Faced with growing pressure to reduce carbon emissions, European countries are increasingly making moves to power trains with electricity instead of diesel. In the UK, however, progress has often appeared hampered. According to the Institution of Mechanical Engineers (IMechE), the UK’s share of electrified railways comes in at 42 per cent. In contrast to the over 60 per cent of other comparable European countries, this figure seems rather meagre.
Why electrify rail?
Many people working in the rail industry feel that recent electrification cutbacks are a mistake. One reason for the reduction is the potential of bi-mode trains, which use both electric and diesel to run on electrified sections of the network before switching over to non-electrified sections when the former aren’t available.
While this technology means that passengers can benefit from electric rail much sooner, bi-mode trains cannot be seen as a silver bullet solution. Uncertainty continues to arise over their performance compared to totally electric locomotives, as well as the implications of using diesel power.
Diesel engines have obvious disadvantages. Fuel is significantly more expensive than electric traction, engines are costly to maintain, and their heavier weight requires additional track maintenance.
In addition, the power output of a diesel engine is limited by its rating, and its traction power is further reduced as the engine also has to supply the train’s hotel load. Electric traction power, however, is limited by its thermal loading and as a result it can operate for short periods at peak power. Partly for this reason, an electric multiple unit has typically twice the acceleration of a diesel multiple unit.
In diesel mode, bi-mode trains suffer from the same cost and carbon drawbacks as diesel-only trains. When in this mode, they only have the traction power of around 60 per cent of their electric mode, lacking the speed and acceleration needed to improve services.
When braking, a train’s kinetic energy cannot be stored on-board. On a diesel train, this energy is dissipated as heat using brake discs or from a roof-mounted rheostat that regulates the current flowing through it by changing the resistance. On electric trains, this braking energy can be fed back into the grid, offering energy savings of up to 20 per cent.
During rheostatic braking, electric energy is dissipated by a bank of onboard resistors, often referred to as the braking grid. In regenerative braking, the electricity is immediately reused by other locomotives or stored for later use. This electricity can be transmitted through overhead wires or an electrified third rail. Alternatively, it can be stored onboard using a flywheel, battery or other energy storage system.
While this method of braking helps make rail transport one of the most energy-efficient forms of mass transportation, it comes with limitations. The ability to return electricity is only available for trains that are electrically powered and in constant contact with their power sources, such as underground rail systems.
In cases where there are no other trains on the track, or the distances between trains is too great, the regenerated energy is dissipated in brake resistors, mounted either on the trains themselves or else at fixed locations alongside the track.
Cressall’s expanded mesh resistors are suitable for this application, being convection cooled and silent, with no moving or wearing parts capable of dissipating very high powers in a compact space.
While there is some reluctance in switching to electric power, it is difficult to ignore the benefits of ditching diesel. For routes with dense traffic and lines that need electric traction to obtain high speeds, electrifying the rail is the most effective and sustainable means of network upgrade.
However, while we must welcome progress towards electrifying the rail, we must also make sure that the technology is in place to boost efficiency on the tracks even when electrifying is not possible. As part of a fully electrified rail system, dynamic braking resistors are critical in ensuring excess traction can be dissipated safely.
The author is Simone Bruckner, managing director at power resistor manufacturer Cressall Resistors.