The rise of the electric vehicle is inevitable. Over the past decade, many car manufacturers, including BMW, Audi and of course Tesla, have blown the lid off this new technology. As EV battery capacity and motor efficiency continue to increase, the ability to charge the batteries within a reasonable timeframe is of the utmost importance. Steve Hughes, managing director of power quality specialist Reo UK, outlines the problem.
The 2018 model of the BMW i3 has a battery capacity of roughly 38 kilowatt hours, which would take roughly ten to twelve hours to charge through a standard UK 13 amp, 240 volt wall socket. By using a dedicated charging infrastructure such as Tesla’s Supercharger network, pushing power at 150 kilowatts (kW), that time drops to less than 20 minutes. At the leading edge, chargers have been produced that provide 300 to 400 kW of power to specially designed EVs, potentially charging the vehicle entirely in five to ten minutes.
Even at that timescale it’s still not comparable to the time taken to fill a car with fuel. This inconvenience is by far the biggest hurdle facing public adoption of electric vehicles. If EVs were as quick to fuel as traditional cars, then the benefits of EVs would entirely outstrip those of conventional cars.
How to get the power to the car
Electrical grids are very tightly maintained systems. The system needs to respond to changes in demand within seconds. Surplus power is simply wasted, and if demand is not met areas of the grid will brown out. The sign of a quality grid is not just its capacity, but its flexibility to adjust that capacity in response to demand.
For a relatively well-known example, when the half-time whistle blows on England football games, thousands of kettles are flicked on almost simultaneously. The national grid must respond to this change in demand by, for example, patching in extra generators in power plants or releasing stored water through a hydroelectric generator.
An average kettle may have a power rating of 2 kW, meaning one EV charging through a Tesla Supercharger is equivalent to 75 kettles running for as long as the vehicle takes to charge, possibly up to half an hour.
When scaled up to a fully electrified road network after rush hour, with hundreds of thousands of people plugging their EVs in as they get home from work, the problems that the grid infrastructure must face become very apparent.
How can we keep the grid stable?
A dead battery in an EV leaves the user uniquely stranded when compared to internal combustion vehicles, so the ability to charge an EV from any AC supply is important. Batteries use DC, meaning somewhere between the mains and the battery a conversion needs to be made. This slow-charge ability requires the EV to be able to rectify AC on board.
Ideally, each EV would be identical and would charge the same. In reality, however, minute flaws and manufacturing variables in the rectifying components create harmonics in the mains AC signal. Multiplied by hundreds of thousands of EVs, all introducing their own perturbations, and the frequency profile of the grid quickly becomes a noisy mess.
This noise represents an obvious waste of electrical energy. Conductors carrying high frequency AC also exhibit far higher resistance than those carrying the same power at lower frequencies, further reducing efficiency. But more importantly, a noisy AC signal causes interference in high frequency applications, which can negatively impact performance.
An ideal way to filter out unwanted frequencies is an electrical choke, such as those offered by REO UK. These devices use inductance to act somewhat like an electromagnetic flywheel, absorbing and smoothing out transient currents or high frequencies. As adoption of the EV increases, the ability to maintain an electrically smooth mains network becomes paramount.
Now that the major automobile manufacturers are driving into the EV market, the need to future-proof the grid presses harder every year. For all the advantages electrifying the road network brings, it’s all for naught without suitable, reliable infrastructure. There’s still a lot to do before we can reap the benefits of the EV as a society. We have the solutions - they just need applying.