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

TL;DR: In this paper, the authors present 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.

About: This article is published in Energy Conversion and Management.The article was published on 2013-11-01 and is currently open access. It has received 357 citations till now. The article focuses on the topics: Energy consumption & Efficient energy use.

Summary

1. Introduction

• Urban rail systems play a key role in the sustainable development of metropolitan areas for many reasons, but mainly because of their relatively low ratio between energy consumption and transport capacity.
• Then, the energy surplus may be returned into the power supply line for use of other vehicles within the same network.
• Moreover, the installation of storage devices in substations or along the track (stationary or wayside ESS) could absorb the surplus regenerated energy, delivering it when required for other vehicles’ acceleration, [3]27] – [41].
• Nowadays recovered braking energy is mainly dissipated in electrical resistors and only a small portion of it is used to supply the auxiliary systems of vehicles or returned to the feeder line.
• The final objective of this paper is to prvide an extensive state-of-the-art review on regenerative braking technologies that can help all the stakeholders to improve the energy efficiency of urban rail systems.

2. Maximising the regenerative energy exchange between vehicles

• According to [3], the network receptivity can be defin d as the ratio of the total energy returned back to the line over the potential energy that could be regenerated in the braking process (kinetic and potential energy).
• In the literature it is possible to find several studies dealing with the optimisation of timetables for energy saving purposes.
• The authors of that paper claim that energy savings of up to 14% can be achieved applying that method, which essentially determines th optimum values of the reserve time (stop time) that maximise the usage of regenerative braking.
• They claimed that energy savings could grow up to 7% by slightly relaxing the imetable constraints.
• It was concluded that the energy savings which can be achieved with high frequencies or low number of running trains are not significant.

3. Energy storage systems for urban rail

• The fast and outstanding development of both energy storage technologies and power electronics converters has enabled ESSs to become an excellent alternative for reusing the regenerated braking energy within its own urban rail system, [58].
• On-board ESSs permit trains to temporarily store their own braking energy and reutilise it in the next acceleration stages.
• On the other hand, stationary ESSs absorb the braking energy of any train in the system and deliver it when required for other vehicles’ acceleration.
• Below, ESSs systems will be generally described and both on-board and stationary applications will be discussed.
• An overview of the technologies currently available for ESS and a list of the most relevant examples of application in urban rail systems will be given.

3.1 Components of Energy Storage Systems

• Regardless of whether they are used for mobile or stationary applications, it can be said that ESSs typically consist of three main functions: the en rgy storage device itself, a power converter to condition the input and output electrical flows, and a controller managing the charge and discharge processes.
• As will be discussed in section 3.2, there are a few technologies meeting these requirements to a different degree.
• ESSs normally work with different input and output conditions than those required in railway networks.
• An overview of the most commonly used topologies for power converters can be found in [59].
• Irrespective of the technology selected for the energy storage device, power flow controllers are needed to optimise the ESS performance.

3.2 Energy storage technologies for urban rail applications

• A brief description and comparison of the most important storage technologies available for urban rail applications will be given in this subsection.
• Table 1 summarises the main features of each technology.

3.2.1 Electrochemical double layer capacitors

• Electrochemical double layer capacitors (EDLC), also known as ultracapacitors or supercapacitors, consist in storage devices that essentially work under the same principle as conventional electrolytic capacitors.
• That is, energy is stored in an electrostatic field by simple charge separation and no chemical reactions take place.
• Supercapacitors have a considerably high power density but, conversely, they present a relatively low energy density, [63].
• By contrast, EDLC are characterised by high self-discharge rates.
• Due to their rapid response they may be effectively used for supplying power peak demands and for voltage stabilisation purposes.

3.2.2 Flywheels

• Flywheels are electro-mechanical storage devices that s ore kinetic energy in a rotating mass so-called rotor.
• The stored energy is proportional to the inertia of the rotor and to the square of its rotational speed.
• Whereas early systems usedlarge steel masses rotating on mechanical bearings, the new generation of flywheels are made of carbon-fibre composite rotors suspended by magnetic bearings, [72].
• [82] and despite the fact that they are typically protected by a multiple-barrier containment system (in which the vacuum chamber acts as the first safety enclosure) this potential danger is regarded as a major safety issue in public transport applications.
• Last but not least, flywheel technology is characterised by high self-discharge rates, which is caused by different factors like internal friction or orientation changes produced by vehicle movements.

3.2.3 Batteries

• Batteries store and deliver energy by means of reversibl electrochemical reactions taking place between two different materials immersed in an electrolyte solution.
• These reactions occur inside cells, which are the basic un ts forming a battery.
• Depending on the core chemistry utilised, batteries may offer a wide range of operational characteristics.
• A brief description of the most common and promising battery configurations available for energy storage in urban rail systems is given below.

3.2.4 Superconducting magnetic energy storage

• Superconducting magnetic energy storage (SMES) enabl s electric energy to be stored in the magnetic field generated by a direct current flowing through a coil cryogenically cooled below its superconducting critical temperature.
• Thecurrent circulates indefinitely in the coil due to the nearly zero resistance of the superconducti g cables, which are typically made of niobium-titanium (NbTi), [101].
• The main advantages of SMES systems are their greaten rgy storage efficiency and very fast responses, see Table 1.
• Another serious issue is the strong magnetic fields generated by these kinds of systems, specially when very large capacities are involved.
• Their features make them potentially suitable for railway applications as well, especially for the case of stationary ESSs, [105], [106].

3.2.5 Techno-economic comparison of different storage technologies for urban rail

• In order to compare and assess the suitability of the above discussed technologies for energy storage in urban rail applications, one of the first c iteria to be considered is technical maturity.
• Table 1 shows that batteries present considerably higher energy capacity per unit of weight and volume than flywheels, supercapacitors or even SMES systems.
• Among batteries, lithium-based technologies offer the greatest energy density range, followed by sodium-based ones.
• The efficiency of charge-discharge cycles and the self-discharge rate of ESSs are two important parameters to consider when evaluating storage technologies as they have a strong influence on the overall system costs.
• Lastly, the capital costs cannot be avoided when comparing different energy storage technologies.

3.3.1 Main characteristics of on-board applications

• On-board ESSs can considerably contribute to energy savings in urban transit systems since the energy recovered and stored during the braking process can be used to power the vehicle itself during the next acceleration, see Figure 4.
• Moreover, from the installation of on-board ESSs the following advantages can be expected: − Shaving of power peaks demanded during acceleration of vehicles, which leads to reduced energy costs and minimum resistive losses in the supply line.
• Limitation of voltage drops in the system network, which might eventually allow for a higher traffic density without further modification i the existing infrastructure.
• Some studies have ass ssed that the additional mass due to on- board ESSs increases the traction energy consumption by 1% to 2%, [4], [21].
• The sizing method for mobile ESSs depends upon their main function; that is, the design requirements will be different when aiming at maximising the energy savings, reducing the voltage drops at the line or running the vehicles in free-catenary mode.

3.3.2 Technologies for mobile storage systems

• Given their fast response, high power density and relatively low costs, it can be said that supercapacitors currently represent the best option for regenerative energy storage on board vehicles.
• Their low energy capacity hinders their use in applications where the main purpose is providing autonomous operation to trains.
• Interestingly, the combination of batteries and EDLCs appears to be a very promising option for on-board ESS, especially if operation without OCL is sought.
• Supercapacitors would absorb the peaks of braking energy and would provide the needed power for vehicle accelerations.

3.3.3 Overview of case studies and commercial systems for on-board applications

• This may be seen as evidence that supercapacitors have been considered as the most suitable option for mobile applications.
• On the one hand, the Railway Technical Research Institute (RTRI) in Japan has developed and successfully tested a hybrid electric light rail prototype incorporating a Li-ion battery to recover braking energy, [19].
• Several simulations have assessed the performance of this system, but no results of real operation have been published.
• EDLC T/ E Control for energy consumption reduction.
• This solution was tested on a RTPA tramway in regular operation from May 2009 to September 2010.

3.4.1 Main characteristics of wayside applications

• Stationary ESSs essentially work absorbing the regen rated braking energy that cannot be used simultaneously in the system.
• Stationary ESSs can be used to reduce the energy deman of the whole system, but also to stabilise the network voltage at weak points of the network, which is a major advantage over reversible substations.
• When designing stationary ESSs, it is also very important to take into account the variability of the traffic conditions, [3].
• The stochasti nature of the design variables has been considered in the sizing methods developed in [31] and [33].
• Then, maximisation of other simultaneous benefits such as peak power shaving or energy consumption reduction could be considered by varying some design parameters.

3.4.2 Technologies for wayside storage systems

• Since stationary ESSs have less weight and volume restrictions than mobile systems, the range of suitable storage technologies is wider in this case.
• EDLCs present excellent characteristics to be used in power shaving and voltage stabilisation functions, but as for mobile applications, the reduced energy capacity could limit their use depending on the specific requirements of each system.
• Flywheels can provide similar power capacities but with slightly higher energy densitie.
• SMES systems appear to be a very suitable alternative for stationary ESSs due to their fast response, but their elevated costs, their high complexity and the associated electromagnetic fields may hinder an extensive application.
• Expected advances in Li-ion and NiMH might make them interesting alternatives as well.

3.4.3 Overview of case studies and commercial systems for wayside applications

• As in the case of on-board ESSs, it is interesting to note that EDLC has been the preferred technology for stationary systems so far.
• Another paper dealing with the use of flywheels in stationary systems is [39], which reports on the performance results of a flywheel ESS developed by the defunct company Urenco.
• The power control and conversion capabilities in that stationary ESS are provided by the EnvistoreTM system, originally developed by Envitech Energy to work with supercapacitors, [131].
• This system is currently being tested in line T2 of the public transport network of Lyon (TCL), where one 1 kWh bay has been in operation since March 2011 with very promising initial result, [128].
• In turn, Woojin Industrial Systems has been contracted by the Korean Railroad Research Institute (KRRI) to install and test an ultracapacitor-based wayside ESS in the Seoul metro system.

4.1 General characteristics

• In DC networks, substations typically provide current only in one direction (power to trains) and are not able to drive the electricity generated in the system back to the distribution grid.
• Additionally, reversible substations are required to minimise the level of harmonics, ensuring a good quality of power supply in both AC and DC sides.
• When compared with ESSs, recuperation of braking energy through reversible substations may be considered a more efficient option as they pr sent fewer transformation losses.
• Besides, the implementation, maintenance and repair do not affect operations in the rail system.
• As a way to reduce the payback period, the energy sent back to the grid could be maxi ised by reducing the interchange of regenerated energy between trains.

4.2 Overview of case studies and commercial systems

• As introduced above, determining the optimal number and location of inverting substations requires a complete study and analysis of the entire transport system.
• The HESOP (Harmonic and Energy Saving Optimizer) system consists of a reversible substation for tramways that has been developed by Alstom in partnership with Converteam within the framework of the RailEnergy project (FP6 – 031458).
• In the traction mode, the rectifier transforms AC into DC, while the inverter acts as an active filter.
• In order to validate the system, two prototype reversible substations were constructed and successfully tested, [46].
• Regarding the 750V system, it has been tested in the Oslo metro, whilst a customised 750 V solution is being developed for the new Singapore downtown line, [146].

5. Conclusions

• A comprehensive overview of the currently available technologies for recovery and management of braking energy in urban rail has been pr sented in this paper.
• The main conclusions that can be drawn from the present study are summarised below.
• The high number of scientific studies, demonstration projects and commercially available systems demonstrates that ESSs can be regarded as a valid solution to improve efficiency and reliability in urban rail systems.
• Interestingly, Li-ion and NiMH batteries may be looked as a valid alternative for on-board ESS when a high degree of autonomy is required.
• Sending the excess regenerated energy back to the main distribution grid with reversible substations may be regarded as a very interesting al ern tive to reduce energy consumption in urban rail systems.

Figures

Table 2

Figure 1 Schematic representation of regenerative energy exchange between trains

Figure 6 Schematic of reversible substations in urban rail

Figure 5 Schematic of wayside ESSs operation in urban rail

Figure 3 Comparison of energy storage capital costs

Table 4

Figure 4 Schematic of on-board ESSs operation in urban rail

Table 3

Table 5

Table 1

Figure 2 Components of an ESS for railway applications

Frequently asked questions

Q1. What is the main function of inverting substations?

Maintaining the output voltage in traction and regeneration modes to reduce losses is another important function that inverting substations have to meet. 

Q2. What are the contributions in this paper?

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. 

Q3. What are the future works in this paper?

However, the economic benefits of reversible substation strongly depend on the possibility to sell the energy to the public network operators and the price set by them. As a final conclusion, it can be said that, even though regenerative braking is a proven technology, its application in urban rail systems remains relatively unexploited. 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 and emissions reduction. 

Q4. What are the main parameters to consider when evaluating storage technologies?

The efficiency of charge-discharge cycles and the self-discharge rate of ESSs are two important parameters to consider when evaluating storage technologies as they have a strong influence on the overall system costs. 

Q5. What is the promising solution for on-board ESSs?

The combination of supercapacitors and batteries has been identified as the most promising solution for on-board systems providing catenary-free operation. 

Q6. What are the main criteria to consider when selecting storage technologies for railway applications?

Energy and power density are decisive parameters to take into account when selecting storage technologies for railway applications, especially for the case of mobile ESSs where both weight and space are critical. 

Q7. What is the main reason why regenerative braking is considered as an interesting alternative?

Sending the excess regenerated energy back to the main distribution grid with reversible substations may be regarded as a very interesting alternative to reduce energy consumption in urban rail systems. 

Q8. What is the importance of a transfer of knowledge between operators, manufacturers and other stakeholders?

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 and emissions reduction. 

Q9. What are the advantages of EDLCs for stationary applications?

EDLCs present excellent characteristics to be used in power shaving and voltage stabilisation functions, but as for mobile applications, the reduced energy capacity could limit their use depending on the specific requirements of each system. 

Q10. What is the main reason why ESSs are being used in urban transit?

The fast and outstanding development of both energy storage technologies and power electronics converters has enabled ESSs to become an excellent alternative for reusing the regenerated braking energy within its own urban rail system, [58]. 

Q11. What are the advantages of lead-acid batteries?

Lead-acid batteries are mainly used in cost sensitive applications where limitations like low energy density of short cycle life do not represent an issue. 

Q12. Why do urban rail systems play a key role in the sustainable development of metropolitan areas?

Urban rail systems play a key role in the sustainable development of metropolitan areas for many reasons, but mainly because of their relatively low ratio between energy consumption and transport capacity. 

Q13. What is the second option to improve the receptivity of the network?

Another option to improve the receptivity of the line is to equip substations with DC/AC inverters (reversible or activesubstations) so that the regenerated energy can be fed back to the medium voltage distribution network, which is naturally receptive, [42] – [46]. 

Q14. What are the main parameters for the control of on-board ESSs?

As for the control of on-board ESSs, different parameters such as vehicle speed, SoC, requested traction power and network voltage must be considered.