Engineering & Consulting

Recuperation of braking energy

According to the government report “ENERGY CONSUMPTION IN THE UK, July 2018”, the UK transport sector takes the largest share of the total energy consumption at 40%, followed by the domestic sector (28%), industry (17%) and the services sector (15%).

Rail transportation accounts for 2% of all transport energy consumption, which is roughly 0.5% of the total national energy consumption. For operators of electrified railway lines, the electricity cost is still one of the major operational costs. In addition to operational costs, railway operators also have responsibility for reducing the carbon footprint of the railway by reducing its electricity consumption.

Recuperation of energy produced during regenerative braking is the most effective way to reduce railway electrical energy consumption.
The classical physics law of Energy Conservation tells us that the kinetic energy of a train mass “m” at speed “v” is Ek=0.5*m*v2. When the potential energy is omitted, this will also be the amount of energy taken to stop the train (i.e. the braking energy) by mechanical means (e.g. friction brake) and/or electrical means (e.g. rheostatic brake or regenerative brake). For a modern electrical traction power supply system (TPS), the energy lost during the acceleration of a conventional train is relatively low, amounting to approximately 20% of Ek (in general, 15% due to propulsion system inefficiency and 5% consumed by auxiliary systems on board).

It follows that the amount of energy remaining potentially available for recuperation through regenerative braking is considerable. In a traction power supply (TPS) system with a good receptivity, up to 75% of Ek can be returned to the TPS network, even when 15% propulsion system inefficiency, 5% auxiliary energy consumption and 5% unrecovered are allowed for.

The amount of energy that can be recovered in one acceleration - braking cycle of train operation can be calculated as:

i.) Energy consumption to reach Ek: E1= 0.2* Ek + Ek = 1.2 Ek
ii.) Energy consumption if 75% Ek back to recovered (25% losses): E2= E1 - 0.75* Ek = 0.45 Ek
iii.) % Energy recovered through TPS: ρ = (E1- E2)/ E1 = 62.5%

It is easily apparent that the TPS system’s receptivity (here represented as 0.75) directly affects ρ. For a closer estimate, the energy losses in the TPS system, typically a few percent, and the train resistance should be taken into account in the calculation. In the case of trains running on long intercity routes, by contrast, most energy is used in maintaining speed and the proportion of energy recovered is much smaller.

The idea of regenerative braking, which is to convert the train’s mechanical kinetic energy back to electricity, has been with us for more than one hundred years. It was not widely adopted until the 1990s, when high-power semiconductor-based traction propulsion systems became available. Nowadays, most newly produced rolling stock is designed with regenerative braking capability. However, while a rolling stock’s capability for regenerative braking is one thing, the amount of regenerative energy which can actually be recovered is, although related, a different matter. The regenerated braking energy will in the first instance feed the train-borne auxiliary supply or train-borne energy storage device (ESD), as determined by the traction control unit. Usually the train auxiliary power and train-borne ESD power is much lower than the maximum braking power. Therefore, there will be considerable braking power remaining and braking energy that can potentially be exported out of the train, depending on the receptivity of the TPS network rather than the train control unit.

Receptivity is a general term describing a TPS network’s capability to absorb the regenerative energy produced by trains during braking. It is directly affected by the TPS system’s design, train operation (signalling, timetable and driving style), the controls of the TPS system, the grid operator’s feed-in policy, and by any energy storage devices installed. The receptivity can be improved by various technologies, which are briefly discussed here:

  1. Running more frequent services (with shorter trains): A shorter train (and hence of less mass) needs less braking power and braking energy. A more frequent service means the distance between a train in acceleration and a train in braking will be generally shorter. Both measures will make the regenerated power more readily accessible for use by other trains. However, this may neither be feasible for mass transit systems that already have low headway, nor for systems that have a low ridership.
  2. Synchronising arrival/departure time: Ideally, we would like to see the energy produced by a braking train being re-used by other nearby accelerating train(s). In reality, there are many constraints on the TPS system design and operation, such as fault isolation requirements that do not allow regular track paralleling (low-cost tramway system might be an exception), timetable constraints etc., which have limited the applicability of this technique. Nevertheless, it still has potential to be explored, especially at the planning stage of a new railway line. 
  3. Inverting substations (for DC TPS): The aim is to transfer part of the braking energy back to the AC supply network. As a pre-condition, the supplier has to be prepared to accept feed-in energy produced by railway operators, which is nowadays a lesser obstacle than in the past, thanks to the drive for a green environment and to better inverter technology. The main factor determining adoption of this solution is Return on Investment, closely ranked by reduction of carbon emissions (depending on the source of electrical energy). The potential benefits of inverting substations are:
    - Recovery of braking energy, which reduces the energy intake from the grid.
    - Reduction in the number of traction power substations.
    - Reduction of waste heat (important for tunnels).
    - Elimination or reduction of the need for on-board braking resistors or wayside resistors.
    The benefits of inverting substations depend on their specific locations, on their design, and on the specific operation of the railway, as do the benefits of Wayside Energy Storage Devices (WESD) discussed below.
  4. Installing wayside ESD (WESD): Many technologies have been developed to store energy, such as those based on batteries, supercapacitors and flywheels, superconducting magnetic energy storage (SMES) or a combination of these. For a railway engineer, the main technical differences between types of WESDs, viewed at their input/output terminals, can be characterised by the energy density vs power density curves, as shown in the graph below.
    The ratio of energy density to power density corresponds to the time it takes to charge or discharge a storage device.
    In addition to offering the same benefits as inverting substation, WESD’s can also
    - lower peak power demand from the grid;
    - mitigate voltage sag;
    - offer greater flexibility in the choice of the installation location.
  5. On-board ESD: Due to the limitation of the ESD size and the total cost to a fleet, on-board ESD has mainly been developed for trams/light rail vehicles (LRVs), based on batteries, supercapacitors, and flywheels. On-board ESDs can eliminate or reduce the size of on-board braking resistors. However, the main advantage of on-board ESDs are their capability to allow LRVs to run on a contactless (or partially contactless) TPS systems, which is a significant aesthetic advantage in city centre transport enhancement schemes.
For a railway engineer, the main technical differences between types of WESDs, viewed at their input/output terminals, can be characterised by the energy density vs power density curves.
For a railway engineer, the main technical differences between types of WESDs, viewed at their input/output terminals, can be characterised by the energy density vs power density curves.

To complete the picture, it should be mentioned that optimizing driving style can result in less energy being used in the first place. Two factors that directly help to reduce electricity consumption are reducing the speed before braking and using a lower deceleration rate during regenerative braking (while still meeting the journey time requirement).

It is easy to imagine that the technologies above are also sensitive to the detailed design, the device locations and the exact train operation (i.e. the time when trains export braking energy). As Return on Investment is always the key consideration, it is very important to get a good estimate of how much regenerative energy can be recovered in real operations. At the design stage, this can only be achieved by time-domain simulation of the whole railway power network.

ENOTRAC is one of the leading specialists in the analysis of railway traction power supply systems. ENOTRAC’s FABEL software simulates the operation of the railway in the time domain by taking into account all key technical elements that affect traction power supply such as detailed route data, the power supply network, rolling stock characteristics (including regenerative braking), operational timetables, and energy storage devices (wayside and on-board). FABEL has been successfully used in many projects for analysing energy saving technologies.

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