Rapid car battery charging may pose grid problems

Siemens is hard at work on technologies for integrating electric cars into the public power grid.

The development of methods to rapidly recharge cars is just one of the company's contributions to Denmark's Edison project, which is the first to plug a pool of vehicles into the grid.

Practical testing will begin next year on the island of Bornholm. Siemens is also researching components for the electric cars themselves.

The company presented three additional demonstration cars at the climate change conference in Copenhagen: the eRUF Stormster based on the chassis of the Porsche Cayenne. The vehicles built by Siemens and the carmaker RUF are being used as shuttles. The 340 hp (250 kW) Stormster has a maximum range of 180 km on a full battery charge, accelerates to 100 km/h in nine seconds and reaches 150 km/h.

Denmark is driving the development of electric cars particularly hard because the cars' batteries are to be used as intermediate storage for the fluctuating supply of electricity from wind power.

Part of the research being conducted by an international consortium for the Edison project is therefore focused on how to optimise the bidirectional flow of electricity between the car and the grid.

For instance, Siemens Energy is developing rapid charging functions for the cars' batteries. Instead of the usual 220 V, the batteries will be charged in an initial step with 440 V and 63 A. The long-term goal is to achieve up to 300 kW of charging capacity so that a car can be recharged in roughly six months.

The development engineers are also investigating how the constant switching on and off of the batteries affects the power grid. The harmonics this generates could knock the grid out of sync. Here Siemens Energy is working directly on the Riso research campus of the Technical University of Denmark, a partner in the Edison consortium.

Siemens Corporate Technology is currently developing the components for the new Greenster II electric car, which will go into small-series production late this year. A central innovation are the two rear wheels, each of which is powered by its own electric motor. This allows the individual wheels to be optimally powered in every driving situation.

The differential, which provides this function in central drive systems, is eliminated, making the car substantially lighter. In the future, each of the four wheels of an electric car is to have its own little drive system. The axle shaft, cardan shaft and central motor will be eliminated, and the car gains installation space while saving weight.