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Recuperation of Regenerative Braking Energy in Electric Rail Transit Systems
Mahdiyeh Khodaparastan, Ahmed A. Mohamed, Werner Brandauer
TL;DR
Electric rail systems often cannot reuse all regenerative braking energy because train demand is uncertain and overvoltage protection can force surplus energy to become heat. This paper reviews and compares timetable optimization, energy storage, and reversible-substation approaches, finding reported savings across these technologies while identifying implementation constraints.
Problem
Uncertain simultaneous demand and third-rail overvoltage limits can prevent reuse of regenerative braking energy, causing surplus energy to be dissipated as heat.
Method
The paper comprehensively reviews and compares research, studies, and implementations involving timetable optimization, energy storage, and reversible substations.
Results
Reviewed studies report 4%–34.5% energy savings from timetable optimization, about 30% from ESS, and up to 13% from reversible substations.
Takeaways & Limitations
Each recovery solution has its own advantages and disadvantages and can be implemented in rail systems with different characteristics.
Takeaways & Limitations
Timetable optimization may be limited by service requirements and requires supervisory real-time train monitoring and control, which some systems may lack.
Abstract
from arXiv · showhide
Electric rail transit systems are large consumers of energy. In trains with regenerative braking capability, a fraction of the energy used to power a train is regenerated during braking. This regenerated energy, if not properly captured, is typically dumped in the form of heat to avoid overvoltage. Finding a way to recuperate regenerative braking energy can result in economic as well as technical merits. In this comprehensive paper, the various methods and technologies that were proposed for regenerative energy recuperation have been analyzed, investigated and compared. These technologies include: train timetable optimization, energy storage systems (onboard and wayside), and reversible substations.
I. INTRODUCTION
Electric rail systems can regenerate braking energy, but uncertain opportunities for immediate reuse and overvoltage limits often force surplus energy to be dissipated as heat. The paper reviews timetable optimization and other technologies for recovering this energy.
- Regenerative braking converts mechanical energy into electrical energy, supplying onboard auxiliary loads before surplus energy is returned to the third rail.
- When neighboring trains are not accelerating nearby, third-rail overvoltage limits can prevent energy injection, requiring surplus energy to be dissipated as heat.The resulting heat can warm tunnels and substations and requires ventilation.
- Proposed recovery approaches include synchronized train timetables, onboard or wayside energy storage, and reversible substations feeding surplus energy to the main AC grid.
- The paper comprehensively reviews academic and industrial research, studies, and implementations, comparing the advantages and disadvantages of each solution.
- III. TRAIN TIMETABLE OPTIMIZATION: 7.31% energy saving is reported for an approach optimizing station dwell times and maximum train speeds.
- III. TRAIN TIMETABLE OPTIMIZATION: An integrated optimization method reduced overall system energy consumption by 21.17% more than timetable optimization and 6.35% more than speed-profile optimization for the same system and headways.
IV. STORAGE BASED SOLUTIONS
Energy storage systems capture regenerative braking energy and release it later, reducing grid consumption while supporting peak-demand reduction and third-rail voltage improvement. Storage may be installed onboard or wayside.
- A properly designed ESS captures energy from braking trains and discharges it when needed, reducing energy consumed from the main grid.
- ESS can reduce peak power demand and provide grid services such as peak shaving.
- Capturing regenerated energy with ESS minimizes reliance on dumping resistors, reducing heat waste and ventilation-system costs.
- Onboard ESS is installed on trains, while wayside ESS is trackside and can serve braking and accelerating trains within the same section.
A. Energy storage Technologies
Rail-transit storage technologies include batteries, flywheels, and supercapacitors, whose suitability depends on capacity, power, durability, lifecycle, and application requirements. The reviewed technologies offer different performance and cost trade-offs.
- ESS technology selection depends on energy capacity, specific energy, charge and discharge rate, durability, and lifecycle.
- Common rail-transit storage technologies are batteries, supercapacitors, and flywheels.
- Common rail batteries include lead–acid, lithium-ion, nickel-metal hydride, and sodium sulfur types.
- Flywheels: Flywheel ESS offers approximately 95% energy efficiency, 5000 W/kg power density, more than 50 Wh/kg energy density, and over 20000 cycles, but has high self-discharge, weight, and cost drawbacks.
- Super Capacitors: Supercapacitors provide approximately 95% energy efficiency, more than 50000-cycle lifecycle, power density above 4000, and low heating losses, but have low maximum voltage and high leakage current.
B. Onboard Energy Storage
Onboard energy storage places storage media on the vehicle to capture regenerative energy, while its effectiveness depends on vehicle characteristics and technology choice.
- Onboard ESS places the storage medium on the vehicle, either on the roof or under its floor.Under-floor placement is relatively costly because space is not readily available.
- Onboard ESS efficiency depends strongly on vehicle characteristics that affect energy produced during braking and consumed during acceleration.
- Onboard storage can reduce peak power, stabilize voltage, and support catenary-free operation.
- Super capacitors have been widely implemented in onboard ESS, whereas flywheels received less attention because of safety and cost limitations.
- 18.6%-35.8% energy saving was reported for Brussel metro and tram lines, while Madrid Metro reported 24% energy saving.
- A Rome metro case study reported theoretical 38% energy recovery using an onboard super capacitor control method.
C. Wayside Energy Storage
Wayside ESS temporarily absorbs regenerative braking energy and returns it to the third rail when needed, with reported savings varying by system characteristics and storage technology.
- Wayside ESS connects a storage medium to the third rail through a power control unit.
- Wayside ESS absorbs energy regenerated during braking and delivers it back to the third rail when needed.
- Wayside ESS can help regulate third-rail voltage and minimize voltage-sag problems by rapidly injecting power during acceleration.
- Up to 30% energy savings have been reported for real-world wayside ESS implementations.The reported amount depends on system characteristics and storage technology.
- Sitras SES can provide 1MW peak power and discharge 1400 A DC into the third rail for 20-30 second.The system uses supercapacitor technology and has been implemented in cities across Germany, Spain, and China.
- Capapost provides scalability from 2.8 to 45 MJ of storable energy and has been reported in Hong Kong and Warsaw metro systems.
D. Energy Storage Control and Energy Management
ESS control and energy management must accommodate frequent fast-charging and discharging cycles while matching control objectives such as energy saving or voltage regulation.
- ESS control and energy management are critical because rail applications require frequent fast-charging and discharging cycles.
- The targeted service, such as energy saving or voltage regulation, changes how the ESS should be optimally controlled.
- Research has addressed ESS design and analysis together with optimal sizing, siting, energy management, and control.
V. REVERSIBLE SUBSTATION
Reversible substations provide an inverter-based path for regenerative energy to reach the upstream AC grid or local AC loads, using several converter architectures with power-quality constraints.
- A reversible substation feeds regenerative braking energy through an inverter to upstream AC equipment or the main grid.Potential local AC loads include escalators and lighting systems.
- Reversible substations must minimize harmonics to maintain acceptable power quality for energy fed back to the grid.
- Two common reverse-path architectures combine a diode rectifier with a DC/AC converter or use a reversible thyristor-controlled rectifier.
- A TCI-based HESOP setup uses a backward-connected thyristor-controlled rectifier, with rated current half that of the forward TCR to reduce cost.
- PWM converters operate at unity power factor but involve high cost and high switching losses.
- Using PWM converters for reversible substations requires a step up DC/DC converter between the PWM converter and the DC bus.
- ABB’s Enviline ERS can return surplus energy to the main grid and operate as a rectifier for rectification support and reactive power support.
- The rectifier operates only in traction mode, and switching between rectifier and inverter modes requires coordinated controller pulses without dead time.
VI. CHOOSING THE RIGHT APPLICATION
Selecting a regenerative-energy recuperation technique requires considering network ownership and characteristics, vehicle attributes, and available alternatives. Catenary-free operation restricts recovery to onboard storage, while ownership and network conditions influence whether reversible substations or wayside storage are preferable.
- Technique selection depends on parameters including catenary-free operation, electric network ownership, and network characteristics.
- Catenary-free tram operation permits onboard storage charged by regenerative braking or station fast charging, and supports operation without overhead-line connection.
- High-price energy sales may favor reversible substations, whereas wayside storage should otherwise be selected when operators have limited network autonomy.
- High traffic can enable natural energy exchange between braking and accelerating trains, which should receive priority in recovery-system design.
- Vehicle age, regenerative capability, and weight affect the value and amount of recoverable braking energy.
VII. ELECTRIC TRANSIT SYSTEM SIMULATION
Electric transit simulation couples train performance with traction-network modeling to represent their interdependence. Simulation supports investment decisions and the selection, sizing, placement, and performance assessment of regenerative-energy recovery technologies.
- Simulation can estimate energy consumption and peak power demand to inform future regenerative-energy recuperation investments.
- Recovery-focused simulation helps determine the optimal technology, size, location, and other performance characteristics of regenerative-energy systems.
- Rail-system load flow differs from conventional analysis because train positions change over time and the system combines DC traction with AC sections.
- Electrified-transit models commonly separate vehicle movement from the electric network, although most studies consider single-train operation.
- Train and network simulators must be coupled because train power demand affects network voltage, while voltage drop affects train power demand.The train simulator advances using the current state and previous voltage; the network simulator calculates voltage for the next time step.
- Transient train models use either forward-facing power-to-speed modeling or backward-facing speed-and-vehicle-property inputs to determine train power.
VIII. NONTECHNICAL ASPECTS
Although core technical challenges of integrating energy storage systems are largely resolved, economic viability, regulation, and energy pricing can strongly influence operator engagement with regenerative-energy recuperation.
- Economic viability, regulatory conditions, and energy pricing substantially influence transit operators’ engagement with regenerative-energy recuperation.
A. Economic Aspects
The paper reviews the economic value of energy storage for capturing regenerative braking energy in rail transit. Potential value comes from energy and demand-cost management, regenerative-efficiency improvements, grid services, and other revenue streams, subject to operating conditions.
- Actual energy-use reductions depend mainly on the number of starts and stops and the traveled route, despite widespread regenerative-braking capability.
- Energy costs represent a substantial share of mass-transit operating costs and may become more important as rail operations become increasingly autonomous.
- Rail-system wayside storage applications include energy-cost management, power-cost management, regenerative-braking efficiency optimization, and renewable-energy optimization.
- Wayside energy storage can create additional revenue through grid services such as reactive-power, voltage, frequency, and reserve support.
- Wayside energy storage supports time-based rate management by charging during off-peak periods and discharging during peak periods when price differences cover storage losses.
- Short ramp-up times allow wayside storage systems to participate in wholesale frequency-regulation markets as on-call services.
B. Ownership Aspects
Ownership, policy, and system characteristics shape which regenerative-energy recuperation option is suitable. The paper reviews timetable optimization, energy storage, reversible substations, simulation tools, and deployment considerations across differing rail systems.
- Ownership and policy: Different asset owners may pursue different efficiency targets, affecting energy-cost priorities and financing arrangements for wayside energy storage.Transportation agencies may operate trains while third parties own infrastructure or build and operate wayside ESS under a service agreement.
- Recuperation methods: Train timetable optimization claims 4% to 34.5% energy savings but may require supervisory real-time monitoring and control unavailable in some systems.The method typically requires no new installations and can also reduce peak power demand.
- Recuperation methods: Energy storage can save about 30% of train energy consumed, while its recuperation performance depends significantly on storage location, size, and technology.Onboard ESS is mainly used for catenary-free operation; otherwise, wayside ESS is preferred because onboard implementation is costly.
- Recuperation methods: Reversible substations can feed up to 13% of vehicle-consumed energy back to the main grid, subject to regulations governing grid power injection.The paper describes diode-rectifier/inverter combinations and reversible thyristor-controlled rectifiers as two common configurations.