The railway systems in various countries are electrified to enhance performance and reduce emissions. 

FREMONT, CA: In the past few years, climate change and environmental sustainability have attracted substantial public attention worldwide. The significance of the extensive use of clean energy has become increasingly prominent. Transportation has been moving towards electrification as one of the biggest consumers of energy, as electricity offers several environmental advantages due to the effective use of renewable energy in electricity systems.

Not only do electrified railway systems decrease emissions of carbon dioxide, but they also improve vehicle performance. There is a rising demand nowadays in enhancing railway vehicles' energy efficiency, especially the braking energy recovery has attracted considerable attention.

Due to the increasing use of regenerative braking technology, kinetic energy (in the braking phase) can be converted into electric energy by electric railway vehicles to reuse. Three solutions for the control of regenerative braking energy (RBE) in railway vehicles are commonly available:

• Storing the RBE in an ESS.

• Synchronizing the loads along the traction power supply lines.

• Feeding the RBE back to the external grid.

Other railway vehicles can make use of the RBE. This approach not only increases energy efficiency but also decreases the railway's peak demand for electricity. The coordination of RBE presence and traction power is essential. To improve such coordination, optimization of railway timetables and driving strategies are commonly studied. Railway vehicles' timing and operating efficiency are limited by the transport market and safety regulations, making optimization very complicated and challenging.

Another solution is to feed the RBE over the substations to the external grid. This method is implemented for DC railway lines, based on reversible substations that enable current to flow bi-directionally through inverters for power electronics. If the reversible substation cannot provide an acceptable level of power quality, harmonics pollution from the traction systems may be passed on to the external grid. This approach also faces the challenges of cost, complexity, and scalability for widespread adoption.

The use of ESSes in rail networks stands out as an alternative method to overcome the limitations of the solutions mentioned above because of the rapid advances in power electronics and energy storage technologies. This approach stores RBE in ESSes for future use, unlike techniques that transmit RBE to other loads or feed RBE back to the external grid. Depending on the necessary functionality of the ESS, the installation of the ESS can be either stationary or on-board. In order to minimize total energy consumption, a comparison of stationary and on-board ESSes is provided.

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