Idaho National Laboratory, in collaboration with Argonne National Laboratory and the National Renewable Energy Laboratory in the US, have been assessing technical issues that might hinder the implementation of extreme fast charging for electric vehicles in the US. Their findings, which are detailed in the Department of Energy’s report Enabling Fast Charging: A Technology Gap Assessment, published in October, and a series of four papers published in the Journal of Power Sources, pinpoint many of the key difficulties for obtaining very fast charging at the battery, vehicle and infrastructure levels.
“Utilities really plan based on how much power you deliver and what your peak needs are – if you don’t appropriately size your battery, then your demand charges are going to be high”Eric Dufek
Although many of the problems of designing electric vehicles able to compete with standard petrol-powered cars in terms of acceleration, handling and safety are steadily being resolved, one of the remaining barriers to widespread adoption has been their longer charging time. Most existing electric vehicles have a range of around 100 miles, while those with a longer range can travel over 200 miles on a fully charged battery. The fastest charger is a 50 kW system that can charge a standard electric vehicle battery in under an hour, although Tesla and Hyundai have also developed vehicles that can be charged by 120 kW systems, bring charging times down to less than half an hour. However, a new approach, extreme fast charging, could deliver 350 kW of power and charge a vehicle’s battery to 80% capacity in only 5 to 10 minutes.
Although a 350 kW capable car is expected to be launched by Porsche in 2020, there are many economic and logistical challenges to handling that power, especially as charging at that rate is a huge load on the electric grid. This is pushing power companies to invest in expensive distribution infrastructure, and to find a way to make the infrastructure investment worthwhile, for the charging to be reliable. One approach is using a large stationary battery at charging stations that would act as a buffer between the electric vehicle and the grid, which gets round the problem of intermittent charge, as electricity from the grid would slowly recharge the battery during the day. Such batteries could also receive further charge from distributed renewable energy resources, including solar panels.
“In most vehicles, the power electronics are designed for 600 volts or less. Now we would need power electronics to handle up to 1,200 volts, which adds cost and complexity”Richard Carlson
Getting the size of the stationary battery right will be crucial to rolling out the technology. As researcher Eric Dufek points out, “Utilities really plan based on how much power you deliver and what your peak needs are – if you don’t appropriately size your battery, then your demand charges are going to be high”. A further challenge for extreme fast charging is the charging equipment, as such delivery needs electronic components able to handle the load, as well as an expensive liquid cooling system to dissipate the heat generated by the transfer of so much power.
The batteries would need to be more robust, and have a voltage of up to 1,200 volts. How the high-voltage charging control unit affects battery life could be problematic, and mean other electronics in vehicles requiring an upgrade. As researcher Richard Carlson said “In most vehicles, the power electronics are designed for 600 volts or less. Now we would need power electronics to handle up to 1,200 volts, which adds cost and complexity”.