The project, set up between September 2022 and March 2024, finds a way to not only outsmart grid congestion, but also reduce greenhouse gas emissions and save on energy costs—all with no impact on daily metro operations. As a result of the positive findings, GVB and AMS Institute will work towards establishing a pilot test location for direct bus charging at Station Noord.
AMS Institute worked closely with the DC Systems and Energy Conversion & Storage department at TU Delft on this project, with the results of the research coming from the work of dr. Ibrahim Diab.
On May 2nd we released a press release with GVB highlighting our results. For a more in-depth look at the research, continue reading here!
As energy grids are congested, options to electrify public busses are limited
At the heart of the energy transition is electrification. Our power generation is decentralized due to increased solar panels and wind energy, while energy demands are becoming more electric: transportation, heating, and much more. However, electricity grids need help keeping up with this increased decentralized energy flow to avoid grid congestion and keep the system operating within its safety limits. The capacity of the electric grid was limited by its designed-in capacity decades ago, which is now proving to be a key hurdle in accelerating today's energy transition.
The issue: Plans for electrification face barriers
Among the many bus lines that GVB runs in Amsterdam, four bus lines service the Amsterdam Noord region. A plan is to add new opportunity charging (charging for a relatively short period on the go, such as at the bus stop), with two opportunity chargers of 360kW at the Noord bus terminal (total 0.72MW), to facilitate increasing demand for public transport and its electrification. Opportunity chargers could also help to avoid the over-cycling of the batteries, increase transport reliability, and increase route range and traffic frequency. However, the introduction of this plan has met practical limitations: a congested local grid.
The solution: Could bus electrification and metro power be supplied under one power contract?
The planned location of the bus chargers is across the street from Amsterdam Noord metro station, the terminal stop of the new Noord-Zuid line 52. Here, transportation grid substations (i.e., the power supply points) are typically oversized and underutilized to cater to the (very infrequent) worst-case traffic scenario. In connecting the electric bus chargers to the metro substation, bus charging power demand can be bundled up with the metro power demand and managed under the existing power contract.
Connecting the chargers to the third rail of the metro grid, which runs alongside the metro grid and supplies energy, would therefore be most beneficial. By being connected to the third rail, the bus chargers would be supplied energy from multiple substations across the Amsterdam Noord region—as metro vehicles are—thereby distributing the load demand across multiple locations and avoiding the appearance of a peak demand at one substation which could breach the power contract limit.
Notably, the bus charger would also harvest some of the otherwise wasted braking energy generated by the metros when stopping. Currently, over 80 to 85% of that energy is wasted via a braking resistor on the metro’s roof, as there is no electric load near the braking vehicles to recuperate it. For the investigated grid section of the Noord-Zuidlijn, (from Centraal Station to Station Noord), this accumulates to 17MWh per day.
However, there are some issues to consider. Firstly, by adding a new, stationary load on the third rail, the transmission losses (energy lost due to the electrical resistance in the network) between the substation and the metro vehicles will increase, adding some energy costs to the operation. Secondly, the voltage drops on the third rail increase. By transport system standards, this voltage level must remain above a certain threshold to keep the metro vehicles running and avoid problems in the transport services of the metro.
The line from Amsterdam Central to Noord was investigated
Key findings: Increased metro operation would be not be impacted by bus charging solutions
The bus charger integration proved feasible and profitable in both 8 and 12 vehicle per hour traffic scenarios. Importantly, while the line voltage did become lower, it was comfortably above the undervoltage threshold: overall, the daily metro operation would not be impacted. Secondly, only small transmission losses were added to the grid, equivalent to only about 5% of what the two bus chargers would consume daily. This loss was well compensated for because so much braking energy that would have been otherwise wasted can be harvested. This harvest is equivalent to about 20% of the daily demand of the bus chargers. This means that 15% of charging energy comes from a free source, leading to tens of thousands of euros in yearly savings. Finally, the bus charging load was indeed spread along the line, and no single substation struggled alone with the full load of the bus chargers: Noord supplies about 57% of the demand, Noorderpark 22%, and Amsterdam Centraal about 6%.
“In order not to slow down sustainability during grid congestion, smart solutions are needed for complex infrastructure issues. This research is an example of the great added value of close collaboration between scientific researchers and partners in the urban environment to achieve innovative and feasible solutions.”
Stephan van Dijk
Director of Innovation
Further research warranted on wasted braking energy
Even with 15% of braking energy being used in bus charging solutions, over 75% of the metro braking energy remains wasted. A solution here could be energy storage systems. This project also investigated supercapacitors as they can accept and deliver high rushes of power, aligned with the short massive peaks of departing and arriving metro vehicles in the traction grid. This makes them suitable for harvesting the braking power peaks and delivering the power peaks demanded by accelerating metro vehicles. Indeed, relatively small supercapacitor systems, at the order of few kWhs, helped bring the used braking energy up to 30% when installed in tandem with a bus charger. Large systems, at the order of tens of kWhs, recuperated almost 40% of the braking energy, but was proven to be economically unjustifiable as supercapacitors are an expensive form of energy storage. This leaves the door open to future research opportunities: bidirectional substation converters and the integration of more electric base loads and eventually the integration of solar panel energy directly into the metro grid, for example.
Data analysis of the functional ten bus chargers present at Amsterdam Sloterdijk Station was key to the study.
Methodology and background
The two key steps of the methodology were a numerical simulation model for calculating the metro grid states (power, voltage, for example) and a realistic charging power profile of the buses in Noord. The first was developed in MATLAB and validated through measurements that were provided by the GVB. The latter was extrapolated through data analysis of the functional ten bus chargers present at Amsterdam Sloterdijk Station. The work was carried out under the present traffic situation of 8 vehicles per hour on the Noord Zuid line as well as under the expected operation for 12 vehicles per hour.
The research was shared periodically with GVB and culminated with a closing event at AMS Institute in March 2024. GVB and AMS Institute are now jointly investigating the possibility of a pilot test location for direct bus charging at Station Noord.
