Batteries on wheels for grid balancing

In July 2021 the so-called LLEC::VxG was initiated as a further extension of the LLEC project, in which various organizational units are involved. We have seen the LLEC-abbreviation (Living Lab Energy Campus) more frequently on this blog, but the abbreviation ‘VxG’ might be new. Very often the term ‘V2G’ is used, which stands for ‘Vehicle-to-Grid’. Basically, this means that the battery of an electric vehicle can be charged with energy from the electricity grid, but can also feed energy back into the grid. Hence, a V2G-capable electric vehicle and charging station support the bidirectional flow of electric power. Nevertheless, we decided to replace the ‘2’ in V2G with an ‘x’ because the project scope also includes unidirectional electric vehicles and charging stations that only support charging an electric vehicle. The charging power and charging moment/time might be controlled by the car user. In this case, ‘x’ can be replaced by ‘1’, making it V1G. The double colon (::) in LLEC::VxG is adopted from programming language and indicates that VxG is part of LLEC. 

In an earlier blog, the LLEC high-power battery system was explained. This is currently being installed close to building 10.14u (Fig. 1). The two functions of the high-power battery system are meant to reduce high peak demands in the grid and to serve as uninterruptable power supply (UPS) for building 10.14u. Within the VxG-project, a 150 kW charging station will be additionally connected to the UPS side of the high-power battery system. By charging electric vehicle batteries with this charging station, it is possible to increase the load on the UPS side of the battery system and to increase the system efficiency. Since energy can only be taken from the UPS to the charging station and not vice versa, the 150 kW charging station is selected as a V1G charging station, i.e. a charging station with unidirectional charging capability. In addition, a bidirectional charging station with a power of 250 kW will be installed. This V2G station will be connected to the transformer station, which is also used to adapt the voltage for the high-power battery system. The 250 kW V2G charging station can therefore also be used for reducing high peak demands in the power grid.

The reason for installing both charging stations in close proximity to the high-power battery system (Fig. 1) is that the cable lengths can be maintained relatively short, which minimizes energy losses and reduces installation costs. Additionally, four parking lots will be available for parking and connecting electric vehicles to the charging stations. Various types of electric vehicles will be acquired within the VxG project, such as the Nissan Leaf, which is one of the few commercially available V2G-capable electric vehicles. 

Both charging stations are custom-made and sourced from an Italian company. An example of the charging post (Ladesäule), containing charging cables and user interface, is shown in Fig. 2 (left). This type of charging post will be used for both charging stations. The charging post contains three cables, each with a different connector: (i) a CCS type 2 connector, (ii) a CHAdeMO connector, and (iii) an AC type 2 connector. This is a rather unique set-up for a (public) charging station and at present these are the three most commonly used connectors in Europe. It is expected that the CCS type 2 connector will become the standard for all (European) electric vehicles in the future. 

Each charging post comes with its own power cabinet (Leistungsschrank). On the right-hand side in Fig. 2 the power cabinet for the 150 kW charging station is shown. The power cabinets contain the power electronics for controlling the charging power and for converting alternating current (AC) to direct current (DC) and vice versa. The 250 kW power cabinet also contains two cooling units for cooling the charging cables between charging posts and electric vehicles. During high-power charging the cable temperature may increase considerably and must therefore be cooled. Another unique feature of both charging stations is their capability to charge both 500 V and 1000 V battery packs. It is to be expected that both charging stations can be taken into operation in the first quarter of 2022. Moreover, additional charging stations and electric vehicles will be acquired and integrated in the course of the project. These charging stations and vehicles are for research purposes only and will be used by VxG project members.

Scientifically, the project focuses on a wide range of topics and thus three organizational units (IEK-9, IEK-10, and TB-P) are involved. In the initial phase of the project, various types of electric vehicles and high-performance charging infrastructure will be integrated into the LLEC energy system. Integrating bidirectional electric vehicles and charging infrastructure contributes to grid balancing. For example, solar energy that is temporarily not consumed by users can be stored in battery packs of electric vehicles. In phases with less energy generation and high demand, the pack can be discharged and that energy can be fed back into the grid for balancing purposes. In this case the battery pack of the vehicle has the same function as both LLEC stationary battery systems. The fact that an electric vehicle is not always connected to the power grid, often parked at other locations, and needs to be partly or fully charged when the user needs to drive, causes major challenges for the control system. For that reason, team ‘Software and Simulation’ (IEK-10) will implement control software for V2G applications. A model, which is based on predictive modelling strategies, determines the optimum between grid requirements, user needs and technical boundary conditions of the electric vehicles at all times. The same team will also focus on integrating the charging infrastructure in the LLEC platform and develop a web-based booking system for the vehicles. Further objectives are to investigate the effects of different charging and discharging modes (bidirectional operation) on the grid as well as answering the question to what extent bidirectional electric vehicles can contribute to grid and system stability.

The research objectives of team ‘Batteries’ (IEK-9) are to investigate battery aging processes under the influence of V2G-processes and to develop appropriate battery aging models to predict aging. The goal of this particular topic is to propose new charging algorithms that reduce battery aging to a minimum under V2G-conditions. A further goal is to design and improve electrochemical and thermal battery models on cell, module and pack level. The designed models will be parameterized and validated with laboratory experiments and test drives with electric vehicles. Ultimately, the models will be used to optimize battery (dis)charging profiles. For this purpose (dis)charging profiles should be developed that do not contribute significantly to battery aging, are suitable for fast charging, or are ideal for V2G-services.

In addition to the technical challenges, user behavior will also play an essential role in this project, on which mostly team ‘Engagement’ (TB-P) will concentrate. Since V2G is comparatively new to most vehicle users, measures for user information and user involvement as well as for knowledge transfer are planned as central elements in the project. This includes surveys to assess user requirements, and to collect possible doubts and concerns. For example, when an electric vehicle driver drives to work and connects to a charging station, the energy that was charged at home might be used for grid balancing purposes at work, with potential battery aging as a consequence. In addition, the battery must be (partially) recharged when the user needs to drive home. For this to be implemented, users must of course accept and allow that their electric vehicles are used for grid balancing purposes, for which compensation measures should be investigated.As a conclusion it should be noted, that with the electrification of the society, the electricity grid could become unstable and/or overloaded. The general consensus is that high investment costs are necessary to make the electricity grid future-proof. However, some experts believe that bidirectional electric vehicles could heavily contribute to grid balancing if control algorithms are accurately designed. Therefore, these investment costs might be significantly reduced. According to current sales forecasts for electric vehicles in Germany, the share of electric vehicles could be almost 25% in 2030. This would correspond to an absolute number of around 11 million vehicles. If, for example, 60% of these vehicles are V2G capable (AC <11kW), have an average battery capacity of around 100 kWh and a share of 30% available for grid stabilization, approximately 100 – 200 GWh of storage capacity and 35 – 72 GW of operating reserve are available. This is about 50 – 100 times more storage capacity than currently available from home storage and large-scale storage systems in Germany.