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EPFL Researchers Suggest Storage System to Speed up Charging of Electric Cars

Researchers from EPFL have discovered a solution that will aid in making electric cars competitive, enabling them to be charged as fast as it takes to fill a tank of gas. The solution is an intermediate storage which does not affect the power grid.

The batteries storage capacity has greatly improved, however the power grid does not meet the expectation of being able to charge thousands of cars at the same time. If ultra-fast charging is required, over 10 times more power is needed.

Filling enough fuel for a diesel car to run approximately 1,000 km takes about one and a half minutes. A high-end electric car would only run for around 6 km if charged for the same duration. Increasing the power flow into the vehicle is the only way the charging process can be quicker. To meet this quick charge, 4.5 MW of power would be required, equal to 4,500 washing machines.

We came up with a system of intermediate storage. With this buffer storage, charging stations can be disconnected from the grid while still providing a high charge level for cars.

Alfred Rufer, Researcher, EPFL’s Industrial Electronics Lab.

This can be achieved by using either the medium-voltage grid, used for regional power distribution or the low-voltage grid, used for residential electricity requirements, thereby cutting down the required investment.

The EPFL’s intermediate storage was built using a lithium iron battery as big as a shipping container. The lithium iron battery was charged constantly using a low level of power from the grid. Now, when a car is in need of a quick charge, this buffer battery is able to provide the stored electricity to the vehicle without even using the grid.

The EPFL Energy Center and Industrial Electronics Lab research team constructed a demonstrator to prove that their intermediate storage system works. The demonstrator was built in partnership with the Swiss Federal Laboratories for Materials Science and Technology (EMPA), the Bern University of Applied Sciences, and the Swiss Federal Institute of Technology in Zurich (ETHZ).

The demonstrator is a trailer, housing the intermediate storage battery. It takes power from the low-voltage grid and within 15 minutes, is capable of supplying the required 20-30 kWh to charge a typical electric car battery.

Our aim was to get under the psychological threshold of a half hour. But there is room for improvement.

Massimiliano Capezzali, Deputy Director, Energy Center

Another advantage associated with this concept is that it can be used to establish the capacity required for future charging stations. Electric charging stations will eventually replace gas stations from the last century. Future electric energy suppliers will need to have a capacity estimate for their buffer storage, just as owners will need to have an assessment of what size their fuel tanks need to be. The team have formulated an equation that takes into account parameters, such as the estimated number of electric cars, the charging capacity of the batteries, traffic statistics on a particular road, and the charging needs of users.

Based on real figures from French-speaking Switzerland, the simulations reveal that the set-up is completely realistic. A 2.2 MWh intermediate storage capacity would be required for a station to be able to quickly charge 200 cars per day. The order of magnitude is the same as the energy used by a single home annually, in terms of volume, it is equivalent to four shipping containers.

Electric cars will change our habits. It’s clear that, in the future, several types of charging systems – such as slow charging at home and ultra-fast charging for long-distance travel – will co-exist.

Massimiliano Capezzali, Deputy Director, Energy Center


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