From parking garage to battery storage

In the age of electromobility, parking garages are transforming into charging stations. In Schwäbisch Hall, research is being conducted into how a parking garage can even become a large, virtual battery storage facility that can be controlled to serve the grid. The calculation of the load forecast, which is influenced by a variety of factors, is of central importance.

As co-operator of the Langer Graben parking garage, which dates back to the 1980s, Stadtwerke Schwäbisch Hall faced the challenge of installing contemporary e-charging infrastructure during the refurbishment. Instead of a complex and expensive power grid reinforcement outside and inside the property, the energy supplier opted for an intelligent charging and load management system from its associated company enisyst GmbH. As a scientific partner for the development of a solution for monitoring and optimizing charging and load management, the Stuttgart University of Applied Sciences is involved via the “Smart_E_Park” research project. This is being funded by the state of Baden-Württemberg as part of the “Intelligent grid connection of parking garages and underground garages” program. The long-term goal is to turn the parking garage into a grid-serving energy storage facility, with the batteries of the parked electric vehicles acting as a bilaterally usable energy buffer, for example for Redispatch 2.0.

Balancing and distributing loading loads

What challenges did and do the project partners face? The new transformer at the Langer Graben parking garage is designed for a maximum of one megawatt of power. In the parking garage, around 100 of the total of almost 500 parking spaces have been equipped with charging boxes, each of which allows up to 22 kilowatts of charging power. If all 100 charging parking spaces were occupied and charging at 22 kW, the transformer capacity would be exceeded by a factor of about 2. The charging system must therefore be able to manage charging operations in such a way that electricity supply and demand, as well as charging capacity, are in balance and the system as a whole remains stable. This is ensured by the cascading connection system with intelligent distribution boxes from enisyst. The system is designed to balance and distribute loads. This means that if more vehicles need to be charged, the specific charging power is throttled. But the system would also be able to prioritize individual charging points, for example, for people who are willing to pay a premium for faster charging. The charging infrastructure has been in operation since the reopening of the parking garage in October 2021 and is functioning as planned. However, the utilization of the e-charging parking spaces is only around 25 percent today.

The particular challenge – and so far there have been at best rudimentary findings on this – lies in determining a typical load curve for a parking garage with e-charging spaces. A reliable load forecast is an indispensable prerequisite for the planned use of the bidirectionally connected e-mobiles in the parking garage as energy storage. Mathematically, 100 vehicles with 80 kWh charging capacity could store up to eight megawatt hours of electricity in the Langer Graben parking garage, which is more than twice the annual electricity consumption of an average household. Although only part of this could be used as control reserve, this would still be an attractive scenario for the grid operator.

Forecasting challenge

To create the load curve, there were and are many parameters to explore: When do e-mobilists charge? What types of vehicles and battery capacities are involved? In which battery charging status do they reach the charging station? How long and how much power is charged? The hardly surprising answer is that the refueling behavior of electric vehicle owners in parking garages is completely different from that of combustion vehicles. With the latter, the tank is often driven almost empty and then filled again completely. Electric vehicles, on the other hand, are charged more often and usually only partially and – depending on the parking time – even when the charge level is still high. The multitude of volatile factors makes forecasting complicated.

Nevertheless, it has been possible to generate a utilization forecast that is quite close to reality. On the one hand, an app was developed that records which vehicles are charging where, for how long, and with what power. The second input source was public data from parking garages in North Rhine-Westphalia, which provided information on how long vehicles typically stay in a parking garage and what the occupancy rate is at what time of day. The data from both sources were combined and a load curve was modeled from them with the support of learning algorithms and artificial intelligence developed by the Stuttgart University of Applied Sciences. This work has not yet been fully completed, but the forecasts already achieve an approximation of the actual situation of over 90 percent.

Further hardening of the data basis

It must be taken into account – and this is one of the reasons why further research is necessary – that the load forecasts to date are based on low occupancy rates of the wallboxes. With further penetration of electric vehicles and more frequent use of the e-parking system, some parameters of charging behavior and system load may change. Very dynamic development can be expected in the electromobility market, and factors such as urban and neighborhood development, for example through changes in shopping behavior, can also have an influence on user behavior. Further data must be continuously collected and insights gained for these scenarios. In the end, a database should be available that makes it possible to use the batteries connected to the charging system in the parking garage as a virtual energy store. The fact that technical rules still have to be created for this is another matter. The operator, Stadtwerke Schwäbisch Hall, says it wants to be ready to act when electric vehicles become the standard means of transportation.

What is already certain is that the data obtained in Schwäbisch Hall will not be directly applicable to other parking garages that are being converted into multi-charging stations: It makes a difference whether people park their e-mobiles in a big-city parking garage during the day while they go shopping, or whether the parking garage is located in an old residential district without associated underground garages and functions as a kind of filling station for the night, so to speak.

Further steps in planning

The next project steps in the Langer Graben parking garage address, among other things, the further broadening and validation of the database and bidirectional charging. enisyst GmbH is already dealing with this future scenario in practice elsewhere. Furthermore, it is about the practical connection of the E-Parkhaus Langer Graben to the network control center of the Schwäbisch Hall municipal utility.

Also on the to-do list is the integration of the charging processes into the consumption billing of Stadtwerke Schwäbisch Hall. Until now, users have paid for the tapped electricity with their parking ticket on site or by using a mobility card from Stadtwerke Schwäbisch Hall. In the future, it will also be possible to invoice the parking garage charging current via normal consumption billing. Consideration is also being given to the implementation of flexible charging rates at quarter-hourly intervals. They are also a tool for grid load management, because customers can fill up on electricity at low cost when the generation supply is currently high.

Conclusion: Implementing intelligent charging management in parking garages and controlling it to serve the grid is a complex task. Their realization requires a lot of technology and know-how. To be able to control an e-parking system in advance and use it as a large battery storage system, a lot of information and data must be processed and evaluated. There is no way around this grassroots work, and there is still a long way to go. It is therefore to be welcomed that a municipal utility like Stadtwerke Schwäbisch Hall is doing pioneering work here and playing a key role in paving the way for a new world of mobility.