials with interest and see if asymmetric sharing of domestic and travel energy sources works better than I am predicting.




About Alison Kidd

Research Psychologist
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