Bidirectional charging in the bus depot

As the largest transport company in Hamburg, Hamburger Hochbahn AG operates the majority of Hamburg’s city bus network and has set itself the goal of achieving climate neutrality by the end of 2030. A key element in achieving this goal is the conversion of its fleet of over 1,100 buses to emission-free vehicles such as battery-electric buses [1] and the installation of the necessary charging infrastructure, including load and charging management systems. Previous discussions on bidirectional charging have been limited almost exclusively to passenger cars. The FfE is already demonstrating that commercial vehicles such as trucks can also be used for this purpose as part of the NEFTON project. Hamburger Hochbahn AG is also asking the question whether bidirectional charging can be used effectively for its battery-electric bus fleet in the future. It therefore commissioned the FfE with this question as part of the “Bidirectional charging in the bus depot” project. The aim was to investigate the revenue potential of bidirectional charging using a real bus depot belonging to Hamburger Hochbahn.

Methodology

In the first step, Hamburger Hochbahn provided data on the movement of buses at one of its depots. This data was processed and converted into annual journey profiles. Various other data, such as annual load profiles and electricity price components, were also provided. The data was used to model the depot for 241 buses. It was assumed that all buses in the depot are electrified. Various use cases were jointly selected and scenarios defined. A combination of tariff-optimized charging, time arbitrage, and peak shaving was chosen for the use cases. Different years (2019-2022) were considered in the scenarios and historical price time series from the intraday auction were taken into account. In addition to bidirectional charging, unidirectional, controlled charging was also optimized, and uncontrolled direct charging was used as a reference. The optimization was carried out using the optimization model eFlame.

Results

Compared to cars, the idle times of the buses are relatively short. However, as most of the buses are parked in the depot overnight, the circulation data already suggests that the depot offers potential for bidirectional charging. The data also shows that around 25% of the depot’s journeys are already covered by electric buses.

The result of the optimization of the bidirectional charging strategy is shown in Figure 3 for an example day in 2021. Depending on the price of the intraday auction, the buses are charged at low prices. Discharging takes place into the grid (V2G) to generate revenue via arbitrage trades. Buses are also discharged to supply the inflexible load (V2B) or other buses (V2V) with cheap energy.

The simulation results vary greatly in the years under consideration. With increasing price fluctuations on the electricity exchanges from 2021, the potential savings from bidirectional charging will also increase. Unidirectional charging optimization can achieve annual savings of several thousand euros per vehicle. Bidirectional charging can slightly increase the savings again. However, the majority of the savings are already achieved through unidirectional optimization. The cost reduction is primarily achieved by postponing charging processes to times when electricity prices are low.

Literature

[1] Hamburger Hochbahn AG: Die HOCHBAHN wird klimaneutral, Sofortprogramm für mehr Klimaschutz, https://www.hochbahn.de/de/verantwortung/umwelt-und-klima (abgerufen am 22.09.2021)