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In the “Demand Response” project, researchers are studying how users of electric cars can recharge their batteries flexibly, help relieve the grids and save money in doing so.
© Fotolia / Kara

Grid-controlled charging to help relieve the electric grids.
© enertec

Electric cars stabilise the grid

How can users of electric cars recharge their batteries flexibly, help relieve the grids and save money in doing so? Researchers examined at what point electric cars become a benefit for grid operators and vehicle owners. Their field test demonstrates the conditions this depends on.

In the Lower Saxonian Electromobility Showcase programme, the Hanover municipal utility company (enercity) is investigating how electric cars could be integrated into the grid to stabilise the energy supply system, and which new business models could be developed as a result. A total of 40 vehicles underwent on-road tests with specially developed smart charging boxes, 30 of which were different private and commercial electric vehicles, as well as ten cars from enercity’s own fleet. In December 2015, the field test ended and it now serves as a basis for the further development of electromobility. “About 90 per cent of the energy demand was charged in a flexible manner and could help stabilise and relieve grids. The first preliminary evaluations confirm widespread user acceptance for grid-controlled charging,” said Project Manager Matthias Röhrig about the on-road tests.

During the first 18-week reference period, the researchers recorded the car-charging habits of the drivers. The most common charging periods were during the hours of the highest daily demand for electricity, between 6 pm and 8 pm. Accordingly, charging a high number of electric vehicles uncontrolled in the future would lead to even higher peak loads, especially in the evening. The basic idea of the project, i.e. to control the charging of electric cars in a way that stabilises the grid, therefore proved to be urgent. The specification of predefined time windows in the second phase, such as charging periods from 11 pm to 7 am or from 11 am to 3 pm, was already very promising. The subjects significantly shifted charging toward later hours, during which the total load curve of the grid drops once again.

Making controlled charging behaviour suitable for everyday use

The initially static time windows did not take account of the fluctuating volume of wind and solar energy. In a next step, the researchers therefore made the charging mode more flexible. Once adapted to feed-in with renewable energy, the grid then specified variable time windows for battery charging. 90 per cent of all charging processes were shifted into more flexible hours. “The specific user decisions made during the course of the project demonstrated that controlled charging times are suitable for everyday use,” said Mr Röhrig.
Flexible charging worked that well because only a fifth of the testers required a battery charge of 80 per cent or more on working days. Half of them managed to cope with batteries at half charge for their daily travels. On average, the batteries were only charged on every second day. While acceptance was very high during the week, the weekends were marked by a stronger desire for security in terms of individualised charging. All options were to remain open for their free time, although the test results showed that actual demand was lower. In addition, it was noted that the attitude of the participants changed during the field test. After the field test, an overall lower desire to control the charging process prevailed.

At the end of the project, the researchers tested ways to utilise electric vehicles in virtual power plants. The users stated departure times by when their vehicles should be charged. Fluctuating electricity generation or demand can be compensated by charging or discharging the electric cars while not in use for several hours.

Electromobility Showcase programme

The Demand Response project is one of 30 projects of the “Electromobility Showcase“ programme in the metropolitan region of Hanover Braunschweig Göttingen Wolfsburg. The German Federal Ministry for Economic Affairs and Energy is spending around 640,000 euros to fund the project as part of the Showcase Initiative of the Federal Government. In summer 2016, enercity will publish detailed project results once the final report has been written.



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Project management
Stadtwerke Hannover AG

Project partner
HBK Braunschweig, ITD