Professional Engineering

Grid balancing is expected to become a fine art in future years as a larger number of variable sources of energy, such as wind and wave, are connected to the electricity network. At present, the overriding principle of operation of the network is to match supply precisely to demand on a second-by-second basis. But that will become even more difficult as intermittency issues associated with renewables increase.

That is why engineers are investigating the potential for using plug-in electric vehicles to provide balancing services to the power grid on a commercial basis. Electric vehicle batteries could act as an energy buffer to support an electricity grid requiring an increasing level of balancing services, while the owners of the vehicles could be financially rewarded for providing such support. It’s what the marketing men call a win-win scenario. 

Ricardo and National Grid have worked together to establish the potential of such a mutually beneficial relationship. And their conclusions bring cause for optimism. “There is no doubt that using plug-in vehicles for grid balancing could represent a flexible solution to the challenges of decarbonising the grid power supplies,” says Simon Wrigley, chief engineer in Ricardo’s technology innovation and strategy department. “Admittedly, there are technical obstacles and limitations, but fundamentally we have found that it’s a very attractive idea.”

Plug-in vehicles of all kinds – pure battery-electric vehicles, range-extended electric vehicles or plug-in hybrid vehicles – could aid grid balancing on account of their relatively high energy draw and power levels. With appropriate control of a large pool of vehicles, the net power flow within the grid could be influenced with a similar effect to that of conventional balancing measures. 

For the purposes of the Ricardo/National Grid research, a nationwide plug-in electric fleet of 600,000 vehicles by 2020 was assumed as a realistic figure. The baseline charger power rating was set as 3kW, this being the capacity available from typical domestic power sockets. However, to enable a full range of charging options and to be able to capture fully the potential of vehicle-to-grid power transfers, a range of charging options up to 50kW was also considered.

For plug-in vehicles, there are two possible modes of balancing operation – demand-side management (DSM) and vehicle-to-grid (V2G). The principle of DSM in the context of plug-in vehicles is simply the interruption or reduction of recharging when required to ease grid imbalances, and the resumption and completion of recharging at a later time. 

DSM mirrors the kind of service that is provided to some large users of electricity whose energy contracts accept variation to ease grid imbalances.

Simulations were carried out to evaluate the total availability of plug-in vehicles to provide balancing services. For DSM, it was found that a standard domestic 3kW charger could provide the same level of balancing as higher-rated chargers. The reason for this is that the value of the grid balancing service provided by reducing demand is proportional to the total energy drawn from the grid; in effect, that a larger charger power could cut a higher power level from demand, but it operates for a shorter period. The research suggests that, using DSM alone, the plug-in electric vehicle fleet in 2020 would be able to provide an average of 6% of daily network balancing requirements. This rises to a maximum of 10% in the evening and overnight. The research also suggests that DSM would provide a modest annual financial return to the vehicle owner of around £50 for zero investment (effectively the equivalent of an 18% saving on annual recharging costs).