Sustainable urban rail systems: strategies and technologies for optimal management of regenerative braking energy

In a society characterised by increasing rates of urbanisation and growing concerns about environmental issues like climate change, urban rail transport plays a key role in contributing to sustainable development. However, in order to retain its inherent advantages in terms of energy consumption per transport capacity and to address the rising costs of energy, important energy efficiency measures have to be implemented. Given that numerous and frequent stops are a significant characteristic of urban rail, recuperation of braking energy offers a great potential to reduce energy consumption in urban rail systems. This paper presents a comprehensive overview of the currently available strategies and technologies for recovery and management of braking energy in urban rail, covering timetable optimisation, on-board and wayside Energy Storage Systems (ESSs) and reversible substations. For each measure, an assessment of their main advantages and disadvantages is provided alongside a list of the most relevant scientific studies and demonstration projects. This study concludes that optimising timetables is a preferential measure to increase the benefits of regenerative braking in any urban rail system. Likewise, it has been observed that ESSs are a viable solution to reuse regenerative energy with voltage stabilisation and energy saving purposes. Electrochemical Double Layer Capacitors has been identified as the most suitable technology for ESSs in general, although high specific power batteries such as Li-ion may become a practical option for on-board applications in the near future. Furthermore, it has been demonstrated that reversible substations are a feasible and commercially available technology, although their economic viability strongly depends on the ability to sell the excess regenerated energy to the public network operators for an appropriate price. Finally, it has been that a transfer of knowledge at international level between operators, manufacturers and other stakeholders is essential to achieve the great potential offered by regenerative braking, both in terms of energy efficiency, emissions reduction and system reliability.