Influence of the charging strategy on the environmental assessment

A marginal observation is one method of calculating the direct environmental effects of one kilowatt hour of electricity charged to an electric vehicle. The crucial question is which power stations will increase their production due to cope with the additional power demands of an electric vehicle compared to to a conventional vehicle. As the operating costs for renewable energy plants are extremely low as they ultimately do not need any fuel, their electricity is fed into the grid in any case. Nuclear power stations also generate only low costs per kilowatt hour, which is why they almost exclusively cover the base load, i.e. through uninterrupted operation. Additional electricity demand is therefore usually covered by thermal power stations, i.e. mainly those running on gas, bituminous coal and lignite.

As part of the Electric Mobility Fleet Test different charging scenarios were investigated for the year 2030 to see which types of power station generate the electricity for recharging (see Fig. 2). The proportion of renewable energies in the average electricity generation was already over 50% in the scenario and the CO₂ factor was around 420 g/kWh.

fig. 2: Comparison of specific CO₂ emissions for different charging scenarios  (IFEU 2011)

The Last Journey scenario describes the situation whereby vehicles are generally recharged immediately after a journey. This leads to peaks in demand at particular times of the day (e.g. evenings after getting home from work) which are met by the increased use of mainly flexible gas-fired power stations.

If the electricity price for the end consumer is adjusted to the current price situation on the electricity market and thus causes temporal displacement of consumption, this is known as Demand Side Management (DSM). In this scenario, the user connects the vehicle to the mains after the journey and states the time at which the vehicle needs to be available again. The charging process is then controlled automatically to minimise the electricity costs. This usually leads to a displacement of the charging process into the night-time hours when many power stations are not working to full capacity. In this case power stations with lower fuel costs are preferred – primarily coal-fired power stations that have high specific CO₂ emissions. The additional electricity demand therefore first and foremost increases the workload of these power stations, giving this electricity a significantly worse climate footprint than average electricity.

A significant improvement of the climate footprint of electric vehicles can only be achieved if the expansion of electric mobility is connected with the construction of additional facilities for the generation of renewable energies, which otherwise would not be built (DSM + EE scenario). In this case, electric mobility can both benefit from the low climatic effect of these facilities and also improve the prerequisites for their integration into the grid through managed charging.

It is conceivable that parked electric vehicles could feed stored electricity into the grid in times of particularly high demand and actively help to stabilise the situation. However, this would require significant improvements in battery technology. In addition, suitable incentive schemes would be needed to encourage customers to make their vehicles available for this grid service.

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Last Updated on Friday, 03 May 2013 20:06