Project GridSurfer: Electric Mobility for a More Stable Electricity Grid
In the GridSurfer project, NEXT ENERGY studied the integration of electromobility into the existing traffic and grid systems. The project took place in the rural area of Oldenburg and Bremen. On the one hand, we wanted to prove the reasonable use of electric vehicles. On the other hand, we tried to analyse the potential of implementing electric vehicle batteries as a stabilising factor into the electricity grid. Among other things, we investigated the so called vehicle-to-grid concept (V2G). By this concept, the traction battery can be used for the vehicle drive as well as for providing storage capacity to compensate for the fluctuating power supply of renewable energy in the grid.
GridSurfer combines the single elements of the Nothwestern energy and traffic systems in a big puzzle.
The outstanding feature of the project, which was funded until September 2011, is the Battery Swapping Station (BSS) for electric vehicles, installed by NEXT ENERGY on its institute premises. The Battery Swapping Station serves as a real testing area. In less than four minutes, the traction battery of an incoming car is replaced by a charged one in a fully automated process. With a cumulative storage capacity of 180 kWh and a maximum power performance of 360 kW, the swapping station provides grid services such as voltage level maintenance or medium-voltage power factor correction.
The Battery Swapping Station on the premises of NEXT ENERGY
Our simulations in the project have shown that using battery storage only for buffering renewable energies is still too expensive and thus uneconomic. An economically interesting solution would be to use vehicle batteries as a combination of traction battery and stationary storage unit in the swapping station. This concept could be an interesting business model especially for taxi companies and other fleet operators.
The project team conducted a three-month field study to obtain detailed information about the every-day use of electrical vehicles and the driver behaviour with respect to the electric driving range. The data was gathered by three converted Audi A2 cars, equipped with batteries made by DBM Energy and a driving range of more than 200 km. After 12,000 kilometers of test driving, the error rate was less than one failure per 1,000 kilometers. Failure causes were, e.g., a gear box failure, an exhaustively discharged 12V-battery of the on-board power supply or water entries into the control system. In contrast, the traction batteries caused no failures. For being able to make reliable assessments about possible failures, degradation of traction batteries under real conditions, and the eventual service life, NEXT ENERGY will continue the field study even after project completion.
Furthermore, NEXT ENERGY investigated the battery charging behaviour and the optimisation of operational management strategies for energy recovery. To this end, we extensively tested the degradation behaviour of lithium-ion batteries. Basing on network and vehicle-specific load profiles, the lithium-ion cells were put through three different laboratory test cycles:
• net driving cycles based on real driving data
• driving cycles with V2G connection
• storage tests with respect to the calendrical degradation
After a defined number of control cycles, we characterised the remaining capacity and the frequency-dependent resistance of the batteries. By means of according tests, we gain more knowledge about the depreciation of battery packs which are additionally used for grid services.
